Speed of Light | Michelson and Morley

The Speed of Light series consist of five parts. Quick access links are here.

Part 1 | Earliest Ideas

Part 2 | The Eclipses of Io

Part 3 | Chopping Light Beams

Part 4 | Done With Mirrors

Part 5 | Michelson and Morley

This final post highlights the work done by Albert Michelson and Edward Morley of the United States to measure c in various directions, and at different times of the year, as the Earth revolved in its orbit around the Sun. They wanted to answer the question of whether light needed a medium upon which to travel.


Wikimedia Commons

Waves are common phenomenon. They can be observed with a Slinky™, ropes, water, flags, earthquakes, as sound, even as crowd movements at a stadium. All of these examples involve some sort of medium upon, or through which, the wave motion propagates.

Should not the same be true of light waves and the various other electro-magnetic waves? Does light travel through an exceedingly thin and rigid medium? Is there a Luminiferous Aether permeating all of space which allows the travel of light but does not impede the motions of all other objects?

This question was posed during the speed of light measurement attempts carried out in the 19th century. The measurements of c were made to increasing accuracy by the likes of Albert Michelson and others. Their instruments always had some sources of error which did not allow them to discern whether the Earth’s motion through the aether of space made a difference in their value obtained. A more sensitive and accurate instrument was needed. Michelson made that instrument, the interferometer, and ultimately won the 1907 Nobel Prize for his work.

Tell Me More About Relative Motion

Consider this simple situation of a motorized boat in a river. You are observing the speed of the boat from the shore as it performs various movements in the flowing river. The river current is 2 mph. The boat speedometer is set to 6 mph. If the boat goes downstream with the current of the river, you would see it pass by going 2 + 6 = 8 mph relative to the shore.

The boat turns around and goes upstream with the same speedometer setting of 6 mph. This time you see it pass by going slower 6 – 2 = 4 mph.

Next, the boat turns to aim directly across the river. The speedometer is still set to 6 mph. The current is still 2 mph. Because of the sideways current, the boat goes off at an angle. The speed is found using the Pythagorean Theorem. It is the square root of 2 squared + 6 squared = 6.3 mph.

This simple exercise is meant to show that the speed of something through a medium depends upon the directions of the motions relative to the observer. In this case, speed is affected by the motion of the medium of the river. It was believed by many scientists in the middle 1800s that the speed of light value should be dependent upon which direction the Earth was moving through the aether medium in space. What was needed in order to show that it was true was an instrument with sufficient accuracy and sensitivity. None had been made until Michelson made the interferometer and tested it in 1881.

What Is Earth’s Motion Through The Aether?

As passengers on spaceship Earth, we are moving in several different ways through space.  Here is a partial list in increasing values of speed. For reference, the speed of light is about 671,000,000 mph.

  1. Earth’s rotation causes objects near the equator to be traveling east nearly 1000 mph.
  2. Earth’s revolution around the Sun causes a speed of nearly 70,000 mph.
  3. The Sun carries Earth and the other planets with it around the Milky Way galaxy at nearly 500,000 mph.
  4. The Milky Way galaxy is moving through space at nearly 1,300,000 mph.

With a sufficiently sensitive apparatus, there ought to be a way to measure such huge relative motions of Earth through the aether of space.

The Michelson Interferometer

Michelson developed a prototype of his instrument prior to 1881. The principle was simple. Send light waves from a source at the left (a) through a half-silvered mirror (b). Part of it goes forward to another mirror and back (c). The other part goes at 90˚ to another mirror and back (d). The waves re-combine and pass to a viewer (e). This is a drawing of the interferometer from Michelson’s paper in the American Journal of Science, 1881, 22: 120-129. The distances to the flat mirrors on each arm were 1.2oo meters. The interferometer was placed on a heavy stone in the basement to stop vibrations. By choosing the time of day carefully, one of the arms would lie along the same direction as the motion of the Earth as it rounded the Sun in orbit. The other arm would lie perpendicular to the motion of the Earth.

A. Michelson

This video by Leonid Kostrykin illustrates the motions of the waves to the mirrors and back.

If the crests of waves in one direction meet and re-combine with the crests from the 90˚direction, they will add to each other, reinforcing and showing a bright fringe of light. Light waves are exceedingly short. Any tiny change in the time of travel for either of the two directions can cause a crest to return when a trough from the other direction returns. A crest and a trough superimposed will cancel and show a dark fringe. Here is a brief video of how those fringes can appear by using a specific wavelength of green light. Notice the bright and dark fringes.

If an aether existed, light waves going over and back perpendicular to the motion of the Earth would take the same amount of time through the aether. The waves going along the same direction as the Earth would take longer going to the mirror and less time coming back. That would alter the total time for their round trip making it slightly more time than the other path. 

