Curvatures in space-time: actual reality or mathematical concept?

[tris_d]

According to GR is space actually supposed to be bending around mass, or is the concept of "curvature" just abstract and purely mathematical idea?

 

[haushofer]

You could ask that question for basically every physical concept. Physics just desribes reality as we perceive it and beyond, without referring to the "actual reality" (whatever that means).

Perhaps you like this story of Feynman:

In the Graduate College dining room at Princeton everybody used to sit with his own group. I sat with the physicists, but after a bit I thought: It would be nice to see what the rest of the world is doing, so I’ll sit for a week or two in each of the other groups.

When I sat with the philosophers I listened to them discuss very seriously a book called Process and Reality by Whitehead. They were using words in a funny way, and I couldn’t quite understand what they were saying. Now I didn’t want to interrupt them in their own conversation and keep asking them to explain something, and on the few occasions that I did, they’d try to explain it to me, but I still didn’t get it. Finally they invited me to come to their seminar.

They had a seminar that was like, a class. It had been meeting once a week to discuss a new chapter out of Process and Reality –some guy would give a report on it and then there would be a discussion. I went to this seminar promising myself to keep my mouth shut, reminding myself that I didn’t know anything about the subject, and I was going there just to watch.

What happened there was typical–so typical that it was unbelievable, but true. First of all, I sat there without saying anything, which is almost unbelievable, but also true. A student gave a report on the chapter to be studied that week. In it Whitehead kept using the words “essential object” in a particular technical way that presumably he had defined, but that I didn’t understand.

After some discussion as to what “essential object” meant, the professor leading the seminar said something meant to clarify things and drew something that looked like lightning bolts on the blackboard. “Mr. Feynman,” he said, “would you say an electron is an ‘essential object’?”

Well, now I was in trouble. I admitted that I hadn’t read the book, so I had no idea of what Whitehead meant by the phrase; I had only come to watch. “But,” I said, “I’ll try to answer the professor’s question if you will first answer a question from me, so I can have a better idea of what ‘essential object’ means. Is a brick an essential object?”

What I had intended to do was to find out whether they thought theoretical constructs were essential objects. The electron is a theory that we use; it is so useful in understanding the way nature works that we can almost call it real. I wanted to make the idea of a theory clear by analogy. In the case of the brick, my next question was going to be, “What about the inside of the brick?”–and I would then point out that no one has ever seen the inside of a brick. Every time you break the brick, you only see the surface. That the brick has an inside is a simple theory which helps us understand things better. The theory of electrons is analogous. So I began by asking, “Is a brick an essential object?”

Then the answers came out. One man stood up and said, “A brick as an individual, specific brick. That is what Whitehead means by an essential object.”

Another man said, “No, it isn’t the individual brick that is an essential object; it’s the general character that all bricks have in common–their ‘brickiness’–that is the essential object.”

Another guy got up and said, “No, it’s not in the bricks themselves. ‘Essential object’ means the idea in the mind that you get when you think of bricks.”

Another guy got up, and another, and I tell you I have never heard such ingenious different ways of looking at a brick before. And, just like it should in all stories about philosophers, it ended up in complete chaos. In all their previous discussions they hadn’t even asked themselves whether such a simple object as a brick, much less an electron, is an “essential object.”

 

[pervect]

It's space-time that's curved, not just space. But with the usual slicing of space-time into space + time, the spatail slices are curved as well.

As far as it being "real" or not - I think of it as "real". But I'm not sure what sort of experiment (if any), would convince you that "curvature" or anything else for that matter was "real" or "not real".

Typically, "reality" is addressed by philosophy and isn't experimentally testable.

 

[tris_d]

You could ask that question for basically every physical concept. Physics just desribes reality as we perceive it and beyond, without referring to the "actual reality" (whatever that means).

Perhaps you like this story of Feynman:

Can you rephrase that as a specific statement?

a.) "curvatures" are only abstract mathematical idea

b.) "curvatures" are abstract idea with a possibility to be actual reality

c.) "curvatures" are ...?


I'm not sure I agree with what you're saying. When physics say there is such thing as electron, do we consider it actual reality or just abstract idea? And when physics say there is such thing as electric or magnetic field, do we consider it actual reality or just abstract idea as well?

 

[tris_d]

It's space-time that's curved, not just space. But with the usual slicing of space-time into space + time, the spatail slices are curved as well.

I think "curvatures" can not really exist in some 3D spatial volume as actual geometrical feature without reference frame against which it would be curved against. Actual curves would require actual reference frame and "empty space" contains nothing, so for it to contain some actual topology seem to be direct contradiction.

As far as it being "real" or not - I think of it as "real". But I'm not sure what sort of experiment (if any), would convince you that "curvature" or anything else for that matter was "real" or "not real".

Typically, "reality" is addressed by philosophy and isn't experimentally testable.

If you can explain some phenomena by supposing space is actually curved that I can not explain with the concept of potentials, gradients or fields, then I will submit space is actually curved as the best explanation.

 

[Dale]

If you can explain some phenomena by supposing space is actually curved that I can not explain with the concept of potentials, gradients or fields, then I will submit space is actually curved as the best explanation.

That is one of the best criteria I have seen for "actual reality". Most people asking similar questions simply avoid trying to define what they mean by it. I congratulate you for making the effort.

However, if you have even the slightest mathematical background then you should be aware that it is always possible to write the same equation multiple ways. That is basically what algebra is all about. Therefore, it is always possible to come up with a different but mathematically equivalent expression. Meaning that there isn't anything in physics which meets this standard.

 

[A.T.]

When physics say there is such thing as electron, do we consider it actual reality or just abstract idea? And when physics say there is such thing as electric or magnetic field, do we consider it actual reality or just abstract idea as well?

Particles and fields are human ideas. They are useful in predicting phenomena in the observed reality.

