European Scientists Performed A Quantum Experiment That Broke Reality | Leonard Susskind

Added: 2026-06-03

[00:00:00]Reality doesn't work the way you think it does. This is not philosophical speculation, not mystical thinking, not interpretative framework that physicists debate over coffee.

[00:00:11]This is experimental fact, demonstrated repeatedly in laboratories around the world, confirmed to extraordinary precision through decades of increasingly sophisticated measurements, tested in every conceivable way that physics allows.

[00:00:26]Quantum mechanics predicts that reality should behave in ways that violate every intuition humans have about how the world works, about what it means for things to exist, about whether objects have definite properties before measurement, about whether distant events can influence each other instantaneously, about whether the universe is fundamentally deterministic or probabilistic. And every time we test these predictions, every time we perform experiments that probe quantum behavior at fundamental level, we find that nature agrees with quantum mechanics and disagrees with classical intuition, forcing us to accept that reality at its deepest level is stranger, more counterintuitive, more fundamentally different from everyday experience than anything humans evolved to comprehend.

[00:01:16]I'm not talking about interpretations of quantum mechanics, not about the different ways of thinking about the what the mathematics means, not about philosophical preferences for how we should understand quantum phenomena.

[00:01:28]I'm talking about experimental results that are independent of interpretation, that don't depend on whether you prefer Copenhagen or many worlds or pilot waves or any other framework for making sense of quantum mechanics. I'm talking about measurements that show definitively that nature violates Bell inequalities, that particles remain entangled across arbitrary distances, that observation affects outcomes in ways that cannot be explained by any classical mechanism, that the universe behaves probabilistically at fundamental level in ways that are not just ignorance about hidden variables, but genuine fundamental randomness built into the fabric of reality itself.

[00:02:10]These experiments are some of the most precise ever performed in physics, controlled to incredible accuracy, repeated countless times by independent groups, verified through multiple different experimental setups and methods, leaving no room for doubt about the results, leaving no possibility that classical physics could account for what we observe, forcing every physicist who examines the evidence carefully to accept that quantum mechanics is not just approximately correct or useful approximation or effective theory that works in certain regimes, but is fundamentally correct description of how nature actually works, how reality actually behaves, how the universe actually operates at its most basic level. And accepting this means accepting that reality is deeply strange, that the world doesn't work the way human intuition suggests, that properties we think of as obviously real and obviously definite are actually contextual and observer dependent and fundamentally probabilistic until measured, that distant events can correlate in ways that seem to violate locality and causality, that observation plays a role in determining outcomes that classical physics cannot accommodate, making quantum mechanics not just counterintuitive, but genuinely revolutionary, genuinely challenging every assumption about what reality is and how it works that humans bring from everyday experience.

[00:03:42]Let me take you through what the experiments actually show, what they definitively prove, what remains debated and what is settled beyond reasonable doubt, and what it all means for our understanding of reality, for our sense of what exists and how existence works, for the deep structure of nature that quantum mechanics reveals through careful measurement and precise prediction, but that remains profoundly difficult to understand intuitively, to visualize clearly, to reconcile with the classical picture of reality that evolution equipped humans to navigate, but that quantum experiments demonstrate is incomplete, is approximate, is valid only at macroscopic scales where quantum effects average out into classical behavior, but fundamentally wrong about how nature works at its deepest level. Start with the simplest quantum phenomenon, wave-particle duality. The fact that entities like electrons and photons exhibit both wave-like and particle-like properties depending on how you measure them, creating immediate tension with classical physics which treats waves and particles as completely different types of things, as mutually exclusive categories that cannot overlap, making an object that behaves as both wave and particle seem contradictory, seem impossible, seem to violate basic logic about what things can be, but the experiments show definitively that quantum entities do exhibit both behaviors. In double-slit experiments, send electrons one at a time through apparatus with two narrow slits and they create interference pattern on detection screen, pattern of bright and dark bands that can only be explained if each electron passes through both slits simultaneously as a wave, interfering with itself, creating the characteristic pattern that wave interference produces.

