How Real Are Virtual Particles?

In summary: They are not "real" in the sense that they exist in and of themselves. Although they are important for understanding particle interaction, they are not physically real. There is much debate surrounding their existence, but from what xepma has said, it seems that there is some evidence that they do exist.
  • #36


Naty1 said:
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.
 
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  • #37


peteratcam said:
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


benk99nenm312 said:
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


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


Dmitry67 said:
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


xepma said:
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


lightarrow said:
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 consensus 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


Dmitry67 said:
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


benk99nenm312 said:
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


DrChinese said:
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!
 
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  • #46


benk99nenm312 said:
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 consensus 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


DrChinese said:
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


p764rds said:
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


lightarrow said:
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


benk99nenm312 said:
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
 
  • #51


peteratcam said:
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


Good point. (There's nothing to argue, what you said is correct) My honest opinion is that red and green dots are wrong. I think once we figure out what a field is, we can then figure out what the excitations in the field are, and therefore 'point particles' will be redefined correctly.
 
  • #52


benk99nenm312 said:
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.

Numbers belong to mathematics which lies outside space-time (INO) and is not a human invention. But 'human invention' should not be derided as 'fake plastic untrue nonsense' because the universe made us so why cannot we think like the universe thinks? Do not underestimate us - we are very very clever IMO. It may just be true that intelligence and mathematics made what we live in - we have both those capabilities.

These logical arguments on physicsforums we have, are often good examples of our intelligence, and and inspirational too.
 
  • #53


p764rds said:
Numbers belong to mathematics which lies outside space-time (INO) and is not a human invention. But 'human invention' should not be derided as 'fake plastic untrue nonsense' because the universe made us so why cannot we think like the universe thinks? Do not underestimate us - we are very very clever IMO. It may just be true that intelligence and mathematics made what we live in - we have both those capabilities.

These logical arguments on physicsforums we have, are often good examples of our intelligence, and and inspirational too.

Guys, you are underestimating the progress of philosophy...
 
  • #54


p764rds said:
Numbers belong to mathematics which lies outside space-time (INO) and is not a human invention. But 'human invention' should not be derided as 'fake plastic untrue nonsense' because the universe made us so why cannot we think like the universe thinks? Do not underestimate us - we are very very clever IMO. It may just be true that intelligence and mathematics made what we live in - we have both those capabilities.

These logical arguments on physicsforums we have, are often good examples of our intelligence, and and inspirational too.

Does a tree know the answer to 2+2? Have we not invented a system of counting? When did the sky decide to rain because it has calculated the proper density of the water vapor?

Mathematics is not nonsense. Humans invented math to represent true observations. We invent theories to represent these observations, and then we see if nature agrees with us. Nature follows rules that we represent with numbers. An electron is not a #.

I know QFT gives you a theory of probability amplitudes, and that the particles are regarded as excitations in the field. This matches what we observe, not what is really there. What is really there is not known. That is all I was saying.
 
  • #55


malawi_glenn said:
Guys, you are underestimating the progress of philosophy...

is there philosophy in virtual particle's?
 
  • #56
virtual philosophy

Super_Jew said:
is there philosophy in virtual particle's?

There is virtual philosophy, which enables a vacuous background to produce a thesis and anti-thesis capable of existing only for short time before destroying each other. :wink:
 
  • #57


It's a bit like negative numbers. Are they real? Well, we can use them to express debt. But debt is not real money, otherwise we wouldn't be in this financial crisis.
 
  • #58


tiny-tim said:
There is virtual philosophy, which enables a vacuous background to produce a thesis and anti-thesis capable of existing only for short time before destroying each other. :wink:
Right! However you forgot to say that this is just the description given by the pertubation method of computing the vacuous background's energy...:smile:
 
  • #59


Super_Jew said:
is there philosophy in virtual particle's?

Well, there are philosophy of quantum field theory, yes

And also philosophy of what is real, if math is invented or discovered. My answer was to p764rds, not to you.
 
  • #60


tiny-tim said:
Virtual particles are … representation, not reality :wink

We should be extremely cautious when trying to apply to the quantum realm such classical statements of "representation" and "reality". We appear to know how to calculate quantumly but we still think way too classically. Depending on interpretations of quantum mechanics, the quantum state may be either viewed as pure representation (Bohr), or as existing (Everett/Deutsch), or as something in-between, where the reality of entities and their objective existence may be the emergent property of more fundamental, "nonexistent", purely representational degrees of freedom (Zurek).
 
  • #61


Again, if you believe in Unruh effect and hawkings radiation then virtual particles are as real as real ones because for some (accelerating) observers virtual particles look perfectly real.
 
  • #62


Dmitry67 said:
Again, if you believe in Unruh effect and hawkings radiation then virtual particles are as real as real ones because for some (accelerating) observers virtual particles look perfectly real.
This is very interesting. Can you expand this idea?
 
  • #63


Dmitry67 said:
Again, if you believe in Unruh effect and hawkings radiation then virtual particles are as real as real ones because for some (accelerating) observers virtual particles look perfectly real.
The same would also imply that real particles are as virtual as virtual ones. Also, note that Fock particle states (eigenstates of the number operator in Fock space) are not necessarily identical to observed local particles, so that we must also be extremely cautious there. The point is that, "real" is an inherently classical concept. We should learn to move on to more sophisticated, fine-grained ontologies when dealing with a quantum world.
 
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  • #64


lightarrow said:
This is very interesting. Can you expand this idea?

http://en.wikipedia.org/wiki/Unruh_effect

An accelerating observer will perceive an apparent event horizon forming (see Rindler spacetime). The existence of Unruh radiation could be linked to this apparent event horizon, putting it in the same conceptual framework as Hawking radiation. On the other hand, the theory of the Unruh effect explains that the definition of what constitutes a "particle" depends on the state of motion of the observer.

The (free) field needs to be decomposed into positive and negative frequency components before defining the creation and annihilation operators. This can only be done in spacetimes with a timelike Killing vector field. This decomposition happens to be different in Cartesian and Rindler coordinates (although the two are related by a Bogoliubov transformation). This explains why the "particle numbers", which are defined in terms of the creation and annihilation operators, are different in both coordinates.
 
  • #65


Dmitry67 said:
Again, if you believe in Unruh effect and hawkings radiation then virtual particles are as real as real ones because for some (accelerating) observers virtual particles look perfectly real.
This is wrong. Unruh effect has nothing to do with virtual particles. This is because the vacuum does NOT contain virtual particles. The vacuum is an eigenstate of the operator of the number of particles, so there are no particle fluctuations in the vacuum. What fluctuates in the vacuum is the field, not particles.
 
  • #66


This your BM view

Put a particle detector in front of a spaceship
Accelerate (with the unrealisticly high acceleration)
Check the tracks of the Unruh particles.
Still believe they are 'virtual'? :)
 
  • #67


experiment has been performed?
 
  • #68


malawi_glenn said:
experiment has been performed?
No. ..
 
  • #69


so why is this guy still referring to things we don't even know exists?
 
  • #70


Dmitry67 said:
This your BM view

Put a particle detector in front of a spaceship
Accelerate (with the unrealisticly high acceleration)
Check the tracks of the Unruh particles.
Still believe they are 'virtual'? :)
You don't read what I write. First, I didn't mention BM in the post above. Second, I did not say that they are virtual.
 

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