ZincMan t1_j1i6t4n wrote on December 24, 2022 at 3:46 PM

Reply to comment by woodlark14 in Did you know it will still take 46 billion years to cross the universe at the speed of light? 65 mph = 4.8 * 10^17 years! by NotAndroid545

Fucking hell. I don’t even understand your first sentence. However what your 2nd paragraph is saying I think I get. Does expansion not count as moving through space ? I understand expansion speed is relative to the observer, but surely the fastest things could expand away from each other has to be twice the speed of light right ? Like if two objects are “expanding” away from each other at the speed of light in opposite directions… their relative speed to each other would seem as though they are traveling twice the speed of light right ? (Assuming you could observe) I guess my questions is can expansion travel faster than this ? Faster than two objects speeding away from each other at light speed ? I don’t get the difference between travel vs expansion. Is expansion actually Stretching reality itself?

woodlark14 t1_j1iczf1 wrote on December 24, 2022 at 4:33 PM

Apologies, relativity is quite hard to explain especially without diagrams. The answer to your other questions is that it is reality that is stretching, the distance between two galaxies becomes larger rather than one of the two galaxies moving into a new space. So it's not limited by the speed of light.

As for your questions about the fastest two things can move away from each other, that's actually a more complicated question than you realise. The issue here is that the maximum speed anything can be observed to be moving at is the speed of light. If you took two objects moving away from you at 0.75c (0.75 times the speed of light) then from the perspective of one of those objects the other wouldn't be moving at 1.5c. Instead it would be moving at 0.96c, and you would notice that clocks on the objects no longer match yours.