Michelson-Morley: Detecting the Ether Wind Experiment

As a result of Michelson's efforts in 1879, the speed of light was known to be 186,350 miles per second.

Still, this did not really clarify the nature of light. Two hundred years earlier, Newton had suggested that light consists of tiny energized particles that bounce off objects and are detected by our eyes and Robert Hooke, on the other hand, thought that light must be a kind of wave, like sound.

At last, different scientific considerations, led to the notion that light is a wave. When a sound wave propagates, it needs some medium to pass through, like air, water, metal, etc. If light is a wave like sound then what medium it passes through?

It was natural to suppose that light must be moving through some mysterious material, which was called the ether (aether), surrounding and permeating everything. This ether 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. And because the Earth's rotation on its axis and its movement around the sun this ether could be described as a ghostly wind blowing through the Earth.

Detecting the motion of the Earth through the ether wind was the next challenge Michelson set himself after his triumph in measuring the speed of light so accurately. For this he teamed up with Morley, a chemistry professor, that was an experienced experimenter.

The Michelson-Morley Experiment
For their experiment, Michelson and Morley build an interferometer instrument that was able to detect light waves interference effects.

The Michelson-Morley Interferometer

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. They both go on to distant mirrors (that are located at equal distance from the half-silvered mirror), 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 ether wind blowing through the Earth, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since both light beams travel the same distance and the one that would have gone upstream and back is more inhibited than the one gone across the stream. It is possible to demonstrate by simple calculations why this difference occurs.

To the researchers surprise, any interference effects were not seen through the telescope.

The only possible conclusion from this series of very difficult experiments was that the whole concept of an all-pervading ether was wrong from the start.

From the notion that the ether is not a physical existing entity came up a few hard solving issues. The whole point of bringing in the ether was to give a picture for light resembling the one we understand for sound. The speed of sound through air is measured relative to air. If there isn't an ether, 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 sound emitted by a loudspeaker. The sound waves come out at a definite speed relative to the loudspeaker. It is found that, despite the expected boost from being emitted by a very fast source, the light is going forward at the usual speed of 186,300 miles per second.

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 or source. Yet when we measure its speed we always get the same result. How can all these facts be interpreted in a simple consistent way?

Albert Einstein's Answer
Einstein in his Special Theory of Relativity postulated that there is no natural rest or relative frame in the universe and that any measurement of the speed of light in any inertial frame will always give 186,300 miles per second.

For more about Einstein's Special Theory of Relativity click here.

Repeat the Michelson-Morley Experiment

The actual apparatus used in the Michelson-Morley experiment

Take in account that this experiment could be dangerous if you use a laser unit as your light source since it can harm your eyes. As a rule: this experiment should be performed under teacher or adult supervision familiar with safety procedures.

This experiment requires a lot of precision work and mechanics skills but not to much for a determined student. This experiment needs some optics and mechanics supplies that can be costy.

First, read carefully the background links and ensure that you understand the basic principals. Now you can read the experiment construction links and even consider to buy a Michelson interferometer kit for your experiment.

Surf further the web and consult your local library, your teacher and other knowledgeable adults and experts.

Links
Background
The Nature of a Wave - The Physics Classroom
The Michelson-Morley Experiment - Michael Fowler, University of Virginia
Michelson-Morley Experiment - HyperPhysics
The Michelson-Morley Experiment - Rochester Institute of Technology
Albert Abraham Michelson - AIP
Michelson-Morley Experiment Applet - Michael Fowler, University of Virginia
Michelson-Morley Experiment - Eric Weisstein's World of Physics
Michelson-Morley experiment - Wikipedia
Crucial Tests: The Michelson-Morley Experiment - Douglas Allchin
Lorentz Explanation of the Negative Results of Michelson Experiment - Milan R. Pavlovic
The Speed of Light - Mark Lawrence
Michelson-Morley Experiment - Neil Moore

Experiment Construction
Build Your Own Michelson Interferometer - Fred Raab, LIGO
Construct a Michelson Interferometer - University of Colorado Physics Department

Buy a Michelson Interferometer Kit
Sargent-Welch
Boreal Laboratories
LightMachinery

Special Relativity
Special Relativity - Michael Fowler, University of Virginia
Relativity - NobelPrize.org
How Special Relativity Works - HowStuffWorks
Think Like Einstein - Nova
It's Relative - KryssTal
A Brief History of Relativity - Time100
Special relativity - MacTutor
Special Relativity - SLAC
Relativity - Mark Lawrence
Relativity: scientific Theory or Illusion? - Milan R. Pavlovic
The Albert Einstein Library

Books
Modern Physics in America
A Michelson-Morley Centennial Symposium (A I P Conference Proceedings 169)

Ethereal Aether: A History of the Michelson-Morley-Miller Aether-Drift Experiments, 1880-1930

Experiment of Michelson-Morley and the Original Formula

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