# B Virtual particles motion

#### Ranku

Do virtual particles travel at the speed of light, or do they have 'virtual rest mass'?

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#### weirdoguy

They do not exist in the first place:

I mean, well, they exist, but it's just a name for propagators appearing in perturbation series used in QFT. And when I look at my notes, I don't see any propagators flying around at the speed of light

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#### Michael Price

They do not exist in the first place:

I mean, well, they exist, but it's just a name for propagators appearing in perturbation series used in QFT. And when I look at my notes, I don't see any propagators flying around at the speed of light

#### A. Neumaier

Virtual particles don't move; they are drawn only.

Their positions are bound variables in an integral ranging over Minkowski space.

To ask about the speed of a virtual particle is therefore like asking for the speed of a point in the graph of a curve enclosing an area that you want to compute by integration.

#### weirdoguy

Well, as you can see: $\frac{-\eta_{\mu\nu}}{k^2+i\varepsilon}$ - it's standing still - in the rest frame of this page of course

#### Michael Price

So let's recap: we've been told that virtual particles:
1) don't exist - or perhaps they do
2) don't move - or perhaps they've stationary

Virtual particles have endpoints in configuration space, so of course they can have defined velocities.

#### A. Neumaier

So let's recap: we've been told that virtual particles:
1) don't exist - or perhaps they do
2) don't move - or perhaps they've stationary

Virtual particles have endpoints in configuration space, so of course they can have defined velocities.
How do you define the velocity of a straight line segment?

#### Michael Price

They do not exist in the first place:
BTW, I read those links when you posted them on another thread, and wasn't convinced. Full of circular reasoning, and never gave an operational meaning of "exist".

#### A. Neumaier

BTW, I read those links when you posted them on another thread, and wasn't convinced. Full of circular reasoning, and never gave an operational meaning of "exist".
I defined exist = having a state, which is necessary to talk about probabilities of observation.

What is your operational meaning of the word?

#### Michael Price

I defined exist = having a state, which is necessary to talk about probabilities of observation.

What is your operational meaning of the word?
Having a measurable effect. Virtual particles mediate forces (which we can measure) and other stuff (which we can also measure).

#### weirdoguy

don't exist - or perhaps they do
When I said that they don't exist I meant that they don't exist the way real particles do - everything is explained in the insight articles.

Wasn't convinced of what? "Virtual particle" is a name for a certain line you draw on a paper when you want to do some calculation. That's what all textbooks on QFT I have seen say. The only books that give them any realness are pop-science books. Have you ever done any calculation using Feynman diagrams? Have you drawn any Feynman diagram? If you did, at what velocity did the internal line you draw on your paper move?
Besides

Virtual particles have endpoints in configuration space
you integrate all that out.

#### Michael Price

When I said that they don't exist I meant that they don't exist the way real particles do - everything is explained in the insight articles.

Wasn't convinced of what? "Virtual particle" is a name for a certain line you draw on a paper when you want to do some calculation. That's what all textbooks on QFT I have seen say. The only books that give them any realness are pop-science books. Have you ever done any calculation using Feynman diagrams? Have you drawn any Feynman diagram? If you did, at what velocity did the internal line you draw on your paper move?
Besides

you integrate all that out.
Yes, you can integrate them out, but you don't have to.

#### weirdoguy

Virtual particles mediate forces (which we can measure) and other stuff (which we can also measure).
Virtual particles appear only in perturbative methods and it's only a name physicists gave to certain part of the mathematical expressions. You seem to be stuck on a pop-science view of them. And as I said in other thread, you can derive Casimir effect without virtual particles.

Um, what? There is no ad hominem. You simply don't accept the very definition of a virtual particle that physicists use.

#### Michael Price

Virtual particles appear only in perturbative methods and it's only a name physicists gave to certain part of the mathematical expressions. You seem to be stuck on a pop-science view of them. And as I said in other thread, you can derive Casimir effect without virtual particles.
Lots of effects in physics have multiple explanations or valid models. One valid model does not invalidate another.

#### weirdoguy

That does not mean that every single part of the model correspond to reality. Expecially when it contradicts the definition of that part. And the definition of virtual particles is as straightforward as it could be.

#### Michael Price

That does not mean that every single part of the model correspond to reality. Expecially when it contradicts the definition of that part. And the definition of virtual particles is as straightforward as it could be.
Unless virtual particles contradict the Casimir effect - which would be an amazing claim - then this is irrelevant, incorrect or confused.

#### weirdoguy

Your view of them contradicts their definition. And also it contradicts the consensus among physicists (probably that's why your last thread was closed), and contradicts what most textbooks on quantum field theory say. Student Friendly Quantum Field Theory by Klauber has even one whole chapter (chapter 10) discussing related issues.

#### Michael Price

Your view of them contradicts their definition. And also it contradicts the consensus among physicists (probably that's why your last thread was closed), and contradicts what most textbooks on quantum field theory say. Student Friendly Quantum Field Theory by Klauber has even one whole chapter (chapter 10) discussing related issues.
From the truncation I gather that you accept that virtual particles do not contradict the Casimir effect. If so, then we are only disputing what "exist" means. Which is not subject to operational resolution.

#### A. Neumaier

Having a measurable effect. Virtual particles mediate forces (which we can measure) and other stuff (which we can also measure).
But only on paper. In this sense, the exponential function also exists since it is ubiqitous in the derivation of measurable effects.

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#### Nugatory

Mentor
B and I threads on this topic are always difficult. It is possible to use English language words like “position” and “mass”, but doing so is terribly misleading to an audience that hasn’t seen the actual mathematical treatment - and that requires an A-level understanding.

Statements like
Virtual particles have endpoints in configuration space, so of course they can have defined velocities
can be justified under a suitably abstract definition of velocity... but it’s pedagogical malpractice to toss them into this discussion.

This thread is closed, as the responses in post 2 and 3 of this thread are reasonable. @Ranku, you should feel free to start new threads with follow up questions after you’ve been through those links.

(As with all thread closures, the thread can be reopened if necessary to allow further constructive discussion. PM me or any other mentor)

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