General Relativity Revisited (What if gravity actully is a force?)

[shanesworld]

Shane,
You have a way of asking questions that really irritates me.

I've noticed.

the above - why did you add the part I've underlined ? That means if I say 'no' to the question I'm implicitly agreeing with the rider. Are the electromagnetic effects in the rider supposed to be replacing the gravitation or in contrast to the gravitational effects ?

Could you please humor me and explain what you are referring to by 'rider' , so I can address your question?

Ignoring the rider, I would say that gravity can change the mechanical momentun but it's frame dependent.

Might agree, but still not sure what you mean by rider.

As to whether Gravity ( not the modeled variety but the actual thing ) is a force - it's pretty obvious that it is in my frame of reference.

Hmmm
If you could somehow fill your body with charged particles and stand next to an electric field, would it feel the same as gravity? Crazy thought of the day.

Something you might like to know is that gravity can be modeled as a force in close analogy to the EM field. As you know, the Lorentz force can be shown to arise purely by insisting on local phase invariance ( symmetry group u(1) ). Gravity can be shown to arise by insisting on local translational invariance. This is very neat because space and time translation have conserved currents, energy and momentum, which are the sources of gravity. This theory does not incorporate the equivalence principle as does GR, but if the EP is added to the gravitational gauge theory, we end up with the same field equations as GR.

Interesting.

The fact that GR incorporates the EP and is thus able to geometrize a force field into an acceleration field has no physical significance whatever.

no physical significance=no new testable predictions :zzz:

What is significant is whether gravitational mass and inertial mass are one and the same.

Agreed.

 

[Mentz114]

Might agree, but still not sure what you mean by rider

I mean the part of the sentence after the comma. I'm using the word in the sense of something 'tacked on'.

RIDER. A schedule or small piece of paper or parchment added to some part of the record; as, when, on the reading of a bill in the legislature, a new clause is added, this is tacked to the bill on a separate piece of paper, and is called a rider.

Source: Bouvier's Law Dictionary, Revised 6th Ed (1856)

I suppose it is rather obscure.

If you could somehow fill your body with charged particles and stand next to an electric field, would it feel the same as gravity? Crazy thought of the day.

My body is already made of gravitational charge.

You keep making associations between EM and gravity. Are you of the opinion that they are related ? Perhaps that gravity is actually the result of some electromagnetic phenomenon ?

 

[JesseM]

I'm not entirely sure of the answer to this question, but I can provide my best take on it. Amazingly, momentum is one of the more abstract concepts in physics. With basic physics, it seems like such a simple concept. But at a certain point its definition changes, and changes, and then one might ask, what exactly IS momentum? My best answer to your question, and I could be wrong, is yes you can define momentum without reference to inertial frames in some cases, for instance in quantum theory particles have intrinsic angular momentum, and fields can store momentum. I don't believe that depends on inertial frames (once again I think I have that correct, but perhaps there are other perspective).

The spin of particles in QM might be a special case that doesn't depend on a choice of reference frame, but I'm pretty sure that any textbook equation involving ordinary linear momentum of particles in QM, as well as any momentum stored by quantum fields, will be written with the assumption we are using an inertial frame (unless explicitly stated otherwise). Certainly the laws of quantum field theory are said to be Lorentz-symmetric, meaning they obey the same equations in the different inertial frames of special relativity.

I'm reasonable certain there are more complex issues that I won't try to get into right now, as well. I'm sure there are better answers than mine, perhaps amongst items like Noether's theory,which is also quite abstract.

Noether's theorem relates conservation of momentum to the fact that the laws of physics are invariant under spatial translation, but again I'm pretty sure this symmetry assumes inertial frames, in an arbitrary non-inertial frame the way a given physical system (a clock, say) behaves can vary with position.

Just saying "there are more complex issues" is very hand-wavey. If you want to say that gravity affects momentum, you need to actually have a clear definition of momentum in mind!

I suppose there is one more thing I would add light carries momentum, but not mechanical momentum. So no, 'force' is imparted upon light if its direction is changed, although technically speaking the momentum is still changed.

Light's momentum is still defined in the context of inertial frames--do you think any of the textbook equations for light's momentum such as p=hf/c would work in arbitrary non-inertial frames? And do you disagree that in a locally inertial frame in GR, by definition light would not be measurably deflected by gravity?

 

[shanesworld]

I mean the part of the sentence after the comma. I'm using the word in the sense of something 'tacked on'.

Ok, then 1) I might bring up that we are talking about a system where there is possibly some kind of combined effect, for instance the dynamics of a proton in gravitational field, where there may be an electric field present. 2) If you say No, then you ARE implicitly agreeing with the rider, there is no way around that in the context...(bearing in mind, the question is presented in the context of how this would be dealt with in GR specifically)
...

