GR question

[lemur]

One of the principles of Relativity is that there is no preferred frame of reference. There is no experiment that you can perform that can tell you if you were "really" moving. IOW, there is no way that the particle can tell that it is moving, thus for it, its mass does not increase, nor does it length contract, so it cannot form a black hole. If it doesn't form a black hole in its own rest frame, it doesn't form one according to someone that it is moving relative to.

Could gravity be as relative as light and thus could a black hole be "gravity-dilated" due to relative velocity/gravity? Why does GR assume relative spacetime and absolute gravitational relations? Is that too abstract a question for this thread?

[swansont]

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Split from http://www.scienceforums.net/topic/57303-dont-mess-with-the-little-guy-small-particle-traveling-fast/
Please follow rule 2.5

[md65536]

Could gravity be as relative as light and thus could a black hole be "gravity-dilated" due to relative velocity/gravity? Why does GR assume relative spacetime and absolute gravitational relations? Is that too abstract a question for this thread?

The force of gravity is proportional to mass, which is relative and depends on relative velocity. I don't think gravity is absolute in any sense.

 

Edit: I just read some posts in the original thread and it looks like this reply is completely wrong.

Edited May 23, 2011 by md65536

[Royston]

Could gravity be as relative as light

 

Stop, what do you mean by this ?

 

and thus could a black hole be "gravity-dilated" due to relative velocity/gravity?

 

What ?

 

Why does GR assume relative spacetime and absolute gravitational relations? Is that too abstract a question for this thread?

 

Not so much abstract, more failing massively to ask coherent questions about GR.

[lemur]

Stop, what do you mean by this ?

Like whether gravitation itself could redshift/blueshift

 

Not so much abstract, more failing massively to ask coherent questions about GR.

The question is why relativity treats gravity levels as absolute while allowing light to shift due to all sorts of causes and conditions. If gravity and light would both be particle/waves travelling at C, then why couldn't gravity vary dynamically in all the same ways light does? But I think that this question is liable to distract from the main crux of this thread, so I think I'll repost it since you responded.

 

 

[lemur]

Why does relativity treat gravity levels as absolute while allowing light to shift due to all sorts of causes and conditions; e.g. redshift, blueshift, and following the curved paths of (fixed) spacetime? If gravity and light would both be particle/waves traveling at C, then why couldn't gravity vary dynamically in all the same ways light does? Certainly it would be confusing if gravitation was influenced by gravitation and spacetime curvature was determined by gravitons that move through curved spacetime because of the chicken-egg type viscous cycle of determination. But on the other hand, why wouldn't it be logical that gravitons behave much like photons, considering they travel at the same speed, etc.?

[swansont]

Scientific theories have to reflect what is observed. It's not a matter of "allowing" something to happen or not. Light traveling at a constant speed in inertial frames is a part of relativity because it was already observed to be true in E&M. Redshift and blueshift is a consequence of relative motion.

 

If you can construct a model where "gravity varies dynamically" (whatever that's supposed to mean), go ahead and construct it. If experiments do not follow its predictions, nobody will care. It will never be accepted as a theory if it doesn't agree with experiment.

[Royston]

Like whether gravitation itself could redshift/blueshift

 

Get a good book on electromagnetism, then move on to special relativity. Then wrap your head around point set / general topology, then get a good book on differential geometery !

 

The question is why relativity treats gravity levels as absolute while allowing light to shift due to all sorts of causes and conditions. If gravity and light would both be particle/waves travelling at C, then why couldn't gravity vary dynamically in all the same ways light does? But I think that this question is liable to distract from the main crux of this thread, so I think I'll repost it since you responded.

 

I'm not sure what you mean, light can loose energy while passing through a gravitional well, therefore it's frequency changes. The geodesics, or the path of light is shaped by the space in which it is defined.You really need to get a grasp of the basics, before you get into GR....it is an incredibly hard subject

[swansont]

!

