[HN Gopher] Virtual Particles: What are they? (2011)
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Virtual Particles: What are they? (2011)
Author : czzr
Score : 83 points
Date : 2021-03-06 16:51 UTC (6 hours ago)
(HTM) web link (profmattstrassler.com)
(TXT) w3m dump (profmattstrassler.com)
| frob wrote:
| This is absolutely the best explanation of virtual particles I
| have ever seen. I've worked through pages of Quantum Field Theory
| (I'm literally looking at my copy of Peskin and Schroeder sitting
| atop Krane right now), expanded and calculated hundreds of
| Feynman diagrams, written pages about these concepts, and yet I
| would say this article gives the best intuitive explanation of
| what is going on I've ever seen. I absolutely love that he mixes
| in the aside about the permittivity of free space arising from
| loops in the vacuum.
|
| I am adding this framework to my set of tools to explain (and
| intuit about) QFT. And going back for a reread now. Well done!
| the8472 wrote:
| > 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.
|
| Are gluons virtual particles then? And all the other heavier,
| unstable particles? What distinguishes them from the other
| disturbances?
| PhillyG wrote:
| By the sounds of it they're neither virtual nor particles!
|
| "A virtual particle is not a particle at all. It refers precisely
| to a disturbance in a field that is not a particle."
|
| So from that I gather, it's a term in physics to describe things
| that look like particles but aren't.
| loopz wrote:
| When we isolate fundamental particles, carefully orchestrated
| experiments may show predictable distributions of outcomes.
|
| On atom and molecular level, chemistry also provides
| distributions of predictability and even more outcome certainty.
|
| This follows the general pattern of interesting chaotic systems,
| and the side-effects of "chaotic attractors". We find areas of
| stability, where big numbers tend to converge to stable and
| predictable values/surfaces. We also find chaos and
| unpredictability between the stable surfaces in outcomes. Without
| full information, we can approximate using ML, though how they
| generalize and explain could also be an iterative ML task.
|
| The 3-body problem means that even on macro scale we run into
| problems calculating some orbits (ie. asteroids). It's just that
| over time, most chaotic orbits tend to stabilize, so we don't run
| into this too much. With full information, everything can be
| calculated theoretically. So it is both the fundamental problem
| of the differential calculus itself, though inaccurate
| information compounds the fundamental.
|
| Virtual particles sound like constructs bridging some of the
| gaps, though still unsolved on most 3+ body problems. Maybe it is
| correct to assume they "work" for stable/semi-stable areas of
| simple models, but break down in between states of chaotic
| systems?
|
| I sense that we search for simple foundational relationships and
| understandable constants. However, the problem itself seems
| intractable, the more you seek to encompass the whole across
| scales.
|
| Ie. given two random real numbers between 1.0 and 10.0, human
| beings expect to find integers. We seek to construct perfection,
| while blinded to the environment sustaining us.
|
| The expectation is not totally unwarranted, if you look at the
| distribution of 2+ random numbers.
| simonh wrote:
| Please help me here to see if I'm getting this. A particle or
| particles can be described by a wave function. Perturbations in a
| wave can be described by the original (clean?) wave plus various
| other waves interfering with it. These other waves, which don't
| actually exist independently of the perturbed particle wave,
| describe the virtual particles. Does that sound right?
|
| I'm wondering what this looks like if you observe these fields
| from a distance. Are 'echoes' of these perturbations and
| interactions detectable at a distance? In the diagrams in the
| article representations of these perturbations are shown between
| the particles, but do these perturbations propagate outside the
| scope of these interactions? If so, I'd have thought these would
| count as e.g. photons.
|
| I suppose not necessarily, not all energy levels in a quantum
| field are achievable, hence discrete discontinuities like
| absorption lines and electron shells, and in fact particles like
| electrons themselves. You can't have a half strength perturbation
| in the electron field that adds up to half an electron. Do these
| perturbations fail to propagate outside the local interaction
| then?
| dexwiz wrote:
| I think you may have the model and reality mixed up. The waves
| in the field actually exist, although not quite like any wave
| we experience. The virtual particle is just a tool to describe
| some types of wave field interactions.
| simonh wrote:
| There really are perturbations in the field yes, so I suppose
| my question is to what extent they have an independent
| existence outside the wave functions of the particles.