[00:05:39]This is not controversial, not debated, not interpretative. The interference pattern appears demonstrating wave-like behavior conclusively, demonstrating that electrons cannot be understood as classical particles following definite trajectories through one slit or the other. But, if you measure which slit the electron goes through, if you place detectors that determine the electron's path, the interference pattern disappears.

[00:06:06]Now, electrons behave like classical particles, each going through one slit or the other, creating pattern of two bright bands rather than many interference fringes, demonstrating particle-like behavior, demonstrating that the act of measurement changes the outcome, that observing which path the electron takes destroys the wave-like interference, creating the the famous observer effect, where observation itself affects the phenomenon being observed in fundamental way that cannot be eliminated, cannot be made arbitrarily small, cannot be avoided through more careful or less invasive measurement. This is deeply strange because in classical physics observation doesn't affect the thing observed, measurement just reveals properties that already existed, that were definite before measurement, that observation merely discovers rather than creating or determining.

[00:07:05]But, quantum mechanics says this classical picture is wrong, that particles don't have definite positions or momenta before measurement, that they exist in superposition of multiple possibilities described by wave function, that measurement causes this wave function to collapse, selecting one possibility, making it definite, creating the outcome rather than just revealing pre-existing property. And this isn't just mathematical formalism, isn't just convenient way of thinking about calculations.

[00:07:38]The experiments show that interference pattern reappears and disappears based on whether path information is available showing that it's not disturbance from measurement device but information itself that matters that if you measure which path but then erase that information before final detection interference returns demonstrating that quantum behavior depends on what information exists about the system not just on physical interaction with measuring apparatus making information fundamental to quantum mechanics in ways that classical physics never required but wave particle duality is just the beginning just a first hint that quantum reality is stranger than classical intuition allows.

[00:08:22]The truly profound strangeness comes from entanglement from the fact that quantum systems can become correlated in ways that have no classical analogue creating connections between particles that persist regardless of distance that allow measurements on one particle to instantaneously affect the other that seem to violate locality seem to allow faster than light influence seem to create what Einstein called spooky action at a distance that he believed showed quantum mechanics must be incomplete must be missing some deeper theory of hidden variables that would restore locality and determinism and make quantum correlations explainable through classical mechanisms create entangled pair of particles separate them by arbitrary distance and measurements on one particle correlate perfectly with measurements on the other in ways that quantum mechanics predicts but that seem impossible classically measure one particle spin in particular direction and you instantly know what the other particle spin will be in that same direction creating perfect correlation across space, creating connection that seems to require instantaneous communication between the particles, seems to violate special relativity's prohibition on faster-than-light signaling, seems to make quantum mechanics inconsistent with basic principles of relativity and causality.

[00:09:56]Einstein, Podolsky, and Rosen formalized this concern in their famous EPR paper, arguing that quantum mechanics must be incomplete because it predicts these correlations without providing mechanism for how they occur, suggesting that particles must have definite but hidden properties, local hidden variables that determine measurement outcomes, that existed all along but were unknown until measured, making the correlations not mysterious, but just consequence of particles carrying complementary information.

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[00:11:00]from their creation, like socks in a drawer, where examining one reveals the other without any communication between them, this seems reasonable, seems to restore classical intuition, seems to explain quantum correlations without invoking non-locality or action at a distance.

[00:11:18]But John Bell showed in 1964 that this cannot work, that any theory based on local hidden variables makes predictions that differ from quantum mechanics in measurable ways, creating Bell inequalities that local realistic theories must satisfy but quantum mechanics violates making the question of whether nature is fundamentally quantum or fundamentally classical with hidden variables an experimental question that could be settled through careful measurement. And the experiments have been done repeatedly with increasing precision and sophistication closing loopholes and addressing concerns demonstrating conclusively that nature violates Bell inequalities that quantum mechanics is correct and local hidden variable theories are wrong that there is no way to explain quantum correlations through classical mechanisms through local realistic theories where particles have definite properties and don't influence each other faster than light.

