Question: what is quantum tunelling?

Robert Woolfson answered on 21 Jun 2013:

Quantum mechanics is weird. I’m starting out by saying that because understanding that is the first step to trying to learn anything in quantum mechanics. One of the greatest scientists in quantum physics, Richard Feynman, once said this:

“If you think you understand quantum mechanics, you don’t understand quantum mechanics”

With that in mind, let’s give this a go. In Quantum Mechanics (which I’m going to abbreviate QM because I’m lazy), things don’t get to be everywhere they want. This is called quantization of states and basically says things are only allowed to be in certain places, and it is impossible for them to be anywhere else.

Between these various states that something (an electron, a neutron, a photon etc) can be in, there are things called energy barriers. These are exactly what they sound like, a barrier that requires the thing to have a certain amount of energy to cross. By tunneling, they are making the change of states more likely to happen that it would be without tunneling.

In QM, things can actually tunnel through the barriers under certain circumstances. There has to be a state that the thing can go in to on the other side, and the barrier can’t be too thick or the thing won’t make it through.

This is seen all the time in fusion reactions in stars, in certain ways DNA can suddenly change and in a machine called a scanning tunneling microscope which scientists use to examine the surfaces of things at a molecular level.

Hope that makes sense. Please comment if you have any questions at all.