experiments in time travel

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Researchers successfully sent a simulated elementary particle back in time

• The second law of thermodynamics states that order always moves to disorder, which we experience as an arrow of time.
• Scientists used a quantum computer to show that time travel is theoretically possible by reverting a simulated particle from an entropic to a more orderly state.
• While Einstein’s general theory of relativity permits time travel, the means to achieve it remain improbable in nature.

In 1895 H.G. Wells published The Time Machine , a story about an inventor who builds a device that travels through a fourth, temporal dimension. Before Wells’s novella, time travel existed in the realm of fantasy. It required a god, an enchanted sleep, or a bonk on the head to pull off. After Wells, time travel became popularized as a potentially scientific phenomenon.

Then Einstein’s equations brought us into the quantum realm and there a more nuanced view of time. No less than mathematical logician Kurt Gödel worked out that Einstein’s equations allowed for time travel into the past. The problem? None of the proposed methods of time travel were ever practical “ on physical grounds .”

So, “Why stick to physical grounds?” asked scientists from the Argonne National Laboratory, the Moscow Institute of Physics and Technology, and ETH Zurich before they successfully sent a simulated elementary particle back in time.

Fair warning: their results are tantalizing but will ultimately dishearten any time lords in training.

A quantum computer mixing chamber (Photo: IBM Research/Flickr)

The great quantum escape

Many of the laws of physics view the future and the past as a difference without a distinction. Not so with the second law of thermodynamics , which states that a closed system always moves from order to disorder (or entropy). Scramble an egg to make your omelet, for example, and you’ve added a whole lot of disorder into the closed system that was the initial egg.

This leads to an important consequence of the second law: the arrow of time. A process that generates entropy — such as your egg whisking — will be irreversible unless you input more energy. It’s why an omelet won’t reform back into an egg or why billiard balls don’t spontaneously reform a triangle after the break. Like an arrow released, the entropy moves in a single direction, and we witness the effect as time.

We are trapped by the second law of thermodynamics, but the international team of scientists wanted to see if the second law could be violated in the quantum realm. Since such a test is impossible in nature, they used the next best thing: an IBM quantum computer .

Traditional computers, like the one you are reading this on, use a basic unit of information called a bit. Any bit can be represented as either a 1 or a 0. A quantum computer, however, uses a basic unit of information called a qubit. A qubit exists as both a 1 and a 0 simultaneously, allowing the system to compute and process information much faster.

In their experiment, the researchers substituted these qubits for subatomic particles and put them through a four-step process. First, they arranged the qubits in a known and ordered state and entangled them — meaning anything that happened to one affected the others. Then they launched an evolution program on the quantum computer, which used microwave radio pulses to break down that initial order into a more complex state.

Third step: a special algorithm modifies the quantum computer allow disorder to more to order. The qubits are again hit with a microwave pulse, but this time they rewind to their past, orderly selves. In other words, they are de-aged by about one millionth of a second.

According to study author Valerii M. Vinokur, of the Argonne National Laboratory, this is the equivalent of pushing against the ripples of a pond to return them to their source.

Since quantum mechanics is about probability (not certainty), success was no guarantee. However, in a two-qubit quantum computer, the algorithm managed a time jump an impressive 85 percent of the time. When it was upped to three qubits, the success rate dropped to about 50 percent, which the authors attributed to imperfections in current quantum computers.

The researchers published their results recently in Scientific Reports .

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Bringing order from chaos

The results are fascinating and spur the imagination, but don’t start investing in flux capacitors yet. This experiment also shows us that sending even a simulated particle back in time requires serious outside manipulation. To create such an external force to manipulate even one physical particle’s quantum waves is well beyond our abilities.

“We demonstrate that time-reversing even ONE quantum particle is an unsurmountable task for nature alone,” study author Vinokur wrote to the New York Times in an email [emphasis original]. “The system comprising two particles is even more irreversible, let alone the eggs — comprising billions of particles — we break to prepare an omelet.”

A press release from the Department of Energy notes that for the “timeline required for [an external force] to spontaneously appear and properly manipulate the quantum waves” to appear in nature and unscramble an egg “would extend longer than that of the universe itself.” In other words, this technology remains bound to quantum computation. Subatomic spas that literally turn back the clock aren’t happening.

But the research isn’t solely a high-tech thought experiment. While it will not help us develop real-world time machines, the algorithm does have the potential to improve cutting-edge quantum computation.

“Our algorithm could be updated and used to test programs written for quantum computers and eliminate noise and errors,” study author Andrey Lebedev said in a release .

Is non-simulated time travel possible?

As Kurt Gödel proved, Einstein’s equations don’t forbid the concept of time travel, but they do set an improbably high hurdle to clear.

Writing for Big Think , Michio Kaku points out that these equations allow for all sorts of time travel shenanigans. Gödel found that if the universe rotated and someone traveled fast enough around it, they could arrive to a point before they left. Time travel could also be possible if you traveled around two colliding cosmic strings, traveled through a spinning black hole, or stretched space via negative matter.

While all of these are mathematically sound, Kaku points out that they can’t be realized using known physical mechanisms. Similarly, the ability to nudge physical particles back in time remains beyond our reach. Time travel remains science fiction for all intents and purposes.

But time travel may one day become an everyday occurrence in our computers, making us all time lords (in a narrow sense).

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Quantum time travel: The experiment to 'send a particle into the past'

Time loops have long been the stuff of science fiction. Now, using the rules of quantum mechanics, we have a way to effectively transport a particle back in time – here’s how

By Miriam Frankel

29 May 2024

Ryan wills/istock/Amtitus

When Seth Lloyd first published his ideas about quantum time loops, he hadn’t considered all the consequences. For one thing, he hadn’t anticipated the countless emails he would get from would-be time travellers asking for his help. If he could have his time over again, he jokes, he “probably wouldn’t have done it”.

