The term “virtual particle” is an endlessly confusing and confused subject for the layperson, and even for the non-expert scientist. I have read many books for laypeople (yes, I was a layperson once myself, and I remember, at the age of 16, reading about this stuff) and all of them talk about virtual particles and not one of them has ever made any sense to me. So I am going to try a different approach in explaining it to you.
The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields.
Analogy time (and a very close one mathematically); think about a child’s swing. If you give it a shove and let it go, it will swing back and forth with a time period that is always the same, no matter how hard was the initial shove you gave it. This is the natural motion of the swing. Now compare that regular, smooth, constant back-and-forth motion to what would happen if you started giving the swing a shove many times during each of its back and forth swings. Well, the swing would start jiggling around all over the place, in a very unnatural motion, and it would not swing smoothly at all. The poor child on the swing would be furious at you, as you’d be making his or her ride very uncomfortable. This unpleasant jiggling motion — this disturbance of the swing — is different from the swing’s natural and preferred back-and-forth regular motion just as a “virtual particle” disturbance is different from a real particle. If something makes a real particle, that particle can go off on its own across space. If something makes a disturbance, that disturbance will die away, or break apart, once its cause is gone. So it’s not like a particle at all, and I wish we didn’t call it that.
Fig. 1: Two electrons approach each other; they generate a disturbance in the electromagnetic field (the photon field); this disturbance pushes them apart, and their paths are bent outward. One says they "exchange virtual photons", but this is just jargon.
For example, an electron is a real particle, a ripple in the electron field; you can hold one in your hand, so to speak; you can make a beam of them and send them across a room or inside an 20th century television set (a cathode-ray tube). A photon, too, is a real particle of light, a ripple in the electromagnetic field, and you can make a beam of photons (as in a laser.) [Can't have one in your hand though, since photons (in vacuum) are always moving.]
But if two electrons pass near each other, as in Figure 1, they will, because of their electric charge, disturb the electromagnetic field, sometimes called the photon field because its ripples are photons. That disturbance, sketched whimsically in green in the figure, is not a photon. It isn’t a ripple moving at the speed of light; in general isn’t a ripple at all, and certainly it is under no obligation to move at any one speed. That said, it is not at all mysterious; it is something whose details, if we know the initial motions of the electrons, can be calculated easily. Exactly the same equations that tell us about photons also tell us about how these disturbances work; in fact, the equations of quantum fields guarantee that if nature can have photons, it can have these disturbances too. Perhaps unfortunately, this type of disturbance, whose details can vary widely, was given the name “virtual particle” for historical reasons, which makes it sound both more mysterious, and more particle-like, than is necessary. [Students of math and physics will recognize real photons as solutions of a wave equation, and virtual photons as related to the Green function associated with this equation.]
Fig. 2: As in Figure 1, for a positron (an anti-electron) and an electron; now the slightly different disturbance causes the two particles to attract one another, and their paths are bent inward.
This disturbance is important, because the force that the two electrons exert on each other — the repulsive electric force between the two particles of the same electric charge — is generated by this disturbance. (The same is true if an electron and a positron pass near each other, as in Figure 2; the disturbance in this case is similar in type but different in its details, with the result that the oppositely charged electron and positron are attracted to each other.) Physicists often say, and laypersons’ books repeat, that the two electrons exchange virtual photons. But those are just words, and they lead to many confusions if you start imagining this word “exchange” as meaning that the electrons are tossing photons back and forth as two children might toss a ball. It’s not hard to imagine that throwing balls back and forth might generate a repulsion, but how could it generate an attractive force? The problem here is that the intuition that arises from the word “exchange” simply has too many flaws. To really understand this you need a small amount of math, but zero math is unfortunately not enough. It is better, I think, for the layperson to understand that the electromagnetic field is disturbed in some way, ignore the term “virtual photons” which actually is more confusing than enlightening, and trust that a calculation has to be done to figure out how the disturbance produced by the two electrons leads to their being repelled from one another, while the disturbance between an electron and a positron is different enough to cause attraction.
Fig. 3: An electron may naively be thought of as a ripple of minimum intensity --- the minimal ripple --- in an electron field. But the electron interacts with the photon field (i.e. the electromagnetic field) and can create a disturbance in it; in doing so it too ceases to be a normal particle and becomes a more general disturbance. The combination of the two disturbances (i.e. the two "virtual particles") remains a particle with the energy, momentum and mass of the incoming electron.
Now there are many other types of disturbances that fields can exhibit that are not particles. Another example, and scientifically one of the most important, shows up in the very nature of particles themselves. A particle is not as simple as I have naively described. Even to say a particle like an electron is a ripple purely in the electron field is an approximate statement, and sometimes the fact that it is not exactly true matters.
It turns out that since electrons carry electric charge, their very presence disturbs the electromagnetic field around them, and so electrons spend some of their time as a combination of two disturbances, one in in the electron field and one in the electromagnetic field. The disturbance in the electron field is not an electron particle, and the disturbance in the photon field is not a photon particle. However, the combination of the two is just such as to be a nice ripple, with a well-defined energy and momentum, and with an electron’s mass. This is sketchily illustrated in Figure 3.
Fig. 4: The Feynman diagram needed to calculate the process in Fig. 3. One says "the electron emits and reabsorbs a virtual photon", but this is just shorthand for the physics shown in Fig. 3.
The language physicists use in describing this is the following: “The electron can turn into a virtual photon and a virtual electron, which then turn back into a real electron.” And they draw a Feynman diagram that looks like Figure 4. But what they really mean is what I have just described in the previous paragraph. The Feynman diagram is actually a calculational tool, not a picture of the physical phenomenon; if you want to calculate how big this effect is, you take that diagram , translate it into a mathematical expression according to Feynman’s rules, set to work for a little while with some paper and pen, and soon obtain the answer.
Fig. 5: As in Figure 3, for a photon. The photon can become a disturbance in the electron field. This disturbance has some regions with negative electric charge and some with positive electric charge, but with total charge zero, like the incoming photon itself. The photon can do the same with other charged fields, such as the muon field.
Another example involves the photon itself. It is not merely a ripple in the electromagnetic field, but spends some of its time as an electron field disturbance, such that the combination remains a massless particle. The language here is to say that a photon can turn into a virtual electron and a virtual positron, and back again; but again, what this really means is that the electron field is disturbed by the photon. But why are we seeing a positron — an anti-electron — and yet I am only referring to the electron field? The reason ties back to the very reason that there are anti-particles in the first place: every field, by its very nature, has particle ripples and anti-particle ripples. For some fields (such as the photon field and Z field) these particle and anti-particle ripples are actually the same thing; but for fields like electrons and quarks, the particles and anti-particles are quite different. So what happens when the electron field is disturbed by a passing photon is that a disturbance is set up that has some electron-like disturbance with net negative electric charge, and some positron-like disturbance with net positive charge, but the disturbance as a whole, like the photon itself, carries no net charge at all.
For those who learned (and recall a bit of) freshman physics, what is happening is that the oscillating electric field that makes up the photon is polarizing the electron field — inducing a dipole moment. Remember dielectrics and how electric fields can polarize them? Well, the vacuum of empty space itself, because it has an electron field in it, is a polarizable medium — a dielectric of sorts.
Fig. 6: The Feynman diagram needed to calculate the process in Fig. 5. One says "photon becomes a virtual electron-positron pair", but this is just shorthand for the physics shown in Fig. 5.
The same is true, by the way, for all the other electrically charged fields, including those of the muon, the up quark, and so forth.
[Here, by the way, we come across another reason why ``virtual particle'' is a problematic term. I have had several people ask me something like this: `` Since the diagram in Figure 6 seems to show that the photon spends some of its time as made from two massive particles [recall the electron and the positron both have the same mass, corresponding to a mass-energy (E = m c-squared) of 0.000511 GeV], why doesn’t that give the photon a mass?” Part of the answer is that the diagram does not show that the photon spends part of its time as made from two massive particles. Virtual particles, which are what appear in the loop in that diagram, are not particles. They are not nice ripples, but more general disturbances. And only particles have the expected relation between their energy, momentum and mass; the more general disturbances do not satisfy these relations. So your intuition is simply misled by misreading the diagram. Instead, one has to do a real computation of the effect of these disturbances. In the case of the photon, it turns out the effect of this process on the photon mass is exactly zero.]
Fig. 7: The electron can generate disturbances in the photon field; the resulting photon disturbance can in turn create disturbances in other electrically charged fields, such as the muon field.
And it goes on from there. Our picture of an electron in Figure 3 was itself still too naive, because the photon disturbance around the electron itself disturbs the muon field, polarizing it in its turn. This is shown in Figure 7, and the corresponding Feynman diagram is shown in Figure 8. This goes on and on, with a ripple in any field disturbing, to a greater or lesser degree, all of the fields with which it directly or even indirectly has an interaction.
Fig. 8: The Feynman diagram needed to calculate the process shown in Figure 7.
So we learn that particles are just not simple objects, and although I often naively describe them as simple ripples in a single field, that’s not exactly true. Only in a world with no forces — with no interactions among particles at all — are particles merely ripples in a single field! Sometimes these complications don’t matter, and we can ignore them. But sometimes these complications are central, so we always have to remember they are there.
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Thanks for a great and understandable explanation.
Is it correct to say that these quantum fields pervade spacetime – or is it better to say the set of fields actually composes (creates?) spacetime? Also, what is the complete list of fields currently known?
Most fields are best thought of as pervading three-dimensional-space and time, except for the graviton field, also known as the “metric” of space-time (the object that is needed to decide how far part two points are) which is really intrinsic to space-time.
In most theories with extra dimensions, some of the fields that we observe would actually form a part of the metric of the higher-dimensional spacetime. In other words, one explanation as to why there are so many fields in nature might be that we live in a world that has some of its dimensions wrapped up (think of how a hose has a large dimension along the hose and small dimension around the hose) and that the metric of the full space-time looks to us, in three-dimensional space, like a metric for three-dimensional space and time along with many other fields whose explanation seems non-obvious.
A complete list of fields is ill-defined, but I can give you the list of apparently-elementary fields. [There are many non-elementary fields too, including the proton field; just as a proton is composite made from quarks, antiquarks and gluons, so is its field made from other fields. And the wind field and temperature fields in air, or a density field in a metal, are composite too.] The list of known apparently-elementary fields is essentially just the list of known particles, http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/, or even more completely, the list of particles before the Higgs gets a non-zero value, http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-particles-if-the-higgs-field-were-zero/ (plus the graviton field, which I didn’t put on the slides.)
