Hacker News Reader

It's both actually. But that gives an apparent contradiction - how can both observers see that the other party has slower clocks? Who is 'right'?

The answer is that the 'moving' traveler needs to flip the ship around at some point and thrust back towards the starting point. Whoever turns back around will find that it's their clocks that passed more slowly.

View on HN

And if the universe is spherical?

I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.

You come back around on a sphere too, e.g. traveling by the equator.

In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.

Isnt a torus curved? What's the argument?

If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont.

See https://en.m.wikipedia.org/wiki/Periodic_boundary_conditions

Reply on news.ycombinator.com