Niels Bohr

The teaching

Taught from his own papers and essays, above all the Como lecture of 1927 and Atomic Theory and the Description of Nature (1934).

Stationary states and quantum jumps

Bohr's atom of 1913 rests on two postulates that broke with the physics of his day. An electron bound in an atom can exist only in certain states, the stationary states, and while it stays in one of them it gives out no radiation, although the old physics said it must. Light is given out or taken in only when the electron passes from one stationary state to another, and the frequency of that light is the difference in energy between the two states divided by Planck's constant. From these two rules the lines of the hydrogen spectrum, measured long before, followed, and with them the constant that ruled their spacing.

To guide the new theory he gave the correspondence principle: where the quantum numbers are large, the predictions of quantum theory must pass over smoothly into those of classical physics, so that the old theory remains the limit of the new one.

The quantum postulate

At Como in 1927 he set out what quantum theory means. Every atomic process, he said, has an essential discontinuity, an individuality, symbolised by Planck's quantum of action. It follows that the interaction between an atomic object and the instruments that observe it cannot be neglected or reckoned away, so that no sharp line can be drawn between the behaviour of the object and the observation of it. The old ideal of a description that is at once a picture in space and time and a strict account of cause and effect must then be given up: the two become complementary, each available only at the cost of the other.

Complementarity

Complementarity, which is taught with his wider use of it in the branch The physicists who read the mystics of Physics and the mystics, is his name for this situation: wave and particle, position and momentum, space-time description and causal account are mutually exclusive and jointly necessary. What is seen depends on the whole experimental arrangement, and he asked that the word phenomenon be kept for an observation made under stated conditions, including an account of the whole arrangement. A question about the atom has an answer only together with a description of the apparatus that asks it.

That description, he insisted, must always be given in the plain language of everyday experience refined by classical physics. However strange the atomic world, the evidence about it is the click of a counter or the mark on a plate, and it must be told in words that others can check; unambiguous communication is the condition of objectivity.

The debate with Einstein

Einstein, who could not accept that nature at bottom is a matter of chance, met Bohr at the Solvay conferences with thought experiments meant to show the theory incomplete. The most famous, in 1930, was a box full of light with a shutter worked by a clock: let one photon out at a known time, weigh the box before and after, and both the time and the energy would be known exactly, against the uncertainty relations. Bohr answered with Einstein's own theory of gravitation. Weighing the box means letting it move in the field of gravity, and a clock that moves in that field changes its rate, so the uncertainty in the box's position brings back exactly the uncertainty in the time. His account of the debate ends in deep respect for the opponent whose challenges, he says, did most to clarify the theory.

An open world

After the war he carried the same honesty into politics. In his open letter to the United Nations of 1950 he argued that the weapons physics had made could be lived with only in an open world, where nations shared knowledge freely and hid nothing from one another, since secrecy breeds the fear that leads to an arms race. He held this to be the lesson of science itself, a community that advances because what it learns is open to all.