# Principle of relativity for proper accelerating frame of reference

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• cianfa72
No, what I'm saying is that, under the same circumstances, the laws of physics in the two frames will be the same.No, what I'm saying is that, under the same circumstances, the laws of physics in the two frames will be the same.

#### cianfa72

TL;DR Summary
Application of principle of relativity for proper accelerating frames of reference in the context of classic Newton theory and SR
Hi,

I've a doubt about the application of the principle of relativity as follows.

Assume as principle of relativity the following statement: It is impossible by any experiment performed inside a "closed" laboratory to say whether we are moving at constant velocity or staying at rest.

Consider the following scenario in the context of Newton classic theory: the analysis of a physical process in two different reference frame sharing the same proper acceleration (i.e. bodies at rest in each of the two frames respectively have the same proper acceleration as measured by an accelerometer attached to them).

The principle of relativity says the equations describing the given physical process in the two reference have to be the same (of course we need to take in account the pseudo forces appearing to act on masses due to non-inertial accelerating reference frames used).

What about in the context of SR ? Does the principle of relativity has to say something only for inertial frames of reference having constant relative velocity (i.e. only for frames in which bodies at rest have got zero proper acceleration) ?

Thanks.

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• Delta2
The principle of relativity is unchanged between SR and Newtonian mechanics.

• valenumr and vanhees71
Dale said:
The principle of relativity is unchanged between SR and Newtonian mechanics.
ok, so even in SR do exist accelerated frames of reference (same proper acceleration for bodies at rest in each of them as measured by accelerometers) having constant relative velocity ?

In those frames the laws of physics should stay unchanged. Is that the case ?

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The idea that accelerating frame is like a non-accelerated frame with the addition of "fictitious forces" works for Newtonian physics, but it needs to be modified for special relativity. Effects happen in accelrating frames that don't fit into this mold. The most common and important example is "gravitational time dilation". As an example of what I mean by this, the bow and stern of a rigidly accelerating spaceship have different proper accelerations, and clocks ticking at the bow and stern of the spaceship do not remain synchronized.

Thomas precession is another example of a SR effect that occurs in accelerating frames that doesn't occur in Newtonian physics. THomas precession however doesn't affect stationary gyroscopes in an accelerated frame, only moving ones.

This does not contradict Dale's remarks, because the notion that the only difference between accelerated and unaccelerated frames is the addition of fictitious forces is not really a good statement of the principle of relativity.

• LBoy, Delta2, vanhees71 and 1 other person
pervect said:
As an example of what I mean by this, the bow and stern of a rigidly accelerating spaceship have different proper accelerations, and clocks ticking at the bow and stern of the spaceship do not remain synchronized.
ok, focusing on this example: if we take two rigidly accelerating spaceships (with the same profile of proper acceleration along each of them) can we apply the principle of relativity to them ?

cianfa72 said:
ok, focusing on this example: if we take two rigidly accelerating spaceships (with the same profile of proper acceleration along each of them) can we apply the principle of relativity to them ?
Yes. Although, some statements of the principle of relativity are better than others.

cianfa72 said:
ok, focusing on this example: if we take two rigidly accelerating spaceships (with the same profile of proper acceleration along each of them) can we apply the principle of relativity to them ?
Why do you need two spaceships?

PeroK said:
Why do you need two spaceships?
My point is the following: Galileo principle of relativity applies not only to inertial frames but even to not-inertial constant (proper) accelerated frames having constant relative velocity (it definitely makes sense in the context of Newtonian mechanics).

Then what about in the context of SR ? I was trying to single out two (proper) accelerated frames (spaceships) having constant relative velocity to ask if we can continue to apply the principle of relativity even to them.

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I don't think this has anything to do with the relativity principle. You are saying that if you have two reference frames that have the same proper acceleration then the physics in them will be the same. Well, of course, under the same circumstances the same thing will happen, what else!