 

[WannabeNewton]

I think "curvatures" can not really exist in some 3D spatial volume as actual geometrical feature without reference frame against which it would be curved against. Actual curves would require actual reference frame and "empty space" contains nothing, so for it to contain some actual topology seem to be direct contradiction.

You seem to be saying that in order to detect \ define curvature in any possible way you would need your hyper - surface to be embedded in some ambient space. This isn't true; even when dealing with regular surfaces in R^3 you will see that Gauss curvature is an intrinsic property of the regular surface. Also, on manifolds endowed with a riemannian metric, the curvature endomorphism is given in terms of the levi - civita connection which make sense without reference to some embedding. I don't understand your statement about "actual" topologies: you don't need to refer to curvature or the likes to even endow a set with a topology.

 

[Dickfore]

Perhaps you like this story of Feynman:

That's a great story! Where is it from?

 

[ZikZak]

If you can explain some phenomena by supposing space is actually curved that I can not explain with the concept of potentials, gradients or fields, then I will submit space is actually curved as the best explanation.

On what basis do you claim that curvature is less-than-real, while fields and gradients are really-real? What does really-real mean anyway? How do you determine when something is really-real?

 

[tris_d]

Sure. There are alternates to GR (and even reformulations of GR) that explain GW's as propagation of a real field against a flat spacetime backdrop. String theory after all demands existence of gravitons as fundamental string excitation that acts as a bona-fide force carrying 'particle'. What I was getting at with GW's is that 'spacetime curving' is a mystical concept without physical content if spacetime is purely a relational entity involving nothing but clocks and rulers. Get's down to what 'physical' implies. There are some that will probably argue that energy and momentum are just mental constructs.

I agree. The thing is people in cosmology field take the idea seriously and so we have these 'shape of the universe' models where the space is actually curved and can be closed and finite. I thought that was not logical assumption and is unreasonable because it could not be proven even if it was true, but people I spoke to felt differently and were firmly believing the space actually bends and curves and pointed me to 'Wilkinson Microwave Anisotropy Probe' measurements for the curvature of the whole universe. They measured zero curvature, as I naturally expected, but apparently NASA and our astronomer friends are far less agnostic about the whole "curved-space" idea.

 

[tris_d]

That is one of the best criteria I have seen for "actual reality". Most people asking similar questions simply avoid trying to define what they mean by it. I congratulate you for making the effort.

However, if you have even the slightest mathematical background then you should be aware that it is always possible to write the same equation multiple ways. That is basically what algebra is all about. Therefore, it is always possible to come up with a different but mathematically equivalent expression. Meaning that there isn't anything in physics which meets this standard.

I would assume that, but I would not claim it without trying it out myself. I think there is a difference when you call abstract things abstract names like "field" or "potential gradient", because even if they are curvatures in reality they would still conform to this abstract definition, relations to distance and mass would stay the same. But if you give abstract things specific names, then you kind of insist that what it actually is. So I understand how there are people who consider 'curvatures' are actual reality rather than abstract idea, I just wondered if that's a bad thing or whether I lack some understanding.

 

[Q-reeus]

I agree. The thing is people in cosmology field take the idea seriously and so we have these 'shape of the universe' models where the space is actually curved and can be closed and finite. I thought that was not logical assumption and is unreasonable because it could not be proven even if it was true, but people I spoke to felt differently and were firmly believing the space actually bends and curves and pointed me to 'Wilkinson Microwave Anisotropy Probe' measurements for the curvature of the whole universe. They measured zero curvature, as I naturally expected, but apparently NASA and our astronomer friends are far less agnostic about the whole "curved-space" idea.

tris_d - just to be clear on this matter, GR's geometrical curved spacetime basis does provide a very successful predictive model - which may however fail at a certain level yet to be tested. Current measurements suggests perfectly flat or very close to perfectly flat overall spatial curvature but this is a prediction of inflationary model of cosmology rather than GR itself. Real problem imo is this conceptual dichotomy where in GR 'gravitational field' has totally different connotation to say electric field. One glaring problem for me is that gravitational field is allowed to have an ambiguously defined energy content but not allowed to act as it's own further source, despite the insistence that all other forms of stress-energy must contribute. Ask an expert why and good luck getting a sensible answer.

The fact of gravitational waves as per binary pulsar data imo screams out one of two things - curvature of an actual physical medium on geometrical formulation of gravity, or physical field propagation through flat spacetime on field formulation of gravity. But definitely not just curvature of spatial and temporal relationships.

 

[haushofer]

Can you rephrase that as a specific statement?

a.) "curvatures" are only abstract mathematical idea

b.) "curvatures" are abstract idea with a possibility to be actual reality

c.) "curvatures" are ...?I'm not sure I agree with what you're saying. When physics say there is such thing as electron, do we consider it actual reality or just abstract idea? And when physics say there is such thing as electric or magnetic field, do we consider it actual reality or just abstract idea as well?

Well, in Newton's days one could say that gravity is actually some sort of invisible arm pulling masses together instantaneously. Einstein says it's because energy curves spacetime. Someone like Verlinde claims gravity is some sort of entropic force. From a string point of view it's because energy is exchanging closed strings with vibrational modes we call a "graviton". Some of these ideas overlap, some don't.

What is "actual reality"? That's rather an abstract notion for a physicist, I think. Perhaps it's some fixed point in "theory-space" of which we hope our theories are converging to. Perhaps it's not fixed at all. Perhaps there are more points. I would say that every theory which describes nature as accurately, consistently and completely as is possible in a certain paradigm is as close as you can get. Personally I don't like concepts as "actual reality" or "absolute truth". It makes you forget that physics is all about description and perception. A wise man once said that description and perception come with deception.

Perhaps I've missed it, but I didn't see a definition of "actual reality".

