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Virtual particles are just math (MWI and MUH)

  1. Jan 10, 2015 #1
    I was reading a lot that "Virtual particles are just math..." and many physicists for some reason get angry explaining it. But I suspect this point of view is interpretation-biased and is outdated for 3 reasons listed below:

    1. The (mathematical) discovery of Quantum Decoherence had provided a solid foundation for MWI. In MWI there is no collapse, and measurements are not "final", "irreversible" and somehow magical. So there is no fundamental difference between observed and unobserved particles.

    2. Different accelerated observers don't agree on the number of "real" particles (check Unruh effect). It suggests that particles, virtual for one observer, could be real for another.

    3. In Tegmark's MUH, everything is just math. So you can't claim something is not real because it is "just math"

    So "Virtual particles are just math..." sounds as something from 1970x and very Copenhagen. I don't want to start interpretation wars. But IF you accept MWI, or MUH, or both, what is your view on the nature of the virtual particles? If you don't accept MWI or MUH, you still can play "devils advocate" and make an analysis from that point of view.

    Thank you
  2. jcsd
  3. Jan 10, 2015 #2


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    Whether virtual particles are just math or not is a matter of taste. It is more important to learn the math. So go learn the math.
  4. Jan 10, 2015 #3

    Vanadium 50

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    No, but we do get angry when people who haven't bothered to put in the time to learn the material tell those of us who have put in that time that they are wrong.
  5. Jan 10, 2015 #4


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    Both these arguments are based on serious misunderstandings. First: although MWI handles the distinction between observed and non-observed things differently than some other interpretations, this distinction is almost completely unrelated to the distinction between virtual and non-virtual particles. "Observed" and "not observed" are not synonyms for "real" and "virtual". And second: "real" in the sense of the Unruh effect is not the same thing as "the opposite of 'virtual'".

    Atyy's advice about learning the math is good. QFT is different from some other branches of physics in that there is no intuitive common-sense model to build on; there's no such thing as "a layman's understanding".
    Last edited: Jan 10, 2015
  6. Jan 10, 2015 #5


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    Its not like that. There is no disagreement amongst physicists about what they are like with how you interpret the QM formalism. They appear in something called a Dyson series:

    This series lends itself very elegantly to pictorial representations called a Feynman diagram:
    'The Dyson series can be alternatively rewritten as a sum over Feynman diagrams, where at each interaction vertex both the energy and momentum are conserved, but where the length of the energy momentum four vector is not equal to the mass. The Feynman diagrams are much easier to keep track of than old-fashioned terms, because the old-fashioned way treats the particle and antiparticle contributions as separate. Each Feynman diagram is the sum of exponentially many old-fashioned terms, because each internal line can separately represent either a particle or an antiparticle. In a non-relativistic theory, there are no antiparticles and there is no doubling, so each Feynman diagram includes only one term.'

    Certain terms that appear in the Feynman diagram are called virtual particles. They could be called Jabberwocky terms or any other name you like - it doesn't matter what you call them they are simply mathematical terms as part of a series expansion. They are an artefact of the mathematical methods used and not in any way real - the confusion resulting from the name used - call them Jabberwocky terms if you like which is probably clearer to start with.

    The good news however is in the past QFT has been a notoriously difficult subject studied at graduate level. But recently some excellent books accessible at the undergraduate level have started to appear:

    I have the book and can attest to how good it is. Suitable background would be Lenny Susskinds book on QM:

    Studying it is the only real way to get to grips with this stuff. It will take time and require your attention with your thinking cap on - but is doable. Take your time - its not a race. Post here with any queries and at the end of it you will have a grasp of QFT way beyond popularisations and the 'its just math' view of virtual particles.

    Last edited by a moderator: May 7, 2017
  7. Jan 11, 2015 #6
    Ok, let me reformulate my question.

    I am familiar with Feynman diagrams on naive level (based on his popular book). My question is: what if we move outcomes to future infinity? Can we make the Feynman diagram "unbounded" ?
  8. Jan 11, 2015 #7


    Staff: Mentor

    What you mean by that beats me.

    Its got nothing to do with the outcomes it describes.

    Until you study the real deal what popularisations say is - how to put it - not to be trusted.

    Last edited: Jan 11, 2015
  9. Jan 11, 2015 #8
    Even the Feynman's book?
  10. Jan 11, 2015 #9


    Staff: Mentor

    Yes - even that book - although its one of the best around. For example its view of particles being absorbed then re-emitted is not correct:
    https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ [Broken]

    Unfortunately in physics things you are told at an earlier level often get corrected later.

    Even Feynman understood this and lamented it was necessary at a pedagogical level.

    It happens to all of us - me included.

    Last edited by a moderator: May 7, 2017
  11. Jan 11, 2015 #10
    Sorry, no, decoherence presupposes a subdivision into systems. It works nicely, if we have such systems - observers, measurement devices, and so on. But how do these systems appear? What defines them as systems? Fundamentally, there are no such subdivisions into systems, so they cannot be used in the foundations of MWI. Except one defines them, as some additional fundamental structure.

    Particles are anyway only excitations of quantized fields, nothing more fundamental than eigenstates of a harmonic oscillator or phonons in condensed matter theory. So this is only an argument that "real" particle are not real too.

    Tegmark is just philosophy going wild. To take the MUH seriously can have side effects for the reputation as a serious scientist, SCNR.

    The same as of the pink unicorn. It is imaginable, thus, exists somewhere in many worlds, and mathematically consistent, thus, exists according to MUH.
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