Is the universe non-local or not real?

The Nobel Prize recognized work showing the universe is not 'locally real.' This doesn't mean the universe isn't real, but rather that it exhibits non-local correlations or quantum behaviors that challenge classical notions of locality and realism. Experiments have shown violations of Bell's inequality, suggesting either reality isn't local or it violates measurement independence.

What is quantum entanglement?

Entanglement is a quantum mechanical phenomenon where two or more particles become linked in such a way that they share the same fate, regardless of the distance separating them. Measuring a property of one entangled particle instantaneously influences the state of the other, but this correlation does not allow for faster-than-light communication.

Can entanglement be used for faster-than-light communication?

No, entanglement cannot be used for faster-than-light communication. While measurements on entangled particles show instantaneous correlations, the outcomes are random and probabilistic. Without a classical communication channel to coordinate or interpret these results, no information can be transmitted faster than light. This is formalized by the no-signaling theorem.

What did John Bell's theorem prove?

Bell's theorem demonstrated that any local hidden-variable theory predicting quantum mechanics must obey certain inequalities regarding measurement correlations. Quantum mechanics violates these inequalities. Experimental verification confirmed these violations, leading to the conclusion that either reality is not locally causal or it violates measurement independence.

What is the significance of violating Bell's inequality?

Violating Bell's inequality experimentally, as recognized by the Nobel Prize, means we must abandon either local causality (the idea that actions are only influenced by their immediate past within the light cone) or the assumption of measurement independence (where measurement outcomes are independent of the measurement setting). This forces a rethinking of our understanding of reality.

What's the difference between quantum non-locality and Einstein's 'spooky action at a distance'?

Einstein's 'spooky action at a distance' referred to a desired physical phenomenon where one particle instantaneously affects another. While entanglement shows non-local correlations, it doesn't involve physical 'action.' The wave function collapse in quantum mechanics can be seen as a non-local update of probabilities, but whether this is a physical 'action' in reality, as Einstein feared, is still debated and depends on the interpretation.

Key Moments

Why is quantum mechanics non-local? (I wish someone had told me this 20 years ago.)

Sabine Hossenfelder

Science & Technology4 min read26 min video

Jul 8, 2023|1,543,012 views|41,263|4,756

science without the gobbledygoook hossenfelder Quantum Mechanics Locality Einstein Physics Non-Locality Local Causality Bell's Theorem Local causality what is nonlocality is quantum mechanics nonlocal

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Key Moments

TL;DR

Quantum mechanics' non-locality means correlations exist, not necessarily faster-than-light action.

Key Insights

Locality means interactions are limited to immediate surroundings, while non-locality implies instantaneous effects over distance.

Faster-than-light travel and non-locality are distinct concepts, neither permitted by Einstein's theory of spacetime.

Quantum mechanics describes probabilities via wave functions; measuring one particle instantly updates the probability for entangled partners.

Entanglement creates non-local correlations, but not 'action at a distance'; information cannot be sent faster than light due to randomness.

Bell's theorem explores local causality and measurement independence, suggesting reality is either non-locally causal or violates measurement independence.

The Nobel Prize recognized experimental violation of Bell's inequality, implying either local causality or measurement independence must fail in reality.

DEFINING LOCALITY AND NON-LOCALITY

Locality dictates that interactions are restricted to one's immediate physical vicinity; you cannot influence something far away without a physical process bridging the distance. This principle applies to travel and all forms of interaction, meaning direct influence is only possible with objects or points adjacent to your own. Non-locality, conversely, suggests the possibility of instantaneous effects across any distance, akin to a 'portal' allowing immediate translocation or influence without traversing the intervening space. While non-locality might seem to imply faster-than-light travel, they are not equivalent; one could travel non-locally but still slower than light, or vice-versa.

SPACETIME AND CONSTRAINTS ON INTERACTIONS

Einstein's theory of spacetime introduces fundamental constraints on both faster-than-light travel and non-locality. In a spacetime diagram, where one axis represents space and the other time, any event is surrounded by 'light cones' demarcating the regions of spacetime that can be influenced by that event (future light cone) or can influence it (past light cone). The speed of light forms the boundary of these cones; nothing can travel faster than light, meaning influences must propagate within these boundaries. Non-local events, or faster-than-light travel, would violate these light cone restrictions, allowing for instantaneous jumps or influences that disregard the structure of spacetime.

