Can Einstein’s spooky action at a distance be explained?A new book attempts to do just this, but does it succeed?
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This year has been designated The International Year of Quantum Science and Technology by the United Nations as it marks the centenary of the development of quantum mechanics (physicsworld.com/a/international-year-of-quantum-science-and-technology-2025-heres-all-you-need-to-know). That designation signifies a kind of ‘coming of age’ for a theory that is typically portrayed not just as highly abstract but also as fundamentally counter-intuitive. Nevertheless, it is empirically successful to an astonishing degree, explaining the behaviour not just of atoms, elementary particles and electro-magnetic radiationbut also of macroscopic phenomena such as superconductivity. What one takes to be the defining characteristic of the theory, however, varies over time and between commentators. Certainly, it was founded on the idea that the properties, such as energy, momentum and so on, of an electron, or a light photon, say, can only have discrete values – they are said to be quantised, where the ‘size’ of such quanta is determined by a constant, h, known as Planck’s constant (h plays a prominent role in Einstein's Entanglement: Bell Inequalities, Relativity and the Qubit). However, some have highlighted the consequence that certain of these properties are related in a peculiar way as expressed by Heisenberg’s Uncertainty (or Indeterminacy) Principle, which states that there is a fundamental limit – again, given by h – on the product of the accuracy of measurements involving certain properties, such as position and momentum. As a result, for example, complete accuracy when it comes to the measurement of the position of an electron, for example, precludes any knowledge of its momentum. Such properties are referred to as ‘complementary’ and this term has also been taken to embrace the famous, or infamous, ‘wave-particle duality’ associated with the theory, where these are regarded as ‘complementary’ aspects of reality. Light, for example, although classically conceived of in terms of waves, also exhibits particle-like behaviour in the form of photons, as in the photoelectric effect, whereby light above a particular frequency (and hence energy) can release electrons from certain metals (and for which Einstein was eventually awarded the Nobel Prize). Electrons, contrariwise, although classically regarded as particles, may also create interference fringes, characteristic of wave phenomena, when they are fired one by one through two slits which are placed side by side. However, the characteristic trait of quantum mechanics according to Erwin Schrödinger (one of the architects of the theory, whose personal reputation has been justifiably besmirched by revelations that he was a serial seΧual abuser), is entanglement, the central topic of Stuckey, Silberstein and McDevitt’s book. This occurs whenever two or more particles interact and as a result, according to the theory, we can only ascribe definite properties to the collective system as a whole and not to the previously distinct components. Significantly, if we were to then undertake certain measurements on these components – two electrons, say – after they have separated, post-interaction, we will find that the results of such measurements are correlated in a way that cannot be understood in terms of classical physics. The states of the particles are said to be ‘entangled’ and it is this that underpins the burgeoning interest in ‘quantum science and technology’, as it forms the basis of quantum computation, encryption and teleportation (three of the early experimentalists in this area won the 2022 Nobel Prize in physics: ‘‘Spooky’ quantum-entanglement experiments win physics Nobel’, Nature, vol. 609, 4th October 2022). Nevertheless, these quantum correlations remain mysterious as measuring the property of one particle in such an entangled pair immediately affects the outcome of a measurement on the other. Dissipating this air of mystery by explaining quantum entanglement is the aim of Stuckey, Silberstein and McDevitt (a physicist, philosopher and mathematician respectively) in their latest co-authored effort (their previous publications together include Beyond the Dynamical Universe: Unifying Block Universe Physics and Time as Experienced, Oxford University Press, 2018, which sets out some of the same themes). And as they note, it was Einstein who first put his finger on this peculiar feature of the quantum world. Despite the growing success of the new theory, following its initial development by Werner Heisenberg in 1925 (hence the centenary celebrations), Einstein had become increasingly disenchanted with it, eventually expressing his concerns in 1935 in what is now regarded as a classic co-authored paper (Einstein, A., Podolsky B., Rosen, N. (1935), ‘Can Quantum-Mechanical Description of Reality Be Considered Complete?’ Physical Review, 47, pp.777–780) – the EPR paper and its paradox. This presented a thought experiment, which embodies the above situation: two systems – two particles, say – initially interact and then go their separate ways. Einstein, Podolsky and Rosen noted that if we were to measure the momentum, say, of one particle, then the momentum of the second could, of course, be predicted, using the conservation of momentum. Likewise, if the position of the first is measured, then that of the other can also be predicted. If we assume that the measurements made on the first particle cannot instantaneously affect the results of the measurements of the second – in accordance with Einstein’s own theory of Special Relativity – then we can conclude that the second particle must possess definite values of position and momentum prior to being measured. Unfortunately, however, according to quantum mechanics these properties are complementary, as mentioned above, in the sense that a system cannot simultaneously possess definite values of both. Hence, Einstein, Podolsky and Rosen concluded, quantum mechanics cannot be the whole story