michelson light experiment

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The Michelson-Morley Experiment

Michael Fowler, University of Virginia

The Nature of Light

As a result of Michelson’s efforts in 1879, the speed of light was known to be 186,350 miles per second with a likely error of around 30 miles per second.  This measurement, made by timing a flash of light travelling between mirrors in Annapolis, agreed well with less direct measurements based on astronomical observations.  Still, this did not really clarify the nature of light.  Two hundred years earlier, Newton had suggested that light consists of tiny particles generated in a hot object, which spray out at very high speed, bounce off other objects, and are detected by our eyes.  Newton’s arch-enemy Robert Hooke, on the other hand, thought that light must be a kind of wave motion , like sound.  To appreciate his point of view, let us briefly review the nature of sound.

The Wavelike Nature of Sound

Actually, sound was already quite well understood by the ancient Greeks.  The essential point they had realized is that sound is generated by a vibrating material object, such as a bell, a string or a drumhead.  Their explanation was that the vibrating drumhead, for example, alternately pushes and pulls on the air directly above it, sending out waves of compression and decompression (known as rarefaction), like the expanding circles of ripples from a disturbance on the surface of a pond.  On reaching the ear, these waves push and pull on the eardrum with the same frequency (that is to say, the same number of pushes per second) as the original source was vibrating at, and nerves transmit from the ear to the brain both the intensity (loudness) and frequency (pitch) of the sound.

There are a couple of special properties of sound waves (actually any waves) worth mentioning at this point.  The first is called interference .  This is most simply demonstrated with water waves.  If you put two fingers in a tub of water, just touching the surface a foot or so apart, and vibrate them at the same rate to get two expanding circles of ripples, you will notice that where the ripples overlap there are quite complicated patterns of waves formed.  The essential point is that at those places where the wave-crests from the two sources arrive at the same time, the waves will work together and the water will be very disturbed, but at points where the crest from one source arrives at the same time as the wave trough from the other source, the waves will cancel each other out, and the water will hardly move.

There's a standard physics lab experiment illustrating this point, it's called two-slit interference. A monochromatic plane wave approaches a screen in which there are two narrow slit openings. If the slit opening width is smaller than the wavelength, a single slit will give approximately semicircular waves coming out the other side, the disturbance radiates out from the slit. The two slits together show the pattern generated by interference. It's best seen by clicking on this applet . Watch how the pattern changes with wavelength and with slit separation.

You can hear the interference effect for sound waves by playing a constant note through stereo speakers.  As you move around a room, you will hear quite large variations in the intensity of sound.  Of course, reflections from walls complicate the pattern.  This large variation in volume is not very noticeable when the stereo is playing music, because music is made up of many frequencies, and they change all the time.  The different frequencies, or notes, have their quiet spots in the room in different places.  The other point that should be mentioned is that high frequency tweeter-like sound is much more directional than low frequency woofer-like sound.  It really doesn’t matter where in the room you put a low-frequency woofer—the sound seems to be all around you anyway.  On the other hand, it is quite difficult to get a speaker to spread the high notes in all directions.  If you listen to a cheap speaker, the high notes are loudest if the speaker is pointing right at you.  A lot of effort has gone into designing tweeters, which are small speakers especially designed to broadcast high notes over a wide angle of directions.

Is Light a Wave?

Bearing in mind the above minireview of the properties of waves, let us now reconsider the question of whether light consists of a stream of particles or is some kind of wave.  The strongest argument for a particle picture is that light travels in straight lines.  You can hear around a corner, at least to some extent, but you certainly can’t see.  Furthermore, no wave-like interference effects are very evident for light.  Finally, it was long known, as we have mentioned, that sound waves were compressional waves in air.  If light is a wave, just what is waving?  It clearly isn’t just air, because light reaches us from the sun, and indeed from stars, and we know the air doesn’t stretch that far, or the planets would long ago have been slowed down by air resistance.

Despite all these objections, it was established around 1800 that light is in fact some kind of wave.  The reason this fact had gone undetected for so long was that the wavelength is really short, about one fifty-thousandth of an inch.  In contrast, the shortest wavelength sound detectable by humans has a wavelength of about half an inch.  The fact that light travels in straight lines is in accord with observations on sound that the higher the frequency (and shorter the wavelength) the greater the tendency to go in straight lines.  Similarly, the interference patterns mentioned above for sound waves or ripples on a pond vary over distances of the same sort of size as the wavelengths involved.  Patterns like that would not normally be noticeable for light because they would be on such a tiny scale.  In fact, it turns out, there are ways to see interference effects with light.  A familiar example is the many colors often visible in a soap bubble.  These come about because looking at a soap bubble you see light reflected from both sides of a very thin film of water—a thickness that turns out to be comparable to the wavelength of light.  The light reflected from the lower boundary has to go a little further to reach your eye, so that light wave must wave an extra time or two before getting to your eye compared with the light reflected from the top of the film.  What you actually see is the sum of the light reflected from the top and that reflected from the bottom.  Thinking of this now as the sum of two sets of waves, the light will be bright if the crests of the two waves arrive together, dim if the crests of waves reflected from the top layer arrive simultaneously with the troughs of waves reflected from the bottom layer.  Which of these two possibilities actually occurs for reflection from a particular bit of the soap film depends on just how much further the light reflected from the lower surface has to travel to reach your eye compared with light from the upper surface, and that depends on the angle of reflection and the thickness of the film.  Suppose now we shine white light on the bubble.  White light is made up of all the colors of the rainbow, and these different colors have different wavelengths, so we see colors reflected, because for a particular film, at a particular angle, some colors will be reflected brightly (the crests will arrive together), some dimly, and we will see the ones that win.

