Electron LOSING energy to the quantum vacuum?

[Pseudo Epsilon]

okay, here goes. Electron A moves through a vacuum. There are quantum fluctuations ocurring all the time, some generate electron positron pairs, which we'll call B. Electron A travels past these. Surely chared Electron A should interact with and lose energy to B electrostaticly in analoge to a cue ball hitting other cue balls? Where am
I wrong?

 

[Bill_K]

okay, here goes. Electron A moves through a vacuum. There are quantum fluctuations ocurring all the time, some generate electron positron pairs, which we'll call B. Electron A travels past these. Surely chared Electron A should interact with and lose energy to B electrostaticly in analoge to a cue ball hitting other cue balls? Where am I wrong?

Well in the first place, electrons do not bump into each other like cue balls. Being charged, they could, however, exchange energy by exchanging a virtual photon.

Secondly, any QED process must conserve energy and momentum. If the incoming state was a single electron, the outgoing state must have the same energy and momentum, and it is not possible to conserve both of these by emitting a real photon or electron-positron pair. So the outgoing state must be a single electron also.

What must happen, therefore, is that the energy comes back in the form of a second virtual photon, and the entire process is just an electron self-energy diagram, which will be absorbed as part of the renormalization.

 

[Pseudo Epsilon]

the universe is determined to conserve, thanks bill_k

 

[OCR]

the universe is determined to conserve

That's good... I think, anyway?

The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.

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

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


This was sometimes misreported as the Higgs boson "ending" the universe.



OCR

 

[Pseudo Epsilon]

whats that got to do with my question?

 

[OCR]

OCR

 

[DarMM]

Well virtual particles are not real in any sense, they're just a useful mental picture when it comes to thinking about perturbation theory. However QED, or any other field theory, does not have virtual particles as actual dynamic states.

 

[Pseudo Epsilon]

what state are they then? What i don't get is if an electron-positron pair is produced in the vacuum they should anhilate into gamma rays, what happens to the photons?

 

[DarMM]

what state are they then? What i don't get is if an electron-positron pair is produced in the vacuum they should anhilate into gamma rays, what happens to the photons?

They are not a state. They don't exist in the theory, they're just a useful picture for understanding perturbation theory, no "pairs" are produced in the vacuum or anything like that.

 

[Naty1]

What i don't get is if an electron-positron pair is produced in the vacuum they should anhilate into gamma rays, what happens to the photons?

I'm not sure anybody does really 'get it'...The vacuum is a slippery subject...
In one sense the vacuum is there, not doing much. Its 'empty' space...but it's not quite that either. Among other things it appears to be powering cosmological expansion...

There is not a definite line differentiating virtual particle pairs from real particles — "real particles" are viewed as being detectable excitations of underlying quantum fields, virtual particles, not so much. The equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more...so they become observable.
Virtual particles are also excitations of the underlying fields, and appear in quantum math, but are detectable only as forces, not particles.

 

[Bill_K]

There is not a definite line differentiating virtual particle pairs from real particles — "real particles" are viewed as being detectable excitations of underlying quantum fields... The equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more...so they become observable.

I agree!

 

[DarMM]

There is not a definite line differentiating virtual particle pairs from real particles

There is, a real stable particle will appear as a pole in the two-point function (in Fourier space), multiparticle states as branch cuts and resonances (at least narrow ones) appear as poles on the second sheet found when you continue through the multiparticle branch cut.

However virtual particles appear absolutely nowhere in the nonperturbative formulation of the theory. They are just convenient labels on diagrams in perturbation theory.

A perfect example is Yang-Mills ghost. There simply are no such particles in Yang-Mills. "Virtual ghosts" appear in the perturbative expansion, but absolutely no such particle appears in the spectrum and they never even appear in a nonperturbative formulation like the lattice.

 

[Bill_K]

A perfect example is Yang-Mills ghost. There simply are no such particles in Yang-Mills. "Virtual ghosts" appear in the perturbative expansion, but absolutely no such particle appears in the spectrum and they never even appear in a nonperturbative formulation like the lattice.

DarMM, We've discussed the virtual particle issue here many times before, and little new can be said. But I think you're the first one to confuse virtual particles with ghosts!

 

[Pseudo Epsilon]

then what is the quantum foam?

 

[DarMM]

DarMM, We've discussed the virtual particle issue here many times before, and little new can be said. But I think you're the first one to confuse virtual particles with ghosts!

I think ghosts are useful example in this regard since the appear in the perturbative expansion just like any other virtual state, yet we know they aren't real. (Don't worry I know you're joking!)

 

[Naty1]

then what is quantum foam

Quantum foam is a description John Wheeler used to explain what might happen when Planck scale [10^-33 cm or 10^-43 seconds as I recall] is approached...

His idea was that space and time become indistinct, merge together in Heisenberg uncertainty ..
that is, space,time,energy uncertainty...analogous to confinement of a particle...the smaller the space the shorter the confined wavelengths...and the higher the energy...

Without a theory of quantum gravity, I'm pretty sure nobody really knows yet what goes on there.

Under "Quantum Foam" Wikipeda says:

...at sufficiently small scales—the energy of these fluctuations would be large enough to cause significant departures from the smooth space time seen at larger scales, giving space time a "foamy" character...