Sadly, Lloyd, a physicist at the Massachusetts Institute of Technology, won’t be revisiting years gone by. Spoiler alert: no one will go back in time during the course of this article. But particles? That is another matter.

Theoretical routes to the past called time loops have long been hypothesised by physicists. But because they are plagued by impracticalities and paradoxes, they have been dismissed as impossible for just as long. But now Lloyd and other physicists have begun to show that in the quantum realm, these loops to the past are not only possible, but even experimentally feasible. In other words, we will soon effectively try to send a particle back in time.

Rethinking reality: Is the entire universe a single quantum object?

If that succeeds, it raises the possibility of being able to dispatch, if not people, then at least messages in the form of quantum signals, back in time. More importantly, studying this phenomenon takes us to the heart of how cause and effect really work, what quantum theory means and perhaps even how we can create a successor theory that more fully captures the true nature of reality.

In physics, time loops are more properly known as closed time-like curves (CTCs). They first arose in Albert Einstein’s theory of general relativity…

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Can we time travel? A theoretical physicist provides some answers

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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Prof. Seth Lloyd and other physicists have begun to show that in the quantum realm, theoretical routes to the past called time loops might be closer to reality, writes New Scientist ’s Miriam Frankel. When first publishing his ideas about quantum time loops, Lloyd says he “probably wouldn’t have done it” given all the questions received about time travel, but now testing time loops is experimentally feasible.

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April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

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“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

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Simulations of ‘backwards time travel’ can improve scientific experiments

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Physicists have shown that simulating models of hypothetical time travel can solve experimental problems that appear impossible to solve using standard physics.

We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics David Arvidsson-Shukur

If gamblers, investors and quantum experimentalists could bend the arrow of time, their advantage would be significantly higher, leading to significantly better outcomes. 

Researchers at the University of Cambridge have shown that by manipulating entanglement – a feature of quantum theory that causes particles to be intrinsically linked – they can simulate what could happen if one could travel backwards in time. So that gamblers, investors and quantum experimentalists could, in some cases, retroactively change their past actions and improve their outcomes in the present.

Whether particles can travel backwards in time is a controversial topic among physicists, even though scientists have previously simulated models of how such spacetime loops could behave if they did exist. By connecting their new theory to quantum metrology, which uses quantum theory to make highly sensitive measurements, the Cambridge team has shown that entanglement can solve problems that otherwise seem impossible. The study appears in the journal  Physical Review Letters .

“Imagine that you want to send a gift to someone: you need to send it on day one to make sure it arrives on day three,” said lead author David Arvidsson-Shukur, from the Hitachi Cambridge Laboratory. “However, you only receive that person’s wish list on day two. So, in this chronology-respecting scenario, it’s impossible for you to know in advance what they will want as a gift and to make sure you send the right one.

“Now imagine you can change what you send on day one with the information from the wish list received on day two. Our simulation uses quantum entanglement manipulation to show how you could retroactively change your previous actions to ensure the final outcome is the one you want.”

The simulation is based on quantum entanglement, which consists of strong correlations that quantum particles can share and classical particles—those governed by everyday physics—cannot.

The particularity of quantum physics is that if two particles are close enough to each other to interact, they can stay connected even when separated. This is the basis of quantum computing – the harnessing of connected particles to perform computations too complex for classical computers.

“In our proposal, an experimentalist entangles two particles,” said co-author Nicole Yunger Halpern, researcher at the National Institute of Standards and Technology (NIST) and the University of Maryland. “The first particle is then sent to be used in an experiment. Upon gaining new information, the experimentalist manipulates the second particle to effectively alter the first particle’s past state, changing the outcome of the experiment.”

“The effect is remarkable, but it happens only one time out of four!” said Arvidsson-Shukur. “In other words, the simulation has a 75% chance of failure. But the good news is that you know if you have failed. If we stay with our gift analogy, one out of four times, the gift will be the desired one (for example a pair of trousers), another time it will be a pair of trousers but in the wrong size, or the wrong colour, or it will be a jacket.”

To give their model relevance to technologies, the theorists connected it to quantum metrology. In a common quantum metrology experiment, photons—small particles of light—are shone onto a sample of interest and then registered with a special type of camera. If this experiment is to be efficient, the photons must be prepared in a certain way before they reach the sample. The researchers have shown that even if they learn how to best prepare the photons only after the photons have reached the sample, they can use simulations of time travel to retroactively change the original photons.

To counteract the high chance of failure, the theorists propose to send a huge number of entangled photons, knowing that some will eventually carry the correct, updated information. Then they would use a filter to ensure that the right photons pass to the camera, while the filter rejects the rest of the ‘bad’ photons.

“Consider our earlier analogy about gifts,” said co-author Aidan McConnell, who carried out this research during his master’s degree at the Cavendish Laboratory in Cambridge, and is now a PhD student at ETH, Zürich. “Let’s say sending gifts is inexpensive and we can send numerous parcels on day one. On day two we know which gift we should have sent. By the time the parcels arrive on day three, one out of every four gifts will be correct, and we select these by telling the recipient which deliveries to throw away.”

“That we need to use a filter to make our experiment work is actually pretty reassuring,” said Arvidsson-Shukur. “The world would be very strange if our time-travel simulation worked every time. Relativity and all the theories that we are building our understanding of our universe on would be out of the window.

“We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics. These simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday’s problems today.”

This work was supported by the Sweden-America Foundation, the Lars Hierta Memorial Foundation, Girton College, and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

Reference: David R M Arvidsson-Shukur, Aidan G McConnell, and Nicole Yunger Halpern, ‘ Nonclassical advantage in metrology established via quantum simulations of hypothetical closed timelike curves ’, Phys. Rev. Lett. 2023. DOI: 10.1103/PhysRevLett.131.150202

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Scientists use quantum entanglement to travel in time

“these simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday’s problems today.”.

Loukia Papadopoulos

Representational image of time travel.

DKosig/iStock  

Researchers at the University of Cambridge have demonstrated that they can mimic what would happen if one could travel back in time by playing with entanglement, a central concept in quantum mechanics that allows particles to be inherently linked.

