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woodlark14 t1_j1i3ljj wrote

Reply to comment by ZincMan 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

From the perspective of relativity, everything is always moving at a constant speed through space-time from every perspective. The faster something is moving through space, the slower it is moving through time and vice versa. Light speed is achieved when an object has no mass, resulting in it travelling entirely through space and experiencing no time passing. There's a whole bunch of effects that result from this like length contraction and time dilation.

Space expansion is very different, It has nothing to do with the object you are observing, it is instead a property of the space between you and the object. In a sense neither object has relative motion, instead the ruler you are using to measure the distance between the two is changing it's length.

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ZincMan t1_j1i6t4n wrote

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?

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woodlark14 t1_j1iczf1 wrote

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.

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