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

Sabine Hossenfelder

8 Jul 202325:08

EducationalLearning

32 Likes 10 Comments

TLDRThis video script delves into the concept of locality and its defiance in quantum mechanics, particularly through entanglement. It clarifies misconceptions about 'spooky action at a distance' and explores the implications of the Nobel Prize-winning experiments that challenge local hidden variables theories. The script explains quantum non-locality, the difference between knowledge correlation and physical action, and critiques the misunderstandings around entanglement's role in faster-than-light communication. It also touches on the philosophical debates between Einstein and Bohr, and the significance of Bell's theorem in understanding the fundamental nature of reality.

Takeaways

🌌 The 2022 Nobel Prize in Physics was awarded for research showing the universe is not locally real, highlighting quantum mechanics' non-locality.
🌐 Locality means interactions and actions can only occur between objects in close proximity, and non-locality would imply instantaneous effects over distance.
✈️ Non-locality is not the same as faster-than-light travel, though they are related in that both are not allowed in Einstein's theory of space and time.
🔍 Einstein's theory of relativity suggests that nothing can exceed the speed of light, and events can only influence what lies within their future light cone.
📦 The concept of non-locality in quantum mechanics is often misunderstood, with entanglement being a correlation over distance rather than an action at a distance.
🤔 Quantum mechanics is described by wave-functions, which predict probabilities of measurement outcomes but not with certainty, making it inherently non-deterministic.
🌀 Entanglement in quantum mechanics involves particles that are correlated in a way that the state of one instantly affects the state of another, regardless of distance.
🚫 Quantum mechanics does not allow for information to be transmitted faster than light, adhering to the no-signalling theorem.
🔄 John Bell's theorem and subsequent experiments have shown that quantum mechanics violates Bell's inequality, suggesting non-locality in the correlations of measurement outcomes.
🎓 Understanding quantum mechanics involves grasping concepts like wave-functions, probabilities, interference, and the implications of non-locality on our understanding of reality.

Q & A

What was the Nobel Prize in Physics awarded for last year according to the headlines mentioned in the script?
-The Nobel Prize in Physics was awarded for demonstrating that the universe is not locally real, showing 'spooky quantum behavior' and 'spooky action at a distance', which relates to the concept of quantum entanglement and non-locality.
What does the term 'locality' mean in the context of the script?
-In the script, 'locality' refers to the principle that physical interactions and influences can only occur between objects that are in close proximity to each other, and not instantaneously across vast distances.
How does the script differentiate between non-locality and faster-than-light travel?
-The script explains that non-locality is the concept of instantaneous appearance or interaction across distances without the need for physical traversal, while faster-than-light travel specifically refers to moving from one point to another at speeds exceeding that of light. They are related but distinct concepts.
What is the 'future light cone' in the context of Einstein's theory of space and time?
-The 'future light cone' is a concept in Einstein's theory of space and time that represents the region of space-time that can be influenced by an event. It is bounded by 45-degree lines emerging from the event, indicating that nothing can travel faster than the speed of light to influence outside this region.
How does the script illustrate the concept of non-local correlation using the example of two numbers in envelopes?
-The script uses the example of two numbers, one in each envelope sent to different locations, to illustrate non-local correlation. When one number is revealed, the other number becomes known instantaneously, showing a correlation that spans distance, even though the physical action of revealing the number is local.
What is the difference between physical non-locality and non-local correlation as described in the script?
-Physical non-locality refers to an action that occurs at a distance, like something going through a portal. Non-local correlation, on the other hand, is about gaining knowledge about something that happened elsewhere instantaneously, but the physical changes associated with gaining that knowledge are local.
What is the significance of the wave-function in quantum mechanics as mentioned in the script?
-In quantum mechanics, the wave-function is used to calculate the probability of obtaining a particular measurement outcome. It does not predict outcomes with certainty, which is why quantum mechanics is considered non-deterministic.
How does the script explain the concept of quantum entanglement?
-The script explains quantum entanglement through the example of a particle decaying into two particles with correlated spins. The entangled particles have a total spin that is conserved, and knowing the spin of one particle instantaneously informs you about the spin of the other, regardless of the distance between them.
What is the 'no-signalling theorem' in quantum mechanics?
-The 'no-signalling theorem' in quantum mechanics states that it is impossible to use quantum entanglement to send information faster than the speed of light. This is because the measurement outcomes on entangled particles are randomly distributed according to the wavefunction probabilities.
What is Bell's theorem and what does it imply about local hidden variables models?
-Bell's theorem states that any local hidden variables model that reproduces the predictions of quantum mechanics must violate measurement independence. It implies that if you want a local model that accurately reflects quantum mechanics, it cannot be locally causal.
How does the script address the idea of 'spooky action at a distance'?
-The script discusses 'spooky action at a distance' as a term originally used by Einstein to criticize the idea of non-local physical processes in quantum mechanics. It explains that while quantum mechanics exhibits non-locality in the form of wave-function collapse, it has not been conclusively shown that this non-locality represents a physical 'action at a distance'.

