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
What is michelson-morley experiment.
About 150 years ago, physicists believed that light waves require a medium to pass through. The Michelson-Morley experiment was performed by American scientists Albert Michelson and Edward Morley between April and July 1887. The purpose of this experiment is to prove the existence of ether. This hypothetical medium permeating space was thought to be the carrier of light waves. This experiment aimed to measure the speed of light in two perpendicular directions relative to the ether. The result was negative. Michelson and Morley did not find any significant difference between the speed of light in the direction of the presumed ether and the speed at right angles.
Experimental Arrangement of Michelson-Morley Experiment
To the effect of ether on the speed of light, Michelson designed a device now known as an interferometer. It is a sensitive optical device that compares the optical path lengths for light moving in two mutually perpendicular directions and utilized the interference of light waves to perform measurements of incredible accuracy. It sent a beam of light from a single source through a half-silvered mirror (beam splitter) that split it into two beams traveling perpendicular to each other. After departing the splitter, the beams traveled out to the long arms of the interferometer, where they were reflected in the middle by two small mirrors. Then, they recombined on the far side of the splitter, producing a pattern of constructive and destructive interference. The pattern of fringes, as observed by an eyepiece, is based on the length of the arms. Any slight change in the length would alter the amount of time the beams spent in transit. This change would then be observed as a shift in the positions of the interference fringes.
Conclusion of Michelson-Morley Experiment
Michelson and Morley measured the speed of light by observing the interference fringes produced by the two beams. They expected that the light would travel faster along an arm if oriented in the same direction as the ether was moving, and slower if oriented in the opposite direction. Since the two arms were perpendicular, there is only one way that light would travel at the same speed in both arms and arrive simultaneously at the eyepiece. It was possible if the instrument were motionless to the ether. If this were not the case, the crests and troughs of the light waves in the two arms would arrive and interfere slightly out of synchronization, thus reducing the intensity. Although Michelson and Morley expected different speeds of light in each direction, they found no noticeable shift in the fringes. Otherwise, that would indicate a different speed in any orientation or at any position of the Earth in its orbit. This null result seriously discredited existing ether theories. Eventually, it led to the proposal by Albert Einstein in 1905 that the speed of light is a universal constant.
Article was last reviewed on Saturday, May 9, 2020
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One response to “Michelson-Morley Experiment”
If the above experiment (as well as Airy’s Failure) did not detect the earth’s movement, how come Focault’s pendulum does? Can’t prove truth via lies.
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and its motion relative to the Earth. The most famous and successful was the one now known as the Michelson-Morley experiment, performed by , which essentially consists of a light source, a half-silvered glass plate, two mirrors, and a . relative to Earth, thus establishing its existence. on their apparatus and introduced the length contraction equation is the contracted length, is the velocity of the frame of reference, and is the . Although the main interpretation of , , . Einstein's idea of space-time contraction replaced Lorentz's interpretation of the contraction equation, and once and for all relegated to the history books. , , Redwood City, CA: Addison-Wesley, pp. 15-3-15-4, 1989. . New York: Dover, pp. 3-7, 1952. Reprinted form Lorentz, H. A. Leiden, 1895. , 120-129, 1881. , 333-345, 1887. , 449-463, 1887.
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Michelson and Morley used a large, sensitive spectrometer to compare the behaviour of light as it travelled along two paths at right angles to each other. As we saw in the , these results were vitally important for Einstein's theory of relativity.
The technical background, which is not necessary for the rest of the discussion, is this: The spectrometer uses the wave property of interference to compare the time that light takes to travel along the two paths. If it takes equal times, then the rays combine in step (technically: in phase) and the resultant beam is bright. If they are out of step by half a wavelength, then they cancel out, and the combined beam is dark (at the particular angle). Looking into the combined beam, what one sees is a series of bright and dark rings, corresponding to reinforcement or cancellation of the light rays that have travelled along the two different paths.
The spectrometer could be rotated, so one of these paths could be parallel to the direction of the Earth's motion around the sun, while the other was at right angles. Further, by doing two measurements twelve hours apart, one could add or subtract the speed of the Earth's surface due to its rotation about its axis. With two measurements 6 months apart, one could add or subtract these speeds from that due to the motion of the Earth around the sun. (The motion of the sun around the centre of the galaxy is effectively constant over our history. The motion of the lab due to the Earth's rotation is rather slower than that due to the motion around the sun.)
