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

  1. Apr 21, 2010 #1
    General Relativity Revisited (What if gravity actually is a force?)

    I will admit a few things. First, I suppose I am posting on here because the chances of provoking some discussion, and recieving some feedback are likely better than a few other places I would try. On the other hand, I am hopful of recieving some well informed responses, if I am lucky; I'm often surprised. That aside, perhaps I'll at least have the delight of communicating some ideas, and getting some benificial feedback.

    I basically understand what is meant by now when I hear about gravity not being a force. In other words General Relativity (GR) opts to model this phenomenon in terms of a curvature of the space-time fabric. I understand that this model works extremely well, and that it is, perhaps, even the best we have currently, or very close to it.

    Yet, it hardly seems fair that now Gravity basically falls down into the category of a pseudo-force, while the electromagnetic interactions still get to sit on that noble pedestool of being a real force. Granted, that is a rather 'emotional' type of statement, but there is a 'reason' I have this sense of dissatisfaction with GR.

    Let's talk about force. My current understanding of force is that it is, literally, is the change in mechanical momentum of a system over time. Noted, there are other forms of momentum which are not mechanical, such as those which may be stored in electromagnetic fields, but force particularly does NOT pertain to those changes, rather it purely belongs to the changes in 'Mechanical momnetum.' Are we to believe that gravity can not itself be the source of changes in the mechanical momentum of a system? The anticipated, typically educated, answer goes something like, "Well, according to the equivalence principle of GR, it cannot." In other words a body following its natural path under the influence of gravity cannot "feel" this effect.

    I understand that is a nice idea. But, as romantic as it is, and as much as that makes sense 'in a way,' I am not satisfied. Why should we believe, a priori, that gravity cannot impart mechanical momentum on a body, and that this is soly up to the electromagnetc interactions? The equivalence of gravitational and inertial mass? Pshhhhhaw.

    I know that the response to my 'a priori' question might go something like this. The effects of the predictions of GR have been experimentally measured through and through, and Gravitational lensing proves that the presence of mass curves the fabric of space time etc...why are you even going on? Well, for one, I have an inquisitive spirit, and I also believe that scientific theories are meant to be probed and questioned; someone like Richard Feynmann might say that is the nature of the game.

    More importantly, let's rewind. GR succesfully models gravitation in terms of curvature in space-time, and through some means (that few of the brightes physicist could possibly reproduce) arrives at that formulation from, well I'm not even completely certain right now, but let's just say Special Relativity, and incorperating the equivalence principle, roughly speaking. But people, THIS IS A MODEL. There are many models that make succesful predictions in their appropriate realms. What if the bending of light by gravitational fields had been discovered prior to GR's creation? If I were not already biased by the beutifully compelling concept of space curvature, my conclusion would NOT be that such 'lensing' proves spacetime is curved.

    I would simply take that to be definitive proof that the GRAVITATIONAL FIELD INTERACTS WITH THE ELECTROMAGNETIC FIELDS (afterall light is an electromagetic field propogatin in the form of a wave and gravity bends that, right?) The funny thing is that the converse of that statement is (and such symmetries are all to present in the the world) the Electromagnetic Field interacts with the Gravitational field. For some reason though, I never hear it put so simply. Instead we must do the space time is curved dance, never mind Occam's Razor. (Side note: I would almost expect, on such simple grounds, there to be way more research, and grants funding said research into how the Electromagnetic field interacts with the Gravitational Theorem, than I'm aware of currently, yes...with repect from the Scientific establishement and all.)

    Why not entertain the idea that gravity IS a force and could hence contributes to the mechanical momentum of a system, just as electromagnetism can. Said symmetry alone to me is very intriguing. Perhaps this effect is so small we simply haven't yet measured it, but that basically makes sense considering how weak the Gravitational field is compared to the Electromagnetic field. If there is anyone that has any input or feed back on this, or is aware of current research going on that is related, please let me know. Thanks.


    (BTW- I bring this up as an effort to provoke discussion on the topic, and not to claim conlusions based on any experimental knowledge, proprietary material, or sensitive information.)
     
    Last edited: Apr 21, 2010
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  3. Apr 21, 2010 #2

    Mentz114

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    Force causes acceleration, acceleration is a change in momentum. So gravity can be modelled either as a force field or an acceleration field. If the equivalence principle is true, a gravitational force field is an acceleration field.

    There is atheory of gravity that does not assume the equivalence principle, but gives the same predictions as GR if the EP is assumed. It also has no spacetime curvature.

