Classically, it is forbidden to escape, but according to the (then) newly discovered principles of quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus.
Traditionally, it's forbidden to escape, but according to the newly discovered principles of quantum mechanics, there exists a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus.