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chentadot
2016-Jul-21, 09:06 AM
Hi.
According to particle physics, some particles communicate different forces. My question is how do they carry the information about that force?

For instance, if a photon need to communicate the EM force between two protons, how does it carry the information about the size and direction of the force? What would be different in the photon if I changed one of these protons into an electron? or into two protons?

Thanks...


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grant hutchison
2016-Jul-21, 12:13 PM
The force carriers are so-called "virtual" particles, and they behave differently from real particles.
You can imagine a proton surrounded by a haze of virtual photons, that don't actually do anything unless there's another charged particle to interact with. If the cloud of virtual particles "finds" another charged particle, the wavefunction of a virtual photon can interact with the wavefunction of the other charged particle, transferring momentum to it. The magnitude and direction of the momentum transfer depends on the mathematics of the quantum wavefunctions - it reproduces the familiar electromagnetic inverse-square law and the rules of opposites attracting and like repelling. The virtual particles don't "know" in advance what task (if any) they're going to do - the nature of the momentum transfer depends on what they "find" to interact with. So there would be no difference in the virtual photons if they encountered differently charged particles, or different numbers of particles; they'd simply interact differently with those particles.
Matt McIrvin gives some non-mathematical detail here (http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html), but it's essentially an artist's impression of the complicated underlying maths.

Grant Hutchison

grant hutchison
2016-Jul-21, 12:29 PM
Oh, by the way:
The spread-out wavefunctions of these virtual photons solves another puzzle, which is how they can transmit attractive forces. That's not at all obvious from a Feynman diagram that shows charged particles bouncing photons off each other like billiard balls.
But because (according to Heisenberg) a particle with well-defined momentum must have poorly defined location, it's perfectly possible for a virtual photon to transfer momentum that nudges particles towards each other, rather than apart.

Grant Hutchison

chentadot
2016-Jul-21, 01:40 PM
Ok I think I can understand the idea... But let me ask it this way-
Let's say I have a proton "standing still" on the table, and another proton is approaching. At some point the the EM force will make the first proton move. What happens at this point with the wavefunctions of the virtual photons around this resting proton? How do they relay back the force causing it to move?
Thnaks


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John Mendenhall
2016-Jul-21, 02:14 PM
Oh, by the way:
The spread-out wavefunctions of these virtual photons solves another puzzle, which is how they can transmit attractive forces. That's not at all obvious from a Feynman diagram that shows charged particles bouncing photons off each other like billiard balls.
But because (according to Heisenberg) a particle with well-defined momentum must have poorly defined location, it's perfectly possible for a virtual photon to transfer momentum that nudges particles towards each other, rather than apart.

Grant Hutchison

(seductive vpoices) "C'mere c'mere c'mere." "G'way g'way g'way."

Cougar
2016-Jul-21, 04:32 PM
You can imagine a proton surrounded by a haze of virtual photons, that don't actually do anything unless there's another charged particle to interact with....

Yes, this is a good model to envision. I think it's Laughlin (https://en.wikipedia.org/wiki/Robert_B._Laughlin) who develops this idea well in his 2005 book A Different Universe: Reinventing Physics from the Bottom Down.

grant hutchison
2016-Jul-21, 07:03 PM
Ok I think I can understand the idea... But let me ask it this way-
Let's say I have a proton "standing still" on the table, and another proton is approaching. At some point the the EM force will make the first proton move. What happens at this point with the wavefunctions of the virtual photons around this resting proton? How do they relay back the force causing it to move?The virtual particles have momentum. If they don't interact with anything, there's no momentum transfer - the energy "borrowed" under the terms of the Uncertainty Principle is paid back, and nothing experiences a force. If they do interact, they impart momentum to the particle they interact with. Both particles are surrounded by these virtual particles, in proportion to their charges, and each cloud of virtual photons has some probability of interacting with the other particle, according to its charge and distance.
It's a cumulative and continuous and bilateral process, not a one-off interaction mediated by a single photon. So there's no single point at which the interaction starts.

