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Comment by alex_young | Hacker News Reader

It’s all a matter of perspective.

If you were to travel at nearly the speed of light, you could cross the universe in a matter of minutes. Of course an external viewer on say Earth would disagree and say it took billions of years, but who’s counting?

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Actually, you couldn't cross it in a matter of minutes. In fact, you would never even reach the edge of the visible universe. This is because the edge of the visible universe is expanding away from us at faster than the speed of light.

I would really like to hear from an actual physicist on this question since both of you seem correct for one of the reference frames and the only way I see to reconcile the two is with weird singularities like "the particle reaches the edge of the visible universe in infinite time according to the rest observer". (And I don't think that is right either.)

The edge of the visible universe functions for us like a cosmic event horizon. Similar to the event horizon around a black hole. A particle leaving earth at light speed can never reach or go beyond that horizon. Even in infinite time. That's assuming the universe continues to expand. If it starts to contract then, yeah, the horizon is going to crash in on us.

What does this look like from the photon's frame of reference, then? Or is it nonsensical to try to describe things from a frame of reference at C?

This is probably a huge reasoning error, but wouldn’t you “expand away” with the expanding away part of the universe at the same rate, sort of riding along with the expansion given that c is the same in the point of reference (the expanding away faster than c part)?

n.b. I obviously lack the vocubulary to communicate properly about this, help needed!

Again, this is a matter of perspective. The amazing long lived Earth observer would see the universe expand out of view, and you along with it wouldn't they?

Yes, after billions of years you would move outside of the sight horizon of the long-lived observer on earth and disappear from view. For you, the traveler, this would happen in mere minutes. But you wouldn't have crossed the universe in that time because the edge of the visible universe is constantly expanding away from us faster than we can travel to catch up with it. Even if we travel at the speed of light.

I remember reading several years ago that there are celestial bodies that we will NEVER be able to see precisely because of this.

Lawrence Krauss has given a talk that mentions something similar. He says that we live in a good time because we can still see "everything" around us. At some point in the far future, any observers won't be able to determine many things about the universe because the "stuff" in it will be too far away to observe.

https://youtube.com/watch?v=7ImvlS8PLIo @ 50:57

Even wilder to me is that one day, anyone living in the Milky Way will look up at the sky and be unable to see any other galaxies. If it weren’t for archaeological evidence they would never have any way of knowing about the existence of those other galaxies.

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