QM and GR

  1. I've heard that if one of these theories is correct then the other must be wrong. What exactly is the conflict between these two theories?
    Last edited: May 17, 2003
  2. jcsd
  3. Is this question too stupid or too hard for anyone to answer? The only reason why i am asking is because i have developed an equation that allows for gravity to be expressed in terms of quanta and im curious if this new expression may be a new solution for physics.
  4. LURCH

    LURCH 2,507
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    As a general rule, this is not a place for "quick" answers, it usually takes a day or two to get a response. Please do not feel as though your topic is being ignored, that's just the pace of things around here.

    The basic conflict between GR and QM as I understand it is as follows. GR presents an image of space-time as being curved, and QM uses probability equations to determine things like location, momentum, etc. Unfortunately, my understanding of these two field is limited by my lack of mathematical education. Therefore, the following paragraph is somewhat speculative in nature.

    Because QM deals mainly in probability, the mathematics utilized must bear a strong resemblance to statistical analysis. I do know that in this field, parameters must first be set. So, when attempting to equate the location of an electron (for example), one would tend to keep one's search within a certain general location, which is then narrowed down (probably through a perturbative process). But all of these probabilities exist within a very small area of space, which led to the conflict in the next paragraph (which is not speculative).

    As Quantum physicists generally deal with phenomena that take place on a scale much smaller than a molecule, the amount that space is curved over this distance is negligible. Being negligible, it tends to get neglected. But under extreme circumstances, such as those found near the center of a black hole, the curvature of space over these short distances is significant. So far, all attempts to include the curvature of space (and the equations of GR) into the probability equations of QM result in infinite numbers. IOW, if you conduct in mathematical search which allows for the fact that the space in which you are searching is curved, you'll find that, for any potential location you investigate, your electron's probability of being in that exact location is infinite.

    Now, as I said before I am no math wiz, but I do know that a probability equation that yields a probability of "p>100%" is incorrect. And one that yields an infinite probability is right out. So, the equations of QM only work for flat space, and GR insists that space is not flat.
    Last edited: May 17, 2003
  5. thank you for the response Lurch. Although it was quite insightful towards my question I'm still hoping for more responses that will further knowledge of disagreements between the theories.
  6. DrChinese

    DrChinese 5,817
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    There really isn't any disagreement at all between the theories. As far as I know, subsequent to the big bang, they both operate quite nicely on their own.

    There is a mystery as to why the two theories are so different, with the hope that one day they will both be shown to be a subset of a greater "theory of everything".

    Some exotic experiments are currently being performed which may shed some light on the situation. For instance, is the force of gravity a constant? Do gravity waves really exist? And so on. Current results leave room for interpretation. But we may see some major breakthroughs in coming years.
  7. Hurkyl

    Hurkyl 15,987
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    Actually, in my understanding of QM, it is not required that one work with normalized probability distribution functions (pdf); i.e. it doesn't have to add up to 100%. The probabilities are actually relative probabilities, so if f(1) = 2 and f(2) = 6, then the odds of the particle being at point 2 are three times as much as it being at point 1.

    For many pdf's, you can divide by the overall total to get a "real" pdf that has the property that it adds up to 100%, but some of them cannot, such as plane wave states.
  8. LURCH

    LURCH 2,507
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    And these total up to values greater than 100%? If f(1) is infinite and so is f(2), then you've done something wrong, yes?
  9. ahrkron

    ahrkron 731
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    A couple differences I can point out:

    1. In GR, there is no objection against simultaneously specifying the location and momentum of particles (or anything), while the structure of QM forbids that. This implies that in GR you can have point-like singularities (including the very initial one, from which the BB started), while QM says such things cannot exist.

    2. QM (and QFT and even String Theory) are based on the assumption that spacetime has a flat (Minkowskian) metric, while GR allows for different spacetime configurations. Curved spacetime has not been accomodated on the framework of QM.
  10. Hurkyl

    Hurkyl 15,987
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    Not necessarily something wrong.

    For example, in calculus, if you're trying to compute a limit and you plug in values and get &infin / &infin, that doesn't mean you did something wrong; it just means what you did was inadequate and you have to apply another technique before trying again to plug in values (such as using L'hopital's rule).

    Same thing here; if you get infinites, that would mean that you need to try and apply some other snazzy technique to get the real answer. (I believe "renormalization" is related to this)

    The problem with gravity & QM, though, is that no technique has been found that gets rid of the infinities (I'm not sure if it has been proven that it cannot be done, though).
  11. ahrkron

    ahrkron 731
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    Yes. I just wanted to comment that there are some cases in which the infinities cannot be removed by selecting a different technique.

    It is, although probability is not the infinite quantity. It can be, for instance, a mass, or a coupling constant. What has to be done (and it does work) is measuring one value for the infinite parameter, and make things so that such infinity is "renormalized" to become the measured number. Then, when you use the renormalized model to compute the value of the same parameter on different conditions (different interaction energy, for example), you do get a number, which can be compared to new experimental data.
  12. marcus

    marcus 24,214
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    First, here is some physics folklore. To make it easy to talk about, assume planck scale units (c = G = hbar =1)

    Imagine an uncharged nonrotating black hole with mass M = 1.

    the radius of such a black hole is always 2M, so in this case the radius is is 2 length units.

    The Compton wavelength of a particle with mass M is 1/M, so it is 1 length unit.

