# Bare and Dressed Photons in space.

#### interested

Am I correct in thinking a "bare" photon in QED is represented as a qq- and a dressed photon is one that is absorbed and re-emitted via particle interactions as it passes through a medium. Does a bare photon as it passes through space interact with the zero point field and become dressed. ?? Does a dressed photon have to be the same properties when it is emitted as when it was absorbed?

#### topsquark

Forum Staff
Am I correct in thinking a "bare" photon in QED is represented as a qq- and a dressed photon is one that is absorbed and re-emitted via particle interactions as it passes through a medium. Does a bare photon as it passes through space interact with the zero point field and become dressed. ?? Does a dressed photon have to be the same properties when it is emitted as when it was absorbed?
I'm not quite sure what you mean by a "dressed photon." Charged particles can be dressed (electron/positron pairs popping in and out) by considering the EM field at very short ranges, which is why we need good renormalization techniques, which we don't really have yet. As photons have no charge they don't have a field to dress with. (There is something less well known here: Photons can scatter off other photons. It depends on electrons and positrons but it's a 4 point Feynman diagram so it is very weak.)

As to the ZPE, it doesn't really exist. The ZPE shows up as an error in the field calculations so no particles are affected by it.

Last comment: photons are made up of photons and nothing else... they are an elementary particle and thus they have no "innards." Photons "decay" into other particles at high enough energies but that is due to photons splitting into, as you suggested, quark/anti-quark pairs. (It can be any particle/anti-particle pair, actually.)

-Dan

#### interested

I'm not quite sure what you mean by a "dressed photon." Charged particles can be dressed (electron/positron pairs popping in and out) by considering the EM field at very short ranges, which is why we need good renormalization techniques, which we don't really have yet. As photons have no charge they don't have a field to dress with. (There is something less well known here: Photons can scatter off other photons. It depends on electrons and positrons but it's a 4 point Feynman diagram so it is very weak.)

As to the ZPE, it doesn't really exist. The ZPE shows up as an error in the field calculations so no particles are affected by it.

Last comment: photons are made up of photons and nothing else... they are an elementary particle and thus they have no "innards." Photons "decay" into other particles at high enough energies but that is due to photons splitting into, as you suggested, quark/anti-quark pairs. (It can be any particle/anti-particle pair, actually.)

-Dan
Oh no!!!!! I have read multiple times Zero point energy of the vacuum, determines the lowest possible energy level any quantum system can have. It is proven to exist via the Casimr effect and the Dynamic Casimir effect. It is also a theoretical candidate for dark energy and the cosmological constant.

Zero point energy of the vacuum, zero point field, zero point energy of particles, and ground state energy are not the same. but they do exist according to lots of sources.

#### topsquark

Forum Staff
Oh no!!!!! I have read multiple times Zero point energy of the vacuum, determines the lowest possible energy level any quantum system can have. It is proven to exist via the Casimr effect and the Dynamic Casimir effect. It is also a theoretical candidate for dark energy and the cosmological constant.

Zero point energy of the vacuum, zero point field, zero point energy of particles, and ground state energy are not the same. but they do exist according to lots of sources.
In developing QFT we run across the concept that fields fill the whole Universe and we can approximate the overall effect of these by approximating the field at point by a Quantum field. (ie. Electrons create "electron fields," neutrons create "neutron fields," etc.) The first non-zero approximation to these fields is to say that we have a Quantum simple harmonic oscillator at every point in space. It is this approximation that lies at the heart of the matter.

QSHOs have a peculiar property: We can calculate the energy of n oscillators as $$\displaystyle E = (n + 1/2) \hbar \omega$$. Even if we have no oscillators we can set n = 0. But that leaves a problem: $$\displaystyle E_{n = 0} = (0 + 1/2) \hbar \omega = (1/2) \hbar \omega$$. If we have no energy in the oscillator we still have a ground state energy! It is this $$\displaystyle (1/2) \hbar \omega$$ added up over all points in space that creates the problem: If no oscillators are present (ie. no particles or fields are present) then we still get a non-zero answer to the ground state of the Universe. The resulting energy, the ZPE, is actually infinite. To say the least it's a problem that we have no solution to. It's supposed to be 0 and it isn't.

As I mentioned in another recent thread I really don't like any problem that is resolved by using the ZPE. If we can't calculate it properly then how can we hope to use it? If I'm wrong, then there is an experiment that can measure it and I'll have to eat my words. Until that happens the value of the ZPE is an open question.

-Dan

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#### interested

Thanks for the clarification.

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