Grant Hutchison

chentadot
2016-Jul-22, 04:33 AM
Thank you for the explanation Grant.
On a related matter, is the fact that these particles are virtual also explains how gravitons would escape a black hole the communicate its gravity?


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Noclevername
2016-Jul-22, 05:43 AM
Thank you for the explanation Grant.
On a related matter, is the fact that these particles are virtual also explains how gravitons would escape a black hole the communicate its gravity?



We aren't sure graviton particles even exist, so we can't really say for sure what properties they have. Gravity appears to be a distortion of spacetime, so it has no need to escape a BH's event horizon, as it's what causes the event horizon.

To simplify, it's like asking how a tsunami wave can "escape" the ocean.

chentadot
2016-Jul-22, 09:16 AM
I can comprehend the general relativistic concept of gravity (the whole ball on a rubber sheet metaphor).
But from the particle physics point of view, at least to my understanding, the universe is run by particles.
Ans the graviton is theorized to be the one who communicate the force of gravity.
And it's a particle much similar to a photon- no mass, speed of light, etc.
And even photons can't escape a black hole so how can thede gravitons can? (Assuming they exist and assuming the particle physics' picture of the universe is correct...)


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grant hutchison
2016-Jul-22, 12:26 PM
I can comprehend the general relativistic concept of gravity (the whole ball on a rubber sheet metaphor).
But from the particle physics point of view, at least to my understanding, the universe is run by particles.
Ans the graviton is theorized to be the one who communicate the force of gravity.
And it's a particle much similar to a photon- no mass, speed of light, etc.
And even photons can't escape a black hole so how can thede gravitons can? (Assuming they exist and assuming the particle physics' picture of the universe is correct...)This is where the concept of a virtual particle becomes important. Although "real" photons can't escape black holes, virtual photons can - a charged black hole will have an electrostatic field, mediated by virtual photons in just the same way as the electrostatic field of a single proton.

There are various ways of translating the mathematics into words - either you can think of virtual particles as having a wavefunction that is very spread out, so they're already outside the event horizon (as well as inside it), or you can say that virtual particles are not bounded by the speed of light during their brief period of existence, any more than they are bound by conservation of energy.
So the particles that quantize a static field (electric, gravitational) just aren't going to behave like the little hard balls that intuition tends to imagine.

Grant Hutchison

chentadot
2016-Jul-22, 12:43 PM
Grant, thank you for the answer and the time!
😊


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Hornblower
2016-Jul-22, 03:35 PM
Something to keep in mind is that the theory of quantum mechanics is derived from observable phenomena at atomic and subatomic scales where GR was not an issue for the purposes of the experiments, and that GR is derived from observable phenomena at large scales at which quantum mechanics was not an issue. So far, attempts to unify the two bodies of theory have not succeeded. The cosmos is what it is and does what it does, and it does not care how much trouble it is for us mortal human beings in our quest for theoretical unity.

Grey
2016-Jul-22, 07:09 PM
So the particles that quantize a static field (electric, gravitational) just aren't going to behave like the little hard balls that intuition tends to imagine.Heck, even non-virtual particles don't behave like the tiny billiard balls that we imagine intuitively.