A wonderful explanation; thank you!
Hi,
I have some questions about the meaning(s) of the word “particle” in the field of high energy physics:
1-What is the safest way to call a quantum object: an elementary particle? or a quantum particle? or a quantum wave? or we just call them by their names like electrons or neutrinos..etc? or the question is just meaningless?
2-What is the meaning of the word particle in “the branch of particle physics”?
3-Which is more fundamental, the quantum particle, or the quantum field? (I did not find consensus in this one)
4-And finally, what confuses me most, why “elementary particles” are irreducible representations of the Poincare group? and what is the meaning of the word particle in this context?
Sing
1) A name’s a name (see Shakespeare) — what matters is not what you call it but whether you understand it… fair? As long as you understand the object does not correspond to any concept in English or in daily life, and that it has some properties that you just have to learn about, you can call an electron an elementary particle, a quantum of the electron field, or a quantized wave in the electron field. Elementary particle is rather short and the least mysterious sounding. “Quantum” is shortest and most accurate, but sounds very mysterious.
2) You mean, what does “particle physics” mean when referring to it as a subject of study? Just that many experiments on the basic properties of nature require studying its particles, and if you do those experiments, propose them, study them, interpret them, etc., you’re doing “particle physics.”
3) Field is more fundamental. Not all fields have particles, while all particles are quanta of fields. That’s why the equations used to describe the physics of the standard model are called quantum field theory, not quantum particle theory. [An example of a quantum field theory that has no particles is a ``conformal field theory'', very important in studying matter at a phase transition, such as a magnet at the critical temperature where it loses its magnetization.]
4) Start with classical physics (no uncertain principle) to keep things simple. Then a massive particle is a localized object with a definite mass, energy, momentum and position, and orientation. Let’s put one in front of you, at rest. I can now generate a representation of the Poincare group — the group of all translations, rotations, and changes of reference frame (boosts with a constant velocity) — by simply moving the particle to a new position, rotating the particle, or looking at the particle from a different frame. That set of particle states (with all those different positions, orientations, and velocities) form a complete representation of the Poincare group.
The only thing different in quantum mechanics is that I can’t precisely localize the particle at the same time I precisely determine the momentum and energy. But I can do a pretty good job on both, or a perfect job on either one. This complication doesn’t change the argument that the full set of things that a particle can do form a representation of the Poincare group.
For a massless particle I can’t start with a particle at rest, but the basic strategy for finding the full representation is the same.
In dense aether theory the water surface can serve as a low dimensional analogy of 4D space-time and the density fluctuations of the underwater are the analogy of virtual particles, after then.
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Matt – This is one of the most helpful pieces I’ve read in years. Thanks so much.
So what you are saying essentially is that transient ripples, which are not stable or sustainable, are caused by every particle in every other field with which they can interact. If another particle is present, their ripples combine and the particles DO interact. These ripples look in *some* ways like the ghost of a particle in that field, enough that we can use the equations of particles to calculate effects. But in other ways, including their stability, they’re not really like particles at all.
Okay. I got everything except how “virtual particles” are involved in particle CREATION. At some point, wouldn’t the transient ripples have to coalesce into wave(s) that ARE stable?
That’s basically right. I wouldn’t use the term “ghost” (for two reasons, one being that ghost is used as a technical term elsewhere, but more importantly that the relation is really that disturbances are the more general case, while a particle is an extremely special form of disturbance.)
How “virtual particles” can create something: the transient ripples themselves are still not stable in this case. They fall apart, into true particles.
For example, if the two electrons in my picture came together with enough energy, the transient ripple shown in the figure could have enough energy to produce an electron-positron pair (so that there would be four particles in the final part of the picture, three electrons and a positron.) The disturbance in the field can carry lots of energy, and that energy can be turned into particles. But the disturbance itself is still transient. Does that answer the question?
Nice article. I noticed a typo: “The language people physicists “.
thanks.
One of the things that tripped me up as a student, was also just how arbitrarily small you could make the difference between real and virtual.
Usually the definition of a virtual particle is a ‘thing’ that does not satisfy the usual energy-momentum rules, or the ‘thing’ which is an internal leg in a Feynman diagram.
But then every particle we have ever observed is an internal leg of a bigger Feynman diagram. For instance, the electron we measured from a particle accelerator eventually is absorbed by an atom somewhere, and hence becomes an internal leg in that diagram. So then most people say, well its just a question of lifetime. Long lived particles are called real, short lived particles are virtual.
But then suppose you have a photon that was emitted from the Pleiades, and it presumably is real (or almost real) all the way until it is absorbed in your retina. So have you just measured a real or virtual photon? It sorta depends on how you draw the diagram!
You are absolutely correct to point out this subtlety! It is, in the end, a matter of degree.
