Virtual Particles Momentum Transfer

[e2m2a]

TL;DR

Can virtual particles transfer momentum?

My understanding is that virtual particles don't really exist. However, they somehow come into existence under certain circumstances. For example, in the Casimir Effect the virtual particles on the outside of the plate now have the capacity to transfer momentum and kinetic energy to the plates to cause the plates to move together. Did these virtual particles on the outside of the plate all of a sudden become "real"?

 

[Keith_McClary]

There are some Insights and associated discussions.

That explanation assumes perfect conducting plates, flat to subatomic scale.

 

[weirdoguy]

in the Casimir Effect the virtual particles on the outside of the plate now have the capacity to transfer momentum and kinetic energy to the plates to cause the plates to move together.

There has been quite a lot of threads here about Casimir effect, and why it is not what most popular presentations say it is. Especially, there is no need to invoke virtual particles in the picture.

 

[EPR]

How would you prove that virtual particles exist?

The theory says they last for 10^-23 seconds.

 

[weirdoguy]

The theory says they last for 10-23 seconds.

Theory (perturbative expansion of scattering amplitude) says no such thing. Only hand-wavy heuristics based on time-energy uncertainty relation may say that, but it's just that - heuristic. Not very meaningfull, regarding what really virtual particles are.

 

[EPR]

Theory (perturbative expansion of scattering amplitude) says no such thing. Only hand-wavy heuristics based on time-energy uncertainty relation may say that, but it's just that - heuristic. Not very meaningfull, regarding what really virtual particles are.

What are they? It they are just a model, it's a rather useful model given that everything around me and you and the whole planet is mediated by virtual particles. From the common sensation of touching objects to subatomic forces and interactions.
I trust the model.

 

[weirdoguy]

What are they?

https://www.physicsforums.com/insights/physics-virtual-particles/
https://www.physicsforums.com/insights/misconceptions-virtual-particles/
https://www.physicsforums.com/insights/what-are-virtual-particles-intro/

I trust the model.

And the model says they are not "real" the way real particles are "real". That's the point! It's not that we say "oh, it's just a model so they are not real". The very model we are discussing states that virtual particles are just calculational tool.

 

[EPR]

Everything is a model. Electrons are labels for observed phenomena... One day this model may and probably will be replaced by strings or loops or something else entirely.

Still, I find it easier to think of everything in terms of virtual particles instead of 'real' particles that most people easily confuse with billiard balls due to the 'real' label. When you think of 'real' particles intercations I read that as virtual particles exchanges. Even mass is thought to be due to virtual particles. You just give more credence to the persisting observed macroscopic phenomena which are easier labelled as their respective 'particles'.
I would not argue if someone were to claim that everything they have seen or touched was virtual particles. This model is very pervasive and can't be ruled out based on 'real' particles alone. Those real particles do not exist as such at all.

 

[weirdoguy]

Everything is a model.

And there is no model in which virtual particles are something more than a name given for internal lines in Feynman diagrams, which by itself, are nothing more than a calculational tool.

When you think of 'real' particles intercations I read that as virtual particles exchanges.

And that picture is not universal since it only applies to perturabtive methods of handling certain processes. You will find nothing like this in e.g. lattice methods, or non-perturbative methods.

Even mass is thought to be due to virtual particles.

Any source for that?

You just give more credence to the persisting observed macroscopic phenomena which are easier labelled as their respective 'particles'.

No, I just know the basics of QFT and calculated one or two things myself

I would not argue if someone were to claim that everything they have seen or touched was virtual particles. This model is very pervasive and can't be ruled out based on 'real' particles alone. Those real particles do not exist as such at all.

Sigh... Did you read the insight articles? "Real particle" in the context we are discussing means an external line in Feynman diagram, something we can construct a state of and make a measurement of. "Virtual particle" is an internal line of Feynman diagram. With one process involving some specific set of particles (called "real") there are infinitely many diagrams corresponding to infinitely many terms in perturbation series. Henece there are infinitely many virtual particles! And we cannot construct any states corresponding to those particles, hence we cannot measure anything about them! They aren't even on-shell in most cases.
You are talkig about some "pervasive model" - but there is no such model! It's just a hand-wavy story told in a lot of popularisations (and unfortunately in some high-school and undegrad general physics textbooks). You will find no such thing in QFT textbooks. Some newer ones even devote some space to discuss this nonsense, eg. Klauber in his "Student Friendly QFT", chapter 10 if I recall correctly.

