# do the particles ever collide in QED

Discussion in 'Physics & Math' started by Arlich Vomalites, Jan 15, 2015.

1. ### Arlich VomalitesRegistered Member

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91
I have the following question: do the particles ever collide into each others in QED?

Or do the particles only act on each others from distance? The thing that that Einstein rejected as spooky
action at distance.

Look at the following picture where two ice skaters act on each others from distance by throwing an invisible
ball back and forth between them:

There seems to be a spooky action at distance that the two ice skaters have on each others.

Last edited: Jan 15, 2015

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3. ### exchemistValued Senior Member

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7,334
I'm far from expert on QED but I think one needs to think about what sorts of "particles" QED is concerned with. Since QED is to do with electromagnetic interactions, I think the particles it deals with will be principally electrons and photons. I don't think photons really collide with each other - I think the waves pass through each other. Electrons, being charged, do, and this is said in the theory to be mediated by exchange of virtual photons (which are not photons, but disturbances in the EM fields that can be treated mathematically in QED as if they are photons, hence the term.) That would be my understanding at least.

I don't think QED can account for what happens during a collision between baryons, for example.

But as I say, I'm not expert on this.

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5. ### Farsight

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3,492
Collide is the wrong word, because the particles aren't little billiard balls. If you can, find a couple of magnets and play repulsion with them. Close your eyes and "feel" the magnetic field. The particles are like that magnetic field, but without the magnets. An electron is "a field excitation". It's just field. It's quantum field theory, not quantum billiard-ball theory or quantum point-particle theory.

They do act upon each other at a distance, but it isn't spooky.

That's popscience rubbish I'm afraid. Like exchemist said, the particles interact because they disturb each other's fields. This is modelled as virtual photons, but there aren't any real photons flying back and forth. Remember this: hydrogen atoms don't twinkle, magnets don't shine.

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7. ### Arlich VomalitesRegistered Member

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91

QED cannot answer what happens at very short distances, or at zero distances. When particles collide, the distance between them becomes zero, but QED does not tell what happens then. QED is incomplete in this respect. QED is supposed to tell about the interaction between matter and radiation, but QED cannot answer what happens when two particles , for example two electrons or like you said two baryons, collide into each others.

8. ### exchemistValued Senior Member

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7,334
I think one has to be careful what one means by "collide". In the case of 2 electrons, the Coulomb repulsion goes up as they approach, deflecting one past the other. That's what I was meaning by a collision. How close they approach during this repulsion/deflection process presumably depends on their kinetic energy. Since the electron has no internal structure or dimensions, I do not think you can speak of a distance at which any specific "contact" between them can be said to occur. It is merely a matter of degree of closeness of approach.

I think it is different with baryons, because these DO have internal structure and the Strong Force will get involved at very close approaches, at which point I rather suspect one moves into the realm of QCD rather than QED.

9. ### Arlich VomalitesRegistered Member

Messages:
91
It seems that you are assuming that the electron is a point-particle, therefore it cannot collide into another electron. We should abandon the point-particle theory of electrons. If we consider the electron as having a finite size, we could talk of a distance at which a specific contact between colliding electrons can be said to happen.

If the electrons have enough kinetic energy, they can overcome their mutual repulsion and collide into each others. We can only imagine the conditions where the electrons could have such an amount of kinetic energy.
I can only think that during the Big Bang the electrons could have had enough kinetic energy to collide
into each others. But as I told, QED does not tell what happens when the electrons collide.
You mentioned QCD, but QCD deals with baryons and strong interaction, not with leptons.
Both QED and QCD are incapable of describing the collisions of leptons.

10. ### exchemistValued Senior Member

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7,334
Well maybe but now you are moving from your original question to new speculations about the nature of the electron. I cant help you there. But possibly a real physicist might. (I do not include home-schooled eccentrics in this category - we have a some of these around the place, as you may realise.)

11. ### Farsight

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3,492
Don't pay any attention to exchemist, his physics knowledge is scant. Amazingly enough, the electron doesn't have a finite size. It's quantum field theory. The electron's field is what it is. And it has no outer edge.

Their mutual repulsion occurs because the two electrons are effectively "colliding" with one another.

