How Real Are Virtual Particles?

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  • #26
DrChinese
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Virtual particles normally exist only if they do not violate the Heisenberg uncertainty principle (delta E delta t < Plancks constant). They are certainly necessary in strong central Coulomb fields (vacuum polarization - - Uehling integral). What would happen if a virtual particle pair were created near a black hole event horizon, and only one escaped?
You'd have Hawking radiation, I believe...
 
  • #27
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You'd have Hawking radiation, I believe...
so that means we have evidence for the existence of virtual particles?
 
  • #28
tiny-tim
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so that means we have evidence for the existence of virtual particles?
Nope! … there is no evidence whatever for Hawking radiation! :biggrin:
… virtual particles? i also thought they exist for like example 1/10,000 of a second physically.
Virtual particles normally exist only if they do not violate the Heisenberg uncertainty principle (∆E ∆t < Plancks constant).
It's well-known (and fairly obvious) that energy-momentum is not conserved at any point on a Feynman diagram if the particles are all real

there are two opposite conclusions to be drawn from this:

i] at least one of the particles must be virtual (ie, has the "wrong mass"), or

ii] they are all real, and energy is "borrowed" for a short time (the more energy, the shorter the time).

But surely there's no justification for somehow combining these two
opposites, and saying that virtual particles are allowed by HUP "borrowing"? :confused:
They are certainly necessary in strong central Coulomb fields (vacuum polarization - - Uehling integral).
Hi Bob S! :smile:

Wot's a Uehling integral? :confused:

I googled the phrase, and got only three hits … one of which was you, about four weeks ago.
 
  • #29
malawi_glenn
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I have to agree with you here Timmy

i) Borrowing energy from HUP makes the particles real

ii) no evidence for hawking, also i have read the derivation, but i see not references to virtual particles there, it is "just" thermal radiation



Nope! … there is no evidence whatever for Hawking radiation! :biggrin:



It's well-known (and fairly obvious) that energy-momentum is not conserved at any point on a Feynman diagram if the particles are all real

there are two opposite conclusions to be drawn from this:

i] at least one of the particles must be virtual (ie, has the "wrong mass"), or

ii] they are all real, and energy is "borrowed" for a short time (the more energy, the shorter the time).

But surely there's no justification for somehow combining these two
opposites, and saying that virtual particles are allowed by HUP "borrowing"? :confused:


Hi Bob S! :smile:

Wot's a Uehling integral? :confused:

I googled the phrase, and got only three hits … one of which was you, about four weeks ago.
 
  • #30
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5


Hi Bob S! :smile:

Wot's a Uehling integral? :confused:

I googled the phrase, and got only three hits … one of which was you, about four weeks ago.
See E. A. Uehling, Physical Review 48, pg 55 (1935). It is mentioned in Bjorken and Drell Relativistic Quantum Mechanics (McGraw Hill (1964) page 158.
The Uehing integral is used to calculate the penetration of the charge renormalization cloud (vacuum polarization) by charged particles in a strong central Coulomb field. Bjorken mentions primarily l=0 atomic states, but it also applies to l>=1 states. It is relatively small in electronic atoms, but is a large effect in muonic and pionic atoms.
 
  • #31
tiny-tim
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Uehling integral

Virtual particles … are certainly necessary in strong central Coulomb fields (vacuum polarization - - Uehling integral)
See E. A. Uehling, Physical Review 48, pg 55 (1935). It is mentioned in Bjorken and Drell Relativistic Quantum Mechanics (McGraw Hill (1964) page 158.
The Uehing integral is used to calculate the penetration of the charge renormalization cloud (vacuum polarization) by charged particles in a strong central Coulomb field. Bjorken mentions primarily l=0 atomic states, but it also applies to l>=1 states. It is relatively small in electronic atoms, but is a large effect in muonic and pionic atoms.
1935? 1964? :frown:

It doesn't seem much-loved. :cry:

Do 21st-century practitioners really regard the reality of virtual particles as necessary for the explanation of vacuum polarisation?
 
  • #32
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Post 5,6,7 and some others which claim virtual particles are merely mathematics artifices run the risk so many times proved in history...when math proves to be accurate and our preconceived notions prove simplistic, even naive but definitely false....Just because we can't imagine them being real doesn't mean they aren't.

When I studied semiconductor theory many years ago it was necessary to theorize electrons and holes to account for electrial behavior....I don't know whether holes are still necessary, I hope not, but it sure always seem an accounting/mathematical rather than a physical reality....

