How Real Are Virtual Particles?

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  • #51


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
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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
malawi_glenn
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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


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
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Guys, you are underestimating the progress of philosophy...
is there philosophy in virtual particle's?
 
  • #56
tiny-tim
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virtual philosophy

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
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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
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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
malawi_glenn
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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
xantox
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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
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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
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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
xantox
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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
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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
Demystifier
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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
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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
malawi_glenn
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experiment has been performed?
 
  • #69
malawi_glenn
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so why is this guy still referring to things we don't even know exists?
 
  • #70
Demystifier
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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.
 
  • #71
Demystifier
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so why is this guy still referring to things we don't even know exists?
Because there are good theoretical reasons to believe that they exist. Still, what he does not want to admit, there are also some serious problems with the theoretical arguments that lead to these results.

But my point is: Even if the Unruh effect exists and if the standard theoretical description of this effect is correct, it has still nothing to do with virtual particles.
 
  • #72
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