Quantum entanglement is an intriguing phenomenon that occurs when two or more particles become correlated in such a way that the state of one particle cannot be described independently of the state of the other(s), even when they are separated by large distances. This means that the properties of one particle, such as its spin or polarization, are dependent on the properties of the other particle(s).

A gift from the past

“Imagine that you want to send a gift to someone: you need to send it on day one to make sure it arrives on day three,” said lead author David Arvidsson-Shukur, from the Hitachi Cambridge Laboratory. “However, you only receive that person’s wish list on day two. So, in this chronology-respecting scenario, it’s impossible for you to know in advance what they will want as a gift and to make sure you send the right one.

“Now imagine you can change what you send on day one with the information from the wish list received on day two. Our simulation uses quantum entanglement manipulation to show how you could retroactively change your previous actions to ensure the final outcome is the one you want.”

Entanglement’s ability to instantly reflect changes in one particle’s state in another particle’s state independent of their physical proximity is one of its most remarkable features. Albert Einstein is credited with using the phrase “spooky action at a distance” to describe what appears to be happening. Due to the fact that two particles can continue to interact even when separated, quantum physics offers a unique solution to time travel.

Co-author Nicole Yunger Halpern, researcher at the National Institute of Standards and Technology (NIST) and the University of Maryland, explained that what the scientists are proposing is the entanglement of two particles.

The first particle is the one used in an experiment. It then acquires new information which leads the experimentalist to manipulate the second particle to effectively alter the first particle’s past state. This process then changes the outcome of the experiment, linking the past to the present.

Linking quantum metrology

The theorists then linked quantum metrology to their model to make it relevant to technology. A common quantum metrology experiment involves shining photons onto an object of interest, and then subsequently registering them with a unique kind of camera. The photons must be prepared in a specific way before they reach the sample for this experiment to be effective. The scientists demonstrated that even if they discover the optimum way to prepare the photons after they have already reached the sample, they can still utilize simulations of time travel to alter the original photons.

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“We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics. These simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday’s problems today,” said Arvidsson-Shukur.

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Loukia Papadopoulos <p>Loukia Papadopoulos is a journalist, writer, and editor with previous experience with the United Nations Momentum for Change, Leo Burnett and Al Arabiya English. She holds a D.E.C. in Pure and Applied Sciences from Marianopolis College, a B.A. in Communications and an M.Sc. in Geography, Urban and Environmental Studies from Concordia University.</p>

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13.7 Cosmos & Culture

Time travel with your fridge.

Jimena Canales

Jimena Canales is a faculty member of the Graduate College at University of Illinois, Urbana-Champaign and a research affiliate at MIT. She focuses on 19th and 20th century history of the physical sciences and science in the modern world. Her most recent book is titled The Physicist and the Philosopher: Einstein, Bergson and the Debate That Changed Our Understanding of Time . You can learn more about her here .

Since H.G. Wells combined the words "time travel" — and used them so systematically to refer to using a machine to travel to a certain date in the calendar — in The Time Machine in 1895, scientists and the public at large have been fascinated with its possibility.

Establishing the rules of time-traveling became part of science in the 19th century. Albert Einstein famously entered these debates, showing us how we could do it. So did the late Stephen Hawking , who wrote a children's book with his daughter about time travel. And Kip S. Thorne (winner of the 2017 Nobel Prize in Physics), too, filling Black Holes & Time Warps with adventures of going down a wormhole where "within a fraction of your second of your own time you will arrive on Earth, in the era of your youth 4 billion years ago." Many other cosmologists and physicists have followed suit, crafting stunning narratives and imagining new laboratory experiments to test them out.

Quantum mechanics gives us particular time-traveling options, that differ from those of relativity theory, by showing how we can fidget with entangled atomic properties. "Physicists Demonstrate How to Reverse of the Arrow of Time," ran a recent headline in MIT Technology Review , while "'Arrow of time' reversed in quantum experiment," was the headline used by Science News .

In these experiments, time is reversed because scientists can make a cold object heat up a hotter one. Thus, these experiments promise to be an entryway into yet another area traditionally considered the realm of science fiction: perpetual motion. But a Second-Law-of-Thermodynamics -breaking-gadget might not be just right around the corner. Even less likely is one that will reverse time.

The announcement relies on the assumption that our sense of time is due to the law of entropy, which is commonly used to explain the "arrow of time."

In the 19th century, scientists' speculations about how they might go about actually reversing time started reaching wide audiences. In a landmark lecture published in the journal Nature on April 9, 1874, the physicist and engineer William Thomson (known as Lord Kelvin) described how the world would look if it suddenly started running in reverse:

"The bursting bubble of foam at the foot of a waterfall would reunite and descend into the water ... Boulders would recover from the mud the materials required to build them into their previous jagged forms, and would become reunited to the mountain peat from which they had formerly broken away ... living creatures would grow backwards, with conscious knowledge of the future, but no memory of the past, and would become again unborn."

Scientists of the Victorian era concluded that the reason why time flowed in one direction was the same one that made heat travel from hot to cold. Hence, they came up with the first idea for manipulating time: Manipulate the direction of atoms in motion. New molecular theories of heat taught scientists that the best way to control the movement of atoms was by changing their temperature. While heated objects tend to reach temperature equilibrium, the reverse operation is highly unlikely.

"It is very improbable that in the course of 1,000 years one-half of the bar of iron shall of itself become warmer by a degree than the other half," explained Thompson. But these effects could sometimes happen spontaneously. Chances were slim, but real. In fact, the molecular view of nature required this possibility to exist. The "probability of this happening before 1,000,000 years pass is 1,000 times as great as that it will happen in the course of 1,000 years, and that it certainly will happen in the course of some very long time," explained Thomson.

The Victorian public listened to Thomson's calculations in awe.

