{"id":901678,"date":"2026-10-20T12:48:10","date_gmt":"2026-10-20T12:48:10","guid":{"rendered":"https:\/\/prometeoliberato.com\/two-but-not-separate\/"},"modified":"2026-10-20T12:57:31","modified_gmt":"2026-10-20T12:57:31","slug":"two-but-not-separate","status":"publish","type":"post","link":"https:\/\/prometeoliberato.com\/en\/two-but-not-separate\/","title":{"rendered":"Two, but not separate"},"content":{"rendered":"

Entanglement: the story of an experiment that forced physics to rethink distance<\/em><\/p>\n

\u201cEverything we call real is made of things that cannot be regarded as real.\u201d The quotation is attributed to Niels Bohr. Yet we know of no primary source that would allow us to establish its authenticity. This is a necessary philological precaution, especially since the great physicists of the twentieth century have become, against their will, an inexhaustible source of apocryphal aphorisms. And yet those words, whoever spoke them, lead us straight into one of the most extraordinary stories in contemporary science. Not because quantum mechanics has demonstrated that reality does not exist, but because it has unsettled some of our deepest assumptions about what reality must be like. Among them is one so familiar that it seldom occurs to us to question it: two things that are far apart are two separate things. It sounds obvious. And for most of our experience, it works perfectly well. We can leave one book in Milan and take another to Paris. The book left in Milan does not change its contents when we open the second. We can divide a pair of objects, send them to different places, and study them separately. If we know all the properties of each, we assume we also know everything about the pair. Quantum mechanics has forced us to discover that this last, seemingly innocent statement is not always true. The discovery is called entanglement. <\/p>\n

Il laboratorio delle domande impossibili<\/h2>\n

Imagine a laboratory in which a source produces two particles of light\u2014two photons\u2014prepared in a shared quantum state. The photons travel in opposite directions. Each reaches a device capable of measuring its polarization, a property of light that we can picture, with some approximation, as its orientation relative to a chosen direction. The instruments are not simply windows through which we observe something. They are devices that put a precise question to the photons. By changing the orientation of the measuring apparatus, we change the experimental question. Here is the crucial detail: each device can be set in different ways. If we repeat the experiment many times, the result recorded by either instrument alone appears unremarkable: a sequence of individual outcomes that cannot be predicted one by one. But when the two laboratories compare their records, a surprising pattern emerges. The results are correlated, and the strength of the correlation varies according to the settings chosen. So far, we might imagine that an agreement was made in advance. After all, if two objects come from the same source, we might reasonably expect them to retain some shared information. Two synchronized clocks can show the same time even after being taken to different continents. No mysterious exchange of messages is required. But quantum experiments raise a harder question: could a set of instructions prepared at the source account for all the correlations we observe, whatever settings the instruments use? Under specific physical assumptions, the answer is no. To understand why this matters, we must go back almost a century. <\/p>\n

Einstein and the right of things to exist independently<\/h2>\n

In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper whose title was already a declaration of intellectual war: Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Quantum mechanics had already demonstrated an exceptional power to predict experimental results. Einstein did not dispute that. But he believed that a theory capable of calculating outcomes correctly was not necessarily a complete description of what exists. His argument had considerable intuitive force. If two systems have interacted and then moved apart, the physical condition of the second should not depend on which experiment we decide to perform on the first, perhaps at a great distance. This demand for independence was connected to a principle Einstein considered fundamental: locality. Broadly speaking, what happens here should not produce an instantaneous physical influence over there. This was not a dispute about the presence of a human observer. Einstein was not asking whether the Moon vanished when nobody looked at it. He was defending the idea that physical reality has properties independent of our experimental choices. Bohr replied by challenging the way classical concepts such as property, measurement and physical reality were being applied to the quantum world. In his view, one could not disregard the experimental conditions that made a particular observation possible. Neither managed to persuade the other. Nor could their disagreement be reduced to a confrontation between someone who believed in reality and someone who did not. Rather, it concerned what we are entitled to say about a physical system when we are not measuring it. That same year, Erwin Schr\u00f6dinger identified a crucial aspect of the problem and introduced the German term Verschr\u00e4nkung, from which the English word entanglement derives: an interweaving or intertwining. When two quantum systems interact under certain conditions, he observed, their joint description can take a form in which assigning each system an independent state no longer allows us to reconstruct the whole. Entanglement, then, was not simply a matter of proximity. It was a property of the physical description of the combined system. For decades, however, the issue remained largely a conceptual challenge. <\/p>\n

