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Einstein was wrong about quantum mechanics

Embracing spooky action at a distance

20th December 2023
| Professor of Philosophy at New York University and author of Philosophy of Physics
3,018 words
Read time: approx. 15 mins

In the first instalment of our series on the Foundations of Quantum Mechanics, philosopher of physics Tim Maudlin revisits Einstein’s famous disquiet with a prediction of quantum mechanics: the possibility of "spooky action at a distance".  As John Bell later proved, quantum mechanics is indeed a non-local theory, allowing for the outcomes of experiments separated by enormous distances to causally influence each other.

 

It seems to be a manifest fact about the physical world that in order for one action or event to have an effect on another there must be some continuous process—an exchange of particles, flow of electricity, flash of light, etc—that connects them. If such continuous processes are required and if those processes are in turn limited by the speed of light, then there would be a strict limitation on what can have a causal effect on what. Indeed, denying this sort of limitation on causation is what Albert Einstein famously referred to as “spooky action-at-a-distance” or “telepathy”, and he roundly rejected the possibility. But quantum mechanics shows that, despite  Einstein's protestations, the physical world is non-local.

In 1935, Einstein, Nathan Rosen and Boris Podolsky published a paper which would—in a way they could not possibly anticipate—result in arguably the most astonishing result in the history of physics. The point of the argument was not to suggest any new and remarkable physical phenomenon. Indeed, the only observable phenomena mentioned in the paper were commonplace and intrinsically uninteresting sorts of correlations which can be trivially explained. The point that EPR (as the three authors are now referred to) were making is that although the correlations can be trivially explained in a pedestrian way, the standard understanding of quantum mechanics does not so explain them. Indeed, the fundamental principles of quantum mechanics as exposited by Neils Bohr and the Copenhagen school forbids explaining them in the obvious trivial way. EPR presented the phenomena not for their intrinsic interest but as a vivid illustration of some bizarre consequences of the Copenhagen approach.

The phenomena that EPR mention are perfect correlations predicted by quantum theory when particular experiments are carried out on separated pairs of particles. Following the standard rules of the quantum formalism, EPR write down a so-called entangled quantum state for the pair. One of the particles is sent off to Alice and the other to Bob, located in arbitrarily distant labs. Alice and Bob each carry out the procedure called a “momentum measurement” on their particle, and since the entangled state is a zero momentum state for the pair, the prediction is that whatever value Alice finds, Bob will obtain the opposite. In this sense, the outcomes of the experiments will be perfectly negatively correlated. Alice and Bob can accurately predict the outcome of the other’s experiment once they have carried out their own.

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Tim Maudlin
20th December 2023

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The reason Tim thinks “spooky actions at a distance” are unavoidable is that he believes the correlated outcomes violating Bell’s inequality must be explained causally. He writes:

“A world that displays these phenomena must be a non-local world—must contain causal relations between arbitrarily distant events that cannot be connected even by light—, because the phenomena violate Bell’s Inequality.”

But there is another way to explain reality that is not causal called “principle explanation.” This is what Einstein did to solve a great mystery at the end of the 19th century with his theory of special relativity. Quantum information theorists in the quantum reconstruction program of the second quantum revolution have rendered quantum mechanics (QM) a “principle theory” (Einstein’s term) like special relativity and thereby solved the mystery of entanglement via principle explanation. Accordingly, their explanation does not entail “spooky actions at a distance” and it conforms beautifully with special relativity, contrary to Maudlin’s claim that entanglement “appears to be incompatible with Relativity.” In doing so, quantum information theorists showed that QM and special relativity follow from the fact that all perspectives on reality are equally valid (relativity principle or “no preferred reference frame"). Let me give a summary.

