In response to quantum mechanics, so the story goes, Einstein proposed a machine, that, based on the uncertainty principle, was a perpetual motion. This showed that quantum mechanics was at odds with evidence that energy is conserved. Bohr later showed that the analysis was flawed; Einstein had failed to take gravity into account (ironically).

Is this story true? If so, what was the machine Einstein proposed, how was it supposed to work, and what did Bohr reveal about it?


2 Answers 2


Actually what Einstein proposed was not a perpetual motion machine but was an experiment design to take down the Uncertainty principle. So Einstein design an experiment known as "Einstein's box". It was a thought experiment. He said that consider a box (ideal one) lined up with mirrors so that it contains light indefinitely. Also there is a shutter (ideal one) in the box so that one can open and let go a photon. So he said that, first weighed the box before a clockwork mechanism and chose a proper instant and open the shutter to let go one photon and measure the time. (It was design to work like that, in a clockwork mechanism). Then he proposed to weighed the box again, the change in the mass will tell us the energy emitted and that's how one can determine with precision the time and amount of energy released simultaneously, contradicting the uncertainty principle.

So Bohr pointed out that Einstein reasoning was flawed due to his own theory of gravity, as gravity effects the time so there is an uncertainty about time due to the gravity of earth and similar other reasoning.

See here

  • $\begingroup$ Did Bohr address what happens if, instead of measuring the box's weight in the local gravitational field, we measure its mass with an accelerometer? No doubt that example introduces important uncertainties in variables of its own. $\endgroup$
    – J.G.
    Commented Apr 10, 2021 at 6:53

Can't find the source for the Einstein quote but it is in the introduction to a proof that he was wrong.

A violation of the uncertainty principle implies a violation of the second law of thermodynamics


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