QUANTUM MECHANICS AND THE WAVE FUNCTION

Quantum mechanics describes the behavior of particles using wave functions, which provide probabilities for measurement outcomes rather than deterministic predictions. When a quantum system, like a photon, is not observed, its wave function evolves locally. However, upon measurement, the wave function appears to instantaneously 'collapse' or reduce across all possible outcomes, updating probabilities. This apparent instantaneous change, wherever a measurement is made, is what leads to discussions of non-locality in quantum mechanics. The debate centers on whether this collapse represents a genuine physical process across space or merely an update in our knowledge about a pre-existing state.

ENTANGLEMENT AND NON-LOCAL CORRELATIONS

Entanglement describes a phenomenon where two or more quantum particles become linked, sharing a correlated fate regardless of their spatial separation. For instance, if a particle with zero total spin decays into two particles, their spins must be opposite. Measuring the spin of one particle instantaneously determines the spin of the other. This correlation can extend over vast distances, creating what seems like a non-local connection. However, this is a correlation, not an 'action at a distance' in the sense of one particle physically affecting the other. Updates to our knowledge about one particle's state instantly inform us about the other's, but no information or signal can be transmitted faster than light due to the inherent randomness of quantum measurements.

BELL'S THEOREM AND LOCAL CAUSALITY

John Bell's theorem provides a framework for testing whether reality adheres to local causality, a principle where an event can only be influenced by its immediate past within its light cone. Bell's theorem, particularly when combined with the assumption of measurement independence (meaning the outcome of a measurement doesn't depend on how it's measured), leads to inequalities that correlations in local hidden variable theories must obey. Quantum mechanics, however, can violate these inequalities, suggesting that either the universe is not locally causal, or the assumption of measurement independence is false.

EXPERIMENTAL VERIFICATION AND INTERPRETATIONS

Experiments testing Bell's inequalities, like those recognized by the Nobel Prize, have consistently shown violations, supporting the predictions of quantum mechanics. This implies that reality must deviate from either local causality or measurement independence, or both. Some physicists interpret this as evidence for genuine non-locality ('spooky action at a distance'), while others propose that local hidden variables exist but require a violation of measurement independence, a concept sometimes misleadingly termed 'superdeterminism'. The debate continues regarding whether the wave function collapse is a physical reality or a representation of our information, with different interpretations of quantum mechanics offering varying explanations.

Mentioned in This Episode

●Software & Apps

●Tools

●Companies

●Concepts

●People Referenced

Common Questions

Locality means that an object can only interact with its immediate surroundings, and actions cannot propagate instantaneously across space. Physical change requires proximity and takes time, prohibiting phenomena like instantaneous travel or 'action at a distance'.

Topics

Quantum Mechanics Hidden Variables Technology & Innovation Science & Mathematics Wave Function Measurement Independence

Mentioned in this video

Tools & Products

Brilliant.org

Sponsor of the video, offering interactive online courses in science and mathematics, including one on quantum mechanics.

New York City

Used as an example to illustrate the concept of locality, emphasizing that travel between locations requires a physical process and is not instantaneous.

Expedia

Humorously mentioned as not offering non-local travel or faster-than-light options, emphasizing their absence in current commerce.

Mach-Zehnder interferometer

A setup discussed to build a local hidden variables model, demonstrating how to reproduce quantum mechanical results locally by violating measurement independence.

Locations

JFK

Mentioned as a destination in New York City, contrasting with Heathrow to illustrate the local nature of travel.

Heathrow

Mentioned as a departure point, contrasting with JFK to illustrate the local nature of travel.

Concepts

Einstein's theory

The theory that prohibits faster-than-light travel and sets boundaries with spacetime, which is relevant to discussions of locality.

Bell's theorem

A theorem that shows if local hidden variables reproduce quantum mechanics, they must violate measurement independence, or reality must be non-local.

Bell's inequality

An inequality derived from the assumption of local hidden variables and measurement independence, which quantum mechanics violates.

Organizations

Tokyo

Used as a distant location for Alice in the envelope thought experiment, highlighting how correlations can span large distances.

People

John Clauser

A recipient of the 2022 Nobel Prize in Physics for experimental work related to Bell's theorem, and who pointed out Bell's oversight regarding measurement independence.

John Bell

A physicist who developed Bell's theorem to distinguish between local hidden variable theories and quantum mechanics, crucial for understanding non-locality.

Companies

Air France

Mentioned humorously in the context of travel delays, reinforcing the idea that travel, even with delays, is a local process.