If Light is a Wave, What is Waving?

Having established that light is a wave, though, we still haven’t answered one of the major objections raised above.  Just what is waving?  We discussed sound waves as waves of compression in air.  Actually, that is only one case—sound will also travel through liquids, like water, and solids, like a steel bar.  It is found experimentally that, other things being equal, sound travels faster through a medium that is harder to compress: the material just springs back faster and the wave moves through more rapidly.  For media of equal springiness, the sound goes faster through the less heavy medium, essentially because the same amount of springiness can push things along faster in a lighter material.  So when a sound wave passes, the material—air, water or solid—waves as it goes through.  Taking this as a hint, it was natural to suppose that light must be just waves in some mysterious material, which was called the aether , surrounding and permeating everything.  This aether must also fill all of space, out to the stars, because we can see them, so the medium must be there to carry the light.  (We could never hear an explosion on the moon, however loud, because there is no air to carry the sound to us.)  Let us think a bit about what properties this aether must have.  Since light travels so fast, it must be very light, and very hard to compress.  Yet, as mentioned above, it must allow solid bodies to pass through it freely, without aether resistance, or the planets would be slowing down.  Thus we can picture it as a kind of ghostly wind blowing through the earth.  But how can we prove any of this? Can we detect it?

Detecting the Aether Wind: the Michelson-Morley Experiment

Detecting the aether wind was the next challenge Michelson set himself after his triumph in measuring the speed of light so accurately.  Naturally, something that allows solid bodies to pass through it freely is a little hard to get a grip on.  But Michelson realized that, just as the speed of sound is relative to the air, so the speed of light must be relative to the aether.  This must mean, if you could measure the speed of light accurately enough, you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and the difference of the two measurements should be twice the windspeed.  Unfortunately, it wasn’t that easy.  All the recent accurate measurements had used light travelling to a distant mirror and coming back, so if there was an aether wind along the direction between the mirrors, it would have opposite effects on the two parts of the measurement, leaving a very small overall effect.  There was no technically feasible way to do a one-way determination of the speed of light.

At this point, Michelson had a very clever idea for detecting the aether wind.  As he explained to his children (according to his daughter), it was based on the following puzzle:

Suppose we have a river of width w (say, 100 feet), and two swimmers who both swim at the same speed v feet per second (say, 5 feet per second).  The river is flowing at a steady rate, say 3 feet per second.  The swimmers race in the following way: they both start at the same point on one bank.  One swims directly across the river to the closest point on the opposite bank, then turns around and swims back.  The other stays on one side of the river, swimming upstream a distance (measured along the bank) exactly equal to the width of the river, then swims back to the start.  Who wins?

Let’s consider first the swimmer going upstream and back.  Going 100 feet upstream, the speed relative to the bank is only 2 feet per second, so that takes 50 seconds.  Coming back, the speed is 8 feet per second, so it takes 12.5 seconds, for a total time of 62.5 seconds.

Figure 1:  In time t , the swimmer has moved ct relative to the water, and been carried downstream a distance vt .

The swimmer going across the flow is trickier.  It won’t do simply to aim directly for the opposite bank-the flow will carry the swimmer downstream.  To succeed in going directly across, the swimmer must actually aim upstream at the correct angle (of course, a real swimmer would do this automatically).  Thus, the swimmer is going at 5 feet per second, at an angle, relative to the river, and being carried downstream at a rate of 3 feet per second.  If the angle is correctly chosen so that the net movement is directly across, in one second the swimmer must have moved four feet across:  the distances covered in one second will form a 3,4,5 triangle.  So, at a crossing rate of 4 feet per second, the swimmer gets across in 25 seconds, and back in the same time, for a total time of 50 seconds.  The cross-stream swimmer wins.  This turns out to true whatever their swimming speed.  (Of course, the race is only possible if they can swim faster than the current!)

Figure 2:  This diagram is from the original paper. The source of light is at s , the 45 degree line is the half-silvered mirror, b and c are mirrors and d the observer.

