Two photons are created together and then separated — one sent to Vienna, the other to New York. The moment the photon in Vienna is measured, the result for the photon in New York is fixed. Not a millisecond later. Not at the speed of light. Simultaneously. Regardless of the distance. Albert Einstein called this "spooky action at a distance" in 1935 and considered it proof that quantum mechanics was incomplete. He was wrong. Over the past four decades, physics has experimentally proved that non-locality is real. In 2022 it was awarded the Nobel Prize.
What entanglement is
Entanglement arises when two particles — such as two photons or two electrons — are created in a single physical process and enter a shared quantum state. From that moment on, the two particles can no longer be described independently; they form a single quantum system — no matter how far apart they are subsequently separated.
The crucial point: as long as no one measures, the state of both particles is not "unknown" but genuinely not yet determined. The photon does not have a particular spin that we merely do not know. It has none. The state comes into existence only at the moment of measurement. And when one of the entangled particles is measured and yields spin "up", the spin of the other particle is "down" at that very same instant — whether the other particle is one metre or a billion light-years away.
Einstein, Podolsky, Rosen: the 1935 EPR paper
Albert Einstein was one of the founders of quantum mechanics — and at the same time one of its sharpest critics. He accepted the mathematical structure but did not believe it to be the complete description of reality. In 1935 he published, together with Boris Podolsky and Nathan Rosen, a paper entitled "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" in the Physical Review. It became one of the most cited papers in the history of physics.
The argument is elegant: if two entangled particles always yield correlated results upon measurement, there are only two possibilities:
- Either the particles carry their properties with them from the moment of creation — like two notes in two sealed envelopes, on which "up" and "down" were already written at the time of sealing. Then opening them merely reveals what was already inside. No spookiness, no action at a distance.
- Or the particles truly do not have their properties until someone measures — and the measurement of one instantaneously determines the state of the other, across any distance. That would be a violation of locality, which Einstein considered physically impossible.
Einstein was convinced the first possibility was correct. The particles must carry "hidden variables" — concealed properties that quantum mechanics simply does not describe. His conclusion: quantum mechanics is incomplete. It gives the right predictions, but it does not capture the whole of reality.
Einstein's relationship with the unexplained was more nuanced than the popular account suggests. In 1930 he wrote a foreword to Upton Sinclair's telepathy book Mental Radio and took the data of the Rhine laboratory seriously. But he looked for a physical explanation that would be classically conceivable — not one that would overturn the foundations of physics. With entanglement he took the same stance.
Thirty years of silence
The EPR paper triggered a brief debate — Niels Bohr responded in the same year — but then it went quiet. For thirty years the question "hidden variables or non-locality?" was considered philosophically interesting but experimentally undecidable. Quantum mechanics worked so well in practice that most physicists ignored the foundational question. "Shut up and calculate" became the unofficial motto of post-war physics.
John Bell: the theorem that changed everything (1964)
In 1964, Irish-born CERN physicist John Stewart Bell (1928–1990) published a short, mathematically precise paper entitled "On the Einstein-Podolsky-Rosen Paradox". It is one of the most important documents in twentieth-century physics.
Bell showed: if Einstein's hidden variables exist — if the particles have already determined their properties before measurement — then certain statistical correlations between the measurement results must obey a mathematical upper bound. This upper bound is known as the Bell inequality.
Quantum mechanics predicts that this upper bound is violated. The correlations between entangled particles are stronger than would be possible with pre-determined properties.
This is the decisive point: Bell turned a philosophical question into an experimentally testable criterion. Either nature obeys the Bell inequality — in which case Einstein was right and hidden variables exist. Or it violates it — in which case there are no hidden variables and non-locality is real.
Bell himself was — remarkably for a physicist of such precision — an admirer of David Bohm's pilot-wave theory. He said publicly on several occasions that his theorem would not have come about without Bohm's work.
Alain Aspect: the experiment (1982)
It took almost twenty years before the technology was mature enough to test Bell's theorem experimentally. In 1982, French physicist Alain Aspect at the University of Paris-Sud (Orsay) carried out the decisive series of experiments.
Aspect produced pairs of entangled photons and measured their polarisation at two spatially separated detectors. The innovation: Aspect switched the measurement direction at each detector while the photons were in flight — faster than a light signal could have travelled from one detector to the other. This ruled out the possibility that the particles were coordinating via a conventional signal.
The result was unambiguous: the Bell inequality was violated. The correlations were stronger than hidden variables permit. Quantum mechanics was right. Einstein was wrong.
The particles had not pre-determined their properties. The state came into existence only at the moment of measurement. And the result of the measurement on one particle determined the state of the other — instantaneously, across any distance.
