Nobel Prize in Physics: a prize from 1935

The story of the 2022 Nobel Prize in Physics, awarded to Alain Aspect, John Clauser and Anton Zelingerdates back to the year 1935. In that year, Albert Einstein, Boris Podolsky and Nathan Rosen published an article that challenged quantum mechanics, criticizing its claims to be a complete theory.

The authors of the article presented the following thought experiment (gedankenexperiment), which Einstein was so fond of. Let us imagine a particle at rest disintegrating into two that travel with opposite speeds, each one of them, in addition, endowed with an internal spin or spin. By the laws of conservation of angular momentum, the spin of each particle must also be opposite to that of the other. So when we measure the spin of one of the particles, we automatically know the spin of the other.

Nobel Prize

The standard interpretation of quantum mechanics holds that the properties of systems are not determined until a measurement is made. But then, when we measure the spin of one of the particles, we are also instantly determining the spin of the particle traveling in the opposite direction.

Is that reasonable considering that the particles could be an arbitrary distance apart? This possibility seemed to Einstein a phantasmagorical action at a distance (spooky action at a distance), which he considered to have definitively defeated with his theory of relativity.

The 1935 article proposed to get out of the quagmire through a fairly logical proposal. In reality, the spin of each particle would already be determined when each of them arose after the disintegration of the initial particle. We would not know their specific values ​​until we measure them because we do not know well the variables that, deep down, would be determining them.

For this reason, and this is the key point in the argument of Einstein and his colleagues, the measurement would not be creating something, but showing something that would already be determined in the two particles from the beginning. Quantum mechanics would be a provisional or incomplete theory because it would not be able to accurately predict the specific value of the spin of each particle, but only give a probability of the measurement that would be obtained.

Are we discussing the sex of angels?

Nothing of that. The story continues in 1964, when the Irish physicist John Bell presented a theoretical framework within which it was possible to discriminate between Einstein’s deterministic interpretation (which would be called “hidden variables”) and the indeterministic interpretation of quantum mechanics (a physical magnitude is not determined until a measurement is made).

Bell’s inequalities, relations between the relevant magnitudes of the problem, have to be satisfied if the system devised by Einstein’s side behaves in a deterministic way, a la Einstein, thanks to those hidden variables; and they do not hold (are violated) if the system is really indeterminate, as the standard interpretation of quantum mechanics advocates. This is where our award-winning Aspect, Clausey and Zelinger come into play.

Nobel Prize: a blow to superdeterminism

A few years later, in 1969, John Clauser, together with Michael Horne, Abner Shimony and Richard Holt, theoretically generalized Bell’s inequalities, rigorously showing the statistical differences between models of local hidden variables (limited to the dimensions of the experiment) and the predictions of quantum mechanics.

At the beginning of the 1980s, after some attempts in other laboratories and by Clauser himself, Alain Aspect succeeded in showing almost unequivocally that Bell’s inequalities were violated using entangled photons of the type proposed by Einstein: the possibility was excluded. that local hidden variables were determining a priori the value of the measured quantities (in this case the polarization of the photons).

quantum mechanics it won the game to the initial critic of Einstein and collaborators. Anton Zeilinger, for his part, is the experimental physicist who has most profitably studied the potentialities of entanglement proposed by Einstein to develop the new science of quantum information, which is at the base of encryption and quantum computing technologies.

But the interest of this Nobel Prize is not only practical. It offers us a key to reading natural processes in which we have not yet delved deeply enough. Either quantum indeterminism is real (not a simple matter of imperfection in our knowledge), or the only possible way out for determinists a la Einstein is that the hidden variables that determine the results are non-local, coming to conspire in the determination of the parameters of experiments that violate Bell’s inequalities to occur in a superdeterministic way.

hidden variables

It would be worth adding here, for the defenders of this desperately deterministic interpretation, the recent experiments by Zeilinger and colleagues showing that such non-local hidden variables could only be affecting our current experiments from over 7.8 billion years ago, carrying their purported influence. practically to the initial conditions of the universe in the Big Bang. A hard blow for superdeterminism, without a doubt, to which is added the media blow of the award of this Nobel Prize in Physics.

Font: Javier Sánchez Cañizares / THE CONVERSATION

Reference article: https://theconversation.com/nobel-de-fisica-un-prize-de-1935-191922