John Bell\u2019s small intellectual miracle<\/h2>\n

John Stewart Bell was born in Belfast in 1928. A theoretical physicist who also worked at CERN, he dealt with very concrete problems in particle physics. Yet he had a passion for the foundations of quantum theory, an area many colleagues regarded as philosophically interesting but experimentally unpromising. In 1964 he published a paper only a few pages long: On the Einstein Podolsky Rosen Paradox. His achievement was to turn a disagreement about principles into a testable prediction. Bell asked what should happen if results obtained in two distant laboratories depended on information shared at the source, without causal influences arriving from the other laboratory during measurement. From these conditions, together with other explicit assumptions, he derived mathematical limits on the possible correlations. Quantum mechanics predicted that, for particular states and experimental settings, correlations could exceed those limits. It was a tremendous breakthrough. Until then, one could argue that two different interpretations merely described the same experimental results in different ways. Bell showed that at least one important family of alternative explanations would yield statistical predictions incompatible with those of quantum mechanics. Nature could now be questioned. But what answer would it give? <\/p>\n

Four questions and a limit that should not be crossed<\/h2>\n

Let us follow the reasoning without using an equation. Suppose the two laboratories, which we will call A and B, can each choose between two settings on their measuring device. A has settings A1 and A2; B has B1 and B2. Each measurement produces one of two possible outcomes, which we will label +1 and \u22121 for convenience. We repeat the experiment thousands of times, varying the settings and recording the results. For each of the four possible combinations\u2014A1 with B1, A1 with B2, A2 with B1, A2 with B2\u2014we can calculate how often the outcomes agree or disagree. We are not looking at a single pair of photons, then, but at four sets of statistics. Here lies the brilliance of the best-known experimental version of Bell\u2019s reasoning, formulated in 1969 by John Clauser, Michael Horne, Abner Shimony and Richard Holt. If the two instruments\u2019 responses depend on information shared in advance and satisfy the local-causality conditions assumed by the model, the four correlations cannot take arbitrary values. A particular combination of them, known as the CHSH parameter, cannot exceed 2 in absolute value. Quantum mechanics, by contrast, predicts that with suitable states and orientations the same combination can reach 2\u221a2, or approximately 2.83. The difference looks small. It is enormous. Not because 2.83 is a particularly mysterious number, but because it is an outcome forbidden by the class of local explanations under examination. The point is not that particles must somehow make decisions while being observed. It is that we cannot reconstruct all those statistics simply by imagining that each pair carried a set of predetermined local answers from the source. This is what the debate of 1935 lacked: a quantitative criterion capable of distinguishing different physical hypotheses. Of course, the argument depends on important conditions, including the appropriate independence of measurement choices from the variables describing the source. Modern experimental tests have been designed precisely to make these conditions increasingly stringent. We have not proved that every conceivable idea of hidden reality is impossible. We have discovered that nature does not obey a particular combination of assumptions that once seemed entirely reasonable. And that is already a revolution. <\/p>\n

A Nobel Prize born of persistent experiments<\/h2>\n

In 1972, John Clauser and Stuart Freedman carried out an experiment at Berkeley that demonstrated a violation of a Bell inequality, in agreement with quantum predictions. Important though it was, the result did not settle the question. Real experiments are material things: imperfect sources, detectors that fail to register every photon, instruments that must be adjusted and synchronized. Each imperfection can leave room for an alternative explanation. In the 1980s, Alain Aspect and his collaborators introduced major improvements, including configurations in which measurement settings changed while the photons were already in flight. The aim was to make it increasingly implausible that the results could be explained by local information about future settings. Anton Zeilinger subsequently contributed to further tests and to the development of techniques for preparing, manipulating and using entangled states. Meanwhile, other groups tackled the remaining experimental difficulties. In 2015, important tests were published that closed several of the principal loopholes in earlier experiments simultaneously, under the experimental conditions considered. The 2022 Nobel Prize in Physics, awarded to Aspect, Clauser and Zeilinger, recognized this long transformation: from a theoretical paradox into a body of experimentally verified phenomena that could be put to work in new technologies. The story deserves telling for another reason. Fundamental research is often judged by its immediate usefulness. Yet here an apparently abstract question about the completeness of quantum description helped, over decades, to give rise to an entire technological field. Scientific curiosity came before applications. Not the other way around. <\/p>\n

