Question

An antiproton has the mass of a proton but a charge of -e. If a proton and an antiproton orbited each other, how far apart would they be in the ground state of such a system? Why might you think such a system could not occur?

Answer 1

Find the Bohr radius for such a hydrogen-like system:

$$a_0=\frac{\epsilon_0h^2}{\pi m_pe^2}=2.88·10^{-14}\text{ m}.$$

Such a system could not occur because the radius of a proton is 0.84·10^-15 m and they would annihilate.