 

[haushofer]

That's a great story! Where is it from?

I'm sure it is in one of its books, but I've forgotten which one and google isn't helping :P

 

[Dale]

So I understand how there are people who consider 'curvatures' are actual reality rather than abstract idea, I just wondered if that's a bad thing or whether I lack some understanding.

That is fine, as long as they understand that EVERYTHING in physics is, according to your classification, an "abstract idea" and not "actual reality". Force, energy, mass, time, potential, momentum, and everything else all fail to meet your criterion for being "actual reality", so it shouldn't be surprising that curvature does also.

 

[jtbell]

I'm sure it is in one of its books, but I've forgotten which one and google isn't helping :P

A Google search for

feynman "essential object"

led me to Surely You're Joking, Mr. Feynman!, which fits because I remember reading that story myself a long time ago, and I think that book was his first "autobiographical" one.

http://books.google.com/books?id=7p...=onepage&q=feynman "essential object"&f=false

 

[PeterDonis]

Real problem imo is this conceptual dichotomy where in GR 'gravitational field' has totally different connotation to say electric field.

This is because gravity is different from electromagnetism. (Specifically, gravity obeys the equivalence principle while electromagnetism does not.) Why is that a problem? The two things are physically different, so the theories that best describe them are different as well.

If it's the word "field" that you're getting hung up on, then drop it; or at least, don't apply it to gravity. "Field" is just a word.

One glaring problem for me is that gravitational field is allowed to have an ambiguously defined energy content but not allowed to act as it's own further source, despite the insistence that all other forms of stress-energy must contribute. Ask an expert why and good luck getting a sensible answer.

We've been here before, of course. You didn't like the answers you got before, but that doesn't mean they're not sensible (or correct, for that matter). But how about the answer I gave above: gravity is different from other things. Does that help?

I really don't understand why people think it's a big problem that gravity is different from other things. What's so hard about that? I agree it would make some theoretical tasks easier if gravity shared more properties with other things, but the fact is that it doesn't.

The fact of gravitational waves as per binary pulsar data imo screams out one of two things - curvature of an actual physical medium on geometrical formulation of gravity, or physical field propagation through flat spacetime on field formulation of gravity. But definitely not just curvature of spatial and temporal relationships.

What is the difference, in your view, between "curvature of an actual physical medium" and "curvature of spatial and temporal relationships"?

Btw, you are correct that, in the particular case of the binary pulsar data, you can also interpret the result as "physical field propagation", but not through a flat background spacetime--through a curved background spacetime. The propagating gravitational waves do not constitute the entire curvature; they only constitute the rapidly varying portion of the curvature.

However, this "physical field propagation" interpretation breaks down in other cases, such as cosmology; I'm not aware of any workable interpretation of the expansion of the universe in terms of "physical field propagation".

 

[tris_d]

Particles and fields are human ideas. They are useful in predicting phenomena in the observed reality.

Yes, everything is human idea, it's all just words. We give things names, and we name their properties as well. We then relate this information to other such things we know, and that's our knowledge, it's all relative. The problem here I see is the question whether this thing we call "curvature" is a property of "space", or property of "mass", or both, whatever those things actually are.

 

[WannabeNewton]

The problem here I see is the question whether this thing we call "curvature" is a property of "space", or property of "mass", or both, whatever those things actually are.

I don't see the problem there. For our purposes, the curvature of a pseudo riemannian 4 - manifold is well - defined and computed on local coordinate charts. The EFEs then give the relation between the curvature and the stress - energy tensor.

 

[harrylin]

According to GR is space actually supposed to be bending around mass, or is the concept of "curvature" just abstract and purely mathematical idea?

Curvature is here a mathematical characteristic of the "world of events" that is called space-time.

Compare: http://www.bartleby.com/173/17.html
http://en.wikipedia.org/wiki/Spacetime (see the intro + "Spacetime in general relativity")

[..] If you can explain some phenomena by [..] fields[..]

As a matter of fact, Einstein and others also used the expression "gravitational field". But "field" is a very vague, abstract concept; its original meaning is merely "area" or "zone".

 

[tris_d]

You seem to be saying that in order to detect \ define curvature in any possible way you would need your hyper - surface to be embedded in some ambient space. This isn't true; even when dealing with regular surfaces in R^3 you will see that Gauss curvature is an intrinsic property of the regular surface. Also, on manifolds endowed with a riemannian metric, the curvature endomorphism is given in terms of the levi - civita connection which make sense without reference to some embedding. I don't understand your statement about "actual" topologies: you don't need to refer to curvature or the likes to even endow a set with a topology.

By 'actual' I mean just "measurable". But as long as the same measurement can be explained with some other concept where the space can stay flat I will believe that explanation is better. Basically, if there is some experiment that strongly suggests gravity indeed curves space rather than being some field or something in flat space, I'd like to hear about it.

 

[Q-reeus]

Q-reeus: "One glaring problem for me is that gravitational field is allowed to have an ambiguously defined energy content but not allowed to act as it's own further source, despite the insistence that all other forms of stress-energy must contribute. Ask an expert why and good luck getting a sensible answer."
We've been here before, of course . You didn't like the answers you got before, but that doesn't mean they're not sensible (or correct, for that matter). But how about the answer I gave above: gravity is different from other things. Does that help?

Hello Peter - I just had this sixth-sense feeling we would meet up again about here . Sure the two (gravity vs EM) are different, and yes it's true I didn't like the answers given before because as I recollect there was no proper dealing with the question of gravity being excluded as source term in the SET. As I further recall you cited ADM definition as evidence gravity (but only as GW's) sort of contributed. But that begs the question - what is fundamentally different about static vs propagating field curvature that one can sort of contribute but absolutely not the other? This is what I mean about consistency issues in GR. There is no such divide with EM as source for instance - static or wave, they both carry energy and both act as contributors in SET. Hocus without the pocus?

