General Relativity

[theprestige]

Having said all that, the earth has a bulge at the equator telling us it is rotating.

Does GR have a problem predicting or explaining that bulge, in any reference frame?

 

[Reality Check]

I can look at the universe quite well at various levels.
...
I can imagine quite well a single frame that takes it all in -- the one with the CMB stationary -- as a naturally occurring preferred frame of reference.

The problem is that all of the other reference frames that you mentioned are also "naturally occurring preferred frame of references"!
Your preference for that frame is personal and subjective.
In GR there is no preferred frame of reference. All frames of reference are equivalent.

I personally prefer the naturally occurring preferred frame of reference where I am the center of the universe and everything rotates around me )). That does not make this a useful or convenient frame of reference.

 

[Ziggurat]

As I mentioned above, the earth has a central bulge revealing its rotation. Saturn, which rotates much faster has an even more marked central bulge, which I believe is a ratio of about 11 to 10 when compared to the distance from pole to pole.

That doesn't address a single thing I said.

 

[Perpetual Student]

Does GR have a problem predicting or explaining that bulge, in any reference frame?

I hope not. Does GR tells us that the bulge is there because the earth is clearly and unambiguously rotating on its axis as opposed to the universe revolving around the earth?

 

[Perpetual Student]

In GR there is no preferred frame of reference. All frames of reference are equivalent.

Yes, in GR.

 

[Perpetual Student]

That doesn't address a single thing I said.

I believe it addresses everything you said.

 

[theprestige]

I hope not. Does GR tells us that the bulge is there because the earth is clearly and unambiguously rotating on its axis as opposed to the universe revolving around the earth?

As I understand it, GR tells us that the real thing that the earth is really doing happens--in reality--independently of what coordinates we use to keep track of when and where it does it.

And as I understand it, when we compare GR with other explanations that do depend on a specific coordinate system to explain what's really going on, we find that such systems without exception fall short of the success we enjoy when we explain things with GR.

 

[Reality Check]

Yes, in GR.

And also in general physics there is no "naturally occurring preferred frame of reference".
There are frames of reference that make calculations easier or harder. There are not "naturally occurring". They are just the ones a person would prefer to make their calculations easier. A different person with a different set of skills might prefer a different frame.

 

[Perpetual Student]

And also in general physics there is no "naturally occurring preferred frame of reference".
There are frames of reference that make calculations easier or harder. There are not "naturally occurring". They are just the ones a person would prefer to make their calculations easier. A different person with a different set of skills might prefer a different frame.

Is the earth really rotating or not?

 

[Ziggurat]

I believe it addresses everything you said.

You are wrong about that. What's more, the only way you could think it did is if you badly misunderstood what I said. But you've done so in a way that I can't even figure out how to address whatever your misconceptions are. I''ve run out of ways to try to explain this. What I've said has not sunk it, and I don't know what else will. So I think I'll call it quits, I don't see any more purpose here.

 

[Perpetual Student]

As I understand it, GR tells us that the real thing that the earth is really doing happens--in reality--independently of what coordinates we use to keep track of when and where it does it.

...

I don't think you will get agreement on that from the GR crowd here.
So, is the earth really rotating? Can GR tell us that?

 

[Michael C]

There are measurable forces associated with rotation. That's a big clue.

That's no answer to my question. I asked you this:

if you describe the motion of your grandchild's top using a frame based on the CMB, does it tell us what is "really happening"? When we use this frame, does it give a better description of the reality of the top's motion than when we use a frame where the floor of the room is at rest?

 

[W.D.Clinger]

As I understand it, GR tells us that the real thing that the earth is really doing happens--in reality--independently of what coordinates we use to keep track of when and where it does it.

...

I don't think you will get agreement on that from the GR crowd here.

Why not? Why do you think theprestige's remarks should be controversial?

So, is the earth really rotating? Can GR tell us that?

If Perpetual Student can figure out what he means by "really rotating", then GR will probably be able to tell us whether the earth is really rotating. Einstein suggested that such definitions should be covariant, which means a satisfactory definition of "really rotating" should be independent of coordinates, as theprestige wrote.

Einstein's development of GR was inspired in part by what Einstein called Mach's principle^WP. Ziggurat mentioned a highly relevant thought experiment related to GR's Lense-Thirring effect, which has been observed in experiments, but Perpetual Student ignored that (along with most everything else Ziggurat had to say).