 

[EPR]

For all these 'arguments' against virtual particles to hold, you must show a model of real particles.
Good luck with that!
If you don't have one, move on.
Reality as far as we know is made up of fields. This is the only real model and substance of the observed reality. Everything else is secondary, temporary and transient.

 

[weirdoguy]

"Real particle" in this context is something very specific! It's not a philosophy discussion!

you must show a model of real particles.

Standard model and quantum filed theory. And within these models "real particles" are perfectly defined. Again - you should read insights! Beacuse you cling to much on a word "real". It has a perfectly defined meaning in the context we are discussing. No philosophy involved.

 

[EPR]

What philosophy? Particles in QFT are momentary excitations of the respective field. Not particles with definite properties.

Every single interaction observed taking place in the universe can be broken down and described by only four types of interactions(4 fundamental forces) mediated by virtual particles.

 

[weirdoguy]

Particles in QFT are momentary excitations of the respective field.

And with that definition virtual particles are not even particles since they don't even have "respective field". Again - it's all in the insights!

 

[EPR]

I am arguing that virtual particles are a necessity and seemingly a more fundamental aspect of what you seem to mistakenly regard as 'real' particles. Both are transient excitations of the respective quantum fields.

 

[weirdoguy]

I am arguing that virtual particles are a necessity and seemingly a more fundamental aspect of what you seem to mistakenly regard as 'real' particles. Both are transient excitations of the respective quantum fields.

And I am arguing that this is nonsense, based on every QFT course I've taken, every textbook on QFT I've read, and every PF discussion I read.

 

[EPR]

You argument is that elementary particles are real and virtual particles are mathematical constructs. Before delving further, a large number of high caliber physicists consider what you label real particles mathematical entities too. I sometimes fall in this trap too.

 

[Motore]

a large number of high caliber physicists consider what you label real particles mathematical entities too

Can you measure the spin, mass and charge of a mathematical entity? Because you can measure these properties of an electron with a measurement aparatus. If that doesn't constitue a real particle I don't know what does.

 

[EPR]

Can you measure the spin, mass and charge of a mathematical entity? Because you can measure these properties of an electron with a measurement aparatus. If that doesn't constitue a real particle I don't know what does.

That you can measure those properties does not mean that the particle is real and had those properties before the measurement. This is what I am arguing against with weirdoguy. This philosophy shines through everywhere.

 

[Motore]

That you can measure those properties does not mean that the particle is real

Can you measure the spin, mass and charge of a mathematical entity?

 

[EPR]

Virtual particles are indeed missing some of the properties of 'real' particles which are also a perturbation in the quantum field. Both are virtual in that they are secondary manifestations of the underlying quantum fields.
Virtual particles are a useful representation and approximation for interactions of the 'real' particles. In that sense, I view them as a necessity for a more complete picture of the events in the micro world. The downside - I asked about it in my first post - how can we measure their existence given the unbelievably short time scale appropriated by the uncertainty principle?

 

[gentzen]

@EPR is it possible that you are confusing quasi-particles like phonons, plasmons, resonances, field excitation, ... with internal lines in Feynman diagrams, which are called virtual particles. Then there is also the bare electron (in the non-interacting theory, and in the interacting theory before renormalization) and the dressed electron, which might both be accused of being mere mathematical entities too. Those too are different from virtual particles. From my POV, the dressed electron is more physical and real than the bare electron, but that is not the point of the disagreement between weirdoguy and you. His point is just that virtual particles is the name given to internal lines in Feynman diagrams, and those aren't even on-shell in most cases.

 

[EPR]

@EPR is it possible that you are confusing quasi-particles like phonons, plasmons, resonances, field excitation, ... with internal lines in Feynman diagrams, which are called virtual particles. Then there is also the bare electron (in the non-interacting theory, and in the interacting theory before renormalization) and the dressed electron, which might both be accused of being mere mathematical entities too. Those too are different from virtual particles. From my POV, the dressed electron is more physical and real than the bare electron, but that is not the point of the disagreement between weirdoguy and you. His point is just that virtual particles is the name given to internal lines in Feynman diagrams, and those aren't even on-shell in most cases.