Six hours later, the arms of the interferometer would have moved due to the 90˚ rotation of the Earth. The light waves traversing the arms would now be in different situations as before. Time of travel should be affected slightly. The observer at the small scope (e) on the nearside should see a shift in the light and dark fringes of light during the course of the 6 hours. The interferometer was capable of showing those fringe shifts if they did occur. However, the fringe shifts were so small as to be inconclusive. They definitely were not obvious and suggested that the aether idea might be wrong. The instrument needed to be more sensitive.

The Improved Interferometer

Several years later at what is now Case Western Reserve University in Cleveland, Ohio, Michelson was joined in this effort by Edward Morley. They redesigned the interferometer to make it 10x more sensitive. They added several sets of mirrors at the ends of the two arms of the instrument. This caused the light waves to travel greater distances before rejoining and going to the viewer. This drawing is from their 1887 paper which is discussed in Wikipedia. The light source of sodium yellow wavelength is at (a). The half-silvered mirror splits the light at (b) into the two 90˚paths to several mirrors at (d – e) and (d’ – e’). The waves rejoined at (b) and were viewed through a telescope (f).

A. Michelson – E. Morley

The entire apparatus was sitting on a large stone slab in the basement of the building. The slab was floating in a tub containing liquid mercury. It could be rotated with nearly no effort and no vibration to watch for the shift in the light and dark fringes.

A. Michelson – E. Morley

Results of the Experiment

The light fringes were watched as the instrument was slowly turned. They were observed at different times of day and at different seasons of the year. The instrument was even taken to a mountain top to see if that made a difference. They always got a null result. No fringe shifts were observed. The experiment offered no supportive evidence for the existence of an aether for light to travel upon. The speed of light was not affected by any changes in the direction of the waves. The aether apparently did not exist.

Albert Einstein, at the age of 16 in 1895, pondered the question of whether one could tell if they were moving or not by measuring the speed of light. He continued to study this question for another 10 years. At the young age of 26, while working in a Swiss patent office, he published his answer in June 1905. The paper was titled On the Electro-Dynamics of Moving Bodies. Here is the English translation. Today, we often refer to this work by Einstein as his Special Theory of Relativity.

“… light is always propagated in empty space with a definite velocity c which is independent of the state of [relative] motion of the emitting body …. The introduction of a `luminiferous ether’ will be superfluous inasmuch as the view here to be developed will not require an `absolutely stationary space’ provided with special properties.” — Annalen Physik 17 (1905).

In other words, the speed of light measured by all observers is the same regardless of their motion. And, the laws of physics are the same in all frames of reference that are not accelerating. He went on to say that those things being constant and unchanging meant that other quantities once assumed to be constants are actually not. The rate of passage of time depends upon the speed of the object. The length of an object is shortened by the fact that it moves rapidly. And, the mass of an object increases if it moves rapidly. These quantities we intuitively accept as constant are variables. That is a discussion to be left for a post another time.

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22 thoughts on “Speed of Light | Michelson and Morley

  1. I’ve read enough of the Wikipedia page on this experiment to know that the Michelson – Morley experiment has been repeated many times over the years with increasing refinements and has consistently yielded the same result. That result is counter-intuitive. It defies common sense and runs counter to experience with relative-motion experiences in material mediums, and yet it is true. It is reality. Space and time are measured separately on our subjective scale and yet they are only separate aspects of one thing, spacetime. The only thing that amazes me more than this reality is that we poor creatures are capable of discovering such a thing.

    One thing I am confused about, though, is how light can have a “frequency” without a medium. I know what frequency is, as in sound waves or ocean waves, and that is the synchronous movement (vibration) of the medium’s particles. But in the case of light, there are no particles to move, and yet, relative movement produces the same effect as if there were a medium, i.e., the red shift of the light of other stars as they move away from us. It makes me think of the Cheshire Cat’s smile. Sorry, can’t help it. 🙂

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    • Light has a source. That source involves the oscillation of some electric charge like an electron in the atomic structure. Oscillation of a charge generates an oscillating electric field E at the same frequency. Coupled with that E is a magnetic field B. As one grows, so does the other. As one decreases, so does the other. The explanation is from Maxwell. (Too heavy for me to lift) This might give you some added info. http://en.wikipedia.org/wiki/Electromagnetic_wave#Wave_model

      These E and B oscillations radiate from the source at the speed of light. Their coupled behavior assures their propagation. They need no medium. They only need each other. Here is an animation from the same Wikipedia source that gives a good visual of the interaction of the oscillations of E and B in x and y planes with propagation in the z direction.

      I need to go to a platelet donation appt until mid afternoon. When I come back, I will check to see if you have more questions. See you later.

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      • I should have looked more carefully at that animation. Make that E in the z direction, B in the x direction, and wave propagation in the y direction. Not that it matters how they are labeled as long as they are each at 90˚ to each other. I didn’t want to confuse.

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      • I followed your link, Jim, and wandered around among other links in Wikipedia without a lot of luck. Physicists have long been at odds over something so seemingly simple as whether light is even “stuff” or whether it is something like a wave without being actual material as we think of it. It gets into semantics.

