Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

HUP in QFT and QM:virtual particles

  1. Nov 8, 2012 #1

    haushofer

    User Avatar
    Science Advisor

    Hi,

    I have a question about reconciling two pictures of virtual particles and the Heisenberg Uncertainty Principle (HUP).

    In QFT "virtual particles" show up in perturbative calculations. We try to calculate an amplitude in interacting theories, this can not be done in an exact way, so we use Taylor expansions, and in this expansion intermediate states show up which we call "virtual particles".

    In non-rel. QM people often say that virtual particles can exist because of the uncertainty principle between energy and time, in which one interprets the "time" in the appropriate way (see e.g. Griffiths).

    My question is: how to reconcile these two pictures? If we would find the mathematical tools to calculate amplitudes in interaction theories in an exact way analytically, what would happen to these "virtual particles"? On the one hand I would say they wouldn't show up in your calculations, just as e.g. all the intermediate steps in

    [tex]
    \sum_{n=1}^{\infty} \frac{1}{n^2} = 1 + \frac{1}{4} + \frac{1}{9} + \ldots = 2
    [/tex]

    wouldn't show up; we know the answer is "2". But on the other hand, if their existence can be argued by the uncertainty principle, their existence should not depend on our ability to solve function integrals analytically in an exact way, right?

    Are my analogies bad, or are the textbook statements not that accurate, or something else?
     
  2. jcsd
  3. Nov 8, 2012 #2
    I don't think it's accurate to say that virtual particles can exist thanks to HUP. When this is stated in a text, it is misleading, I would say.
     
  4. Nov 8, 2012 #3

    strangerep

    User Avatar
    Science Advisor

    Yes.
    IMHO, such "explanations" are rubbish. (I always become suspicious of physics authors who are subtly dismissive about mathematical rigor.) Is it only in Griffiths where you've seen such arguments, or are you thinking of other common textbooks also?

    Right. Virtual particles are not real. :-)
    The latter.
     
  5. Nov 9, 2012 #4

    tom.stoer

    User Avatar
    Science Advisor

    as strangerep says: it's rubbish
    1) it's s difficult to state the time-energy HUP in non-rel. QM in a general sense; it's even more unclear how to formulate it in QFT
    2) virtual particles /as defined in perturbation expansion) are not Hilbert space states but "integrals of propagators"; so you can't define Δt and ΔE (or Δx and Δp) for these expressions in the usual sense
    3) somethimes people claim that virtual particles "borrow energy" or "violate energy conservation for some short time Δt"; this is nonsense as well b/c in the Feynman diagrams energy and momentum are conserved exactly at each vertex; what is violated is the mass-shell condition m² = E² - p²
    4) last but not least it's b...sh.. to discuss the "existence" of virtual particles


    In the Graduate College dining room at Princeton everybody used to sit with his
    own group. I [Feynman] sat with the physicists, but after a bit I thought: It would be nice to see what
    the rest of the world is doing, so I'll sit for a week or two in each of the other groups.
    When I sat with the philosophers I listened to them discuss very seriously a book
    called Process and Reality by Whitehead. They were using words in a funny way, and I
    couldn't quite understand what they were saying. Now I didn't want to interrupt them in
    their own conversation and keep asking them to explain something, and on the few
    occasions that I did, they'd try to explain it to me, but I still didn't get it. Finally they
    invited me to come to their seminar.
    They had a seminar that was like a class. It had been meeting once a week to
    discuss a new chapter out of Process and Reality some guy would give a report on it
    and then there would be a discussion. I went to this seminar promising myself to keep my
    mouth shut, reminding myself that I didn't know anything about the subject, and I was
    going there just to watch.
    What happened there was typical so typical that it was unbelievable, but true.
    First of all, I sat there without saying anything, which is almost unbelievable, but also
    true. A student gave a report on the chapter to be studied that week. In it Whitehead kept
    using the words "essential object" in a particular technical way that presumably he had
    defined, but that I didn't understand.
    After some discussion as to what "essential object" meant, the professor leading
    the seminar said something meant to clarify things and drew something that looked like
    lightning bolts on the blackboard. "Mr. Feynman," he said, "would you say an electron is
    an 'essential object'?"
    Well, now I was in trouble. I admitted that I hadn't read the book, so I had no idea
    of what Whitehead meant by the phrase; I had only come to watch. "But," I said, "I'll try
    to answer the professor's question if you will first answer a question from me,
    so I can have a better idea of what 'essential object' means. Is a brick an essential object?"
    What I had intended to do was to find out whether they thought theoretical
    constructs were essential objects. The electron is a theory that we use; it is so useful in
    understanding the way nature works that we can almost call it real. I wanted to make the
    idea of a theory clear by analogy. In the case of the brick, my next question was going to
    be, "What about the inside of the brick?" and I would then point out that no one has
    ever seen the inside of a brick. Every time you break the brick, you only see the surface.
    That the brick has an inside is a simple theory which helps us understand things better.
    The theory of electrons is analogous. So I began by asking, "Is a brick an essential
    object?"
    Then the answers came out. One man stood up and said, "A brick as an
    individual, specific brick. That is what Whitehead means by an essential object."
    Another man said, "No, it isn't the individual brick that is an essential object; it's
    the general character that all bricks have in common their
    'brickness' that is the essential object."
    Another guy got up and said, "No, it's not in the bricks themselves. 'Essential
    object' means the idea in the mind that you get when you think of bricks."
    Another guy got up, and another, and I tell you I have never heard such ingenious
    different ways of looking at a brick before. And, just like it should in all stories about
    philosophers, it ended up in complete chaos. In all their previous discussions they hadn't
    even asked themselves whether such a simple object as a brick, much less an electron, is
    an "essential object."


