Do we move in the Universe? Or is everything stationary

  • #26
PeterDonis
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Relative to its non-spinning self.
If you mean, will the moving astronaut cause the one at rest to spin as he passes by, no, not under the assumptions of the OP, which are, as I said, that both astronauts are test objects, with negligible gravity.

If you want to talk about objects that are massive enough to have non-negligible gravity, passing by each other at relativistic speeds, you should open another thread; that's getting way off topic for this one.
 
  • #27
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No, you got me wrong. I would say, the moving astronaut will gain less spin than the one that is at rest. So the astronaut that faces the other astronaut first, will know he is the one that is slower. Even if it takes 3 billion years, in the theoretical universe that is the OP.

I even got curious how you did not question OP about how these two astronauts can see each other in the void with no light?
 
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  • #28
PeterDonis
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I would say, the moving astronaut will gain less spin than the one that is at rest.
If neither astronaut is exerting any gravity on the other, why would either one's spin change at all?

I even got curious how you did not question OP about how these two astronauts can see each other in the void with no light?
In thought experiments like this, we assume that there is a reasonable way for anything that is postulated to happen, to happen, as long as it is not inconsistent with the laws of physics. It's perfectly possible for the astronauts to see each other with light beams of low enough intensity that the beams have negligible energy and momentum compared to the astronauts. Since the exact method of how they see each other is irrelevant to the OP's question anyway, I just assumed the above was what he meant.
 
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  • #29
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What is negligible gravity? Our very existence is based on two specs of dust pulling each other and clumping up you know...

Exerting no gravity? These astronauts have no mass? Because every mass has its own gravitational field.

OP asked "Can any of you prove which one is moving?", i don't remember the OP talking about the masses of the astronauts, or neglecting any force in the process of answering this question.
 
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  • #30
PeterDonis
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What is negligible gravity?
"Negligible" means "small enough to be neglected at whatever level of accuracy we are dealing with".

Our very existence is based on two specs of dust pulling each other and clumping up you know...
No, it's based on a huge cloud of dust collapsing and forming clumps within it. If it were just two specks of dust to start with, the solar system would never have been formed.

every mass has its own gravitational field.
In principle, yes. In practice, lots of objects have negligible gravitational fields--we need extremely accurate measurements to detect them, and for practical purposes we can ignore them.

OP asked "Can any of you prove which one is moving?", i don't remember the OP talking about the masses of the astronauts, or neglecting any force in the process of answering this question.
The OP is welcome to clarify if he intended to take into account the gravity of the two objects themselves. The usual interpretation of the question the OP asked is that it's a question purely about the relativity of motion, in the idealized case where gravity is neglected.
 
  • #31
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I was was not including the interference of gravity, but I like the discussion. I was more leaning towards the frame of reference of each body of mass to the universe.
 
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  • #32
PeterDonis
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I was more leaning towards the frame of reference of each body of mass to the universe.
I think the point of the previous discussion is that there is no such thing in any absolute sense. We can construct reasonable definitions of what it means to be "moving with respect to the universe" (for example, vanhees71's suggestion that "moving" means "not seeing the CMBR as isotropic"), but no one of them can be singled out as "the" definition of what it means to be "moving". It all depends on your choice of coordinates and what problem you are trying to solve.
 
  • #33
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I was was not including the interference of gravity, but I like the discussion. I was more leaning towards the frame of reference of each body of mass to the universe.
Then in this context, the answers you got from previous posters are correct. My thought process only applies to your experiment where i take into account the gravity of the masses.

I was never against what PeterDonis was suggesting, he is writing in the context of reality. Which i agree is in the spirit of the forum. I just found the idea that i suggested interesting to think about.
 
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