06/03/2024

Folha: The paradox of Einstein, Podolsky, Rosen

Reproduction of Marcelo Viana's column in Folha de S. Paulo.

In 1935, Albert Einstein , Boris Podolsky, and Nathan Rosen published an article in the scientific journal Physical Review entitled "Can the description of physical reality given by quantum mechanics be considered complete?".

The uncertainty principle, formulated eight years earlier by Werner Heisenberg, implies that the position and momentum (mass x velocity) of a subatomic particle can never be simultaneously and precisely determined: any experiment to measure one of these values disturbs the particle, making the value of the other imprecise.

The prevailing interpretation at the time, led by Niels Bohr, was that these quantities have no physical meaning in themselves: it is the experiment that, by measuring one of them, confers physical existence to it, to the detriment of the other.

Einstein, Podolsky, and Rosen rejected the idea that physical reality is determined by the observer, arguing that the problem lies in the fact that quantum mechanics does not cover all of reality: there are "hidden variables" not contemplated in the theory that would explain the behavior of nature in a more reasonable way.

To prove their thesis, they proposed the following thought experiment: Consider two particles, A and B, that interact for a brief moment and then move apart. They showed that by measuring the position of A it is possible to determine the position of B and, analogously, by measuring the momentum of A we can determine the momentum of B, without performing any experiment with B, even if the two particles are millions of light-years apart!

Since information cannot be transmitted instantaneously from A to B, as nothing moves faster than light, the position and momentum values at B cannot be the result of measurements at A; they must have a real-world existence.

Bohr responded in an article with the same title, published in the same year in the same journal. His arguments convinced experts that the argument of Einstein, Podolsky, and Rosen was not the death blow to quantum mechanics that the three authors intended to deliver. But he did not satisfactorily address the most important question for Einstein: how to explain that what we do with A here determines the reality of B elsewhere, in a "spooky action at a distance"?

To read the full text, visit the newspaper's website.