    We should send Griffiths et al. to those seminars 24 hours a day and 7 days a week in order to prevent them writing books!
     
  6. Nov 9, 2012 #5
    @strangerep
    " Virtual particles are not real."
    OTOH, Feynman said (in his lectures on gravtitation) that all particles we ever observe are "virtual", because when they are measured, they correspond to internal lines of Feynman diagrams.
    But of course, in a sense, particles themselves are not real but just a convenient way of looking at fields.
     
  7. Nov 9, 2012 #6

    haushofer

    User Avatar
    Science Advisor

    Plenty of textbooks, like those of Beiser, but also popular literature. That's why it's hard for me to believe that all those people are sloppy, and there is no rigorous justification for it.
     
  8. Nov 9, 2012 #7

    haushofer

    User Avatar
    Science Advisor

    Yes, this is also how I understand it; I've even once wrote a more or less popular article about it, and saw that the editor added the "Δt and ΔE" HUP to the text which I tried to circumvent. As I said, I find it rather amazing that so many people quote this explanation for virtual particles, and it made me doubt my own understanding of QM and QFT.

    In a topic about virtual particles I onced asked the question what happens with our notion of virtual particles and "our vacuum filled with virtual particles" if we were able to solve our amplitudes in an exact way, without perturbation theory. You would then agree, looking at your post, that the whole notion of virtual particles would disappear, and that they solely are artifacts of doing pertubation theory, right?

    Somehow people make the notion of vp's in popular literature much more romantic. I guess a popular explanation like "artifacts of the fact that our mathematical skills are not developed enough" is not flashy enough. Thinking about it, also regarding your topic on the definition of energy and entropy in GR elsewhere, we could almost start a topic in which all these popular notions in physics literature are being critized :P



    Yes, I love that story, and recently also quoted it here in some topic :D
     
  9. Nov 9, 2012 #8
    I think there is some justification for quoting the HUP in this context:
    Think of the ground state of the Harmonic oscillator: I think it is permissible to say that the x-positon of the particle cannot be exactly zero because of the HUP.
    If we accept this, then the fact that the probability of finding a non-zero amplitude for a field in QFT is also in some sense due to the HUP because this is (in each mode) equivalent to a H.O. The fact that this probability is smaller if k²+m² is larger fits into this picture, ath least intuitively: It is less probable to find an excitation of a higher-energy mode. (At least intuitively, this smaller probability might be thought to realte to a "shorter time", although this is nothing I would ever use to calculate something).

    And with an interaction added to the theory, this probability amplitude may be described via the concept of virtual particles.

    So I think there is some justificaton for this in a non-rigorous explanation, but in a good textbook you should add a bazillion of caveats.

    And to finish with a question: Could one use the fall-off of a not-on-mass-shell propagator to make this more rigorous (the fall-off is faster the greater the violation of the mass-shell condition is)?
     
