[Calamity]

Indeed - something often misunderstood as always recorded from the perspective of a third party observer.

Quick follow up question. If you did depart and travelled at near-ligth speed to another galaxy, arrived there in a few seconds and then looked back at the earth - would it appear as only a few seconds have passed for the earthlings back home as well?

By the same token, if you are actively watching another galaxy as you travel at near-light speed towards it - will you be watching that galaxy "age" at an extremely fast pace throughout the journey?

[jonplackett]

I think the downside is that for earth, time is passing by at 'normal' speed. So as soon as you get up to speed you're leaving behind, possibly the entire human race.

Weird to think that too if aliens show up. Their entire civilisation is probably already gone (unless it's VERY stable over hundreds of thousands of years).

[m463]

that's an interesting question.

If you traveled one light year at the speed of light, when you looked back you would see light from right after you left. But since you were traveling at the speed of light, your relativistic time for the journey would be zero.

So the light you observed would be proportional to your subjective time.

[Calamity]

So, by the same token, do you observe the galaxy you're approaching age quickly? On "fast-forward" if one could imagine that?