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Can virtual particles break the law that energy cannot be created or destroyed?

  1. Jun 4, 2010 #1
    Im new and not that advanced in science so can you try to keep your answers simple. My question is can virtual particles break the law that energy cannot be created or destroyed?
    Have virtual particles been proven/observed? thanks in advance for your answers
     
  2. jcsd
  3. Jun 4, 2010 #2
    Hey, Virtual Particles do not break the law that energy cannot be created or destroyed. This is because Virtual Particles only "borrow" the energy for a very short amount of time. In fact, it's so short that we can't observe them. They actually arise from time/energy uncertainty principle.
     
  4. Jun 5, 2010 #3
    I asked a particle physicst from this website http://phy.syr.edu/HEPOutreach/ [Broken] lady named marina aurtuso and she said yes can somebody explain that
     
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  5. Jun 5, 2010 #4

    DaveC426913

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    Explain what? We don't know what she said.
     
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  6. Jun 5, 2010 #5
    all she said was yes it does violate the conservation of energytheory no explaination
     
  7. Jun 5, 2010 #6

    tom.stoer

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    virtial particles do not violate energy-momentum conservation. At each vertex in a Feynman diagram energy and momentum are conserved.

    But the virtual particles can be off-shell. Usually for a particle with rest mass m there is the relation

    E² - p² = m²

    This relation can be violated, so virtual photons can have non-zero m (but this is a mathematical artefact).
     
  8. Jun 5, 2010 #7
    For a 'very' short time, it might be said that conservation of mass/energy is locally violated. Averaged over even a whole second, or a sufficient region of space, no such violation occurs. Virtual particles come in particle/antiparticle pairs, which quickly annihilate each other. Averaged over space and time, such particle pairs remain stable, so the total energy remains constant. The Casimir Effect is taken as the most straightforward experimental evidence. In QM a vacuum may be basically empty, but that does not mean it's 'nothing'.
     
  9. Jun 5, 2010 #8
    I'm not unfamiliar with the argument but not the particulars. Can you give a simple example where energy conservation doesn't momentarily occur?
     
  10. Jun 5, 2010 #9
    It only occurs when you consider a suficiently small region of space. The vaccuum fluctuations can vary as a result of the uncertainty principle. In the bigger picture this is similar to saying an air conditioner violates conservation because it decreases entopy in some limited area. It of course didn't because the area of entropy decrease is not an enclosed system, and the entire system must be considered. Which of course increases overall entropy.

    Note also that there is a 3rd law of thermodynamics, which doesn't allow absolute zero. This implies that, even at maximum entropy, small local random fluctuations will remain. Classically this is a small random variations in temperature with random molecular motion, which average over to a constant. In QM this occurs as a result of the uncertainty principle. The Casimir Effect works by suppressing random fluctuations of certain wavelengths between 2 masses.
     
  11. Jun 5, 2010 #10

    tom.stoer

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    Please show me a calculation from which violation of conservation of energy can be derived.
     
  12. Jun 5, 2010 #11
    I didn't say it did. I only said it would appear that way if you restricted your description to some part of the system. I even used an air conditioner, where only the thermodynamic effects inside the building is considered, as a classical analogy. The second law applies to enclosed systems only. Thus when you only consider some subset of an enclosed system it can appear as if the 2nd law is violated, when in fact it's not.
     
  13. Jun 6, 2010 #12

    tom.stoer

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    So you say that quantum fluctuations may carry away energy from a certain region of space. OK, I agree.

    Please understand why I insist on energy conservation. There is this argument you can read quite frequently in some popular books that particles can borrow energy and that this indicates that energy conservation is violated at short time scales.

    I agree that you can have quantum fluctuations and non-zero energy fluctuation

    [tex]<E^2> - <E>^2[/tex]

    But at the same time you have

    [tex]\partial_\mu T^{\mu\nu} = 0[/tex]

    as an operator identity; and you certainly have

    [tex]<\partial_0 H> = 0[/tex]
     
  14. Jun 6, 2010 #13

    I'm trying to learn some QFT at the moment (haven't got very far yet), and I must say that I've been a bit confused by all this talk of energy-time uncertainty relation in connection with vacuum fluctuations.

