# Tunneling in quantum mechanics

We have this well known case of ~4.2 MeV α-particle being emitted from the U-238 nucleus but the α-particle emitted from the Po-212 which has the energy of ~9MeV cannot penetrate it. This phenomenon is explained by the tunneling effect from quantum mechanics.

Now, of the tunneling can happen from the inside to the outside, then why can't it happen from the outside to the inside?

• It can happen the other way, but it's not what you consider when you want to describe a nucleus leaking an alpha-particle. – Raskolnikov Jan 28 at 13:50
• Yes, it can go inside. – Raskolnikov Jan 28 at 14:01
• @Korra: it would if it was directed properly! – Gert Jan 28 at 14:23
• Directed toward the $Po-212$ nucleus. – Gert Jan 28 at 14:37
• No, any old $\alpha$ will do. Directed properly, of course. – Gert Jan 28 at 14:47

Given the correct initial state ─ an $$\alpha$$ particle directed at the nucleus at the right energy, going radially inwards, and for a sufficient length of time ─ the reverse tunnelling process can indeed happen. (Indeed it must happen, because unitary evolution in QM is invariant under time reversal.)
However, that reversed initial state is far les common than the $$\alpha$$-particle-inside-the-nucleus state, so you don't see it in practice.