IMHO you're looking at this wrong. Think of two hurricanes colliding. They interact at some distance, they don't bounce off each other like cannonballs. Now have a google on electron and spinor.

12. ### QuarkHeadRemedial Math StudentValued Senior Member

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1,555
By whose judgement? Yours, perhaps? Personally, I should expect someone formally tried in chemistry to have working knowledge of quantum mechanics, if not the latest developments

Like any good scholar, exchemist freely admits to the limits of his knowledge, but is willing freely to share what he does know.

Such people you bombastically and arrogantly dismiss. This would be in all circumstances rude, in yours it is pathetic.

Question for all: in mathematics the concept of a field is well defined, once we establish the context in which the term is being used Let's have a definition of a field in the present context. Anyone?

13. ### exchemistValued Senior Member

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7,334
Thanks for the support QH, though in fairness I did rattle his cage first.

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I'd be interested in comments from physicists on electron-electron collisions - let's hope we get some.

14. ### Farsight

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3,492
A field is "a state of space". I didn't invent that, see Einstein talking about it here:

Expanding the Theory
This theory having brought together the metric and gravitation would have been completely satisfactory of the world had only gravitational fields and no electro-magnetic fields. Not it is true that the latter can be included within the general theory of relativity by taking over and appropriately modifying Maxwell's equations of the electro-magnetic field, but they do not then appear like the gravitational fields as structural properties of the space - time continuum, but as logically independent constructions. The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds, and it is natural to suspect that this only appears to be so because the structure of the physical continuum is not completely described by the Riemannian metric."

Exchemist is not a good scholar. His physics knowledge is scant. He knows a lot less physics than I do, and he is envious. Thus he could not resist the urge to snipe, and he will not follow up the references I provide. So his physics knowledge is going to stay scant. That isn't the mark of a good scholar.
.

15. ### Farsight

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3,492
See my discussion with James R in this thread for information about electron-electron interactions.

16. ### QuarkHeadRemedial Math StudentValued Senior Member

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1,555
And you regard this as a "definition"?

Pathetic. Try this.....

A field assigns to each and every point in space (or spacetime, if relevant) a geometric object - scalar, vector, tensor etc.

So how do you define the field or fields that you referring to? Particularly the "quantum field"

17. ### Arlich VomalitesRegistered Member

Messages:
91

That is a good question. We need to learn to understand the electric field. We need to know what is the quantum of the electric field.

I have already asked several times on this forum : what is the quantum of the electric field?

The main problem in QED is that it does not quantize the electric field. The quantization of the electric
field has been always neglected because it is often thought that the electric field does not exist. This is due to
idea that there is only the electromagnetic field, but not the separate electric field alone.

18. ### Farsight

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3,492
Quarkhead: you quoted the "mathematical definition" of a field. A "wind field" is then perfectly legitimate in that the wind has a direction and a strength at every location. But a "wind field" isn't what we mean when we talk about fields in physics. We talk about gravitational fields, and electromagnetic fields, and so on. And whilst you can assign values to points in space, this isn't defining the field, it's defining the result of field interactions. Like things fall down in a gravitational field, with an acceleration of g, which reduces with distance from the centre.

You describe the state of space.

Which one? If you mean the electron field, again you describe the state of space.

19. ### Farsight

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3,492
I understand the electric field. Note that people say the electron has electric charge, but it has an electromagnetic field rather than an electric field. If you were another charged particle such as a positron, and if I put you down near an electron, you'd move towards it, and you'd say you were in an electric field. But if I threw you past the electron, you'd swirl around too, and you say you were in a magnetic field as well. Generally speaking, if the force on you is linear, we talk about an electric field, and if the force is rotational, we talk about a magnetic field. But it's important to note that the fields that are present are electromagnetic fields. When they interact, the result is linear "electric" force and/or rotational "magnetic" force. Note that what Quarkhead thinks of as a field, is actually a plot of the magnitude and direction of this resultant force at every location. He confuses the force resulting from field interactions, with fields. And sadly, he is not alone.