What is the difference, in your view, between "curvature of an actual physical medium" and "curvature of spatial and temporal relationships"?

Gravitational waves for one!

Btw, you are correct that, in the particular case of the binary pulsar data, you can also interpret the result as "physical field propagation", but not through a flat background spacetime--through a curved background spacetime. The propagating gravitational waves do not constitute the entire curvature; they only constitute the rapidly varying portion of the curvature.

I understand that source - binary pulsars - as gravitating objects necessarily generates a static/quasi-static curvature through which GW's must propagate, but that's in geometric interpretation. You are well aware there exist other formulations of gravity where everything is owing to fields existing in an unobservable flat background spacetime. That's what I meant there.

However, this "physical field propagation" interpretation breaks down in other cases, such as cosmology; I'm not aware of any workable interpretation of the expansion of the universe in terms of "physical field propagation".

I'm no expert, but there are alternate descriptions/formulations using field concept - Yuri Baryshev comes to mind but there are others. In any case, sticking to geometric concept, how do you explain energy-momentum transport in GW's without it directly implying a physically 'stressed' medium of some sort?

 

[harrylin]

By 'actual' I mean just "measurable". But as long as the same measurement can be explained with some other concept where the space can stay flat I will believe that explanation is better. Basically, if there is some experiment that strongly suggests gravity indeed curves space rather than being some field or something in flat space, I'd like to hear about it.

As far as I know, experiment cannot distinguish such things. Note that also "flat space" is a mathematical term; it has no meaning that I know of in a physical sense (how can empty space be literally "flat"?; that's like saying that it is "green"!).

 

[Q-reeus]

As far as I know, experiment cannot distinguish such things. Note that also "flat space" is a mathematical term; it has no meaning that I know of in a physical sense (how can empty space be literally "flat"?; that's like saying that it is "green"!).

Sum of angles in a triangle! Greater than 180 degrees = +ve curvature, less than 180 degrees = -ve curvature. Note that this can be explained also using a non-geometric field theory = 'effective' curvature.

 

[tris_d]

On what basis do you claim that curvature is less-than-real, while fields and gradients are really-real? What does really-real mean anyway? How do you determine when something is really-real?

What's measurable is real enough for me. Only, we can explain measurements with all sorts of mathematical concepts, so the question is which one is better. For example, I see these equations that are supposed to be about curves are more about some tensors really, but I don't see anyone suggesting space is actually made of little bouncy springs. You see what I mean?

 

[harrylin]

Sum of angles in a triangle! Greater than 180 degrees = +ve curvature, less than 180 degrees = -ve curvature. Note that this can be explained also using a non-geometric field theory = 'effective' curvature.

Ah, then you actually mean with "flat space" something else, it's about "geometrical measurements with tools" - not the vacuum (nothingness?) itself to which tris seems to refer.

Compare: http://www.bartleby.com/173/23.html

The point is, as several people here mentioned, that one can map phenomena to a flat background - which is just as much the fruit of our imagination as a curved one. In a parallel thread we are now even discussing a locally flowing background!

 

[ZikZak]

What's measurable is real enough for me. Only, we can explain measurements with all sorts of mathematical concepts, so the question is which one is better. For example, I see these equations that are supposed to be about curves are more about some tensors really, but I don't see anyone suggesting space is actually made of little bouncy springs. You see what I mean?

No, because your preference for fields is at least as arbitrary. A field is a mathematical object which assigns a numerical value to each point in space, or space-time. There's nothing more mathematically abstracted from reality than that.

 

[Dale]

By 'actual' I mean just "measurable".

Curvature is definitely measurable. It is tidal effects. So I guess that curvature is "actual" by this definition but not "actual reality" by the previous definition.

 

[tris_d]

One glaring problem for me is that gravitational field is allowed to have an ambiguously defined energy content but not allowed to act as it's own further source, despite the insistence that all other forms of stress-energy must contribute. Ask an expert why and good luck getting a sensible answer.

I am not aware of it. What theory, what equation are you referring to?

The fact of gravitational waves as per binary pulsar data imo screams out one of two things - curvature of an actual physical medium on geometrical formulation of gravity, or physical field propagation through flat spacetime on field formulation of gravity. But definitely not just curvature of spatial and temporal relationships.

I don't know about that either. What did we measure and what was the reading?

 

[PeterDonis]

I just had this sixth-sense feeling we would meet up again about here.

As I further recall you cited ADM definition as evidence gravity (but only as GW's) sort of contributed. But that begs the question - what is fundamentally different about static vs propagating field curvature that one can sort of contribute but absolutely not the other?

I think you're either misremembering or misinterpreting--and to be fair, these issues are not easy to disentangle given all that has been said about them by various experts in the field. To briefly restate the key points from earlier discussions:

(1) Neither "static" nor "propagating" curvature contributes to the SET; *no* aspect of gravity contributes to the SET. (I've always been consistent about this. Also, I should note that I am assuming a zero cosmological constant; including that would just get us into a further argument about whether the cosmological constant should be viewed as an "aspect of gravity" or not, depending on which side of the EFE we choose to put it on. I don't think we need to go there.)

(2) If one really, really wants to salvage some kind of "contribution of gravity to total energy", one can define various pseudo-tensors that allow one to do that. However, they are all pseudo-tensors, meaning they are dependent on how you slice up spacetime into space and time, and they can only be consistently defined in certain kinds of spacetimes. None of this changes #1 above.

(3) One can also, in certain kinds of spacetimes, define "total energy at a given time" in different ways, which can sometimes give different results. In the case of a spacetime containing gravitational waves, such as a binary pulsar spacetime, one can show that the ADM energy and the Bondi energy are different, and the difference between them is standardly viewed as "energy carried away by gravitational waves". But this energy cannot be localized; it can only be "seen" when you look at the global properties of the spacetime. None of this changes #1 above either.

