Graham writes …In October’s post, John discussed the winner of the Nobel Prize of Physiology and Medicine, and this month I’d like to say something about the work of the winners of the Physics Prize, and the extraordinary things it says about the nature of reality. There were three winners, Alain Aspect, John Clauser and Anton Zeilinger, and broadly speaking they earned the prize for their work on the topic of quantum entanglement. So what is quantum entanglement, and why is it important? I’d like to say something about this concept, hopefully in language that is accessible to non-physicists As you may know, prior to the 1900s, the laws devised by Isaac Newton reigned supreme. It is also fair to say that for many engineering and science applications today, Newton’s theory still works. This classical theory is elegant, compact and powerful, and is still part of the education of a young science student today. One of the main aspects of Newton’s physics is what it says about the nature of reality. Put simply, if you tell me how the world is now, then the theory will tell you precisely how the world will be tomorrow (or indeed yesterday). In other words, if the positions and velocity of all the particles in the Universe were known at a particular time, then in principle Newton will be able to determine the state of all the particles at another time. This total determinism is a defining facet of Newtonian physics. However, in the early years of the 20th Century, the comforting edifice of classical physics collapsed under the onslaught of a new theory. Physicists investigating the world of the very small – the realm of molecules, atoms and elementary particles – found that Newton’s laws no longer worked. A huge developmental effort on the part of scientists, such as Einstein, Planck, Bohr, Heisenberg, Schrödinger and others, ultimately led to an understanding of the micro-world through the elaboration of a new theory called quantum mechanics (QM). However, it was soon realised that the total determinism of classical physics was lost. The nature of reality had changed dramatically. In the new theoretical regime, if you tell me how the world is now, QM will tell you the probability that the world is in this state or in that. Einstein was one of the principal founders of the theory of QM, but it is well known that over time he came to reject it as a complete description of the Universe. Much has been made of Einstein’s resistance to QM, summed up by his memorable quote that “God does not play dice with the world”. However, Einstein could not deny that QM probabilities provided a spectacularly accurate prediction of what was going on in the microworld. Instead, he believed that QM was a provisional theory that would ultimately be replaced by a deeper understanding, and the new theory would eliminate the probabilistic attributes. He could not come to terms with the idea that probabilities defined the Universe, and felt there must be an underlying reality that QM did not describe. He believed that this deeper understanding would emerge from a new theory involving what has become known as ‘hidden variables’. On a personal note, I have to say I have great sympathy with Einstein’s view. As an undergraduate, with a very immature appreciation of QM, I too could never get to grips with it from the point of view of its interpretation of how the Universe works. This is one of the reasons why I studied general relativity – Einstein’s gravity theory – at doctorate level, which is inherently a classical theory. Getting back to the discussion, Einstein strived to find his ultimate theory until the end of his life. Along the way, he was always attempting to find contradictions and weaknesses in QM. If he believed that he had found something, he would throw out a challenge to his circle of eminent ‘QM believers’. This stimulating discourse continued for many years. Then in 1935, with the publication of a paper with coauthors Podolsky and Rosen, Einstein believed he had found the ultimate weakness in QM in a property referred to as quantum entanglement (QE). This publication became known as the ‘EPR paper’. In broad terms, QE can be summarised along the lines of – if two objects interact and then separate, a subsequent measurement of one of them revealing some attribute would have an instantaneous influence on the other object regardless of their distance apart. You might ask, why does this have such a profound impact on our understanding of reality? To grasp this, we need to discuss in a little more detail what this means when we consider quantum objects, like subatomic particles, and their quantum qualities, such a quantum spin. We have discussed the enigma of quantum spin before (see the March 2022 blog post). If we measure the quantum spin of a particle about a particular axis, then the result always reveals that it is spinning either anti-clockwise or clockwise (as seen from above), with the same magnitude. The former state is referred to ‘spin up’ and the latter ‘spin down’. There are just two outcomes, and this is a consequence of the quantised nature of the particle’s angular momentum (or rotation). As I have said before – nobody said quantum spin was an intuitive concept! It is possible to produce two particles in an interaction in the laboratory such that they zoom off in opposite directions, one in a spin up state and the other in a spin down state (for example). In the process of their interaction the two particles have become entangled, and we can measure their spin in detectors placed at each end of the laboratory. Another part of this story is understanding the nature of measurement in QM. In the example we have chosen above, the conventional interpretation of QM says that the particle’s spin state is only revealed when a measurement takes place. Prior to this moment the particle is regarded as being in a state in which it is neither spin up nor spin down, but in a fuzzy state of being both. The probability of one or other state is defined by something called the wave function, and a collapse of the wave function occurs the moment a measurement is made, to reveal the actual spin state of the particle. This process is something of a mystery, and is still not fully understood. However, that is another story. For interested readers, please Google ‘the collapse of the wave function’ for more detail. So, in our discussion, we have two ways of interpreting our experiment. That of QM which says that the spin state of the particle is only revealed when a measurement is made, and that of Einstein who believed in an underlying reality in which the spin state has a definite value throughout. If you think about the two entangled particles created in the lab, discussed above, then QE only presents us with an issue if QM is correct and Einstein is wrong. In this case, the measurement of the spin of one particle reveals its value (up or down), and an instantaneous causal influence will reveal the state of the other (the opposite value), even if the two particles are light years apart. Einstein called this "strange spooky action at a distance”, and it troubled him deeply, particularly as both his theories of relativity forbid instantaneous propagation of any physical influence. QM could not, in his view, give a full final picture of reality. For years, nobody paid much attention to the EPR paper, mostly because QM worked. The theory was successful in explaining physics experiments and in technology developments. Since no one could think of a way of testing Einstein’s speculation that one day QM would be replaced by a new theory that eliminated probability, the EPR paper was regarded merely as an interesting philosophical diversion. Einstein died in 1955, and the debate about QE seemed to die with him. However, in 1964 an Irish physicist called John Stuart Bell proved mathematically that there was a way to test Einstein’s view that particles always have definite features, and that there is no spooky connection. Bell’s simple and remarkable incite was that doable experiments could be devised that would determine which of the two views is correct. Put another way, Bell's theorem asserts that if certain predictions of QM are correct then our world is non-local. Physicists refer to this ‘non-locality’ as meaning that there exist interactions between events that are too far apart in space and too close together in time for the events to be connected even by signals moving at the speed of light. Bell’s theorem has been in recent decades the subject of extensive analysis, discussion, and development by both physicists and philosophers of science. The relevant predictions of QM were first convincingly confirmed by the experiment of Alain Aspect (one our Nobel Prize winners) et al. in 1982, and they have been even more convincingly reconfirmed many times since. In light of these findings, the experiments thus establish that our world is non-local. I emphasise once again that this conclusion is very surprising, given that it violates the theories of relativity, as mentioned above. In summary then, this year’s Nobel Prize for Physics has been awarded to Alain Aspect, John Clauser and Anton Zeilinger, whose collective works have used Bell’s theorem to establish to most people’s satisfaction (1) that Einstein’s conventional view of reality is ruled out (2) that quantum entanglement is real and (3) that quantum mechanics and quantum entanglement can be used to develop new technologies (such as quantum computing and quantum teleportation).
Usually, the Nobel Physics Prize is awarded to scientists whose work makes sense of Nature. This year’s laureates reveal that the Universe is even stranger than we thought, and in addition they achieved the rarest of things – they proved Einstein wrong! Graham SwinerdSouthampton, UK November 2022
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