In quantum physics, which explains the subatomic world, probability rules fundamentally: it tells us that never, no matter how hard we try or how good our measurement instruments are, we will be able to know the value of a variable below a minimum level of uncertainty.
For Albert Einstein this was unacceptable . For him, nature should be well defined and that quantum theory spoke of intrinsic probabilities was a warning that it was not a complete theory, that there had to be some “hidden variables” so that if we knew them we could eliminate that uncertainty. So during the 1920s and 1930s Einstein set about laying mines on the quantum path , trying to show that it was the wrong path. But those mines were being deactivated – not without great effort – by another giant of physics, the Danish Niels Bohr.
But Einstein did not let up and in the May 15, 1935 issue of the journal Physical Review he signed, together with the physicists Boris Podolsky and Nathan Rosen, the article “Can the quantum mechanical description of reality be considered complete?”. In it they implied that quantum theory was incomplete and, therefore, could never be a description of reality: since then it has been known as ‘the EPR paradox’ .
The hammer blow was órdago. Wolfgang Pauli, one of the fathers of quantum theory, was furious: “Once again Einstein has spoken publicly about quantum theory…every time it happens it’s a catastrophe.” Erwin Schrödinger, another of the greats, wrote to Einstein: “You have grabbed dogmatic quantum mechanics by the throat.” The one who felt the blow the most was Niels Bohr . After reading the article, he returned home dejected. He abandoned all his projects to dedicate himself body and soul to answering Einstein’s article which, by the way, has become in its own right one of the most important in physics of the 20th century.
The proposal was brilliant and from it came a concept that today is the foundation of teleportation and quantum computers: entanglement . According to him, if two particles have been “bound” in some way in the past, they will be forever, regardless of how much time passes and how far away they are from each other. To understand the scope of the EPR paradox we must remember that quantum mechanics tells us that we cannot know the exact value of the properties of the particles if we do not measure them , and only when making the measurement does the system “collapse” and acquire a certain value .
Now, says Einstein: let’s take two entangled particles that we send to two very distant places. Now imagine that we want to measure the direction in which the particles rotate, whether clockwise or counterclockwise. Without making the measurement, quantum mechanics tells us that this property will be in a superposition state, a mix of “turn clockwise” and “turn left.”
But if we measure how one of them rotates, thanks to the fact that they have been intertwined, we will instantly know which is the direction of rotation of the other: just the opposite. There was the catch! How is it possible – Einstein reasoned – that we can know the direction of rotation of a particle without measuring it? Doesn’t that violate the sacrosanct quantum mechanical principle? Einstein argued that if this happened it was because the particles do have the defined properties. What happens is that quantum mechanics cannot calculate it because it is an incomplete theory.
Now, under Einstein’s approach there are two underlying assumptions that the great physicist considered obvious as true. The first is realism : objects have definite properties that hold whether we look at them or not (my car is going 100 km/h whether or not it has a speedometer). The second is locality : there is no way to influence who is far away unless we send a signal that it must travel, as required by special relativity, at a maximum speed equal to the speed of light.
Thus, if both assumptions are true, entanglement leads to a paradox and quantum theory has a huge conceptual flaw. Bohr thought about it a lot and finally came to a conclusion: there is no paradox because nature behaves as the EPR thought experiment claims . Saying that is a depth charge to our way of understanding the universe; what Bohr was saying is that the world is non-local and non-realistic.
That was the case until a freckled, red-haired Irish physicist named John Bell came into play in the 1960s, realizing that Einstein, Poldoslky, and Rosen had discovered not a paradox but something crucial to our understanding of the universe. Quantum theory is not incomplete, it is the assumptions of realism and locality that contradict the soul of quantum theory . Now, how to prove experimentally that he was right? To do this, he had to discover a mathematical formulation that would allow discerning between both situations. Thus was born his famous theorem containing certain inequalities known since then as Bell’s inequalities . With them, an experiment can be set up that allows us to decide between two situations: either we live in a classical world and Einstein’s hidden variables create the illusion of quantum entanglement, or entanglement is real, the subatomic world is as strange as it seems, and the non-locality is a basic feature of our world.
The first to design such an experiment were Abner Shimony and Mike Horne in Boston, John F. Clauser in New York, and Richard Holt at Harvard in 1969. The idea was to use entangled photons and measure their polarization. Clauser, who believed in the local realism of Einstein, made a bet with the Israeli Yakir Aharanov: two to one against quantum mechanics. The first result proved quantum mechanics right: the world is intrinsically non-local.
Except for Clauser and company, Bell’s paper went completely unnoticed by physicists. Very few were interested in the philosophical aspects of quantum theory; it was enough for them to know that it worked perfectly. In fact, in 1979 the best reviews on the subject were an unpublished CIA memo and a feature article in Oui magazine, Playboy ‘s response to the more erotically explicit Penthouse.
The one who hit the quantum table was a Frenchman who in 1971 had gone to Cameroon to help people not live in adverse conditions: Alain Aspect . Away from academic influence, he spent his free time studying one of the deepest and most complete books on quantum mechanics, that of Claude Cohen-Tannoudji, Bernard Diu and Franck Laloë. There, in the heart of Africa, he discovered the EPR paradox and read a peculiar article by an obscure physicist working at CERN, John Bell.
Back in France, he decided to elucidate once and for all who was right (and Aspect was convinced it was quantum mechanics), and to do this he designed three experiments. The end result was that quantum theory totally defeated hidden variables . But the last nail had to be put in the grave of realism. And, among others, that is what Johannes Handsteiner and Anton Zeilinger of the University of Vienna did in 2017: they used the light of two different stars, each observed by a telescope and both separated by 2 km, to test the reality of the interlacing. And so it happened: Einstein had lost.
Aspect A (1999). “Bell’s inequality test: more ideal than ever” , Nature 398 (6724): 189–90
Bell, J.S. (1964). “On the Einstein Podolsky Rosen Paradox” , Physics Physique Физика. 1(3): 195–200
d’Espagnat, B. (2003) Veiled Reality. An Analysis Of Present-Day Quantum Mechanical Concepts . Westview Press
Einstein, A; B. Podolsky; N Rosen (1935-05-15). “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” , PhysicalReview. 47(10): 777–780
Kaiser, D. (2011) How the hippies saved physics , Norton & Co.
Kumar, M. (2009) Quantum , Icon Books