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bobtheblob6 t1_j26s5mx wrote

Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

> the Sun is moving at about 370 km/s (1/800 the speed of light)

That's fascinating. One question that's always been in the back of my mind for some reason is "if speed can only be measured relative to something else, what if our 'absolute speed' in the universe is actually quite high (fast enough to cause time dilation), and everything around us in the known universe is also moving similarly, such that we have no indication of our actual movement, and no one has ever actually experienced something near the baseline, dilation-free flow of time?"

I guess there are reference points out there that we can use to show that is not the case!

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nicuramar t1_j284u3s wrote

> “if speed can only be measured relative to something else, what if our ‘absolute speed’ in the universe […]

The fact that speed can only be measured relative to something else implies that there is no absolute speed. Also, the flow of time is always normal for yourself. It’s only that you see others, moving at very different speeds, to have dilated time, compressed space.

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bobtheblob6 t1_j28b6qh wrote

Isn't the speed of light a limit on your "absolute" velocity though? I figured it must exist in some form

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tnaz t1_j29hqqs wrote

The laws of physics do not specify an absolute velocity, and the speed of light is a maximum that any observer can measure any object going, no matter how fast the observer is going relative to some reference point.

The important context here is that velocities do not compose by simple addition, but by a Lorentz transform. If I see two objects moving away from me in opposite directions at half the speed of light, those two objects will see each other moving away at less than the speed of light. This is where you also see phenomena such as time dilation, length contraction, etc... come from.

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