The word quantum is derived from "quantus" which means "how much". There is no "how much" to the electric or electromagnetic field. The question doesn't make sense. Of course, people will tell you that the photon is the quantum of the electromagnetic field, but that doesn't make much sense either. There are no photons flying back and forth between an electron and a positron. In similar vein hydrogen atoms don't twinkle and magnets don't shine. In QED electromagnetic field interactions are modelled using virtual photons, which is reasonable because a photon is an electromagnetic field variation. However some change to some electromagnetic field is not actually a photon.

There's definitely an issue with massive bosons, in that the only massive subatomic particles that are stable are fermions. Groups of these are then said to make up massive bosons, such as Helium 4, but that isn't a subatomic particle.

I don't think that's the main problem myself. I think the main problem is that QED doesn't correctly describe gamma-gamma pair production, and then presents the electron field as something totally different to the photon field.

20. ### James RJust this guy, you know?Staff Member

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31,445
Arlich Vomalites:

How do you propose we measure or calculate the size of an electron?

If the electron had a finite size, then calculating the energy needed for electrons to collide would be straightforward, wouldn't it?

Forget the details of what happens in the collision for a moment. Think of a collision as a black box. Things go in; things come out.

My question to you is: if you think two electrons can collide, what comes out of the collision?

I'm not entirely sure what you're asking. If you're asking what the force carrier is for the electric field, QED's answer is the photon.

Now you have me confused. QED is a quantum field theory. Photons, electrons and so on are excitations of the field. A photon is nothing but a quantum of the electromagnetic field. Electric forces, in the QED picture, are transmitted by (virtual) photons from one charged particle to another.

An electromagnetic field is just electric + magnetic fields. Thus, an electric field is just a special case of an electromagnetic field.

Farsight:

Technically, a field is a mathematical concept. QuarkHead defined it pretty well, above.

This looks like speculation from Einstein. And could you check the quote? The word "not" should probably not start the second sentence.

exchemist has not claimed knowledge that he doesn't have. You, on the other hand, presume yourself to be an authority on these matters. But you and I both know that you can't do the math. And since the math is kind of important to the physics, it's a bit rich to claim that you know a lot about physics. You have, at best, a kind of pop-science knowledge. Quite often you get caught out in inaccuracies, and sometimes in blatant, ludicrous errors.

The mark of a good scholar is not to attempt to bluff others. A good scholar knows what he doesn't know. If he needs to know something, he tries to learn it. A good scholar also knows his own limits.

Last edited: Jan 18, 2015
21. ### Farsight

Messages:
3,492
Quarkhead gave the mathematical definition of a field. See this on Wikipedia: "A field is a physical quantity that has a value for each point in space and time.[1] For example, in a weather forecast, the wind velocity is described by assigning a vector to each point on a map. Each vector represents the speed and direction of the movement of air at that point". This Wikipedia article is entitled Field (physics), but I put it to you that no serious physicist would say there's a "wind field" outside. He'd talk to you about electromagnetic fields and gravitational fields, and he wouldn't entertain the notion that a gravitational field is merely some abstract set of arrows labelled g pointing down.

It's from http://www.rain.org/~karpeles/einsteindis.html , please don't dismiss what Einstein said as speculation. The quote contains some typographical errors. There's an "of" in the first sentence that should be an "if". The "Not" in the second sentence should be a "Nor". Perhaps there's an original version in German.

Expanding the Theory.
This theory having brought together the metric and gravitation would have been completely satisfactory of the world had only gravitational fields and no electro-magnetic fields. Not it is true that the latter can be included within the general theory of relativity by taking over and appropriately modifying Maxwell's equations of the electro-magnetic field, but they do not then appear like the gravitational fields as structural properties of the space - time continuum, but as logically independent constructions. The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds, and it is natural to suspect that this only appears to be so because the structure of the physical continuum is not completely described by the Riemannian metric.

I'm well read, so I have good knowledge, I can refer to Einstein etc to back up what I say, and I can do the math. I understand it. I understand what the terms mean, and what's wrong with $\mathbf{F} = q\left(\mathbf{E} + \mathbf{v} \times \mathbf{B}\right)$ and the Wikipedia Lorentz force article.

No, I don't get caught out with inaccuracies or blatant ludicrous errors. Would you care to link to an instance?