You keep making associations between EM and gravity.

These associations are for two reasons, 1) For the sake of analogy 2) We are discussing how the presence of combined effects would be addressed in GR, and perhaps in alternative frameworks.

Are you of the opinion that they are related ? Perhaps that gravity is actually the result of some electromagnetic phenomenon ?

NO, I have no affirmative or negative opinion on that matter at this point... it would seem natural to inquire if any such relations are present though...in itself that doesn't mean they are though; it also doesn't initially assume the negative. A unification approach would be one that sought to analyze each as different aspects of the same phenomenon, right?

 

[Dale]

Noether's theorem relates conservation of momentum to the fact that the laws of physics are invariant under spatial translation, but again I'm pretty sure this symmetry assumes inertial frames, in an arbitrary non-inertial frame the way a given physical system (a clock, say) behaves can vary with position.

Noether's theorem is a very general result and does not require an inertial reference frame. In fact, it can be applied to very weird coordinate systems. If a Lagrangian is invariant under a differential shift of some generalized coordinate you would then have conservation of the corresponding generalized momentum. This would not necessarily be equivalent to any component of momentum as defined in an inertial frame using orthonormal coordinates, but whatever the generalized momentum is would be conserved.

 

[JesseM]

If a Lagrangian is invariant under a differential shift of some generalized coordinate you would then have conservation of the corresponding generalized momentum.

But in an inertial frames the Lagrangian for any possible physical system will be invariant under a shift of the position coordinate, right? I imagine this wouldn't be true in general for arbitrary systems analyzed in arbitrary non-inertial frames?

 

[Frame Dragger]

Ok, then 1) I might bring up that we are talking about a system where there is possibly some kind of combined effect, for instance the dynamics of a proton in gravitational field, where there may be an electric field present. 2) If you say No, then you ARE implicitly agreeing with the rider, there is no way around that in the context...(bearing in mind, the question is presented in the context of how this would be dealt with in GR specifically)

Are you in any way related to Conway? Anyway, what kind of combined effects? This sounds like a hodgepodge of your personal views of what MIGHT be discovered at some later date, sans math or citations, or even a clear mission. I've noticed that what began as (a fairly crudely and nastily put) question has evolved into something else entirely once you didn't receive the feedback you wanted/expected. As for #2... given how long you've been on conjecture and how short you've been (with others here) anything concrete, I wouldn't throw stones.

These associations are for two reasons, 1) For the sake of analogy 2) We are discussing how the presence of combined effects would be addressed in GR, and perhaps in alternative frameworks.

Alternate frameworks? Such as? That would be a "Beyond The Standard Model" bit, far removed from anything you're actually talking about. This is physics, so vague hints at a framework you have deigned not to share is singularly unilluminating.

NO, I have no affirmative or negative opinion on that matter at this point

Or, in common parlance: "I have no idea whatsoever", or, "I have a notion, AND an agenda."

... it would seem natural to inquire if any such relations are present though...in itself that doesn't mean they are though; it also doesn't initially assume the negative. A unification approach would be one that sought to analyze each as different aspects of the same phenomenon, right?

This is a deflection. Each time you're challenged by Dalespam, Vanadium, or Mentz you retreat to a slightly different position, from which you then begin to make similar "points". This fits some patterns I've seen here (and elsewhere), which end badly. If this is just a reflection of the same attitude that has you and Mentz at daggers-drawn, maybe you could discuss matters in a manner that seeks to provoke THOUGHT, not merely a reaction.

 

[HarryWertM]

A very primitive side question here. From Kev's post no. 9:

It might also be worth noting that the GR equations for orbital motion contains terms that look like Newtonian gravitational force opposed by centrifugal force, but there is an additional term that that is unique to GR

Kev, could you give a link to show exactly what you are talking about? Most every website concerning GR displays general vector-tensor PDE's and it is impossible to find anything comprehensible as you have implicated.

P.S. Kev; Your contribution to this thread:
https://www.physicsforums.com/showthread.php?t=398358"
is so beautiful it makes the rest of the thread a waste of time.

 

[Frame Dragger]

A very primitive side question here. From Kev's post no. 9:

Kev, could you give a link to show exactly what you are talking about? Most every website concerning GR displays general vector-tensor PDE's and it is impossible to find anything comprehensible as you have implicated.

I believe this will be useful. https://www.physicsforums.com/showthread.php?t=170293

EDIT: Heh, Kev is damned good isn't he? He's one of the guys/gals here who routinely spills concepts and math that make my noggin ache in a good way.