Michelson’s great idea was to construct an exactly similar race for pulses of light, with the aether wind playing the part of the river.  The scheme of the experiment is as follows: a pulse of light is directed at an angle of 45 degrees at a half-silvered, half transparent mirror, so that half the pulse goes on through the glass, half is reflected.  These two half-pulses are the two swimmers.  They both go on to distant mirrors which reflect them back to the half-silvered mirror.  At this point, they are again half reflected and half transmitted, but a telescope is placed behind the half-silvered mirror as shown in the figure so that half of each half-pulse will arrive in this telescope.  Now, if there is an aether wind blowing, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since one would have gone more upstream and back, one more across stream in general.  To maximize the effect, the whole apparatus, including the distant mirrors, was placed on a large turntable so it could be swung around.

We have an animation, including the aether wind and the rotating turntable, here !

Let us think about what kind of time delay we expect to find between the arrival of the two half-pulses of light.  Taking the speed of light to be c  miles per second relative to the aether, and the aether to be flowing at v  miles per second through the laboratory, to go a distance w  miles upstream will take w / ( c − v )  seconds, then to come back will take w / ( c + v )  seconds.  The total roundtrip time upstream and downstream is the sum of these, which works out to be 2 w c / ( c 2 − v 2 ) ,  which can also be written

t upstream + downstream = 2 w c ⋅ 1 1 − ( v 2 / c 2 ) .  

Now, we can safely assume the speed of the aether is much less than the speed of light, otherwise it would have been noticed long ago, for example in timing of eclipses of Jupiter’s satellites.  This means v 2 / c 2  is a very small number, and we can use some handy mathematical facts to make the algebra a bit easier.  First, if x  is very small compared to 1 ,     1 / ( 1 − x )  is very close to 1 + x .   (You can check it with your calculator.)  Another fact we shall need in a minute is that for small x ,  the square root of 1 + x  is very close to 1 + x / 2.    

Putting all this together,

t upstream + downstream ≅ 2 w c × ( 1 + v 2 c 2 ) .

Figure 3 This is also from the original paper, and shows the expected path of light relative to the aether with an aether wind blowing.

Now, what about the cross-stream time?  The actual cross-stream speed must be figured out as in the example above using a right-angled triangle, with the hypoteneuse equal to the speed c ,  the shortest side the aether flow speed v ,  and the other side the cross-stream speed we need to find the time to get across.  From Pythagoras’ theorem, then, the cross-stream speed is c 2 − v 2 .

Since this will be the same both ways, the roundtrip cross-stream time will be

t cross and back = 2 w c 2 − v 2 .  

This can be written in the form

2 w c 1 1 − v 2 / c 2 ≅ 2 w c 1 1 − ( v 2 / 2 c 2 ) ≅ 2 w c ( 1 + v 2 2 c 2 )

where the two successive approximations, valid for v / c = x ≪ 1 ,  are   1 − x ≅ 1 − ( x / 2 )  and 1 / ( 1 − x ) ≅ 1 + x .

t cross and back   ≅ 2 w c × ( 1 + v 2 2 c 2 ) .

Looking at the two roundtrip times at the ends of the two paragraphs above, we see that they differ by an amount

t upstream + downstream  − t cross and back ≅ 2 w c ⋅ v 2 2 c 2 .  

Now, 2 w / c  is just the time the light would take if there were no aether wind at all, say, a few millionths of a second.  If we take the aether windspeed to be equal to the earth’s speed in orbit, for example, v / c  is about 1/10,000, so v 2 / c 2  is about 1/100,000,000.

This means the time delay between the pulses reflected from the different mirrors reaching the telescope is about one-hundred-millionth of a few millionths of a second.  

It seems completely hopeless that such a short time delay could be detected.  However, this turns out not to be the case, and Michelson was the first to figure out how to do it.  The trick is to use the interference properties of the light waves.  Instead of sending pulses of light, as we discussed above, Michelson sent in a steady beam of light of a single color.  This can be visualized as a sequence of ingoing waves, with a wavelength one fifty-thousandth of an inch or so.  Now this sequence of waves is split into two, and reflected as previously described.  One set of waves goes upstream and downstream, the other goes across stream and back.  Finally, they come together into the telescope and the eye.  If the one that took longer is half a wavelength behind, its troughs will be on top of the crests of the first wave, they will cancel, and nothing will be seen.  If the delay is less than that, there will still be some dimming.  However, slight errors in the placement of the mirrors would have the same effect.  This is one reason why the apparatus is built to be rotated—see the animation !  On turning it through 90 degrees, the upstream-downstream and the cross-stream waves change places.  Now the other one should be behind.  Thus, if there is an aether wind, if you watch through the telescope while you rotate the turntable, you should expect to see variations in the brightness of the incoming light.

To magnify the time difference between the two paths, in the actual experiment the light was reflected backwards and forwards several times, like a several lap race.

