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High energy virtual particles

  1. Sep 14, 2011 #1
    I am told that the space surrounding me is filled with virtual particles, popping into and out of existence. I could believe that they exist for such short times and at such low energies that sophisticated equipment is needed to detect them. However, a collision between even 1 virtual electron and a starship going 0.99999999999999999999999999 c would have a significant effect on the ship because in the ships frame of reference the electron would have a large kinetic energy. Since all reference frames have equal claim to be "at rest", why are we not constantly bombarded by virtual particles that may be at rest relative to the afore mentioned starship, but would tear us apart?
     
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  3. Sep 15, 2011 #2

    Bill_K

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    mrspeedybob, Couple of misconceptions here. In the first place, virtual particles do not "pop into and out of existence." The vacuum state is time independent, there are no time dependent things happening. This is no more true than the idea that an electron "spins" on its axis or "orbits" about the nucleus. The vacuum does not boil with activity. This popsci idea results from forcing a classical interpretation of a quantum situation. The vacuum is a quantum state in which virtual particles exist with a certain (constant) probability amplitude.

    Second is the implicit notion that "you can violate energy conservation if you're quick about it." Not so. Energy is exactly conserved at all times, even by virtual particles. Virtual particles can only transfer energy from one object to another. In order to be struck by a high energy virtual particle, that energy does not simply materialize out of the vacuum, it must have come from somewhere else. Whenever one particle gains energy, another (real) particle loses energy.
     
  4. Sep 15, 2011 #3

    Ken G

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    I'm not sure I agree that we can state categorically that energy is exactly conserved at all times. I agree it is exactly conserved in any measurement, but all measurements "take awhile." I would say that time is a rather weird beast in QM (for example there isn't a formal time operator but there can be operators that function like time in given situations), so we need to talk about it carefully. There is a parameter "t" in QM, that we loosely regard as time, but it is actually only time in the sense that it will correspond to real time over the real time it takes to make a measurement to be able to say what happened. That really doesn't afford us the liberty of being able to talk about "at all times", because the ability to establish this correspondence over some given time t will always require an energy E~h/t, and that energy might simply not be available to test the expectation. It could easily hold that the correspondence breaks down at some energy scale that is so far undiscovered, and it is even expected to break down at the Planck energy scale.

    So I would agree that saying that virtual particles "pop into and out of existence" is a very awkward and informal picture, but I don't think we can say they don't do that either. I would simply say that the universe takes no position on the existence of virtual particles because it is never called out to take such a position, and the question that is never asked is also never answered. So I would say the whole question of "what exists" becomes murky at the scale of virtual particles, and it may be as wrong to exclude them from a transitory existence as it is to assert they do have a short-term existence. Can't we just say we don't have an empirical language to even talk about existence or non-existence at that scale?
     
  5. Sep 15, 2011 #4
    But apparently precisely that is what virtual particles are there for. They are the representation of how "you can violate energy conservation if you're quick about it" and get away with it. But this is not so weird due to the fact (as Ken pointed out) that it is not clear at all that energy is exactly conserved at all times, certainly that is not the case in GR where as Hilbert liked to say: general relativity has only improper energy theorems. And the HUP for time and energy precisely allows for energy not being exactly conserved at all times at certain energies. Virtual particles are just a visual device (and a calculational one) to justify this energy conservation violations.
     
  6. Sep 15, 2011 #5

    Bill_K

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    Well, KenG and TrickyDicky, it's difficult to believe that anyone would seriously entertain the nonconservation of energy, but that seems to be what we're doing. Energy conservation is the bedrock of all of physics, and it holds exactly in quantum mechanics, and in relativity too. Not only is energy conserved "at all times", it is conserved locally, meaning "at all points in spacetime." When you draw a Feynman diagram, the four-momentum at each vertex must add to zero. There is no "missing energy" feature that will let energy disappear at one point and magically reappear at another.

    Please do not confuse this with measurement, or the Heisenberg uncertainty principle. Uncertainty is not the same as nonconservation. If the energy of particle A is uncertain, fine; but that same uncertainty must be present in the opposite sense somewhere else, in particle B. The sum of the two must in all cases be exactly equal to the initial energy.

    Quantum mechanically, energy conservation can be thought of as analogous to charge conservation. The fact that electromagnetism is gauge invariant demands that it couple to a locally conserved current Jμ, meaning that electric charge is conserved, not just globally but locally at each spacetime point. You cannot have charge disappear in one place and pop up somewhere else later. Likewise, the fact that gravity is gauge invariant (general covariance) demands that it couple to a locally conserved quantity Tμν, meaning that energy-momentum is rigorously conserved at each spacetime point. Without this property the theory cannot be inconsistent.
     
