Nobel Prize in Physics 2022
Commented by Ana Nunes, leader of the Biophysics and Nanosystems Research Group
Ana Nunes | Photo by Ciências ULisboa
THE NOBEL PRIZE IN PHYSICS 2022 AND THE STRANGE WORLD OF QUANTUM MECHANICS
The greatest achievement of Physics over the past one hundred years is the development of quantum theory. Yielding an impressive range of predictions that were always found in excellent quantitative agreement with experiment, no other theory has been so fully tested. Yet, twentieth century physicists never loved their most successful theory, and its foundations have been the subject of controversy and unprecedented scrutiny for many decades.
This happened because quantum theory challenges some of the basic tenets of any worldview inspired by our everyday experience. Perhaps the best indicator of that mismatch was the coining of the term wave-particle duality to make more palatable the unintuitive behaviour of quantum objects, which indeed may exhibit wave-like or particle-like properties. This notion establishes a connection with concepts every physics student is familiar with, but at the same time it is telling us that none of the players of classical physics is suitable to describe quantum reality.
The quantum skier, a popular cartoon illustration of wave particle duality | from Surrey Physics Blog
Apart from particles and waves, another casualty of quantum theory is local realism, or the notion that a physical universe exists objectively (i.e. whether or not we can observe or measure it), in which changes are brought about through local interactions. And this was thought of as being much too much to take in by many prominent twentieth century physicists, among them Einstein, perhaps the most influential physicist of his time. Einstein was the first to realize that simultaneity, a seemingly unchallengeable notion of everyday experience, is unphysical, but he was unable to accept that the same could happen with local realism. For these physicists, quantum mechanics had to be seen as an incomplete theory, a blurred image of a full theory formulated in terms of as yet unknown hidden variables. This theory would be compatible with the worldview of classical physics while reproducing all the results of quantum mechanics.
In the absence of concrete predictions to test the alternative, this debate between worldviews went on for decades. The breakthrough is due to John Bell (1928-1990), and it came in the form of a very deep thought experiment, devised to distinguish between the two. Bell’s theorem applies to entangles states, states of composed systems that entail perfect correlations for the measured values of certain properties on each subsystem. A popular classical analogue for this is a pair of gloves. If I send each glove of the same pair in closed boxes to a different city, if in Paris a right glove is reported, in London a left glove will be observed: the chirality of the gloves is perfectly (anti-) correlated. If the pairs of gloves come from a batch of black pairs and white pairs, the observations made of the colour of the gloves upon opening the boxes in Paris and in London will also be perfectly correlated. For quantum gloves, if chirality and colour correspond to complementary physical quantities then it is impossible to measure both at the same time. We must choose either to touch the glove eyes closed, and determine its chirality, or to peek into the box and find out its colour. According to the classical worldview, and common sense, this does not preclude that each box contains at all times a glove with a well-defined colour and chirality, nor can the correlations mean that the observations done in Paris influence the results obtained in London, or vice-versa.
Bell’s theorem proves that for entangled states these combined assumptions lead to quantitative predictions that are different from the predictions of quantum mechanics. The results of many observations of complementary quantities made on each subsystem are more strongly correlated when computed according to quantum mechanics than allowed by any hidden variables theory.
Bell´s theorem (1964) turned philosophy into physics but performing Bell’s test in a laboratory remained a formidable challenge. The first realization of Bell’s test was done in 1967 by one of this year’s Nobel laureates, John Clauser, with results that pointed towards the quantum worldview, but there were still loopholes in the setup that allowed some hope for local realism.
In the past 40 years, more sophisticated experiments due the other two 2022 Nobel laureates, Alain Aspect and Anton Zeilinger, closed these loopholes, proving beyond reasonable doubt that nonlocality is present in many natural systems, and that it is not a microscopic phenomenon in the sense that it manifests itself over distances as large as several kilometres. The scale and complexity of the systems involved in recent experiments of this type has also increased, showing that quantum rules apply to macroscopic objects too.
The strange world of quantum mechanics is the world we live in, it is our world!