I Is acceleration absolute or relative - revisited

[Peter Leeves]

But you apparently do not agree that in the steady state, the bucket and the water are at rest relative to each other?

Why say that ? To try and make me "wrong" yet again ? Of course in the steady state the bucket and water are at rest relative to each. That defines "steady state". I certainly didn't realize you'd assume every statement I didn't quote would be held against me, like - he didn't answer that bit so therefore I'll simply assume he didn't agree. Once again, I'd prefer to stick to physics rather than semantics.

 

[Peter Leeves]

The traditional Newton's bucket is at rest relative to the water. I.e. the spin-up or spin-down happened a long time ago, viscous forces have caused all of the relative motion between the water and the bucket and between different parts of the water to dissipate.

"When friction between the water and the sides of the bucket has the two spinning together with no relative motion between them then the water is concave."
https://mathshistory.st-andrews.ac.uk/HistTopics/Newton_bucket/

Edit: never mind, I see that you just discussed that for completeness

I understand every word of that.

If you don't wish to work towards accuracy and completeness (especially when it costs nothing), that's absolutely fine. The issue I tripped up over is now included MANY times (many more than necessary now) so I'm all good.

 

[Dale]

especially when it costs nothing

Actually including the spin-up or spin-down would cost an enormous amount. Fluid dynamics are messy, especially when you have to include viscosity. But I see that you are not actually including them, just mentioning them in passing. That is fine. I don't think that it adds anything substantive to the discussion precisely because we are not actually analyzing those periods, but it does no harm either to just mention them.

 

[Peter Leeves]

Actually including the spin-up or spin-down would cost an enormous amount.

Wrong context. Didn't mean "cost anything" with respect to fluid mechanics (something I'm more familiar with by profession). I meant it doesn't "cost anything" but our toil and the sweat of our brows to strive for completeness and accuracy

Here's the context. I said "If you don't wish to work towards accuracy and completeness (especially when it costs nothing), that's absolutely fine." On reflection I'm sure you'll agree there's nothing in there hinting at fluid mechanics. No need to reply.

 

[PeterDonis]

Why say that ?

Because from your previous post that's what it seemed like you were saying. Now you have clarified that it wasn't, which is good.

You need to seriously take a step back and consider the possibility that the way you are phrasing things might be misleading.

 

[Peter Leeves]

Because from your previous post that's what it seemed like you were saying.

"Seemed" ?

Physics please. The physics in this thread is absolutely fascinating.

 

[Peter Leeves]

You need to seriously take a step back and consider the possibility that the way you are phrasing things might be misleading.

Purposely misconstrued is also a possibility. But please, can we stick to physics ?

 

[PeterDonis]

Purposely misconstrued is also a possibility.

No, it isn't. I am not purposely trying to misunderstand what you mean. I am honestly having difficulty understanding what you mean. You really should stop assuming that that is not possible.

 

[PeterDonis]

can we stick to physics ?

The things you are saying that I am having trouble understanding are about the physics. So I have to clarify them if we are going to talk about the physics.

 

[Peter Leeves]

I confess although I've heard of the Coriolis force/effect, it's been a long time so I looked it up:

"In physics, the Coriolis force is an inertial or fictitious force that acts on objects that are in motion within a frame of reference that rotates with respect to an inertial frame."

This does sound exactly as I described earlier this evening, "We agreed just before post #87 all matter in the visible universe communicates gravitationally with all other matter at the speed of light c in a vacuum, and this must have some influence on the water. In a proper stationary universe [edit for clarity - containing a proper stationary bucket], this would likely have little (or no) net influence on the water, especially if you choose to assume matter is more or less evenly distributed. Perhaps just a tiny, quite possibly imperceptible, bulge in all directions would be my guess.

It then occurred that if the entire visible universe was proper rotationally accelerated, the effect on the [edit for clarity - proper stationary] water [further edit for clarity - but proper rotating WITH RESPECT TO the now proper rotating universe] might be far from net zero. Now the entire universe's mass would exert a circumferential torque on the water."

For my description to equate to a coriolis effect, the water would be the object in motion within a frame of reference that rotates WITH RESPECT TO an inertial frame (the entire universe). Here's the killer chaps ... WITH RESPECT TO the entire universe's reference frame (which is proper rotating), the water reference frame IS proper rotating.

Furthermore, the observation (water bulging) MUST be identical in both scenarios for the reason I also gave. The proper acceleration is invariant in one scenario so it MUST remain in the other scenario. Literally all you're doing is choosing whether to view from the water reference frame or the entire universe reference frame.

It's actually similar (in fact equivalent) to the coordinate solution you provided earlier. The proper acceleration is invarient so remained identical. All we had to do was choose the water as the stationary reference frame. Nothing more, and nothing less.

However, the mechanism to make to the two scenarios equivalent is different in each case, centripetal in one (proper stationary universe with proper rotating water), coriollis in the other (proper rotating water WITH RESPECT TO a proper rotating universe). Just like Einstein's thought experiment after all.

This would also verify both scenarios take place in a single spacetime geometry. I did say that earlier, but was informed in no uncertain terms I was wrong and confusing reference frames with spacetime geometries. That I could only be proposing two spacetime geometries. That was incorrect.

