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We say that motion isn't absolute.but why?if we measure motion relative to "space" it will be absolute for all observers. isn't it? Is space also relative like motion? If space has no "special point" so how we measure acceleration. If we rotate instead of straight motion , we feel acceleration . But the entire universe also rotating relative to me , so why the others doesn't feel acceleration? Isn't Acceleration relative?

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  • $\begingroup$ How do you define "space" in the first place against which or "relative" to which motion is being measured? We don't do this. We measure the location of objects relative to other objects. $\endgroup$ – ggcg Sep 7 at 19:38
  • $\begingroup$ How do you measure motion relative to "space"? How will you pick a specific point in space? If you pick the location of a planet, for instance, the point will be moving in space - or will it?? For there is no reference point in "static space" to compare the movement of another point to. As they say, "there is no preferred inertial reference frame." $\endgroup$ – electronpusher Sep 7 at 19:40
  • $\begingroup$ In "free floating" outer space, what do you anchor your fixed point to, such that you can measure motion relative to this point? $\endgroup$ – David White Sep 7 at 20:40
  • $\begingroup$ Related: physics.stackexchange.com/questions/366301/… $\endgroup$ – D. Halsey Sep 9 at 17:06
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You cannot determine a point in space that is or is not moving. Choose Earth, for example, Earth moves around the Sun, which moves around the center of the Milky Way, which moves through the universe relative to other galaxies, which also move relative to each other, yet any of these points could be said to be sitting still and everything else moving around them. So you cannot say that a determined point is absolutely moving or not moving. There is no test you can do in an inertial frame of reference to determine that you are still and everything else is moving past you, or that you are moving past everything else. When two people move past each other they can both say, with equal validity, that they are still and the other is moving. This is one of the basics of relativity, a frame of reference's motion or stillness relative to any other frame of reference.

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The notion of motion usually means a change of position (in some generalised coordinates), and in order to define a position and a frame of reference you have to choose any particular object as an origin. Assuming that we live in an infinite, flat, homogeneous world, there are no special points in the space, every point is as bad or good as any other point. So the only sensible way is to define a motion as with respect to some object.

And this object is taken to be a point-like, for the case of finite body one may take the center of mass, or any other specific point, relative to which measure the distance. As for infinite space, there is no way to take an specific point, so motion relative to "space" seems meaningless.

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  • $\begingroup$ Just out of curiosity how would pick a special point in a closed finite space, like a sphere or hyper sphere? $\endgroup$ – ggcg Sep 7 at 20:08
  • $\begingroup$ @ggcg on any homogeneous space there are no distinguished points - on a hyperplane, hypersphere, Anti-De-Sitter space. I've just mentioned the simpliest example. $\endgroup$ – spiridon_the_sun_rotator Sep 8 at 4:43
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In the context of this question, one is generally considering flat Minkowski space, that is, we're not going to muddle things up with stuff that can provide a local rest frame (e.g., the CMB).

In flat space, there is simply no such thing as motion relative to space. In Pop-Relativity, one often hears, "As you approach the speed of light, time slows down". This is very misleading.

As you approach the speed of light, relative to your 'stationary' friends, your time slows down according to them, but their time slows down according to you...so who's more correct?

If you lack 'stationary' friends, then all you have is light signals, and no matter how you move inertially, the light signals that you emit with propagate outward uniformly and isotropically at the speed of light. Moreover, any incoming light wave (or photon) will arrive at $c$, independent of direction.

Finally, if you devise more complex experiments that look at the quantum vacuum state, or say you can directly measure the Higgs field (the ultimate Lorentz scalar), nothing you observe will depend on what you think you velocity is.

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We know that we are moving when we see all (or almost all) other things moving around us.

When inside a car, a ship, a train or a airplane we say that we are moving. We accept that the moving world is only a consequence of it.

But when looking at the moon and stars moving the same way during the night, there was a lot of quarrel at Galileo's times on the same argument...

There is no meaning in a movement in relation to empty space.

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  • $\begingroup$ How do we know we are moving and not sitting still with everything else moving past? $\endgroup$ – Adrian Howard Sep 7 at 20:39
  • $\begingroup$ It is only a convenient convention. Like to use an foreign currency as a reference when (almost) all prices increase every month in the local currency. We can say than that the local currency is becoming cheaper every month. $\endgroup$ – Claudio Saspinski Sep 7 at 21:45
  • $\begingroup$ Yes, but do we have an absolute currency that never changes, by which all other currency can be measured? $\endgroup$ – Adrian Howard Sep 7 at 23:39
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    $\begingroup$ There is no absolute currency or absolute frame of reference. What I am trying to say is that some frames are more convenient than others, depending on the problem. $\endgroup$ – Claudio Saspinski Sep 8 at 0:31

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