Moderator Note

Topics merged

[DrRocket]

I'm not sure what you mean, light can loose energy while passing through a gravitional well, therefore it's frequency changes. The geodesics, or the path of light is shaped by the space in which it is defined.You really need to get a grasp of the basics, before you get into GR....it is an incredibly hard subject

 

GR is a mix. The basic idea, while a bit abstract, is incredibly simple. But the devil is in the details, and solving the equations explicitly is indeed incredibly hard. The fundamental equation, up to a constant, [math]G= 8 \pi T[/math], is simplicity itself. The hard part is deducing the implications.

[lemur]

I'm not sure what you mean, light can loose energy while passing through a gravitional well, therefore it's frequency changes. The geodesics, or the path of light is shaped by the space in which it is defined.You really need to get a grasp of the basics, before you get into GR....it is an incredibly hard subject

This doesn't address what I was talking about. My question was not in regards to how spacetime is traditionally understood in GR. My question was whether it seems logical that photons should move along geodesic paths determined by gravity but gravitons, if such a particle would be recognized, would presumably be determined by nothing but the matter emitting them. I.e. spacetime would be treated as curved for photons and matter but would it be treated as flat for gravitons?

[csmyth3025]

This doesn't address what I was talking about. My question was not in regards to how spacetime is traditionally understood in GR. My question was whether it seems logical that photons should move along geodesic paths determined by gravity but gravitons, if such a particle would be recognized, would presumably be determined by nothing but the matter emitting them. I.e. spacetime would be treated as curved for photons and matter but would it be treated as flat for gravitons?

Let me see if I understand your question correctly:

 

Essentially you're asking if you are moving at a very high velocity relative to, for example, two closely orbiting neutron stars that are producing measurable gravitational waves, would those gravitational waves be "blue shifted" (higher frequency) if you're going towards the neutron star binary or "red shifted" if you're going away from them. This is actually a Special Relativity question.

 

If this is your question then on the basis that gravitational waves are thought to travel at the speed of light, my thought would be that yes, this is the effect you would observe.

 

On the General Relativity side of things, if you were standing on a distant neutron star observing the gravitational waves generated by this binary system, my thought would be that you also would "see" these gravitational waves blue shifted in the same way that light from ordinary stars would be.

 

I'm not very knowledgeable about these things so I could be completely wrong. Perhaps others in this forum will have a better understanding.

 

Also, if I've misunderstood your question, please let me know.

 

Chris

[Royston]

GR is a mix. The basic idea, while a bit abstract, is incredibly simple. But the devil is in the details, and solving the equations explicitly is indeed incredibly hard.

 

Yes, qualitatively it's not particularly hard, quantitatively it becomes a nightmare ! Even with simple metrics, solving the geodesics ends up with a whole page of calculations.

 

Lemur, I'm not sure where to start...you stated one thing in your OP (which didn't make much sense) and now you're talking about gravitons !? Please stick to distinct, and manageable questions, rather than shifting about, thanks

Edited May 29, 2011 by Royston

[lemur]

Let me see if I understand your question correctly:

 

Essentially you're asking if you are moving at a very high velocity relative to, for example, two closely orbiting neutron stars that are producing measurable gravitational waves, would those gravitational waves be "blue shifted" (higher frequency) if you're going towards the neutron star binary or "red shifted" if you're going away from them. This is actually a Special Relativity question.

 

If this is your question then on the basis that gravitational waves are thought to travel at the speed of light, my thought would be that yes, this is the effect you would observe.

 

On the General Relativity side of things, if you were standing on a distant neutron star observing the gravitational waves generated by this binary system, my thought would be that you also would "see" these gravitational waves blue shifted in the same way that light from ordinary stars would be.

 

I'm not very knowledgeable about these things so I could be completely wrong. Perhaps others in this forum will have a better understanding.

 

Also, if I've misunderstood your question, please let me know.