Ken G
2016-Jul-23, 12:21 PM
Also bear in mind that "virtual particles" are a topic that elicit some significant disagreement among physics experts. Asking a room full of experts "do virtual particles actually exist" is a bit like inserting a hive of African killer bees into the room! Some experts will say that of course virtual particles don't exist, that's why we call them "virtual", while others will say that it is the "real" particles that don't exist, they are merely idealizations of what virtual particles can do in the presence of energy that makes them seem "more real." But the one thing that seems pretty clear is that virtual particles are an evocative metaphor for picturing the mathematical terms in quantum field theory when you choose to write the effects of that theory in the form of perturbation theory, which is why Grant brought up Feynman diagrams (which encode how to generate the terms in a perturbative expansion like that). So the situation is a bit like imagining that the ex function, which can be written as the sum from n=0 to infinity of xn/n!, is in some sense created or materialized by "virtual" effects that somehow act to create each one of those terms, such that the full function is the superposition of all the effects of those individual terms. This is the approach we take when we think about the "Taylor expansion" of an analytic function-- we say the behavior of the analytic function for any x can be approximated to increasing levels of precision by looking at longer and longer lists of the higher derivatives of that function at x=0. So we say we can approximate an analytic function at x=100 by looking super closely at the behavior of that function around x=0, where that super close look at what is happening near x=0 is a bit like saying the function comes from the superposition of the contribution of a bunch of "virtual functions" that behave more simply than the original analytic function, and work together to produce its attributes. Then ask if those virtual functions are "really there" inside the ex function, as if the ex function couldn't be what it is without them, and watch the mathematical sparks fly.

At the end of the day, I'd say that we must always notice the difference between what our theories predict, and what pictures we choose to invoke to help guide us into a useful understanding of what our theories predict. Only the former is the actual theory. So we only need to manipulate the concept of virtual particles if it provides us with insight into understanding the outcome of an experiment, but we never need to take them any more seriously than that. Since this statement is generally true of all interpretations of all scientific theories, it is perhaps the greatest scientific value in the "virtual particle" concept-- for what could expose any more clearly these artifices we invoke than the concept of a particle that does not actually exist?

George
2016-Jul-23, 10:36 PM
Are virtual particles today similar to the atoms of Democritus in their "realness"? It took a while to catch those slippery devils but we snagged them, then hit them with special tiny hammers and found more stuff inside. The Casimir experiment, DE, Hawking radiation are all hints that virtual particles may need a name change in the future, or are there barriers like Uncertainty, or something, that prevent enough... certainty?

Ken G
2016-Jul-24, 08:38 PM
So far, I don't believe there are any consequences of virtual particles that are not already in the theories that inspired that picture, without the virtual particles' explicit presence. So when you take the theory, you can still take or leave virtual particles, you never actually need them. What you're asking would require some new experiment that is not explained by the theories we have, but is explained by an expanded virtual particle concept that has to be part of the new theory because the new theory is built from them. We don't have a theory like that, but someday we certainly might. Or, we might go the opposite direction and get rid of the particle concept altogether-- I'm reminded of the cryptic quote of Heisenberg that "We will have to abandon the philosophy of Democritus and the concept of elementary particles. We should accept instead the concept of elementary symmetries."

George
2016-Jul-25, 03:33 PM
If I understand your point, if we discover that we can grab some of these virtual particles and hit them with a hammer it will involve a new theory. Is this a reasonable & tangible (struggled as it is) view to the nuanced apparent intangibles?

Ken G
2016-Jul-26, 01:07 PM
Yes. And I'm saying something more-- I'm saying that being able to hit something with a hammer is what we mean by it being real, whereas being able to picture something as we apply some theory is not so clearly what we mean by it being real. So it comes down to what we mean by something being real, moreso than what the universe hands us that either is, or is not, real of its own accord.

George
2016-Jul-26, 10:19 PM
Yes, but given that lucid account, is there any hope that it will be hammerably real? If so, some other concept of virtual stuff will, no doubt, come into play and the virtual stuff now will need new names, and a new theory, apparently. Is this a fair way to see possible future things? [I avoided the use of "real" and stayed with "certain", though the hammer action would make things certainly real. ;)]

Ken G
2016-Jul-28, 03:26 PM
Yes, but given that lucid account, is there any hope that it will be hammerably real?I doubt it-- hammerably real generally involves an existence that is maintained for a long time, but given the uncertainty relation between time and energy, that requires a fairly precise energy. That's what virtual particles lack-- the energy it takes to be a hammerably real boy. Some new theory that gives virtual particles such hammerable reality sounds like it would violate the spirit of what we need virtual particles to do in the first place. However, it is of course hard to anticipate the imagination used in whatever new theories could be out there. The most likely result seems to me to be that our modern concept of virtual and real particles will morph into some rather new way of looking at both, such that we will chuckle at the naivete of those prior physicists who thought they understood what it meant to be real.