Gravitational waves for one!

Then in GR, curvature is "curvature of an actual physical medium" by your definition, and *not* just "curvature of spatial and temporal relationships", since GR predicts that gravitational waves can exist. What's the problem? (Also see further comments at the end of this post on spacetime as a "physically stressed medium".)

I understand that source - binary pulsars - as gravitating objects necessarily generates a static/quasi-static curvature through which GW's must propagate, but that's in geometric interpretation. You are well aware there exist other formulations of gravity where everything is owing to fields existing in an unobservable flat background spacetime. That's what I meant there.

This is a valid interpretation, but note that if you adopt it, the quasi-static part of the field does not "propagate". It's just there.

I'm no expert, but there are alternate descriptions/formulations using field concept - Yuri Baryshev comes to mind but there are others.

AFAIK there isn't any such alternate formulation that works for cosmology. But there is a lot of literature out there that I have not read, so I may be missing something. Any relevant links would be welcome.

In any case, sticking to geometric concept, how do you explain energy-momentum transport in GW's without it directly implying a physically 'stressed' medium of some sort?

To me, curved spacetime *is* a "physically stressed medium" that can transport energy. But looking at it that way dives below the level that GR addresses; it involves trying to come up with a more fundamental theory that underlies GR, such as string theory or loop quantum gravity. AFAIK it is generally accepted that GR is not a "fundamental" theory in this sense; it is a low-energy approximation to some other more fundamental theory. So I don't expect GR to tell me *how* curved spacetime can be a physically stressed medium; I need the more fundamental theory to do that. Unfortunately we don't have any way to probe the structure of spacetime at a small enough distance scale to investigate this sort of thing experimentally.

 

[Dale]

Well if it is just mathematical abstraction in GR then GR is severely self-inconsistent since it's eq'ns predict gravitational waves as purely a disturbance of spacetime curvature

I am labeling this "DS2Qr(1)": Do you have any solid evidence for this supposed self-inconsistency, or is this just another of your unsubstantiated assertions? By solid evidence I mean either a rigorous derivation according to the accepted mathematical rules of GR that leads to a self-contradiction, or a mainstream scientific reference describing the inconsistency you allege. A non-rigorous description of a scenario that you are unable to analyze does not qualify as evidence.

I suspect not and await your usual response, perhaps we can label it "Qr2DS(1)" and save ourselves a lot of typing in the future.

I can also provide DS2Qr(2): That is not solid evidence, it is up to you to provide evidence to support your claim, not up to me to provide evidence against it. Please come back when you actually have some evidence.

 

[Q-reeus]

Ah, then you actually mean with "flat space" something else, it's about "geometrical measurements with tools" - not the vacuum (nothingness?) itself to which tris seems to refer.

Well this is the uneasiness of GR isn't it - what exactly does 'curved spacetime' entail - apart from the relational aspects that is? The latter allows a well enough definition of 'flat' space as per my #28 and good old Wiki fleshes out here

Compare: http://www.bartleby.com/173/23.html

Just caught this addition in time -will check out but initial impression is it seems to concentrate on Ehrenfest type situation - rotating disk.

The point is, as several people here mentioned, that one can map phenomena to a flat background - which is just as much the fruit of our imagination as a curved one. In a parallel thread we are now even discussing a locally flowing background!

Right - well I got a shoulder banged up bad going over some rapids way back, and sort of have an inbuilt aversion to 'river models'. At one time I thought to have exposed a fatal flaw in that model, based on expected quantity of dark energy being continually sucked in. Quickly discovered though that net effect was zero owing to the peculiar cancellation of energy and pressure terms.

 

[tris_d]

This is because gravity is different from electromagnetism. (Specifically, gravity obeys the equivalence principle while electromagnetism does not.) Why is that a problem? The two things are physically different, so the theories that best describe them are different as well.

Is that about charge can be both positive and negative while gravity only attracts? Is there more? And while we at it, is there any reason why we could not propose electric or magnetic fields are perhaps some curvatures in space as well?

I really don't understand why people think it's a big problem that gravity is different from other things. What's so hard about that? I agree it would make some theoretical tasks easier if gravity shared more properties with other things, but the fact is that it doesn't.

First time I hear about. Tell me everything. What's the problem for people to accept?

What is the difference, in your view, between "curvature of an actual physical medium" and "curvature of spatial and temporal relationships"?

That confuses me. Is space in GM considered "empty" or "physical"?

Btw, you are correct that, in the particular case of the binary pulsar data, you can also interpret the result as "physical field propagation", but not through a flat background spacetime--through a curved background spacetime. The propagating gravitational waves do not constitute the entire curvature; they only constitute the rapidly varying portion of the curvature.

Are you saying there is no way to explain that with flat space?

However, this "physical field propagation" interpretation breaks down in other cases, such as cosmology; I'm not aware of any workable interpretation of the expansion of the universe in terms of "physical field propagation".

At least the whole universe needs not to be curved for that. Current data suggests we live in infinite flat universe.

http://en.wikipedia.org/wiki/Shape_of_the_Universe
- The recent Wilkinson Microwave Anisotropy Probe (WMAP) measurements have led NASA to state, "We now know that the universe is flat with only a 0.5% margin of error." Within the Friedmann–Lemaître–Robertson–Walker (FLRW) model, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model...