I appreciate there are a lot of statements above. I am not qualified to know if those statements are correct or not. I am just asking if this is the conclusion currently being considered ?

 

[PeterDonis]

I looked it up

Where? Please give a reference.

the Coriolis force is an inertial or fictitious force

Note that this means it is not a "proper" force--it is an artifact of choosing a particular reference frame, and you can make it vanish by choosing a different frame. It is not an invariant.

That means it can't be the same as anything that happens because of "proper rotation", because "proper rotation" is an invariant.

I am just asking if this is the conclusion currently being considered ?

No, for the reason given above. Physics is contained in invariants. If you go back and read my post #149, you will see that I described four different scenarios, corresponding to the four possible ways of combining the two binary variables "bucket/water not rotating vs. rotating" and "the rest of the matter in the universe not rotating vs. rotating". I described those scenarios entirely in terms of invariants; you won't see "Coriolis force' or "centrifugal force" or any other frame-dependent thing anywhere in my descriptions. I strongly suggest that you read those descriptions carefully and think carefully about what they are telling you.

(Btw, for clarity, what you and @Dale have been calling "proper rotation" is the same as what I called "nonzero vorticity" in that post.)

 

[vanhees71]

How would it imply that? In science the “correct” quantity is the one that matches experiment.

Well, yes, and the empirical evidence is that the universe is not rotating.

 

[anuttarasammyak]

As for the words "Is acceleration absolute ?”, say the referring (local) system or frame of reference does not satisfy Newton's first law, i.e. free objects change speed with time, the FR is not a (local ) IFR. In the sense that everybody in any FR agrees with this observation, we may say it is not a (local) IFR absolutely.

 

[anuttarasammyak]

Stars are moving against each other with proper speed. We may well consider their proper speeds to make them rotate uniform. We should do it for dark matter also. No other place but the bucket on the Earth is chosen to Origin of the rotation. Say all the stars start to rotate at the same time for Origin, farther star starts to rotate earlier. Far stars of more than c/$\omega$ distance move with speed more than c in current IFR which might be retained by closer stars which does not start rotation yet. Now farther stars have radial coordinate speed > c due to inflation of the universe. How should we do... They are a part of difficulties to do experiment of rotating universe. If we were lucky enough new comer stars beyond the event horizon could be rotating uniform to effect the bucket.

 

[Peter Leeves]

If correct how do they verify that their universe is rotating?

I have a thought with respect to "their universe is rotating". If this is "old news" please just ignore. I'm an amateur and most of the physics in this thread is new to me. But I can see two possible uses of the phrase "proper rotating universe".

1) Reality
Neglecting local effects and ongoing expansion, I don't think it's ever possible to claim the entire universe as a reference frame is proper rotating in reality. An acceleromoter (proper acceleration) and gyroscope (proper rotation) should always confirm this (as long as they avoid local effects). 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. In that case, you could legitimately say reality shows the larger reference frame to be proper stationary and the visible universe is indeed proper rotating - but is now merely a local effect within the larger construct. However, since there is nothing larger (causally connected at speed of light c in a vacuum) than our entire visible universe (Earth as a datum), we can never say "in reality" the universe is rotating.

In short, everything within the largest available causily connected reference frame, moves with respect to it. This is both logical and reflects our reality.

2) Thought Experiment
However, I may see a case for saying "the entire universe is proper rotating" in a thought experiment, should you desire, but only if it adds something of value to physics.

However, it occurs to me from earlier discussion, this is merely changing choice of reference frame (coordinate systerm). I'm still struggling with this, but it does seem to be just saying "lets pretend the universe is proper rotating" - but that statement has no physical impact whatsover on anything happening within the reality of the thought experiment. Even in the thought experiment, the universe remains the largest reference frame we have, and the thing that everything within moves with respect to. Given that it changes nothing physical within, I can see no purpose whatsoever to consider a proper rotating universe, not even in a thought experiment.

Conclusion
Everthing within the largest available causily connected reference frame, moves with respect to it. This reflects reality. Any measurement by accelerometer (proper acceleration) and gyroscope (proper rotation) within, is a local effect. There is no case for considering the universe to be proper rotating, either in reality or thought experiment.

I believe this to be the correct conclusion. I also look forward to being shot down

 

[timmdeeg]

I have a thought with respect to "their universe is rotating".

Please note that my post 173 refers to the Godel universe which has been discussed between @Dale and @PeterDonis. So your conclusions seem to fail in this respect.

I believe this to be the correct conclusion. I also look forward to being shot down

Don't worry I've no gun.

 

[Peter Leeves]

Please note that my post 173 refers to the Godel universe

Timmdeeg, I've literally never come across the term "Godel universe" before this thread. Every time I come across it I've let it go for the moment a) because I'm lazy and b) because I find the physics in this thread fascinating and difficult to find time to break away and research (Google etc).

Do you have time to briefly define "Godel universe" here ? If not, just tell me to stop being lazy and go look it up.

