The second postulate of relativity is easy to state, but rather subtle in its implications. You see, never before had something like the constancy of the speed of light occurred. In fact, after physicists realized that light is an electromagnetic wave, they believed that there must be a medium in which the wave propagates. All other waves -- sound, water, string vibrations -- propagated in a medium. So, went the thinking, there must be a medium, which they called the ether, for the propagation of electromagnetic waves. If such an ether existed, then the speed of light would equal c only when measured from a reference frame at rest relative to the ether. In reference frames that are moving with respect to the ether, the speed of light could be greater than or less than c.
This situation is akin to an observer moving towards or away from a stationary source of sound. As seen in the discussion of the Doppler effect in section 14.6, the speed of the observer adds or subtracts from the speed of sound in air, resulting in a different relative speed. This results in a change in the number of cycles of the wave that reach the observer in a given time, causing the frequency of the sound to change according to the Doppler formula.
The most famous experiment to detect the ether is the Michelson-Morley experiment. The experiment was performed in Cleveland, at what is now Case Western Reserve University, and to get to the punch line, the result was negative, no ether was detected.
The experiment is based on an interferometer, a device capable of detecting small changes along one light path relative to a second light path. The change is detected by observing a shift of the interference pattern produced by recombining the light from the two paths. In this interferometer, the two light paths are aligned perpendicularly. A measurement is made by rotating the entire interferometer through 90° and watching for a small change due to the change of orientation with respect to the velocity of the earth relative to the ether. No change of the interference pattern is observed.
Michelson interferometers are once again at the forefront of Physics research. Two large interferometers are being built for the LIGO experiment. LIGO stands for Laser Interferometer Gravitational-Wave Observatory. The facility housing one of the interferometers is pictured. This facility is in Hanford, Washington, and the second is in Livingston, Louisiana. In the photo you can see two long tubes emerging from the central building. Each tube is a 4 kilometer (2-½ mile) long vacuum for the light to travel in. Laser light is used for its superior properties in interferometry.
As the name implies, the LIGO experiment will search for gravitational waves, a prediction of Einstein's general theory of relativity which will be briefly discussed at the end of this chapter.
The null result of the Michelson-Morley experiment pretty much killed the ether hypothesis. (There were proposed modifications to make the results consistent, but as Einstein pointed out, the simple conclusion is 'you don't need the ether; it doesn't add anything to the Physics, so drop it'.
Maxwell's equations predict one speed for electromagnetic waves, and without an ether to provide a universal reference frame, the speed of light has to be the same in all valid (that is, inertial) reference frames. In order to make sense of the fact that everyone measures the same speed of light, we will have to alter our basic notions of space and time.
I mean, isn't this stuff speculative, not well proven, and subject to change at almost any moment?
No. Special relativity is very well established and relied upon daily by millions of people around the world. The Global Positioning System (GPS) satellites in orbit about the Earth carry very precise clocks; it is the precision of the clocks that enable the GPS system to accurately determine your location. Effects from special relativity cause the clocks to run slow by 7.11ms per day. That's not much, but it must be corrected for the system to function properly.
One of the results of special relativity is that no object or particle can travel faster than the speed of light. In my research work, we accelerate protons and electrons to extremely high speeds, nearly the speed of light, every day. The protons are accelerated to a speed of 0.9999995c inside of a large ring of magnets. If special relativity were not correct, the speed of the protons would reach 44 times the speed of light, and we wouldn't be able to follow them properly around the ring -- distance traveled = speed × time would be 44 times greater!
Special relativity is now deeply ingrained in Physics and technology.
There are a number of consequences of special relativity, some of which you may be familiar with: