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I Virtual photons [in electrostatic interactions]

  1. Aug 15, 2016 #1
    I've been looking through the internet and I haven't found anything too clear on this: is it correct to assume a electrostatic interaction, say, between two stationary electrons as an exchange of virtual photons?
     
  2. jcsd
  3. Aug 15, 2016 #2

    jfizzix

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    Yes.
    However, once those charges start moving due to their attraction or repulsion, it's not electrostatics anymore.
    Physically speaking, we can't force a pair of charges to be stationary, except through other interactions.

    Also, remember that electrostatics and magnetostatics is just a particular limit of electrodynamics. Fundamentally, the forces are the same.
     
  4. Aug 16, 2016 #3
    Thanks!
     
  5. Aug 16, 2016 #4
  6. Aug 16, 2016 #5

    haushofer

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    The exchange of photons-story is a heuristical description of the fact that such an interaction is calculated by a perturbation series in which terms can be bookkeeped by terms we call 'virtual particles' because somehow they look like what we call a particle. To look like is however not the same as to be like.

    So I'd answer your question with 'on paper, viewing virtual particles as bookkeeping devices, yes. In reality, as an ontological statement about nature, most certainly not'.
     
  7. Aug 16, 2016 #6

    vanhees71

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    To describe electrostatic interaction in terms of QED is a somewhat advanced subject which requires the resummation of perturbation theory (soft-photon ladder resummation; note that here only virtual spacelike inner photon lines in Feynman diagrams are addressed). As usual the most clear explanation can be found in Weinberg, Quantum Theory of Fields, Vol. 1.
     
  8. Aug 16, 2016 #7
    Well, there is no way to decide whether a mathematical formalisation, which fully describes a physical phenomenon, is in fact "real" or not. If you are a platonist or even a supporter of the mathematical universe hypothesis, you can very well describe virtual particles as real.
     
  9. Aug 16, 2016 #8

    bhobba

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    That's not it.

    It's not real 100% for sure:
    https://en.wikipedia.org/wiki/Dyson_series

    Virtual particles are pictorial representations of terms in the Dyson series. They are not real despite what popularization's and beginner texts tell you. This causes all sorts of problems in the forum because for some reason they believe what those sources say despite the assurances of those that have studied the theory here - some of who are professors that actually teach it (I am not - just a guy with an interest). That's right - they teach it, tell posters the truth and they still do not believe it. You are left shaking your head and try everything you can think of to get the message across. It still doesn't work. Its unbelievably exasperating - people come here to get answers from genuine experts yet refuse to believe what those experts say. Its a very very interesting psychological phenomena - but that's all it is.

    Thanks
    Bill
     
  10. Aug 17, 2016 #9
    As I previously implied, the ontological status of a mathematical expression is not a question of physics at all. It's like asking whether the Lorentz manifold in GR is "real" or merely a tool to describe nature. It isn't even possible or at least quite difficult to properly define the meaning of the term "real". One should take great care when talking about the semantics of a mathematical expression, especially when this expression is used in physics. There are simply many philosophical interpretations.
     
  11. Aug 17, 2016 #10

    bhobba

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    Terms in a perturbation expansion are not real regardless of what philosophical gymnastics you want to dress it up in. Its simply a mathematical 'trick'.

    Its exactly the same as expanding a function in a power series and wanting to say the terms are real particles - its nonsense - obviously.

    Thanks
    ill
     
    Last edited: Aug 17, 2016
  12. Aug 17, 2016 #11

    bhobba

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    In fact its not possible, which is why physicists don't even try. Instead simple common-sense notions are used and philosophical subtleties are not worried about. Wienbergs view on Kuhn is very typical of physicists attitude:
    http://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html

    Note we do not discuss philosophy here by forum rules. Occasionally its get touched on and its both unavoidable and OK. But a full blown discussion is best done elsewhere on for example the philosophy forums. I occasionally post there but get done like a turkey dinner because philosophy is not my bag.

    Thanks
    Bill
     
    Last edited: Aug 17, 2016
  13. Aug 19, 2016 #12
    Sorry for the late response, I studied the board's rules and seemingly, this question about philosophy of mathematics is certainly in correspondence with the rules.