and there must be a deeper account of what is going on. The EPR paper formed part of an on-going debate with Niels Bohr who responded to it by insisting that when it came to such a thought experiment due consideration should always be taken of the measurement context, in the sense that any experiment set up to measure position precluded the possibility of measuring momentum and vice-versa. These two quantities reveal complementary aspects of the world, which is the core message of quantum physics, and so there is no need to search for any ‘deeper’ alternative. Many physicists were content to take Bohr’s word for it and get on with just applying the theory and with the shift in the centre of theoretical physics from Europe to the USA where a more pragmatic attitude dominated, “Shut up and calculate!” became the order of the day. Nevertheless, Schrödinger, having done so much himself to lay the foundations of this new mechanics, also had serious doubts about the spin put on it by Bohr and his followers. He had expressed these in correspondence with Einstein and in the same year as the EPR piece appeared he published two further papers of his own. In one he introduced another thought experiment, now famously known as ‘Schrödinger’s Cat’. The aim was to undermine Bohr’s distinction between the microscopic world, where quantum mechanics applies, and the macroscopic setting of the measurements, by showing how quantum effects would, in fact, spread from the former to the latter. Despite its current iconic status, cropping up everywhere from an Ursula LeGuin short story (‘Schrödinger’s Cat’, which historian Robert Crease credits with bringing the idea into popular culture: "Ursula Le Guin: the pioneering author we should thank for popularizing Schrödinger's cat". Physics World, 27/05/2024.) to an episode of the sitcom The Big Bang Theory, it was largely ignored by physicists themselves for at least the next twenty years. Schrödinger’s other paper was even more profound, introducing as it does the above notion of ‘entanglement’, presented in terms of each of the two systems in the EPR set-up leaving a ‘trace’ on the other, resulting in what he called an ‘entanglement of our knowledge of the two bodies’ (Schrödinger, E. (1935) Discussion of Probability Relations between Separated Systems. Proceedings of the Cambridge Philosophical Society, Mathematical and Physical Sciences, 31 (4), 555–563). However, it was the later work of John Stuart Bell, then a young physicist at CERN (Conseil Européen pour la Recherche Nucléaire) that threw the spotlight onto this notion. In a paper published in a comparatively obscure and short-lived physics journal, Bell returned to the EPR result and showed that any attempt to complete the quantum story, as Einstein hoped, would involve a certain mathematical constraint on how the outcomes of the measurements on the two particles in the EPR set up were correlated (Bell, J. S. (1964), ‘On the Einstein Podolsky Rosen Paradox’, Physics 1, pp. 195-200). This constraint became known as ‘Bell’s Inequality’ and Bell went on to show that quantum mechanics predicts correlations that violate it, a result that is now called ‘Bell’s Theorem’. Crucially, although alternative forms of the theorem can be constructed, involving different assumptions, later versions proved amenable to experiment and so far, all such tests have come down in favour of quantum mechanics (it was this that led to the award of the 2022 Nobel Prize mentioned above). As a result, what Einstein had dismissed as ‘spooky action at a distance’ and Schrödinger had elevated to the status of the characteristic trait of quantum mechanics, now entanglement lies at the heart of the new ‘quantum science and technology’.  'Two particles generated together at the same time are said to be 'entangled'. The state of one cannot be described independently of the state of the other. © NASA, used under its non-commercial copyright terms Nevertheless, as noted above, entanglement calls out for explanation and Stuckey, Silberstein and McDevitt’s book, Einstein's Entanglement: Bell Inequalities, Relativity and the Qubit, represents the latest in a long line of such efforts. Ironically, as the authors themselves acknowledge, they take their cue from the recalcitrant Einstein who, in 1905, famously cut through the various attempts to reconcile Newtonian mechanics and Maxwell’s electromagnetic theory by laying down two clear principles which together form the basis for the Special Theory of Relativity (Einstein, A. (1905), ‘Zur Elektrodynamik bewegter Körper’ (On the Electrodynamics of Moving Bodies), Annalen der Physik, 17, pp. 891-921). The Relativity Principle: the laws of physics are the same in all inertial reference frames; and, The Constancy of c: the speed of light in vacuum, c, is a constant for all observers. Using these principles, Einstein explained length contraction, time dilation and, of course, in a subsequent paper, derived that most famous of equations, E=mc². Essentially Stuckey, Silberstein and McDevitt (SSM) try to do the same with quantum mechanics by showing how it too can be ‘reconstructed’ from two straightforwardly graspable principles, thereby explaining entanglement. For their first principle, they introduce a generalisation of The Relativity Principle which they call, ‘No Preferred Reference Frame’ (NPRF). They then replace c, above, with h, Planck’s constant, as introduced earlier and argue that the combination of NPRF + h resolves Einstein’s concerns as expressed in the EPR paper. Furthermore, and crucially, they maintain that it also explains the fact that interacting quantum systems become entangled, just as the above two principles of Special Relativity explain the fact that moving clocks run slow (time dilation). Now, this is certainly a novel approach and one that is less radical metaphysically speaking than some of the alternatives they briefly consider and dismiss… But here’s the thing – and this will turn out to be the fly in this particular ointment – as Einstein emphasised, and SSM acknowledge, Einstein’s theory was actually misnamed because what