But does something travel instantaneously?<\/h2>\n

It is the inevitable question. If the photons are far apart and their measurements reveal correlations that cannot be explained by a local model of pre-existing instructions, must we conclude that one sends something to the other faster than light? No. At least, not in the ordinary sense of sending a signal. The person measuring the first photon cannot choose which result to obtain. And the person measuring the second, looking only at their own data, cannot discover which setting was chosen in the other laboratory. To observe the correlations, the two teams must compare their records. That comparison requires ordinary communication, subject to the limits imposed by relativity. We can debate whether, and in what sense, certain interpretations of the theory involve forms of nonlocality. What we cannot legitimately do is turn entanglement into a technology for sending instantaneous messages. The apparent contradiction often arises from confusing three different things: the existence of a correlation, the ability to influence an outcome, and the ability to use that influence to communicate. Quantum mechanics forces us to reconsider the first two within the framework of its different interpretations. But it does not give us the third. Einstein, then, may continue to worry about the theory\u2019s underlying structure, without his concerns licensing the construction of a telephone that could speak to Mars without delay. <\/p>\n

Entanglement becomes a resource<\/h2>\n

Once physicists learned to prepare and control entangled systems, they began to put them to use. In quantum teleportation, for example, an unknown quantum state can be transferred from one system to another through a protocol combining shared entanglement, appropriate measurements and classical communication. The name evokes science-fiction journeys, but no body is transported and the speed of light is not circumvented. A quantum state is reconstructed elsewhere, while the original state does not remain available as an independent copy. Some quantum cryptography protocols use quantum correlations to establish secret keys and, under stringent conditions, verify the security of communications. In quantum computers, entanglement plays an important role in manipulating systems made up of many qubits, although it is not sufficient on its own to guarantee that a quantum machine will outperform a classical computer. And in quantum networks, researchers are investigating how to distribute entanglement between different nodes, overcoming substantial practical difficulties. The most obvious is the fragility of controlled quantum states. The environment is no mere spectator. Interactions with a system\u2019s surroundings can disperse the information needed to observe and exploit quantum correlations. That is why experiments require precise apparatus, isolation, control and verification procedures. Entanglement is not an indestructible sentimental bond between two particles. It is a physical property that must be produced, identified and preserved. <\/p>\n

Il confine fra una scoperta e una metafora<\/h2>\n

Entanglement has enjoyed a cultural popularity out of all proportion to the public\u2019s familiarity with its foundations. The word has appeared in books about consciousness, destiny, love and coincidences. It has been invoked to suggest that all living beings are mysteriously connected and that separation between individuals is merely an illusion. The appeal is understandable. But physics has demonstrated nothing of the kind. An entangled state is not the same as a generic relationship between two objects. The fact that two people have met, or that two events display a striking coincidence, does not mean that they are entangled in the quantum-mechanical sense. Nor do we have evidence that the correlations observed in Bell experiments explain telepathy, emotional intuition or the endurance of human bonds. That does not mean science should police poetry. We may speak of two intertwined lives, a shared memory, an affinity that survives distance. Language has the right to use images that are not literal descriptions of the physical world. But when we invoke a scientific experiment to prove a belief about how the mind or emotions work, we have entered different territory. And we must provide appropriate evidence. This distinction does not diminish the beauty of the discovery. It restores its particular kind of beauty. <\/p>\n