 

Chris

No, you basically clarified it; except the example of a binary neutron system is more specialized than the terms I was thinking in. I was just thinking of any source of gravitation and 'gravitons' generally, if such a particle is responsible for carrying gravity. I.e. if photons move through curved geodesics, why not gravitons? But if gravitons do, then it raises the problem of what causes the curvature of the geodesics that gravitons move though?

[Royston]

No, you basically clarified it; except the example of a binary neutron system is more specialized than the terms I was thinking in. I was just thinking of any source of gravitation and 'gravitons' generally, if such a particle is responsible for carrying gravity. I.e. if photons move through curved geodesics, why not gravitons? But if gravitons do, then it raises the problem of what causes the curvature of the geodesics that gravitons move though?

 

Right, 'now' I understand what you're asking, we got there eventually What you are asking is how can a particle not only define but create the space in which it resides (sounds contradictory doesn't it), yes ? It maybe best to think of gravitons as quantified bits of curvature, in the same way as a photon could be considered as a quantified bit of a light wave (I suppose)...but that would be muddying the issue (but at this level it will do as an explanation!) It is a very good question, and I'm simply not qualified to give a thorough answer. I do know someone who specializes in QFT, if you would like me to submit this question to him ?

Edited May 29, 2011 by Royston

[imatfaal]

Can I make sure I understand the problem in its most basic form; is the question whether a photon and a graviton emitted by an object will follow the same geodesic path through space time? And if so; what causes the curvature upon which the graviton follows its geodesic?

 

Isn't this dangerously close to attempting to merge general relativity and quantum field theory? - ie getting pretty difficult

[lemur]

Can I make sure I understand the problem in its most basic form; is the question whether a photon and a graviton emitted by an object will follow the same geodesic path through space time? And if so; what causes the curvature upon which the graviton follows its geodesic?

That sums it up, yes.

 

Isn't this dangerously close to attempting to merge general relativity and quantum field theory? - ie getting pretty difficult

Idk, it's just something that occurred to me for some reason earlier in this thread. Since I don't do math, these kinds of issues appear more logical/philosophical to me. It's just odd to say, on the one hand, that gravity curves the spacetime through which light and other things travel, yet on the other hand say that gravity propagates at the speed of light without mentioning anything about the grid through which it propagates being curved or straight or whatever. In other words, it's just a logical conundrum to me.

 

 

[DrRocket]

That sums it up, yes.

 

 

Idk, it's just something that occurred to me for some reason earlier in this thread. Since I don't do math, these kinds of issues appear more logical/philosophical to me. It's just odd to say, on the one hand, that gravity curves the spacetime through which light and other things travel, yet on the other hand say that gravity propagates at the speed of light without mentioning anything about the grid through which it propagates being curved or straight or whatever. In other words, it's just a logical conundrum to me.

 

 

 

You are indeed mixing quantum theory and general relativity. Nobody knows how to do that, yet.

 

The gravitational waves of general relativity do propagate at c. Buthe they are waves of spacetime curvature, "disturbances of the grid" rather than something that propagates 'through the grid". Worse, there is no universal grid. There is no easy answer.

 

I think it will take an eventual unification of gravitation with quantum theory to answer your question. Such a theory has remained elusive.

 

The problem with "logical/philosophical" rather than mathematical formulations of and approaches to such questions, is that the "logical/philosophical" language of everyday experience is completely inadequate for questions of quantum theory and relativity which do not fit that everyday experience. The language of physics is mathematics. In order to understand it you must speak the language.

[MigL]

Interesting topic which shows the application of theories to specific subjects.

 

Yes, gravity gravitates, ie gravity will act on gravitational energy. But GR in of itself does not predict gravitons. They would come out of a quantum gravity theory ( in the future ). Gravitons have never been observed, or even their by-products, and may never be. They are postulated.

 

Its unfortunate that we have very differing theories of the fundamental forces, geometric deterministic for gravity nd quantum probabilistic for the others, but it means you have to have some understanding of the 'physical' problem before applying the approriate theory.