If so, some other concept of virtual stuff will, no doubt, come into play and the virtual stuff now will need new names, and a new theory, apparently. Is this a fair way to see possible future things? I would expect so.

George
2016-Jul-28, 05:58 PM
However, it is of course hard to anticipate the imagination used in whatever new theories could be out there. The most likely result seems to me to be that our modern concept of virtual and real particles will morph into some rather new way of looking at both, such that we will chuckle at the naivete of those prior physicists who thought they understood what it meant to be real. And that seems to be what I see in history. I would guess the mid-19th century ideas of what might be internal to an atom, if anything, would be a theory too far and, thus, not really a theory -- if not quite testable enough--, though still a theory if practical in solving problems (though I don't think they had such at that time) -- an early theorino, perhaps. ;) Yet and atom is a macro sort of thing compared to quarks and fermions. Being ~ 97% Newtonian does not allow me to see what you see, especially that which may prove to be an actual theoretical limit for any hammerable event found within particle physics.

Planck Time might serve as an example, not that I understand it. Events requiring divisions of a unit of Planck time are beyond a defined theoretical limit, I think. We have been driving science for a while, so "are we there yet"? Your suggesting that may, but may not, be the case. That helps me get a better picture, so thanks. [I am enjoying Butterworth's "Most Wanted Particle" account, which presents particle physics in a clear and helpful way. The LHC foreground for his book is a great way to hold my attention especially given my apparent fondness for hammers.]

Yet, perhaps a virtual thing or two will become hammerable certainty and things like Gauge theory will be re-calibrated, which is typical of gauges. (pun, sorry)

trinitree88
2016-Aug-04, 12:58 AM
I've spoken before of the business of George Gamow touting the graviton as a neutrino/antineutrino pair in his text "Gravity", a Harvard Project Physics Reader from the sixties....and of the SU(5) symmetry in Howard Georgi's 1981 Sci. American article as touting the Z as coupling universally to all particles....and of submitting the successful prediction of the coincidences between the Rome, Maryland, bar gravitational wave detectors, and the IMB, Kamiokande, Baksan, and Mont Blanc neutrino detectors during SN1987A @ 3.2 sigma (Larry Sulak, IMB principal investigator).
So what you need is to see a graviton in action in a tiny setting. The Tau/Theta puzzle. Resolved in the fifties by C.N.Yang & T.D.K. Lee...they proposed that the seemingly anomalous decay of the particles into 2 or 3 pions, inferring that parity was not conserved in the decay was a failure of Conservation of Parity, and a richly deserved Nobel for both.
But, that might not be the whole story. When a particle absorbs or emits another particle, including photons, and Z's ...(gravitons)...it changes it's parity. So if a kaon normally decays into 2 pions when "free" of an attached Z, indicating + parity.......if it picks up a Z, it changes to minus, and while so coupled, must decay into 3....restoring the law of Conservation of Parity, and dynamically indicating the quantum interaction.
Is that testable in a lab? Yep. A particle accelerator tuned to generate a kaon beam, produces one which fluctuates in composition as the beam travels, and the measurable rate of that fluctuation should be sensitive to the ambient neutrino...(and hence Z) flux.
Set up the kaon beam near a reactor, turn on the reactor, measure the rate for the quiescent reactor, and the powered up one. You should see a change in the halflife of the kaons. Betcha a hot fudge sundae...2 scoops, lots of whipped cream, walnuts, jimmies, and two cherries on top.
pete

Shaula
2016-Aug-04, 05:45 AM
I've spoken before of the business of George Gamow touting the graviton as a neutrino/antineutrino pair in his text "Gravity", a Harvard Project Physics Reader from the sixties..
He did not tout it. What he said was caveated with "All this is, of course, the sheerest speculation but a connection between neutrinos and gravity is an exciting theoretical possibility.". The sole line or argument given is very weak. In fact it can be used to argue that the graviton is a bound state of a million photons. There exists precisely as much theoretical justification (and as many issues with) this idea. Then there is the issue of neutrino mass and the broken symmetry this would lead to in gravity. The idea simply isn't viable.


