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VoilaVoilaWashington t1_j295nnw wrote

Reply to comment by Bladestorm04 in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

CMB isn't any sort of special reference frame. It's just one that can be described as somewhat universal, but in theory, you could use the exact same thing with my neighbour Steve - the rotation of Andromeda is measured relative to Steve. That'll make your math nice and fucky, right?

But then the same is true of the CMB. Imagine if we tried to calculate highway speed limits based on the CMB reference frame. Or even the velocity of Sol relative to the CMB, to pick a larger example - how would that help us calculate how long it takes to get to the other side of the galaxy?

So, you always pick a reference frame that makes your math easy. Speed limits are based on the car's relative speed to the road, ignoring the rotation of the earth and all that. Your ability to juggle on a train is unaffected by the speed of the train relative to the tracks. Earth's rotation around the sun is measured against the sun, not the center of the galaxy.

You can still calculate highway speed limits using Andromeda's approach as a frame of reference, you'll just end up doing a LOT more math depending on where you are on earth and what time of day it is. But you could do it if you really wanted to!

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Nattekat t1_j2afp4g wrote

Doesn't there have to be some reference frame whereby a body is not moving through space at all? Not under influence of any force, just the expansion of the universe all around it making it seem as if it's accelerating from any observer.

Under the laws of special relativity an observer in a space ship travelling at 0.1c will see Earth speed up, while an observer on Earth will see the ship slow down. But if all speed is relative, both should see the other speed up, which feels paradoxical.

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VoilaVoilaWashington t1_j2ahqjh wrote

> Doesn't there have to be some reference frame whereby a body is not moving through space at all?

Relative to what? If you're measuring against the expansion of the universe, then you'd have to take VERY precise measurements against the most distant objects, and they're moving in all kinds of directions, but if you could, then sure, you could do that, somehow.

You'd just be moving at an insanely high velocity relative to anything local to you.

And because you're moving at that insane speed relative to, say, earth, you'd have to apply a massive force to actually get up to that insane speed.

> But if all speed is relative, both should see the other speed up, which feels paradoxical.

Welcome to relativity. Say you have 2 objects approaching Earth at 0.1c, relative to Earth. They'd see each other as moving less than 0.2c, because it's not additive, and if they could each see a clock on the other ship, they'd see that clock moving slower than their own.

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Ape_Togetha_Strong t1_j2cge19 wrote

> Under the laws of special relativity an observer in a space ship travelling at 0.1c will see Earth speed up, while an observer on Earth will see the ship slow down.

No they won't. They'll both see each other slow down, since motion is truly relative. From the perspective of the Earth, the ship is moving away. From the perspective of the ship, the Earth is moving away. Neither is right or wrong. Both see symmetrical time dilation.

The unintuitiveness of this fact is why the twin paradox is so famous, despite not really being a paradox: https://en.wikipedia.org/wiki/Twin_paradox

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