# What if Einstein's light clock is on the platform and the observer on the train?

Hoping someone can help me understand the time clock thought experiment. The normal version of the experiment has the clock on the train and observer on the platform. The experiment concludes that the passage of time is slowed down on the train from the perspective of someone on the platform. I've understood it so far.

But if we consider the experiment in reverse (observer on train watching clock on platform), wouldn't we reach the conclusion that time is slower on the platform from the perspective of the observer on the train? If time slowed down in both locations from the perspective of the other then there is no difference in elapsed time in when the two locations come together? Help!

• Just to clarify, when you say 'the two locations come together', are you asking about the moment that the train is at the same location as the platform, passing at constant velocity, or are you asking about the hypothetical scenario where the clock on the train, after passing the platform and seeing it slowed, somehow turns around, goes back to the platform location, and then stops? – Dragonsheep Jan 5 '18 at 7:28
• I was thinking of the train on a circular track doing a few laps before slowing down and stopping at the platform.But the other scenario you describe (the moment the clocks pass each other) is interesting too. – Jonesgh Jan 6 '18 at 6:31

Yes, an observer on the train watching a clock on the platform will also conclude that time on the platform is passing more slowly.

The last part of your question is unclear. I'll guess that you're asking what happens if one brings the two clocks together, which sounds like it should lead to a contradiction. However, it doesn't - to bring the clocks together it's necessary for the train to turn around. To turn around, there must be an acceleration, and that breaks the symmetry between the two observers. Both clocks will show that less time has passed for the clock on the train compared to the one on the platform. This is the essence of the twin paradox - it's the twin that gets on the spaceship that ages less.

• Sorry, my question was unclear, but you have pointed me in the right direction. The twin paradox seems to describe the problem that was troubling me. I see that it is covered in a later chapter of the book I'm reading so I will wait until I get there rather than trouble you further. – Jonesgh Jan 6 '18 at 7:01