quantum tunneling
Roll a ball at a hill. If it lacks the energy to reach the top, common sense says it rolls partway up and comes straight back — it can never appear on the far side. A quantum particle facing a barrier breaks this rule. Because it is described by a spread-out wave that leaks a little way into the wall, there is a real chance it simply turns up on the other side, even though it never had enough energy to climb over. That impossible-looking passage is quantum tunneling.
More precisely, quantum tunneling is the ability of a particle to pass through an energy barrier that classical physics says it could not cross. Its wavefunction does not stop dead at the barrier; it decays inside it, and if the barrier is thin enough the wave emerges on the far side with a small but genuine amplitude — meaning a small but genuine probability of the particle being found there. The thinner and lower the barrier, and the lighter the particle, the more likely tunneling becomes.
The honest significance is that tunneling is not a rare curiosity — it is essential to how the world works. It lets protons and electrons slip through barriers in enzymes and in some chemical reactions, speeding them up far beyond what classical theory predicts, especially at low temperatures. It is how the Sun fuses hydrogen, how radioactive nuclei decay, and how the scanning tunneling microscope images single atoms.
In some enzymes a proton must cross an energy barrier to move from one atom to another. Being light, it tunnels straight through the barrier rather than climbing over it, so the reaction runs far faster than its activation energy alone would allow — and it keeps running even at low temperatures where classical hopping would nearly stop.
Too light to climb the barrier, the proton simply passes through it.
Tunneling falls off extremely fast as the barrier gets wider or the particle gets heavier. This is why electrons and protons tunnel readily, whole atoms much less, and everyday objects never — a thrown ball's chance of tunneling through a wall is unimaginably small.