In general, what we have in quantum fields are disturbances of many types. There is a very special disturbance we may call a particle, which is a ripple that can in principle travel forever. But this is an idealization: any real particle interacts with other objects, and this means nothing is ever exactly this precise, idealized ripple. So the issue is how close is it to the ideal case. In most physical processes one deals with objects that are clearly either close to the ideal or very far from the ideal. A photon traveling from the Pleiades is clearly about as close as you are going to get to the ideal; its energy and momentum are almost the perfect match that you would expect for a massless particle. The disturbance between [a "virtual photon exchanged"] between an electron and a nucleus in an atom has very little energy and a lot of momentum; it is very far from what you would call a particle.
If I’m understanding correctly, your saying virtual ‘particles’ arise because of ‘nearby’ real particles disturbing a field, those disturbances being the virtual ‘particles’. That left me wondering what then explains the ‘vacuum energy’ of empty space? I naively thought virtual ‘particles’ where popping in and out of existence with no real particles nearby. I took it that the cosmological constant was somehow ’powered’ by vacuum energy ?
Thanks for the nice article. When reading about Hawking radiation, one is told that when a pair virtual particles is created near the event horizon, one may fall into the black body and the other escape as radiation. Are the virtual particles in this situation somewhat different than the ones you describe? Do they become “real” particles in this case?
A wonderful picture of the world on “Quantum field theory ”, recall wave-particle duality ,what about “Quantum entanglement ”？
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My lay understanding was that virtual particles “challenge” conventional notions of cause and effect, but you use the word “cause” in very conventional ways in this article.
Could you try to help me figure out where this notion comes from? Do you know where you read it? I have some guesses as to where this conception comes from, but I wonder whether there are modern books promulgating the idea. While it is true that one has to be careful in general about assuming that all processes can be described in terms of cause and effect (even before accounting for quantum mechanics), and also true that quantum mechanics is weird, no doubt about it , there is no profound challenge to basic causality in this context. Certainly I do not think you will not find any discussion of challenges to causality from “virtual particles” (i.e. generalized disturbances in fields) in any modern quantum field theory book.
Is this “virtual particle=>causality issue” maybe coming from discussions like those in Bjorken and Drell section 12.3 ? My reading of their conclusion is that there isn’t really a problem though.
I’m confused as to what explains the ‘vacuum energy’ where there are no real particles disturbing any fields ? what powers the cosmological constant ?
There’s plenty of energy in those disturbances. You seem to be assuming that the only way fields can carry energy is through their particles, but this is not the case. Analogy: one way for a spring to carry energy is to go back and forth at its resonant frequency, but a spring could carry energy even if you pushed it back and forth in an arbitrary way. And a quantum spring, thanks to the uncertainty principle, will move around, in a limited but arbitrary way, and with non-zero energy, even if you just leave it alone. The difference in the math is the same for particles (resonant behavior that can go on its own) versus general disturbances (which require an outside push, or just the jiggling assured by the uncertainty principle.)
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As an electrical / electronics engineer with a strong interest in physics (but not great at advanced math) your articles have explained more to me in a few hours than in the several years I spent in secondary and tertiary education. You write and diagram with amazing clarity. I do notice that when questions are asked about speculative physics (such as the possibility of extra dimensions), you confine your responses to answers that can now (or may be in the near future) subject to experiment.
Your explanation of “virtual particles” here eliminates most of my confusion about articles on the subject I have read elsewhere, but two bits of confusion persists; if “virtual particles” are complex disturbances in a field, then, can the disturbances (through Fourier transforms) be described as an agglomeration of resonances (particles)? also, what did / does Prof. Hawking mean by HIS use of the term in relation to what happens at the event horizon of a black hole? (I know I should ask Prof. Hawking, but he is somewhat less accessible
)
In your article “What’s a proton?” you state that there are “zillions of gluons, antiquarks, and quarks in a proton”. Do most of these qualify as virtual particles as described in this article, i.e. as mere disturbances of their fields that are in many respects quite unlike real particles?
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Once again, brilliant. I understand so much more about particle fields after reading this article.
If a particle is “a nice, regular ripple in a field”, how would you describe a string from string theory? What will they appear like in the field?
Well, a string is a ripple in a string-field, something for which we have no daily intuition. We have intuition for fields because we live in a wind field, a temperature field, a air-density field, and so forth, and wind, sound, heat diffusion are all things we can imagine and study rather easily. But a string field actually contains an infinite number of types of ordinary fields — and we don’t encounter one in daily life — so you can imagine building an intuition for that would not be easy. Let me think about how I might explain this one someday — this will not be coming soon.
What are the grounds for calling this ripple a “string”, in contrast to a point-like “particle”?
More or less what you’d think: if you strike it, you will find an elementary string can respond in all sorts of ways — by wiggling in all sorts of different configurations — while an elementary particle cannot do that.
If a particle is “a nice, regular ripple in a field” would it be similar to a phonon concept in a virtual-particle “Dirac-sea”?
No, what I have in mind is simpler. Think of light — an ordinary ripple in the electric and magnetic field. Now remember that in quantum mechanics that light cannot be arbitrarily dim — there is a dimmest possible flash, a ripple of lowest possible intensity. That dimmest possible flash — the ripple of lowest possible intensity — is a photon.
Does that help?
Nice. Thanks for this.
Sorry if this is a double-post. I tried to comment earlier but it didn’t seem to go through (which is hopefully a good thing, because I’ve since edited the post.)
