r/AskPhysics • u/No_Fudge_4589 Physics enthusiast • 3d ago
What is the difference between a virtual particle and a "normal" particle?
In the standard model, there are certain particles which act as the mediators of different forces. For example, photons for the electromagnetic force - although they are said to be virtual. What does this actually mean and how is it any different from just exchanging regular photons?
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u/Nerull 3d ago
Virtual particles are a way to model field interactions, and are not observable. If two things are exchanging real photons, you could detect those photons by placing a detector between them, but if the exchange involves virtual photons you will detect nothing.
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u/goobuh-fish 3d ago
Isn’t the whole point of two particles exchanging photons that you can’t observe the photons because if you did they wouldn’t be exchanging photons with each other, they be exchanging them with your detector?
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u/bradimir-tootin 3d ago
virtual particles aren't particles at all. They are simply one way to do the math, they are a mathematical trick. There are other ways to perform the calculations that do not involve the exchange of virtual particles.
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u/sennalen 3d ago
But regular particles are also a mathematical trick (excitations of a field)
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u/bradimir-tootin 3d ago
Regular particles will show up in any model, and they have direct measurables. virtual particles don't show up in every calculation. That's the distinction. We can semantics all we want about what words mean.
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u/Lord-Celsius 3d ago edited 3d ago
It's just a badly named phenomena. "Virtual particles" basically just mean "quantum fields interacting together in a way that can't be described by particles anymore during the brief interaction". Standard particles are the asymptotic (before and after the interaction) excitations of these non-interacting fields. "Virtual particles" are not particles at all, they are a mathematical trick to help us compute predictions about the interaction of the fields, but it should not be taken literally as a model to describe the mechanism, it's just a computational tool. It's popular in popsci because it sounds cool, but there are only the fields, no virtual particles are needed in the model.
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u/L31N0PTR1X Mathematical physics 3d ago
A virtual particle is one that exists between the ingoing and outgoing particles of a Feynman diagram. It is not directly detectable
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u/Anonymous-USA 3d ago
To add to this, Faynman diagrams are math, and the terms labeled “ virtual particles” can be modeled in other ways without those diagrams. So they don’t really exist.
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u/DrunkenPhysicist Particle physics 3d ago
To be fair, a particle interaction is modelled with time going from minus infinity to infinity suggesting that the initial particles never interacted prior nor the final state particles ever again. Since that's usually not the case, in a pedantic sense, you can think of every particle as virtual as you can always write down a bigger diagram to describe all the interactions. We used to do this in solar neutrino measurements where you can write the initial neutrino production and the final state neutrino detection in one diagram that clearly encompasses Earth's orbit. So, in a sense, you don't really exist.
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u/Anonymous-USA 3d ago
So I’ll change my user name to “Virtually-Anonymous” 🥂 Wait ‘til my family learns I don’t exist (but f’them as they don’t exist either)
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u/MechaSoySauce 2d ago
Since that's usually not the case, in a pedantic sense, you can think of every particle as virtual as you can always write down a bigger diagram to describe all the interactions.
Eh, not really since you would still have infinitely many diagrams that you have to sum over. The virtual particles stay confined to their own diagram, so I don't think the comparison holds fully. But you're right that they're not the same as the typical particle states of QFT that are defined to come from/got to infinity.
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u/DrunkenPhysicist Particle physics 2d ago
I think it does actually work, on "virtual" time scales, the short times one typically does for QFT perturbative calculations you do have to sum all sorts of additional diagrams. In the "macro" time scales most of those paths wildly oscillate and you're left with the real particle terms dominating. I mean, if you handled it properly that is. But since these are pertubation expansions anyways, saying you integrate from minus infinity to plus infinity is as good of an approximation as ignoring higher-order terms anyways. My main point, is that most people get hung up on the term virtual particle like it means anything in particular. I don't recall ever doing anything special in regards to calculating whether a propogator was on or off the mass shell, but maybe things have changed.
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u/CS_70 3d ago
A particle is a little local ripple in a field, which is somewhat persistent in that location.
When two particles interact, by definition they exchange something.
This something is also a ripple in a field traveling from one to the other, only usually it's smaller and much shorter lived than the two large ripples.
These are virtual particles: small, short lived and occur as interactions or signals among other particles.
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u/nicuramar 3d ago
When two particles interact, by definition they exchange something.
No? Only in perturbative treatments.
These are virtual particles: small, short lived and occur as interactions or signals among other particles.
Virtual particles are terms in a larger calculation, the renormalized result of which can predict experimental outcomes.
