The courage of ones convictions

The second postulate, that the speed of light is the same for all observers, takes an additional and crucial step beyond the postulate of Galileo. Galileo restricted his thoughts to mechanical laws governing falling droplets or flying insects. The propagation of light did not enter into his thoughts — not surprisingly, since at that time the speed of light was considered to be infinite.

Einstein went beyond darting fishes and flying bullets. His second postulate is a logical extension of the first. It may look like a small step but could be likened to Neil Armstrong's 'giant leap for mankind'. It led to a new understanding of the symmetry and universality of the laws of physics.

If we are convinced that all frames are equivalent, it must follow that we will get exactly the same result for the speed of light, whether we measure it in a laboratory on the earth, in another laboratory in a jet plane, or indeed in a spaceship. Otherwise, of course, a measurement of the speed of light would distinguish one frame of reference from another. For light in free space there is no upstream or downstream. All directions are equivalent. That is why the experiment of Michelson and Morley gave a negative result, or at least gave a result which was not in line with current thinking at the time. Galileo was not aware that he had an even stronger argument, using light rather than drops of water in his Gedanken experiment!

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