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Just how many different theories of gravity are there?

  1. Dec 25, 2007 #1

    DSR

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    Hello.

    I’m new to this forum and I’m unsure of the best way to start out. Can I just jump in and begin a new thread? I was just wondering about something and am interested to see if anyone might think this is at least worth note and have something to say.

    Just how many different theories of gravity are there? And how many of them are actually accepted by the scientific community other than General Relativity (GR)? How many of them actually try to explain gravity in terms of simpler phenomena? In the same way that magnetism was explained as electricity plus special relativity creating electromagnetism, can gravity be explained in terms of other well understood phenomena? A theory such as GR provides a good mathematical description of gravity. It can be interpreted as describing gravity as a curvature of spacetime, but then, although it provides a good description, there’s something unsatisfying about considering spacetime as a physical object.

    Gravity can be described, like the other forces, as an exchange of bosons – gravitons in this case, but gravitons have not been detected yet.

    String theory, or M-theory, envisions strings that span higher multiple dimensions, but it sort of seems like this is borderline mysticism in that how can you observe these higher dimensions, or even the strings upon which everything else is supposed to be built?

    Has anyone heard of Stochastic Electrodynamics (SED) which tries to explain gravity in terms of an electromagnetic interaction of the zero-point field? At least that one has the advantage of describing gravity in terms of other fundamental phenomena of electromagnetism and zero-point energy.

    Is “Mach’s Principle”, where the inertia of a body is determined in relation to all other bodies in the universe, of any use at all as an explanation of gravity? It seems a little vague. How would you make an experiment to test this?

    Could gravity be explained using Relativity where an accelerating atom appears distorted due to its motion and that distortion makes the atom appear electrically polarized? The other accelerating atoms nearby “see” the electric field and an attractive force. Thus gravity is reduced to an electric force.

    Or if you think of the “cloud” that is the electron shell as extending far beyond each atom, there is a small probability of it extending near the nucleus of an atom of an adjacent object. The positively charged nucleus is concentrated over a very small space compared to the electron shell being smeared out over a larger volume, which intersects only a small part of the adjacent electron shell when they are far apart leaving a small net attractive force between the atoms. This would also reduce gravity to an electric force. When the atoms are close then the repulsion of the electron shells will overwhelm any other force between them, which is what accounts for the apparent “solidity” of an object that we observe. While the probability of this kind of interaction may be extremely low and the resulting net force low, it will add up when there are a large number of interacting atoms. When you consider the number of atoms in something the size of the Earth and a person you are still only left with a force of only a couple hundred pounds between them. What do you think?

    How many other theories about gravity are there out there?

    Thanks.
     
  2. jcsd
  3. Dec 25, 2007 #2

    pervect

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    GR is currently the "gold standard" as far as theories of gravity go. While there are/were quite a few other proposals (Branse-Dicke, Noretveldt, etc.) they are for the most part more complex than GR, with tunable parameters that have to be tuned near the GR values to be compatible with experiment.

    You don't have to consider spacetime to be a physical object for it to be curved. All you need to consider is that gravity acts uniformly on all types of matter (and energy) - the equivalence principle. If this is true, then by virtue of acting uniformly on everything, you can't really detect gravity as a force in the usual manner. For instance, for an electric field, you can define the field at a point by virtue of the fact that there are neutral charges, so you can compare the motion of a neutral charge to a positive test charge. With gravity, there is no known equivalent of "neutral charges", so you can't characterize gravity via a force. There is also the fact that gravity causes gravitational time dilation - this cannot easily be accomodated without using the geometrical approach. Electric fields don't cause clocks to tick at different rates, but gravity does, so it's more than just a "force". This can be traced back to the fact that gravity affects everything equally.

    Careful. It is possible that gravity may be described as an exchange of virtual bosons, but it's not entirely clear yet if this will work. The best forum for this sort of question is outside the scope of this forum, youll get the best answer in a different forum. See for example life without the gravition in the "Beyond the standard model" forum (it may be a bit technical).

    Note that even if gravity can be described by an exchange of bosons (this is apparently an open question except in the weak field where it comes out of perturbative theory) that real boson exchange would correspond to gravitational waves, just as real photons correspond to light. Virtual photons would describe the force between charges, and virtual gravitions would describe the force between masses, *if* the idea of bosons works at all outside the perturbative formalism.