Michelson calculated that an aether windspeed of only one or two miles a second would have observable effects in this experiment, so if the aether windspeed was comparable to the earth’s speed in orbit around the sun, it would be easy to see.  In fact, nothing was observed.  The light intensity did not vary at all.  Some time later, the experiment was redesigned so that an aether wind caused by the earth’s daily rotation could be detected.  Again, nothing was seen.  Finally, Michelson wondered if the aether was somehow getting stuck to the earth, like the air in a below-decks cabin on a ship, so he redid the experiment on top of a high mountain in California.  Again, no aether wind was observed.  It was difficult to believe that the aether in the immediate vicinity of the earth was stuck to it and moving with it, because light rays from stars would deflect as they went from the moving faraway aether to the local stuck aether.

The only possible conclusion from this series of very difficult experiments was that the whole concept of an all-pervading aether was wrong from the start.  Michelson was very reluctant to think along these lines.  In fact, new theoretical insight into the nature of light had arisen in the 1860’s from the brilliant theoretical work of Maxwell, who had written down a set of equations describing how electric and magnetic fields can give rise to each other.  He had discovered that his equations predicted there could be waves made up of electric and magnetic fields, and the speed of these waves, deduced from electrostatic and magnetostatic experiments, was predicted to be 186,300 miles per second.   This is, of course, the speed of light, so it was natural to assume light to be made up of fast-varying electric and magnetic fields.  

But this led to a big problem: Maxwell’s equations predicted a definite speed for light, and it was the speed found by measurements.  But what was the speed to be measured relative to?  The whole point of bringing in the aether was to give a picture for light resembling the one we understand for sound, compressional waves in a medium.  The speed of sound through air is measured relative to air.  If the wind is blowing towards you from the source of sound, you will hear the sound sooner.  If there isn’t an aether, though, this analogy doesn’t hold up.  So what does light travel at 186,300 miles per second relative to?

There is another obvious possibility, which is called the emitter theory: the light travels at 186,300 miles per second relative to the source of the light.  The analogy here is between light emitted by a source and bullets emitted by a machine gun.  The bullets come out at a definite speed (the muzzle velocity) relative to the barrel of the gun.  If the gun is mounted on the front of a tank, which is moving forward, and the gun is pointing forward, then relative to the ground the bullets are moving faster than they would if shot from a tank at rest.  The simplest way to test the emitter theory of light, then, is to measure the speed of light emitted in the forward direction by a flashlight moving in the forward direction, and see if it exceeds the known speed of light by an amount equal to the speed of the flashlight.  Actually, this kind of direct test of the emitter theory only became experimentally feasible in the nineteen-sixties.  It is now possible to produce particles, called neutral pions, which decay each one in a little explosion, emitting a flash of light.  It is also possible to have these pions moving forward at 185,000 miles per second when they self destruct, and to catch the light emitted in the forward direction, and clock its speed.  It is found that, despite the expected boost from being emitted by a very fast source, the light from the little explosions is going forward at the usual speed of 186,300 miles per second.  In the last century, the emitter theory was rejected because it was thought the appearance of certain astronomical phenomena, such as double stars, where two stars rotate around each other, would be affected.  Those arguments have since been criticized, but the pion test is unambiguous.  The definitive experiment was carried out by Alvager et al., Physics Letters 12 , 260 (1964).

We have to mention here that the most spectacular realization (so far) of Michelson's interference techniques is the successful detection (first in 2016) of gravitational waves by a scaled-up version of the interferometer. The basic idea is that as a (transverse, polarized) gravitational wave passes the two arms have their lengths slightly altered by different amounts. The tiny effect is made detectable by having arms kilometers in length, and the beams go back and forth in the arms many times. 

Einstein’s Answer

The results of the various aether detection experiments discussed above seem to leave us really stuck.  Apparently light is not like sound, with a definite speed relative to some underlying medium.  However, it is also not like bullets, with a definite speed relative to the source of the light.  Yet when we measure its speed we always get the same result.  How can all these facts be interpreted in a simple consistent way?  We shall show how Einstein answered this question in the next lecture.

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The Site Where Albert Michelson Measured the Speed of Light 145 Years Ago

On a seawall at the u.s. naval academy, now an aps historic site, michelson measured the speed of light more accurately than anyone had before..

For more than 175 years, the United States Naval Academy has educated future officers along the scenic shores of Maryland’s Severn River, just before it converges with the Chesapeake Bay. In the late 1870s, a young physics teacher, Albert A. Michelson, meticulously aligned optical equipment against that backdrop.

There, on the school’s seawall, he measured the speed of light far more accurately than anyone had before.

That seawall is gone now; as the Academy grew, it filled in much of the shoreline to make room for new buildings. Today, the original path of Michelson’s experiment is marked by round aluminum disks set into the terrace between Michelson Hall and Chauvenet Hall (named for mathematician William Chauvenet, who helped found the Naval Academy). Michelson Hall houses the school’s science wing.

