Virtual Particles: Not Particles At All

Among the many tricky concepts which the layperson has to grapple with when learning about particle physics is something called “virtual particles”, which show up in cute pictures called “Feynman diagrams” along with “real particles”.  In most books for the public, some words are mumbled about the uncertainty principle and how virtual particles are particles that exist for a short time and then disappear.  Well, this isn’t really wrong, and it is the language that physicists use.  But I have found that the language is so misleading for non-experts that it leads to many confusions among my readers.   For this reason, I’ve taken a different tack in this pedagogical article, with the thinking that the most important thing for a layperson to understand about virtual particles is that they really are not particles at all, despite the name, and they don’t behave that much like them.  They are more of a generalized disturbance in a field, while a real particle, a nice ripple in a field, is a special one.

Why would you want to read this article?  Because in addition to the Cohen-Glashow argument against the OPERA experiment’s result on superluminal neutrinos, which appeals to a process similar to Cerenkov radiation, there’s a more powerful, but more subtle, argument that involves virtual particles.  I’ll explain that in my next post.

34 responses to “Virtual Particles: Not Particles At All

  1. Pingback: Virtual Particles: Not Particles At All | Physics and Co. | Scoop.it

  2. Matt, Im confused. This forum: http://www.physicsforums.com/showthread.php?t=658412
    clearly states all quarks are created and annihilated from the vacuum and that none are special and persistent. This would mean all quarks in QFT are virtual particles right and there are no real particles?
    please help, Lee

    • If you read an assortment of comments written by non-experts and partially educated students, you will end up deeply confused. Most of the comments made in that forum were mostly wrong. A few are partially correct. The only correct ones are those that said “this is too complicated for this forum”. Yes, the forum makes clear statements, all of which are incorrect to a greater or lesser degree. In my opionion, you should mostly ignore forums like that; they are unintentional purveyors of misinformation. You should get your information from established experts, and when someone tells you something about particle physics, you should insist they also tell you how many years of study of quantum field theory they themselves have undertaken.

      I have taught quantum field theory for many years; you are more likely to get correct information here.

      As for the statement “all quarks in QFT are virtual particles right and there are no real particles”, that is both wrong and an extremely subtle point.

      Let’s start simple: if you made the statement “all electrons in QFT are virtual particles right and there are no real particles” that would be incorrect in all but the most arcane senses. The electrons in atoms, or in an electron beam, are real; they can be measured to be real because they are both stable and weakly-interacting with their environment. (The fact that they are “created from the vacuum” is a mathematical statement about how we physicists calculate their behavior, and is not a statement about the physics of where electrons come from in nature.)

      Now, what makes quarks and gluons in a proton more complicated is that they are constantly interacting strongly with each other, and indeed they are constantly disappearing and appearing, such that there are always three more quarks than antiquarks in a proton. [Same for a neutron.] You can’t follow any one of these quarks or gluons long enough to say whether it is real or virtual; indeed the distinction doesn’t really exist in a clear, well-defined sense. To define a particle as real you must be able to measure its properties, and that takes time. There’s no time to set up such an experiment inside a proton; an up quark will be gone, or will at least have changed its velocity drastically, before you can study it.

      However, in a bottom-quark meson like the B_s meson mentioned here, http://profmattstrassler.com/articles-and-posts/tevatron-news/a-rare-and-interesting-decay-of-a-b-meson/ , there are many quarks, antiquarks and gluons, but (almost all the time) only one bottom quark. That bottom quark is real, to a substantial extent. It is much heavier than the other quarks and antiquarks, so it barely moves inside its meson. Because it is moving so little, you *can* to a substantial extent measure its properties. It is most certainly persistent!!!

      All of this is to say that the meaning of real and virtual is subtle and requires a lot of proper discussion of quantum field theory if you’re going to state it correctly. And you will not learn that from physicsforums.

  3. Matt, thank you very much for your reply. It was this comment here that got me confused:” Maybe the best way of saying it in English is that the thing that we call an “electron” behaves mostly like a little teeny charged grain of sand if you don’t get too close; but if you could look at it closely enough you’d see a continuously churning maelstrom of N electrons and N-1 anti-electrons with N constantly changing as creation and annihilation is happening” is this comment true? because thats what makes electrons sound like there virtual…

    • Ok, I see what is bothering you.

      A real electron is one that you look at from a distance; it has the mass of 511 MeV/c^2, it is stable, and it is not being probed too hard.