    It doesn't actually matter if you model gravity as a force field or an acceleration field.

    You still fall down when you trip.:biggrin:
     
  4. Apr 21, 2010 #3
    Hmmm, interesting thought, but not entirely what I was getting at. Here's an thought that could interest you. Acceleration is not neccesarily a change in momentum, for intance there must be a 'mass' present. By analogy an electric field causes an acceleration of charge, however is not an acceleration field because the same charge can have a different mass and therefor accelerations will be different. Anyway thanks for your thoughts.
     
  5. Apr 21, 2010 #4

    Mentz114

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    I think it is.

    There always is. What else can be accelerated ?


    Are there any massless charged particles ?
    Which is why the Lorentz force is a force, not an acceleration

    They were entirely wasted since you've dismissed them.
     
  6. Apr 21, 2010 #5

    Actually I did NOT dismiss them. I don't know what would make you assume that. I actually said they were interesting explicitly, but that I didn't believe it was exactly hitting the nail on the head as far as my inquiry, and even said something I thought would interest you. I'm sorry if your upset by such. If anyone has any other view points or information I envite you to share it. Thanks.
     
    Last edited: Apr 21, 2010
  7. Apr 21, 2010 #6
    From a theoretical stance,YES; I believe it is important to analyze such concepts, just as we have the theoretical machinery to treat a massive electrically neutral body, we should just as well be able to deal with a massless electrically charged particle in principle.
     
  8. Apr 21, 2010 #7

    Mentz114

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    Massless particles travel at the speed of light and cannot be accelerated.

    Might interest me ? That's patronizing coming from someone who thinks massless charges exist and can be accelerated.

    The truth is, mate, in your waffly piece you haven't actually said anything worth discussing.

    If you search this forum you'll find a lot of hot-air about gravity being a force or a pseudo-force or whatever. It isn't important, it's just semantics.

    This statement seems completely off the wall. When you drop something does its momentum in your eyes not increase ?

    If you want to know about similarities between the gravitational field and electric field there's a wealth of literature on the subject on the internet.
     
  9. Apr 21, 2010 #8
    You are missing the point. Please post elsewhere, and stop cluttering my thread.
     
  10. Apr 21, 2010 #9
    I am not sure of the answer to your question, but here are some thoughts to add to the pot. In the Newtonian context, an body in circular orbit has the gravitational force balanced by an equal and opposite centrifugal force. One problem with this definition is that Newtonian physics also states that a body continues in a straight line if no forces are acting on it. If the centrifugal force exactly cancels out the gravitational force, then the orbiting body should be moving in a straight line and not in circle. This is at the root of banishing centrifugal force as a real force. In GR, both gravitational force and centrifugal force are treated as pseudo forces and the orbiting particle simply follows a geodesic (which is the equivalent of a straight line in curve space), when no forces are acting on the particle. For a point particle it is difficult to prove if no forces are acting on it or if all the forces are cancelling each other out. For a large orbiting body, not all the composite parts of the orbiting body can follow their individual geodesics simultaneously and as a result the orbiting body can be torn apart by tidal forces at the Roche limit. Those tidal forces are real enough! When a particle is forced to deviate from its geodesic the gravitational force becomes real. I am not sure how one would go about proving that there are no forces acting on a body in freefall other than defining force as something that is measured by an accelerometer and when something is in freefall an accelerometer will show a zero reading. Maybe Mentz is right and maybe it is just a case of interpretation and either viewpoint is correct. 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 as well. At the moment there are a couple of other threads going on in this forum, where we can not even agree if the gravitational force acting on a horizontally moving particle is independent of the particle's kinetic energy or not.
     
  11. Apr 21, 2010 #10

    Mentz114

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    Sorry you're not satisfied. Do you actually think that gravity does not impart momentum ?
    Pshhhhhaw to you too.

    You haven't understood a single word I've written, and you're talking nonsense.

    Your problem is you don't understand the subject you're talking about, but you have prejudices which you are arrogant enough to think override centuries of thinking.
     
  12. Apr 21, 2010 #11
    My problem is that I have done much advance study in the area, and that you continue to harrass and provoke arguments. As I said, please stop posting here, that will simply lead to further reports.
     
  13. Apr 21, 2010 #12

    JesseM

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    Re: General Relativity Revisited (What if gravity actually is a force?)

