# Movement in a Vacuum

• B
• Atari_Me

#### Atari_Me

A very basic level question.

Two objects are floating in a vaccum, Object A and Object B. Then distance begins to grow between the two objects. Object A feels acceleration being exerted, Object B does not.

What does this show?

From a classical perspective, it would be my understanding that this would show that Object A is in motion and that Object B is not. Do you agree?

From a Relativity perspective, it would show that Object A is moving away from Object B at the same velocity as Object B is moving away from Object A and that the acceration being exerted upon Object A does not show anything other than Object A is being accelerated. Would this be an accurate interpretation?

Considering that both objects are in a vacuum and there is no independent third perspective, does the fact that one object is feeling acceleration and the other is not, show anything?

Thx

## Answers and Replies

Even in classical physics, there is no notion of a preferred universal rest frame. One cannot distinguish between a situation where both objects were initially moving and A slowed to a stop and a situation where both objects were initially at rest and A began accelerating away. Both descriptions of the situation are equally valid.

Atari_Me
From a classical perspective, it would be my understanding that this would show that Object A is in motion and that Object B is not. Do you agree?
Definitely not. This is inconsistent with the principle of relativity (either the Galilean or Einstein’s version).

Didn’t we already discuss this?

Considering that both objects are in a vacuum and there is no independent third perspective, does the fact that one object is feeling acceleration and the other is not, show anything?
It shows which object is inertial and which is non inertial. This is a different concept from claiming that one or the other is at rest.

berkeman
Didn’t we already discuss this?
Yup, in this previous thread which was locked...
Newtonian Physics states in part that 'every object has an absolute state of motion relative to absolute space, so that an object must be either in a state of absolute rest, or moving at some absolute speed.'

Granting that if absolute space were to exist, we have no means to measure it at present and therefore irrelevant, is there anything in the above aspect of Newtonian Physics that is incompatible with Relativity?

@berkeman, @Dale, obviously prior comments on the subject did not provide a clear answer and this is a different aspect regardless. The level of condescension in this forum is astounding. I expect more of physics students and professors, but the reality is that they behave just like every one else on every other forum, with reckless disregard for the impact of their statements. This is not a classroom, there is no heirarchy, and clearly there is a disfavor in discussing the question of what physics is beyond the math that governs it. Which, I find astounding considering that without the theoretical side of physics, none of the math would exist.

Under various versions of classical physics, there is a concept of absolute space where an object's position can always be determined, therefore one object can be shown to be moving where the other is not. Relativity changed all that. So I am trying to gain a better understanding of what modern concepts of physics say is happening in the hypothetical provided. Meaning, which object is moving given the facts presented.

My understanding of modern physics would be that the answer is both a) it doesn't matter, and b) either/both depending on perspective and that acceleration by itself is not indicative of showing an object is in motion. I am looking for confirmation that this is a correct/incorrect interpretation of how Relativity/modern physics addresses the situation.

...how Relativity/modern physics addresses the situation.
1) One object is experiencing proper acceleration.
2) The two objects are moving relative to one another.

Dale
I expect more of physics students and professors, but the reality is that they behave just like every one else on every other forum,
Wait, so you've had issues in other science discussion forums that you've participated in?

Under various versions of classical physics, there is a concept of absolute space where an object's position can always be determined,
This is not true now and was never true, not even in Newton’s conception. In his conception there was an absolute space, but an object’s position could never be determined relative to this absolute space. That is why it was discarded and the Galilean principle of relativity was accepted.

I am looking for confirmation that this is a correct/incorrect interpretation of how Relativity/modern physics addresses the situation.
This is correct, but this concept long predates Einstein. It was already well accepted by his time, which is one reason that his first postulate was immediately accepted by the community.

clearly there is a disfavor in discussing the question of what physics is beyond the math that governs it
I don’t think this is true at all. The experiments are equally or even more important.

For my part, my disfavor is with having to repeat myself. I was as clear as I could be in our previous thread that classical physics does not have a concept of absolute space, and yet here you are again claiming not only that it is a feature of classical physics, but that it is detectable. I put quite a bit of effort into writing that statement well and clearly, and I disfavor wasted effort. So what you are sensing from me personally is not condescension but frustration.

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anorlunda
Relativity changed all that.
That is Galilean relativity, 400 years ago:
https://en.wikipedia.org/wiki/Galilean_invariance

https://en.wikipedia.org/wiki/Absolute_space_and_time
https://en.wikipedia.org/wiki/Absolute_space_and_time said:
Even within the context of Newtonian mechanics, the modern view is that absolute space is unnecessary. Instead, the notion of inertial frame of reference has taken precedence, that is, a preferred set of frames of reference that move uniformly with respect to one another. The laws of physics transform from one inertial frame to another according to Galilean relativity, leading to the following objections to absolute space, as outlined by Milutin Blagojević:[10]

• The existence of absolute space contradicts the internal logic of classical mechanics since, according to Galilean principle of relativity, none of the inertial frames can be singled out.
• Absolute space does not explain inertial forces since they are related to acceleration with respect to anyone of the inertial frames.
• Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon.
Newton himself recognized the role of inertial frames.[11]

The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.​
So basically even Newton knew that uniform motion relative to his absolute space didn't have any consequences and wasn't detectable.

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Dale
A very basic level question.

Two objects are floating in a vaccum, Object A and Object B. Then distance begins to grow between the two objects. Object A feels acceleration being exerted, Object B does not.

What does this show?

From a classical perspective, it would be my understanding that this would show that Object A is in motion and that Object B is not. Do you agree?
It only shows that there is a force on object A that is not on object B. Motion is only relative as others have adequately addressed.

It appears to me that a large part of the problem is that the term "absolute space" has not been fully defined. If "absolute space" means to you that there existed (at some point) in classical physics a preferred inertial frame, then the answer is no, there was not, unless you go back in time before Galileo. (This assumes that you are talking about kinematic motion of bodies, and not light propagation in a preferred "ether" frame. Speculation about a preferred ether frame appeared in the later 1800s.) If however you mean that if an observer in inertial frame A would measure the same separation and acceleration of two objects in that frame as an observer in the inertial frame B, then yes, classical physics assumes there is an "absolute space" in which the two observers would obtain the same result. In other words, the measured separation between objects in space do not depend on the motion of the observer. There is also a corresponding absolute time in classical physics. Note that classical kinematic physics was developed before the speed of light could be really measured or have an effect upon experiments of the time.

Einstein's contribution was stating that the laws of physics should be the same in all inertial frames, including the measured speed of light, and not just the kinematic laws of motion. This was driven in part to be consistent with the outcome of the earlier Michelson–Morley experiment. The result was the theory of special relativity.

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Orodruin
Einstein's contribution was stating that the laws of physics should be the same in all inertial frames, including the measured speed of light,
What Einsteins Relativity actually did is to change the speed that remains invariant under coordinate transformations, from ∞ (in Galilean Transformation) to c (in Lorentz Transformation). The rule how velocities transform, is something that connects different inertial frames. So it doesn't make sense to say that it applies in an inertial frame (like the kinematic Laws). In both versions of Relativity the rule how velocities transform is consistent across all inertial frame pairs.

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