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Quantum Mechanics and the Universe

  1. Sep 17, 2007 #1
    As a beginner to Science I want to pose the following question.

    Can QM account for how something came form nothing? This being the sincularity that perpetrated the Big Bang.

    If not, why not? Is it because energy conservation is required? What about Quantum fluctuation?
  2. jcsd
  3. Sep 17, 2007 #2
    On what basis do you affirm that "something came form nothing"? There is no evidence that such an event occurred. Therefore there is no need for an explanation.
  4. Sep 17, 2007 #3


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    Not that I know of. QM is just a physical theory like mechanics, it's not something magic that will solve all our problems. Basically, QM tells us if we have a particle (or a wave :tongue:) how it will move, just like classical mechanics does but better. Quantum fluctuation (if you will, violation of conservation of energy) is only possible on very short time-scales.

    It is possible though, that particles are spontaneously "created", e.g. a virtual pair being created, one particle of which gets sucked into a black hole before they can annihilate. You could view this as "something from nothing", with some imagination, but I don't think it's what you meant.
  5. Sep 17, 2007 #4
    The only relation to cosmology that QM has that I know of is in the fine-scale anisotropy of the background microwave radiation. It is postulated that the structure present is actually caused by quantum or perhaps simply statistical fluctuations which were subsequently expanded in the inflation phase.
  6. Sep 17, 2007 #5
    So is it impossible for Quantum Fluctuation to make the laws of physics etc?

    If so, why?
  7. Sep 17, 2007 #6
    What do you imagine quantum fluctuations to be, if not a part of the laws of physics? Nothing *makes* the laws of physics. They just *are*.
  8. Sep 17, 2007 #7
    Well I read that the laws of physics were instigated at the Big Bang. I'm just looking as to what may have made them since apparently they were created.
  9. Sep 17, 2007 #8


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    cosmology is a funny little subject. sure, it's physics, but it is different in some ways from the majority of physics...

    anyway... astronomers can look out and see that the galaxies are in general all moving away from each other as time goes on. thus, it is not entirely unreasonably to think that if one were to run the clock backwards the galaxies would all be moving towards each other. keep running the clock backwards and you get to a point where all the galaxies are pretty damn close to one another. what happens before that? well, there's a whole bunch of literature on the subject and many people have given their best scientific efforts at figuring out just that problem. if it is an interesting problem to you then I encourage you to take some more physics classes and begin to figure out the solution... warning: there's not too much funding around for that type of research these days, but good luck all the same. cheers.
  10. Sep 17, 2007 #9
    There is no theory capable of explaining the Big-Bang. Everything you've read was pure speculation. The only thing we know is that 15 bln years or so in the past the universe was very dense and hot. That's it. Period. We don't know why the universe was in such a state or for how long it was in that state or what it was before that state.
  11. Sep 18, 2007 #10
    So is there definitely no way quantum fluctuation could explain how the singularity came to be?
  12. Sep 18, 2007 #11


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    It depends what you mean by "singularity." As has been said many times on here, a singularity is not a physical entity, but is merely a mathematical tool that tells us that a theory does not cover a certain point. If we have a theory that explains the big bang, then we expect it to be non-singular.

    I think genneth nails the quantum fluctuation part quite well above. To the best of my knowledge, quantum fluctuations are suggested so that they might describe how the universe has a structure nowadays. If there were small fluctuations in the quantum field at the big bang, then it is suggested that these were increased by inflation in the early universe and so form the universe that we see today. Of course, none of this has been proven yet-- it's all an area of active research.
  13. Sep 18, 2007 #12
    1. QM cannot explain gravity, therefore I wouldn't expect it to shed much light on how mass/energy could appear.

    2. It is difficult to apply QM to the whole universe. Bohmian interpretation (BI) could do that but we don't have a relativistic BI yet.

    3. One cannot extrapolate a law that holds in an already existing universe to the creation of an universe. It is a logical fallacy.
  14. Sep 18, 2007 #13


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    Classical cosmology is grounded in general relativity, a theory of gravitation that breaks down when describing the evolution of the universe at a time before the Planck time. Before the Planck time the quantum effects of the gravitational field become important due to the high energies and small distances involved. The description of the initial singularity is therefore in the realm of quantum cosmology.

