How does it work? I get lost a gravity and relative observer stuff.
Say we earthlings devolop the ability to travel at, say, .5c. We build a spaceship and send it to a destination 5 light years distant. They reach the point and immediately return for some reason(hostile aliens maybe). Im observing from earth so I perceived not 10 year passing between departure and return, but some longer period. How long am I stuck here waiting?
Is time dilation the same for the return trip?
What if their route take them in close proximity to something massive, perhaps a large star or black hole. Is only thier percieved time affected, if at all, or do I wait longer as well?
They’re traveling 10 light years total at .5c (ignoring time to accelerate, etc.) so it will take 20 years Earth-based time for the round trip. The astronauts will experience 17.3 years. They experience the time dilation. The formula for the time they experience is sqrt[1-(v/c)[sup]2[/sup]] multiplied by the time in the inertial reference frame.
And yes basically to your other question. Only they would experience the time effect of a large gravity field.
Thank you for the answer to to the first question.
On the second:
How would this affect their experience? Perhaps they “slingshot” a massive gravitational body at the mid point. Once for outbound and again for inbound. Is their 17.3 years affected by this? Im sure it depends on the gravitational forces.
Yes there is gravitational dilation. Time runs slower deeper in a gravity well, but you don’t give enough information to determine an answer. You’d need to know how much time was spent at each potential.
However, unless the mass was huge, and the ship went quite close. I think the effect wouldn’t be noticeable. Furthermore, unless the mass was enormous, I don’t think you could get close enough to have much of an effect without the tidal forces literally ripping the ship apart.
There’s almost certainly nothing that massive within 20 light years of Earth or I’d think we’d know about it.