  10. Nov 9, 2012 #9

    haushofer

    User Avatar
    Science Advisor

    That means that even if we would be able to solve for an interacting theory exactly, we still would find "virtual particles"?

    I'm not familiar with these things, but aren't we able to solve some 1+1 dimensional interacting QFT's exactly? Does the notion of virtual particles appear in such theories?
     
  11. Nov 9, 2012 #10

    haushofer

    User Avatar
    Science Advisor

    John Baez also seems to justify the relation between vp's and the HUP:

    http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

     
  12. Nov 9, 2012 #11
    That's not been the previous consensus in these forums[nor in the posts here so far]:

    Here is How Zapper explains it in his blog....



    [hence, no support for virtual particles.]


    On QFT: explanation from prior discussions in these forums:



    and a reminder from relativity:

     
  13. Nov 9, 2012 #12
    Probably I'm misunderstanding that, but that seems to contradict what I've read on QM (and it also seems to contradict all those Gedankenexperiments of Bohr and other people who are discussing the HUP). So I'd be glad if you could explain (or put a link here).
    Edit: See for example this experiment:
    http://en.wikipedia.org/wiki/Uncertainty_principle See einstein's slit
     
    Last edited: Nov 9, 2012
  14. Nov 9, 2012 #13
    Sonderval:
    Likely you ARE understanding what I posted......

    the best interpretation is the second quote from Zapper's blog [above]..

    try these sources on for size....and make your own judgments:


    I suspect the issue to which you refer is captured in post 2 and 3 here:

    Is Heisenberg Uncertainty a problem with our measuring techniques
    https://www.physicsforums.com/showthread.php?t=538854


    John Baez has a mathematical perspective here:

    http://www.cbloom.com/physics/heisenberg.html

    [Seems to me he is discussing a lower bound involving standard deviations, an ensemble of measurements, and NOT a single the measurement of conjugate observables. If so, this seems to me consistent with Zappers blog comments I quoted previously.

    This is the discussion that led me to my interpretation.....

    Loooong discussionhttps://www.physicsforums.com/showthread.php?t=516224

    I have zero confidence that everyone accepts this interpretation.

    [edit: a related discussion brought this interesting piece:

    "particles *may* have well-defined positions at all times, or they may not ... the statistical interpretation does not require one condition or the other to be true."
     
    Last edited: Nov 9, 2012
  15. Nov 9, 2012 #14
    In rereading my notes on HUP, I came across this explanation from PAllen
    that I really like [unsure which thread this is from]:

    [Reading Wikipedia on HUP appears to give a different impression than these descriptions.]
     
    Last edited: Nov 9, 2012
  16. Nov 9, 2012 #15
    I don't see why a momentum measurement necessarily involves two position measurements, at least not of the particle concerned. I could have a particle bounce from a ball or wall and measure the momentum change of the ball or wall at leisure. (As in the Einstein slit experiment I quoted above).
     
  17. Nov 9, 2012 #16

    strangerep

    User Avatar
    Science Advisor

    That's a bit misleading, imho. I suppose he's referring to how a measurement corresponds to establishing a correlation between a system's initial state and an apparatus' final state by having them interact. (Ballentine covers this reasonably well in his QM text.)
    I, too, prefer to focus on thinking about fields.
     
  18. Nov 9, 2012 #17

    strangerep

    User Avatar
    Science Advisor

    Er,.... how do you "measure the momentum change of the ball" ?
     
  19. Nov 9, 2012 #18

    strangerep

    User Avatar
    Science Advisor

    I've yet to anyone seriously advocating that stuff who is also capable of explaining the notion of "unitarily inequivalent representations" and "Haag's theorem" in QFT properly. ;-)
     
  20. Nov 9, 2012 #19

    strangerep

    User Avatar
    Science Advisor

    The way to be rigorous is to solve the theory exactly rather than via perturbation theory. :-)
     
  21. Nov 9, 2012 #20

    strangerep

    User Avatar
    Science Advisor

    In the (exactly solvable) Lee model, one basically finds the correct Hilbert space for the interacting theory, rather than trying to shoehorn the interacting theory into a Hilbert space which is only appropriate for the free theory.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: HUP in QFT and QM:virtual particles
  1. HUP: Particles <=> ? (Replies: 2)

Loading...