Closing the loopholes: 2015
Aspect's 1982 experiment still left theoretical loopholes open — tiny logical possibilities that the result might yet be explained by local effects. In 2015 three independent teams in Delft (Ronald Hanson), Vienna (Anton Zeilinger) and Boulder (Lynden Shalm) conducted loophole-free Bell tests that closed all known loopholes simultaneously.
The result remained the same: the Bell inequality is violated. Non-locality is real. There are no hidden variables in Einstein's sense.
The 2022 Nobel Prize
On 4 October 2022 the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Alain Aspect (France), John F. Clauser (USA) and Anton Zeilinger (Austria) — "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science."
The Nobel Prize confirmed what physics had known since Aspect's experiment but had only proved beyond all loopholes in 2015: the non-locality of quantum mechanics is an experimentally established fact. Two entangled particles respond to each other instantaneously, regardless of distance, without any signal travelling between them.
John Bell, whose theorem was the foundation of all three laureates' work, died in 1990 and could not receive the prize. Many physicists consider this one of the great injustices of Nobel history.
Faster than light?
The correlation between entangled particles is not "very fast" — it is instantaneous. No signal travels from A to B. There is no delay. The simultaneity has been experimentally confirmed: when the photon in Vienna is measured, the state of the photon in New York is fixed at that same moment.
Nevertheless, entanglement cannot be used to send controllable messages faster than light. The reason: one cannot control which result the measurement yields. One randomly gets "up" or "down" — one can only determine after the fact that the result in New York was always the opposite of Vienna. To see this correlation, the two sides must compare their results — and that requires classical communication, which travels at most at the speed of light.
Physics has thus proved two things simultaneously:
- There are instantaneous, non-local connections in nature.
- These connections cannot be used to send controllable messages.
Einstein's theory of relativity stands as the limit for signal transmission. But the notion that distant parts of the universe are entirely independent of each other has been experimentally refuted.
What nobody knows: what non-locality is
Physics has proved that non-locality exists. It has not explained what it is. Competing interpretations remain:
- Copenhagen interpretation: There is no physical reality below the level of measurement. The question "what happens between measurements?" is meaningless.
- Bohm: There is a pilot wave that connects both particles instantaneously across any distance. Non-locality is real and explicit. Bohm's implicate order describes a deeper level at which separation is an illusion.
- Many worlds: At each measurement the universe branches. The correlation is a property of the branching structure, not action at a distance.
- Wigner/von Neumann: The consciousness of the observer plays a constitutive role in the measurement process.
None of these interpretations has been experimentally refuted. None has been experimentally confirmed. The mathematics of quantum mechanics works regardless of which interpretation one prefers. The question of what non-locality ontologically means — what it says about the structure of reality — remains one of the open foundational questions of physics.
Entanglement and consciousness research
The experimentally confirmed non-locality of quantum mechanics has consequences for the question whether non-local connections exist beyond particle physics:
- Wolfgang Pauli suspected in quantum non-locality a bridge to what he called "synchronicity" in his correspondence with C.G. Jung.
- Roger Penrose and Stuart Hameroff propose that quantum processes in neuronal microtubules play a role in consciousness.
- The PEAR lab and the Global Consciousness Project have documented statistically significant correlations between human consciousness and physical random number generators — correlations reminiscent of the non-local connections of entanglement.
- Rupert Sheldrake's morphic resonance postulates non-local connections between organisms of the same species — a concept that seemed unthinkable before the experimental confirmation of quantum entanglement.
None of these connections has been proved. Entanglement so far operates only at the level of individual particles, not at the level of brains or organisms. But entanglement has changed one thing: the notion that instantaneous, non-local connections are "physically impossible" has been officially untenable since 2022. They are not merely possible — they are a Nobel Prize-confirmed fact.
What this means for consciousness research is open. That it means nothing is unlikely.
Sources: Albert Einstein, Boris Podolsky & Nathan Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", Physical Review 47, 1935, pp. 777–780. John S. Bell, "On the Einstein Podolsky Rosen Paradox", Physics Physique Fizika 1 (3), 1964, pp. 195–200. Alain Aspect, Jean Dalibard & Gérard Roger, "Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities", Physical Review Letters 49 (2), 1982, pp. 91–94. B. Hensen et al., "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres", Nature 526, 2015, pp. 682–686. M. Giustina et al., "Significant-Loophole-Free Test of Bell's Theorem with Entangled Photons", Physical Review Letters 115, 2015, 250401. L.K. Shalm et al., "Strong Loophole-Free Test of Local Realism", Physical Review Letters 115, 2015, 250402. Nobel Prize Committee, "The Nobel Prize in Physics 2022", nobelprize.org. John S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge University Press 1987.