The most prevalent theory of ether stated that it was stationary and the Earth moved
through it. An object stationary on the Earth would be moving through the ether.
Michelson and Morley decided to measure the speed of light when the light was moving
in different directions relative to the Earth’s motion around the Sun. If ether existed, the
speed of light would be different, and would be slowest when it moved in the same
direction as the Earth’s motion through the ether.
Here is an analogy that may make this clearer. Imagine two birds flying at the same
speed in opposite directions, while you observe them from a slow-moving train, with the
train’s motion causing you to feel air in your face. If the train is moving in the same
direction as one of the birds, that bird will appear to fly slower than the other bird. The
birds are like light, the air is like the ether, and the train is like the Earth.
The two physicists searched for changes in the speed of light using an apparatus now
known as a Michelson interferometer. Michelson estimated that the Earth’s motion
through the ether would cause a change in the speed of light on the order of
30,000 m/s. The interferometer was capable of detecting this 0.01% change in the
speed of light.
Michelson made multiple observations, measuring the speed of light with different
orientations relative to the Earth’s rotation and motion around the Sun. He observed no changes in its speed. As he tersely summarized: “The
result of the hypothesis of a stationary ether [with respect to the Earth] is thus shown to be incorrect.” (Michelson also considered the case that
the ether was dragged along with the Earth; this possibility was later shown to be false as well.)
Michelson’s conclusion was correct. His experiment indicated that the effects expected when waves propagate through a moving medium did
not occur. Despite some efforts to revive the theory of ether, the clarity and simplicity of Michelson’s experiment was a major and conclusive
blow to the theory of ether.
If this experiment had proven only that light could travel in a vacuum, disproving the existence of the ether, it would have been a landmark in
the history of science. But Einstein realized that something even more interesting was being shown by the experiment: The motion of the
equipment was not affecting the observed speed of light in the way classical physics predicted it should. As the light moved in the
interferometer, the equipment itself was moving, which according to the physics as Einstein had learned it should alter the effects of the
experiment. In other words, even without an ether, the equipment itself was moving, and that should have affected the measurement of the
light.
In his 1905 paper on special relativity, Einstein wrote, “...the unsuccessful attempts to discover any motion of the Earth relative to the ‘light
medium,’ suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute
rest.” Michelson’s experiment helped Einstein to formulate his second postulate: The speed of light is absolute; it does not depend on the
reference frame. It also supported Einstein’s first postulate. If the ether frame existed, the laws of light propagation would have been different in
that frame, which would have contradicted the requirement that the forms of the laws of physics be the same for all observers in inertial
reference frames.
Light travels through vacuum
No difference in speed measured
·There is no ether. The speed of light is
constant35.4 - Simultaneity, or the lack thereof
Einstein’s simultaneity thought experiment: The relative motion of two
observers determines whether they perceive two events as simultaneous.
One of the consequences of Einstein’s postulates is that the perception of “simultaneous” events is relative to an observer’s reference frame.
Einstein demonstrated that his postulates proved that two observers would disagree on how much time elapsed between two events, or on
whether two events occurred simultaneously. In this, he disagreed with Sir Isaac Newton. Newton had stated: “Absolute, true and mathematical
time, of itself, and from its own nature, flows equably without relation to anything external.” Einstein was to prove this wrong.
To make his point, he used a specific means to define simultaneity, noting that if an observer sees two events that are the same distance away
occurring at the same moment in time, she thinks the two events occurred at the same moment in time. It seems a commonsense conclusion
that another observer, also equidistant from the events, would see them occurring simultaneously. Einstein’s genius lay in challenging this
conclusion. He created a scenario showing that the relative motion of two observers influenced whether they perceived two events as occurring
simultaneously, or as occurring one after the other.
Einstein made his case in part with a famous “gedanken experiment,” or “thought experiment.” A thought experiment is one that is conducted in
the mind, as opposed to in a laboratory. Einstein used his thought experiment to make the following revolutionary point: The measured length
Simultaneity
Events perceived as simultaneous in
one reference frame
Not simultaneous in another