Thomson had matured quite a bit as a scientist and intellectual by the time he offered these numbers. As a younger man, he and his brother had chased with jejune enthusiasm a host of possible entropy-busting perpetual motion machines. Their contraptions proved not only to not function at all, but they had often already been thought up by ingenious others. (A friend of James Thomson politely told him he should go back to studying before talking so big: "It seems to me to be nearly as great a waste of time, making attempts at useful discovery without this previous knowledge, as for a person to labour at working out the highest problems in Astronomy without having first gone through the Calculus." He signed off, "believe me my dear James.")

But eventually, James's inventive genius led to significant improvements to water wheels, pumps and turbines. William's contributions to science, in turn, led him to be elevated to the peerage by Queen Victoria (as first Baron of Kelvin) and to have a temperature scale named after him.

Enter the refrigerator.

When history delivered a temperature-equilibrium-reversing-machine in the form of the refrigerator, one of the first domestic fridge companies adopted the name Kelvinator . They had once been so fascinating that they even attracted the attention Einstein, who applied for a patent. Historian Gene Dannen recounts how the attorney in charge of it was so taken aback when he read the name Einstein in the application that he wrote back: "I would be interested to know if Albert Einstein is the same person who propounded the theory of relativity."

Before the commercialization of refrigerators in the late 1920s by Electrolux, Frigidaire and others, the dream of reversing time by reversing the flow of heat had captivated headlines. It still does. Although the refrigerator did not reverse the entropy of the universe, it did so locally, inside a well-insulated enclosure. It did not deliver on the promise of making time run backwards, but it did permit milk and vegetables to last a bit longer.

Scientists today have succeeded in using a strong magnetic field to make the nuclei in hydrogen particles of chloroform get hotter, while their colder carbon partners got colder.

Have they reversed time?

If this history of thermodynamics can teach us anything, it is that these modest temperature reversals have not taken us back in time at all. But it is more fun to think otherwise. So next time you open the fridge door, let your mind wander off as if on a voyage to the past. If you want to go into the future, you might try your oven.

But if you want to really travel in time, you might try the old fashion way of doing it: Turn to history and literature.

"For to converse with those of other ages and to travel is almost the same thing," wrote René Descartes, in the 17th century.

• Albert Einstein
• thermodynamics
• time travel
• entropy reversals

The Time Dilation Experiment: How Physicists Prove Its Real

As a team of physicists, we are fascinated by the concept of time dilation. It is a fundamental aspect of Einstein's Theory of Relativity that describes how time can appear to pass differently for two observers in different frames of reference. This theory has been proven experimentally time and time again, and today we want to take you through some of the most compelling experiments that have been conducted to demonstrate this phenomenon.

The first thing we need to understand is what time dilation actually means. In simple terms, it refers to the fact that time appears to move slower for an observer who is moving relative to another observer who is stationary. This may sound counterintuitive, but it has been demonstrated repeatedly through carefully designed experiments. These experiments not only help us better understand the nature of our universe but also have practical implications in fields such as GPS technology and space travel. So let's dive into the exciting world of physics and explore some fascinating examples of how physicists prove that time dilation is real!

Understanding Time Dilation Theory

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Understanding the mind-bending theory of time dilation is essential for grasping the intricacies of Einstein's theory of relativity. In simple terms, time dilation can be defined as the difference in elapsed time between two events that occur at different distances from a gravitational mass or relative to each other's motion. This means that time passes slower for an object in motion or near a massive object than it does for an observer who is stationary and far away.

To understand this concept better, let's take an example. Imagine two synchronized clocks placed at different altitudes - one on top of Mount Everest and another at sea level. According to the theory of general relativity, because gravity is weaker at higher altitudes, the clock on Mount Everest would tick faster than the one at sea level. This phenomenon can be explained mathematically using equations such as Lorentz transformations and special relativity formulas.

With this understanding of time dilation, we can now delve into the concept of time dilation experiment without missing any crucial details.

](/blog/time-travel-theories/time-dilation/time-dilation-experiment-physicists-prove-real)As we delve deeper into the concept of measuring time in different ways, a mind-bending realization starts to take shape. The theory of relativity suggests that time is not constant and can be influenced by various factors, such as gravity and motion. To prove this theory, physicists have conducted numerous experiments over the years using advanced measurement techniques and observational evidence.

To further illustrate the concept of time dilation, here are some key points to consider:

• According to the theory of relativity, time passes more slowly in strong gravitational fields or at high velocities.
• This means that if two individuals were traveling at different speeds or in different gravitational fields, they would experience time differently.
• The first experimental evidence for time dilation came from the famous Hafele-Keating experiment in 1971, which involved atomic clocks being flown around the world on commercial airliners.

With these ideas in mind, let us explore how physicists were able to conduct their first time dilation experiment.

You will delve into the first demonstration of time's non-constant nature through an experiment using advanced measurement techniques and observational evidence. The first time dilation experiment was conducted by two physicists, Joseph Hafele and Richard Keating, in 1971. They flew atomic clocks on separate commercial airplanes that traveled around the world in opposite directions. This experimental setup allowed them to compare the elapsed time of one clock with respect to another.

The data collection process involved comparing the readings of the clocks after they returned from their journeys. The results showed that the clock traveling westward experienced a slower passage of time than the stationary clock on Earth, whereas the clock flying eastward experienced a faster passage of time than its counterpart on Earth. This finding provided strong evidence for Einstein's theory of relativity and proved that time dilation is not just a theoretical concept but a real phenomenon that occurs in our universe.

This groundbreaking experiment paved the way for further research into understanding how gravity affects space-time and led to more recent time dilation experiments exploring new frontiers such as black holes and neutron stars.

In recent years, there have been several groundbreaking experiments that further prove the existence of time dilation. One such experiment involved atomic clocks, which are incredibly precise timekeeping devices. By measuring the differences in time between two identical atomic clocks (one stationary and one in motion), scientists were able to observe time dilation effects predicted by Einstein's theory of relativity.

