An electron (mass = 9.11x10-31 kg) and positron annihilate each other resulting in two identical gamma ray photons. If a proton (mass =1.67x10-27 kg) and antiproton annihilate each other, will the resulting photons have longer or shorter wavelengths than those from the electron/positron process? Explain your reasoning.

Answer:

An electron has a mass of 9.11 x 10^-31 kg which corresponds to a rest energy of 0.511 MeV. A proton has a mass of 1.67 x 10^-27 kg which corresponds to a rest energy of 938.257 MeV which is about 1876 times the rest energy of the electron.

So, in a proton-antiproton annihilation, the energy released will be significantly larger (1876 times large) than electron-positron annihilation.

Since Energy of a photon is inversely related to its wavelength, the wavelength of the photon from the proton-antiproton annihilation will be shorter than the electron-positron annihilation.