# What happens to pressure exerted by photons in pair production?

Photons exert (positive) pressure which is a source of gravity.

What happens to pressure exerted by a photon after converting into an electron-positron pair or a proton-antiproton pair? Does it still exert the same pressure as a photon?

• Gravity is not caused by photons exerting pressure. Take away all the photons and you still have gravity. – S. McGrew Aug 19 '18 at 21:27
• Energy-momentum is the source of gravity. The nature of the photon em distribution is quite different from that of the ep pair, so the same holds for the gravitational field. – my2cts Aug 19 '18 at 23:09

1. Gravity is caused by stress-energy.

2. Photons can interact with an atom in three ways:

  1. elastic scattering

2. inelastic scattering

3. absorption

3. When photons scatter off an atom inelastically, they may exert pressure on the atom, or the macro object (that the atom is part of). In this case the photon gives part of its energy to the atom, changes phase and angle.

The atom will get some of the photon's momentum, and this is the way how the photon exerts pressure on the atom.

Photons do have stress-energy, and can cause gravity.

You are asking, whether when a photon converts into an electron positron pair, does the electron positron pair have the same gravitational effects as the photon.

Now since the mass of the electron positron pair is the same as the photon's (energy), the electron positron pair will have the same stress-energy as the photon. So the electron positron pair will have the same gravitational effects as the photon.

Now as per the comment, this is a little more complicated, since in your case, the photon has zero rest mass, we need to look at the energy-momentum tensor. In this case, it needs a third component, another photon, and the photon pair needs to have stress-energy at least the sum of the electron positron pair's mass.

• it is not the mass of the photon pair, but the energy momentum tensor which needs also a third component , for energy momentum conservation as the photon has zero mass and the pair at least the sum of two electron masses . see physics.stackexchange.com/questions/182656/… – anna v Aug 20 '18 at 5:17
• OK, the electron positron pair have the same gravitational effects as the photon. But does it still exert (positive) pressure like a photon? – Forge Aug 20 '18 at 9:29
• @Forge it is not just the electron positron, there is a third four momentum in the equation, the nucleus the provided the field on which energy momentum is balanced, input=output. Already from this one can see that the results will be different as far as spatial distribution goes, three fouvectors in three different directions so any pressure will be a complex extrapolation. – anna v Aug 20 '18 at 12:38