﻿ Michelson-Morley: Detecting the Ether Wind Experiment

Michelson-Morley
Detecting The Ether Wind Experiment
For the Advanced Science Hobbyist: Build an Interferometer and Repeat the Michelson-Morley Experiment

### Background

 The luminiferous aether: it was hypothesised that the Earth moves through a "medium" of aether that carries light Cronholm144 - CC 3.0
As a result of Michelson's efforts in 1879, the speed of light was known to be 186,350 miles per second (299,909 kilometers 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 "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.

### Michelson-Morley Experiment

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

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 (beamsplitter), so that half the pulse goes sraight on through the glass and half is reflected. As a result, the half light pulses arrive to the 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, the light beams 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 arrive at slightly different times, since both light beams travel the same distance from the beamsplitter to the two mirrors, and the one that would have gone the ether wind upstream and back is more inhibited than the one gone across the stream and back. It is the same case as with a swimmer that swims at a constant speed the same distance upstream a river and back and across the river and back - upstream and back time will be longer that the across and back time. It is possible to demonstrate by simple calculations why this difference occurs.

The interferometer was rotated in order to locate those upstream and across "ether winds" directions that were supposed to be perpendicular to each other.

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.

### The Emergence of Special Relativity

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?

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.

### Repeat the Michelson-Morley Experiment

 The actual apparatus used in Michelson-Morley experiment
The original Michelson–Morley experiment is quite complicated to repeat. In order to locate the small expected interference shift the apparatus was located in a closed room in the basement of a stone building, eliminating most thermal and vibrational effects. Vibrations were further reduced by building the apparatus on top of a huge block of marble, which was then floated in a pool of mercury.

The mercury pool allowed the device to be turned, so that it could be rotated through the entire range of possible angles to the "aether wind." such that one arm rotated into the direction of the wind and the other away.

Instead we suggest the building of a simple and relatively cheap Michelson interferometer by which you are going to demonstrate light interference and the relationship between the movement of an interferometer mirror and the resulting movement of the pattern of interference fringes.

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 teachers and other knowledgeable adults and experts.

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
The Speed of Light - Mark Lawrence
Michelson-Morley Experiment - Nobel Media

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

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 - towson.edu
Relativity - Mark Lawrence
Lorentz Transformation - HyperPhysics
Why Time Dilation (and Special Relativity Theory) Is Illusion
The Albert Einstein Library

### Books

Modern Physics in America
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