Antiparticles and vacuum

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1.does antiparticles really travels backwards in time or is it just used to describe feynman diagrams and diracs negative energy states.
2.what does vacuum really means physically is it just a state in the fock space from which other particle states are created or it really means something different ?Iam totally confused!
 

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  • #2
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Anitiparticles do not literally travel back in time - its just a way to describe it mathematically.

Physically it means exactly what the math says it means - the vacuum can be viewed as particles constantly being created and destroyed. The issue I have with it is its supposed to be infinite and you subtract the infinity to give the ground state, with differences from the ground state being what is supposedly observable. Never really understood that one. It cant be infinite - obviously the fact the theory has it infinite is an issue - it cant be correct - a more fundamental theory - perhaps string theory - should correct it. In the meantime I do not view it as infinite but rather very large with some cutoff we do not quite know the value of yet - similar to the way you can view renormalisation - although that is not personally the way I view it.

Thanks
Bill
 
  • #3
Bill_K
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the vacuum can be viewed as particles constantly being created and destroyed.
Likewise, I suppose, you could view Schrodinger's cat as constantly being killed and reincarnated. :uhh:

In fact the vacuum state is a time-independent superposition of components with a varying number of particles.
 
  • #4
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In fact the vacuum state is a time-independent superposition of components with a varying number of particles.

Point taken.

Yea - just to be careful about the other thing I said - I simply cant stomach infinite vacuum energy so you imagine that maybe space-time has some kind of granular structure at the Plank scale for example. You don't get infinite - but a really large number. It cant be like that either so there is something really sick about QFT (I think its because gravity hasn't been included and when we have a theory of gravity valid at all energies it will be resolved - but that just a guess on my part) but at least the subtracting a large value to give a zero ground state doesn't sound like total nonsense.

Thanks
Bill
 
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2.what does vacuum really means physically is it just a state in the fock space from which other particle states are created or it really means something different ?Iam totally confused!

Physically, the vacuum is just the lowest energy state of a quantum field theory.
 
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what Ireally wanted to ask is that whether the field theoretic vacuum has anything to do with the real vaccum empty space?because i dont get it how particles and antiparticles being created and destroyed from empty space
 
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what Ireally wanted to ask is that whether the field theoretic vacuum has anything to do with the real vaccum empty space?because i dont get it how particles and antiparticles being created and destroyed from empty space

The Casimir effect is usually cited as evidence for it to actually exist, and certainly it can account for it but some recent alternate explanations based on van der wals forces have been put forward:
http://en.wikipedia.org/wiki/Casimir_effect

Thanks
Bill
 
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vanhees71
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The Casimir effect is not about the vacuum, because there are always charges involved. In the most simple textbook treatment you simplify the presence of the charges in the uncharged plates by applying appropriate boundary conditions, but from a microscopic point of view there are charges present.

The vacuum is, as was posted in one posting before, the ground state of a quantum field theory. There are of course many subtleties with this idea. Usually what we consider are free fields in empty space and built the bosonic or fermionic Fock space out of occupation-number basis-vectors (i.e., totally antisymmetrized product states of N one-body basis states; usually chosen as momentum-spin or energy-angular-momentum states).

Already when you consider external classical fields, e.g., the famous case of a strong electrostatic field, interesting features concerning vacuum states occur. In this case, the socalled Schwinger-pair-creation mechanism is predicted but not yet experimentally confirmed: there are spontaneously electron-positron pairs created in this electrostatic field, because you have different in- and out-vacuum states that are connected by a Bogoliubov transformation. Of course, again here you don't deal with empty space but with space + a classical electric field, which itself has to be created somehow by charges.
 
  • #9
Vanadium 50
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but some recent alternate explanations based on van der wals forces have been put forward

Recent = 1948: c.f. "The Influence of Retardation on the London-van der Walls Forces", Casimir and Polder.
 
  • #10
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The Casimir effect is not about the vacuum, because there are always charges involved. In the most simple textbook treatment you simplify the presence of the charges in the uncharged plates by applying appropriate boundary conditions, but from a microscopic point of view there are charges present.

Hmmmm. I think there could be an issue here. Certainly textbooks I have such as Zee QFT In A Nutshell - page 65 - don't say that. He calculates it from a vacuum disturbance. Certainly the vacuum itself is not observable - as is correctly said it simply defines the ground state - but changes in the vacuum certainly are and that's precisely what Zee (and other texts I have seen) calculate.

Is it being said that these treatments are incorrect? If so that's a bit of a worry. I know more elementary textbooks on occasion can simplify things to the point what they say is not quite true eg in textbooks explaining how transistors work they say holes are the absence of electrons but if you think about it, it doesn't explain things like the Hall effect. But that one is fairly well known - the better textbooks explain that's just a simplification to help getting grip on it at a more elementary level - QM provides the correct framework where the holes are 'quasi' particles. Never heard about this one though.

Thanks
Bill
 
  • #11
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what Ireally wanted to ask is that whether the field theoretic vacuum has anything to do with the real vaccum empty space?because i dont get it how particles and antiparticles being created and destroyed from empty space

There can't be empty space in our universe, since we can observe background radiation anywhere in our universe, we can easily deduct that there is something everywhere. At least that's my understanding.
 
  • #12
vanhees71
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I forgot to give a link to an interesting paper, showing that the Casimir effect is not about zero-point energy, which is unobservable. The Casimir force is due to the presence of charges and currents and quantum fluctuations of the em. field.
 
  • #13
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I forgot to give a link to an interesting paper, showing that the Casimir effect is not about zero-point energy, which is unobservable. The Casimir force is due to the presence of charges and currents and quantum fluctuations of the em. field.

I have done a bit of research and now think you are correct - books like Zee are - well I wont put too fine a point on it - wrong. Its a bit annoying though - but certainly shows how worthwhile it is posting on this forum.

Thanks
Bill
 

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