    There exist some models of quantum cosmology that would correspond somehow to what you imagine or consider an origin of the universe due to a quantum fluctuation. These are models of canonical quantum cosmology, that describes the dynamics of the wavefunction of the universe in a similar way than quantum mechanics describes the dynamics of the wavefunction of a single particle. Based on this results, similar consistency conditions that lead to a theory of multiple particles that can be created and annihilated (quantum field theory), may allow to formulate a model of multiple (baby) universes that may be created and annihilated as quantum fluctuations.

    These are imaginative ideas but far too speculative, with too many open problems (for example, all three issues mentioned by ueit would apply to such models), and without any expectation to be experimentally verified in a near future.
    Last edited: Sep 18, 2007
  15. Sep 18, 2007 #14
    How is it even possible that particles can appear without any energy conservation (ie quantum fluctuation)?
  16. Sep 19, 2007 #15


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    Energy is conserved between measured events in quantum mechanics and quantum field theory (for example scattering experiments). These theories do not include gravitation. Here we are dealing with a different scenario that involves the dynamics of spacetime and gravitation. In a classical regime this interaction is described by general relativity in which the notion of energy for a system is not always a well defined one (see for example this).
  17. Sep 19, 2007 #16
    A Platonist responds

    I doubt there's any physical law (now or in the future) which could explain how something can come from absolute nothing. It seems philosophically impossible.

    I suspect the universe just exists in an infinite mathematical space of possibilities, like PI or the Mandelbrot set. Asking where a particular universe came from is a bit like asking where a digit of PI or a point on the Mandelbrot set came from. It didn't come from anywhere!

    Where did the fundamental laws of physics come from? They didn't come from anything- they're just rules that exist in the mathematical space of all possible rules.

    It doesn't much matter to this argument if there existed an infinite number of universes preceding this one. There still appears to be a problem of how and why the whole structure is arranged this way.

    I strongly suspect the answers lie outside the laws of physics and in the realm of philosophy.
  18. Sep 20, 2007 #17
    Hmm, but isn't that the whole idea? Quantm fluctuation explains how something can come from nothing as no energy is required.
  19. Sep 21, 2007 #18


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    It is in your interest to read 2000 more years worth of philosophy... You can skip ahead to Russel...

    Oh! I think I just puked a little in my own mouth. This is a physics forum. Not a metaphysics forum.

    Observation. Observation; it's the scientific method... and it is taught to 2nd graders... Review that subject.

    Philosophy is fine subject and worth the study, but much of it deals with the non-physical. Is it a worthwhile pursuit? Maybe... I don't think so... But go ahead and pursue it. Just don't pretend that it can say a damn thing about physics... or, really, anything whatsoever.
  20. Sep 21, 2007 #19


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    Dear Platonist,
    This link takes you to post #30 of a recent PF Cosmology forum thread with discussion of pre-bang

    One of the things I quote in that thread is a bit from the August 2007 issue of Nature Physics journal by Carlo Rovelli. Here's an exerpt:

    Science has frontiers; sometimes these frontiers move. One of the most impressive of science's frontiers is the Big Bang, and now a quantum theory of gravity — loop quantum gravity — is providing equations with which to explore it. Although these equations are still tentative, and rely on drastic approximations, they introduce a definite method of exploration, and are capable of describing the Universe not only close to the Big Bang but also beyond it. It is in this context that Martin Bojowald reports, in this issue, on the possibility of a peculiar limitation to our ability to observe fully the 'other side' of the Big Bang — whatever that expression might mean (Nature Phys. 3, 523–525; 2007).

    Have fun.
  21. Sep 21, 2007 #20


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    In response to the original post:

    "There are something like ten million million million million million million million million million million million million million million (1 with eighty [five] zeroes after it) particles in the region of the universe that we can observe. Where did they all come from? The answer is that, in quantum theory, particles can be created out of energy in the form of particle/antiparticle pairs. But that just raises the question of where the energy came from. The answer is that the total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus, in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero." - Hawking, 1988

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