How Did One Exception Cause So Much Trouble? (1)

For simplicity, any motion used in this section means uniform motion.

Just open any book about Special Relativity, some paradoxes are certainly to show up. Only light is singled out, and the whole universe is ruined. How did that happen?

In books where Special Relativity is touched, it is not rare to see that the theory is first introduced by using a light clock, and then the author claims that any other types of clock will have the same effect. Is that true?

Let's try to use a mechanical clock this time.

Based on Newtonian mechanics, no matter the clock is at rest or in uniform motion, all the physical forces acting on it work exactly the same way, thus the clock's time will not depend on the motion state of the clock.

Can we use Newtonian mechanics here?

The answer is absolutely yes.

The first postulate is just an extension to the idea of relativity from Galileo and Newton. This point will be much clearer if the second postulate is removed temporarily. In fact, without Galileo velocity addition rule and Newton's second law of motion serving as its solid foundation, the first postulate won't exist at all.

As for the second postulate, it talks nothing but light. So it does not change any point in the first postulate, thus Newtonian mechanics is not affected in any way.