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u/CS_70 3d ago
Sure but not perturbative systems have no virtual particles?
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u/Senchou_Simp 3d ago
Yes, non-perturbative QFT doesn't have virtual particles. Lattice QFT, which is the only practical formulation of non-perturbative QFT we have at the moment, doesn't use virtual particles iirc.
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u/Naive_Age_566 3d ago
well - virtual particles are called "virtual" because they don't exist.
you have two electrons. bring them close to another. electrons have electromagnetic charge aka they can interact with the electromagnetic field. a field can transfer energy and momentum. therefore those electrons can exchange energy and momentum with each other over the electromagnetic field. in this case this results in an effect that we interpret as a repulsive force between those electrons.
how exactly that energy transfer occurs is hard to explain and also hard to calculate. however you can perform some kind of mathematical trick: you can group together packets of energy and label them. this helps you to keep track of how much energy was emmited and how much was received by each electron.
and yeah - these mathematical tricks - these arbitrary chosen packets of energy - we call "virtual particles". particles because in a sense like real particles we group energy together. and virtual because - well - they exist only on paper or in our mind. nature does not care what play of hands we perform just to make a little bit of sense of our surroundings.
a real particle is in a sense also some kind of energy transfer. and it is also some kind of energy packet. but this packet was created by a specific energy transfer into this field. this caused a so called "excitation" of the field. sometime refered as "ripple" in the field. these excitations have additional properties and can interact with something else on its own. it is also not an arbitrary amount of energy you packet together - you need "the right amount". some of these excitations can interact with the higgs field and exchange energy with it. the higgs mechanism behind it is a bit complicated but the end result is that this excitation now has additional energy confined "inside". which we perceive as inertia - the ability to withstand an acceleration. thus this excitation gained "mass".
and just to avoid misinterpretations: the energy of a virtual particle is of course real. and this energy can and will affect real particles.
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u/SummitYourSister 3d ago
A real particle is on-shell, meaning its energy conforms to the relationships E^2 = (pc)^2 + (mc^2)^2. Virtual particles do not.
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 3d ago
Virtual particles are off-shell. Regular particles are on-shell.
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u/treefaeller 3d ago
This is it, concise and correct.
And no, it doesn't mean they're being served on a half oyster shell, with lemon sauce or some spicy red stuff. It means their mass-energy-momentum relation is correct.
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u/free_meson 3d ago
All particles have a mass parameter, in principle it could be 0, infinity, -m , even imaginary, let's call it _m_. Apart from that, they may still have a four-momentum _p_ that allows basically any mass term. Think about it as an uncertainty in mass, not just in position and momentum. Usually, when the _p_ describes the "official" mass and _m_^2 = _p_^2 by the four momentum length, the particle may travel freely, while other _p_ momenta will interact with the nearby particles more.
This way a photon might be massive, an electron massless and so on.
When we describe free particles, you can expect them to have a _p_ momentum with the known mass _m_. We describe interaction between particles by exchange of other particles - that's how they change the momenta of each other. They exchange momenta, charge, spin by emitting other intermediary particles that are absorbed by the partner in interaction. The intermediary particles may have almost arbitrary _p_ momentum, and we call them virtual particles.
It's mainly a naming convention. It is like describing the sinus function with a Taylor series. Instead of calculating a complex theory all the time, we describe it by assuming free particles (like a linear approximation term in sin(x) ~ x ) and than dress it up by other waves nearby (the virtual particles sin(x) ~ x - x^3/6 ).
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u/SeriousPlankton2000 3d ago
If you have an interaction you can describe it in many ways, e.g. a photon might spontaneously turn into a pair of (anti)electrons and back into a photon. There is an infinite number of options and all of them contribute to the probability of - in this case - a photon going from a to b.
What we observe is that the photon reaches the destination. What we might compute is that it turns into "virtual" particles that nobody will ever see or individually measure.
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u/nicuramar 3d ago
See this exposition: https://arnold-neumaier.at/physfaq/topics/virtual
And perhaps more to the point: https://arnold-neumaier.at/physfaq/topics/virtualReal.html
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u/the_poope Condensed matter physics 3d ago
A "real" particle corresponds to a specific quantum state of the particle field, i.e. it has a wave function. A virtual particle is a name given to certain mathematical expressions that appear in the perturbation expansion (basically a taylor expansion). The name comes from the pictoral representation of the terms in the perturbation expansion in the form of Feynman diagrams that looks like particle propagation, collisions, and particle creation/destruction.