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Why don't atoms collapse?
The short answer (tap to open)
Old physics predicted that a circling electron would spiral into the nucleus in a hundred-billionth of a second. Quantum physics explains why it doesn't. An electron behaves like a wave, and squeezing a wave into a smaller space costs energy. The lowest rung is the best balance between the nucleus's pull and that squeezing cost.
What went wrong with the old picture?
In 1911 Ernest Rutherford showed that an atom has a tiny, heavy nucleus with electrons around it. The obvious picture was a little solar system, with electrons orbiting like planets.
But old physics had a firm rule: a charged particle moving in a curve gives off light, and loses energy as it does. An orbiting electron should radiate, sink and spiral into the nucleus. The calculation gives about a hundred-billionth of a second for a hydrogen atom. Every atom in the universe would have collapsed long ago. Yet here we are.
How did Bohr fix it, and why wasn't that enough?
In 1913 Niels Bohr simply declared that electrons can only sit on certain rungs, and that the bottom rung is stable. His rule matched hydrogen's colours perfectly. But it was a rule without a reason. Why should a bottom rung exist?
What's the real reason?
The answer came with the wave picture of 1924 to 1926, and it rests on one idea: squeezing costs energy.
An electron behaves like a wave (chapter 1, Part C). Confine a wave to a smaller space and it has to wiggle faster. For an electron, faster wiggling means more energy of motion. Physicists know this as Heisenberg's uncertainty principle: pin down where a particle is more tightly, and its motion becomes less certain and more energetic.
So the nucleus pulls the electron in, which lowers its energy, while squeezing it closer raises its energy. Too far out and the pull wins. Too close in and the squeezing cost wins. Somewhere in between sits the lowest total energy. That balance point is the bottom rung, about a twentieth of a billionth of a metre from the centre for hydrogen. There's nowhere lower to go.
Find the lowest total energy.
Too far out: the pull of the nucleus wins, so the total energy can still go lower by moving in.
Too far, too close and just right, in words
- Too far out: the pull of the nucleus wins, so the total energy can still go lower by moving in.
- Too close in: the squeezing cost wins, so the total energy rises.
- Just right: the pull and the squeezing cost balance, and the total is as low as it can go. That's the bottom rung.
Why doesn't solid matter collapse either?
One more rule finishes the job. In 1925 Wolfgang Pauli found that no two electrons can share exactly the same state. So electrons in bigger atoms can't all crowd onto the bottom rung. They stack up rung by rung, which gives every element its own chemistry and stops atoms squashing into each other. That's why tables are solid. In 1967 Freeman Dyson and Andrew Lenard proved mathematically that, thanks to Pauli's rule, ordinary matter is stable.
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Atoms are stable because of quantum rules, not in spite of them.
Squeezing an electron wave costs energy, so there's a lowest rung.