by Amara Graps (firstname.lastname@example.org)
The properties of light have perplexed scientists ever since humans were capable of giving it thought. Newton thought of light as showers of particles. Young and Fresnel gave evidence for light as waves. Maxwell concluded: "Light consists of electromagnetic waves," after combined electricity and magnetism in his electromagnetic wave theory. If Maxwell's statement is true, then what do the waves travel in, since mechanical waves have to propagate in some medium?
This paper is a brief investigation of that medium- called the ether. If light truly is a wave, then an ether is essential.
The properties of the hypothesized ether are very unusual. One type of medium is required by Maxwell's electromagnetic equations. Yet another type of medium is required from the noninterference of the ether with motions of bodies in our universe.
Maxwell derived his electric and magnetic field equations from his technique of analogy where he likened magnetic lines of force to incompressible fluid flow. However the waves in his electromagnetic field are transverse, that is, in a sinusoidal up and down (or sideways) motion. Transverse waves, cannot travel through a body of liquid or gas. These types of waves can only be conducted through solids, in a gravitational field, or along the surfaces of water. Therefore the ether cannot be a fluid because transverse waves cannot pass through a fluid. The ether has to be a solid.
A solid medium carries a transverse wave in the following way. As the wave passes through the ether, that portion of the ether has to be distorted at right angles to the transverse light wave. Then the forces holding that portion of the solid have to snap the ether back. The rate at which the light wave travels through the medium depends on the size of the force that snaps back the distorted region. The greater the force, the greater the snap-back, the more rapid the progression of the wave. Since we know that light travels at over 186,000 mi/sec, the snap-back by the forces must be extremely rapid- in fact the force holding each portion of the ether in place was calculated to be considerably stronger than steel.
A second type of medium results from our experience of seeing bodies able to move freely throughout the universe. Because we know that the motions of the bodies in our universe are not interfered with in any detectable way- it seems reasonable to assume that ether is nothing more than an extremely rarefied gas.
So we have a combination of properties that is very hard to visualize. The ether must be an extremely tenuous gas and possess a rigidity greater than that of steel. About a dozen experiments have tested the existence of an ether. The most famous is the Michelson- Morley (M-M) experiment. I will focus on that one. After I describe the experiment, I will state the contradictory results and how some scientists have resolved the contradictions.
Michelson and Morley's experiment was designed to measure the motion of the earth through the ether. We are fixed on the earth, so the ether should move relative to us. The velocity of light (c) traveling through this either would change for angles that ranged from light was traveling in the same direction as the ether (c+v) to light traveling in the reverse direction of the ether (c-v). The key instrument in their experiment was an interferometer which allows one to see light interference fringes. The role of the interferometer was to detect whether a beam of light, split into paths at right angles to each other and then recombined, has a difference in velocities over the two paths. The interferometer was set with one path parallel to the motion of the earth in its orbit, and then rotated to put the other path parallel to the motion of the earth.
There was a half silvered mirror (e.g. it's coated with enough silver to reflect half the light and allow the remaining half to be transmitted) at P. The beams are reflected at two mirrors S1 and S2 respectively and return through the half-silvered mirror to the telescope at the other end, where Michelson and Morley noted the number of interference fringes n. To calculate n they first calculated a time for the light to traverse the paths PS1P and PS2P. Then they calculated the difference in optical path defined as D=c(t1- t2). Michelson and Morley rotated the experimental set-up 90 degrees and repeated the calculations. They calculated a new optical path: D'. If ether exists then the interference fringes should shift by n fringes where n is defined by (D'- D)/l, and l is the wavelength of the light source.
The result of the experiment was that they found no shifts in the interference fringes. The accuracy of their result was 10 km/sec. E.g., although the earth's orbital speed is 30 km/sec and the light's speed of 300,000 km/sec, the velocity or the earth relative to any ether frame must be less than 10 km/sec.
The experimental result introduces a conflict. Light waves, whatever their form, could not be mechanical waves in a physical medium. And if they were not waves in a physical medium, how could they be said to be waves at all? Two resolutions of the conflict exist. Either an ether exists, and the M-M experiment didn't measure it or an ether doesn't exist, and light is not a wave.
Some scientists say that the ether exists and that the M- M experiment didn't measure it. One such scientist is H. Aspden, who claims that the ether is attached to the earth--it is a "localized ether." Consequently the M-M experiment didn't measure the ether because it was only designed to measure the linear motion of the earth through space, not rotational motion of the earth through space. Another scientist is E. W. Silvertooth, who claims that any laser interferometer experiment analogous to the M-M experiment would give a null result. His idea is that the frequencies of the interfering beams are themselves dependent upon velocity relative to a fixed frame. Therefore the frequency will adjust exactly to cancel any effect due to the motion through the light-reference frame, and a null result is an inevitable consequence.
Lorentz and FitzGerald had a related idea that they called the "contraction hypothesis." They postulated that, as a result of the motion of the stationary ether, all bodies are contracted by a factor in the direction of the ether. Therefore an arm of the M-M interferometer parallel to the motion of the ether would be shortened by this amount, and no fringe shift would be obtained when the instrument was rotated. (Later the contraction hypothesis was discarded because an effect of the hypothesis was that the velocity of the interferometer should change every twelve hours due to the earth's rotation, and the effect was never found.)
Other scientists say that an ether doesn't exist, but that a better explanation must exist for the appearance of light as waves in many situations (one example is double-slit experiments). D. Larson promotes the idea that light are particles that travel in a sinusoidal fashion. On this basis, he can easily explain why radiation can have wave-like properties, such as that of polarization, even though it consists of discrete particles. Scott Murray promotes the idea that light are particles that travel in rarefractions and compressions, i.e. concentrations of photons, like sound waves traveling in concentrations of air molecules.
The resolutions listed above are only a small number of the many that creative scientists have thought up to explain the properties of light and the null result of the M-M experiment. This paper up to this point has been about whether ether exists with relation to light's properties. The question is also important with relation to absolute frames of reference in physics. An ether signifies a fixed frame of reference that scientists can use in their measurements of the universe. Einstein's Special Theory of Relativity says that no such frame of reference exists, i.e. all motion is relative. The finding of an ether would shatter that hypothesis. Therefore, it is doubly important to investigate the ether hypothesis and alternate light theories further. I am not in a position to state a conclusion on whether or not ether exists, but I think that we should think seriously about what light is, and not laugh at what is typically presented as absurd.
Aspden, Harold, Physics Unified, Sabberton Publ., 1980, p. 52-69.
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November 25, 1996.