    Not in the Kaluza-Klein theory which attempts to explain electromagnetism in terms of 5-dimensional spacetime curvature (with the extra spatial dimension wrapped into a microscopic circle so we don't notice it). As I understand it this approach has in some sense been incorporated into string theory, which attempts to unify all four forces (and thus can't treat them as fundamentally different sorts of things).
    Can you define "momentum" without reference to inertial frames? After all momentum is usually treated as a function of velocity in inertial frames. But in GR there are no global coordinate systems in large regions of spacetime (like a region that's large enough to observe the bending of light by a star) that satisfy the requirements of an "inertial frame", though you can talk about locally inertial coordinate systems in any small region where the curvature can be treated as negligible (thanks to the equivalence principle). Such a local inertial frame can be defined on the tangent space to a point in GR which I believe is how the momentum vector would normally be defined in a GR context. And in such a local inertial frame I'm pretty sure the light won't experience any change in momentum.
     
    Last edited: Apr 22, 2010
  14. Apr 22, 2010 #13

    Vanadium 50

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    It's not "your thread". Everyone is free to participate.
     
  15. Apr 22, 2010 #14

    Mentz114

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    I wasn't going to post here again, but you've talked me into it. I am not harassing you, I'm having a discussion which means I am allowed to ask you questions - which you have ignored.

    You seem to have an objection to curved space, because you think it models gravity in a way that deprives it of its force-like nature. That isn't true. As you have said 'it's a model' - so why are you taking it literally ? In my first post I pointed out that gravity can be modelled as a force, and that with the added assumption of the equivalence principle, these models make the same predictions as GR.

    If the EP is assumed to be false, the models do not make predictions that agree with observation. The EP is supported by experiment and observation, and is NOT a 'romantic' idea !
     
  16. Apr 22, 2010 #15

    D H

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    That is true only if you look at the orbit from a perspective of a rotating frame in which the planet is not moving.
    There is no problem. The planet is not moving in this frame. Stationarity is the trivial case for Newton's first law.

    If you are going to pick on Newtonian physics, do it right.
     
  17. Apr 22, 2010 #16
    Even if their speed is constant, they can change direction, which is an acceleration. And since massless particles can still have momentum, you could also still talk about a force on them starting from the definition F=dp/dt.

    I find it strange that you and shane's comments quickly became so adversarial.

    Shane,
    To help drive home Mentz comments, note that Newtonian gravity can be rewritten as a metric theory. Does this no longer make gravity a force in Newtonian mechanics? just because we can write it with other mathematical representations?

    Basically, to answer your question "What if gravity actually is a force?" we need to first come to some agreement on what you even mean by this. In trying to progressively make the question more precise, it is quite likely that will answer the question itself. Either way, this discussion needs to be had, otherwise people will only be talking past each other instead of with each other.
     
  18. Apr 22, 2010 #17

    Dale

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    Re: General Relativity Revisited (What if gravity actually is a force?)

    I would just like to point out that "works extremely well" is the one and only scientific criterion for judging a theory. It doesn't matter if it is "fair" or any of your other complaints; scientifically they are all completely irrelevant. The only scientific concern is that it accurately predict the results of experiments.
     
    Last edited: Apr 22, 2010
  19. Apr 22, 2010 #18

    Mentz114

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    Point taken - for now:smile: ( Angular momentun is an axial vector. The field would be doing work on a massless object ... hmmm).

    But we don't know of any massless charged particles.

    The guy did not understand what I said and lectured me. The phrase 'something you might like to know' is just polite for 'you've got it wrong (fool) because ...' followed by an elementary lecture.

    I was actually saying this

    but perhaps not as elegantly. Let's see if he ignores you as well because of his 'unsatisfaction'.
     
  20. Apr 22, 2010 #19

    JesseM

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    Re: General Relativity Revisited (What if gravity actually is a force?)

    While true, it is also true in the history of physics that "elegance" of some kind has often been a motivation for physicists when looking for new theories, and it doesn't seem unreasonable to hope a "theory of everything" would put all four fundamental forces on the same footing.
     
  21. Apr 22, 2010 #20

    Dale

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    Re: General Relativity Revisited (What if gravity actually is a force?)

    True, but such considerations are philosophical rather than scientific, which is why I emphasized the word "scientific" in my post. E.g. in the SR vs. LET debate, both are equally valid scientifically since each predicts the same experimental results in all cases, but SR is generally preferred on philosophical grounds like Occham's razor. You can certainly use philosophical or aesthetic considerations to select between two theories which have equal experimental validation, but you cannot reject an experimentally validated theory on mere philosophical grounds.
     
    Last edited: Apr 22, 2010
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