  7. Sep 15, 2011 #6
    Bill_K, this is kind of funny because until a few weeks ago I used to be firmly convinced that "Energy conservation is the bedrock of all of physics", but in this time interval and after some sour debates with several science advisors in these forums I have to say that I have come to understand the fact that energy is not strictly conserved in GR (see the cosmology FAQ "What is the total mass-energy of the universe? " that starts with the phrase "Conservation of energy doesn't apply to cosmology"), or the article by Sean Carroll: "Energy is not conserved" http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/
    At the very least it is something debatable and there are contradictory views even from reknown physicists.
    Now keep in mind that what you have talked about in your post is actually "momentum-energy" conservation, and absolutely everybody agrees that that is an exactly conserved quantity. But in general relativity at least the distinct quantities energy, mass, momentum and angular momentum on their own are not globally conserved, (some of them according to some authors not even well defined yet).
     
  8. Sep 15, 2011 #7
    Also I don't think anybody is confusing uncertainty with nonconservation, even if some people conflates the general meaning of uncertainty with the HUP. What is true is that it is a known fact that what allows virtual particles to have at least mathematical existence is the enrgy-time uncertainty principle.
    And I don't think measurement and HUP should be confused.
     
  9. Sep 15, 2011 #8
    so what your saying is that virtual particles don't in reality pop in and out of existence?
     
  10. Sep 15, 2011 #9

    Vanadium 50

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    It's very difficult to have a discussion about virtual particles. There are 100x as many people whose understanding of virtual particles comes only from popularizations as those who have actually studied them. Unfortunately, when someone from the second group says something in conflict with the popularizations, people from the first group inevitably jump all over them telling them they are wrong.
     
  11. Sep 15, 2011 #10

    Ken G

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    Let me give you a different perspective. The issue at hand is not whether conservation of energy works essentially exactly "at the end of the day" when we compare to observations, it's whether it should be regarded as a fundamental truth underpinning everything that can be conceived of as having some claim on existence, or if it is just a kind of emergent truth that results from the more fundamental theory in the appropriate limits. To me, the beating heart of quantum mechanics is the concept that what actually happens is a kind of coherent sum over a vast number of barely conceivable things, and what culls out the actual from the barely conceivable is just one thing-- constructive interference. That's essentially the Feynman path integral picture, in spacetime a path integral involves paths that correspond to completely different energies, so the very principle of conservation of energy emerges from other possibilities that don't conserve it. The same idea underpins the principle of least action in classical physics. In this view, energy is conserved because nonconservation of energy gives rise to destructive interference, and for no other reason.

    However, constructive interference is not a fundamental law, it emerges from the sheer vastness of the number of contributing amplitudes, and it comes at a price-- it isn't instantaneous, it takes a little time to happen. Every example Bill_K gave above, albeit good physics, all involves outcomes that were fully time integrated over the interactions they refer to, so do not suffice to answer the question that was posed-- is energy conserved at all times. By that we don't mean was it in 1956 and will it be in 2012, we mean at every moment in time in the evolution of a system, because we are wondering if "virtual particles" can have some claim on existence during the short intervals of time where that constructive and destructive interference is actually happening, those intervals when what can happen and what cannot is still being decided.
     
  12. Sep 15, 2011 #11

    jtbell

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    See for example the following threads:

    https://www.physicsforums.com/showthread.php?t=302923

    https://www.physicsforums.com/showthread.php?t=75307

    https://www.physicsforums.com/showthread.php?t=460685

    https://www.physicsforums.com/showthread.php?t=506228
     
    Last edited: Sep 18, 2011
  13. Sep 15, 2011 #12

    Ken G

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    So it's a topic that you are tired of hearing about. What is demonstrably correct, and seems to be agreed on by all experts, is the quote from the field theory textbook:
    "The correspondence between the integrals that make up the Dyson series and Feynman diagrams is perfectly precise and well-defined. However, it is customary to go further and think of the Feynman diagrams as schematic pictures of physical processes, and here the interpretation acquires a more imaginative character. ... They are, in short, the infamous virtual particles that are so ubiquitous in physicists' discourse. In the final analysis, the only existence they possess for certain is as picturesque ways of thinking about the ingredients of the integrals in the Dyson series."
    Now, what does this mean? Apparently, physics can now be divided into two categories, ontological constructs that are actually real, and those that are "picturesque ways of thinking about the ingredients of the integrals." Just stop for a moment and think about this interesting dichotomy we have within physics, and you see how quickly it falls apart. In actuality, anything that is a picturesque way of thinking about the ingredients of integrals is just precisely what is meant by ontology in physics. But yes, you are tired of the question.