All that from watching the changes in bright and dark stripes of light in an interference pattern — what a cool experiment! The diagram shows schematically how this works. Qualitatively, one can see that both paths should be slowed a little by motion with respect to the aether. But the amounts do not cancel out. Go to for a quantitative treatment.
In retrospect, it was indeed a cool experiment, but not in the way the experimenters thought. It is now considered as one of the most famous null results in history. In all of the different times and seasons, the orientation of the spectrometer failed to add or to subtract the putative motion of the lab through the aether.
The simplest interpretation of the results is that light travelled at the same speed with respect to the lab, whether or not the arm of the spectrometer were travelling with the Earth through the aether or at right angles to it. (One could also make explanations in which the speed of light varied, but the shape of the spectrometer changed according to its orientation, in such a way that it exactly cancelled the effect of the lab's motion.)
Many further experiments have been performed to look for variations in the speed of light with respect to relative motion, usually by looking at the speed of light in different directions, as Michelson and Morley did. One can never show that the difference is zero, one can only give an upper limit to the possible ratio of speeds in the different directions. Currently, Stephan Schiller's team in Dusseldorf gives an upper limit to the possible variation of 6 parts in 10 . The ratio of the speeds is 1.000 000 000 000 000 0 plus or minus 0.000 000 000 000 6. (Reported in Nature's research highlights, 9/6/05.)
Basically, no-one has yet found a case in which the speed of light in vacuum is different.
is not isotropic, but that the effect is very small compared with that expected from a stationary aether, so the data are not immediately persuasive. For example, by Reg Cahill of Flinders University puts the case for retaining the Lorentz equations but abandoning Einstein's principle of special relativity.
" ." " ."
These are common responses — and they were certainly the responses of this author when I first read about relativity. The answer to the first is simply that it is not up to us to decide in advance what is and what is not so (in spite of what Plato might have said). That is the job for observation and experiment (as Plato's student Aristotle, and even more emphatically Galileo, might have told Plato). To the second objection, we might say that it is not up to us to tell the universe what to do. The universe just is. It is up to us to make sense of it. For scientists, this means finding theories and laws whose predictions are in agreement with what we observe in the universe. Relativity is a theory that has been very thoroughly and precisely tested, and whose predictions are in spectacularly good agreement with the behaviour of the universe.
The principle of Special Relativity, including its weird consequence that the speed of light is the same for different observers, is not illogical. It is not false. It may be surprising or upsetting. Deep down, I think that most people who object to the principle of Special Relativity are saying "It may be true, but it wouldn't be true if I had designed the universe" or "I don't like the principle of relativity". To this objection, the universe is unlikely to register offence.
Notice that the objections mentioned in the previous paragraphs are in fact not objections to a theory, but objections to the results of experiments. The invariance of the speed of light for different observers is an experimental observation. That clocks run at different rates in different frames is an experimental observation. . You can't wait for some new theory that isn't weird: any theory that did not make the same predictions as relativity (in the cases that we have measured so far) would be immediately recognised as false, because it wouldn't agree with experiments.
This argument doesn't stop the observations from being weird. But those are the observations. It seems that we have three options: (i) accept them, (ii) forget about them or (iii) go and live in a universe where they aren't true.
See also .
I have offered little sympathy for Platonic idealists or postmodernists in the paragraph above. I have never met any Platonic idealists: people who believe that one can decide how the world is without looking to see how it is and believe that it is preferable to do so. On the other hand, there are some people who believe (or at least loudly claim to believe) that physics is an arbitrary construct with no special advantage over other systems of thought addressing the same phenomena.
I am a physicist, so you may think that I am biassed about physics and about the scientific method. Further, physics is an orthodoxy: only on a small number of questions at the very periphery of knowledge will you find serious disagreement among physicists. Further, the orthodoxy is institutionalised: someone who believed that apples fell up would find it difficult to obtain a employment as a professional physicist. So the physics world view reinforces itself. Can you trust it?
Let's make the question stronger: Many readers will have flown in an aircraft, which had been designed by engineers with a good understanding of the relevant areas of physics. It was navigated using a system designed by engineers and physicists with a good knowledge of relativity. How would you feel about travelling in an aircraft designed according to the principles of a one of the systems of thought that arrives at quantitatively different conclusions from those of physics and engineering? How many times do you trust your life to physics and to its applied science, engineering?
