The history of the speed of light involves, among others, Galileo, two chaps named Michelson and Morley, a FirzGerald, a guy named Lorentz, and, of course, Albert.
You might think that the speed of light was a relative thing. That it would go faster coming from car headlights than a lamp post. But you’d be wrong. You might also think that relativity was Albert’s original idea, and you’d be wrong there too, it was Galileo that first came up with a theory of relativity. He apparently said : The laws of mechanics valid in one frame of reference are valid in all frames of reference that move uniformly in relation to it.
Of course Albert took that idea and really put it to work. He expanded Galieo's relativity to include all the laws of physics. In particular, he included the laws governing electromagnetic radiation (which includes light).
But before Albert, there was Michelson & Morley, who measured the speed of light. Actually they weren’t as interested in its actual speed as they were in proving the existence of a medium for it to travel through. The thought of the day was that as a wave, light had to propagate through something. They called the something, the ether.
Of course if the ether existed, the earth had to move through it (it was assumed that the ether was motionless). Therefore a light wave moving in the direction of the earth’s motion should travel at a different speed than one traveling perpendicular to it. Unfortunately when Michelson and Morley did their experiment, both light beams traveled at the same speed. The conclusion: either the earth was not moving through the ether, or the ether did not exist. Neither alternative was very attractive to scientists of the day.
The next guy to throw his hat in the ring was George Francis FitzGerald. He reasoned that the ether may just compress matter traveling through it, including the “yardstick” used to measure the light speed. He suggested that this compression might just be enough to render measurement of the two light beams the same.
Then, Hendrik Antoon Lorentz independently worked the same idea out mathematically and laid it out in now famous theorem called the Lorentz Transformations. Albert put Galileo’s principle of relativity together with Lorentz’s transformation mathematics and came up with the Special Theory of Relativity, which says that:
1) The velocity of light in a vacuum is the same for all observers (i.e. in all frames of reference) moving uniformly in relation to each other, and
2) All laws of nature are the same in all frames of reference moving uniformly relative to each other.
The speed of light being constant in all situations is the fly in the ointment. It just couldn’t be right according to classical physics, but if it is right, if light speed is constant, then something else has to change, and Albert figured out that the something that changes is space-time.
(Even though the Michelson-Morley experiment had been published 18 years prior to Albert’s 1905 special relativity paper, and had led to the Lorentz Transformations, Albert claimed that it was not part of his reasoning process that led to his special theory.)
The perplexing part about measuring the speed of light is that since Albert threw out the idea of a fixed, unmoving, reference point, it is impossible to determine which is moving, the source or the observer. Maybe they both are. Therefore you can only talk about their motion relative to each other and only take measurements from one or the other point of view (or frame of reference).
If you are in the source frame of reference, the light leaving you will always be leaving at the speed of light, regardless of your motion relative to anything else in the universe. Consider the idea that light from a car’s headlights travels faster than light from a lamp post. Now, why would you expect such a thing? Does the sound from a car’s horn travel faster when the car is moving than when it is stopped? Of course not. Sound travels at a speed determined by the medium it travels through, i.e., the atmosphere, or you neighbours wall. Since light is a wave it propagates at a specific velocity too, regardless of the motion of the source, just as sound waves do.
However, if you are measuring from the observers frame of reference the situation is a little different (or the same, depending on how you look at it). The fact is that mass is compressed when it is in motion, just like FitzGerald said it would be (but because of a change in space-time rather than compression by the ether). The result is that the measuring tools (rulers and clocks), moving along with the observer, change just enough so that the speed of the light wave will still measure 186,000 miles/second. But that’s not all; the frequency will also measure the same as if it were measured at the moving source. The confusion comes in when you try to talk about stationary and moving observers. The fact is that you can only state your motion relative to the source of the light.
Therefore, as an observer, if you are stationary relative to the source you will measure the speed at 186,000 m/s. If, on the other hand you are moving toward the source at some incredible velocity, your measuring tools will shrink and you will still measure the speed at 186,000 m/s. No matter what the relative speeds are, or who is moving and who is stationary, the light speed will always be the same.
The classic example of a train whistle changing pitch as the it passes by is the same for light, hence the red-shift for stars moving away from us and blue-shift for those approaching us. In both cases, the velocity stays the same but the frequency changes.
Light leaving a moving star will still travel at 186,000 m/s, but the frequency will have to shift. If we were to suddenly begin to move away from that star, thus adding to the speed we are moving away from each other at, we would still see the same light speed and frequency because both our yardstick and our clock would change resulting the same measurement we got when we were "still" and the star moving away from us.
There are several other interesting points:
First; it appears that the classical physicists may have been right about a propagating medium. That medium may have been resurrected by Quantum field theory, which states that particles (including photons) are excited states of the featureless ground state of the all-pervading quantum field. (It seems to me that this field would be the fixed reference point 19th century scientists were seeking??)
Second: If light does not propagate through some medium, if it just whizzes unimpeded through empty space, then why does it settle on a specific speed? What makes 186,000 miles/second the speed limit? Well, if there is a propagating medium, i.e. the quantum field, then it make a little more sense that energy would propagate through it at a specific speed, just as sound through air. It is also in keeping with the fact that light travels slower when it encounters matter, such as glass, or the fibre used in fibre optics.
Of course, you have to wonder what also makes that particular speed an ultimate one that nothing can exceed. Is mass and energy that closely entangled that the same rules have to be obeyed regardless of which form it is in?
And finally, just to warp your mind, consider one of Albert’s thought experiments:
Imagine yourself in a moving room. The room moves with a uniform velocity close to the speed of light. Exactly in the centre of the room is a light bulb that flashes on and off periodically. The room is made of glass so that an outside observer can see what goes on inside. At the precise moment that we pass an observer the light flashes on.
Now you might think that you in the room and the observer will see the same thing, but you’d be wrong. Inside the room we would see the light travel out in all directions at the same speed and hit all four glass walls at the same time.
The outside observer also sees the light travel out in all directions at the same speed. However, he also sees that the room is moving, and therefore the light travelling backward hits the rear wall before the forward moving light can catch up to the forward wall.
And Albert’s conclusion? Events that are simultaneous for one observer may occur at different times for another observer, depending on their relative motion. The terms “sooner”, “later”, and “simultaneous” * are local terms and have no meaning unless they are tied down to a specific frame of reference.
*(this is apparently true only for “space-like separated events”. “Time-like separated events” can never appear simultaneous in any frame of reference travelling at less than the speed of light)
Just when you think you have some of it all figured out, Albert will throw something like that at you.
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