• Grasshopper
Dale said:
Yes. Although, some statements of the principle of relativity are better than others.
That does mean the two frames (rigidly accelerating spaceships with same profile of proper acceleration along them) have constant relative velocity ?

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martinbn said:
I don't think this has anything to do with the relativity principle. You are saying that if you have two reference frames that have the same proper acceleration then the physics in them will be the same. Well, of course, under the same circumstances the same thing will happen, what else!
Why ? It is in force of the principle of relativity that we can say that.

cianfa72 said:
Why ? It is in force of the principle of relativity that we can say that.
Why? You have to spaceships with the same acceleration. The people inside perform experiments. Why do you need the principle to say that they will have the same results?

martinbn said:
Why? You have to spaceships with the same acceleration. The people inside perform experiments. Why do you need the principle to say that they will have the same results?
Sorry...but if not involved in this or similar cases, which is the content/purpose of the principle or relativity ?

That's more or less the same scenario (with no proper acceleration) of the Galileo description of experiments performed in two different 'closed' cars having constant relative velocity.

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Suppose a ball sitting on a frictionless table is held in place in an accelerating rocket ship. If that ball is released by the holder so that the frictionless table is now the only object interacting with the ball, does the ball remain in place in that ship?

If not, measure the position after 1 sec.

Suppose another accelerated ship performs the same experiment, does the ship obtain the same displacement?

Do the accelerations matter?
By saying “same velocity”, in what frame?

robphy said:
Suppose a ball sitting on a frictionless table is held in place in an accelerating rocket ship. If that ball is released by the holder so that the frictionless table is now the only object interacting with the ball, does the ball remain in place in that ship?
No, I would say no. Nevertheless I didn't get your point.

I updated my post

cianfa72 said:
That does mean the two frames (rigidly accelerating spaceships with same profile of proper acceleration along them) have constant relative velocity ?
I don't think that is implied. What is implied is the converse: if they have constant relative velocity then they will have the same profile of proper acceleration. However, as I said before, some formulations of the principle of relativity are better than others. This one is a little poor when applied to accelerating reference frames because the concept of constant relative velocity becomes more difficult to define. It can still be made to work, but it is not straightforward.

• vanhees71
Dale said:
I don't think that is implied. What is implied is the converse: if they have constant relative velocity then they will have the same profile of proper acceleration. However, as I said before, some formulations of the principle of relativity are better than others. This one is a little poor when applied to accelerating reference frames because the concept of constant relative velocity becomes more difficult to define. It can still be made to work, but it is not straightforward.
So, which could be a better formulation of the principle of relativity in this case ?

For special relativity I prefer: the laws of physics are the same in all inertial reference frames. And for general relativity I prefer: the laws of physics are the same in all reference frames.

Another formulation that I like is an explicit statement of the symmetries: (SR) the laws of physics are unchanged for rotations, translations, and boosts. (GR) the laws of physics are unchanged for any change of coordinates. The last means that the laws can be written as tensors, which in some sense is an almost trivial statement but was not obvious for many years so I think it is worth stating explicitly.

• LBoy and vanhees71
This is a somewhat dangerous formulation of the meaning of "general covariance" for GR. You can in fact make any theory generally covariant. Also in SR you work with tensor (and sometimes also various kinds of spinor) fields and you can use any non-Lorentzian spacetime coordinates you like (it's as in Euclidean affine space, where you can use any "curvilinear coordinates" you like; you are not restricted to Cartesian coordinates).

The physical symmetry principles are global Poincare invariance for special relativity with the proper orthochronous Poincare group as the spacetime symmetry and making this global Poincare invariance local to formulate GR. The physical principle of relativity is then the statement that there's the class of global inertial reference frames in SR and for GR there's at any point in spacetime a class of (local) inertial frames. Note that using this point of view you get standard GR with a pseudo-Riemannian (Lorentzian) manifold as the spacetime model as long as you restrict yourself to the macroscopic classical physics (i.e., (continuum) mechanics + the em. field), while you get Einstein-Cartan theory, i.e., a differentiable pseudo-metrical Lorentzian manifold with torsion, when including also spinor fields.

