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# Annihilation of two particles / Rest mass energy watch

1. When two particles annihilate (e.g. proton and antiproton), the resulting radiation will have a minimum energy of the sum of the rest mass energies of the two particles, right? But how does the kinetic energy of the particles factor in? Does the energy of the resulting radiation, or in this case, photons, equal the rest mass energies of the proton and antiproton PLUS the the kinetic energies of both particles?

Like this:

hfmin = E0 + KE

?!
2. (Original post by laughingbagel)
When two particles annihilate (e.g. proton and antiproton), the resulting radiation will have a minimum energy of the sum of the rest mass energies of the two particles, right? But how does the kinetic energy of the particles factor in? Does the energy of the resulting radiation, or in this case, photons, equal the rest mass energies of the proton and antiproton PLUS the the kinetic energies of both particles?

Like this:

hfmin = E0 + KE

?!
Yeah, that's almost right, except you just haven't written algebraically what you described. For particle-antiparticle annhilation, you meant:

- minimum photon energy is the sum of the rest mass energies (where each has rest mass energy ).

but

- actual photon energy is the rest mass, plus whatever KE they were carrying when they annhilated.
3. Unless of course the proton and anti proton are virtual pairs, and one of them has negitive energy...
4. (Original post by QuantumOverlord)
Unless of course the proton and anti proton are virtual pairs, and one of them has negitive energy...
Negative energy what does that even mean.
5. Side note:
When dealing with an annihilation you will always end up with at least 2 photons. The reason is that momentum is conserved, photons have momentum and in the 0 momentum frame momentum is 0 => you need at least 2 photons
6. (Original post by mf2004)
Side note:
When dealing with an annihilation you will always end up with at least 2 photons. The reason is that momentum is conserved, photons have momentum and in the 0 momentum frame momentum is 0 => you need at least 2 photons

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