..and of the SU(5) symmetry in Howard Georgi's 1981 Sci. American article as touting the Z as coupling universally to all particles..
Thing is the Z particle doesn't couple universally. If it did then we'd see effects at the energies modern accelerators get up to. This model also predicts proton decay at a rate we would have observed. Which is why it was abandoned.


including photons, and Z's ...(gravitons)..
Oh so Zs are also pairs of neutrinos now? What awfully flexible particles...

Suggest that if you want to carry on promoting this stuff you take it to ATM. Because this is not mainstream, it is you connecting up a load of very old references and making up stories around them. There is plenty of strong experimental and theoretical evidence against them. But if you think you can challenge this then, please, present your theories in ATM. You've dropped this stuff into a number of threads over the years and it'd be good to finally discuss it in enough detail to pin down just what this idea you tout is.

trinitree88
2016-Aug-04, 10:38 PM
"1 In fact it can be used to argue that the graviton is a bound state of a million photons. There exists precisely as much theoretical justification (and as many issues with) this idea." This is not what Gamow said, nor what I said, but rather an ATM comment you made up in your rebuttal post and an administrator should remind you that ATM belongs in ATM not in Questions and Answers. Defend it there as you see fit. There is nowhere on the internet nor in any peer-reviewed journal where such a comment has ever been made.





]

2. " Thing is the Z particle doesn't couple universally". That's odd try p. 39 from the Summary Table from CERN's own site...weak interaction couples to leptons and hadrons...that'd be universal. Either you have difficulty reading or petty issues are cropping up. SEE:file:///C:/Users/Library%20Patron/Downloads/9781441982667-c1.pdf


3." Oh so Zs are also pairs of neutrinos now?" According to the ATLAS training site @ the LHC, ~ 20 % of Z's decay into neutrino /antineutrino pairs....that'd be just as if they were made up of the two of them as I said. I never said that ALL Z's must have that composition, only SOME of them will do nicely. I'm not the one making things up here ...that comment is another distortion in your rebuttal post and has nothing to do with the reality of the world of physics.
see:http://atlas.physicsmasterclasses.org/en/zpath_lhcphysics2.htm

Shaula
2016-Aug-05, 05:15 AM
"1 In fact it can be used to argue that the graviton is a bound state of a million photons. There exists precisely as much theoretical justification (and as many issues with) this idea." This is not what Gamow said, nor what I said, but rather an ATM comment you made up in your rebuttal post and an administrator should remind you that ATM belongs in ATM not in Questions and Answers. Defend it there as you see fit. There is nowhere on the internet nor in any peer-reviewed journal where such a comment has ever been made.
I know Gamow did not say it, and I know you did not. My point was that the line of reasoning used to support that idea is incredibly weak. It is based on emission probabilities and nothing else. Therefore it is precisely correct to say that other solutions based on emission probabilities are equally justified and just as well supported by the argument. I also know that Gamow did not tout this idea - it was a one sentence comment based on a very weak argument. More a kind of "Look at this" than a theory. This concept of a graviton as a bound state of a neutrino pair is not mainstream and no substantive theory based on it has passed peer review as far as I know. Please provide me with any references you have if I am wrong. A single reference to it as an idea in a 1960s book is nothing like peer review.


2. " Thing is the Z particle doesn't couple universally". That's odd try p. 39 from the Summary Table from CERN's own site...weak interaction couples to leptons and hadrons...that'd be universal. Either you have difficulty reading or petty issues are cropping up. SEE:file:///C:/Users/Library%20Patron/Downloads/9781441982667-c1.pdf
Nope. It couples to particles with weak isospin. Left handed leptons and hadrons we know of do have weak isospin. Gluons do not. Photons do not. The Z itself does not. Not universal at all, it is just like any other boson.