 

[Farsight]

Say you have a spaceship travelling along and on one side of it there's a laser firing a beam across the width of the ship to a detector on the other side, does the point that the laser hits the detector vary as the ship speeds up or slows down?

Yes and no. Yes, in that if the ship is accelerating, the light beam appears to curve downwards. That's the principle of equivalence. But if the ship isn't acclerating, you can't tell how fast it's going by looking at the laser beam. That's the principle of invariance, which was Einstein's original name for relativity. Your horizontals go a bit awry.

 

[Perpetual Student]

Why not? Why do you think theprestige's remarks should be controversial?

My question is very simple. Is the earth really rotating or is the universe revolving around the earth? Does GR shed any light on that Question?

If Perpetual Student can figure out what he means by "really rotating", then GR will probably be able to tell us whether the earth is really rotating. Einstein suggested that such definitions should be covariant, which means a satisfactory definition of "really rotating" should be independent of coordinates, as theprestige wrote.

We know when objects are rotating because there are forces associated with rotation. Put a loosely structured object into rotation and those forces will cause parts to fly off. When we are aware of these forces we can say something is really rotating. Is it really that difficult for you to understand?

Einstein's development of GR was inspired in part by what Einstein called Mach's principle^WP. Ziggurat mentioned a highly relevant thought experiment related to GR's Lense-Thirring effect, which has been observed in experiments, but Perpetual Student ignored that (along with most everything else Ziggurat had to say).

I am well aware of the history of Newton's bucket, Mach's thoughts and Einstein's inspiration and I don't think we need Ziggurat's Gedankenexperiment about frame dragging, when no one seems to be able to answer the question whether the earth is really rotating or is the universe revolving around the earth. Is it really that painful for you and Ziggurat to have me pass on his Gedankenexperiment? Well, tell me if the earth is really rotating and then perhaps I will relieve your pain.

 

[Perpetual Student]

That's no answer to my question. I asked you this:

if you describe the motion of your grandchild's top using a frame based on the CMB, does it tell us what is "really happening"? When we use this frame, does it give a better description of the reality of the top's motion than when we use a frame where the floor of the room is at rest?

All frames should describe the motion of the top equally well, based on my layman's understanding of GR. But, apparently, there is no information forthcoming from GR about what is "really happening."
Does that answer your question?

 

[sol invictus]

We know when objects are rotating because there are forces associated with rotation. Put a loosely structured object into rotation and those forces will cause parts to fly off. When we are aware of these forces we can say something is really rotating. Is it really that difficult for you to understand?

Is it really so difficult for you to understand that GR's predictions for all experiments are absolutely identical in all coordinate systems?

Forces that cause stresses inside an object can be measured, and the results of those measurements will be identical regardless of whether one uses coordinates in which the object is rotating or coordinates in which it isn't.

 

[Perpetual Student]

Is it really so difficult for you to understand that GR's predictions for all experiments are absolutely identical in all coordinate systems?

Forces that cause stresses inside an object can be measured, and the results of those measurements will be identical regardless of whether one uses coordinates in which the object is rotating or coordinates in which it isn't.

I have no basis to believe otherwise. What is your issue?
Do you agree that the earth is really rotating even though, under GR, we have a frame where it is not rotating?

 

[sol invictus]

I have no basis to believe otherwise. What is your issue?

You keep referring to bulges at the earth's equator and the stresses that rotating objects feel as though they had some relevance. They don't.

Do you agree that the earth is really rotating even though, under GR, we have a frame where it is not rotating?

I don't think the statement "the earth is really rotating" means anything until you define it more carefully.

 

[Michael C]

All frames should describe the motion of the top equally well, based on my layman's understanding of GR. But, apparently, there is no information forthcoming from GR about what is "really happening."
Does that answer your question?

Well, my question was asking you which frame you consider to best describe what is "really happening". I know that all frames are equally valid in GR, but you have been saying that there must be a "preferred" one that somehow describes reality better than the others. You have also said that:

I can imagine quite well a single frame that takes it all in -- the one with the CMB stationary -- as a naturally occurring preferred frame of reference.

Do you consider that a frame where the CMB is stationary is a naturally occurring preferred frame of reference for describing the motion of the top?