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Examples of no gravity in quantum mechanics?

  1. Mar 1, 2010 #1
    I seem to be missing the resources that describe how gravity is not found in quantum mechanics. What phenomena in quantum mechanics illustrate that gravity is [thus far] not a part of it?

    This is not a homework question.

    I know that gravity is incompatible with what we know about QM. What I DON'T know is WHY. I'm not interested in theories on quantum gravity. I have books about them and that's not what my question is about. My question is more about the reason we are trying to find quantum gravity in the first place. What is it SPECIFICALLY about QM that is not compatible with gravity? What QM behavior "defies" gravity?
    Last edited: Mar 1, 2010
  2. jcsd
  3. Mar 1, 2010 #2
    It's not so much that a particular phenomena shows the lack of gravity at the quantum level... the opposite in fact. Rather, QM just doesn't describe gravitiational interactions well (or at all). It's the gap between Quantum Mechanics and Relativity.

    EDIT: The reverse is true in terms of Relativity explaining the "world of the very small". That's why people are so excited about any possiblity of a working theory of quantum gravity.

    Now, if this is a simple homework question, and you need help understanding why that is, we kind of need to know, and this should be in the help section. I will say that either way, examining what theories purporting to be a roadmap to quantum gravity claim to bring to the table is a roadmap to understanding what SR/GR and QM lack in terms of unity.

    Of course, the NAME "QUANTUM GRAVITY" should be the big hint... what is gravity NOT, that all other forces ARE shown to be? What particle has no clear evidence of its existence, or for which a mechanism may or may not exist (not the Higgs)?
  4. Mar 1, 2010 #3
    It is not true that "QM does not describe gravitational interaction". There is no difficulty in computing quantum corrections to general relativity at moderate energies. We obtain a non-renormalizable theory, which therefore is not valid at arbitrarily large energies (more precisely, it looses its predictivity as more and more counterterms must be included, while performing only a finite number of measurements), but it works at low energy and the first corrections are expected to be fine on general grounds.

    It will be best to understand the problem if one looks at John Donoghue's contributions, such as :
    Perturbative dynamics of quantum general relativity
  5. Mar 1, 2010 #4
    I suppose that's why I said it doesn't "describe graviational interactin well". I would call the complete breakdown of a theory a "not well" outcome for a description of an interaction. :grumpy:
  6. Mar 1, 2010 #5
    You may call it that way if you please, however it is not a surprise if we make little progress in this field : there is no data. I do not expect data in the deep ultraviolet will come before we have data to test first order corrections, as in the above effective approach. All expectations for this theory suffering a "complete breakdown" is that, when we have data at intermediate energy, they will agree with first order corrections, and this will therefore constitute possibly the most beautiful success of fundamental physics in history, until we obtain data in the deep ultraviolet, which is so difficult it may never happen.

    I believe it is important to know this approach well, before even embarking into any other speculation.
  7. Mar 1, 2010 #6
    I wasn't speculating, but noting an absence of a theory's predictive abilities. There IS a UV catastrophe, just as HR seems to break Unitarity. When it's solved I'll start to refer to things differently, but I'm not couching my every phrase in terms of what progress MIGHT be made and how beautifull it will be then.
  8. Mar 1, 2010 #7
    I appreciate your input so far. I'm still looking for clarification, though. I think the answer I'm looking for is easier than it seems. I think the answer I'm looking for is something like this:

    Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes. Consequently, quantum particles do not abide by the same spacetime laws that other things in the universe do. Since spacetime is fundamental to gravity, this means that quantum particles are not compatible with the laws of gravity.

    Is this right? Because spacetime does not apply to quantum particles neither does gravity????
  9. Mar 1, 2010 #8
    What I am referring to only awaits measurement. The theory is available in the papers I quoted. Just as Fermi theory, it works effectively, and so much so that the so-called ultraviolet "catastrophe" can also be interpreted as an amazing prediction of the theory itself : its own failure signals new physics. This is an interpretation of the results which does not deserve the qualification of "catastrophe" I believe.

    Hoku, you propose a qualitative line of reasoning which may or may not turn out to be true in an ultimate theory which we do not have. Again, I think if you want to understand the problem with naive quantum gravity, you need to study it. It's simply exposed in the papers above. Your reasoning is not valid in the above papers for instance. The theory is not renormalizable perturbatively, which means that as energy increases, you will have to include more and more correction terms in your theory, until you have more terms than the number of experimental observations, at which point all predictive power is lost, and we do not have a "theory" anymore. A century ago, that was considered a "bad catastrophe". Today, I dare say it's been a while we do not think like this anymore. Yes, it is possible that our picture of a spacetime with particles moving inside is only "effective", and so ? You will never experimentally prove anything more than a theory with a limited range of validity, even if you came up with a pertubatively renormalizable theory of quantum gravity, what good would it be for ? You would only be able to test at forever higher energies, until you get tired and quit. A non-renormalizable theory has this very desirable feature that it tells you how far you need to go (worse case) before you know you will find something new.