“When I was a kid coming to the Academy with my dad, I would walk that path,” says Naval Academy Provost Samara Firebaugh. Years later, as an electrical engineering professor, she would take her students there, and today, she still points out the markers to visitors and shares Michelson’s story.

The site now has newfound fame: In a ceremony on April 12, APS President Young-Kee Kim and CEO Jon Bagger formally recognized it as an APS Historic Site, an honor given to a select number of places each year where important events in physics history took place. The Naval Academy was chosen because of Michelson’s enduring contribution to physics.

Michelson’s family immigrated to the United States from Prussia in 1856, opening a dry goods store in a California mining town, and later in Nevada. His parents sent him to school in San Francisco, where he excelled in science and became fascinated with optics.

“Albert’s brothers and sisters were swept into the drama of Western life,” wrote Michelson’s daughter Dorothy Michelson Livingston in The Master of Light: A Biography of Albert A. Michelson , but Albert “had acquired a different set of values.”

Michelson sought to continue his science education after high school. He saw his chance when the paper announced a statewide competition for a coveted spot at the U.S. Naval Academy, which had a strong optics program. Michelson tied for first place, but the appointment went to a co-winner.

With the support of his U.S. representative, Michelson appealed to President Ulysses S. Grant for a special appointment to the Academy. President Grant initially declined, having already made the 10 appointments he was allotted, but later reconsidered and awarded Michelson an 11th appointment.

At the Naval Academy, Michelson led his class in optics and thermodynamics but lagged in seamanship. Around his graduation, the Naval Academy’s superintendent, Rear Admiral John Lorimer Worden, told Michelson, “If in the future you’d give less attention to those scientific things and more to your naval gunnery, there might come a time when you would know enough to be of some service to your country,” according to a 1931 New York Times article.

After a two-year tour, Michelson returned to the Academy as a chemistry and physics teacher. In the fall of 1877, his supervisor suggested Michelson start an advanced physics class by demonstrating Léon Foucault’s 1862 speed of light measurement. Michelson reluctantly agreed — demonstrations were a new teaching method at the Academy — and studied the original experiment.

To find the speed of light, Foucault had measured how much a rapidly rotating mirror displaced a reflected beam of light, and then he had correlated the displacement to the mirror’s angular velocity. The result was solid, but Michelson saw room for improvement.

“The objection to Foucault’s method is that the displacement, a quantity which enters directly in the formula, is very small, and therefore difficult to measure accurately,” Michelson wrote in a paper published in The Scientific Monthly . By increasing the displacement, Michelson realized he could measure it more precisely. That meant he could more accurately determine the speed of light — a fundamental measurement for navigation at sea and for science broadly.

With a few months and scavenged parts, Michelson built a proof-of-concept apparatus. He spent the next year refining a large-scale design that, in 1879, yielded a 115 mm displacement (Foucault’s was less than 1 mm) and a value of 299,940 km/s for the speed of light. The most accurate value of its time, Michelson’s measurement was within 0.05% of the currently accepted speed of light.

This work kicked off his groundbreaking career in optics, during which Michelson experimentally challenged the existence of ether, invented the interferometer bearing his name, and, in 1907, received the Nobel Prize in Physics, becoming the first American laureate in science. He resigned from the Navy in 1881 to pursue physics but always stayed connected. “Father’s devotion to the Navy took second place only in relation to his romantic feeling about light,” wrote Dorothy Michelson Livingston in an article for the U.S. Naval Institute Proceedings.

Each year, the Naval Academy honors Michelson by inviting a distinguished researcher to share their expertise with midshipmen during the Michelson Memorial Lecture. The event is a campus-wide reminder that although technology landscapes and seawalls may shift, clever instrument design and careful measurement have lasting impact.

Kendra Redmond

Kendra Redmond is a writer based in Bloomington, Minnesota.

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Michelson-Morley Experiment

Once it was clear that light was a wave, the obvious question was: what is waving?

Everyone knew sound waves were compression waves in air, and on a windy day the sound would be carried by the wind. Presumably there was some mysterious background material in the Universe, they called it the aether, that played the same role for light. And, the Earth must be moving through the aether, so that would boost the light speed in the wind direction.

This experiment is designed to detect the Earth’s movement by setting up a race between a blip of light going across stream and back, and one going upstream then downstream. The apparatus can be rotated (by touch on the applet) to switch the two laps, which should change who wins. Try it with various speeds of the “aether wind”. (Note: the expected value of aether windspeed, say the velocity of Earth in orbit, is only one ten-thousandth of the speed of light, easily detectable by these guys, but obviously not something we can show here—we're just illustrating the idea! And, we’re neglecting the (small) change in light direction the supposed wind would cause.)

Albert Abraham Michelson and the Famous Experiment that lead to Einstein’s Special Relativity Theory

Albert Abraham Michelson (1852 – 1931)

On December 19, 1859, US-american physicist Albert Abraham Michelson   was born. Together with his colleague Edward Williams Morley he conducted an experiment that proved the by the time famous ether theory to be wrong and is considered to be one of the pilars of the theory of relativity .