[00:12:22]The results are unambiguous have been confirmed by dozens of independent experiments using different methods and different types of particles and different detection schemes leaving no room for doubt that quantum entanglement is real that non-locality is genuine feature of nature that reality at quantum level does not obey locality in the way classical physics assumes. Let me explain Bell's theorem more carefully because this is where quantum mechanics most directly contradicts classical intuition where experimental physics provides clearest evidence that reality doesn't work the way humans naturally think it should.

[00:13:01]Bell showed that any theory satisfying locality and realism makes predictions about correlations between entangled particles that can be expressed as mathematical inequality with certain correlation values being impossible if both locality and realism are true. Locality means influences cannot propagate faster than light.

[00:13:23]Measurements on one particle cannot instantaneously affect the other particle regardless of distance. Realism means particles have definite properties before measurement. Measurement just reveals pre-existing values rather than creating them through observation. These seem like reasonable assumptions, like basic features of reality that any sensible theory should respect, making Bell inequalities seem like obvious constraints that nature must satisfy.

[00:13:50]But quantum mechanics predicts violations of Bell inequalities for certain entangled states and measurement settings, predicting correlations stronger than any local realistic theory allows, making quantum mechanics incompatible with the combination of locality and realism, forcing us to abandon at least one of these classical assumptions if quantum mechanics is correct. And experiments definitively show that nature violates Bell inequalities exactly as quantum mechanics predicts, measuring correlations that exceed the bounds that local realistic theories allow, demonstrating that we must give up either locality or realism or both. The quantum reality is fundamentally different from classical reality in ways that Bell's theorem makes mathematically precise and experimentally testable.

[00:14:44]This is not interpretation dependent, not philosophical debate about what quantum mechanics means, but experimental fact about how nature actually behaves, about what correlation values we actually measure when we perform these experiments carefully and precisely. Alain Aspect's experiments in the early 1980s were pivotal, using sophisticated setups with rapid switching of measurement settings to close timing loopholes, demonstrating violations of Bell inequalities under conditions where any classical explanation based on local hidden variables would require faster-than-light communication between particles or between measurement apparatus particles.

[00:15:25]More recent experiments have closed additional loopholes, including detection efficiency loopholes and freedom of choice loopholes, using better detectors and random number generators to select measurement settings, demonstrating violations of Bell inequalities under increasingly stringent conditions that eliminate potential classical explanations one by one, leaving quantum non-locality as the only viable explanation for observed correlations.

[00:15:54]This is profound because it means locality or realism or both must be abandoned at quantum level.

[00:16:02]Most physicists accept that realism fails, that particles don't have definite properties before measurement, that quantum states are fundamentally probabilistic descriptions of potential outcomes rather than ignorance about actual definite states, making measurement outcome genuinely random, genuinely undetermined before measurement, genuinely created through measurement process rather than revealed. This preserves locality in the sense that measurement on one particle doesn't causally affect the other particle faster than light, but sacrifices realism, abandons the classical assumption that physical properties exist definitely before measurement, making quantum mechanics fundamentally probabilistic at level that cannot be reduced to ignorance about hidden variables.

[00:16:47]Now let me address delayed choice experiments, another class of quantum measurements that reveal how deeply strange quantum behavior is, how thoroughly it violates classical intuition about causality and temporal ordering of events.

[00:17:02]In Wheeler's delayed choice experiment, you decide whether to measure particle-like or wave-like properties after the quantum system has already evolved, after it has already passed through the double slits in the wave version, making the decision about measurement type come after the event you're measuring, creating apparent retroactive causation where later measurement affects earlier behavior. Set up double slit experiment where you can choose to detect which path information or to observe interference pattern, but make this choice randomly after each particle has passed through the slits, but before it reaches detection screen. Classical intuition says the particle must have either gone through both slits as wave or through one slit as particle, that this behavior is determined when the particle passes the slits, that later measurement choice cannot affect this earlier behavior.