Simple question - how do those 'real' particles interact? Because they certainly do interact.

This model of virtual particles mediation is very useful and can't be discarded

 

[gentzen]

Simple question - how do those 'real' particles interact? Because they certainly do interact.

OK, so you really mean those internal lines in the Feynman diagrams. Very good, fine with me.

It is true that there are some interaction for which the dressed electrons will undress before the interaction. But those interactions are rare (just like in real life, and note that animals don't even have the option to undress), most interactions with other electrons happen from the distance (because electrostatic repulsion often prevents that they come sufficiently close together). The situation might be different for interactions between electrons and photons, I am not sure at the moment and would have to check.

 

[EPR]

You certainly know this but just about everything in reality except for gravity depends on electromagnetic forces. Like everyone else on Earth you assume everything you touch to be real. This realness in QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles that transmit electromagnetic forces. These photons are “virtual”.

If you consider them non-existent, what is that exists?
I contend that they are a very useful model and approximation.
Every theory in physics is an approximation and a model.

 

[EPR]

If by now everyone understands that both virtual and 'real' particles are modes of QFT whereby 'particles' are generated as excitations of the respective quantum field(and their interactions are modeled via virtual particles exchanges generated by the same underlying quantum field), I think this debate will become more clear to the general audience.

The physicists above don't have a better model than QFT but in it, virtual particles are used extensively and very successfully(as a very good approximation).

 

[EPR]

It seems to me lots of physicists can't shake off the notion of classical particles.

Classical particles are point-like with defined trajectories and they do not exist. Classical particles cannot exhibit self-interference the way a single electron can or form diffraction patterns when sent at the double-slit screen as electrons do.

The 'particles' in QT are field quanta, not classical particles. They are generated by the same quantum field that produces the virtual particles.

 

[Motore]

It seems to me lots of physicists can't shake off the notion of classical particles.

Huh? Nobody is talking about classical particles. We are talking about:

In relativistic QFT, observable – hence undisputably real – particles of mass are asymptotic objects, sufficiently far (in theory infinitely far) away that their interactions can be neglected.

Source https://www.physicsforums.com/insights/physics-virtual-particles/

But to emphasize the misconceptions about virtual particels (I don't think you read the insight articles @weirdoguy posted):

States involving virtual particles cannot be created since quantum field theory has creation operators only for observable particles whose 4-momentum satisfies the mass-shell constraint. For lack of a state, virtual particles have none of the usual physical characteristics of real particles: They cannot be said to exist in space and time, have no position, no meaningful probabilities to be created or destroyed anywhere, no lifetime, cannot cause anything, interact with anything or affect anything – since all these things are (within the unavoidable uncertainty) determined by the state. Therefore there are also no dynamics, speed of motion, or world lines. (In physics, dynamics is always tied to states and an equation of motion. Neither exists for virtual particles.)

Though QFT computations generally use the momentum representation, there is also a Fourier-transformed complementary picture of Feynman diagrams using space-time positions in place of 4-momenta. In this version (relevant for perturbation theory in curved space-time), the integration is overall of space-time, so in this picture, virtual particles have space-time positions but no momentum, and again no states, no probabilities, no cause and effect, no dynamics, and no world lines.

In the nonperturbative lattice approach to quantum field theory, virtual particles are completely absent, another proof of their unreal, ”virtual” nature.

The only way the usual dynamical language for virtual particles is justified by the theory is as purely figurative analogy in ”virtual reality”, useful for an informal talk about complicated formulas and for superficial summaries in lectures capturing the imagination of the audience.

This has to be kept in mind when reading in professional scientific publications statements involving virtual particles. Otherwise many statements become completely misleading, inviting a magical view of microphysics and weird speculation, without the slightest support in theory or experiment.

Source https://www.physicsforums.com/insights/misconceptions-virtual-particles/

So virtual particles are (as the name suggests) not real in any meaningful sense.

 

[berkeman]

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