    This is a fringe theory at best, a crank theory at worst. For instance, you don't usually hear grad students giggle when the names of prominent scientists are mentioned, but I have seen them giggle when they hear the name Puthoff and Haisch (two proponents of SED according to the wikipedia). This should give you an indication that there may be some respect issues here. Since *some* papers by these two authors are published in respectable journals, it's within PF guidelines to discuss it, but you may have trouble finding anyone who knows a lot about it. You'll also have to be careful to keep the discussion focussed on specific claims by these authors that have been published in peer reviewed journals - one of the reasons they take some flack, aside from the novelty of their theories, is that they publish a lot of stuff that isn't good enough to make it into the journals (as well as some stuff that is).

    The current wikipedia seeems to have some good comments, including a quote by Unruh. But you'll have to check out the details for yourself as always with wikipedia.

    http://en.wikipedia.org/w/index.php?title=Stochastic_electrodynamics&oldid=144573967


    That's the 64 dollar question. I regard Mach's principle as a philosophical guiding idea, not a scientific principle. Some people like it more than I do, it has inspired at least one alternate gravity theories like Garth's Self Creation Cosmology (SCC). Unless there is a big surprise and upset in the current GPB results, SCC is due to be falsified any time now by the GPB experiment (though I think I heard it could be saved by adding tunable parameters).

    SCC is defintely a dark-horse candidate as far as a theory goes.
     
    Last edited: Dec 25, 2007
  4. Dec 25, 2007 #3
    A discussion of Stochastic Electrodynamic (SED) theory is found at this link, with a number of links to published papers:
     
  5. Dec 25, 2007 #4

    russ_watters

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    How is electromagnetism a simpler phenomena than gravity? Seems to me that with electric currents, magnetic currents, electromagnetic radiation, electrochemistry, etc., it is pretty complicated.
     
  6. Dec 26, 2007 #5

    Garth

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    You may be interested in the latest post I made about the GP-B results: Inconsistent with GR to 1 [itex]\sigma[/itex].

    Garth
     
    Last edited: Dec 26, 2007
  7. Dec 26, 2007 #6

    haushofer

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    Gravity is described by a tensor field, namely the metric tensor which is a (0,2) type of tensor. Electrodynamics is described by the vector potential, which is a one-form, or an (0,1) type of tensor. Aside from some QED effects, the Maxwell equations are linear, whereas the Einstein equations are non-linear; gravity "self-interacts". This makes the equations very nasty to solve; with two solutions we can't construct a new solution by simple addition.

    Thereby, gravity is another kind of "force" then the electromagnetic force; gravity is described by the curvature of space-time, whereas electrodynamic forces just take place in that space-time.

    I think these are some arguments :)
     
  8. Dec 27, 2007 #7

    pervect

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    Note that the "discussion" on this site has a highly positive bias towards SED. But the papers given on this website are a somewhat useful as a reference to the pro-SED literature. The site would be considerably more useful if it were more neutral, however.

    The wikipedia article in the version I quoted actually gives a much better overall description of the standing of SED in the scientific community than this webiste does, (IMO).

    I don't know if Unruh's counter-argument has been published in detail elsewhere, but to me it sounds like a remarkably simple and serious obstacle to people who want to believe in SED.
     
  9. Dec 27, 2007 #8
    It's still all just theories isn't it? I mean, there doesn't seem to be a real explanation for any of the "force at a distance" types, like gravity or magnetism. There's plenty for formulas that describe how they work, but I've never really read anything that actually describes what it is.

    Which might be impossible since we can only see the effect of the forces, not the actual force itself.

    Don't know if I made any sense here, English isn't my first language ;)
     
  10. Dec 27, 2007 #9

    pervect

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    While it is "all just theories", GR is the current favorite because it's made testable predictions that weren't predicted by previous theories (and in some cases that were completely unexpected). And so far the predictions made by GR have all came true. Every single one of them, to date.

    This is the best that a theory can do. A short list of examples of testable predictions that are in agreement with GR to date, and were not in agreement with the previous theory of gravitation (Newton's theory, which was already pretty good).

    The precession of mercury's orbit (previoulsy observed, but never explained)
    The bending of light by the sun of twice the Newtonian value.
    The shapiro time delay effect (associated with gravitational time dilation).
    The Nobel prize winning observation of the inspiral of a pair of binary stars.

    Now, look at the confirmed list of predictions for SEC that are different from the current theory:

    {}

    Pretty short list. That's the main reason SEC appears to me to be mostly "blue sky" speculation. IMO.
     
    Last edited: Dec 28, 2007
  11. Dec 28, 2007 #10

    haushofer

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    Every idea in science which explains, predicts and endures falsification is "just a theory". The fact that we name the theory of relativity a theory is the greatest compliment an idea can get.

