Quantum and General Relativity

Albert Einstein, the founder of Relativity, and Niels Bohr, the principal founder of Quantum mechanics, had had a long debate on the foundation of quantum mechanics until their death. Relativity requires that no signal can travel faster than light thus causality should be always obeyed, while in Quantum mechanics causality can be violated through collapse of the de Broglie wave caused by measurements and thus events can only be predicted with probabilities. This fundamental inconsistency between Relativity and Quantum mechanics, which continues to be the biggest mystery in theoretical physics today, was the cause for the debate between Einstein and Bohr. Einstein strongly believed causality, while Bohr advocated the "complementarity" which implies that the probabilistic nature of Quantum theory is an unavoidable result of the impossibility in separating the behavior of atomic objects from the interaction with the measuring instruments.

These pictures are a glimpse of the debate between Einstein and Bohr. The left picture shows Bohr walking with Einstein in Brussels during the Solvay Conference of 1927, which is the start of the debate. The right two pictures show a famous Gedankenexperiment constructed by Einstein to beat the uncertainty relation which is a fundamental principle of Quantum mechanics. The right top picture is a sketch made by Bohr in London, May 1944, to illustrate for R. V. Jones and F. C. Frank his account of the debate with Einstein on the uncertainty relation with the Gedankenexperiment. The right bottom picture is George Gamow's make-believe apparatus for Einstein's Gedankenexperiment.

The Gedankenexperiment illustrated above was made by Einstein at the sixth Solvay Conference in 1930 which both Bohr and Einstein attended. Einstein suggested a box filled with radiation with a clock fitted in one side. The clock is designed to open a shutter and allow one photon to escape. Weigh the box again some time later and the photon energy (through the famous relation E = m c² in Special Relativity) and its time of escape can both be measured with arbitrary accuracy. This seems to violate the uncertainty relation which demonstrates that energy and time cannot be simultaneously determined with arbitrarily high accuracies. Bohr is reported to have spent an unhappy evening, and Einstein a happy one, after this challenge by Einstein to the uncertainty relation. However Niels Bohr had the final triumph, for the next day he had the solution by cleverly using Einstein's General Relativity. The mass is measured by hanging a compensation weight under the box. This in turn imparts a momentum to the box and there is an error in measuring the position. Time, according to General Relativity, is not absolute depends on the position of the box in gravitational field. Thus the error in the position of the box translates into an error in measuring the time which just satisfies the uncertainty relation. (Imagine: if at the conference Einstein had not discovered General Relativity yet, how could Bohr recover from headache?)

(Click here for a review of the debate by Niels Bohr, including
a detail discussion of the above Gedankenexperiment. )