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Comment by jofo25 | Hacker News Reader

I seem to remember from physics class that mass cannot travel faster than the speed of light, so how is it possible for galaxies to be unreachable if you managed to travel at the speed of light?

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It is possible because the galaxies are not travelling faster than light. Space is expanding.

Consider if you were travelling on the surface of an expanding balloon. Your distance from the starting point would increase faster than your speed relative to the surface at your current spot, because the part of the surface you'd already travelled across would continue expanding. Now do that in three dimensions.

>Now do that in three dimensions.

You just broke my brain.

Imagine raisins inside a lump of dough expanding as it is baked into a fruit loaf.

Now imagine the dough expanding faster than the speed of light.

You're going to need a very big oven.

If you're going to bake an Apple pie from scratch, first you must invent the universe. But before that, buy a high dimensional superluminally expanding hyper oven.

Just use a spherical oven and perform a geometric inversion with origin the centre of the oven and radius the oven's radius. Then all the pie inside will become outside and the outside will fold inside and you'll have infinite space for pie. As easy as capturing a lion.

Imagine that you go running with a friend. You run at 0.9 * speed of light and your friend runs at 0.8 * speed of light in the exact opposite direction. How fast are you getting away from each other?

I'm not sure I understand. It seems like you're implying that the two people would be moving away from each other faster than the speed of light, but I was under the impression that that's not the case. I thought that each person would observe the other travelling at C?

I think they would observe each other traveling at C but would experience visible time dilation (in the other person's frame of reference).

1.7c for an observer just standing around and watching the two guys run. I am not sure what it looks like from the point of view of the runners without calculating it but they should see a speed smaller than the speed of light.

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