 

[harrylin]

Well this is the uneasiness of GR isn't it - what exactly does 'curved spacetime' entail - apart from the relational aspects that is? The latter allows a well enough definition of 'flat' space as per my #28 and good old Wiki fleshes out here

Sure. The problem is likely related to different meanings of the word "space"; and "space" means here perhaps not what tris means with that word. Did you see the Wikipedia intro on spacetime that I linked? I think that it's quite good, Mr & Ms Anonymous phrase it as follows:

"In physics, spacetime (or space–time, space time or space–time continuum) is any mathematical model that combines space and time into a single continuum. [..] it is assumed that spacetime is curved by the presence of matter "
(they should perhaps phrase it as "has curvature" for grammatical consistency).

Just caught this addition in time -will check out but initial impression is it seems to concentrate on Ehrenfest type situation - rotating disk.

It was merely to illustrate how physical interpretation is not always evident and differs from "curvature" (which is not used once in that chapter).

Right - well I got a shoulder banged up bad going over some rapids way back, and sort of have an inbuilt aversion to 'river models'. At one time I thought to have exposed a fatal flaw in that model, based on expected quantity of dark energy being continually sucked in. Quickly discovered though that net effect was zero owing to the peculiar cancellation of energy and pressure terms.

I also dislike them; but it strikingly illustrates how one can map the same observations with different models.

 

[tris_d]

As far as I know, experiment cannot distinguish such things. Note that also "flat space" is a mathematical term; it has no meaning that I know of in a physical sense (how can empty space be literally "flat"?; that's like saying that it is "green"!).

I was using term used in cosmology, by flat they mean 3D Euclidean space. They think they could measure the curvature of the universe by examining some differences in temperature of background radiation. I say extrinsic curvature would be impossible to detect in any case, we would need to step into higher dimension, but in our realm all those extrinsic curves would be straight lines. So I have no idea what in the world did they hope to measure, but they did measure it, and it turns out the curvature is zero. Heh.

 

[harrylin]

I was using term used in cosmology, by flat they mean 3D Euclidean space. They think they could measure the curvature of the universe by examining some differences in temperature of background radiation. [..]

Right - and what "they" mean with "3D Euclidean space" is described in the link that I gave; it's a measurement relationship that depends on how an operator handles and defines his measurement tools. In particular the "space" (more precisely, Einstein calls it "space data" and "space co-ordinates") of a rotating disc is "non-Euclidean". Perhaps that is not exactly what you mean with that expression.

 

[PeterDonis]

Is that about charge can be both positive and negative while gravity only attracts?

No, it's about what I said: gravity obeys the equivalence principle, electromagnetism does not. Put another way, all objects have the same ratio of inertial to gravitational mass; but different objects have different ratios of charge to mass.

And while we at it, is there any reason why we could not propose electric or magnetic fields are perhaps some curvatures in space as well?

Not in ordinary spacetime, no. But you can interpret the math of EM fields, and of the weak and strong interactions as well, as describing curvature in an abstract space. That probably deserves a separate thread, though, and not in this forum (the Quantum Physics forum would probably be a better place).

First time I hear about. Tell me everything. What's the problem for people to accept?

I personally don't have a problem accepting it, so I'm the wrong person to ask.

That confuses me. Is space in GM considered "empty" or "physical"?

Yes. It's empty because it has zero stress-energy. It's physical for the reasons given in my response to Q-reeus about whether spacetime is a "physical medium".

(Btw, you keep saying "space" instead of "spacetime". They're not the same thing; see my comment at the end of this post about what is and is not flat.)

Are you saying there is no way to explain that with flat space?

As Q-reeus pointed out, you can view the quasi-static part of the spacetime curvature as being due to a field on an unobservable flat spacetime background as well. But as I pointed out, if you adopt that interpretation, the quasi-static part of the field does not propagate; it's just there.

At least the whole universe needs not to be curved for that. Current data suggests we live in infinite flat universe.

It suggests that we live in an infinite *spatially* flat universe. That does *not* mean the *spacetime* of the universe is flat. It isn't; it's curved, but the curvature (in the standard FRW coordinates in which the spatial slices are flat) is all in the time dimension.

 

[tris_d]

I don't see the problem there. For our purposes, the curvature of a pseudo riemannian 4 - manifold is well - defined and computed on local coordinate charts. The EFEs then give the relation between the curvature and the stress - energy tensor.

Are these curvatures around some mass radial or circular? Could you say where some curve starts, where it ends, where it curves and how much it curves? And what happens to space, does it get stretched and squashed?

 

[Q-reeus]

Q-reeus: "Well if it is just mathematical abstraction in GR then GR is severely self-inconsistent since it's eq'ns predict gravitational waves as purely a disturbance of spacetime curvature"

I am labeling this "DS2Qr(1)": Do you have any solid evidence for this supposed self-inconsistency, or is this just another of your unsubstantiated assertions? By solid evidence I mean either a rigorous derivation according to the accepted mathematical rules of GR that leads to a self-contradiction, or a mainstream scientific reference describing the inconsistency you allege. A non-rigorous description of a scenario that you are unable to analyze does not qualify as evidence.

I suspect not and await your usual response, perhaps we can label it "Qr2DS(1)" and save ourselves a lot of typing in the future.

I'll save you even more typing - just read again that above passage you quoted - bold emphasis added. Read it in context of post I was addressing. There is nothing to deal with here - you are barking up the wrong tree.

I can also provide DS2Qr(2): That is not solid evidence, it is up to you to provide evidence to support your claim, not up to me to provide evidence against it. Please come back when you actually have some evidence.

Have no idea what all this 'DS2Qr(2)' code is supposed to mean. And btw, it would pay to tread a bit lightly rather than being aggressive. You should know that sort of thing can come home to roost. Let you off very lightly here Fact is if you were honest there would have been a simple admission in #15 you got it plain wrong in #5.