[00:17:56]But quantum mechanics predicts and experiments confirm that the measurement choice determines whether interference or which path behavior occurs.

[00:18:05]That if you choose to measure which path you destroy interference, even though this choice comes after the particle passed the slits, that if you choose to observe interference pattern, it appears even though this choice comes after the path was determined, creating results that seem to violate temporal ordering of cause and effect. This doesn't mean quantum mechanics allow sending signals backward in time, doesn't create grandfather paradoxes or causality violations in the usual sense, because the correlations only become apparent after comparing results, after bringing together information from different measurements, preventing faster than light or backward in time communication. But it does mean that classical temporal intuitions about when things are determined, about how present choices affect past events, about the relationship between measurement and measured system, all break down at quantum level, all fail to describe how nature actually behaves, requiring fundamentally different framework where measurement and system cannot be cleanly separated, where temporal ordering of events doesn't determine causation in classical way where quantum correlations transcend simple notions of earlier and later, cause and effect. More recent quantum eraser experiments extend this strangeness, showing that you can erase which path information after it's been recorded and restore interference pattern, or you can preserve which path information and prevent interference with a choice made after particles have been detected, after the events have occurred from classical perspective, yet still determining whether interference appears in the data when properly analyzed. These results are not controversial, not disputed, have been confirmed by numerous experiments, demonstrating conclusively that quantum behavior cannot be understood through classical temporal causation, through simple story where earlier events determine later outcomes independent of what measurements we choose to perform.

[00:20:08]Let me now address what all this means for the concept of objective reality, for the idea that the world exists in definite state independent of observation, that properties of objects are real whether or not anyone measures them, that the universe has objective features that don't depend on consciousness or measurement or observation. Quantum mechanics seems to challenge this classical realism, seems to suggest that observation plays fundamental role in determining outcomes, that properties don't exist definitely before measurement, that the universe at quantum level is fundamentally about information and probability rather than about definite states and objective properties existing independently of observers.

[00:20:53]Different interpretations of quantum mechanics handle this differently, give different answers to whether reality is objective or observer dependent, whether wave function collapse is physical process or just update of information, whether measurement creates outcomes or reveals them. Copenhagen interpretation emphasizes measurement and observation, treats wave function collapse as fundamental, suggests that quantum systems don't have definite properties before measurement, making observation central to quantum physics in ways that challenge classical objectivity. Many worlds interpretation preserves objective reality by saying all outcomes occur in different branches, that there's no collapse, that wave function always evolves unitarily, that observation just determines which branch you find yourself in, rather than creating outcomes, restoring determinism and objectivity at cost of accepting that all possibilities are realized somewhere in the multiverse. Other interpretations, like pilot wave theory or objective collapse models, try to restore realism in different ways, either by adding hidden variables that determine outcomes or by modifying quantum mechanics to include physical collapse mechanism independent of observation.

[00:22:10]But all interpretations must account for the experimental facts, must explain why we observe violations of Bell inequalities, why delayed choice experiments give the results they do, why quantum behavior exhibits the strange features that countless experiments have confirmed, making the choice between interpretations more about philosophical preference and what the aspects of classical intuition you're willing to sacrifice than about empirical adequacy, since all viable interpretations reproduce quantum predictions and thus all agree about what we should observe, even if they differ about what this means metaphysically.

[00:22:52]What's not in dispute is that classical realism fails, that the naive picture where objects have definite properties existing independently of observation cannot account for quantum phenomena, cannot explain Bell inequality violations, cannot accommodate the experimental results that quantum mechanics predicts and nature confirms.