This is all extremely fascinating—thank you!
The impression I’m getting about the fundamental nature of matter is as follows, and I’d love to know if I’m more or less on the right track. Please forgive me if I’m way off-base; the last thing I want to do is confuse other lay-readers like myself with falsehoods!
–Quantized fields that permeate the fabric of space-time—rather than particles—might be thought of as truly “fundamental.”
–A “fundamental particle” is a stable ripple of contained energy on a field.
–At least partly because of the quantized nature of fields, stable ripples in a given field (that is, all “real” particles of a given type) can’t just have any old values for their properties. Instead, they must have certain characteristics in common (mass, spin number, etc.) and are essentially indistinguishable from one another.
–There are plenty of unstable disturbances in a field, too (more of them than stable ripples even?), which can only be very short-lived and need not behave the same way as stable ripples (particles) do in the same field. They are known confusingly as “virtual particles.”
–Stable ripples (“particles”) and unstable disturbances (“virtual particles”) alike interact with other ripples/disturbances in their own field and/or in (some) other fields (the specifics of which other fields’ disturbances a given field can interact with vary from field to field).
–The results of these interactions among field ripples/disturbances are matter and forces as we know them.
If I’m basically on the right track, I have a few questions to throw at you:
I understand that the Higgs field is thought to pervade all of space-time. Is the same true of other fields associated with elementary particles, such as the “electron field” and “photon field” you’ve mentioned? Is it thought that there is just one “electron” field that exists everywhere?
When you speak of unstable disturbances (short-lived “virtual particles”) in a field—do these phase in and out everywhere, all the time, in every field, in something like a uniform fashion? Is this what is meant by “quantum fluctuations” in a vacuum, or “vacuum energy”? If so, is there a theory as to what “causes” these disturbances? In other words, are these disturbances always the result of the presence of other nearby particles (real or virtual) interacting with the given field? Or is this just something fields do?
Am I right to think of a field as something like a fabric of quantized energy?
I really appreciate what you are doing on this site. Many thanks in advance for any corrective feedback you can provide for my evolving conceptual comprehension of the nature of reality. I know I can never fully understand this stuff without the math, but I really want to try!
Remarkable. Almost everything you’ve said is basically right. A small issue:
“A “fundamental particle” is a stable ripple of contained energy on a field”
well, the words “contained energy” aren’t really necessary and don’t really have content. Any ripple in any field has some energy. So let’s just say “A “fundamental particle” is a stable ripple on a field” and be done with it…
… except for one additional subtlety with the word “stable”… it needs to be “relatively stable”, because most particles eventually decay to other particles, though it takes a while.
Now, to your questions.
Yes, all fields related to the known elementary particles are believed to exist everywhere in space, and at all times. There is just one electron field that is everywhere in the universe. What makes the Higgs field different from the other known fields is that it is non-zero on average everywhere in the universe, while, say, the photon field [i.e. the electric and magnetic fields treated together] and the electron field are on average essentially zero.
Quantum fields are constantly fluctuating, and the unstable disturbances that we call “virtual particles” are always there. It is just something that quantum fields do, and the mathematics known as “quantum field theory”, which I teach to first and second year graduate students and which is very well established both theoretically and experimentally, does a great job of predicting the details of these fluctuations/disturbances/virtual particles. No lingering mysteries here, not for many decades.
I don’t know what a “fabric of quantized energy” is or means. You can’t really explain fields in terms of more fundamental things, at least not at this time; as far as we know, they are the fundamental things. Fields are just the basic ingredients of our universe, in our current view. Of course this picture of the universe is likely to evolve over time as we learn more, and so my point of view may someday have to change. Right now it is consistent with all experiments.
Great! Thanks for this. Conceptual explanations of this material are incredibly hard to come by, which is really a shame, since it’s endlessly interesting. Have you ever thought about writing a book on particle/field physics for laypeople? I’m sure you have real science to attend to, but it’s worth keeping in mind that there are probably very few people in the world who possess both your level of expertise in the discipline and your knack for dumbing things down juuust the right amount when explaining mind-blowing and unintuitive quantum truths to the mathematically challenged.
Here is another one, if you find a few free moments: does QFT have implications for making sense of wave-particle “duality” and wave-function “collapse”? Given your understanding of particles as (relatively stable) ripples in constantly fluctuating fields, how do you wrap your mind around the observed effect of a single electron apparently “interfering” with itself in the double-slit experiment?
Thanks again,
Mike
I am writing a book for laypeople. Slowly.
Quantum field theory does not help make quantum mechanics less weird. It doesn’t make it more so…
There are reasons I have not discussed the difficulties of making sense of a quantum mechanical world. It’s hard to do it well, and I’m not ready.
The way I wrapped my mind around the double slit experiment is that I watched the experiment being done (with photons, not electrons, but it’s the same effect.) So then I had no choice in the matter. I’ll write about this someday.
This is a great article.
I do have a few questions though. I am studying particle physics as an undergraduate and I am struggling with this concept a bit. After immense amounts of research, I have found no explanation as to why virtual particles are needed to explain forces. What do particles offer that fields do not? Can oscillators in a field not be explained by quanitizing the field? I’ve read most all of Feynman’s lectures and many others and have found no answer.
