## An imaginary experiment with light

Let us go back to the last example but instead imagine that we have light rather than a bullet chasing the astronaut. It is quite irrelevant whether we talk about a wave or a photon; let us just say that instead of a bullet, we have a light signal. Again, both the observer on earth and the astronaut measure the speed in their respective reference frames S and S'. c = speed of light relative to earth and v = speed of astronaut relative to earth.

Earthman and astronaut measure the speed of the same photon.

Both the observer on earth and the observer in the rocket measure the speed of a light signal, by timing it, as it crosses a marked distance in their respective laboratories.

This time, however, they both get the same result! 15.6.3 A paradox?

How can they possibly get the same result?

Common sense' tells us that as the light signal is overtaking the astronaut, it will pass him, the same as the 'chasing bullet', at a lower relative speed than its speed in the earthframe. Einstein's second postulate, 'The speed of light is the same for all observers', tells us something different. We either accept it or we do not. If we accept it, we must also accept the consequences, strange as they might seem!

Paradox (Chambers dictionary definition): A statement that is apparently absurd but is, or may be, really true.

We have to accept that the contradiction here is not between the laws of Nature themselves, but between the laws of Nature as they are, and the laws of Nature as we think they should be.

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