        For now I’m satisfied that I’m not alone in my confusion and I found some amusement in reading this paragraph under the subject of “wave-particle duality”:

        Semantic artifact view

        Robert Anton Wilson suggests that many of the so-called quantum paradoxes represent semantic artifacts that disappear when using E-Prime, a variant of English avoiding the use of the verb to be, for reporting observed scientific phenomena.[46] Wilson frequently used the wave-particle duality example to illustrate his idea. To wit, let us propose one group of scientists performs a set of experiments and reports, “One experiment demonstrates light is a wave while another demonstrates light is a particle.” Let us also propose that another group of scientists halfway around the world also perform an identical set of experiments and reports, “When constrained by one experiment, light behaved as a wave and while constrained by another experiment, light behaved as a particle.” Although both groups of scientists aim to report similar empirical observations, the first group makes an existential conclusion about the ‘is-ness’ of light. The second group reports their observations operationally, describing what they actually observed light doing rather than jumping to an existential conclusion about what light ‘is’.

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      • You’ve landed on a good question in duality. It seems there is not an answer to which is it. It depends on what your experiment is designed to measure. That bothers a lot of people. Maybe this will help. 🙂

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  2. Interference of light is hard to visualize without concrete examples. The MM setup really helps to understand light waves. Polarized lenses do too (I’m guessing my camera has a polarized lens somewhere because sometimes I can’t see the image on the lcd screen when I turn it for a vertical photo). The wave interference I’m most familiar with is water waves which varies every moment as I watch from the beach. Rogue waves are a memorable reminder of wave interference!

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    • You’ve come up with some good examples. The LCD screens on camera displays, etc, are polarized. If you wear polarized sunglasses, you can block it out by rotating your head.

      You water waves and rogue wave are good examples. Even sitting in the right or wrong place at a theater can give you sound wave cancellations or additions. Not a good thing. That’s why a gymnasium is not a good auditorium for music.

      Nice to see you.

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      • In confirmation of what you said, the LCD screen on my Canon EOS 5D Mark III is polarized, and I have to take my polarized sunglasses off to read menus on that LCD screen.

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  3. I looked up what a Black Hole was again, particularly the “Event Horizon”:

    “The particle horizon of the observable universe is the boundary that represents the maximum distance at which events can currently be observed. For events beyond that distance, light has not had time to reach our location, even if it were emitted at the time the universe began. How the particle horizon changes with time depends on the nature of the expansion of the universe. If the expansion has certain characteristics, there are parts of the universe that will never be observable, no matter how long the observer waits for light from those regions to arrive. The boundary past which events cannot ever be observed is an event horizon, and it represents the maximum extent of the particle horizon.

    One of the best-known examples of an event horizon derives from general relativity’s description of a black hole, a celestial object so massive that no nearby matter or radiation can escape its gravitational field. Often, this is described as the boundary within which the black hole’s escape velocity is greater than the speed of light. However, a more accurate description is that within this horizon, all lightlike paths (paths that light could take) and hence all paths in the forward light cones of particles within the horizon, are warped so as to fall farther into the hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time, and can actually be thought of as equivalent to doing so, depending on the spacetime coordinate system used.”

    Seems like ‘particles’ or “waves” of light are in there somewhere. Seems like light does need a medium too. I thought that Einstein had agreed that light was both a wave and particles. It also seems like gravitational forces are also going to affect this.

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    • I was out of town until just now. Give me some more time to collect my thoughts and get a few chores done. I will try to respond to your comments.

      Thanks…

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    • It has taken me longer than expected to get back to you. Things come up and cause diversion, as you know.

      Light has properties that make it act the waves and also the way particles behave. Sort of like saying a bird has the human-like property of two legs. But, it has wings which is an insect-like property. It isn’t either one.

      Because light is an electro-magnetic wave, it is self propagating. The E and the B fields oscillate in a coupled interaction. Each creates the other, in a sense. Nothing else is needed.

      The travel of these packets of energy, photons, is affected by gravity. Einstein predicted the path of light was not actually straight. Instead, it took curved paths through space. The amount of curvature depended upon the mass of the nearby objects is passed. The idea was tested and confirmed by Arthur Eddington in 1919 during a total solar eclipse.
      http://thethoughtstash.wordpress.com/2011/01/03/how-eddington-demonstrated-that-einstein-was-right/

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  4. I’m impressed by the amount of research you do for each post, and by the wonderful diagrams and visualization methods that you use to make complex topics more understandable. I always learn new things and get a deeper understanding of a science-related topics when I visit this blog.

    An aside–the boat and current diagrams bring back memories by the word problems we had in my 5th grade math class. 🙂

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    • Thank you very much. It means a lot to me. That helps me feel like I doing this in a good way.

      Good for you making the kids do those problems. They are good exercises.

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