Some middle-aged men have trains in their attics. Niels Bohr had Werner Heisenberg. In the winter of 1926-1927, the brilliant young German was working as Bohr's chief assistant, billeted in a garret at the top of the great Dane's Copenhagen institute. After a day's work, Bohr would come up to Heisenberg's eyrie to chew the quantum fat. They often sat up late into the night, in intense debate over the meaning of the revolutionary quantum theory, then its infancy.
One puzzle they pondered were the trails of droplets left by electrons as they passed through cloud chambers, an apparatus used to track the movements of charged particles. When Heisenberg tried calculating these seemingly precise trajectories using the equations of quantum mechanics, he failed.
One evening in mid-February, Bohr had left town on a skiing trip, and Heisenberg had slipped out to catch some night air in the broad avenues of Faelled Park, behind the institute. As he walked, it came to him. The electron's track was not precise at all: if you looked closely, it consisted of a series of fuzzy dots. That revealed something fundamental about quantum theory. Back in his attic, Heisenberg excitedly wrote his idea down in a letter to fellow physicist Wolfgang Pauli. The gist of it appeared in a paper a few weeks later: "The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa."
Thus Heisenberg's notorious uncertainty principle was born. A statement of the fundamental unknowability of the quantum world, it has stood firm for the best part of a century. But for how much longer? Rumblings are abroad that a second quantum principle - entanglement - could sound the death knell for uncertainty. Is it goodbye Heisenberg, hello quantum certainty?
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