Clock in a box

This is one of the classic thought experiments Einstein designed in order to undermine the Copenhagen Interpretation of Quantum Mechanics. Most of these were designed in order to simultaneously measure a pair of Complementary variables, thus undermining the Uncertainty Principle upon which CHI is based.

The theory works like this: A clock is placed in a box full of light, connected to a shutter so it is pre-programmed to open at a known time and release a single photon of light. The box can be weighed before and after, and so the energy of the photon can be determined from the relation E=mc^2. We now know both the energy of the photon and the time it was emitted, which according to the Uncertainty Principle is impossible.

Niels Bohr, the principal architecht of CHI, demonstrated the flaw in this argument by examining how the experiment would have to be performed. The box needs to be weighed, so it must be suspended in a gravitational field. When the photon is emitted, the box will recoil, which not only causes an uncertainty in the weighing but also - according to Einstein's own general theory of relativity, which the master had conveniently forgotten - causes an uncertainty in the time reading. CHI was thus restored.

Einstein was so pissed off at this that he went away and dreamed up the EPR Paradox in response.