[rewgs]

It's very late and I don't have the energy to look up the math, but:

Assuming you're traveling at light speed across the Milky Way, a distance of 100,000 light years, the trip would take 100,000 years for the person traveling, no? I wonder then how long it would be for everyone left on Earth. Millions of years?

[ncmncm]

No.

The closer you get to light speed, the less time it seems to you to take getting there. If you could get close to light speed, "only" 100,000+ years would pass back home, never less, but could be any larger amount. At 1/10 light speed, a million years pass back home and almost a million on board.

If you don't get really, really close to light speed, it still takes thousands of years, on board.

Getting close takes way, way more energy than (e.g.) getting to half light speed did, which itself takes unimaginably much; getting to 3/4 takes way more than getting to 1/2.

You probably can't carry enough material to produce that much energy, but everything you encounter on the way is blasting you at a good fraction of lightspeed. Maybe a laser from home can keep providing you energy, but that has to keep working for 100,000+ years. The faster you go, the less energy it can provide; and the farther away you are.

[rewgs]

So, even though “100,000 light years” means “it will take 100,000 years to travel this distance at the speed of light,” those traveling won’t experience it as 100,000 years, but rather a much shorter time period? How long would the travelers experience that as?

To be clear I’m not talking about “almost light speed,” I’m talking about traveling at exactly the speed of light (ignoring for a moment whether that’s possible).

[ncmncm]

At exactly the speed of light you would experience no duration at all. But it would take infinite energy to get going that fast, and infinite energy again to stop, and would still take 100,000 years as seen by anybody watching at either end.

[rewgs]

Ah got it. Mind-boggling. Thank you!