| dmingod666 wrote:
| Looks like AWS is extending its serverless platforms to add
| support for some physics APIs.
| cwmoore wrote:
| Lacking rigor, but some elements of this remind me of Moire
| Patterns:
|
| https://en.m.wikipedia.org/wiki/Moir%C3%A9_pattern
| fielrdss wrote:
| You see a red circle moving across the computer screen, but in
| reality there is no red circle, the only real thing is a
| excitation moving though the R,G,B fields of the screen.
|
| The same with particles. They are as real as the red circle. A
| convenient illusion which simplifies things and computations.
| varjag wrote:
| There is a red circle in reality, measurable in intensity,
| shape and wavelength of its radiation.
| thamer wrote:
| From the article:
|
| > 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.
|
| Another comment here mentioned Hawking radiation. Isn't Hawking
| radiation when a pair of virtual particles appears just at the
| event horizon of a black hole, with one "falling in" and the
| other escaping out? Is it not in fact going "off on its own
| across space"? Or as the Wikipedia article[1] on Hawking
| radiation puts it: "the other escapes into the wider universe
| ('to infinity')". I fail to see the difference here, especially
| when both pages use very similar language about particles
| traveling the universe. If the one that escapes is "not like a
| particle at all", how is it different? Does it not behave exactly
| like any other particle of the same kind?
|
| Or is the article calling a "disturbance" the creation and
| annihilation of a pair of virtual particles? I certainly see how
| that's different from a particle, but it's also pretty clear that
| it's not one particle but a pair. Of course this very special
| pair of particles that spawns and disappears is not like "a"
| particle.
|
| [1] https://en.wikipedia.org/wiki/Hawking_radiation
| akiselev wrote:
| Hawking Radiation has never been observed so we don't know if
| it's a real physical phenomenon but if it is, it's that
| weirdness at the intersection of relativity and quantum
| mechanics that makes it a phenomenon worth naming. QM is chock
| full of these caveats, especially at the event horizon, and
| Hawking Radiation is an extreme example.
| canjobear wrote:
| Sean Carroll's Youtube series covers these kinds of things very
| clearly including the math.
|
| https://www.youtube.com/watch?v=PaRGj5Phpm0
| frongpik wrote:
| I'd call them purely mathematical constructs that arise from the
| need of modern QM theories to mediate all interactions with
| particles.
| Twisol wrote:
| The article seems to be saying that there really is some
| physical phenomenon occurring here -- effectively, "cross-feed"
| between coupled fields -- but that this phenomenon doesn't obey
| the relations that define a "particle". Or, more
| metaphorically, a particle is more of a squishy, wobbly blob
| (with a rather ill-defined boundary) than a rigid packet, and
| as it wobbles "in and out" of the individual fields it's
| coupled to, its effect on those fields will vary.
| ISL wrote:
| They are mathematical constructs, but they can sometimes lead
| to viable intuition. Classic (and necessarily simplifying) "how
| it works" explanations for Hawking radiation, the Casimir
| effect, deep inelastic scattering results, and more rely upon
| invoking virtual particles.
|
| Matt Strassler was among several theorists who helped me, as a
| graduate student, to develop a deeper intuition for quantum
| fields -- it is important to develop an understanding of the
| strengths and weaknesses of both the simplistic and nuanced
| ways of thinking about and discussing these interactions.
|
| It is tempting to regard Feynman diagrams as if they are
| actually what is happening. One should only use them as guides
| to intuition. They are actually compact expressions of
| successive terms in a perturbative expansion, terms that can
| interfere with one another sometimes. Theorists (like Matt),
| who work with these things every day come to develop a more-
| nuanced understanding, generally one that places greater weight
| on fields (not just the bosonic force-mediating fields, but the
| fields for the fermions, too) than understood by even many
| practicing physicists in other specialties.
|
| Another helpful insight that may aid understanding of virtual
| particles as you start to understand things more-deeply: the
| photons we observe with our eyes from distant stars need not be
| exactly on-shell, just very (very!) close.
| pixel_fcker wrote:
| How do virtual particles play into the Casimir effect?
| [deleted]
| frutiger wrote:
| I'd be surprised if no one has done this already, but someone
| needs to write an article that literally enumerates out the first
| dozen terms of the perturbation expansion for a scattering matrix
| along with the Feynman diagram for each term.
