What is a virtual photon and how is it used in quantum field theory?

[big_bounce]

Hello all
Does virtual photon have energy ?
Does virtual photon travel same as speed of light ?
That's energy how can calculate ?

when a photon absorbed by electron , photon destroyed or changes to virtual photon ?

 

[Bill_K]

big_bounce, Virtual particles carry energy and momentum, but they are not required to obey the "mass shell" relation E² = p²c² + m²c⁴. (In the case of a photon, m = 0, it is not required that E = pc.) Energy and momentum are separately conserved, each time they interact with other particles. Only real photons must travel at the speed of light.

 

[phinds]

when a photon absorbed by electron , photon destroyed or changes to virtual photon ?

When a photon is absorbed by an electron, the energy of the photon is added to the energy of the electron, which now assumes a higher state, so I guess you could say that the photon is "destroyed", although I think the more common terminology is "ceases to exist". No virtual photon is involved.

 

[Naty1]

big bounce: you might be interested in some related background:

“...Vacuum energy is the zero-point energy of all the fields in space...the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields...

[This means the ground field state has energy and that is synonymous with virtual particles...]


I posted this in another discussion:

A particle [or matter] is a quanta of a quantum field...a concentration of energy, momentum, etc. Regarding the big bang, it is possible quantum fluctuations in the inflationary vacuum become quanta [particles] at super horizon scales. Particle production via changing gravitational fields and expansion is believed a real phenomenon. It seems that expansion of geometry itself, especially inflation, can produce matter.

We're familiar with other theoretical cases where geometric circumstances create real (not virtual) particles, like Hawking radiation at a BH horizon and Unruh radiation caused by an accelerating observer. With the Unruh effect, it is theorized that two adjacent observers, one inertial and one accelerating will measure different temperatures and make different counts of particles. In other discussions in these forums, there are theories that at the Hubble radius the accelerating Hubble Horizon is sufficient for the production of particles.

And here is another interesting tidbit:


rindler-vacuum.pdf


“An observer at rest has his own definition of a vacuum: it is the state in which he sees no particles. An accelerated observer also has his own vacuum, using the same definition. We will show that these two vacuums are not the same, so that an accelerated observer actually sees particles in the inertial observer's vacuum. In other words, vacuum" is a relative concept that depends on the observer.

 

[big_bounce]

Thanks all .
But it is not still clear for me

Does photons that send message particle's charge between electron and nucleus in atom are virtual photon ?
And they are traveling at C ?

 

[andrien]

Virtual photons more or less are calculational tools because they can not be observed(detected).That's why there name is virtual photon.

 

[Sonderval]

Please note that the concept of a "virtual" photon (or any virtual particle) is tricky - it is nice to draw these little feynman diagrams to describe the exchange of a photon between two particles, but the correct physics involves summing over all possible Feynman diagrams.

The "virtual photon" is a handy device to allow us to keep a kind of particle picture in mind instead of dealing with quantum fields (where the quantum photon field is usually a complicated superposition of different field values at each space-time point), but to give you the correct picture, you should always keep in mind that you have to sum over all possibilities (as described in feynman's QED-book).

You should also note that if you define a "virtual photon" as a photon that belongs to the internal line of a feynman diagram, all photons that are ever observed are virtual, because they are absorbed. For photons that travel any appreciable distance, their deviation from the "mass shell" (as explained by Bill_K) is so small as to be negligible - that is why the concept of a "real" photon is useful, although it is an idealisation.