    The Compton describes how quantum mechanically spread out a particle is---how difficult it is to localize.

    To localize a particle within 1/M requires expenditure of energy M (heisenb. uncert. princ.) and that much energy would be enough to create another similar particle!

    So in the case of our black hole GR says it is a very precisely shaped geometrical structure with radius exaclty 2. But QM says it cannot even be localized that well. Its very location is fuzzy by 1 length unit.

    this is not the real reason that it is challenging to reconcile and unify QM and GR. It is more of a little parable, or even a joke.
    But there is GREAT interest in unifying QM and GR, these days, into a quantum theory of gravity.

    The research is always done at planck scale because those are the natural units to work with in dealing with basic things and because some of the problems that must be faced appear at this scale.

    Notice that in the story if the black hole were more massive like M=1 million = 106
    then the compton would be a MILLIONTH and would not give
    any problems. The radius 2M would be 2,000,000 and the uncertainty in position would be a millionth which is very small
    compared with the size of the hole and good enough for government work.

    For masses much larger than planck mass, the quantum fuzziness does not bother the theory of gravity----all is well.

    For masses much smaller than planck mass, like a proton which is on the order of a quintillionth of planck mass, the theory of gravity is kept out of the picture and quantum theory reigns.

    the high-energy-particle people say keep GR far from our door and we will not imagine any black holes! We will neglect gravity
    and that will be fine because it is very weak compared with our particle forces.

    So the planck mass (22 micrograms) is a kind of vague no-mans land border between two jurisdictions----and a place where the two jurisdictions can sometimes come into conflict.


    this too is only a story. Instead of fables, why not go
    directly to a key player and get it from the horses mouth:
    Lee Smolin is a major figure in quantum gravity.

    The greatest recent paper on the unification of QM and GR is by
    Lee Smolin and is online and is called
    "How far are we from the quantum theory of gravity"
    March 19,2003 and recently reviewed by John Baez in his
    weekly theoretical physics column
    The Smolin paper is
    but it is 90 pages and you are guaranteed to understand only
    a few paragraphs, such is life :-(
  13. marcus

    marcus 24,214
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    There is another link to Smolin's paper

    It is not a question of "if one theory is correct then the other fails"

    Both theories are almost certainly wrong, or rather, they are only
    approximations which are good at certain scales but do not work if applied at an inappropriate context.

    The aim is to have a quantum theory of gravity which reproduces GR in the limit for large objects and low particle energies
    and is in harmony with quantum mechanical principles that have worked so well in the microscopic and high energy arena
    and so to reconcile the two by somehow improving both.

    GR is merely our theory of gravity. That is all the curved spacetime stuff is, really, just a model of how gravity works.
    But it depends on having space be bendable and dynamic, stretchy. This is totally foreign to QM which is built like a brick
    house on a determinedly rectangular foundation. The space
    on which QM is built is rigid and not dynamic. So our model of
    gravity is different from our models of all the other forces.
    This difference is at the root of why it is so difficult to reconcile the two.

    this too is not my own view but folklore or gleaned from Smolin.
    Just giving you my take on it. Not authoritative, just sharing my
    personal opinion and perspective. Good luck. the whole field is
    very hard
  14. Yup... the basic conflict is that quantum field theory (QFT) assumes that spacetime is perfectly flat; whereas GR says it is curved by matter. GR also assumes that everything is smooth continuous: but quantum vacuum fluctuations ought to severely distort space and time on a very small (Planck probably) scale (the 'quantum foam') mucking everything up.

    In QFT this manifests itself as gravity being non-renormalizable (can't get rid of the infinities)...
  15. Re: Re: QM and GR

    You must be joking. You can't neglect spin of elementary BH because graviton has a spin (=2, I believe).

    That is why your futher conclusions are wrong.
  16. I've read alot of books on general relativity and Im pretty sure i know what the theory is offering. However, Im not exactly an advanced mathematician so can't understand stand all of it completely. What im curious about is how matter actually "bends" space-time or in other words what is it that matter is doing that bends space-time?
    Last edited: May 19, 2003
  17. Tom Mattson

    Tom Mattson 5,526
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    Re: Re: Re: QM and GR

    No, he is not joking. He is talking about nonrotation in the classical sense, not quantum mechanical spin. This case is worked out in all GR textbooks. You are correct in that graviton is spin-2, but that does not imply a spin on the black hole itself. If the gravition field is spherically symmetric, then angular momentum would be conserved for a spinless black hole, in much the same way as a spin-0 meson decays to two (spin-1) photons without violating conservation of angular momentum (the photons travel in antiparallel directions).
  18. LURCH

    LURCH 2,507
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    Nobody really knows. QM sometimes suggests that the "curvature" is really the probability curve or potentiality curve (or some such) of a force-carrying partical called a Graviton. So far, this partical remains undiscovered.

    I have my own crackpot theory, but that's over in the "...favorite pet theory..." thread (in this same Forum).
  19. I am very confident in a theory that I myself have created about what mass is actually doing when it is working. Its actually quite simple and provable and I think it may shed some light on the current views of Quantum gravitation. Where could I submit such a theory so that I could get the recognition for it? Im not in college so i don't really no what to do with it.
  20. Tom Mattson

    Tom Mattson 5,526
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    Why not post it in the Theory Development section here at PF?
  21. I'm not trying to sound overly confident in my theory but i don't necessarily want to post a potentially correct theory on the internet without knowing that I will get the full recognition for it.
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