So why is physics, in general, an orthodoxy? Suppose that there are different ways of looking at a problem. Do they give different answers? If they do, we conduct experiments and we abandon, or at least qualify, those that give the wrong answers. If they give the same answers, then we look to see whether they are actually equivalent. For instance, Newton's and Hamilton's laws of mechanics appear to be quite different ways of looking at the world. Newton's system is more like a philosopher's 'cause and effect', whereas Hamilton's is more like 'purpose'. However, one can be derived from the other, so we see them as logically equivalent and related to each other fairly simply via calculus.
So, in most cases, we can look at competing theories and discard some of them because they fail the test of experiment. Those that remain are usually equivalent. Sometimes, theories only differ in exotic ways that are (currently) impossible to test. So, for instance, there are currently competing theories of quantum gravity, and none of them is accepted: they are often regarded as speculative.
Sometimes we retain for everyday use theories that, in exotic situations, are shown to be false. So, for instance, when speeds and energies are low, we use Galilean relativity and Newtonian mechanics, because they are considerably easier to use and because, when speeds and energies are low, they give the same answers as relativistic mechanics, to excellent approximations.
If you're interested, we show film clips of measurements of the speed of light and of (UHF) radio waves in a multimedia tutorial on .
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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A.A. Michelson's 1879 Determinations of the Speed of Light
In 1879, Albert Abraham Michelson conducted an experimental study to determine the speed of light using a rotating mirror apparatus at the U.S. Naval Academy in Annapolis, Maryland in 1879.
Details on the apparatus, the optical theory, and the conduct of the experiment are given in the reference. An abbreviated summary of these follows the variable descriptions.
A data frame with 100 rows and 15 variables
The determined speed of light in air in kilometres per second.
Number of beats per second between tuning forks.
Correction for temperature to a standard fork in beats per second.
Day of experiment in progress (June 5 is day 1) on which these measurements were taken.
Difference betweeen the greatest and least values of revolutions.
Subjective measure of the quality of the image `I`; the more distinct was the image the higher the quality (1 = poor, 3 = good).
Displacement of image `I` from slit `S` in divisions of the micrometer.
Micrometer position of the deflected image.
Radius of measurement in feet.
Number of times the mirror revolved per second.
Measure of one screw turn in millimetres.
Micrometer position of the slit providing the light source `S`.
Air temperature measured in degrees Fahrenheit.
Time of day at which the observation was recorded. `AM` means one hour after sunrise and `PM` one hour before sunset.
Unusual remarks recorded for that observation.
The experiment is conducted within a closed and darkened small building at the U.S. Naval Academy. Light enters the building from one corner passing through a slit `S` whose location is precisely determined using a micrometer.
The light then proceeds to hit a rotating mirror at the other end of the building's interior from whence it is reflected out of the building through an opening in a corner different from that of the source.
The light beam travels outside to strike another (stationary) mirror which reflects it back into the building through the same corner it exited whereupon it then strikes the rotating mirror.
Depending on the position of the rotating mirror, the returning light will be reflected off it to land at some position `I` near the original source given by the slit `S`.
The speed of the rotating mirror is controlled using an adjustable pump to blow air across a surface to rotate it. If the speed of rotation is just right, a crisp image `I` of the reflected slit will appear near the original source `S`. The speed is adjusted until this is the case.
The speed of rotation is determined using an electric tuning fork connected to the rotating mirror and whose frequency was measured by comparing it to a second standard tunig fork of known frequency. The electronic fork frequency was compared to the standard fork by determining the number of beats per second difference the two (by counting over 60 seconds).
With a speed of revolution and the displacement measured between `S` and its returned image `I`, a measurement of the speed of light could be had.
See reference for more details.
R.J. MacKay and R.W. Oldford 2000, `Scientific Method, Statistical Method, and the Speed of Light`, Statistical Science, Volume 15, No. 3, pp. 254-278. <doi:10.1214/ss/1009212817>
lightspeeds
R.W. Oldford
AIR & SPACE MAGAZINE
The pipeline that measured the speed of light.
In 1931, a corrugated steel tunnel figured in one of history’s greatest science experiments.
Nick D’Alto
The speed of light (the “c” in Einstein’s E = mc2 ) is 186,282 miles per second. Knowing its precise value is critical to understanding almost everything in the universe, from measuring the distance between stars to precisely locating where you are, such as with GPS. Galileo was the first to attempt to calculate the speed, by shining lanterns across hilltops in 1638. Various quirky methods followed. So what would you use to measure this universal standard?
Albert A. Michelson had a new idea. Born in 1852 and raised in gold-rush Virginia City, Nevada, he was eventually appointed to the U.S. Naval Academy and assigned to lecture on physics.