    Firstly, I should mention that I've only got as far as learning about perturbation treatments of "scattering" type scenarios, where I have some incoming particles, a few vertices, some internal lines and some outgoing particles. In these cases, for the momentum space Feynman diagrams, I have a delta function which imposes conservation of four momentum, so all is clear.

    However, I'm confused about what precise QFT process the popular accounts are referring to when they talk of virtual pair creation via the energy/time uncertainty relation. The talk is usually of the vacuum as something like a "choppy sea in which particle/antiparticle pairs are constantly being created and annihilated". This is usually used in explaining effects like screening of bare charge and suchlike.

    But in the more rigorous treatments of vacuum polarization I see no mention of the energy/time uncertainty relation. Am I right in saying that it's completely misleading to talk about energy/time uncertainty in this context ?
     
  15. Jun 6, 2010 #14

    tom.stoer

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    That's exactly the point: energy-momentum conservation at each vertex!

    I agree, that's confusing.

    Not completely, but to a large extend, yes!
     
  16. Jun 6, 2010 #15
    So the consensus is energy is never violated at anytime
     
  17. Jun 6, 2010 #16
    is not strange that nature would regulate so tightly the energy/matter resources available to the universe at any given instant? Wouldn't that perhaps suggest that the universe is finite? Why the laws of conservation if energy.matter is availabe in unlimited quantities?
     
  18. Jun 6, 2010 #17

    tom.stoer

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    Because these conservation laws are local! Not only is the total energy conserved, but even for the local energy-density there is a conservation law, namely

    [tex]\partial_\mu T^{\mu\nu}(x) = 0[/tex]
     
  19. Jun 6, 2010 #18
    thats all well and good, but it doesnt answer my question. Why would nature be so tight with resources?
     
  20. Jun 6, 2010 #19

    tom.stoer

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    These conservation laws can be derived from symmetry arguments, namely Noether's theorem. For energy conservation it's rather simple: physical laws are time-independent (laws look the same at every point in time). That causes energy to be conserved.
     
  21. Jun 6, 2010 #20
    okay, so if the laws regulating quantum fluctuations are time-independent then in theory they exist for all universes (assuming a multiverse)?
     
  22. Jun 6, 2010 #21

    tom.stoer

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    of course this depends; what do you mean by "multiverse" ? the problem is that - as multiverses are invisible to us - so are their laws ...
     
  23. Jun 6, 2010 #22
    its actually an argument against many-worlds theories of qm. Not evidence, just a line of argument. For instance we see in our universe at almost every level of physical law (not just at quantum scale) how nature runs a very tight ship on resource allocation and efficiency. ie. virtual particles.

    The point is that it would seem contrdictory for nature to be such a strict mistress only in our universe, but then spend like a drunken sailor through an infinite amount of such universes.
     
  24. Jun 6, 2010 #23
    Sorry for posting again but
    I just want to confirm this energy is never created or destroyed with virtual particles or any other circumstance and why on other topics about virtual particles on the forum does it say it can and not taking into account multiuniverses does it violate
     
    Last edited: Jun 6, 2010
  25. Jun 6, 2010 #24
    There is a wide ranging overview here that is pretty decent:

    http://en.wikipedia.org/wiki/Conservation_of_energy

    that IS the current consensus.

    But I would NOT take irrevocable solace in that: over history the "consensus" have almost never been absolutely correct. But maybe we are getting better!
     
  26. Jun 7, 2010 #25

    tom.stoer

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    I see what you are saying. I tend to agree, but I would not mix it up with conservation laws!

    What you are refering to is a principle like Ockham's razor: If there are two theories explaining (predicting) exactly the same phenomena, the one which is simpler should be taken as the correct one. My problem with many worlds is that
    - they are invisible by construction
    - nevertheless they are taken to be real
    - the are postulated in order to interpret QM, not to predict physical phenomena

    I can't see any benefit. I have to introduce a plethora of meta-physical entities which I can neither prove nor disprove; I have to believe in them. This is obviously not physics but meta-physics - which is not wrong per se - but in the realm of meta-physics again Ockhams razor applies.

    To me the conclusion is as follows: I have two alternatives:
    1) QM which works physically but which can't be interpreted meta-physically
    2) QM + MW which is physically the same as QM but which adds not observable, not understandable, ... entities.
    All what I achieve by alternative 2) is to shift my ignorance from QM to MW.

    So by Ockhams razor I reject the many-worlds interpretation.
     
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