(This note on Alain Aspect getting the 2022 Nobel prize was shared by Prof. Sivakumar Srinivasan of Krea University. I’m posting this note here with his permission. I’ve taken a bit of liberty to split into paragraphs for easier reading. No other changes to his note.)
Albert Einstein argued that the formalism of quantum theory is incomplete. For him, a probability-based approach to understanding nature is an incomplete prescription. He firmly believed that the need for a probabilistic description is a consequence of our inability to know or access an even more fundamental layer of information. Imagine tossing a coin. We speak of chances of getting head or tail. But given enough information on the force applied, gravity, atmospheric conditions, etc., it is possible to predict the outcome with certainty. However, the complexity of knowing and using so much of intricate details forces us to use the language of chance(probability).
In essence, ignorance (lack of information) necessitates using a probability-based description. Therefore, according to Einstein, there should be a more fundamental theory whose “averaged” version is the quantum theory we know. Such a proclamation may sound very convincing, requiring no further substantiation. However, David Bohm set out to construct a more fundamental theory that did not require probability. He could do that for a physical system such as an electron. This formalism is an example of what is known as local, realistic, hidden-variable theory. In this context, “local” means that there is no instantaneous messaging and “realistic” implies that the outcome of an ideal measurement reveals a preexisting value of the physical quantity measured. The phrase “hidden variables” (HV) refers to a set of parameters we cannot access. But these variables influence the values of the physical quantities so that the experimental results are consistent with the predictions of quantum theory.
The success of Bohm’s formulation encouraged the scientific community to attempt such formulations for other systems. But such attempts did not yield any success. Efforts to replace quantum theory with a more fundamental theoretical framework was losing steam. As luck would have it, John S Bell articulated a marvellous scheme for experimentally testing whether there is an underlying local-realistic-HV theory. His suggestion was to make suitable measurements whose possible outcomes are predicted differently by quantum theory and HV theory. But doing the experiment was still challenging as it required a copious supply of correlated particles moving in opposite directions and performing measurements on each pair.
Though such experiments had the possibility of proving the objections of Einstein and Bohm as unsustainable, the latter, in the true spirit of science, suggested a doable version of Bell’s suggestion. It is this experiment that Alain Aspect carried out using photons (“light particles” in a non-rigorous sense) that are correlated in their polarizations (direction of their electric field). The experiment demonstrated that there was no underlying local-realistic-HV theory to explain away the outcomes and the results were consistent with the predictions of the quantum theory.
It is indeed a towering achievement of Aspect, one among the three who are sharing this year’s Nobel Prize in Physics. This experiment vindicated the stand of Neils Bohr, Heisenberg, Schrodinger and Dirac that the quantum theoretical framework is not a coarser version of the finer theory that Einstein and Bohm posited. For the time being, we are to content ourselves with an inherently probabilistic approach to understanding nature. The story has three elements: Einstein’s philosophical position, Bell’s constructive suggestion for a phenomenological verification and Aspect’s experimental realization of Bell’s idea. The result of these different outlooks is an achievement that belongs to the Nobel league. We will have another story another time to discuss the contributions of Clauser and Zeilinger.