 

[Frame Dragger]

I mean the part of the sentence after the comma. I'm using the word in the sense of something 'tacked on'.



I suppose it is rather obscure.

My body is already made of gravitational charge.

You keep making associations between EM and gravity. Are you of the opinion that they are related ? Perhaps that gravity is actually the result of some electromagnetic phenomenon ?

Rider shouldn't be obscure to anyone... at least, no one who signs a contract in their life should be unaware of the concept. "The Doctrine of Stale Demand" aka Laches... now that is an obscure term of art. Rider actually makes sense, even without inserting it as a term of art. Ergo... you shouldn't be the one aplogizing...

 

[Dale]

But in an inertial frames the Lagrangian for any possible isolated physical system will be invariant under a shift of the position coordinate, right? I imagine this wouldn't be true in general for arbitrary systems analyzed in arbitrary non-inertial frames?

With the caviat inserted in bold, I agree.

 

[HarryWertM]

From Shanesworld post no.

My question remains: in gravitational theory, does Gravity itself have the capability of contributing to changes the mechanical momentum in a system, or is that entirely up to electromagnetic effects? If gravity can contribute to such effects then Gravity IS a Force.

Consider: Two uncharged neutron stars are in binary orbit. They emit gravitational waves. They lose momentum.

It appears to this civilian that everyone, even the most expert, constantly make mistakes when using English, or 'natural' language. Maybe not with math, but constantly with natural language.

 

[Frame Dragger]

With the caviat inserted in bold, I agree.

This is my own lack of understanding here: there is no special meaning of "isolated" beyond the norm for a system in Relativity, right?

P.S. Caveat... I think you might be in the mood for caviar! If so, I recommend it be taken upon blinis, with creme freche and pepper vodka (none of the off-the-shelf **** either) and don't be afraid of sevruga or osetra, beluga isn't the only... um... fish in the sea. Ok, technically the fish is the sturgeon, but I give no respect to dinosaurs that masquerade as fish. Don't like caviar?... how about the same deal, but with salmon and some fresh chive?


P.P.S... Damn.. I think I'm hungry.

 

[Frame Dragger]

From Shanesworld post no.


Consider: Two uncharged neutron stars are in binary orbit. They emit gravitational waves. They lose momentum.

It appears to this civilian that everyone, even the most expert, constantly make mistakes when using English, or 'natural' language. Maybe not with math, but constantly with natural language.

Ok, you're right on the physics, but where do you see the mistake in the use of language? Remember that physics is a pastiche of past and present, including the language. People then demand it all "in 'english' not math", but so many of these concepts only start to make sense WITH the math. Math is the language of physics... it's not trite, but true.

language is far more approximate, and limited, and less precise.

 

[Dale]

This is my own lack of understanding here: there is no special meaning of "isolated" beyond the norm for a system in Relativity, right?

Correct.

P.S. Caveat... I think you might be in the mood for caviar!

 

[Frame Dragger]

Correct.

Ok, got it. I just wanted to make sure I wasn't going down the old "left hand path" on this one. As for caviar... I'm going to make a damned sandwich... I'm REALLY hungry!

 

[Dale]

My wife has a lasagne in the oven, so fish eggs hold no appeal right now!

 

[shanesworld]

Just saying "there are more complex issues" is very hand-wavey. If you want to say that gravity affects momentum, you need to actually have a clear definition of momentum in mind!

Light's momentum is still defined in the context of inertial frames--do you think any of the textbook equations for light's momentum such as p=hf/c would work in arbitrary non-inertial frames? And do you disagree that in a locally inertial frame in GR, by definition light would not be measurably deflected by gravity?

Thanks for your insights. I disagree about that being hand wavy. 1) It is a fact, there are more complex issues. 2)I am not getting into them. QED

I wholly agree about having a clear definition of momentum in mind, in fact that is just the nature of the inquiry.

Do I think any textbook equations for light's momentum such as p=hf/c would work in arbitrary non-inertial reference frame? Yes. I'm pretty sure pushing the gas pedal on my car does not change the momentum of light.

 

[shanesworld]

From Shanesworld post no. Consider: Two uncharged neutron stars are in binary orbit. They emit gravitational waves. They lose momentum.

It appears to this civilian that everyone, even the most expert, constantly make mistakes when using English, or 'natural' language. Maybe not with math, but constantly with natural language.

That is NOT from my post. Hmmm...maybe I'm not getting the From Shanesworld no. clause at the beginning?

 

[Frame Dragger]

Thanks for your insights. I disagree about that being hand wavy. 1) It is a fact, there are more complex issues. 2)I am not getting into them. QED

I wholly agree about having a clear definition of momentum in mind, in fact that is just the nature of the inquiry.

