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Question:
How did scientists determine the exact speed of light? Why cannot anything travel faster than the speed of light?



Replies:
You determine any speed by measuring the distance something travels in a given amount of time. Since light goes so fast, you either need to use a very long distance to do the measuring, or else have a very short time. The original measurements of the speed of light were done using rather long distances - one of the first used the difference in timing between the apparent positions of Jupiter's planets as seen from earth, according to whether the earth was on the same or opposite sides of the sun: the relevant distance there is the diameter of earth's orbit. However, we do not actually know that diameter all that precisely, so more accurate measurements required much shorter times (the Jupiter measurement involved a time of about 20 minutes). The best way to do it, until recently, was using an interferometer that takes advantage of the wave nature of light, relative to the distance between two mirrors, for example. If you know exactly how many wavelengths of light lie between the two mirrors, and also know the precise frequency of that light (which requires other methods to determine) you can determine the speed (in the laboratory atmosphere) according to the product of those two numbers. In fact, the speed of light no longer needs to be measured: it is defined as an international standard, and the units of distance are then derived from it (since we can measure time much more precisely than distance). The reason nothing can go faster than the speed of light (as far as we know) is because of relativity, which is based on the observed fact that light goes at the same speed relative to you no matter how fast you are traveling. As you approach the speed of light, at first the energy associated with your motion goes up as the square of your speed, but that is only accurate for relatively slow speeds - the closer you get, the more energy is required, and it would take infinite energy for a massive object to actually reach the speed of light. This has all been tested and confirmed very nicely for elementary particles, and for some larger objects as well.

Arthur Smith


In 1880, Albert Michelson measured the speed of light by measuring the time it took for a beam of light to hit a mirror on one facet of an octagonal rotating mirror, bounce to a mirror on a mountain 35 km away and then back to his octagonal mirror. If he adjusted the rate of rotation correctly he could be sure that the light upon returning to his lab hit the same mirror. Then, knowing the rate of rotation, he knew the time it took to make the round trip of 70 km and BOOM!-- he is nabbed the speed-o-light. He noticed one problem, however. He was expecting to discover the existence of ether, a "stuff" that 19th century scientist figured must be the medium in which electromagnetic waves must propagate. If the relative motion of this ether was in the direction of the light beam, the beam should move faster, and vice versa. Well, assuming that this ether permeated the universe, the earth rotated in it, so Michelson should have seen different speeds of light if he did his test into or out of (so to speak) of the earth's rotation. This did not happen. The speed of light was a constant regardless of the frame of reference of the light or the observer. Einstein explained this in his special theory of relativity. Photons can go at the speed of light because they have no mass. Nothing having mass can hit the speed of light because it would take infinite energy to accelerate it to that speed. Why? The glib answer would be that, "that is just the way nature is. Tough!" Basically that is a heavy philosophical question that I cannot answer. I hope this helps.

Nick P. Drozdoff



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