Also, I am a confused about the state of the virtual particles before and after the interaction between two field particles. Do they only form when approaching other field particles? Are they emitting these virtual particles at all times? What actually happens to the particle after the interaction?
Thank you very much for this article. It has helped me very much conceptually.
Do you understand how Green’s functions are used to explain the force between two electrons in classical electrodynamics?
Not exactly. I know Green’s functions are used to solve advanced differential equations to sometime correlate functions but I have a weak background in electrodynamics and have not taken any classes on it yet. Is this where I should look to find my answer?
Yes. A virtual particle is to a real particle as a Green’s function is to a wave.
A wave is a solution to a wave equation, with zero on the right hand side of the equation. Such a wave can propagate on its own. Light is an example; it can travel through space even if there are no electric charges around. Quantum light is made from quanta [i.e. photons] and these are what we call “particles” of light.
A Green function is a solution to the wave equation in the presence of a point source on the right-hand side. You learn how two electrons affect each other by studying the field at one electron’s location due to an electron at a second point; this involves the Green function with a point source at the location of the second electron. A simple example of a Green function is the 1/r electrostatic potential between two static electrons. The potential would never take this 1/r form on its own, if the charges were not there.
Hey Matt
Discovered your blog over the weekend am a second year physics student at the university of south australia but all the people here are dopes and not interested in fundamental questions. I’m interested in the background independent aspect of the field theories ie: what if we throw away space time altogether and just haver the fields and their interactions? . Bothered by you’re inclusion of the “muon field” as seperate from the electron field in this discussion tho. The muon and the tau should just be other kind of excitations in the “electron field” right? the question is how many fields and how many ways to fold them? the less the better
Your field question is slightly ill-posed, though probably you can refine it. A field in classical physics is a function f(x,t), with a set of differential equations that govern its behavior. If you throw away space and time it becomes a number f with a set of algebraic equations that determine it. Quantum mechanics of such a field is just ordinary integrals. This is probably not what you had in mind, but I’m not sure yet what you really did have in mind. Did you want to remove the *metric* on space and time [i.e. our ability to measure distances?] while keeping the space and the time around?
There is strong evidence against thinking of the muon and tau as excitations in the electron field. When you excite an atom, the thing you expect is that the excited state can decay to the ground state by the emission of a photon. But muons decay to electrons only via the weak nuclear interaction, during which they spit off a neutrinos and an anti-neutrino. The process muon –> electron + photon has been searched for, but very extensive experiments have never seen this process (or the corresponding ones for taus) and the result so far is that less than 1 in 100,000,000,000 muons decays this way.
You might still wonder whether that just means there’s something special about the way that a muon and a tau are excited forms of the electron that is different from atoms. Well, here’s more evidence. If the electron is special, in that the muon and tau are excited versions of it, then you would not expect a tau to decay to a muon (plus a neutrino and anti-neutrino) at the same rate that it decays to an electron (plus a neutrino and anti-neutrino). But in fact the rates for these two processes are the same.
Finally, the electron and muon and tau are in some sense intrinsically massless; they only develop a mass when the Higgs field becomes non-zero (see http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-particles-if-the-higgs-field-were-zero/ ). So their masses are determined by how they individually interact with the Higgs field, not by some internal dynamics.
I guess I should mention that in atomic physics there are many excited states with heavier and heavier masses, but beyond the tau there’s no sign of any fourth lepton even up to masses over 100 times larger than the tau’s mass.
You may contrast this with the proton, for which there are many excited versions, all of which decay preferentially back to a proton plus some pions (which for proton-like objects, made from quarks, gluons and antiquarks, is an even more efficient process than emitting photons).
Why should we have three versions of almost the same type of particle? And why would they all interact very differently with the Higgs field? There have been many, many proposals, but so far nothing is yet known about the answers to these questions.
Dear Sir,
In the vacuum of space, if there are two bodies A and B and if there is absolutely nothing between them they should come together since there is nothing to keep them apart. They must be some form of energy field that creates the space to keep them apart. This energy field which keep the planets and stars apart could have been created during the big bang.
Gravity could also be explained by assuming that any body with a mass will be capable of absorbing this field and creating a decreasing density of this field as we go nearer and nearer to this body.
So before the big bang since there is really absolutely nothing, with no time and space, all virtual particles that are being continuously created out of nothing should come together and occupy the same point and as such would ultimately form a singularity with almost infinite density and triggers off the big bang.
This could happen if there is a mechanism which separate these virtual particles from their anti-particles.
So has anybody found such a mechanism?
Dr HW Looi
gmail: looihw88@gmail.com
Your first point is correct, but incomplete.
” if there are two bodies A and B and if there is absolutely nothing between them they should come together since there is nothing to keep them apart. ”
Two bodies A and B that are stationary relative to one another will gradually come together under the force of gravity.
However, if they are moving relative to one another, conservation of angular momentum will drastically slow the rate at which they can come together. That is why the planets can orbit the sun for billions of years under the force of gravity.
So there is no need for an energy field to keep objects apart and to explain why objects do not come together.
“before the big bang since there is really absolutely nothing, with no time and space, ”
We don’t know that.
Dear Sir,
Thank you for answering my question. But I think you have completely misunderstood what I was trying to say.