I'm not bluffing you. I'm telling you, and I'm backing up what I tell you with robust references such as Maxwell's theory of molecular vortices. The mark of a good scholar is to pay attention to the likes of Einstein and Maxwell when you read something they said that doesn't square with what you've been taught.

22. ### James RJust this guy, you know?Staff Member

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31,445
Farsight:

A physical quantity that has a value, eh? What kind of value? You mean, like a numerical value? Wouldn't that make it a mathematical concept?

I'd have no problem talking about the velocity field of the air. This kind of thing is often done in fluid dynamics.

That's what it boils down to.

To be fair, this is almost a philosophical argument rather than a physics one. We don't ever measure fields directly. We infer things about them from measurements of measurable quantities like force or charge or whatever. The field is a useful mental and mathematical picture that helps us to model the processes behind our actual observations. But it's an abstraction.

You're right that some of the errors are typos, like the "of/if" error. But that "Not" at the start of the second sentence shouldn't be there at all. Look - the sentence doesn't even make sense with that word. And it doesn't make sense if you replace "Not" by "Nor", either. Take out the "Not" and it's ok. So, it looks like this transcription is not particularly accurate or reliable.

I'm not dismissing what Einstein said there, by the way (reading it without the "Not", of course). Everything in that quote is solid except for the last sentence, which is more of a statement of Einstein's philosophical position than a statement about any established physics. That's what I'm saying is speculation.

I see no evidence of mathematical competence from you.

You think there's a problem with the Lorentz force equation? Here we go again. Please explain what the problem is. And, importantly, please explain why the equation gives results that match experimental tests, given that it's wrong and all.

I'll keep a look out. It's only a matter of time until you make another blunder. I don't want to go searching back through old posts. You can take this as an unsupported assertion right now, if you wish.

Do you think something in that quote doesn't square with what I've been taught?

23. ### Farsight

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3,492
Yes. That is a mathematical concept. What I'm telling you about is what Einstein said: a field is a state of space.

Well I do. Because the wind is not a state of space.

But that's not what a gravitational field is. It's a region of space where the state of space is described by the stress-energy-momentum tensor, and your measurements of distance and time made using light moving through that space, are described by the metric tensor:

$G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu}$

A field isn't an abstraction. The space in the room you're in is "neither homogeneous nor isotropic". Hence light curves downward and your pencil falls down. See this thread. The gravitational field is there because "the state of space" has been altered by the energy tied up as the matter of the Earth. The effect diminishes with distance such that when you plot it using say light clocks, your plot is curved.

Then look for a better transcript, or maybe a German original.

I think you need to think some more about what a field is. Imagine your room is a test chamber, and we empty out the air. A light beam still curves. Why? If you've been following any of the other threads you'll know it isn't because "the spacetime is curved" or because "the space is curved". If you follow the links you can read Einstein saying it's because the space is inhomogeneous, rather like sonar. This sort of stuff isn't mere philosophy or speculation, it's understanding, it's fundamental physics.

I'm no dummy.

It infers point particles when it's quantum field theory rather than quantum point-particle theory, it suggests there's an electric field E and a magnetic field B when "one should properly speak of the electromagnetic field Fuv rather than E or B separately". It totally ignores the situation where you have two charged particles, each with an electromagnetic field. It ignores the fact that it takes two to tango. All it's saying is that the force is a product of the electric linear force and the magnetic rotational force.

The mistakes I make are minor, and few and far between. Because I'm not some my-theory guy. I'm not making this stuff up. When I say something, I'm usually backing it up with some rock-solid references, and quotes by the likes of Maxwell and Minkowski and Einstein.

Yes. That's the story of my life. What's a field? I take note of Einstein saying it's a state of space, you've been taught it's some scalar/vector/etc value at every point in space. What's an electromagnetic field? I take note of Maxwell and Minkowski and Dirac and say it's twisted/vorticial/spinorial space, you've been taught it's a combination of E and B. It's a fresh eyes thing I guess. I am reminded of Avatar:

Moat: It is hard to fill a cup that is already full.
Jake Sully: My cup is empty. Trust me. Just ask Dr. Augustine. I'm no scientist.
Moat: Then what are you?
Jake Sully: I was a marine. A warrior... of the uh... Jarhead Clan.