    Also, the suggestion that descriptions that take virtual particles seriously, or more correctly, as seriously as any of the many ontological crutches that we physicists routinely adopt without apology, is just pop sci, might seem insulting to well-known physics pedagogs like John Baez. Consider what he has to say about virtual particles:http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html. Is he saying they are really real? Of course not, no physicist ever needs to say that about anything but the outcomes of measurements. Is he saying they have as good a claim to existence as any of the many other ontological elements that are invented to help us understand the observable phenomena we encounter? Yes, I think he is saying that, but only in the usual "virtual", ephemeral, or too-short-lived-to-call-real sense of existence. So they are ephemeral, or they are just terms in an expansion, what difference does it make? Why is it so important to reject them as fantasy? Pedagogy is more of an art than anything else in physics. This doesn't make anyone wrong about virtual particles, it makes the issue quite nebulous, and perhaps that's what it is supposed to be when we are at the edges of what we can really describe.
     
    Last edited: Sep 15, 2011
  14. Sep 18, 2011 #13
    Hmm... nobody here even mentioned that in particle physics "virtual" has a very specific meaning for particles. It simply means "off mass shell". It is a real particle that doesn't obey the Einstein relation. However, they are only indirectly detectable.

    Best,

    Fred Diether
     
    Last edited by a moderator: Sep 22, 2011
  15. Sep 18, 2011 #14

    Ken G

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    That was my impression also, that in the spirit of a Feynman path integral, virtual means contributions from processes that cannot persist "at the end of the day", but can contribute to the amplitudes on short times, essentially timescales over which the interference emerges. So if something contributes to what actually happens, then it is "kind of" a real contribution, but not quite as real as what actually happens, ergo "virtual." I get that many other people just see it as labeling terms in approximate expansions, but I would say this just comes down to language, and whether or not one wants to use language that makes the Feynman path integral (or whatever correlation function calculation is needed) sound like it is "really happening", or if it is just some kind of calculation that has no direct correspondence to what is really happening. I'm not sure that issue can be adjudicated, it seems like "tomato tomahto" to me, but perhaps the field theorists have a stronger view about the evils of virtual particle language.
     
  16. Sep 18, 2011 #15

    Vanadium 50

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    There is very little to be gained with a model that allows people to learn wrong things quickly.

    Two points:

    • Any problem that can be solved using virtual particles can be solved some other way.
    • There are problems that cannot be solved by the virtual particle paradigm that can be solved some other way.

    These are facts, and because of these facts, it is difficult to ascribe some sort of independent reality to virtual particles. In many ways, they are a lot like the Method of Images in E&M.

    Unfortunately, writers of popularizations either do not know or gloss over this, and people get the idea that these are real entities, with measurable quantities like velocity and mass.
     
  17. Sep 18, 2011 #16
    There is a good deal of ontological talk going on here. What about the Casimir effect, supported by a relatively simple experiment, which offers evidence that virtual particles really exist in a demonstrable way?
     
  18. Sep 18, 2011 #17

    Vanadium 50

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    No, it doesn't. Read the threads that JT Bell posted.

    And you see the problem? People who have read some popularizations think they know things that simply are not true.
     
  19. Sep 18, 2011 #18
    "No, it doesn't. Read the threads that JT Bell posted.

    "And you see the problem? People who have read some popularizations think they know things that simply are not true."

    With all respect, I have read those threads and do not see what you are talking about. Is it possible you could avoid bidding me to look through scores (or hundreds) of replies and simply address the issue of the Casimir effect directly here?
     
  20. Sep 18, 2011 #19

    jtbell

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    See the following post, which references a paper that calculates the Casimir effect without using virtual particles:

    https://www.physicsforums.com/showthread.php?p=3370377#post3370377

    (I've just added this thread to the list in my earlier post. It wasn't there when you looked at the post earlier, and I'm not trying to make you look stupid for not having seen it before. I found it by doing a Google search for "casimir effect virtual particles site:physicsforums.com".)
     
    Last edited: Sep 18, 2011
  21. Sep 18, 2011 #20
    Well, I would like to see how Vanadium 50 would model decay of pions and muons without virtual W bosons. The Casimir effect is actually still a bit contraversial.

    Fred
     
    Last edited by a moderator: Sep 22, 2011
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