      To see the effects you mention, you must look at an electron up close, quickly, and using a lot of energy. Through fast and energetic and highly probing experiments, you discover that a real electron is not a simple object. And that maelstrom (which contains not only electrons and positrons but also muons and antimuons and positive pions and negative pions and lots and lots of photons and even neutrinos and antineutrinos and so on) is indeed part of what an electron is… when you probe it this way.

      The fact that these two statements are not in conflict takes some getting used to.
      Note that it doesn’t bother you that (a) a proton is a real object, and (b) when you probe a proton with a rapid, short-distance and energetic experiment, it turns out to be a composite object made out of other things. It has a structure, and a size, and internal stuff that it is made from.

      The surprise is that even elementary particles — because of their interactions with the other fields of the universe — have structure. The electron doesn’t have a size the way a proton does, but still, it has some internal structure — which, with specific measurements, we can uncover. That structure reveals the interacting nature of the universe.

      What you must not do is confuse a real electron — the one with mass of 511 MeV/c^2 that can live forever — with a BARE electron, the one that appears as a line in a Feynman diagram. Lines in Feynman diagrams are just figments of a mathematical toolbox. Real physical electrons are things we measure — and in a universe where fields and their particles interact with one another, they are not the same things as the lines in the Feynman diagrams.

  4. * they are virtual, excuse my punctuation

  5. my main issue is with fermions, whats the position on them being virtual and not real?

  6. sorry I can see you have answered the question on fermions. Physics forums the way they put it is that virtual particles we dont if they exist, and hence if all matter is virtual particles we dont know if macroscopic matter exists. This was what I was getting from them that confused me. I mean obviously macroscopic matter exists right?

    • Electrons, protons and neutrons, atomic nuclei, photons, neutrinos, muons — all of these particles and their anti-particles exist as well-defined real particles. You can measure them — they are very much real. They are ripples in quantum fields with definite energy, momentum and mass, and their lifetimes are very long. Virtual “particles” aren’t particles; even if you try to write them down as sort of like particles, they can have any mass (including apparently imaginary mass) and their lifetimes are generally very short — as short as the processes which bring them into existence.

      • but as the forum said electrons are not well defined, if you look at an electron closely you see a continuously churning maelstrom of N electrons and N-1 anti-electrons with N constantly changing as creation and annihilation is happening. Same with protons. A proton IS just a mess of quarks and gluons creating and annihilating so how can that and the electron be a well defined ripple? im starting to think physics forums has corrupted me with confusion

        • A real electron is absolutely well defined. There’s exactly one real electron in a hydrogen atom. You are confused about what the maelstrom does and doesn’t mean. This maelstrom is part of what an electron is.

          I suspect one source of the confusion is this: the “N electrons and N-1 anti-electrons with N constantly changing” in the maelstrom are not REAL electrons — they are virtual, and they are not really things you can count. The REAL electron contains these virtual objects as part of its identity.

          A ripple in a non-interacting quantum field is simple, but the real world has interacting fields, and the resulting ripples are more complicated. Perfectly well-defined, but complicated. I have alluded to this in Figure 7 of http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ .

  7. lots of people also refer to virtual particles as mathematical and we dont know if they exist, whats your view on this? last question in a row, I promise

    • A given virtual particle, written as a line in a Feynman diagram, is just a mathematical device useful for a particular calculation.

      But I stated very clearly that virtual particles are non-particle generalized disturbances in quantum fields. These disturbances have many important physical consequences. So in this sense, they are very much real, and are responsible for all sorts of important properties and processes in the real world. For instance: Particle-antiparticle annihilation proceeds through effects in quantum fields that are not particle-like. The fact that quarks are confined in protons is due to non-particle-like behavior of fields. Now you can write formulas for some of these things down, using the internal lines in Feynman diagrams that people call “virtual particles”; that step is pure mathematics. But the physical effects that the Feynman diagrams are helping us calculate are certainly real, and certainly not due to real particles.

      In short — many things happen in the world that are due to non-particle-like behavior of quantum fields. If you want to describe that behavior as due to “virtual particles” you will not be far from what physicists usually mean by the term; the terms “off-shell particles” and “quantum fluctuations” and “vacuum fluctuations” are often better. If you want to write down Feynman diagrams to compute these things, you can call the internal lines in those diagrams “virtual particles” — but at that stage you’re doing math.

  8. but how can an electron be a well defined ripple when by looking at it closely we see a “continuously churning maelstrom of N electrons and N-1 anti-electrons with N constantly changing as creation and annihilation is happening” and a proton IS a flux of “quarks and gluons creating and annihilating” where is the fine ripple definition in that. A proton is a mess?

