The Ether and Michelson-Morley Experiment; Most Famous Failure
The Ether and non-locality
In doing some reading into my research area in the foundations of quantum mechanics, I was surprised to learn that the existence of the Ether was suggested as a possible explanation for non-locality. The famous mathematician, John Bell, whose theorem established non-locality in the first place, had a hard time, as many of us do, in accepting that non-local connectivity exists between separated entangled particles.
In a nutshell Bell thought that different reference frames would allow superluminal communication between particles. Einstein’s special relativity predicts the well-known space and time contractions in different inertial frames, and perhaps these could explain the EPR paradox and rationalize non-locality.
Indeed with Han Geurdes, we found extra terms after a pair of entangled particles were boosted from their rest frame. My take on those terms is they account for the local correlation that exists between EPR pairs and do not explain non-locality. However to date no-one has come up with a local realistic explanation of the EPR paradox, so maybe the Ether approach has merit.
Moreover, the idea of the Ether is not inconsistent with quantum field theory which assumes a background of quantized harmonic oscillators from which particles like photons can be created and destroyed. Could this be a manifestation of the Ether?
Whatever the answers are to these questions, there is no evidence for the existence of the Ether, although that does not prove that it does not exist, only that it has not been detected. The famous Michelson-Morley experiment of the 19th century is often referred to as the most famous failure of an experiment. In the remainder of this entry I will review the Michelson-Morley experiment.
The Ether for light
It was thought that electromagnetic waves, like mechanical waves, required a medium to propagate. Nineteenth-century physicists postulated the existence of the Ether as a mass-less rigid medium which was fixed to the stars, and did not inhibit the motion of celestial bodies.
The reference frame attached to the Ether was postulated to be the absolute frame of reference in which the laws of electromagnetism would be valid and in which electromagnetic waves would propagate at speed c. Hence all other frames, such as the Earth’s, would travel at speeds relative to c.
Assume that the Ether does exist and in it the laws of electromagnetism are defined. If the Ether frame is fixed to the stars, then the Earth must move relative to that frame.
During a year, as the Earth rotates about the Sun with velocity v, it could be lined up with the Ether and so the speed of light in the Ether frame would be (c+v). Other times, the Earth would move opposite to this absolute frame and the speed of light would be (c-v).
Another way of saying this is that since the Earth frame is different from the Ether frame, light traveling in one direction on Earth would have a different speed than light traveling in another direction. The Michelson-Morley experiment was designed to measure this difference in the speed of light in different direction on Earth.
About half of the light from the source is reflected from the half silvered mirror onto mirror 1 and the rest is transmitted onto mirror 2. Both beams of light travel the same distance.
If the speed of light is different in the two directions, then an interference pattern would be observed because one light beam will slightly lag behind the other.
The speed at which the Earth moves around the Sun is approximately 28.8 km s-1. This is fast enough to be easily detected in the Michelson-Morely experiment.
From the above figure, suppose the Earth, and therefore the experimental setup, is moving in the same direction as the light beam moves towards mirror 2. That beam should therefore move faster than the beam going to mirror 1. The Ether is supposed to “blow” the light beam off course.
The experiment poses the question: Is the time taken for the two split light beams to cover the same distance on different paths the same?
Times of Flight
Before the light beam is split, it travels at the same speed. After it is split, one part travels parallel and the other perpendicular to the Ether frame as seen below. The part that moves perpendicular will be blown off course in the time it takes to go from the half-silvered mirror to mirror 1, and back again.
However the part going parallel to the Ether frame would not be blown off, but move faster in one direction than the other.
Different times of flight along different paths
If the Ether does blow light off course, then the time to travel back and forth to mirror 1 is
Using Pythagorean Theorem from the triangle above
and substitute this gives the time of flight as
In contrast, the time to travel back and forth to mirror 2 is
Using the fact that the Earth moves much more slowly than the speed of light
the binomial theorem can be used to expand the denominators to give the time difference between the paths as
The difference in times of flight is large enough to lead to interference patterns in the experiment. In fact no interference was observed thereby indicating that the Ether could not be detected.
No matter how the apparatus was oriented, the Michelson-Morley experiment could not produce interference patterns. They even waited six months for the Earth to move in the opposite direction and did the experiment again, but still no interference.
Of course this experiment was done before Einstein predicted that the speed of light in all inertial frames is a constant. Einstein knew that Special Relativity could be formulated using an Ether frame, but the ideas were mathematically simpler not to predict its existence.
Interested in the world of Quantum Mechanics? See my previous posts on the subject
The interactive software used in this video is the General Physics Tutorial from MCH Multimedia: It can be used alone or in conjunction with any introductory physics text book. All the topics found in AP (Advanced Programs in High School), and college level General physics courses. Also available for General Chemistry.