Of course I know very well that the conventional Einstein concept of gravity is that it bends space-time and as such causes two objects to move towards each other. And the Newtonion concept is that it is nothing more than just an attractive force.
What I am trying to say is that there maybe another more simple concept.
If there are 2 objects in the vacuum of space, and if there is really absolutely nothing in between, the two objects do not just move towards each other, but rather they should be next to each other!
Take another analogy. If there are 2 chairs that are 10 feet apart. There is a “space” of 10 ft and if there is absolutely nothing in between, including no space and time, the chairs would be next to each other and they do not need the conventional gravity to get them moving towards each other.
The fact that objects in the vacuum of space are separate is because there is “space” in between them and this space must have been created by some form of energy field.
Thanking you especially for you great patience and endurance,
Dr HW Looi
email: looihw88@gmail.com
Your statements are correct until the last one.
“this space must have been created by some form of energy field.”
This is both a bit illogical and also somewhat confused. A field is one thing; energy is something it can have, but there is no such thing as an “energy field”. There are electric fields; particles that are ripples in these fields are called photons and they can carry energy from one place to another. There are gravitational fields; particles (hypothetical but deeply plausible) that are ripples in these fields, essentially ripples in space itself, are called gravitons, and they too can carry energy from one place to another. But an “energy field” wouldn’t make any sense. You must mean something else.
In general, we don’t know why there is space, or why there is anything that can be called a “universe”, but in principle it need not be created by some other field, any more than the electron or quark fields that are found throughout space need to be created by some other field. There are many theories as to how space comes to be, including theories that have time but not space at first, and then develop space through a very subtle mechanism that is far too difficult to explain here. But once you have space, you can have objects in it, and they will remain separate.
Am I right that your real question is “why is there any such thing as space, through which waves and particles can move and in which one can find objects?” If so, the answer is not known, but it does not have anything to do with an energy field.
Dear Sir,
Thank you very much for your great patient and explaination.
I really do appreciate it.
So I will just have to change the last part of my statement to “there is a tiny possibility that this space may be created by some form of matter or energy, but we really don’t know what space is made of.”
Thanking you,
Dr HW Looi.
The importance of virtual particles is in whether they have an effect on measurable quantities.. If they had no effect, then studying them is just a mental exercise. Vacuum polarization of virtual particles is a real effect, and in the case of pionic atoms, the atomic energy levels are shifted by a large amount. The first precise measurement of the pion mass was done by measuring the atomic transition energies of pions in pionic atoms, and calculating the level shift due to vacuum polarization. See Appendix B in Robert E. Shafer, “Pion Mass Measurement…”, Phys. Rev. 163, 1451 (1967) The vacuum polarization effect was first calculated by Uehling in Phys Rev (1935).
This atomic level shift is related to charge renormalization, in that virtual particles in strong Coulomb field partially shield the bare Coulomb field..
Robert Shafer
Hi, do you know how I found this? At a depression website bulletin board labeled as off topic. Some of the smartest people I know are nuts. And of course, I didn’t get there by accident, even though I got here by accident.
I love this stuff. Total lay person here. I was a surveyor though and I can use sine, cosine and tangent to lay out the corner of the latest Walmart building to within…well, a thousandth of a foot. Close enough for concrete. And using those 3 functions and a 20 dollar calculator and a hundred foot tape, I can check into known points, inverse some rectangular coordinates into angle and distance, and voila! Attention shoppers, Brian has caused all this.
You write so well…better than Feynman. That’s right, I said it. Q E D (major and minor premises, with a happy ending type of Q E D)
Thanks, excellent. The analogies, ie. the spring, the swing, are huge, Feynmanesque. My spatial faculties have grown a lobe.
layman Brian, eating in a corned beef and cabbage field, interacting with a coffee swallow field. It is almost time for a cigarette field. (Did you ever look at cigarette smoke in a sun ray in a overly windowed room? It is 1.) blue. and 2.) made up of tiny particles.)
“layman Brian, eating in a corned beef and cabbage field, interacting with a coffee swallow field. It is almost time for a cigarette field.”
this made me smile
I love these articles. Like you, I’ve read many books for laypeople on particle physics (and even glanced into a few for experts), and your articles are the first that have given me even a glimmer of understanding of quantum field theory.
The explanations of virtual particles I’ve read have generally been along the lines of “the uncertainly principle allows conservation of energy and momentum to be violated over very short times, and virtual particles are the result.” How does this relate to what you say here? Or is it another “white lie”?
Another white lie, but it gives roughly the right estimates (when you apply the uncertainty relations) for how common are quantum disturbances of a given size. That’s why people talk that way.
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Prof. Strassler,
Thank you for this article. I have a few questions:
1) I always thought that, in the context of two “real” particles interacting with each other, Feynman diagrams were a mathematical convenience, with each diagram representing a term in the perturbation expansion, and virtual particles were nothing more than a pictorial way of representing the propagator. You characterize a virtual particle as a “disturbance” in the field; how does one reconcile the “mathematical convenience” view of virtual particles with the “disturbance in the field” view of it?