 

[Q-reeus]

I think you're either misremembering or misinterpreting--and to be fair, these issues are not easy to disentangle given all that has been said about them by various experts in the field. To briefly restate the key points from earlier discussions:
(1) Neither "static" nor "propagating" curvature contributes to the SET; *no* aspect of gravity contributes to the SET. (I've always been consistent about this. Also, I should note that I am assuming a zero cosmological constant; including that would just get us into a further argument about whether the cosmological constant should be viewed as an "aspect of gravity" or not, depending on which side of the EFE we choose to put it on. I don't think we need to go there.)

Thanks for that clarification which I will strive to remember clearly - unlike obviously faulty recollection of previous occasion.

(2) If one really, really wants to salvage some kind of "contribution of gravity to total energy", one can define various pseudo-tensors that allow one to do that. However, they are all pseudo-tensors, meaning they are dependent on how you slice up spacetime into space and time, and they can only be consistently defined in certain kinds of spacetimes. None of this changes #1 above.

(3) One can also, in certain kinds of spacetimes, define "total energy at a given time" in different ways, which can sometimes give different results. In the case of a spacetime containing gravitational waves, such as a binary pulsar spacetime, one can show that the ADM energy and the Bondi energy are different, and the difference between them is standardly viewed as "energy carried away by gravitational waves". But this energy cannot be localized; it can only be "seen" when you look at the global properties of the spacetime. None of this changes #1 above either.

Got the picture. Which imo is still a big problem for GR conceptually. What is the bedrock justification for such conditionally allowing an energy content but never a gravitating energy content to gravitational field? There is afaik no argument that say field 'pressure' can cancel field energy. So what gives here? Equivalence principle makes it all too problematic to define field energy - so hang it, just excise it altogether. Null results for Lunar etc. Nordtvedt effect imo suggests strongly there has been an unjustified sweeping under the rug.

Then in GR, curvature is "curvature of an actual physical medium" by your definition, and *not* just "curvature of spatial and temporal relationships", since GR predicts that gravitational waves can exist. What's the problem? (Also see further comments at the end of this post on spacetime as a "physically stressed medium".)

See above comments.

AFAIK there isn't any such alternate formulation that works for cosmology. But there is a lot of literature out there that I have not read, so I may be missing something. Any relevant links would be welcome.

Following article has been mentioned here at PF before: http://xxx.lanl.gov/abs/gr-qc/9912003 ,there are various others from same author there at arXiv. A recent one by what may be termed a more mainstream theoretician is http://arxiv.org/abs/1209.3511. Suggest a Google search if one wants to chase up more.

To me, curved spacetime *is* a "physically stressed medium" that can transport energy. But looking at it that way dives below the level that GR addresses; it involves trying to come up with a more fundamental theory that underlies GR, such as string theory or loop quantum gravity. AFAIK it is generally accepted that GR is not a "fundamental" theory in this sense; it is a low-energy approximation to some other more fundamental theory. So I don't expect GR to tell me *how* curved spacetime can be a physically stressed medium; I need the more fundamental theory to do that. Unfortunately we don't have any way to probe the structure of spacetime at a small enough distance scale to investigate this sort of thing experimentally.

Right, but to me this is an admission there is this conceptual shortfall in GR. As I wrote in earlier post - ether banished in SR but seems to return in GR as some kind of elephant in the living room.

 

[tris_d]

Curvature is here a mathematical characteristic of the "world of events" that is called space-time.

Compare: http://www.bartleby.com/173/17.html
http://en.wikipedia.org/wiki/Spacetime (see the intro + "Spacetime in general relativity")

As a matter of fact, Einstein and others also used the expression "gravitational field". But "field" is a very vague, abstract concept; its original meaning is merely "area" or "zone".

Is there any diagram depicting these geodesics in 3D?

 

[Dale]

I'll save you even more typing - just read again that above passage you quoted - bold emphasis added.

OK, so to be clear you are not claiming that an inconsistency exists in GR?

Have no idea what all this 'DS2Qr(2)' code is supposed to mean. And btw, it would pay to tread a bit lightly rather than being aggressive. You should know that sort of thing can come home to roost. Let you off very lightly here Fact is if you were honest there would have been a simple admission in #15 you got it plain wrong in #5.

DS2Qr(2) is meant to be short hand for DaleSpam to Q-reeus standard response number 2.

I didn't get #5 wrong, I addressed what I saw as the content of the question, and you focused on the obvious fact that it was not directly related to the context of the question. I said exactly what I meant and there was nothing incorrect in what I said. It certainly is possible that I misunderstood phinds' intent, but at the most that would have made #5 irrelevant, not wrong.

There wasn't any dishonesty, and whatever you may say about my "aggressiveness", you are the one went immediately to casting character aspersions here by calling me dishonest.

 

[WannabeNewton]

Are these curvatures around some mass radial or circular? Could you say where some curve starts, where it ends, where it curves and how much it curves? And what happens to space, does it get stretched and squashed?

The curvature is determined by the mass - energy distribution. It doesn't have to be a fixed spherical mass; we can, for example, have dust. Once you know what the metric tensor is you can compute the christoffel symbols and, in principle, solve the geodesic equation for the geodesics on the manifold, given initial conditions, which will of course determine the future behavior of the geodesics assuming the solutions are well behaved. You can also use geodesics to describe the curvature at points on a riemannian manifold; this is called sectional curvature if you want to look that up. I'm not sure how to characterize "stretched" or "squashed".

 

[PeterDonis]

What is the bedrock justification for such conditionally allowing an energy content but never a gravitating energy content to gravitational field? There is afaik no argument that say field 'pressure' can cancel field energy.

You've got things backwards. None of the things you are talking about are part of the basic conceptual foundation of GR. As I've said many times, that is the EFE. The EFE says unequivocally that, as my #1 said, gravity does not appear in the SET. *That* is the only "bedrock justification" you need. GR simply does not treat the stuff you are talking about as fundamental; the questions you are asking are questions about a particular conceptual scheme you want to overlay on GR, not about GR itself. See next comment.