|
| That should very clearly explain what "virtual" particles are.
| analog31 wrote:
| When I was a student, one of the profs gave a talk on his work
| in atomic structure theory. I remember this only dimly, but the
| gist is that he showed slides with the first and second terms
| of the perturbation expansion. Then he showed a page of
| expressions generated by a computer algebra system. He said:
| "These are the third order diagrams. There are a few dozen more
| pages. The fourth order are unfathomable."
|
| Of course that was many years ago, and maybe they're fathomable
| now. There may even be code for it.
| geuis wrote:
| You're gonna need something to break this down first
| "perturbation expansion for a scattering matrix". I'm a semi
| literate layman and have no idea what you're referring to here.
| tobmlt wrote:
| It might also help to know that the scattering matrix has
| classical counterparts in engineering. For example the
| "transfer function" in circuits and signal processing, etc,
| and the "response amplitude operator" (RAO) in ship design. I
| can speak from experience only about the RAO, but once you've
| got one built and for your system, you know quite a lot about
| the response of the system to input (in this case typically
| water waves). A higher order transfer function in ship design
| might be constructed to give you say, drift response (or
| force) as a function of wave frequency, and give you wave
| drift response spectra when "hit with" a wave spectrum.
|
| (First order response might regular 6DOF motions (motion
| spectra) in response to wave spectra)
|
| Here the analogies are pretty fun, because you are scattering
| and radiating waves in the water. Very physical!
| ajkjk wrote:
| Well -- that comment was not targeted at a layperson, I
| think.
|
| But the idea is: the scattering matrix
| (https://en.wikipedia.org/wiki/S-matrix) is a matrix that
| says for each possible input and output state, what the
| amplitude of that transition happening.
|
| The perturbation expansion is roughly a Taylor series in the
| number of interactions that occur in a scattering problem.
| So: no interaction is order-0, exchanging one particle is
| order-1, etc.
|
| The first dozen terms or so then tell you the dozen or so
| highest-amplitude processes that occur in a given
| interaction. Most of these will involve virtual particles.
| frutiger wrote:
| That's totally a fair point; the comment wasn't supposed to
| be the explanation itself! But let me try without actually
| writing such an article.
|
| The elements of the scattering matrix determine how two (or
| more) colliding particles will interact and emit zero or more
| particles as a result. These elements are defined by an
| integral that has no closed form. For certain fields
| (electromagnetic being the prominent example) the integral
| can be approximated by the sum of terms in an infinite
| series.
|
| Each term in the series includes a multiplicative factor of
| the coupling constant (typically denoted by alpha which is
| approximately equal to 1/137) raised to a certain power.
| Since this number is quite small, the higher the power, the
| less the term contributes to the overall sum.
|
| Additionally, there is a bidirectional mapping of each term
| in the series to a Feynman diagram (you are likely familiar
| with examples of these diagrams). The number of vertices in
| the diagram correspond to the power of the constant mentioned
| above. So, the terms that dominate the integral are the ones
| that have a small number of vertices, but you can keep going,
| adding more and more vertices to get a more accurate sum.
|
| These additional vertices can be added as long as you satisfy
| the rules of the diagram for the particular field you are
| considering (different fields have different constraints on
| invariants that must hold at each vertex). For EM, you can
| add additional photon/electron vertices as long as the
| overall electric charge is preserved.
|
| These additional photon edges in the diagram are "virtual"
| particles. Nothing more than a pictorial representation of a
| term in an infinite series that approximates an integral.
| eperdew wrote:
| Thank you for the explanation. This is a very comfortable
| level of detail in my opinion, if you were to write a full
| article.
| tobmlt wrote:
| As a rejoinder, maybe Richard Mattuck's book would be helpful
| for those interested in Feynman diagrams generally:
|
| https://www.amazon.com/Guide-Feynman-Diagrams-Many-Body-Prob...
|
| Accessible to anyone who can sum a geometric series and take a
| Fourier transform!
|
| (Feynman diagrams for dummies basically, for any who run across
| this comment - yes I'm being cheeky about "dummies" but I
| believe it calls itself something similar in the front matter.
| A self interaction term, then.)
| andi999 wrote:
| Virtual particles are an artefact of perturbation theory.
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