Critics of the Copenhagen interpretation

Albert Einstein found it very difficult to come to terms with the new ideas. In correspondence between Einstein and Bohr, which lasted for many years, we find a lively discussion on the principles of the Copenhagen interpretation, with Einstein asking clever questions, and Bohr supplying equally clever answers. It is there that we find Einstein's much quoted saying 'I cannot believe that God will throw dice' and Bohr's riposte 'Do not tell God what to do'.

Other physicists and philosophers expressed similar scepticism. Erwin Schrodinger, who himself had played a key role in the development of quantum mechanics, could not come to terms with the indeterminate nature of quantum reality. How could physical reality depend on whether or not this 'reality' had been observed? He devised what is perhaps the best-known example of the apparent absurdity of the theory — the paradox of the Schrodinger cat. This describes an imaginary situation in which a cat is placed in a sealed box. Its fate is determined by a small amount of radioactive substance, any atom of which may decay at any moment causing the release of a deadly gas which poisons the cat. Since the probability of radioactive decay is statistical we have no knowledge of the fate of the unfortunate cat, and hence the cat has a dual existence, one of which is observed and the other not observed. In the not-observed existence the cat is simultaneously alive and dead!

A physically more convincing example was presented in 1935 by Einstein together with Boris Podolski (1896-1966) and

* By its nature, antimatter is extremely difficult to contain and observe. Some more details of the experiment are given in Appendix 17.1.

Nathan Rosen (1909—1995), which demonstrated that, according to the Copenhagen interpretation, a measurement of the angular momentum of an electron could have an immediate effect on another electron located at an arbitrary distance somewhere else in the universe — the EPR paradox. If quantum mechanics was a correct representation of the laws of Nature, then these laws had the amazing quality of being non-local, in that observation made in one place, instantaneously changed things far away. Einstein in particular was unhappy with this notion, since according to his theory of relativity no effect could ever be transmitted with a speed greater than the speed of light!

All of the above paradoxes could be resolved by postulating the existence of hidden variables'. These could be described as 'hidden gears and wheels', which determine the results of a physical measurement but may not be susceptible to our techniques of detection. Hidden variables maintain the classical point of view that the basic laws are indeed deterministic, and that the apparent probabilistic behaviour of Nature is due to our lack of knowledge of these variables.

Albert Einstein never really came to terms with quantum theory. Courtesy of An Post, Irish Post Office.
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