    What would you call "an actual description" of phenomena?
     
  12. Dec 28, 2007 #11
    I don't know in the case of things like gravity or light etc. It's hard to explain, I just feel something is missing. Even though GR describes it in a way that's gives accurate results it doesn't actually explain why spacetime bends, it doesn't actually say what spacetime is besides a way to calculate things. It still ends up as a "force at a distance", same goes for magnetism or any other force at a distance. We've got plenty of formulas to describe the effect and we know how to create the effect. But i've never seen an actual description of what it is.

    It might be cause i'm a programmer myself, but as I see it we can give an actualy description of all the higher level functions (things like fluid mechanics, pressure etc). But can't really describe the lower level functions (gravity, electromagnetism, strong, weak).

    Just like a programmer might be able to describe what a c++ program compiles to in assembler language. But doesn't actually know how the assembly mnemonics are constructed in the CPU itself. While there actually is an underlying mechanism of transistors, a programmer will never be able to describe that mechanism without actually opening the CPU and looking inside it, but he can most definitely describe every mnemonic and what the effect of each one is without ever doing so (of course, you can go even deeper after the transistor, but this is just an example of what I'm trying to say).

    Anyhow, it might actually be impossible to get to know the real underlying mechanism of our universe since we're actually living in it. Just as it would be impossible for a piece of software to find out that it's running on a CPU consisting of transistors, simply because it has no way to look "outside".


    Not sure if that made any sense at all...
     
  13. Dec 29, 2007 #12

    haushofer

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    Sure it does. What we call gravity, is the geometry of space-time, which is determined by the distribution of energy and momentum:

    [tex]
    G_{\mu\nu} = 8\pi G T_{\mu\nu}
    [/tex]

    If we use the principle of least action, we see that an object follows a geodesic in that space-time if no other forces are in play. Actually, in general relativity gravity is not exactly a force; it is just the consequence of objects moving along geodesics in a curved space-time. Because we can't give a clear destinction between a gravityfield and acceleration locally, it makes no sense to speak of "acceleration due to gravity".

    What is energy? Or momentum? Just definitions with which we can calculate things. I'm not sure what you are expecting here :)


    I hope you can see now why this is not true.

    Same here: what is "an actual description of what it is"? Electromagnetism can be viewed in different ways: as a force propagated by photons between charged objects. Or as a consequence of some symmetries. Because some very general symmetries hold in an internal space, a gauge field is introduced in your equations which plays the role of the vector potential responsible for the electrogmagnetic fields. The same can be said of gravity: by diffeomorphism invariance of the theory in space-time we have to include some sort of connection to preserve this invariance.

    Maybe you want a some more simple explanation. But you could ask yourself the question: what exactly is "the true nature of things"? Science gives a nice description in my opinion, with which we can predict, describe and explain. "True natures" are play toys for philosophers and maybe religious people. The validity of the scientific description is determined by its ability to predict, describe and explain.
     
  14. Jan 18, 2008 #13

    DSR

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    Hello,

    Thank you all for responding to my initial post. It’s good to see that it started such a discussion. In his post haushofer says that gravity and EM are fundamentally different kinds of forces where EM is linear, but gravity is non-linear that self-interacts. But they are both inverse square forces described by similar equations, at least at the Newtonian level. Wouldn’t that imply some possible connection?

    I was afraid that I might be a bit out of my element with this topic. I just have an interest in this stuff, but when the discussion begins to go to things like tensors is where I start getting lost. Can someone suggest a good source for learning about tensors? Is there a book: Tensors for Dummies?

    I am a little surprised, though, that there was no response to the remark about String Theory.

    Thanks.
     
  15. Jan 18, 2008 #14
    It might suggest a possible connection, but that suggestion is quickly found to be little more than a tease when one actually sets about trying to describe both interactions with a single theory.

    The mathematics of tensors is what makes General Relativity so much more difficult to learn than its cousin, Special Relativity, which can be understood completely using no more than high school algebra.

    There are plenty of books on the mathematics of tensors, but they require some grounding in other areas of math with which you might not be familiar. Have you had any experience with differential geometry? I think that is one of the realms where tensors come to light in a good basic way. Short of that, you can probably find descriptions in some books on Classical Mechanics or Classical E&M, both of which use it. There might also be a "Tensors for Dummies" book, but I haven't seen it! :cool:

    As for String Theory, discussion of its merits or lack thereof are a regularly reappearing thread on this site. It supporters are as vehement as its detractors, so you might find many of us hesitant to start down that road again, unless explicitly asked to.
     
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