Also, thanks for letting me know my idea contradicts Godel. I don't mind if I'm wrong. I just have a thirst to learn. In the meantime I'll go back and re-read #173

No, nothing in #173 defining Godel universe. I'll go to Google.

 

[PeterDonis]

Do you have time to briefly define "Godel universe" here ?

The Godel universe is a spacetime geometry--a known mathematical solution to the Einstein Field Equation. It was originally discovered by Godel, hence the name.

Several papers describing this spacetime geometry have already been linked to in this thread.

 

[PeterDonis]

my idea contradicts Godel

More precisely: the Godel spacetime is a valid mathematical solution to the Einstein Field Equation, so it is a perfectly valid basis for a thought experiment. So your statement in post #176 that there is no valid basis for considering a "rotating universe" as a thought experiment is wrong. We have been using the Godel spacetime as the basis for considering "rotating universe" thought experiments in this thread.

However, as multiple people have posted, our observational evidence is not consistent with the Godel spacetime as a description of our actual universe; so your statement in post #176 that there is no valid basis for considering a "rotating universe" in reality is correct.

 

[Peter Leeves]

Several papers describing this spacetime geometry have already been linked to in this thread.

Doing a thread search on Godel now, and will catch up on what I skipped over earlier (time restraints etc). Thank you.

 

[PeterDonis]

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)

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.

 

[PeterDonis]

I don't think it's ever possible to claim the entire universe as a reference frame is proper rotating in reality. An acceleromoter should always confirm this

You don't detect proper rotation with an accelerometer; an accelerometer detects proper acceleration, not proper rotation. You detect proper rotation with gyroscopes.

 

[Peter Leeves]

The actual universe is not a reference frame.

My mistake. I thought reference frames could be inertial and non-inertial and the entire visible universe could be regarded as the largest of all available inertial reference frames. I stand corrected and thanks again.

 

[PeterDonis]

I thought reference frames could be inertial and non-inertial

They can. But either way, they're human abstractions.

There is a separate concept of inertial vs. non-inertial motion of an object; "inertial motion" means "zero proper acceleration", and "non-inertial motion" means "nonzero proper acceleration". But that concept has nothing to do with reference frames.

 

[PeterDonis]

the entire visible universe could be regarded as the largest of all available inertial reference frames

In a curved spacetime, such as the spacetime of our actual visible universe, there are no global inertial frames. The global frame we use to describe our universe is not inertial. Objects at rest in this frame are moving inertially (zero proper acceleration), but the frame as a whole is not inertial.

 

[Peter Leeves]

They can. But either way, they're human abstractions.

Understood

 

[Peter Leeves]

The global frame we use to describe our universe is not inertial.

In post #183 you state "The actual universe is not a reference frame." but in #187 you say "The global frame we use to describe our universe is not inertial."

This has immediately confused me. Can you please clarify:

a) By "The global frame we use to describe our universe" did you mean "The global reference frame we use to describe our universe" ?

b) If this is the case, it logically directly contradicts the statement "The actual universe is not a reference frame.

Put simply, is the universe a reference frame or is it not a reference frame ?

Even though reference frames are human abstractions, the two statements "is" and "is not" can't both be true simultaneously. It must be either "is a reference frame" or "is not a reference frame". Or we might as well give up on language and logic.

 

[PeterDonis]

By "The global frame we use to describe our universe" did you mean "The global reference frame we use to describe our universe" ?

Yes.

If this is the case, it logically directly contradicts the statement "The actual universe is not a reference frame.

No, it doesn't. The thing we use to describe the universe is not the same as the actual universe. If I use a map to describe the city I live in, the map is not the same as the actual city. A reference frame is like a map.

 

[Peter Leeves]

No, it doesn't. The thing we use to describe the universe is not the same as the actual universe. If I use a map to describe the city I live in, the map is not the same as the actual city. A reference frame is like a map.

I understand, but this is merely toying with semantics.

If we as humans decide to assign the abstact concept "reference frame" to the entire universe - then it is perfectly valid for a human to say "For the purpose of doing physics I'm going to assign the entire universe as my reference frame". Off he goes and does his physics. There is no claim by the human that the reference frame is in fact the entire universe.

In your map analogy, it's equally valid for the human to say "For the purpose of navigating my way around the city, I'm going to assign the map as my guide." Off he goes and finds his way around. There is no claim by the human that the map is in fact the city.

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."

God help us if every time we speak or write something we have to qualify it by saying "In no way do I consider a reference frame to be the actual universe, but I'm going to assign the universe as a reference frame for the following purpose ... etc". Wow, what a fun world that would be.

From this point forward, I (and every other right thinking person) will consider that the entire universe can be assigned as a legitimate reference frame for the purpose of conducting physics.

Please don't feel a need to respond. My preference will always be physics above toying with semantics.

 

[PeterDonis]

I understand, but this is merely toying with semantics.

No, it's standard practice in physics. 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). You can either use the standard terminology that physicists use, or you can insist on your own idiosyncratic terminology and continue to have problems whenever you want to have a discussion. Your call.

I (and every other right thinking person) will consider that the entire universe can be assigned as a legitimate reference frame for the purpose of conducting physics.

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.

 

[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