In 1926, about 20 years after becoming the first American scientist to win a Nobel Prize, Michelson first tried picking up where Galileo left off. He timed arc lamp flashes between Mt. Wilson Observatory in California and a mountain 22 miles away. But realizing that the air in between might have introduced errors, Michelson decided he needed an air-free environment.
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This story is a selection from the February/March issue of Air & Space magazine
In 1931, he decided to construct a mile-long segment of corrugated steel pipe, all but evacuated of air, to measure the speed of a light beam. After local municipalities refused to lend him an existing pipeline, Michelson commenced building his own. Sealing the 60-foot pipe sections proved especially daunting. The final recipe for the home-brew sealant included rope, canvas, glue, friction tape, and tire inner tubes. Propped on wood trestles between flimsy steel shacks, the pipe looked like a sloppy public works project. But inside, pumps could drop the pressure to half an inch of mercury (equivalent to the atmosphere at about 110,000 feet). This otherwise-unassuming pipeline effectively simulated deep space, smack in the middle of California farm country.
On each run, a “sun bright” beam from an arc lamp bouncing off a 16-sided whirling mirror completed five round trips. To clock elapsed time, Michelson adjusted the mirror’s rotation until the returning beam met the next mirrored face exactly.
A fascinated public devoured regular news reports on the experiment, then flocked to see it in such numbers that Michelson implored them to stop. Albert Einstein visited in March 1931. As work progressed, even tides and tectonic shifts became concerns. Michelson worked at night to avoid any heat expansion in the pipe’s rarefied atmosphere.
“I plan to retire,” Michelson vowed, as doctors warned the now-79-year-old that his “beautiful obsession,” as he called it, might risk his health. By April, he was directing the work from a sickbed. Elated after completing around half the trials, Michelson was beginning to draft his report when he died. The results, published posthumously in Astrophysical Journal , gave a figure of around 186,271 miles per second, just slightly lower than we accept today.
The Irvine experiment was damaged in a 1933 earthquake, and the pipe was later bought and reused by the city. Pieces of this unique aerospace artifact may be draining farmlands or marshes in Southern California right now. Today, CERN’s Large Hadron Collider, among other experiments, continues the quest to understand the universe by sending beams of light through a long tunnel.
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Nick D’Alto | READ MORE
As an aerospace engineer, Nick D’Alto already knew that great aircraft design isn’t always easy to recognize. After studying aviation camouflage, he’s more convinced than ever.
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Why does physics work in SI units?
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.
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previous G&E index Modern Physics index next Spanish Version Applet of the experiment
The Michelson-Morley Experiment
Michael Fowler, UVa
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. You can hear this 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 layer 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 layer. What you actually see is the sum of the light reflected from the top layer and that reflected from the bottom layer. 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.
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!)
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.
An animated applet of the experiment is available here –it makes the account above a lot clearer!
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 wc /( c ²- v ²), which can also be written (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, the upstream--downstream roundtrip time
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 the square root of ( c ²- v ²).
Since this will be the same both ways, the roundtrip cross-stream time will be
This can be written in the form
Therefore the across-stream roundtrip time, assuming the aether velocity is much less than that of light, is
Looking at the two roundtrip times at the ends of the two paragraphs above, we see that they differ by an amount (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 lightwaves. 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. 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 experiments on how these fields link together, would be 186,300 miles per second. This is, of course, the speed of light, so it is natural to assume that light is made up of fast-varying electric and magnetic fields. But this leads to a big problem: Maxwell’s equations predict a definite speed for light, and it is the speed found by measurements. But what is 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 (called 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).
Einstein’s Answer
The results of the various 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 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.
The Michelson-Morley experiment was an attempt to measure the motion of the Earth relative to the luminiferous aether, [A 1] a supposed medium permeating space that was thought to be the carrier of light waves.The experiment was performed between April and July 1887 by American physicists Albert A. Michelson and Edward W. Morley at what is now Case Western Reserve University in Cleveland ...
The Site Where Albert Michelson Measured the Speed of Light 145 Years
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.
Albert A. Michelson
Albert A. Michelson
Michelson-Morley Experiment: Explanation, Apparatus & Result
The Michelson-Morley experiment was performed by American scientists Albert Michelson and Edward Morley between April and July 1887. The purpose of this experiment is to prove the existence of ether. This hypothetical medium permeating space was thought to be the carrier of light waves. This experiment aimed to measure the speed of light in two ...