So what gives here? Equivalence principle makes it all too problematic to define field energy - so hang it, just excise it altogether.

GR does not say this. GR says that there is no *need* to "define field energy" at all. You can explain all gravitational phenomena without it; just use the EFE, in which "field energy" does not even appear. All this worrying about "field energy" is *your* problem, not GR's.

Nordtvedt effect imo suggests strongly there has been an unjustified sweeping under the rug.

The best evidence we have indicates that there is no Nordvedt effect, so I don't understand why you think an effect that doesn't exist shows some problem with GR, since GR predicts that it does not exist.

Following article has been mentioned here at PF before: http://xxx.lanl.gov/abs/gr-qc/9912003 ,there are various others from same author there at arXiv. A recent one by what may be termed a more mainstream theoretician is http://arxiv.org/abs/1209.3511.

I'll look at these in more detail when I have a chance, but on a quick skim they don't seem to cover cosmology at all; they only talk about asymptotically flat cases like the spacetime around a single gravitating body or a small system of bodies like the binary pulsar. This is the same limitation I've seen in all treatments of this subject: the assumption of a flat background spacetime (even if it's unobservable) forces the actual observed solution to be asymptotically flat.

Right, but to me this is an admission there is this conceptual shortfall in GR.

It's only a conceptual shortfall if you expect GR to be a complete fundamental theory. By that criterion, we have never had a scientific theory without a conceptual shortfall.

 

[Q-reeus]

Q-reeus: "One glaring problem for me is that gravitational field is allowed to have an ambiguously defined energy content but not allowed to act as it's own further source, despite the insistence that all other forms of stress-energy must contribute. Ask an expert why and good luck getting a sensible answer."
I am not aware of it. What theory, what equation are you referring to?

Sorry - forgot about your #33. Check out here Go down to the very first equation and then read the following few lines after that.

Q-reeus: "The fact of gravitational waves as per binary pulsar data imo screams out one of two things - curvature of an actual physical medium on geometrical formulation of gravity, or physical field propagation through flat spacetime on field formulation of gravity. But definitely not just curvature of spatial and temporal relationships."
I don't know about that either. What did we measure and what was the reading?

There are lots of articles on this but here's one.

 

[surajt88]

After I saw the Einstein Rings, I was convinced that spacetime curvature was "real".

 

[Q-reeus]

OK, so to be clear you are not claiming that an inconsistency exists in GR?

Of course I'm claiming an inconsistency exists - but not as per your misconstrued quotes!

DS2Qr(2) is meant to be code for DaleSpam to Q-reeus standard response number 2.

Stick to plain english - there is less typing in the end.

I didn't get #5 wrong, I addressed what I saw as the content of the question, and you focused on the obvious fact that it was not directly related to the context of the question. I said exactly what I meant and there was nothing incorrect in what I said. It certainly is possible that I misunderstood phinds' intent, but at the most that would have made #5 irrelevant, not wrong. There wasn't any dishonesty, and your insinuation that there was is typical of your own hypocritical aggressiveness.

You may choose to persist in that line, but the thread is there for all to read and judge. Your answer in #5 was quite misleading given the entirely clear context of what it was addressing and best to admit as such.

 

[Q-reeus]

Q-reeus: " What is the bedrock justification for such conditionally allowing an energy content but never a gravitating energy content to gravitational field? There is afaik no argument that say field 'pressure' can cancel field energy."

You've got things backwards. None of the things you are talking about are part of the basic conceptual foundation of GR. As I've said many times, that is the EFE. The EFE says unequivocally that, as my #1 said, gravity does not appear in the SET. *That* is the only "bedrock justification" you need. GR simply does not treat the stuff you are talking about as fundamental; the questions you are asking are questions about a particular conceptual scheme you want to overlay on GR, not about GR itself. See next comment.

I don't accept having gotten things backwards above at all. I asked for the conceptual justification. You simply stated the GR formal position without offering any explanation for why in GR gravity is excised from being SET source term.

Q-reeus: "So what gives here? Equivalence principle makes it all too problematic to define field energy - so hang it, just excise it altogether."

GR does not say this. GR says that there is no *need* to "define field energy" at all. You can explain all gravitational phenomena without it; just use the EFE, in which "field energy" does not even appear. All this worrying about "field energy" is *your* problem, not GR's.

Owing to typically extreme weakness of contribution, no seeming need to include up to present levels of observational test. I believe Nordtvedt results actually do undermine that position - more below on that.

Q-reeus: "Nordtvedt effect imo suggests strongly there has been an unjustified sweeping under the rug."
The best evidence we have indicates that there is no Nordvedt effect, so I don't understand why you think an effect that doesn't exist shows some problem with GR, since GR predicts that it does not exist.

Please - don't stoop to excising part of a quote that completely changes it's import. The bit you left out: "Null results for Lunar etc. Nordtvedt effect..."
It's precisely the null result(s) that imo strongly implies, if not outright demands, that gravity does gravitate: null results imply m_a = m_p = m_i, with all m's owing to gravitational binding energy which I trust you agree is inclusive of gravitational field itself.

 

[tris_d]

In any case, sticking to geometric concept, how do you explain energy-momentum transport in GW's without it directly implying a physically 'stressed' medium of some sort?

I have no idea what are you guys are talking about, but when you said "medium" I thought of what Lorenz commented about Einstein's theory.

- Lorentz on his side continued to use the aether concept. In his lectures of around 1911 he pointed out that what "the theory of relativity has to say ... can be carried out independently of what one thinks of the aether and the time". He commented that "whether there is an aether or not, electromagnetic fields certainly exist, and so also does the energy of the electrical oscillations" so that, "if we do not like the name of "aether", we must use another word as a peg to hang all these things upon."