The idea that travelling at the speed of lìght makes one age at a different rate to those of their home planet.

Question

I need someone to make sense of it for me. I appreciate the clock scenario where it stays at 12 o’clock if you move away from it at the speed of lìght, but regardless of how fast someone travels, their body will still age just as fast as anyone else (roughly). I don’t understand how just putting distance between someone’s self and the rest of Earth would somehow make them age at a slower rate? You’re aging, just further away..

Hope this makes sense!

In: Physics

Answer 1

Before I can explain this, there is an assumption we must make: the speed of light is constant for everyone. That is, if I’m moving at a certain spzed relative to you, and we both measure the speed of light, we both will get the same result.

Now, with that out of the way, let’s simplify everything down to you and me. Both of us sitting in a spaceship out in space, and nothing else is near. At some point , you see me passing with some speed. And I see you passing with the same speed, but in the other direction. So far nothing special.
Now, let’s imagine we have a clock on board. But not just any clock. A special one. Two parallel mirrors, with a beam of light going between them. It takes a certain amount of time for the light to move between the mirrors.

Here’s where it gets interesting. When you look at my clock, the beam of light is going diagonally. If it weren’t, then the light wouldn’t hit the mirror for you, but it would for me. Which can’t be. And since we have already established that the speed of light is constant, the beam on my end must take longer to get to the mirror. [This image](https://upload.wikimedia.org/wikipedia/commons/8/8e/Time-dilation-002-mod.svg) shows it well. On the left is my clock as I see it, on the right as you see it.

But there’s more! I see the same effect for your clock. To me, your clock is ticking slower. At firdt glance, this is weird, but the thing is, there is nothing special about my spaceship, or yours. An anaology to this symmetry of weirdness is perspective. If we’re fzr away from each other, each will see the other as smaller.

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And as a final note, I leave with the cavezt that this only works for constant linear motion. In other words, there are no forces involved, no changing of direction or whatever. As soon as you get to that, you’ll need general relativity. Which is a lot more complex, and I’m not privvy to its secrets.