Interpreting the Quantum World

The Clock in the Box

Bohr presented his ideas at the historic Solvay conference in 1927. Schrödinger and especially Einstein disliked his ideas and in particular, the Heisenberg uncertainty principle. Einstein really wanted a theory which could describe the objects themselves and not probabilities – he would not believe that 'God plays dice with the world.'

He tried to develop thought experiments to destroy the uncertainty principle over the coming years. But Bohr always won. A good example is Einstein's 'Box of Light.' Einstein described a box full of light and said that it was possible to measure both the energy 'E' of a single photon and the time 't' when it was emitted. This was not allowed by a variant on Heisenberg’s uncertainty principle which stated:

Einstein said that the box could be weighed at first and then a single photon be allowed to escape through a shutter controlled by a clock inside the box. The box would then be weighed again and the mass difference 'm' determined. The energy of the photon 'E' is simply
E = m c². It appeared that both the photon's energy and its time of emission could be determined! Bohr finally realized that there was a flaw in Einstein's reasoning. When the photon is released, the box will recoil (to conserve momentum) and the position of the box in the earth's gravitational field will be uncertain. Einstein's very own general theory of relativity said that this would cause a corresponding uncertainty in the time recorded. Bohr had been saved by Einstein forgetting his own theory!

This was the last serious assault – approximately 28 years after its inception at the hands of Planck, the foundations of quantum mechanics were complete.

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