# Photons are massless or negligible

## Sexl Physik 8, textbook

We come to the idea that every electrical charge is surrounded by a cloud of virtual photons. These photons are constantly emitted and absorbed and in their entirety form the field in the vicinity of the charge. If a second charge is brought close, virtual particles are exchanged between these charges, whereby energy or momentum is transferred. The further away the charge is, the longer the virtual particle needs, the less energy and momentum it transmits: the force decreases with distance. It has been shown that the exchange particles are bosons for all types of forces (see Table 73.3). The electrical interaction between charged particles including the creation and annihilation of electron-positron pairs is successfully described by quantum electrodynamics (QED). The QED was developed in the 1940s. One of her first successes was the calculation of the contribution of the virtual photons to the magnetic moment of the electron. The QED is the model for the description of the strong interaction between the quarks and the weak interaction (e.g. β-decay). Forces between particles are mediated by exchange particles. The photon is the exchange particle of the electromagnetic interaction. Range of forces between particles With the idea of ​​exchange forces, one can understand the short range of forces (approx. 10 −15 m) between protons and neutrons in atomic nuclei in nuclear physics. The exchange particles are pions with a mass of approx. 140 MeV / c 2, which each consist of a quark and an antiquark. If a virtual particle with the mass m is to be generated, the energy Δ E = mc 2 is required for this. This energy can only be applied for the period of time Δ t = h / Δ E. During this time the virtual particle can cover a maximum distance of Δ s = c · Δ t = c · h / Δ E = h / (m · c) ≈ 10 - 15 m. The range of the force field is therefore limited and depends on the mass m of the exchange particle. At distances that are significantly greater than the range, the forces are negligibly small. The range of the electric field is infinite because the photon is massless. The force between electrical charges at a distance r is proportional to r −2; it is still effective at any distance. The vacuum The concept of virtual particles leads to a revision of the ideas about empty space, the vacuum. We have already seen in the theory of electricity that matter-free space in the vicinity of charges is the carrier of a field. As we can now see, virtual particles are linked to these fields. Not only are virtual particles of charges continuously emitted and absorbed, but virtual particle-antiparticle pairs (mass 2 m) for periods of time Δ t