PDF The Michelson-Morley Experiment
The Michelson-Morley Experiment Spanish Version Flashlet of the Experiment! Michael Fowler U. Va. Physics 9/15/08 The Nature of Light As a result of Michelson's efforts in 1879, the speed of light was known to be 186,350 ... you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and ...
Michelson-Morley Experiment
In 1878, Michelson had made an excellent measurement of the speed of light at the age of 25, and he thought the detection of motion through the ether might be measurable. Michelson proceeded to invent a new instrument with accuracy far exceeding that which had been attained to that date, and that instrument is now universally called the ...
Michelson-Morley Experiment -- from Eric Weisstein's World of Physics
The most famous and successful was the one now known as the Michelson-Morley experiment, performed by Albert Michelson (1852-1931) and Edward Morley (1838-1923) in 1887. ... Michelson and Morley were able to measure the speed of light by looking for interference fringes between the light which had passed through the two perpendicular arms of ...
Michelson, Morley and the speed of light. From Einstein Light
Schematic and animation of the Michelson-Morley experiment. A beam of light (actually continuous, not pulsed as in the animation) is split when it strikes a transparent plate: part is transmitted, part reflected. ... (One could also make explanations in which the speed of light varied, but the shape of the spectrometer changed according to its ...
Michelson-Morley Experiment
Michelson-Morley Experiment. Once it was clear that light was a wave, the obvious question was: what is waving? ... 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 ...
Albert Abraham Michelson and the Famous Experiment that lead to
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. ... 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 ...
Speed of Light Experiment by Michelson
Michelson's speed of light experiment is presented in Science class by Ural in May 2019. The idea behind the setup and the experiment is brilliant.
Foucault's measurements of the speed of light
Michelson's final 1931 attempt to measure the speed of light in vacuum was interrupted by his death. Although his experiment was completed posthumously by F. G. Pease and F. Pearson, various factors militated against a measurement of highest accuracy, including an earthquake which disturbed the baseline measurement.
The Speed of Light
These techniques gave the speed of light with an accuracy of about 1,000 miles per second. Albert Abraham Michelson . Albert Michelson was born in 1852 in Strzelno, Poland. His father Samuel was a Jewish merchant, not a very safe thing to be at the time. ... The next lecture is on the Michelson-Morley experiment to detect the aether.
Michelson and the Speed of Light :: USNA News Center
Michelson died on May 9th, 1931. The memorial outside of the academic buildings of Chauvenet and Michelson (named in his honor) shows where Albert Michelson conducted his experiment to accurately measure the speed of light along the old sea wall of the Naval Academy. USNA
A.A. Michelson's 1879 Determinations of the Speed of Light
In 1879, Albert Abraham Michelson conducted an experimental study to determine the speed of light using a rotating mirror apparatus at the U.S. Naval Academy in Annapolis, Maryland in 1879. Details on the apparatus, the optical theory, and the conduct of the experiment are given in the reference. An abbreviated summary of these follows the ...
The Pipeline That Measured the Speed of Light
In 1931, a corrugated steel tunnel figured in one of history's greatest science experiments. The Albert Michelson pipeline in California carried light, not fuel. After the experiments, the tube ...
A.A. Michelson
A.A. Michelson | Nobel Prize-Winning Physicist
Mount Wilson Observatory
By measuring the tiny change in angle of the mirror during a round trip of a burst of light, taking only two ten-thousandths of a second, the speed of light was measured.This elegant experiment, devised and carried out during 1924-26 by Albert A. Michelson, America's first Nobel Prize winner in a science field, provided the first modern and ...
The Michelson-Morley Experiment
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.
Emission theory (relativity)
Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment of 1887. Emission theories obey the principle of relativity by having no preferred frame for light transmission, but say that light is emitted at speed "c" relative to its source instead of applying ...
Speed of Light Experiment
The Speed of Light Experiment uses laser light and a high speed rotating mirror to determine this fundamental constant using the Foucault method. Laser light passes through a series of lenses to produce an image of the light source at a measured position. The light is then directed to a rotating mirror, which reflects the light to a fixed ...
One-way speed of light
Round-trip light path in a Michelson interferometer. The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. ... and Kim repeated the Greaves et al. experiment intending to obtain with greater accuracy the one-way speed of light. [28] This experiment assumed that the signal return ...
The Michelson-Morley Experiment
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.