In 1996, Carlo Rovelli wrote:

“[Q]uantum mechanics will cease to look puzzling only when we will be able to *derive* the formalism of the theory from a set of simple physical assertions (‘postulates’, ‘principles’) about the world. Therefore, we should not try to *append* a reasonable interpretation to the quantum mechanics *formalism*, but rather to derive the formalism from a set of experimentally motivated postulates. … The reasons for exploring such a strategy are illuminated by an obvious historical precedent: special relativity. ... Special relativity is a well understood physical theory, appropriately credited to Einstein’s 1905 celebrated paper. The formal content of special relativity, however, is coded into the Lorentz transformations, written by Lorentz, not by Einstein, and before 1905. So, what was Einstein’s contribution? It was to understand the physical meaning of the Lorentz transformations.”

What Einstein did to produce a “physical meaning of the Lorentz transformations” was to bypass causal accounts of the light postulate, i.e., everyone measures the same value for the speed of light c, regardless of their relative motions. At that time, people were trying to explain the light postulate causally using length contraction via the luminiferous aether. Einstein gave up such “constructive” attempts, writing:

“By and by I despaired of the possibility of discovering the true laws by means of constructive efforts based on known facts. The longer and the more despairingly I tried, the more I came to the conviction that only the discovery of a universal formal principle could lead us to assured results.”

So, instead of finding a causal mechanism for the observer-independence of c, Einstein said it had to follow from the relativity principle. That is, since c is a constant of Nature according to Maxwell's electromagnetism, the relativity principle says c must be the same in all inertial reference frames. And, since inertial reference frames are related by uniform relative motions (boosts), the relativity principle tells us the light postulate must obtain. So, special relativity is a “principle theory” (Einstein’s terminology) because its kinematics (Lorentz transformations) follows from an empirically discovered fact (the light postulate). And, importantly, the light postulate is justified by the relativity principle.

Given this “historical precedent” Rovelli suggested using principles of information theory to render QM a principle theory and in 2001, Lucien Hardy produced the first so-called reconstruction of QM via information-theoretic principles. The empirically discovered fact that gives us the Hilbert space kinematics of QM is Information Invariance & Continuity (wording from 2009 by Caslav Brukner and Anton Zeilinger). If you couch that physically, it means everyone measures the same value for Planck’s constant h, regardless of their relative spatial orientations or locations. Let me call that the “Planck postulate” in analogy with the light postulate. Since h is a constant of Nature per Planck’s radiation law, just like c is a constant of Nature per Maxwell’s equations, and since inertial reference frames are related by spatial rotations and translations as well as boosts, the relativity principle says the Planck postulate must be true just like it says the light postulate must be true.

This means quantum information theorists have rendered QM a principle theory, just like special relativity, exactly per Rovelli’s 1996 challenge. Accordingly, quantum superposition and entanglement are not dynamical effects resulting from some nonlocal or superdeterministic or retro causal mechanism. They are kinematic facts that result from the observer-independence of Planck's constant h as required by the relativity principle and Planck's radiation law. This is in total analogy with the fact that length contraction and time dilation are not dynamical effects resulting from some causal mechanism like the luminiferous aether. They are kinematic facts that result from the observer-independence of the speed of light c as required by the relativity principle and Maxwell's equations.

Thus, QM and special relativity tell us that:

“Reality is the collection of all of our different dynamical subjective experiences, from all of our different perspectives, unified self-consistently such that no one’s perspective is privileged (“no preferred reference frame” NPRF). Each person can then explain their past and predict their future dynamical experiences as provided by principle explanation based on NPRF + c and NPRF + h.”

Accordingly, it’s not the case that all phenomena are most fundamentally explicable via causal mechanisms. Events, data, observations, etc. are distributed in spacetime according to the equivalence of all perspectives, and that means sometimes our dynamical experience of those events, data, observations, etc., is simply not amenable to dynamical or causal explanation.

For details, see our book, "Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit" (Oxford UP, 2024) or our open access paper, “Unifying Unifying Special Relativity and Quantum Mechanics via Adynamical Global Constraints,” J. Phys.: Conf. Ser. 2948, 012009 (2025).