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The Ether |
Although Sir Isaac Newton
believed that light consisted of particles, numerous experiments by others
demonstrated that light was a wave phenomena. Even 
. Therefore, it is not surprising that by the
turn of the 20'th century the scientific community was convinced that light was
a wave.
But this presented a problem.
In order for a wave to exist, there must be something "waving". There
must be some kind of medium through which the wave travels, a medium capable of
being disturbed or distorted. But in the vacuum of space there is nothing. How
then could a wave travel through free space?
Yet, light and other electromagnetic radiation do travel through free space. So Maxwell proposed a medium through which light traveled. He called this medium the luminiferous ether and assumed that it permeated the universe everywhere, including the vacuum of space. This mysterious ether had very unusual properties. It was completely invisible and completely frictionless, allowing planets and other material objects to travel freely through it. Yet was extremely rigid when it came to electromagnetic waves because it transmitted such waves with such an extremely high velocity.
But as mysterious as its properties were, the hypothesized ether seemed essential for explaining the propagation of light through empty space. And the explanation concluded that light traveled through the ether with a speed c relative to the ether. Therefore, the ether itself could be used as an absolute rest frame uniquely determined by the properties of the propagation of light. And one should be able to measure the motion of the earth relative to this luminiferous ether rest frame as it revolves around the sun and moves through the galaxy.
Michelson and Morley attempted to measure the earth's motion relative to this ether with null results. They tried again and again. At every attempt they found the earth to be at rest with respect to the ether.
These experimental results suggested that the ether was not a universally static medium but rather a fluid-like medium that could cling to large gravitating bodies and be dragged along with the earth. This ether drag theory never gained widespread acceptance because it was inconsistent with the observation of stellar aberration.
Because all attempts to detect Maxwell's hypothesized ether have failed, the concept is no longer in vogue. Indeed, Einstein's application of the principle of relativity to the speed of light demonstrated that the ether concept necessitating an absolute or preferred rest frame was invalid.
Today we recognize that Maxwell's equations are actually relativistically invariant. This means that when they are transformed consistent with the theory of special relativity from one inertial frame to another they take on the same form in all inertial frames. In other words, Maxwell's equations do not imply a preferred or absolute rest frame. They imply that every inertial frame behaves as if it were at rest.
As far as a wave needing a medium through which to travel, something that can be disturbed, today we recognize that the something being disturbed by light rays are the electric fields and magnetic fields that easily can exist in a vacuum. These fields are not independent of one another but interact according to Maxwell's equation in a manner consistent with the theory of special relativity.
Therefore, the concept of luminiferous ether is no longer necessary or
desirable. It is an idea that has come and gone, useful only to help one
understand the thought process that lead to the development of special
relativity.
Questions:
Q1. Which are true? (A) One reason
for assuming the existence of the luminiferous ether is that light is a wave
and waves need a medium in order to have something to be “waving.”
(B) One reason for assuming the existence of the luminiferous ether is that
Maxwell’s equations predicted that the speed of light was equal to c only
for one particular reference frame. (C) The hypothesized ether had very unusual
properties, being both extremely fluid regarding matter and extremely rigid
regarding light. (D) Two of these. (E) Three of these.
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