Another experiment involved observing gravitational time dilation, which occurs when an object is located near a massive body causing it to experience slower time than an observer farther away from the massive body. Scientists observed this effect by using extremely sensitive atomic clocks placed at different heights above sea level.

The results and analysis of these experiments provide even more evidence for the reality of time dilation and its importance in our understanding of physics.

You'll feel the ticking of an atomic clock in your bones as you imagine the precision and accuracy required for this experiment. Atomic clocks are the standard for measuring time with extreme accuracy, relying on the natural vibrations of cesium atoms to keep incredibly precise time. The recent atomic clock experiment conducted by physicists tested whether or not time dilation occurs at different altitudes above Earth's surface.

The test involved comparing two identical atomic clocks: one kept on the ground and another taken up to a high altitude via airplane. The results confirmed that time dilation does indeed occur, with the higher altitude clock running slightly faster than its grounded counterpart due to gravitational differences. This level of atomic clock accuracy is essential for measuring even the smallest differences in time dilation, providing crucial data for theories like Einstein's theory of relativity.

Now, let's move on to the next step where we explore how physicists conduct experiments that prove gravitational time dilation is real.

Get ready to feel the thrill of discovery as we delve into the fascinating world of gravitational time differences and how they can be measured with incredible precision. The gravitational time dilation experiment involves measuring the difference in time between two clocks placed at different altitudes in a gravitational field. As Einstein's theory of general relativity predicted, time moves slower closer to a massive object due to the curvature of space-time caused by gravity.

Experimental evidence for this effect was first observed in 1962 when atomic clocks on board airplanes flew around the Earth and were found to be out of sync with identical clocks on the ground. More recent experiments have used highly precise atomic clocks flown on airplanes or launched into space satellites to measure these effects even more accurately. These experiments have also been able to detect other factors that can affect time dilation, such as changes in velocity and gravitational waves. With this technology, physicists are able to confirm that general relativity is indeed an accurate description of our universe.

As we move onto discussing results and analysis, it's important to note that these experiments have not only provided evidence for Einstein's theory but also opened up new avenues for research into fundamental physics, including investigations into dark matter and quantum gravity.

Now we can finally see the fascinating and groundbreaking results that confirm Einstein's theory of general relativity. The gravitational time dilation experiment has provided evidence that time slows down in stronger gravitational fields, which is consistent with the predictions made by the theory. By using precision measurement techniques to compare atomic clocks at different altitudes, scientists have demonstrated that time passes more slowly closer to massive objects.

The results obtained from this experiment are statistically significant and provide strong support for Einstein's theory. They indicate that gravity affects not only space but also time, which is a fundamental concept in physics. These findings have important implications for our understanding of the universe and its behavior. As we move on to discussing the implications of time dilation, we must keep in mind how crucial these experimental results are for advancing our knowledge of physics.

So now that we understand the basics of time dilation and how it has been experimentally proven, let's look at some of its implications. First, there are practical applications for space travel: as objects near the speed of light experience less time than those at rest, astronauts on long space missions could age slower than their counterparts on Earth. Secondly, time dilation has theoretical implications for our understanding of the nature of time itself and its relationship to space. Finally, continued research and development in this area could lead to new technologies and a deeper understanding of fundamental physics.

You'll be fascinated to learn that space travel could become more efficient and faster with the use of time dilation, as demonstrated by the fictional spacecraft in the movie Interstellar. The concept behind this is simple: if astronauts travel at a speed close to the speed of light, their time will slow down relative to those on Earth. This means that they can effectively age slower than their counterparts back home, allowing them to spend more time exploring and less time aging.

This has huge implications for astronaut travel, as it means that we can potentially send humans on long-duration missions without worrying about the effects of prolonged exposure to zero gravity. Furthermore, it also opens up possibilities for interstellar travel and even time travel (in theory). Of course, there are still many technical challenges that need to be overcome before we can realize these dreams, but it's an exciting prospect nonetheless. With all of this in mind, let's delve deeper into the theoretical implications of time dilation.

We can hardly contain our excitement as we explore the mind-boggling theoretical implications of time slowing down at high speeds. Philosophical considerations arise when we ponder how this phenomenon challenges our understanding of the nature of time itself. Our traditional view of time as an absolute and constant entity is shattered by the reality that it can warp and distort depending on relative motion.

The practical implications are equally fascinating. Time dilation has been observed in experiments involving atomic clocks, which have shown that even fractions of a second can make a significant difference over long distances or high velocities. This has important implications for GPS systems, where precise timing is critical for accurate location tracking. As we continue to unravel the mysteries of time dilation, future applications in fields such as space travel and telecommunications may become possible. But first, more research and development is needed to fully harness this incredible phenomenon.

You're about to discover the exciting possibilities that lie ahead in the field of researching and developing new technologies that can harness the incredible effects of time distortion at high speeds. With the confirmation of time dilation through experiments, scientists are now exploring ways to apply this phenomenon in innovative timekeeping devices and space travel. One potential application is using atomic clocks on spacecraft to accurately measure time in space, where the effects of gravity and velocity can distort time.

Technological advancements in quantum mechanics and nanotechnology are also paving the way for more precise measurements of time dilation. Researchers are experimenting with using quantum entanglement to create ultra-precise clocks that could be used for navigation or even detecting gravitational waves. As we continue to uncover more about this fascinating aspect of physics, it's clear that there are countless possibilities for future research and development in this field.

When discussing time dilation theory, it's impossible not to mention Einstein's contributions to the field of physics. His theory of relativity revolutionized our understanding of space and time, showing that they are intertwined and not absolute. Time perception is a crucial aspect of this theory, as it suggests that time can appear differently depending on one's frame of reference. This idea has been tested and proven in various experiments, including the famous Hafele-Keating experiment where atomic clocks were flown around the world to measure differences in elapsed time due to changes in velocity and gravity. Overall, Einstein's work on relativity paved the way for further exploration into the nature of time and how it relates to our physical universe.

