Press release: The 1973 Nobel Prize in Physics

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   A ball without enough energy never gets over the hill. A quantum particle, because it also behaves like a wave, has a small chance of turning up beyond a wall it could not jump: that is tunnelling, and the chance falls very fast with the wall's thickness, its height and the particle's mass.

   Press release: The 1973 Nobel Prize in Physics · 1973

   A quantum particle neither jumps the wall nor breaks it: its wave passes through weakened, and what is left on the far side is a chance.

   Picture a ball you roll towards a hill. If it is not fast enough, it climbs a little, stops and rolls back. Never, not even once in a million tries, does it turn up on the other side. That is how the big world works, the world of balls, cars and people. In the very small world of electrons and protons the rule changes. There, a particle that does not have enough energy to get over an obstacle still has a small chance of being found beyond it. Physicists call this tunnelling, although no tunnel is dug and the wall stays whole. Why does it happen? In the lessons on the double-slit experiment we saw that a particle also behaves like a wave, and the wave tells you where you are likely to find it. When the wave hits a wall it does not stop dead at the surface. It goes a little way into the wall and fades gradually, much as sound comes through your neighbour's wall, weaker. If the wall is thin enough, something of the wave is left on the far side. That remainder means a real chance that the particle is detected beyond. The sound analogy has a limit: the sound arrives quieter, whereas the particle that gets through arrives whole, with all its energy. Not a ‘piece’ of an electron passes, but the whole electron, only more rarely. The most important part is how fast the chance falls. It does not halve when you double the wall's thickness; it collapses. For an electron, each extra layer as thick as a small atom divides the chance by roughly ten. The height of the wall matters too and, very much, the mass. A proton, almost two thousand times heavier than an electron, gets through the same wall vastly more rarely. That is why tunnelling shows up with electrons, with atomic nuclei and in the cores of stars, but never with a ball or with a person who bumps into a wall.

   “These waves can penetrate a barrier that would be a forbidden area if the particle was considered in the classical way. The term tunneling phenomenon refers to this property – the particle “tunnels” through the forbidden area.”

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