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer/13/66/1366dcd8-3dcd-4f27-b9f3-658c40294be5/michelson_pipeline.jpg "speed of light experiment michelson")
COMMENTS
The Michelson-Morley experiment was an attempt to measure the motion of the Earth relative to the luminiferous aether, [A 1] a supposed medium permeating space that was thought to be the carrier of light waves.The experiment was performed between April and July 1887 by American physicists Albert A. Michelson and Edward W. Morley at what is now Case Western Reserve University in Cleveland ...
Light - Michelson-Morley, Experiment, Wave-Particle
Michelson-Morley experiment | Description, Results, & Facts
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.
Albert A. Michelson
The Michelson-Morley experiment was performed by American scientists Albert Michelson and Edward Morley between April and July 1887. The purpose of this experiment is to prove the existence of ether. This hypothetical medium permeating space was thought to be the carrier of light waves. This experiment aimed to measure the speed of light in two ...
The Michelson-Morley Experiment Spanish Version Flashlet of the Experiment! Michael Fowler U. Va. Physics 9/15/08 The Nature of Light As a result of Michelson's efforts in 1879, the speed of light was known to be 186,350 ... you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and ...
In 1878, Michelson had made an excellent measurement of the speed of light at the age of 25, and he thought the detection of motion through the ether might be measurable. Michelson proceeded to invent a new instrument with accuracy far exceeding that which had been attained to that date, and that instrument is now universally called the ...
The most famous and successful was the one now known as the Michelson-Morley experiment, performed by Albert Michelson (1852-1931) and Edward Morley (1838-1923) in 1887. ... Michelson and Morley were able to measure the speed of light by looking for interference fringes between the light which had passed through the two perpendicular arms of ...
Schematic and animation of the Michelson-Morley experiment. A beam of light (actually continuous, not pulsed as in the animation) is split when it strikes a transparent plate: part is transmitted, part reflected. ... (One could also make explanations in which the speed of light varied, but the shape of the spectrometer changed according to its ...
Michelson-Morley Experiment. Once it was clear that light was a wave, the obvious question was: what is waving? ... 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 ...
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. ... 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 ...
Michelson's speed of light experiment is presented in Science class by Ural in May 2019. The idea behind the setup and the experiment is brilliant.
Michelson's final 1931 attempt to measure the speed of light in vacuum was interrupted by his death. Although his experiment was completed posthumously by F. G. Pease and F. Pearson, various factors militated against a measurement of highest accuracy, including an earthquake which disturbed the baseline measurement.
These techniques gave the speed of light with an accuracy of about 1,000 miles per second. Albert Abraham Michelson . Albert Michelson was born in 1852 in Strzelno, Poland. His father Samuel was a Jewish merchant, not a very safe thing to be at the time. ... The next lecture is on the Michelson-Morley experiment to detect the aether.
Michelson died on May 9th, 1931. The memorial outside of the academic buildings of Chauvenet and Michelson (named in his honor) shows where Albert Michelson conducted his experiment to accurately measure the speed of light along the old sea wall of the Naval Academy. USNA
In 1879, Albert Abraham Michelson conducted an experimental study to determine the speed of light using a rotating mirror apparatus at the U.S. Naval Academy in Annapolis, Maryland in 1879. Details on the apparatus, the optical theory, and the conduct of the experiment are given in the reference. An abbreviated summary of these follows the ...
In 1931, a corrugated steel tunnel figured in one of history's greatest science experiments. The Albert Michelson pipeline in California carried light, not fuel. After the experiments, the tube ...
A.A. Michelson | Nobel Prize-Winning Physicist
By measuring the tiny change in angle of the mirror during a round trip of a burst of light, taking only two ten-thousandths of a second, the speed of light was measured.This elegant experiment, devised and carried out during 1924-26 by Albert A. Michelson, America's first Nobel Prize winner in a science field, provided the first modern and ...
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.
Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment of 1887. Emission theories obey the principle of relativity by having no preferred frame for light transmission, but say that light is emitted at speed "c" relative to its source instead of applying ...
The Speed of Light Experiment uses laser light and a high speed rotating mirror to determine this fundamental constant using the Foucault method. Laser light passes through a series of lenses to produce an image of the light source at a measured position. The light is then directed to a rotating mirror, which reflects the light to a fixed ...
Round-trip light path in a Michelson interferometer. The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. ... and Kim repeated the Greaves et al. experiment intending to obtain with greater accuracy the one-way speed of light. [28] This experiment assumed that the signal return ...
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.