“While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measurement shows its importance — where quantitative work is more to be desired than qualitative work. An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals.” — Albert A. Michelson, 1894 [7]

Early Years

Michelson was born in Strelno. His Jewish parents emigrated to the United States when he was two years old. They first settled in Virginia City and then moved to San Francisco. After his High School graduation, Michelson began studying at the United States Naval Academy in 1869 and graduated in 1873.  He soon became an instructor in physics as well as chemistry, and a few years later continued his studies in Berlin, Heidelberg, and Paris. After leaving the Navy in 1883, Michelson began his academic career as a professor in physics at various universities across the United States.

The Speed of Light

The speed of light is the speed at which light propagates in a vacuum. In addition to light, all other electromagnetic waves as well as gravitational waves propagate at this speed. It is a fundamental natural constant whose significance in the special and general theory of relativity goes far beyond the description of electromagnetic wave propagation. The speed of light is so high that for a long time it was assumed that the ignition of light can be perceived simultaneously everywhere. In 1676, Ole Rømer  [ 4 ] discovered that the observed orbital time of the Jupiter moon Io fluctuated regularly depending on the distance of Jupiter from Earth. From this he correctly concluded that light propagates at a finite velocity. The value determined by him already had the correct order of magnitude, but still deviated by 30 percent from the actual value. The measuring methods used to determine the speed of light became more and more accurate in the following years.

Inspired by translations of the works of Adolphe Ganot and his explanations of a universal ether, he was particularly interested in the problem of measuring the speed of light. After two years of study in Europe, he left the navy in 1881. In 1883 he accepted a position as professor of physics at the Case School of Applied Science in Cleveland and concentrated on the development of an improved interferometer. Already in 1877, while he was still an officer in the US Navy, Michelson began planning to improve Léon Foucault ‘s rotational mirror method for measuring the speed of light.[ 3 ] He wanted to use improved optics and a longer distance. In 1878 he made some preliminary measurements with strongly improvised equipment. When Simon Newcomb , director of the  Nautical Almanac Office,  found out about Michelson’s results, he invited the young scientist to Washington D.C. and a long lasting scientific friendship evolved.

Michelson and Morley’s interferometric setup, mounted on a stone slab that floats in an annular trough of mercury

Newcomb received with its better financed project a value of 299,860±30 km/s, within the measurement uncertainty consistent with Michelson’s value. In the 1920’s, Michelson began his studies on new measurements at the Mount-Wilson-Observatory in California and improved his results along with Francis Pease in 1927, but unfortunately passed away before the complete results were published in 1935. Michelson further improved his measurement method and in 1883 published a measurement of 299,853±60 km/s much closer to that of his mentor.

The Michelson-Morley-Experiment

The famous Michelson-Morley-Experiment he became best known for was performed already in 1881 during his stay in Berlin and Potsdam and repeated six years later in Cleveland. In the experiment, Michelson and Morley were to calculate the Earth’s movement in relation to the stationary ether. Physics theories of the late 19th century assumed that just as surface water waves must have an intervening substance, i.e. a “medium”, to move across, and audible sound requires a medium to transmit its wave motions (such as air or water), so light must also require a medium, the “ luminiferous aether “, to transmit its wave motions. Because light can travel through a vacuum, it was assumed that even a vacuum must be filled with aether. To Michelson’s surprise the results of his experiment showed that the Earth’s movement could not me measured this way and therefore disproved the existence of the ether. This is to be seen as one of the foundations to Albert Einstein ‘s theory of relativity . Michelson and Morley were able to carry out precise measurements of fine structure decomposition in atomic spectra with their interferometer for the first time , which was theoretically explained by Arnold Sommerfeld in 1916 and is still the subject of current research with the introduction of fine structure constants.

The period after 1927 marked the beginning of new measurements of the speed of light with new optoelectronic sensors, all of which were significantly below Michelson’s 1926 level. Michelson was looking for another measurement method, but this time in an evacuated tube, to avoid difficulties in image interpretation due to atmospheric effects. In 1930 he started a collaboration with Francis G. Pease and Fred Pearson to perform measurements in a 1.6 km long tube in Pasadena. Michelson died after completing 36 of 233 measurements. The experiment was significantly affected by geological instabilities and condensation problems before the result of 299,774±11 km/s, consistent with previous optoelectronic values, was published in 1935 after his death.