On the other hand, Einstein refused to believe in his own GR mathematics..so he stuffed in the cosmological constant to keep the universe in a steady state; Einstein also refused to believe in black holes which were also predicted by his own theory....and as a father of quantum theory he believed "God does not place dice" ...and missed reality a third time.

Sometimes experiment teases out new information; sometimes math does.....what's "real" and what's not is often not obvious....
 
  • #33
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Students specializing in experimental particle physics often get a dumbed down introduction to perturbation theory and the Feynman rules. If you follow such course then you may get the impression that virtual particles are real.

But if you read a decent book on QFT, e.g. the one by Itzykson and Zuber, you would never think that.
 
  • #34
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1935? 1964? :frown:

It doesn't seem much-loved. :cry:

Do 21st-century practitioners really regard the reality of virtual particles as necessary for the explanation of vacuum polarisation?
Virtual vacuum polarization particles are not real, but the vacuum polarization concept in treating strong Coulomb fields and charge renormalization is necessary to explain the hyperfine structure of hydrogen atoms (Lamb shift) and measurement of muonic and pionic atoms. Vacuum polarization is a necessary vertex corrention in all but the lowest order of charged particles interacting with the external field.

Please read this recent article:

The Uehling correction to the energy levels in a pionic atom

S. G. Karshenboim, E. Yu. Korzinin, and V. G. Ivanov
Can. J. Phys. 84(2): 107–113 (2006)

Abstract: We consider a correction to energy levels in a pionic atom induced by the Uehling potential, i.e., by a free electron vacuum-polarization loop. The calculation is performed for circular states (l = n–1). The result is obtained in a closed analytic form as a function of Zα and the pion-to-electron mass ratio. Certain asymptotics of the result are also presented.
==================================
If you can get a copy, please forward one to me. Thanks.
 
  • #35
171
1


The important point is that particles are not 'real' either.

We have interacting quantum fields, and it is a nightmare to calculate anything: we are partly rescued by some conservation laws (what is conventionally called 'particle number' might be conserved) but still it is impossible to work anything out.

One scheme for doing calculations is to split the QFT action into a quadratic (harmonic) part, and a non-quadratic part. A harmonic oscillator has quantised energy levels, with equally spaced levels. We consider a harmonic oscillator in its 5th excited state as being 5 particles: the point being that the cost of increasing the level, or decreasing the level is independent of which level we are in already and this feature allows us to pretend we have added 'a particle', or taken away 'a particle'. This is what particle means - a harmonic excitation. This also explains away the confusion surrounding 'identical particles'. It is trivially obvious that a harmonic oscillator in its 7th excited state is identical to the same harmonic oscillator in its 7th excited state!

So *if* we commit to doing calculations by splitting the action into a quadratic (non-interacting) and a non-quadratic part (interacting), then we can benefit from the language of particles (which is easy to deal with because harmonic oscillators are easy to deal with) and then hope to deal with the interaction term using perturbation theory. Virtual particles are an essential tool of perturbation theory (a mathematical technique).

Why is the particle picture so useful and ubiquitous? It is the same reason the harmonic oscillator is so useful and ubiquitous: that any sensible potential well is quadratic as a first approximation.

Both particles and virtual particles are features of the necessity to use perturbation theory on a harmonic (ie solvable) problem to work out anything.

I'm sure some people will disagree.
Peter
 
  • #36


Post 5,6,7 and some others which claim virtual particles are merely mathematics artifices run the risk so many times proved in history...when math proves to be accurate and our preconceived notions prove simplistic, even naive but definitely false....Just because we can't imagine them being real doesn't mean they aren't.

When I studied semiconductor theory many years ago it was necessary to theorize electrons and holes to account for electrial behavior....I don't know whether holes are still necessary, I hope not, but it sure always seem an accounting/mathematical rather than a physical reality....

On the other hand, Einstein refused to believe in his own GR mathematics..so he stuffed in the cosmological constant to keep the universe in a steady state; Einstein also refused to believe in black holes which were also predicted by his own theory....and as a father of quantum theory he believed "God does not place dice" ...and missed reality a third time.

Sometimes experiment teases out new information; sometimes math does.....what's "real" and what's not is often not obvious....
I couldn't agree with you more. And yes, holes are no longer necessary to account for electrical behavior.
 
  • #37


The important point is that particles are not 'real' either.

We have interacting quantum fields, and it is a nightmare to calculate anything: we are partly rescued by some conservation laws (what is conventionally called 'particle number' might be conserved) but still it is impossible to work anything out.