[00:23:14]Whether you think this means reality is fundamentally observer dependent or that all possibilities are realized or that there are hidden variables we cannot access or the collapse is objective physical process, you must abandon the simple classical picture with a world exists definitely and measurement just reveals pre-existing facts, making quantum mechanics genuinely revolutionary regardless of which interpretation you prefer.

[00:23:40]Now, let me address quantum decoherence, the process that explains why quantum strangeness disappears at macroscopic scales, why everyday objects behave classically even though they're made of quantum particles, why we don't observe superposition and interference in uh human scale phenomena despite the fact that fundamental physics is quantum.

[00:24:06]When quantum system interacts with environment, when it becomes entangled with many degrees of freedom in surrounding matter and radiation, decoherence occurs rapidly, quantum coherence is lost, superposition effectively collapses into classical mixture, making quantum behavior invisible at macroscopic scales, making the world appear classical even though underlying reality is quantum. This happens because environment constantly monitors quantum systems, constantly becoming entangled with them, constantly spreading quantum information into many degrees of freedom that become impossible to track or reverse, making interference terms in quantum superposition decay exponentially fast, making different branches of wave function become effectively independent, unable to interfere, behaving like classical alternatives where one occurs rather than quantum superposition where both occur simultaneously.

[00:25:06]For macroscopic objects, this decoherence is extraordinarily rapid, happening in tiny fractions of a second, far faster than any time scale relevant to human perception or macroscopic dynamics, making quantum behavior impossible to observe, making classical behavior emerge naturally from quantum substrate through environmental interaction. This explains why we don't see superposition in everyday life, why chairs and tables and people behave classically, why the strangeness of quantum mechanics only appears in carefully isolated systems where environmental interaction is minimized, where decoherence is slow enough that quantum effects survive long enough to be measured. It's not that macroscopic objects obey different physics, not that there's separate classical and quantum realms, but that environmental interaction erases quantum behavior so quickly that it's invisible at scales we experience directly, making classical behavior emergent from quantum mechanics rather than separate domain requiring separate laws.

[00:26:12]But decoherence doesn't solve measurement problem, doesn't explain why we observe definite outcomes, doesn't remove need for interpretations of quantum mechanics. Decoherence explains why interference disappears, why superposition becomes invisible, why different branches stop interfering and behave classically, but it doesn't explain why we experience one branch rather than superposition of branches, why observation gives definite outcome rather than perceiving all outcomes simultaneously, making decoherence important for understanding quantum to classical transition, but not sufficient for complete interpretation of quantum mechanics, not eliminating the deep puzzles about observation and measurement and the role of consciousness or observation in determining outcomes, let me address quantum information now because modern quantum mechanics increasingly describes reality in informational terms, in terms of qubits and entanglement entropy and quantum channels, treating information as fundamental and physical properties as derived, reversing classical hierarchy where matter and energy are fundamental and information is abstraction describing states of physical systems. In quantum information theory, the universe is fundamentally about information processing, about quantum states evolving according to unitary transformations, about entanglement and measurement and decoherence understood as information theoretic processes, making information more basic than space or time or matter or energy, making physics fundamentally about how information behaves rather than about how particles move through space. This connects the holographic principle to the idea that space-time itself might be emergent from quantum information, that three-dimensional space might arise from two-dimensional boundary where fundamental degrees of freedom live, where quantum information is processed in ways that create bulk space-time as emergent structure, as approximate effective description valid at large scales but not fundamental, not existing at deepest level where reality is purely informational, purely quantum, purely about entanglement and correlation and information content without geometric structure, without spatial relationships except as emergent features of how information is organized.