References and Further Reading:

• [1]  Nineteenth Century astronomy at the U.S. Naval Academy [PDF]
• [2]  Michelson’s Biography at nobelprize.org
• [3]  Fizeau, Foucault and Astronomical Photography , SciHi Blog
• [4]  Ole Rømer and the Speed of Light , SciHi Blog
• [5] Works of or about Albert A. Michelson at Wikisource
• [6]  Albert Abraham Michelson at Wikidata
• [7] D edication of Ryerson Physical Laboratory, quoted in Annual Register 1896,  p. 159
• [8]  Markus Klute,  3.3 Michelson-Morley Experiment,  MIT 8.20 Introduction to Special Relativity, January IAP 2021,  MIT OpenCourseWare  @ youtube
• [9]  Guide to the Albert A. Michelson Papers 1891-1969   Archived ,    University of Chicago Special Collections Research Center
• [10]  Livingston, D. M. (1973).   The Master of Light: A Biography of Albert A. Michelson .  
• [11]  Timeline for Albert A. Michelson, via Wikidata

Harald Sack

Related posts, karl pearson and mathematical statistics, nathan rosen – wormholes and time travel, wilhelm wien and the distribution law for blackbody radiation, satyendra nath bose and the einstein-bose statistics.

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Concerning the history of the Michelson experiment, we read the Commentary ‘How gravitational waves went from a whisper to a shout’, published in Physics Today August 2016, volume 69, number 8, pages 10-11. We are thinking that one may wish to emphasize that the detection was accomplished by using one of the most groundbreaking instruments in physics: The Michelson Interferometer, developed by the first American Nobel Prize winner Albert Abraham Michelson. The interferometer is so extraordinary powerful for detecting gravitational waves–LIGO’s interferometers are designed to measure a distance 1/10,000th the width of a proton! Interferometers were actually invented in the late 19th century by A.A. Michelson. The Michelson Interferometer was used in 1881 in the so-called “Potsdam Michelson Experiment”, which set out to prove or disprove the existence of a “Luminiferous Aether”–a substance at the time thought to permeate the Universe. All modern interferometers have evolved from this first one since it demonstrated how the properties of light can be used to make the tiniest of measurements. The invention of lasers has enabled interferometers to make the smallest conceivable measurements, like those required by LIGO. Interestingly, the basic structure of LIGO’s interferometers differs little from the interferometer that Michelson designed over 135 years ago. We would like to make the readers of this article aware of some new insights into the history of the Michelson Interferometer as published recently as contained in the papers available at https://aip.scitation.org/doi/abs/10.1063/1.37840 and http://leibnizsozietaet.de/wp-content/uploads/2016/03/HJH_BH2016_Michelson-Experiment-002.pdf

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MICHELSON-MORLEY EXPERIMENT

The MICHELSON-MORLEY EXPERIMENT was performed in the basement of a WESTERN RESERVE UNIV. dormitory in July 1887 by ALBERT A. MICHELSON of the Case School of Applied Science and EDWARD W. MORLEY of Western Reserve Univ. it was designed to detect the motion of the earth through the "luminiferous aether," a theoretical substance which, according to 19th century physicists, was essential to the transmission of light.

At the heart of the experiment was an interferometer—a device invented by Michelson—which utilized the interference of light waves to perform measurements of incredible precision. Although an identical experiment undertaken by Michelson at Potsdam in 1880-81 failed to detect any motion of the earth relative to the ether, leading physicists were anxious for a repetition. Thus encouraged, Michelson and Morley first undertook a series of preliminary measurements of the velocity of light in moving fluids. After these measurements were successfully completed, the two men constructed an interferometer which was larger and more sensitive than the original Potsdam interferometer. Earlier difficulties caused by extraneous vibration were resolved when Morley designed an ingenious mounting for the device, in which the interferometer rested on a large sandstone slab which rotated while floating in a pool of mercury. Despite these improvements and the collaboration of these two great experimental scientists, the experiment again failed to detect any motion of the expected magnitude.

This incongruous result puzzled the physicists of the world until 1905 when Einstein published his theory of relativity. Viewed in the light of Einstein's revolutionary work, the null results of the Michelson-Morley experiment were not only predictable, but provided experimental confirmation of Einstein's theory. In 1995 the American Chemical Society declared CWRU 's Adelbert Hall, the site of the experiment, a national historic chemical landmark, only the 4th location to be so designated in the country.

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Searching for Light's "Ether"

Part of the Einstein exhibition.

Most physicists of the time believed that light traveled through what they called the "luminiferous ether." In 1887, two American scientists, Albert Michelson and Edward Morley, built a device known as an interferometer, which they hoped would enable them to prove the existence of the ether.

Michelson and Morley ran their interferometer experiment numerous times but never saw any evidence of the ether. Other scientists, sure that the ether theory was correct, continued searching for it. It wasn't until 1905, when Einstein published his Special Theory of Relativity, that the physics community began to accept that the ether does not exist.

The Michelson-Morley interferometer worked by splitting a single beam of light in two. The two beams bounce off mirrors and arrive at a detector.

If the ether existed, it would remain still while the Earth moved through it. The ether would then change the speed of light depending on whether the light was moving in the direction of Earth's motion or at a right angle to that motion.