One scheme for doing calculations is to split the QFT action into a quadratic (harmonic) part, and a non-quadratic part. A harmonic oscillator has quantised energy levels, with equally spaced levels. We consider a harmonic oscillator in its 5th excited state as being 5 particles: the point being that the cost of increasing the level, or decreasing the level is independent of which level we are in already and this feature allows us to pretend we have added 'a particle', or taken away 'a particle'. This is what particle means - a harmonic excitation. This also explains away the confusion surrounding 'identical particles'. It is trivially obvious that a harmonic oscillator in its 7th excited state is identical to the same harmonic oscillator in its 7th excited state!

So *if* we commit to doing calculations by splitting the action into a quadratic (non-interacting) and a non-quadratic part (interacting), then we can benefit from the language of particles (which is easy to deal with because harmonic oscillators are easy to deal with) and then hope to deal with the interaction term using perturbation theory. Virtual particles are an essential tool of perturbation theory (a mathematical technique).

Why is the particle picture so useful and ubiquitous? It is the same reason the harmonic oscillator is so useful and ubiquitous: that any sensible potential well is quadratic as a first approximation.

Both particles and virtual particles are features of the necessity to use perturbation theory on a harmonic (ie solvable) problem to work out anything.

I'm sure some people will disagree.
Peter
I can see where you are coming from, but I can also see that sometimes we become too engrosed in the math to focus on what the math really means. Particles might not be exactly what we think they are, but if particles are not real, then what in the world is it that I am sitting on?
 
  • #38
1,901
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Particles might not be exactly what we think they are, but if particles are not real, then what in the world is it that I am sitting on?
Fields are not enough?
 
  • #39
2,471
1


Virtual particles are as real (or unreal) as the 'normal' ones.

In different inertial frames observers always agree on the existence of the particles, but it is not the case for the accelerating frames. Virtual particle in one frame may appear a normal one in another frame.

As an example check the Unruh effect: accelerating obserer observes normal particles while in the rest frame these particles are virtual.

So you can convert virtual particles into the normal ones and back just by changing the frame -> there is no real difference.
 
  • #40
525
7


Virtual particles are as real (or unreal) as the 'normal' ones.

In different inertial frames observers always agree on the existence of the particles, but it is not the case for the accelerating frames. Virtual particle in one frame may appear a normal one in another frame.

As an example check the Unruh effect: accelerating obserer observes normal particles while in the rest frame these particles are virtual.

So you can convert virtual particles into the normal ones and back just by changing the frame -> there is no real difference.
You are NOT talking about virtual particles. The Unruh effect is merely a statement about the disagreement of two observers in different frames. What appears to be a vacuum state for one observer is a thermal bath for the other. There is no such thing as conversion of virtual particles into real ones.
 
  • #41
2,471
1


1 You are NOT talking about virtual particles. The Unruh effect is merely a statement about the disagreement of two observers in different frames. What appears to be a vacuum state for one observer is a thermal bath for the other.
What is a "thermal bath" is made of?
Accelerating observer with a detector on a winshield will see his detector counting REAL particles! Then the windshield melts from the Unruh radiation :)
Unruh radiation is equivalent somehow to the Hawking radiation. Like hawking radiation it consists of the REAL particles.
 
  • #42


Fields are not enough?
We would need a slightly better definition and interperitation of fields, because right now, we understand that distant particles interact because of 'fields,' but what is the reason or cause of these fields? 300 years ago, the concensus agreed that there was such a thing as an 'ether,' but what was that?

In my opinion, the way we generally understand fields today, they are the slightly more sophisticated modern versions of ethers.
 
  • #43
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7


What is a "thermal bath" is made of?
Accelerating observer with a detector on a winshield will see his detector counting REAL particles! Then the windshield melts from the Unruh radiation :)
Unruh radiation is equivalent somehow to the Hawking radiation. Like hawking radiation it consists of the REAL particles.

Well, but then you could say that what is "really real" are the quantum fields which always exist even in vacuum, as the vacuum is merely the ground state of QFT.
 
  • #44
DrChinese
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Particles might not be exactly what we think they are, but if particles are not real, then what in the world is it that I am sitting on?
To add to lightarrow's comment about fields:

You are definitely NOT sitting on real particles. There are no solids on this planet that would support a person against the effect of gravity if virtual fields (Pauli exclusion principle etc.) did not exist.
 
  • #45
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To add to lightarrow's comment about fields:

You are definitely NOT sitting on real particles. There are no solids on this planet that would support a person against the effect of gravity if virtual fields (Pauli exclusion principle etc.) did not exist.
Even worse than sitting on virtual particles - IMO you are sitting on numbers!
 