Do I think any textbook equations for light's momentum such as p=hf/c would work in arbitrary non-inertial reference frame? Yes. I'm pretty sure pushing the gas pedal on my car does not change the momentum of light.

So why are you here, talking about this, and expecting that you would get anything in return? "A cat for a hat, or a hat for a cat, but nothing for nothing."

 

[shanesworld]

... Anyway, what kind of combined effects? This sounds like a hodgepodge of your personal views of what MIGHT be discovered at some later date, sans math or citations, or even a clear mission. I've noticed that what began as (a fairly crudely and nastily put) question has evolved into something else entirely once you didn't receive the feedback you wanted/expected. As for #2... given how long you've been on conjecture and how short you've been (with others here) anything concrete, I wouldn't throw stones.




Alternate frameworks? Such as? ... This is physics, so vague hints at a framework you have deigned not to share is singularly unilluminating.




Or, in common parlance: "I have no idea whatsoever", or, "I have a notion, AND an agenda."

Addressing this is simply to tempting to pass on. Oh well

1) 'Alternative framework' is simply the basis for comparing ANY theory to something else, besides its own content. Without such there is simply no external thesis or antithesis with which to conduct such analysis. Ex Theory1: 2*5=10 Theory 2: 2*5=(-2)(-5) Theory 3: 2*5=5+5. Conclusion: Theory 1 is consistent with the alternative frameworks of theories 2 and 3. I am not "hinting" towards any particular framework, simply making reference to the smorgasbord of any possibly alternative framework by which GR may be compared.

2) Combined effects hodgepodge? If you didn't read first paragraph very carefully, very specifically defined an example of the kind of thing I'm referring, which would classically speaking be adressed by, F=mg+qE a.k.a 'combined effects of both a gravitational source and an electric force on particle with electric charge q and gravitational charge m in static limit, what is so outlandish here?

3)Who's throwing stones?

4) 'I have no opinion is simply' saying I am not exercising any preconceived notions about what experimental results on such logical analysis/ inquiry would be at this time, it's probably better to take such statements as having no opinion'more literally, than trying to make it into something different that it is not.

 

[Mentz114]

Shane,

all your answers are vague and hand-wavey. You have shown no understanding or insight in this thread and ignored pretty much everything told to you. Now you want to define momentum 'one of the most abstract and subtle concepts ...' which is a change from your 'revisiting' of GR and 'gravity is a force' ( why 're-visit ? Have you been away ?) .

Regarding momentum, you could have mentioned that translational invariance implies conservation of momentum, or that momentum is a source of gravity, or Hamilton's conjugate momenta - but, you never say anything ! Are you a politician by any chance ?

[This was posted while you were posting, darn it]

I've read your post. The only thing worth commenting on is the idea of comparing GR to something ( a smorgasbord). Comparing GRs predictions with the facts is what matters. If another theory gave better predictions, it would be adopted.

 

[Frame Dragger]

Addressing this is simply to tempting to pass on. Oh well

1) 'Alternative framework' is simply the basis for comparing ANY theory to something else, besides its own content. Without such there is simply no external thesis or antithesis with which to conduct such analysis. Ex Theory1: 2*5=10 Theory 2: 2*5=(-2)(-5) Theory 3: 2*5=5+5. Conclusion: Theory 1 is consistent with the alternative frameworks of theories 2 and 3. I am not "hinting" towards any particular framework, simply making reference to the smorgasbord of any possibly alternative framework by which GR may be compared.

2) Combined effects hodgepodge? If you didn't read first paragraph very carefully, very specifically defined an example of the kind of thing I'm referring, which would classically speaking be adressed by, F=mg+qE a.k.a 'combined effects of both a gravitational source and an electric force on particle with electric charge q and gravitational charge m in static limit, what is so outlandish here?

3)Who's throwing stones?

4) 'I have no opinion is simply' saying I am not exercising any preconceived notions about what experimental results on such logical analysis/ inquiry would be at this time, it's probably better to take such statements as having no opinion'more literally, than trying to make it into something different that it is not.

That's what I thought... Pure sophistry without a hint of physics. Tiring... you really should read the rules you agreed to when you joined.

 

[shanesworld]

Shane,


Regarding momentum, you could have mentioned that translational invariance implies conservation of momentum, or that momentum is a source of gravity, or Hamilton's conjugate momenta - but, you never say anything ! Are you a politician by any chance ?

(Damn it ,this whole forward slash thing keeps throwing me off with respect to the backslash thing in TeX..)
..Actually I'm a physicist by profession. From here, it appears there is a lack of understanding amongst many or the people whom have replied here...