When it comes to performing time dilation experiments, there are certainly limitations and potential drawbacks to consider. One major limitation is the accuracy of the experiment itself. In order to measure time dilation accurately, physicists must use incredibly precise instruments and methods. Even small errors in measurement could lead to inaccurate results, which could have serious implications for our understanding of the universe. Another potential drawback is that time dilation experiments can be incredibly complex and difficult to carry out. They require a great deal of planning, resources, and expertise, which may not always be available. Despite these challenges, however, time dilation experiments remain an important tool for physicists seeking to better understand the nature of time and space.

When it comes to practical implications of time dilation, physicists have developed experimental methods that help account for its effects. For instance, GPS systems rely on precise timing to determine a user's location. However, the satellites that send signals to GPS devices are in motion relative to the Earth and therefore experience time dilation. To ensure accurate timing, scientists must adjust the clocks on the satellites based on calculations of their velocity and altitude. By doing so, they can correct for the effects of time dilation and provide users with reliable location data. Overall, while time dilation can pose challenges in certain applications, physicists have found ways to mitigate its impact through careful experimentation and analysis.

Everyday examples of time dilation can be observed in our daily lives. One example is the aging process, where time appears to pass more quickly for those who are moving at higher speeds relative to a stationary observer. Experimental methods have also been used to prove the existence of time dilation, such as high-speed particle accelerators and spacecraft traveling at high velocities. These experiments have shown that time dilation is not just a theoretical concept, but a real phenomenon that occurs in extreme conditions as well as everyday situations.

Future implications of time dilation theory are vast and exciting. Technological advancements in the field will allow for more precise measurements, leading to a deeper understanding of the universe's fundamental workings. To put this into perspective, consider that the world's most accurate atomic clock loses only one second every 15 billion years due to time dilation effects. This level of precision is necessary for research in areas such as space exploration, satellite communication, and GPS technology. As we continue to push the limits of our understanding of time and space, time dilation theory will undoubtedly play a crucial role in shaping our future discoveries and innovations.

So, there you have it – time dilation is not just a theory, but a proven fact. Through various experiments conducted over the years, physicists have demonstrated that time really does slow down when an object moves at high speeds or experiences intense gravitational forces.

But what does this mean for us? Well, it has implications for everything from our GPS systems (which rely on precise timing) to our understanding of the universe itself. It's mind-boggling to think about how much we've learned through these experiments and how much more we still have yet to discover. The possibilities are endless and truly exciting.

In conclusion, time dilation is one of those concepts that can seem too abstract and outlandish to be believed at first glance. But thanks to the hard work and ingenuity of countless scientists over the years, we now know that it's real – a verified phenomenon that shapes our world in ways we're only beginning to understand. It's proof that sometimes even the wildest theories can turn out to be true – a testament to human curiosity and perseverance if ever there was one.

A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time. 

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future. 

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly. 

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them? 

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers 

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible. 

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time 

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox 

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on … 

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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Looks like time travel is possible... for particles of light. 

Using a photon, physicists have managed to simulate quantum particles traveling through time . Studying the photon’s behavior could help scientists understand some inexplicable aspects of modern physics.  

"The question of time travel features at the interface between two of our most successful yet incompatible physical theories -- Einstein's general relativity and quantum mechanics," University of Queensland’s Martin Ringbauer says in a news release . "Einstein's theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules."

Time slows down or speeds up depending on how fast you move relative to another object . Einstein's theory suggests the possibility of traveling backwards in time by following a space-time path that returns to the starting point in space -- but at an earlier time. This is called a closed timelike curve (pictured above). It’s a traversable wormhole. 

In a quantum regime, the authors say, the paradox of time travel can be resolved, leaving closed timelike curves consistent with relativity. Near a black hole, for example, the extreme effects of general relativity play a role. 

Pictured above, a space-time structure exhibiting closed paths in space (horizontal) and time (vertical). A quantum particle travels through a wormhole back in time and returns to the same location in space and time.

"The properties of quantum particles are 'fuzzy' or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations," UQ’s Tim Ralph explains . "Our study provides insights into where and how nature might behave differently from what our theories predict." These include the violation of Heisenberg's uncertainty principle, cracking of quantum cryptography, and perfect cloning of quantum states. 

The work was published in Nature Communications this week. 

[Via University of Queensland ]

Image: Martin Ringbauer

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What We Know About The CIA's Alleged Secret Time Travel Program

In October of 1943, an alleged experiment took place at the Philadelphia Naval Shipyard that opened the proverbial door to time travel (via Military.com ). According to legend, the USS Eldridge (DE 173) — a Cannon-class battleship used primarily to hunt and destroy enemy submarines — left our timeline in an experiment in invisibility gone wrong, per USS Slater . Part of a CIA research project known as Project Rainbow , the alleged incident with the USS Eldridge at the Philadelphia Naval Yard has become unofficially known as "the Philadelphia Experiment," per  The Guardian .

In an effort to put an end to World War II, the story says that the U.S. military began experimenting with ways to end the dragging world war in the quickest, most efficient manner. One of the ideas floating around was the idea of cloaking — or making invisible to radar — battleships. Using a device called a "time zero generator," the military attempted to do just that, per  The Guardian . What allegedly happened, however, was completely unexpected.

Where did they go?

On October 28, 1943, the switch was allegedly thrown on the time zero generator. Eyewitness' claim to have seen the USS Eldridge suddenly begin to glow in a green-blue haze that surrounded the vessel (via Military.com ). The Eldridge began to fade, leaving just the outline of the ship remaining. And with that, the ship blinked out of existence, according to The Guardian . Time slowly ticked by. No one knew exactly what happened to the ship or where it went. After a long 20 minutes, the Eldridge reappeared, but with horrifying results. Much of the vessel was on fire, members of the crew — who allegedly left our reality along with the ship — were found insane. And those were the lucky ones. Reports swirl that many crew members of the Eldridge had "fused" with the ship upon its return to our reality; torsos, limbs, and other miscellaneous body parts were found amalgamated into the ship's steel hull.