Last edited:
  • #46
1,901
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We would need a slightly better definition and interperitation of fields, because right now, we understand that distant particles interact because of 'fields,' but what is the reason or cause of these fields? 300 years ago, the concensus agreed that there was such a thing as an 'ether,' but what was that?

In my opinion, the way we generally understand fields today, they are the slightly more sophisticated modern versions of ethers.
You ask: "what is the reason or cause of these fields"? and I ask: what is the reason or cause of particles? What are they made of? What is a photon, an electron, a quark, a gluon, a neutrino made of? How is it made? It doesn't seem likely that an elementary particle is without explanation of what holds its energy (or mass) or charge together, so every time you talk about particles you should explain that.

We found atoms and then we asked what holds an atom together; we discovered the answer is essentially EM fields;

we found nuclei and then we asked what holds a nucleus together; we discovered the answer is other fields ;

we found necleons and then we asked what holds a nucleon together; we discovered the answer is other fields...

...
...
...

Now we now that mass (I mean, invariant mass) is nothing else than energy confined in a specific region of space.
What need you do have of particles at all?
 
  • #47


To add to lightarrow's comment about fields:

You are definitely NOT sitting on real particles. There are no solids on this planet that would support a person against the effect of gravity if virtual fields (Pauli exclusion principle etc.) did not exist.
I know. I was trying to make a point.
 
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  • #48


Even worse than sitting on virtual particles - IMO you are sitting on numbers!
Numbers are not what the universe is made of. There has to be something there, whether we know exactly what it is or not is debatable. Numbers are an invention of man, to represent things we observe. That is what I was trying to get across with that post.
 
  • #49


You ask: "what is the reason or cause of these fields"? and I ask: what is the reason or cause of particles? What are they made of? What is a photon, an electron, a quark, a gluon, a neutrino made of? How is it made? It doesn't seem likely that an elementary particle is without explanation of what holds its energy (or mass) or charge together, so every time you talk about particles you should explain that.

We found atoms and then we asked what holds an atom together; we discovered the answer is essentially EM fields;

we found nuclei and then we asked what holds a nucleus together; we discovered the answer is other fields ;

we found necleons and then we asked what holds a nucleon together; we discovered the answer is other fields...

...
...
...

Now we now that mass (I mean, invariant mass) is nothing else than energy confined in a specific region of space.
What need you do have of particles at all?
I can't explain what a particle is because no one knows. What need do we have for particles? Quote "we found necleons and then we asked what holds a nucleon together; we discovered the answer is other fields." If particles exist, what are they? If fields exist, what are they? All we know is that they both depend on each other. A particle can't interact without a field. A field is nothing without particles.

No one knows exactly what a particle is. No one knows exactly what a field is. A particle appears to be confined energy, but then, what is energy? Also, E=mc^2 relates mass and energy. We know they are proportional, but not for sure that they are equal in some manner (although I think they are).

My post was explaining the fact that fields are ill defined because we don't know what a field is. We have no idea, and yet we use fields to explain everything.

(note)- My quote states that a field is another definition for an ether. Prove me wrong. I guarantee you that the math will come, but I want a conceptual understanding of a field.
 
  • #50
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I can't explain what a particle is because no one knows. What need do we have for particles?
...
If particles exist, what are they? If fields exist, what are they? All we know is that they both depend on each other. A particle can't interact without a field. A field is nothing without particles.

No one knows exactly what a particle is. No one knows exactly what a field is.
...
My post was explaining the fact that fields are ill defined because we don't know what a field is. We have no idea, and yet we use fields to explain everything.
No, the point of QFT is that everything is fields, there are no particles. In a quantum field theory, the energy levels of excited states of the field are quantised, just like the energy levels of a harmonic oscillator are quantised: the finite bundle of energy arising because of quantisation is what one calls a particle.

It is an accident of history that the phenomenon of light was first successfully addressed by classical field theory, whereas the phenomenon of massive particles was first addressed by classical point-particle mechanics, with particles interacting with fields. If the wave nature of matter and the particle nature of light had been noticed earlier, maybe things would have developed differently.

For those who really believe in 'particles', I'd be interested to hear how you understand the 'indistinguishability of identical particles' in QM if it isn't with reference to an underlying field excitation picture for quantum particles. I was never happy with those red and green electrons which far too often appear in explanations!

As for what a field really is, sure that might remain a bit of an ontological mystery, but if we're reductionists its better to only have one mystery to explain than two.
Peter
 

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