[00:28:45]If this is correct, if space time is emergent from quantum information, then locality becomes approximate, becomes feature of emergent geometry rather than fundamental principle, helping explain how quantum mechanics can violate locality without contradicting relativity because relativity describes emergent space time and locality is feature of this emergent structure, while quantum mechanics operates a deeper level where space time hasn't emerged yet, where quantum entanglement creates correlations that appear non-local from emergent space time perspective, but are actually describing information structure at more fundamental level where spatial separation doesn't exist, where distance hasn't emerged, where everything is connected through quantum information in ways that create correlations transcending emergent geometric separation. This is speculative, not established, remains active area of research without consensus, but it demonstrates how quantum mechanics might be pointing toward fundamentally different conception of reality, toward picture where information is primary and physical properties, including space time, are derived or emergent, are approximate descriptions of underlying quantum information structure, making the universe more like computer simulation or mathematical structure than like collection of particles moving through space, making reality fundamentally abstract, fundamentally informational, fundamentally quantum in ways that our classical geometric intuitions cannot capture, cannot visualize, can only describe mathematically. Let me end by reflecting on what all this means for human understanding of reality, for ability to comprehend the universe, for the relationship between how nature works and how human minds evolved to think about nature.

[00:30:37]Quantum mechanics reveals that reality at at deepest level is profoundly different from the classical world that human evolution equipped us to navigate, that the universe operates according to principles that violate every intuition we develop through interacting with macroscopic objects at human scales, making deep understanding of quantum reality perhaps impossible through intuition alone, requiring mathematical formalism to describe what we cannot visualize, what we cannot imagine, what we cannot grasp through ordinary cognitive faculties.

[00:31:11]Evolution optimized human cognition for survival in classical macroscopic world, for understanding medium-sized objects moving at moderate speeds, for predicting trajectories and manipulating tools and navigating environments where quantum effects average out into classical behavior, where decoherence is so rapid that quantum strangeness never manifests observably, making classical intuitions accurate enough for evolutionary purposes, even though they're fundamentally wrong about deep structure of reality. We evolved to think in terms of objects with definite properties existing independently of observation, to believe in locality and determinism and objective reality, not because these are true, but because they're useful approximations at scales relevant to survival and reproduction, making our intuitions reliable guides to macroscopic world, but misleading about fundamental physics, about how nature actually works beneath the classical appearance. This means accepting quantum mechanics requires abandoning intuition as reliable guide, requires trusting mathematics and experiments over feelings about how reality should work, requires accepting that nature is stranger than human minds can comfortably grasp, that the universe at its foundation operates according to principles that seem paradoxical or contradictory or impossible from classical perspective, but are actually just different.

[00:32:37]Just describing reality at scales where different physics applies, where quantum effects dominate, where classical approximations break down completely.

[00:32:47]We can calculate quantum mechanics, can predict its results with extraordinary precision, can use it to build technologies and explain experiments and make progress in physics, but we cannot truly visualize it, cannot make it feel intuitive, cannot reduce it to simple classical picture that human minds find natural and obvious.

[00:33:09]Perhaps the most disturbing discovery in modern physics is not that quantum mechanics is strange, not that particles exhibit wave-particle duality or entanglement, creates non-local correlations, or observation affects outcomes, but that reality itself may never have obeyed classical intuition in the first place, that the classical world we experience is approximate, emergent, fundamentally misleading about deep structure of nature, making our everyday experience more like useful fiction than accurate representation of how things actually work, making the universe fundamentally quantum in ways that classical physics never suspected, in ways that human evolution never prepared us to understand intuitively, leaving us dependent on mathematics and experiments to reveal truths about reality that lie forever beyond direct intuitive comprehension.

[00:34:06]And human beings may spend centuries building theories and performing experiments and developing technologies to understand the universe, may achieve extraordinary precision in predicting quantum phenomena, may harness quantum mechanics for computation and communication and measurement, accumulating vast knowledge about how quantum systems behave and what quantum mechanics predicts and how to manipulate quantum states for practical purposes, only to discover that the deepest layer of reality behaves in ways the human mind was never evolved to intuitively comprehend, that understanding quantum mechanics fully means accepting that intuition fails, that visualization breaks down, that the only way to grasp quantum reality is through abstract mathematics describing relationships and probabilities and information that cannot be