From this point of view the general covariance of GR (or Einstein-Cartan theory) is rather a gauge symmetry, i.e., a local symmetry. It is rather an redundancy in the description of the physical observables, which are necessarily gauge invariant to make physical sense.

• Dale
Dale said:
For special relativity I prefer: the laws of physics are the same in all inertial reference frames. And for general relativity I prefer: the laws of physics are the same in all reference frames.
Surely, this way we're basically limit the scope of the principle of relativity only to inertial frames (actually excluding proper accelerated reference frames).

Dale said:
For special relativity I prefer: the laws of physics are the same in all inertial reference frames. And for general relativity I prefer: the laws of physics are the same in all reference frames.

Another formulation that I like is an explicit statement of the symmetries: (SR) the laws of physics are unchanged for rotations, translations, and boosts.
Are the two spaceship's rest frame of reference with the same profile of proper acceleration related by a boost ?

cianfa72 said:
Surely, this way we're basically limit the scope of the principle of relativity only to inertial frames (actually excluding proper accelerated reference frames).
Yes, that is correct and intended. SR in this way can still handle non-inertial frames, but the application of the principle of relativity is deliberately restricted. This avoids the difficulties I mentioned above with your approach.

cianfa72 said:
Are the two spaceship's rest frame of reference with the same profile of proper acceleration related by a boost?
If two spaceships are related by a boost and one is undergoing proper acceleration then the other will undergo the same proper acceleration.

• vanhees71
vanhees71 said:
You can in fact make any theory generally covariant.
Yes, this is specifically what I was referencing by "almost trivial", but still worth stating.

• vanhees71
Dale said:
If two spaceships are related by a boost and one is undergoing proper acceleration then the other will undergo the same proper acceleration.
ok, but as you said that does not imply they have a constant relative velocity, though. Nevertheless we can continue to apply the principle of relativity in terms of symmetries formulation (boost in this case).

Hence, since related by a boost, the laws of physics are the same in both the spaceships undergoing the same profile of proper acceleration (basically the equations of the physics laws are the same in the two reference frame).

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Dale said:
This one is a little poor when applied to accelerating reference frames because the concept of constant relative velocity becomes more difficult to define. It can still be made to work, but it is not straightforward.
Do you mean employing the concept of MCIRF (Momentarily Comoving Inertial Reference Frame) ?

cianfa72 said:
ok, but as you said that does not imply they have a constant relative velocity, though. Nevertheless we can continue to apply the principle of relativity in terms of symmetries formulation (boost in this case).

Hence, since related by a boost, the laws of physics are the same in both the spaceships undergoing the same profile of proper acceleration (basically the equations of the physics laws are the same in the two reference frame).
Yes, it's indeed a bit mind-boggling if it comes to non-inertial frames in relativistic physics. The most simple example coming to mind is Bell's spaceship paradox, which nevertheless illustrates well the obstacles with accelerated (in this case even non-rotating) reference frames in SR. As a side effect it also illustrates the pretty strange properties of Born-rigid bodies. Rumor has it that Bell kept his colleagues at CERN busy for some time discussing his "spaceship paradox"...

For an elementary treatment, see

https://itp.uni-frankfurt.de/~hees/pf-faq/srt.pdf

• Dale
cianfa72 said:
That does mean the two frames (rigidly accelerating spaceships with same profile of proper acceleration along them) have constant relative velocity ?
https://www.geogebra.org/m/ETgBGFSX
(My Bell spaceship/rocket visualization.)

In my earlier questions above,
I had this visualization in mind.

• vanhees71
robphy said:
https://www.geogebra.org/m/ETgBGFSX
(My Bell spaceship/rocket visualization.)
ok, so even if the two spaceships have the same (fixed) proper acceleration and the same (increasing) velocities w.r.t LAB inertial frame, they do not maintain a constant relative velocity between them, however.

