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I am not a physicist :)

I (think I) understand the idea that motion can be relative in the sense that, I'm standing still inside a moving train in regards to the moving train itself, but I'm moving in regards to an external observer who is standing still in regards to the earth but who is moving in regards to someone floating in space looking at the earth spinning and rotating around the sun etc..etc..

I also (think I) understand the thought experiment by which we would not be able to determine whether a ball that is floating in space by itself, with no reference frames around it, is moving or is standing still.

But I'd like to ask the following:

  1. Does relativity only suggest that what we observers determine as movement or stillness, when observing other objects, is relative (and I agree with that)? Or does it also suggest that objects in themselves, regardless of what experience/observation we might have of them cannot be said to move or stand still in an absolute fashion (which I do not agree with)?
  2. Why are we determining the movement of an object by relying on reference frames which are indeed relative by nature, instead of simply determining movement by the object itself, via the energy/momentum that it holds (or doesn't hold) which cannot be relative in itself.

In other words, we are not able to determine whether a ball floating in space is moving or not by relying on spatial reference frames if there aren't any available, but that doesn't change the fact that the ball itself will contain some kinetic energy/momentum if it's moving, or none if it's not, right? And yes, we might not be able to measure this either and determine whether the object is moving or not, but that doesn't mean that the object won't contain that energy. In other words, if physicists are not able to observe/determine absolute "states" that doesn't mean that those states do not exist by themselves regardless of what experience we can have of them right ?

To clarify, I understand that even measuring the kinetic energy of an object can be a "relative" process but that that doesn't mean that the existence/fact of the energy itself in itself "contained" in the object is relative as well, right?

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  • $\begingroup$ Kinetic energy and momentum are always relative to some frame, they are not intrinsic. $\endgroup$ – PM 2Ring Feb 24 at 9:56
  • $\begingroup$ If the energy is not an intrinsic property of the object then why does the glass always shatter regardless of whether it's the glass moving and crashing into a wall, or whether it's the wall crashing into the glass? The same event (wall and glass colliding) can be analyzed & defined from a 1000 different reference frames, I understand that. But the glass will always shatter, regardless of reference frames. If the energy is not an intrinsic, absolute property, then is there a reference frame by which for the same singular spatiotemporal event the glass won't shatter and stay intact? $\endgroup$ – Sprout Coder Feb 24 at 10:12
  • $\begingroup$ Maybe I don't quite get your question, but in that example the glass has a certain speed $v$ relative to the wall, and observers in any frame wll calculate the same $v$ no matter what speeds they measure for the glass & wall separately. $\endgroup$ – PM 2Ring Feb 24 at 10:18
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    $\begingroup$ I see what you mean in the Q and that you are aware of the comment by @PM 2Ring. I might feel along your line. But in order to shatter the glass the system must contain energy, there won't be a reference frame in which both the wall and the glass have both zero kinetic energy but the glass shatters. Glass is just the fable part, but energy must be there in one or the other or both. If you choose the wall (or the glass) again you refer to something that can't be absolute. The only thing I see has absolute is some proper motion different from hubble flow. But I got reprimands in others Q/A. $\endgroup$ – Alchimista Feb 24 at 10:21
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    $\begingroup$ Ok I understand. I kept thinking that I need to assign the energy to one of the two objects, but it's the system that "contains" the energy. $\endgroup$ – Sprout Coder Feb 24 at 10:43

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