I Is acceleration absolute or relative - revisited

[Dale]

Any right thinking person would say "Yes, it's valid to consider the entire universe as a reference frame." In which case, at best your original statement "The actual universe is not a reference frame." was unintentionally misleading. Since you were making an equivocal statement, it would have been more instructive to say "The actual universe is not a reference frame in and of itself. It can however be validly assigned as one."

I am a right thinking person and I would be very hesitant to say that. It could be made correct in certain spacetimes, but not for example in the FLRW spacetime that we believe best represents our actual universe.

 

[Doc Al]

Any right thinking person would say "Yes, it's valid to consider the entire universe as a reference frame."

With statements like this, you might want to rethink your attitude.

 

[Peter Leeves]

Physicists do not say our actual universe is a reference frame. They say that we use reference frames to describe the universe (or whatever part of it we are interested in).

I refer you back to post #183. You quote me saying, "if the entire universe reference frame sat inside some larger reference frame (to which it was causally connected at speed of light c in a vacuum)".

It was this that prompted your response, "This doesn't make sense. Reference frames are human abstractions, not real things. Reference frames can't be causally connected to anything; they aren't real things. The actual universe is not a reference frame."

I've never believed the actual universe to be a reference frame, any more than I believe a map is an actual city. I have only ever believed the actual universe may be assigned as a reference frame. But my description was insufficiently clear for you "This doesn't make sense." which leads you to continue further and end your post with the statement I found misleading "The actual universe is not a reference frame."

In the spirit of "good physicist speak", I'll attempt to clarify my idea which didn't make sense to you. Replace my original statement, "The only way this could be wrong is if the entire universe reference frame sat inside some larger reference frame (to which it was causally connected at speed of light c in a vacuum) and was proper rotating with respect to it." with the following items 1 through 4:

1) Assign the entire visible universe as a reference frame (while not considering the actual universe to be considered in any way a reference frame in and of itself).

2) Assign some larger body, theoretically beyond the bounds of the visible universe, as a larger reference frame (while not considering it's actual contents to be considered in any way a reference frame in and of itself).

3) Consider that the mass content of the actual universe is causally connected (at speed of light c in a vacuum) to the mass content of the larger body.

4) Then it follows, the only way this could be wrong is if the entire visible universe reference frame sat inside the larger body reference frame and was proper rotating with respect to it.

I trust this clarifies my original intent.

This attitude is bringing you very close to getting a warning. We are doing our best to help you improve your understanding. Remarks like this are uncalled for.

I'm sure everyone can agree that an interest in, and love of, physics should be the only reason anyone is in here. That's the only reason I'm here.

 

[Doc Al]

This is a good point to close this thread.

 

[PeterDonis]

Moderator's note: Discussion regarding rotation in the Godel universe has been moved to a separate thread:

https://www.physicsforums.com/threads/rotation-in-godel-spacetime.999543/

 

[bhobba]

Physics is contained in invariants.

Exactly. SR, like much of physics, is in fact a theory about symmetry. The language of symmetry is group theory, in which the important things are the invariants. If anyone has not seen the connection here is the link I often give:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

The assumption is inertial frames (frames that have the almost obvious from everyday experience properties of the laws of physics are the same regardless of where you are, what direction you are, or what time it is) at least conceptually exist. The POR is the law that leads to SR (OK, strictly speaking it is a meta law ie a law about laws). It says the laws of physics are the same in all inertial frames or frames moving at constant velocity relative to an inertial frame. But do they actually exist? Well it is thought deep in interstellar space they do to a very high degree of accuracy. But strictly speaking they do not. It is like a point is supposed to have no size - only position. Such does not actually exist, but to a good degree of accuracy they do. Such conceptualisations are very useful in developing models. SR is a model about inertial frames. Acceleration is always assumed to be relative to some inertial frame. It is a conceptualisation used in modelling it. Then what is GR? That is not easily answered, but one (rough) way of looking at it is the idea that locally you can always find some space-time coordinate system that is inertial (suggested by things like Einstein's freely falling elevator thought experiment). This implies space-time may be curved and described by what is called a 4 dimensional Pseudo-Riemannian Geometry, the mathematics of which is well known. We then have a very powerful theorem, called Lovelock's Theorem, that more or less implies GR. You can find the detail in Lovelock and Rund:
https://www.amazon.com/dp/0486658406/?tag=pfamazon01-20

Let's take the famous bucket experiment. It is rotating relative to some local inertial frame, which from GR we know always exists, and that is what leads to the behaviour of the water. We could also say, as I have heard some physicists say, it is rotating relative to the local metric tensor, but that is obscure until you understand the detail of GR. We do not need to delve in Mach etc to see what is going on.

Thanks
Bill