## How Did One Exception Cause So Much Trouble? (2)

To remove any doubt on this point, let's review a typical example in high school physics.

In a moving train, an apple on a table fell to the floor. If to one observer A at the table, it took a specific amount of time for the apple to reach the floor, another observer B on the platform would get exactly the same result. Although to observer B on the platform, the apple traveled a much longer distance, but it also got a "free ride", so its overall speed to observer B was also increased.

Now let's replace this apple with light. Can we get the same conclusion? No. The reason lies in the second postulate. Since light does not follow the rule of relative motion, it cannot get a “free ride”.

That's why a mechanical clock is not influenced by the motion of its carrier, but a light clock does.

There is another evident difference between a mechanical clock and a light clock. In all books about Special Relativity, the light clocks in the spaceships are always facing the same direction. This is not a coincidence. Just change the direction a little bit, and the formula of Special Relativity will be destroyed. A light clock depends on direction but a mechanical clock does not. This difference is also caused by the absolute motion of light and the relative motion of all other objects.

So the types of clock do matter. Different types of clock can lead to different conclusions. Just replace light clocks with any other types of clocks, the universe will be back to normal again.