2) I have read the “Schwinger limit” described as the electric field of a laser which is strong enough to pull virtual electron-positron pairs out of the vacuum and make them real. Using your language, the laser field creates disturbances in both the electron and positron fields, i.e. polarizes the vacuum, and if these disturbances are large enough, they become ripples in the fields, i.e. a real electron-positron pair. Is that an accurate description of what happens when a strong laser field interacts with the vacuum and creates electron-positron pairs?
3) How can the following statement from the Wikipedia page for virtual particles be reconciled with your description of virtual photons as “disturbances” in the photon field:
“Virtual photons are also a major component of antenna near field phenomena and induction fields, which have shorter-range effects, and do not radiate through space with the same range-properties as do electromagnetic wave photons. For example, the energy carried from one winding of a transformer to another, or to and from a patient in an MRI scanner, in quantum terms is carried by virtual photons, not real photons.”
4) At one point you said, “Exactly the same equations that tell us about photons also tell us about how these disturbances work; in fact, the equations of quantum fields guarantee that if nature can have photons, it can have these disturbances too.” Could you be more specific? I am trying to relate what you said here to what I have read in QFT books. How does your description relate to the S matrix, propagators, and perturbation expansion? I have seen the QFT formulation of electron-electron scattering which leads to a scattering cross section, but how does the repulsive force follow from that?
Thanks,
Neil
1) propagator = Green function = off-resonance disturbance in the field caused by a source. Example: 1/r electric potential between charges.
singularity in the propagator = resonance = ripple in the field that can travel indefinitely without a source. Example: photon
2) Yes. If the energy is large enough the singularities in the propagators can be accessed and there is a non-analytic change in the response of the vacuum. This also happens for quarks and antiquarks in a strong gluon field, in the context of jet formation; see http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/jets-the-manifestation-of-quarks-and-gluons/
3) See (1); the near-field effects are off-resonance. Photons are on-resonance.
4) See (1): the equation for a Green function G for a field phi is O(G) = J, where O is some differential operator and J is a source; the equation for a resonance is O(phi) = 0, for the same operator O.
Said differently: the “virtual particle” i.e. Green function i.e. propagator satisfies an inhomogeneous linear differential equation, while the real particle satisfies the homogeneous version of the same equation.
Thank you!
The effect of virtual particles (electron positron pairs) on the Coulomb field of nuclei was first calculated by E. A. Uehling in 1935 (see http://philoscience.unibe.ch/documents/physics/uehling35/uehling35.pdf ). and this seems to be very accurate in predicting atomic level shifts in muonic atoms (atoms with an electron replace with a muon)..
Dear professor Strassler
I am so happy to have found your website. It has been extremely educating. thank you. I hope you write the book(s). There at least two books: the perspective for the layperson; an undergraduate level for budding physicists.
I finished my doctorate in physics back in 1995 and became a patent consultant. My doctorate thesis was on magnetic thin films. I did some modelling work on the experimental results. The model explained a number of things especially interlayer magnetic spin coupling and it made some predictions that could be tested. After Presenting the model at a conference, a friend asked me a profound question: Abu is this what is really happening at the spin level in the thin films? I replied that the model fitted the data well and made predictions that could be tested and where tested. And so based on that it was a valid model. However, regarding his question in the final analysis I have to be honest and say I don’t know.
Now coming to ‘virtual particles’, ‘quantum fields’, ( quantum mechanics) etc and their underlying models they are indeed useful, powerful but in the final analysis do they describe reality that actually exists?
Regarding quantum fields for some reason I always imagined them to be one-dimensional lines: is it better to think of them as volumes – since they fill all space? And is regarding the different (particle, force) fields as being intertwined a useful analogy when thinking about particle decay?
Many thanks
Abuisa
In the case of the thin films, we do understand that there is another layer of reality below the layer that you were using to describe the phenomenon — so the question you were asked was well-posed.
But when you ask “in the final analysis do they describe reality that actually exists?”, the answer has two parts. First, how do we know we are anywhere near a final analysis? There could certainly be more layers of reality beyond the ones we’ve encountered so far. What experiment can you imagine doing that could ever answer your question with finality? And if no experiment exists, is your question a scientific question, in the end?
Furthermore, what does “actually exist” mean? can you imagine an experiment that tells you whether something really exists or whether it is just a useful tool for describing and predicting the world? Again, if there is no experiment, can this question be answered scientifically?
As far as we know, the description of the world using quantum fields and so forth is an excellent way to think about the world that allows for a vast array of measurements to be predicted in advance. But we neither know this is a unique nor a final way to think about reality.
Probably the reason you think of fields in terms of one-dimensional lines is because of what you know about the electric field, which is indeed drawn as field lines. But that does not work for all types of fields. Generally, a field is a function of space and time — start with that. It might just be a number at every point: F(x,y,z,t). In the case of an electric field, it would be a vector at every point: E_x(x,y,z,t), E_y(x,y,z,t), E_z(x,y,z,t); in fact you need to generalize this to include also the magnetic field to make it consistent with relativity, but this isn’t so important here. After Einstein, the gravitational field is a function with even more indices on it. For an electron field, you need a function that is a one-by-four matrix — etc. So this is to say that all fields are functions of space and time, but different types of fields, depending on their spin and perhaps on other properties, will actually be collections of functions, ones that can only be thought of in terms of field lines in the special case that the functions form a vector.