3." Oh so Zs are also pairs of neutrinos now?" According to the ATLAS training site @ the LHC, ~ 20 % of Z's decay into neutrino /antineutrino pairs....that'd be just as if they were made up of the two of them as I said. I never said that ALL Z's must have that composition, only SOME of them will do nicely. I'm not the one making things up here ...that comment is another distortion in your rebuttal post and has nothing to do with the reality of the world of physics.
see:http://atlas.physicsmasterclasses.org/en/zpath_lhcphysics2.htm
No, because something decays into particles is not at all like it is made of them. Unless you are now also claiming that Zs are made up of fermion/anti-fermion pairs too? And neutrons made of protons and electrons? And are you now making some kind of claim that Zs are not only composite particles but composite particles that behave similarly but actually have wildly varying internal structures?

You are trying to dodge the point here. You have made a number of decidedly non-mainstream claims here. Again. Gravitons are not pairs of bound neutrinos in any current mainstream theory. You are basing your speculations of out of date material, some of which has direct experimental evidence against it. So the challenge to you is either prove me wrong by proving links to peer reviewed papers setting out a current, widely accepted model that encompasses your claims or take it to the correct forum.

01101001
2016-Aug-05, 01:17 PM
SEE:file:///C:/Users/Library%20Patron/Downloads/9781441982667-c1.pdf

I'd look, but I think you might not have granted me access to your C: drive.

You may have meant to reference Springer where one can sample or buy: Symmetry and the Standard Model: Mathematics and Particle Physics (http://www.springer.com/us/book/9781441982667). The specific material might have been "Chapter 2: A Preview of Particle Physics: The Experimentalistís Perspective". One can also search Google, to get the Springer link, for: 9781441982667-c1 (https://www.google.com/search?q=9781441982667-c1)

Copernicus
2016-Aug-05, 06:14 PM
The virtual particles have momentum. If they don't interact with anything, there's no momentum transfer - the energy "borrowed" under the terms of the Uncertainty Principle is paid back, and nothing experiences a force. If they do interact, they impart momentum to the particle they interact with. Both particles are surrounded by these virtual particles, in proportion to their charges, and each cloud of virtual photons has some probability of interacting with the other particle, according to its charge and distance.
It's a cumulative and continuous and bilateral process, not a one-off interaction mediated by a single photon. So there's no single point at which the interaction starts.

Grant Hutchison

I like this. Best explanation ever.

Swift
2016-Aug-08, 07:53 PM
Suggest that if you want to carry on promoting this stuff you take it to ATM. Because this is not mainstream, it is you connecting up a load of very old references and making up stories around them. There is plenty of strong experimental and theoretical evidence against them. But if you think you can challenge this then, please, present your theories in ATM. You've dropped this stuff into a number of threads over the years and it'd be good to finally discuss it in enough detail to pin down just what this idea you tout is.

"This is not what Gamow said, nor what I said, but rather an ATM comment you made up in your rebuttal post and an administrator should remind you that ATM belongs in ATM not in Questions and Answers.
I would appreciate it if both of you would stop posting like a Moderator/Administrator of CQ and telling the other one what should or should not be posted in ATM. As you both well know, if you have a problem with someone else's post, you Report it, you don't make such comments in-thread.

Since neither of you is the asker of the initial question, and are just using Q&A to argue, if you don't behave yourselves, I'll just close this thread.

publiusr
2016-Aug-13, 05:51 PM
Yes, this is a good model to envision. I think it's Laughlin (https://en.wikipedia.org/wiki/Robert_B._Laughlin) who develops this idea well in his 2005 book A Different Universe: Reinventing Physics from the Bottom Down.

I think a lot of Mr. Laughlin