Roll a ball at a hill too tall to climb and it should roll back — yet a quantum particle sometimes appears on the far side, having 'tunnelled' through. This leak-through is how the Sun shines and how some computer chips work.
A particle where it 'can't' be
Classically, a ball without enough energy cannot cross a hill. Quantum mechanically, a particle's wavefunction extends into and beyond a barrier, so there is a nonzero probability of finding it on the far side even without the energy to go over — this is tunnelling.
The probability of tunnelling falls off sharply with barrier width and height. Thin, low barriers are 'leaky'; thick ones are effectively impassable.
Tunnelling powers the sun (protons fuse despite repulsion), scanning tunnelling microscopes, and flash-memory storage.
- A particle meets a barrier it lacks the energy to climb.
- Recall that its wavefunction penetrates the barrier.
- State whether it can appear on the other side, and how that likelihood changes if the barrier is made thicker.
What you should see: There is a nonzero chance it tunnels through; making the barrier thicker sharply reduces that probability.