[csmyth3025]

You are indeed mixing quantum theory and general relativity. Nobody knows how to do that, yet.

 

The gravitational waves of general relativity do propagate at c. But they are waves of spacetime curvature, "disturbances of the grid" rather than something that propagates 'through the grid". Worse, there is no universal grid. There is no easy answer...

Now I'm somewhat perplexed. Are gravitational waves ("...waves of spacetime curvature - 'disturbances of the grid' - rather than something that propagates through the grid...") red-shifted and blue-shifted as electromagnetic radiation would be in Special and General Relativity or are they immune from this effect? Is there a theory about how gravitational waves propagate or, alternately, are the details of gravitational wave propagation contained in other existing theories?

 

Chris

[lemur]

You are indeed mixing quantum theory and general relativity. Nobody knows how to do that, yet.

Maybe not, but it's no crime to think about issues that might be dealt with one day as the science evolves in that direction, is there?

 

The gravitational waves of general relativity do propagate at c. Buthe they are waves of spacetime curvature, "disturbances of the grid" rather than something that propagates 'through the grid". Worse, there is no universal grid. There is no easy answer.

That's what makes it such an interesting topic, the mysteriousness of the frontier.

 

I think it will take an eventual unification of gravitation with quantum theory to answer your question. Such a theory has remained elusive.

I know what you're saying with this, but I cringe to hear it put this way because whenever people think of something as a "unification of paradigms," they either avoid touching it because they wouldn't want to appear professionally arrogant or it attracts excessive crackpottery just because it is a frontier issue. Maybe such crackpottery just comes with the path-breaking process, though, and maybe it actually helps valid theorizing along by illuminating potential avenues forward and their pitfalls.

 

The problem with "logical/philosophical" rather than mathematical formulations of and approaches to such questions, is that the "logical/philosophical" language of everyday experience is completely inadequate for questions of quantum theory and relativity which do not fit that everyday experience. The language of physics is mathematics. In order to understand it you must speak the language.

You shouldn't eschew logic and philosophy just because it doesn't include math. This is a discussion that happens over and over and it never goes anywhere except that everyone who's good at math goes on and on about it until those that aren't concede inferiority and the inability to do real physics. It's really not necessary to have this discussion again. There are many physics issues that can be dealt with using logical philosophizing. Just conservation of energy and mechanics can actually get you very far without any math. Obviously you're never going to be able to calculate anything without math but you can make sense of things such as why light has to have a finite speed to have limited energy or why elements on the left of the periodic table are more prone to losing electrons than those on the right, etc.

 

 

[DrRocket]

, are the details of gravitational wave propagation contained in other existing theories?

 

Chris

 

general relativity

[imatfaal]

You shouldn't eschew logic and philosophy just because it doesn't include math. This is a discussion that happens over and over and it never goes anywhere except that everyone who's good at math goes on and on about it until those that aren't concede inferiority and the inability to do real physics. It's really not necessary to have this discussion again. There are many physics issues that can be dealt with using logical philosophizing. Just conservation of energy and mechanics can actually get you very far without any math. Obviously you're never going to be able to calculate anything without math but you can make sense of things such as why light has to have a finite speed to have limited energy or why elements on the left of the periodic table are more prone to losing electrons than those on the right, etc.

 

 

 

But a science built upon solely philosophical principles and logic, but without maths and experimentation is wasteful and sterile. Whilst some beautiful extrapolations and theories can emerge they are a dead end - because being logical consistent and philosophically pleasing does not imply they will even approximate reality. You might say that some of the theories may be the breakthrough that mathematically-constrained science was unable to make - but again this fails; science without maths/experimentation has no way of testing itself apart form self-consistency. Whilst a non-rigorous approach may generate a delicate spectrum of elegant hypotheses; to test these we need to move back to the lumpen reality of maths and measurement. Conversely, the great breakthroughs of the twentieth century came from believing in the inviolability of the maths and experimentation and rejecting the dogma and restrictions of the more philosophical view (ie rejection of absolute space and time to create a system of transformations/IRs in which c is constant). I fail to see how the concept of conservation of energy (and your other examples) can really help to make progress without maths - sure it can produce an ephemeral surface veneer of knowledge - but without the knowledge of the intricacies it comprises this is nothing more than a pleasing aphorism that allows no progress.

Edited May 30, 2011 by imatfaal

[swansont]

You shouldn't eschew logic and philosophy just because it doesn't include math. This is a discussion that happens over and over and it never goes anywhere except that everyone who's good at math goes on and on about it until those that aren't concede inferiority and the inability to do real physics. It's really not necessary to have this discussion again. There are many physics issues that can be dealt with using logical philosophizing. Just conservation of energy and mechanics can actually get you very far without any math. Obviously you're never going to be able to calculate anything without math but you can make sense of things such as why light has to have a finite speed to have limited energy or why elements on the left of the periodic table are more prone to losing electrons than those on the right, etc.

 

We wouldn't have conservation of energy or mechanics without the math and experimentation to confirm it. We'd have the logic that it requires a force to move and that the natural state of anything is to be at rest. The logic and philosophy of Aristotle. We'd have four elements, and the earth would be at the center of the universe.

 

Logic is useless when the premise is incorrect.

[lemur]

But a science built upon solely philosophical principles and logic, but without maths and experimentation is wasteful and sterile. Whilst some beautiful extrapolations and theories can emerge they are a dead end - because being logical consistent and philosophically pleasing does not imply they will even approximate reality. You might say that some of the theories may be the breakthrough that mathematically-constrained science was unable to make - but again this fails; science without maths/experimentation has no way of testing itself apart form self-consistency. Whilst a non-rigorous approach may generate a delicate spectrum of elegant hypotheses; to test these we need to move back to the lumpen reality of maths and measurement. Conversely, the great breakthroughs of the twentieth century came from believing in the inviolability of the maths and experimentation and rejecting the dogma and restrictions of the more philosophical view (ie rejection of absolute space and time to create a system of transformations/IRs in which c is constant). I fail to see how the concept of conservation of energy (and your other examples) can really help to make progress without maths - sure it can produce an ephemeral surface veneer of knowledge - but without the knowledge of the intricacies it comprises this is nothing more than a pleasing aphorism that allows no progress.

I never said that I eschew experimentation and measurement. Philosophy and logic can be applied to design experiments/observations that settle philosophical disputes. The point is that reason and logic are methods of rigorous thinking that are as much a part of science as math and maybe more so. The thing is that it doesn't actually matter what is more or most essential to science, though, because each question generates its own discourse. When the discourse leads to experimentation and measurement to settle questions, there's usually no way to avoid that. When a critique/question arises involving reason/logic, you can't get around that by reference to math.

 

 

We wouldn't have conservation of energy or mechanics without the math and experimentation to confirm it. We'd have the logic that it requires a force to move and that the natural state of anything is to be at rest. The logic and philosophy of Aristotle. We'd have four elements, and the earth would be at the center of the universe.

 

Logic is useless when the premise is incorrect.

I agree that empiricism is central. There are, however, direct applications of philosophy/logic to empiricism that allow issues to be settled without counting or measuring something quantitatively. When the two objects were dropped from the tower of Pisa, for example, the question was whether they hit the ground simultaneously or not. You could also probably come up with non-quantitative experiments to falsify the aristotelian belief that continuous motion requires force-addition. The fact that satellites remain in motion without thrust is an example, or the fact that vehicles continue moving when you stop pushing them demonstrates inertia. Anyway, it really doesn't matter because reason and logic should stand on their own in practice or not. If an argument is rational/logical, it should be sufficient and if further empirical testing, measurement, and/or math is necessary to evaluate and argument, logic and reason can be used to make that clear.