    It is also possible that the above theory is renormalizable after all, non-perturbatively. It's a long story, but we have good hints, you may ask what Marcus thinks. So far, Nature always had more tricks in Her leaves when we thought we finished laying down the laws. It's only speculation, personally I prefer to keep my mind open.
    Last edited: Mar 1, 2010
  10. Mar 1, 2010 #9
    I think I'd better get out of this discussion, but before doing so, I will emphasize once again that I do not understand how one could know there is something wrong with canonical perturbation of gravity if one has not studied it. So I think the resources I provided are essential. From the above paper, one can read
    This is all I am trying to say here.
  11. Mar 2, 2010 #10
    Humanino, I want to thank you again for your contributions. I have printed the papers you suggested and will spend time reading through them. Although the math is a little over my head, I think I can find something in it to grab onto and move forward with. HOWEVER... I realize that I need to be more clear with what I'm looking for. What I'm specifically looking for with this question is not "the facts" about gravity as it does or does not relate to QM. I'm looking for the history of our perspective on it.

    If I asked why we thought the sun revolved around the Earth, I wouldn't expect the answer to be, "Well, the sun doesn't revolve around the earth. That's an outdated way of thinking". I would expect the answer to be more like, "Because it doesn't feel like we're moving and that's the way it looked to us as the sun rose and set." Do you understand where I'm coming from? I'm not looking for the "right way" to view the problem, I'm looking for a basic history of how we have viewed the problem.

    That being said, would you say that my mock answer is a good representative of why people have viewed QM and gravity to be incompatible?
    Last edited: Mar 2, 2010
  12. Mar 2, 2010 #11
    Maybe I need to think a little while longer to provide an alternative answer. For me, it's essentially technical, both the history and the conceptual reasons. I am not especially a string proponent, but I so happen to be in the mood right now, so I will provide a link to a more historical discussion by Distler
  13. Mar 2, 2010 #12
    That is most definitely an opinion, and no more right or wrong for that, but still just an opinion. You may well be right, but for now the catastrophe exists, and until those measurements are made and MATCH predictions... well... every time you make an assumption, a virtual pair dies. :wink:
  14. Mar 2, 2010 #13
  15. Mar 2, 2010 #14
  16. Mar 2, 2010 #15
    Well, I think I spent enough time justifying why your initial answer was neither helpful nor appropriate, and why Hoku, if he is serious about understanding the question he initially posted, should start by studying canonical perturbative quantum gravity. I have a hard time finding a constructive contribution of yours in this discussion indeed. It is a fact that the expectation is widespread, you may disagree, that does not change anything nor is it really interesting.
  17. Mar 2, 2010 #16
    One of the major incompatibilities, though I would not say the incompatibility with QM and Gravity, is time... Time is like the craziest thing ever...

    Quantum uses a background time, which GR says does not exsist. I know you said you got QG books so this should be in there, but a Quantum theory of gravity will be gackground independent, which is different then current quantum theory which uses a background time to be successful, and it is very successful...

    to the point, A quantum theory of gravity is needed to explain gravitational interactions inside of a plank distance becuase of quantum fluctucations in space-time at those scales.

    The time issue is a big one, and maybe, just maybe, the coolest discrepency, but that is the reason why we need a QToG.
  18. Mar 2, 2010 #17
    Thanks jfy4 for your input. Although I was first excited by the idea, I've since become skeptical of the "background independant" issue, as was described in my post, "LQG and relational spacetime" posted in "beyond the standard model". I'm formulating an idea for a new post about the issue and I hope that it will bring more responses that my LQG post did. Unfortunately, there are so many different perspectives to all of this that people seem to loose their calm, objective science skills and become almost fanatical about the perspective they've chosen. I think that might be why its dificult for people answer the basic questions I have.

    The question I raised in this post seems like a basic one, indeed. It should not be so difficult to answer. I even created a mock answer as a starting point (see reply #7). I think my mock answer is pretty good and don't see why it doesn't explain the historic view of incompatability. Maybe we can break it down:

    "Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

    Has this statement not, at some point in our history, been viewed as true? I believe it has. Can we first nail down an agreement on this? If no agreement is possible, perhaps we can alter it to a more agreeable statement, without sacrificing simplicity and clarity...
  19. Mar 2, 2010 #18
    Well, first i would say that particles are things, and things are things! we have right now two different treatments for things, i guess three, a Quantum treatment, a classical treatment, and a semi-classical treatment... I would not be inclined to call a particle a "quantum particle", but thats what physics is, interpretation backed by observation, you can if youd like, but it might lead to misunderstanding because interpretation varies.

    How we treat the particles seems to matter when it comes to observing results... which is what you were saying in a sense i think.

    The last part

    "That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

    Is a interpretation, and i think you are right, it is shared by many. Interpretations matter, there are two parts if you will. First, we as a community have chosen math as the language of physics, so english is a fantastic start, but our math must match with experiment. Second, even our english must be scientific, which it seems you are wholly committed too IMO, so it must be correctable and testable.

    these being said, I dont interpret, along with others, that say a quantum particle has a constantly changing position and momentum, and my meaning would obviously vary from circumstance to circumstance, but rather that these properties of a particle are probabilistic.

    That is to say, i can imagine a situation where a momentum is changing, and that is some physics to be sure, but the quantum part is that that momentum is probabilistic. And our math should say that, along with our english i think...

    so after all this i would say: "particles can be treated in various fashions, however, depending on the situation one should be more inclined to use a a treatment which yields consistent results with experiment. (here's the part i think you are interested in) The quantum treatment tells us that the various aspects of particles which are observable, are probabilistic, and so they follow the rules of probability and wave mechanics."

    that would be how i would write, what i think is, your statement. I hope we make some ground here! :)
  20. Mar 2, 2010 #19


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