Michelson and Morley expected to find that two light beams arrived at the detector at different times. Instead they found that no matter which direction light traveled, it always moved at the same speed—indicating that the ether does not exist.

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A.A. Michelson

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• Linda Hall Library - Scientist of the Day - Biography of Albert A. Michelson
• Famous Scientists - Biography of Albert Abraham Michelson
• The Nobel Prize - Biography of Albert A. Michelson
• Albert A. Michelson - Student Encyclopedia (Ages 11 and up)

A.A. Michelson (born December 19, 1852, Strelno, Prussia [now Strzelno, Poland]—died May 9, 1931, Pasadena , California , U.S.) was a German-born American physicist who established the speed of light as a fundamental constant and pursued other spectroscopic and metrological investigations. He received the 1907 Nobel Prize for Physics.

Michelson came to the United States with his parents when he was two years old. From New York City the family made its way to Virginia City, Nevada, and San Francisco, where the elder Michelson prospered as a merchant. At age 17 Michelson entered the United States Naval Academy at Annapolis, Maryland, where he did well in science but was rather below average in seamanship. He graduated in 1873 and served as science instructor at the academy from 1875 until 1879.

In 1878 Michelson began work on what was to be the passion of his life, the accurate measurement of the speed of light. He was able to obtain useful values with homemade apparatuses. Feeling the need to study optics before he could be qualified to make real progress, he traveled to Europe in 1880 and spent two years in Berlin, Heidelberg, and Paris, resigning from the U.S. Navy in 1881. Upon his return to the United States, he determined the velocity of light to be 299,853 km (186,329 miles) per second, a value that remained the best for a generation, until Michelson bettered it.

While in Europe, Michelson began constructing an interferometer , a device designed to split a beam of light in two, send the parts along perpendicular paths, then bring them back together. If the light waves had, in the interim , fallen out of step, interference fringes of alternating light and dark bands would be obtained. From the width and number of those fringes, unprecedentedly delicate measurements could be made, comparing the velocity of light rays traveling at right angles to each other.

It was Michelson’s intention to use the interferometer to measure Earth’s velocity against the “ ether ” that was then thought to make up the basic substratum of the universe. If Earth were traveling through the light-conducting ether , then the speed of the light traveling in the same direction would be expected to be equal to the velocity of light plus the velocity of Earth, whereas the speed of light traveling at right angles to Earth’s path would be expected to travel only at the velocity of light. His earliest experiments in Berlin showed no interference fringes, however, which seemed to signify that there was no difference in the speed of the light rays and, therefore, no Earth motion relative to the ether.

In 1883 he accepted a position as professor of physics at the Case School of Applied Science in Cleveland and there concentrated his efforts on improving the delicacy of his interferometer experiment. By 1887, with the help of his colleague, American chemist Edward Williams Morley , he was ready to announce the results of what has since come to be called the Michelson-Morley experiment . Those results were still negative; there were no interference fringes and apparently no motion of Earth relative to the ether.

It was perhaps the most significant negative experiment in the history of science . In terms of classical Newtonian physics, the results were paradoxical. Evidently, the speed of light plus any other added velocity was still equal only to the speed of light. To explain the result of the Michelson-Morley experiment, physics had to be recast on a new and more-refined foundation, something that resulted eventually in Albert Einstein’s formulation of the theory of relativity in 1905.

In 1892 Michelson—after serving as professor of physics at Clark University at Worcester , Massachusetts , from 1889—was appointed professor and the first head of the department of physics at the newly organized University of Chicago , a position he held until his retirement in 1929. From 1923 to 1927 he served as president of the National Academy of Sciences . In 1907 he became the first American ever to receive a Nobel Prize in the sciences, for his spectroscopic and metrological investigations, the first of many honours he was to receive.

Michelson advocated using some particular wavelength of light as a standard of distance (a suggestion generally accepted in 1960) and, in 1893, measured the standard metre in terms of the red light emitted by heated cadmium. His interferometer made it possible for him to determine the width of heavenly objects by matching the light rays from the two sides and noting the interference fringes that resulted. In 1920, using a 6-metre (20-foot) interferometer attached to a 254-cm (100-inch) telescope, he succeeded in measuring the diameter of the star Betelgeuse (Alpha Orionis) as 386,160,000 km (300 times the diameter of the Sun). This was the first substantially accurate determination of the size of a star .

In 1923 Michelson returned to the problem of the accurate measurement of the velocity of light. In the California mountains he surveyed a 35-km pathway between two mountain peaks, determining the distance to an accuracy of less than 2.5 cm. He made use of a special eight-sided revolving mirror and obtained a value of 299,798 km/sec for the velocity of light. To refine matters further, he made use of a long, evacuated tube through which a light beam was reflected back and forth until it had traveled 16 km through a vacuum. Michelson died before the results of his final tests could be evaluated, but in 1933 the final figure was announced as 299,774 km/sec, a value less than 2 km/sec higher than the value accepted in the 1970s.