    • Internally a proton is a mess; externally, a proton is simple. It is not obvious that these two statements are consistent, but it is true. And the consistency is a consequence of aspects of quantum mechanics that are not obvious. Even a hydrogen atom is simpler externally than it is internally.

  9. sorry this response didnt save first time but now appears to of repeated itself. My only issue is with how can an electron be a real particle/a well defined ripple if when observing closer it becomes a churning maelstrom as above, and a proton is only a mess of virtual quarks and gluons that isnt well defined. Thats my only confusision. Your saying there well defined ripples but when studied in more detail, there not. whats your opinion on that?

  10. * sorry for my spelling and persistence. I wont bother you again after this, Im just too confused at the moment and its really frustrating me

  11. what about electrons? are they the same, a mess of electrons and anti electrons annihilating?

  12. electrons are mainly what confuses me as I am aware protons are just a bunch of virtual quarks and gluons, but electrons, whats the story with them…

    • That was the point of my earlier comment; electrons have structure too, as do all elementary particles in an interacting quantum field theory. They are more like protons than you might naively have expected. But they differ from them also; a proton has a definite size, and at any given time, some of the quarks and gluons inside it are closer to real than virtual. (And as I mentioned, in a bottom quark meson, the bottom quark is pretty darn real.) For a real electron, which does not have a definite size, all of the electron-positron and muon-antimuon pairs that you might find if you probe it closely are virtual.

      At some point, if you really want to understand this in detail, you have to take a quantum field theory class. There is, in the end, no substitute for the mathematics. On extremely subtle points like this, I can’t get you any further with just words. I explain this stuff in class to 2nd year graduate students, taking an hour to do it, and working out the math carefully.

  13. ive found your reply thanks

  14. the point is though you say when you probe an electron closely you see all the mess of it, the mess is what it really is and the single electron you see from outside is an illusion. correct?

    • I don’t think you want to use the word “illusion” here. Objects are what you measure them to be. If you look at an electron calmly, with low-energy, long-time probes, you will see it as a single simple object. If you look at an electron with a high-energy, short-time probe, you will see it behave differently, with a maelstrom of virtual disturbances around it. You cannot say what an electron’s true nature is; you can only ask: if I measure it in this way or in that way, what will I find? In other words, both statements are true; the electron is a simple object when not carefully probed, and is a complicated object when probed in detail. Neither statement is an illusion.

  15. oh so the fact it is made of virtual particles doesnt make it a virtual particle. so even though a proton is made of virtual particles, a proton isnt virtual. is this where your coming from…

  16. you stated this “The REAL electron contains these virtual objects as part of its identity.” but doesnt this mean a real electron is these virtual objects rather than “contains”

    • The word “contains” is probably not the best term, I agree. The virtual objects (which you find when you probe with high-energy and short time and distance) are part of the electron (which behaves simply when you only probe it with low energy and long-distance and time.)

  17. so its a sort of duality then, the virtual objects are part of the electron and the simple well behaved electron by itself with all its properties are also part of the electron?

    • Not exactly;, the simple well-behaved electron is what an electron is when viewed by a sufficiently low-energy probe. It’s not part of it, unless what you mean by the word “part” includes also the word “whole”.

  18. oh so its a kind of duality, the virtual parts are part of the electron when probed deeply and the simple single electron with its properties are also part of the electron?

  19. virtual when probed deeply, single well behaved when not probed deeply, both parts….

    • Look at Figure 7 in the Virtual Particles article. If you don’t probe deeply, you’re effectively averaging over a long time period, and you’re not going to see all those complicated happenings inside the electron… they might as well not be there, because your probe can’t interact with them. If you probe deeply, you are effectively taking a quick snapshot of the electron, and then your probe can potentially interact with those complexities, allowing you to detect them.

      Your probe, typically, is another particle — low-energy or high-energy — that interacts with the electron you’re probing. (Note it is not clear who is probing who — is your particle probing the electron, or is the electron probing your particle? This is yet another layer of subtlety…)

  20. ok im with you. But there is still a sort of duality in some respects that part of an electron is its total properties that appear as a single well behaved electron which isn’t probed in detail

  21. and the virtual particles ARE what the electron is when probed more deeply if we include the forces involved as well. correct? some properties though of the electron despite the flux always remain the same though such as charge and other quantum numbers?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s