Nevertheless, since the two spaceships reference frames are related by a Lorentz boost, we can apply the principle of relativity in terms of symmetries formulation to get the result that physical laws stay unchanged in both the frames.

Make sense ? Thank you.

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cianfa72 said:
ok, so even if the two spaceships have the same (fixed) proper acceleration and the same (increasing) velocities w.r.t LAB inertial frame, they do not maintain a constant relative velocity between them, however.

Nevertheless, since the two spaceships reference frames are related by a Lorentz boost, we can apply the principle of relativity in terms of symmetries formulation to get the result that physical laws stay unchanged in both the frames.

Make sense ? Thank you.
I don't think the statement is specific enough.

There are certainly laws of physics, independent of any observer.
The principle of relativity suggests that the equations take the same form among
inertial observers.

Are you trying to extend this to arbitrary observers?
I would think the principle fails in relating a noninertial uniformly-accelerating observer and an inertial one.

Or are you trying to extend this to equal-proper-acceleration observers?
If their worldlines are distinct, no boost transformation will map one worldline onto the other.
A translation is needed.

Maybe the analysis of a simple thought-experiment would be more specific.

robphy said:
There are certainly laws of physics, independent of any observer.
The principle of relativity suggests that the equations take the same form among
inertial observers.

Are you trying to extend this to arbitrary observers?
I would think the principle fails in relating a noninertial uniformly-accelerating observer and an inertial one.
My point is quite straightforward.

Galilean principle of relativity makes no assumption about the state of motion of frames involved. Its statement involves just the constant relative velocity between the two frames (that's fine in Newtonian mechanics). Then for frames having constant relative velocity it follows that if one frame is inertial then even the other one is inertial too.

In the context of SR instead, as @Dale pointed out in post #22, the principle of relativity is actually deliberately 'restricted' to inertial frames only.

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cianfa72 said:
In the context of SR instead, as @Dale pointed out in post #22, the principle of relativity is actually deliberately 'restricted' to inertial frames only.
And as I pointed out in post 2 the principle of relativity is the same in SR and Newtonian physics.

• valenumr, cianfa72 and vanhees71
cianfa72 said:
ok, so even if the two spaceships have the same (fixed) proper acceleration and the same (increasing) velocities w.r.t LAB inertial frame, they do not maintain a constant relative velocity between them, however.

Nevertheless, since the two spaceships reference frames are related by a Lorentz boost, we can apply the principle of relativity in terms of symmetries formulation to get the result that physical laws stay unchanged in both the frames.

Make sense ? Thank you.
Sorry, is my understanding right ?

cianfa72 said:
Sorry, is my understanding right ?
Since your understanding appears to be that there is a difference between SR and Newtonian mechanics regarding the principle of relativity then I would say, no, your understanding is not right.

Dale said:
Since your understanding appears to be that there is a difference between SR and Newtonian mechanics regarding the principle of relativity then I would say, no, your understanding is not right.
Sorry, not sure to understand We said that the content of the principle of relativity is the same both in Newtonian mechanics and in SR, however in SR its applicability is deliberately 'restricted' only between inertial frames, right ?

In the case of spaceships undergoing the same proper acceleration, are their rest reference frames related by a Lorentz boost ? In that case the principle of relativity in terms of symmetries formulation should be applicable, dont' you ?

The special (sic!) principle of relativity states that there are global inertial frames, where Newton's 1st Law holds. Together with the assumptions that for inertial observers time is homogeneous and space is Euclidean as well as that the transformations between inertial frames, connected continuously to the identity, and the spacetime manifolds admits a causality structure, you get only two kinds of spacetime, the Galileo-Newtonian one (a fiber bundle), or Minkowski space (a pseudo-Euclidean affine space with a fundamental form of signature (1,3)) underlying special relativity.

• dextercioby