it is really all about is what remains invariant between reference frames, namely the space-time interval, which combines space and time (and c). For their explanation to run along similar lines, SSM need something that plays the same role in quantum theory. Here they draw on quantum information theory, a major on-going research programme within which there exists a diverse range of attempts to reconstruct quantum mechanics using what is known as the ‘qubit’. This is the fundamental unit of quantum information that differs from the classical ‘bit’ in that whereas the state of the latter is binary (usually represented by 0 or 1), that of a qubit can be a superposition of possible states (being simultaneously both 0 and 1), thereby allowing it to hold more information (upon measurement, however, a definite outcome is always obtained). The total information embodied in such a qubit is invariant across all reference frames and with h representing a limit on how much information observers can simultaneously access, SSM show that they are able to obtain the sequences of measurement outcomes associated with entanglement. Now, back to that fly: the interval which remains invariant in Special Relativity only makes sense in the context of Hermann Minkowski’s 1908 reformulation of the theory in terms of four-dimensional space-time, (Minkowski space) which led him to famously announce that, ‘[h]enceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.’ (from ‘Space and Time’, in The Principle of Relativity (ed. by A. Sommerfeld), Eng[J1]. tr. W. Perrett and G. B. Jeffery, London 1923 and Dover, N.Y. n.d., pp.75–80). Einstein was initially resistant to this move but as historian and philosopher of physics John Norton has argued, it turned out to be a crucial step on the path to the more comprehensive – and even more radical – theory of General Relativity (Norton is cited several times by SSM and his website is a real treasure trove of items on relativity theory, much of it accessible to non-specialist readers; see for example: sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/spacetime). And subsequently, generations of physics students have honed their skills using spacetime diagrams in terms of which length contraction and time dilation are explained in ways that truly are ‘simple, beautiful and compelling’, as SSM repeatedly claim for their own account. They are, however, resistant to taking a similar step – making their explanation accessible to non-specialist readers – and it is no wonder why. If they did, SSM would have to regard their invariant quantity, based on the qubit, as an element of ‘an independent reality’, just like the spacetime interval. However, such a realist view of quantum information has long been comprehensively demolished (Timpson, C., Quantum Information Theory and the Foundations of Quantum Mechanics, Oxford University Press, 2013). The obvious alternative, then, is to take the qubit as reflecting our knowledge about a system and adopt the view attributed to Bohr as cited by SSM themselves: ‘There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.’ (p. 166). At this point, however, those working in the foundations of physics, philosophers and physicists alike, will no doubt cry out as one, “Been there, done that!” The history of the field is littered with such retreats from the attempt to understand quantum mechanics in terms of what it says about how the world is. And the problem with all of them is that the sense of explanation they provide – whether of entanglement or any other quantum phenomenon – just seems too thin. Now in fairness, SSM haver between taking the qubit to be an element of reality and regarding it as reflecting our knowledge but, like all such fence-sitting, that’s just not sustainable in the long-run: they must either grasp the former horn of the dilemma and tackle Timpson’s critique head-on, or adopt the alternative and accept that many will find their purported explanation insufficient. By this stage the average reader of SF² Concatenation (if there is a such a person!) may well be thinking that these are deep waters indeed. And they would not be wrong. Although the book opens with a presentation of the nature of quantum entanglement aimed at the layperson, couched in terms of gloves that are not only right- and left-handed but also of different colours, the bulk of it is peppered with terms – the ‘Bloch sphere’, ‘wave-function realism’, ‘superdeterminism’… – that require a pretty solid background in the foundations of physics. There is also a great deal of repetition, as when we are told time and time again that alternative accounts are not fit for purpose and that only the authors’ is up to the job, although such claims might carry more weight if the alternatives were given their due. As yet another exercise in the philosophy of quantum mechanics, albeit one that offers, as I said, a novel approach, the book will no doubt be subjected to the same kind of acute critical analysis in the academic literature as all the others. And I can envisage it being used as the focus for discussion in a postgraduate reading group, for example. However, as an attempt to offer an accessible explanation of a phenomenon that is becoming increasingly technologically significant, I’m afraid that I cannot recommend it to readers of this forum. Steven French
This review article builds on an earlier review by bioscientist Jonathan who felt he was out of his depth without a graduate-level background in physics.
Steven French is a member of the SF2Concatenation book review panel. He is also a historian and philosopher of science (now retired) with a background in physics. His latest book is A Phenomenological Approach to Quantum Mechanics: Cutting the Chain of Correlations, Oxford University Press, 2023. He is also co-editor, with Juha Saatsi, of Scientific Realism and the Quantum, Oxford University Press, 2020, which presents a series of essays by leading philosophers of physics on explanation in the context of different interpretations of quantum mechanics.
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