A hoax or horrifying accident

According to the surviving members of the ship's crew, during the vessel's alleged 20-minute disappearance, the ship seemingly re-appeared 600 miles away in Newport News, located in Virginia. Of course,  The Guardian  calls the story "hokum" concocted by UFO enthusiast Carl Allen. And with no real concrete proof of the event ever taking place, the U.S. Navy outright denies the Philadelphia Experiment ever happened. 

Nevertheless, the USS Eldridge did exist — sold off to Greece in 1951 and finally decommissioned and sold for scrap in the '90s — and Project Rainbow did occur. But the Office of Naval Research (ONR) stated that force fields to make a ship invisible don't "conform to known physical laws" (via the Black Vault ).  Coupled with the fact that there are no official documents, military or otherwise about the event, it's very well likely the story of the Philadelphia Experiment is likely to remain just that ... a story.

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Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

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• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

causation: backward | free will: divine foreknowledge and | identity: over time | location and mereology | temporal parts | time | time machines | time travel: and modern physics

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Library of Congress Catalog Data: ISSN 1095-5054

Physical Review D

Covering particles, fields, gravitation, and cosmology.

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Prospects for measuring the time variation of astrophysical neutrino sources at dark matter detectors

Yi zhuang, louis e. strigari, lei jin, and samiran sinha, phys. rev. d 110 , 043037 – published 26 august 2024.

• No Citing Articles
• INTRODUCTION
• EVENT RATES AT DIFFERENT DETECTORS
• PHYSICAL PERIODIC SIGNALS OF INTEREST
• STATISTICAL METHODS FOR TIME-VARYING…
• SYNTHETIC DATA SIMULATION STRATEGY
• CONCLUSIONS
• ACKNOWLEDGMENTS

We study the prospects for measuring the time variation of solar and atmospheric neutrino fluxes at future large-scale xenon and argon dark matter detectors. For solar neutrinos, a yearly time variation arises from the eccentricity of Earth’s orbit and, for charged current interactions, from a smaller energy-dependent day-night variation due to flavor regeneration as neutrinos travel through Earth. For a 100-ton xenon detector running for ten years with a xenon-136 fraction of ≲ 0.1 % , in the electron recoil channel a time-variation amplitude of about 0.8% is detectable with a power of 90% and the level of significance of 10%. This is sufficient to detect time variation due to eccentricity, which has amplitude of ∼ 3 % . In the nuclear recoil channel, the detectable amplitude is about 10% under current detector resolution and efficiency conditions, and this generally reduces to about 1% for improved detector resolution and efficiency, the latter of which is sufficient to detect time variation due to eccentricity. Our analysis assumes both known and unknown periods. We provide scalings to determine the sensitivity to an arbitrary time-varying amplitude as a function of detector parameters. Identifying the time variation of the neutrino fluxes will be important for distinguishing neutrinos from dark matter signals and other detector-related backgrounds and extracting properties of neutrinos that can be uniquely studied in dark matter experiments.

• Received 29 February 2024
• Accepted 11 July 2024

DOI: https://doi.org/10.1103/PhysRevD.110.043037

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP 3 .

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• 1 Department of Physics and Astronomy, Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University , College Station, Texas 77843, USA
• 2 Department of Mathematics and Statistics, Texas A&M University-Corpus Christi , Corpus Christi, Texas 78412, USA
• 3 Department of Statistics, Texas A&M University , College Station, Texas 77843, USA

Article Text

Vol. 110, Iss. 4 — 15 August 2024

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Neutrino-electron ES spectra for xenon (left) and argon (right) for solar and experimental background components. For ES argon, we consider Rn 222 as the background [ 23 ].

CE ν NS spectra for xenon (left) and argon (right). Shown are the components of the solar, atmospheric, and DSNB spectra. The atmospheric spectra are shown for the SURF detector location.

Different detector efficiency models from nest simulations (LZ, G3, Xe100t-5) and current experiments (Argon-Darkside, Xe1T). Left: solid curves are LZ, G3, and Xe100t-5 for xenon ES. The dotted curve is the efficiency from Xenon1T [ 27 ]. Right: solid curves are LZ, G3, and Xe100t-5 for the xenon nuclear recoil CE ν NS channel. The orange solid curve is the efficiency from Darkside-50 [ 28 ] for nuclear recoils in argon.

Example of histograms for the assumptions of known periods (left) and unknown periods (right) under the null hypothesis H 0 and the alternative hypothesis H 1 . Assumed parameters are run-time T = 10     yr , time binnings Δ t = 0.2 , 10, 30 days, period of P = 365.25     days , amplitude A d = 0.024 , R s = 100     ton − 1     yr − 1 , and no experimental backgrounds R b = 0     ton − 1     yr − 1 . Left: the solid curve is the χ 2 2 distribution, the vertical line is the critical value χ 2 , α 2 with α = 0.1 . The dotted histogram in each panel is the distribution of 5000 simulated values of 2 L S max under the alternative hypothesis. Right: the filled histogram in each panel is the empirical distribution of 5000 simulated values of L S max under the null hypothesis H 0 using the LS method as described in the text. The solid curve is f X ( x | ν ) with ν = M scan , the vertical line is the critical value L S max , α , 1 with α = 0.1 . The dotted histogram in each panel is the distribution of 5000 simulated values of L S ( ω known ) under the alternative hypothesis with ω known = 2 π / P at the detection period P = 365.25     days .

Power curves (the probability of rejecting H 0 ) for the GLR and Lomb-Scargle methods when α = 0.1 and P = 1     yr and under the unknown period scenario. The critical values are L S max , α , 1 , L S max , α , 2 for LS and S max , α , 1 and S max , α , 2 for GLR. The vertical line is A d , α = 0.1 , β = 0.1 under the LS method and when the critical value is L S max , α , 1 .