    Let me clarify some things;

    I think it is perfectly accurate to think about virtual particles as being "non-real", though I am also well aware that while this statement is reasonable, it can never be formally proven (And I really mean a Formal Proof). All I am saying, that from the perspective of mathematical logic, you can certainly also think about "virtual particles" and ANY other mathematical "trick" as true in some sense. It has good reasons that giving semantics to certain mathematical statements (like virtual particles) on most universities is "punished", as it is pure guessing, which is a good thing, since it is pure guessing. What a tautology.


    When I talk about experts in the field of particle physics about the semantics of mathematical equations, they somehow can't prove certain statements about quantum field theory. However, it is not just limited to quantum field theorey, it is seemingly a general "observation" that it isn't a trivial task to assign a truth value/ semantics to some particular mathematical statements, used in physics, in respective theories. In mathematics, it is up to model theorey to determine the semantics of a mathematical statement, but in this case, I talk about physical semantics.
    You can very well express many things in theoretical and mathematical physics differently. But for every statement of any physical theory, there is no algorithm which tells you that certain mathematical expressions are physically true, while others are just "tricks", unless you can provide me FORMAL algorithm witch exactly assigns semantics to every mathematical expression used in any physical theory. Seemingly, you declare some physical statements, expressed in pure mathematical means, as reflecting reality, while others are declared as pure "tricks". You might or might not be able to give a general algorithm, which is able to distinguish between "real physical"-mathematics and "tricks"-mathematics. - Oh... well, I forgot - It is even harder than unify GR and QFT...

    So please, if you can't provide an algorithm, don't randomly declare physical statemants as "pure tricks" while declaring other statements as "physical reality", that is in no way accurate. "Punishment" of such statements in mathematical physics has good reasons.
     
    Last edited: Aug 19, 2016
  14. Aug 19, 2016 #13

    bhobba

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    Because no one can agree what real is, until such an agreement is reached its a meaningless statement of zero physical content. It utterly vacuous.

    Instead physicists adopt, by and large, a common-sense view of such things. Expanding a function as a perturbation series under any usual view of what makes a particle real does not make it real, and is one of the many myths about QM:
    http://arxiv.org/abs/quant-ph/0609163
    .
    The reason some textbooks, often at the populist or beginner level, but surprisingly frequently even more advanced treatments, consider it real has to do with promoting intuition, not because they are real particles - they aren't - and obviously so. If you cant see that then there is really nothing to gain continuing the discussion - it will not get anywhere. Its like discussions of solipsism. Can I prove solipsism wrong? Of course not. But nearly everyone rejects it. And discussions about don't really get anywhere because there is no way to answer it.

    This is the reason the following guidelines are in effect:
    https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/
    Generally, in the forums we do not allow the following:
    • Discussion of theories that appear only on personal web sites, self-published books, etc.
    • Challenges to mainstream theories (relativity, the Big Bang, etc.) that go beyond current professional discussion
    • Attempts to promote or resuscitate theories that have been discredited or superseded (e.g. Lorentz ether theory); this does not exclude discussion of those theories in a purely historical context
    • Personal theories or speculations that go beyond or counter to generally-accepted science
    • Mixing science and religion, e.g. using religious doctrines in support of scientific arguments or vice versa.
    • Philosophical discussions are permitted only at the discretion of the mentors and may be deleted or closed without warning or appeal.
    By and large philosophy discussions go nowhere. Every now and then threads touch on philosophy and that is perfectly OK. But a dialectic on what is a real particle, rather obviously, just like discussions on solipsism, will not go anywhere and the above mentioned mentors will, correctly IMHO, shut it down. After all if they were easily resolvable philosophy would have resolved them many moons ago instead of the constant rehashing you find in philosophical discussion. I need to make clear such is a perfectly valid subject of intellectual discourse, but as far as physics goes doesn't really go anywhere.

    You have pointed out the whole issue of what is real, especially what is a real particle, is murky. I think we all get that, but now is the time to move on or the thread will be shut down.

    Thanks
    Bill
     
    Last edited: Aug 19, 2016
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