The whole and its parts<\/h2>\n

Finally, the hardest question remains. What does entanglement teach us about reality? Not that objects are illusions. Not that the human observer invents the world by looking at it. Not that everything is mysteriously linked to everything else. It teaches us that some quantum systems have an overall state that cannot be reconstructed by assigning each part an independent state. It is possible, for example, to know precisely the pure state of an entangled pair while being unable to assign either component its own pure state which, combined with the other, would reproduce the state of the pair. The parts exist. They may be far apart and may undergo different measurements. But the correlations of the joint system cannot be explained as a mere collection of pre-existing local properties, within the assumptions tested by Bell experiments. Classical physics has accustomed us to a world in which, at least ideally, we can take an object apart, understand every piece and reconstruct the whole. Quantum physics warns us that this conceptual operation has limits. That is why the quotation attributed to Bohr, despite its uncertain provenance and its excessively categorical scientific claim, retains its provocative power. It is not necessarily things themselves that are unreal. Rather, some of the categories through which we have learned to regard them as real prove inadequate. Perhaps the most important achievement is not discovering surprising correlations between distant particles. It is finding a way to test experimentally an idea that once seemed to belong exclusively to philosophy: that every part of the world possesses, independently of the rest, all the properties required to describe it. Einstein was right to insist that such a fundamental question be taken seriously. Bohr was right to distrust the careless application of classical categories to the quantum world. Bell\u2019s extraordinary achievement was to find a way of asking nature to settle one precise part of their disagreement. The answer did not erase reality. It made the task of describing reality more difficult\u2014and far more interesting. <\/p>\n

—<\/p>\n

Le fonti fondamentali<\/h3>\n

– A. Einstein, B. Podolsky, N. Rosen, Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?, \u00abPhysical Review\u00bb, 47, 1935, pp. 777\u2013780.<\/p>\n

– N. Bohr, Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?, \u00abPhysical Review\u00bb, 48, 1935, pp. 696\u2013702.<\/p>\n

– E. Schr\u00f6dinger, Discussion of Probability Relations between Separated Systems, \u00abProceedings of the Cambridge Philosophical Society\u00bb, 31, 1935, pp. 555\u2013563.<\/p>\n

– J. S. Bell, On the Einstein Podolsky Rosen Paradox, \u00abPhysics\u00bb, 1, 1964, pp. 195\u2013200.<\/p>\n

– J. F. Clauser, M. A. Horne, A. Shimony, R. A. Holt, Proposed Experiment to Test Local Hidden-Variable Theories, \u00abPhysical Review Letters\u00bb, 23, 1969, pp. 880\u2013884.<\/p>\n

For further reading on experiments and interpretations<\/h3>\n

– Royal Swedish Academy of Sciences, scientific and explanatory materials relating to the 2022 Nobel Prize in Physics, awarded to Aspect, Clauser and Zeilinger.<\/p>\n

– Stanford Encyclopedia of Philosophy, voce Bell\u2019s Theorem, per l’esame delle ipotesi del teorema e delle sue implicazioni filosofiche.<\/p>\n

– Istituto Nazionale di Fisica Nucleare (Italian National Institute for Nuclear Physics), explanatory materials on the 2022 Nobel Prize and entanglement.<\/p>\n","protected":false},"excerpt":{"rendered":"

Entanglement: the story of an experiment that forced physics to rethink distance \u201cEverything we call real is made of things that cannot be regarded as real.\u201d The quotation is attributed to Niels Bohr. Yet we know of no primary source that would allow us to establish its authenticity. This is a necessary philological precaution, especially […]<\/p>\n","protected":false},"author":1756,"featured_media":901679,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[97],"tags":[],"class_list":["post-901678","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-isobare"],"acf":[],"yoast_head":"\nTwo, but not separate - Prometeo Liberato<\/title>\n<meta name=\"description\" content=\"Entanglement: the story of an experiment that forced physics to rethink distance\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/prometeoliberato.com\/en\/two-but-not-separate\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Two, but not separate - Prometeo Liberato\" \/>\n<meta property=\"og:description\" content=\"Entanglement: the story of an experiment that forced physics to rethink distance\" \/>\n<meta property=\"og:url\" content=\"https:\/\/prometeoliberato.com\/en\/two-but-not-separate\/\" \/>\n<meta property=\"og:site_name\" content=\"Prometeo Liberato\" \/>\n<meta property=\"article:published_time\" content=\"2026-10-20T12:48:10+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-10-20T12:57:31+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/prometeoliberato.com\/wp-content\/uploads\/2026\/10\/Entanglement-illustrazione-1024x683.png\" \/>\n\t<meta property=\"og:image:width\" content=\"1024\" \/>\n\t<meta property=\"og:image:height\" content=\"683\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"J. 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