The unknown period scenario plot for 90% detection ( α = 0.1 , β = 0.1 ) amplitude A d , α , β , P as a function of P for different ( T , Δ t ) and when R s = 100     ton − 1     yr − 1 , R b = 0     ton − 1     yr − 1 , and D = 100 ton in the simulation study. First column: T = 10     yr , Δ t varies from 5 to 30 days. Second column: Δ t = 0.2     days and T varies from 1 to 10 yr. Third column: Δ t = 10     days and T varies from 2 to 20 yr. Fourth column: Δ t = 10     days and T varies from 22 to 77 yr. In each panel, the white region is where the amplitude A d , α , β , P can be approximated by the average of the amplitudes over different frequencies A ¯ d , α , β , and the red horizontal line is A ¯ d , α , β .

Same as Fig.  6 but for known periods.

90% detection ( α = 0.1 , β = 0.1 ) amplitude A d , α , β ′ for solar components through electron scattering obtained from Eq. ( 13 ) as a function of 2 ν β β fraction f 2 ν β β . The top horizontal dashed line shows the expected amplitude due to the eccentricity A e c c = 0.03342 and the bottom horizontal line shows the bound on the amplitude of the day-night modulation from Borexino A d , D N , max = 0.00891 . The label “BG” indicates “background components” and “allBG” includes 2 ν β β , Rn 222 , and Kr 85 backgrounds. The assumed run-time is T = 10     yr , and the detector size D = 100     ton . The time binnings and periods are Δ t = 0.2     days for P = 1     day , Δ t = 10     days for P = 1     yr . The curves show the spectrum including energy resolution and Xe100t-5 detector efficiency for xenon (Fig.  3 ) and 10% energy resolution with recoil energy threshold 100 keV only for argon. Each curve is for a different time binning Δ t , as indicated.

90% detection ( α = 0.1 , β = 0.1 ) amplitude A d , α , β ′ for B 8 solar components through CE ν NS obtained from Eq. ( 13 ) as a function of recoil energy threshold, using Δ t = 10     days . The assumed run-time is T = 10     yr , and the detector size is D = 100     ton . Curves are shown for different detector and efficiency models as indicated. The dashed line is the amplitude of yearly modulation A e c c = 0.03342 .

The power for detecting time variation of solar components (summing over pp, Be 7 , CNO, and pep components) through electron scattering, under the assumption of the known period (left) and unknown period (right) scenarios described in the text. We take α = 0.1 and plot the results as a function of detector run-time T . The yearly modulation has an amplitude of A e c c = 0.03342 , due to the eccentricity of Earth’s orbit, and the daily modulation has an amplitude of A d , D N , max = 0.00891 , which matches the Borexino upper limits on the daily modulation. The assumed detector size is D = 100     ton . Top row: xenon detection of A d , D N , max for a time binning of Δ t = 0.2     days . Middle row: xenon detection of A e c c with Δ t = 10     days . Bottom row: argon detection of A e c c with Δ t = 10     days , and A d , D N , max with Δ t = 0.2     days . For xenon, all BG includes 2 ν β β , Rn 222 , and Kr 85 . For argon, the background includes Rn 222 .

The detectable amplitude for solar components through electron scattering with 90% power and 10% level of significance ( α = 0.1 , β = 0.1 ), estimated from Eq. ( 14 ) as a function of detector run-time T for D ′ = 50 ton, under known period and unknown period scenarios. Dashed lines are the amplitude of yearly modulation A e c c = 0.03342 and the day-night modulation A d , D N , max = 0.00891 . Top row: xenon detection of A d , D N , max for a time binning of Δ t = 0.05     days . Middle row: xenon detection of A e c c with Δ t = 10     days . Bottom row: argon detection of A e c c with Δ t = 10     days and of A d , D N , max with Δ t = 0.05     days . Points indicate the run-time T when A ¯ d , α = 0.1 , β = 0.1 ′ reaches A e c c or A d , D N , max . For xenon, all BG includes 2 ν β β , Rn 222 , and Kr 85 . For argon, the background includes Rn 222 .

The power of detecting time variation of B 8 through CE ν NS under known period (left column) and unknown period (right column) scenarios with α = 0.1 , as a function of detector run-time T . The assumed detector size is 100 ton. For xenon, assumptions are for an ideal detector and the Xe100t-5, LZ, and G3 detector efficiencies, and for argon, an ideal detector and the Darkside detector efficiency.

The detectable amplitude for B 8 neutrinos through CE ν NS with 90% power and 10% level of significance ( α = 0.1 , β = 0.1 ), estimated from Eq. ( 14 ) as a function of detector run-time T , for D ′ = 50 , 100 ton, under known period and unknown period scenarios, when Δ t = 10     days . The dashed line is the amplitude of yearly modulation A e c c = 0.03342 . Crosses indicate the run-time T when A ¯ d , α = 0.1 , β = 0.1 ′ reaches A e c c . Configurations of ( R s ′ , R b ′ , D ′ ) from different detector target, resolution, and efficiency are indicated.

Power for detecting atmospheric neutrino time variation at SURF and SNOlab through nuclear recoils under known period (left) and unknown period (right) scenarios for α = 0.1 , as a function of detector run-time T . The assumed detector sizes D are 200, 600, 1000 ton. Bands indicate the differences between Δ t = 10 and Δ t = 30     days for assumptions of an ideal detector.

The detectable amplitude for atmospheric neutrinos through CE ν NS with 90% power and 10% level of significance ( α = 0.1 , β = 0.1 ), estimated via Eq. ( 14 ) as a function of detector run-time T , for D ′ = 200 , 600 ton, under known period and unknown period scenarios, when Δ t = 30     days . The dashed horizontal lines are A a t m in Sec.  3c , and each color is a different location as indicated. Crosses indicate the run-time T when A ¯ d , α = 0.1 , β = 0.1 ′ reaches A a t m at the corresponding location. A xenon detector with ideal resolution and efficiency is assumed.

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