When is a Photon Real or Virtual?

[Tyger]

The issue arises whether virtual particles can be considered to be "real" in the same sense that photons of light can be. So exactly when is a photon real and when is it virtual, and do virtual photons have as much right to be thought of as existing as "real" ones?

A real photon (free field photon) is one that is propagating from one part of space to another without being in any way "anchored" to the originating charges, such as one coming from the Sun and going to your eye.

A virtual photon is one that is in some sense anchored to a charge or charges. It may go directly from one charge to another, or return to the issueing charge if it isn't absorbed by another one.

Real photons have four possible polarization states, and any admixtures of them. They can be vertically or horizontally polarized, or have right or left handed spin. By suitably adjusting the phases we can make any polarization state out of any other pair of states. The polarization refers to the direction of the electric field vector.

Electric fields always act to change the momentum and energy of a charged body at the same time, while magnetic fields always act transversely on a moving body, i.e. at right angles to both the velocity and the magnetic field vector. To act otherwise would be to change the rest mass of the body, which Nature doesn't allow.

Virtual photons can be longitudinaly polarized or have timelike polarization. They can't propagate very far from the source charge because the timelike and longitudinal amplitudes cancel out at long distances. Magnetic and static electric fields can be thought of as being made up of virtual photons, so they have as much right to being considered real as real photons do.

 

[Creator]

Originally posted by Tyger
The issue arises whether virtual particles can be considered to be "real" in the same sense that photons of light can be. So exactly when is a photon real and when is it virtual, and do virtual photons have as much right to be thought of as existing as "real" ones?

Tyger; Quantum Electrodynamics (QED) and QFT provide for a description of charge interaction by intermediate particles called 'virtual' particles (sometimes called 'off-mass shell'). According to QED it is not a REAL physical particle because 1.) it is unobservable; and 2.) it is not subject to the usual energy-momentum relation, E^2 = p^2 + m^2. It has nothing to do with being "anchored" or not.

Real photons have four possible polarization states, and any admixtures of them. They can be vertically or horizontally polarized, or have right or left handed spin. By suitably adjusting the phases we can make any polarization state out of any other pair of states. The polarization refers to the direction of the electric field vector.

Not exactly. Photons only have two fundamental polarization states---> 'left' and 'right' circular polarization corresponding to +1 and -1 helicity. All other polarizations (including linear polarizations) are derived from these two.
Hope that helps.

Creator

 

[ranyart]

Originally posted by Tyger
The issue arises whether virtual particles can be considered to be "real" in the same sense that photons of light can be. So exactly when is a photon real and when is it virtual, and do virtual photons have as much right to be thought of as existing as "real" ones?

A real photon (free field photon) is one that is propagating from one part of space to another without being in any way "anchored" to the originating charges, such as one coming from the Sun and going to your eye.

A virtual photon is one that is in some sense anchored to a charge or charges. It may go directly from one charge to another, or return to the issueing charge if it isn't absorbed by another one.

Real photons have four possible polarization states, and any admixtures of them. They can be vertically or horizontally polarized, or have right or left handed spin. By suitably adjusting the phases we can make any polarization state out of any other pair of states. The polarization refers to the direction of the electric field vector.

Electric fields always act to change the momentum and energy of a charged body at the same time, while magnetic fields always act transversely on a moving body, i.e. at right angles to both the velocity and the magnetic field vector. To act otherwise would be to change the rest mass of the body, which Nature doesn't allow.

Virtual photons can be longitudinaly polarized or have timelike polarization. They can't propagate very far from the source charge because the timelike and longitudinal amplitudes cancel out at long distances. Magnetic and static electric fields can be thought of as being made up of virtual photons, so they have as much right to being considered real as real photons do.

What Virtual Photons relate to inside 3-Dimensional domains(Galaxies, Stars, Atoms etc, is a product that is >-<, always inclined to Attract.

The opposite occurs for Virtual Photons in the EM field of far off Inter Galactic flat Space, which is two dimensional and the VPs are
<-> always in opposite expansion, the less matter around(obvious!)the longer the Length of expansion moment appears, ie, <--->, this is why the Vacuum of Space at far away locations produce high expanding force, it is in effect a Quantum of Space.

It is hard for us to percieve as we look up and outward from our Galaxy, that the intervening Space is actually constructed at a lower dimensional framework, a 2-Dimensional flat space matrix, this has the consequence of only allowing a directional movement in opposite dynamics, or back to back.

The geometric makeup of 2-D Space connects every Galaxy, which is structured into Spacetime, and has 3-Dimensional spatial movements, but any movement from Galaxy to Galaxy has to be of a Non-Time dependent motion through a Quantum Field, this of course would be Quite difficult for Items that are themselves made up oF 3-d matter!

Rather than you warping the intergalacitc space, I am afraid intergalactic space would 'flatten/collapse/contract' you into 2-d energy! not so much as you leave our Galaxy, but the further you travel, the more(actually LESS) you become, as you radiate from a 3-D entity, into a 2-D dynamical energy.

 

[Boffin]

Originally posted by Tyger
The issue arises whether virtual particles can be considered to be "real" in the same sense that photons of light can be. So exactly when is a photon real and when is it virtual, and do virtual photons have as much right to be thought of as existing as "real" ones?

If I told you that a bicycle was a virtual automobile, you would look at me kind of funny. But I think that virtual photons are about as different from real photons in the same way.

So far I've concluded that it's better to use the old name of force field instead of the newer name virtual photons. It appears that a force field has completely different properties than real photons, so it's not really appropriate to use a similar name. That confuses people.

But what is a force field, really. I'm trying to understand what this is also.