Outlines

00:00

🌌 The Concept of Locality and Quantum Mechanics

The paragraph introduces the concept of locality in the context of the universe's behavior, contrasting it with the non-local 'spooky' quantum phenomena. It sets the stage for a discussion on whether the universe is locally real or subject to quantum mechanics' non-local effects. The speaker outlines a plan to explore the meaning of locality, its implications in everyday life, and how it conflicts with quantum theory. The explanation includes the idea of traveling to New York City as an analogy for locality and the concept of 'non-locality' as an instantaneous action across distances, which is not allowed by Einstein's theory of space and time. The paragraph concludes with an introduction to the space-time diagram and the concept of light cones, which are boundaries within which the effects of an event are confined.

05:04

🔬 Quantum Mechanics and Non-local Correlations

This section delves into the principles of quantum mechanics, focusing on the wave-function and its role in calculating probabilities of measurement outcomes. It explains the non-deterministic nature of quantum mechanics and uses the example of a photon passing through a semi-transparent plate to illustrate how the wave-function evolves upon measurement. The paragraph introduces the concept of entanglement, where particles can have correlated properties across distances, and how this correlation is updated instantaneously upon measurement of one particle, regardless of distance. The discussion also touches on Einstein's 'spooky action at a distance' and the debate between Einstein and Bohr on the reality of the wave-function and the existence of hidden variables.

10:06

🔗 Entanglement and the Misunderstanding of Non-locality

The paragraph clarifies misconceptions about entanglement, emphasizing that it does not imply instantaneous 'action' or change in entangled particles when one is manipulated. It explains that entanglement is merely a correlation that extends over distance and that no information can be transmitted faster than light using this phenomenon, adhering to the no-signalling theorem. The speaker also introduces John Bell's work on non-locality, differentiating between physical non-locality and non-local correlations, and setting the stage for Bell's theorem and its implications for local hidden variable theories.

15:09

📡 Bell's Theorem and the Challenge of Local Hidden Variables

This section discusses Bell's theorem and its implications for local hidden variable theories. It explains the concept of local causality and how quantum mechanics violates this principle, indicating non-locality in the wave-function collapse. The paragraph uses the Mach-Zehnder interferometer to illustrate the challenges of creating a local hidden variable model that can reproduce quantum mechanical phenomena without violating measurement independence. It suggests that to achieve local causality, such a model would need to incorporate a form of 'superdeterminism,' which is a controversial concept in the interpretation of quantum mechanics.

20:12

🏆 Nobel Prize for Quantum Mechanics and the Reality of Non-locality

The final paragraph summarizes the discussion by highlighting the Nobel Prize awarded for experimental confirmation of quantum mechanics' violation of Bell's inequality, which challenges local hidden variable theories. It points out that while quantum mechanics is non-local due to wave-function collapse, it has not been conclusively proven that this non-locality is a physical reality. The speaker also addresses the philosophical implications of the many-worlds interpretation and invites viewers to explore these topics further through a linked course on quantum mechanics, offering a comprehensive understanding of the subject.

Mindmap

Keywords

💡Locality

Locality refers to the principle that physical processes and interactions occur only within the immediate vicinity of an object or event, without instantaneous effects at a distance. In the video, the concept of locality is juxtaposed with the idea of non-locality in quantum mechanics, where effects can seemingly occur faster than light. The script uses the analogy of traveling from Heathrow to JFK to illustrate the concept, emphasizing that in everyday experience, locality is a fundamental aspect of reality.

💡Non-locality

Non-locality is the concept that information or interactions can occur instantaneously over any distance, which challenges the principle of locality. The video discusses non-locality in the context of quantum mechanics, where phenomena like entanglement and the collapse of the wave-function suggest that particles can be correlated in a way that defies local constraints. The script contrasts non-locality with the idea of faster-than-light travel, clarifying that they are distinct concepts.

💡Quantum Mechanics

Quantum mechanics is the branch of physics that describes the behavior of particles at the atomic and subatomic scale, where classical physics no longer applies. The video delves into quantum mechanics to explore its non-local characteristics, such as the probabilistic nature of wave-functions and the phenomenon of entanglement. The script uses the example of a photon passing through a semi-transparent plate to illustrate the uncertainty principle inherent in quantum mechanics.

💡Wave-function

In quantum mechanics, the wave-function, denoted by the Greek letter Psi, is a mathematical description that encapsulates the probability distribution of a particle's position, momentum, and other physical properties. The script explains that the wave-function does not predict outcomes with certainty but allows for the calculation of probabilities, which is a key aspect of quantum mechanics' non-deterministic nature.

💡Entanglement

Entanglement is a quantum phenomenon where two or more particles become correlated in such a way that the state of one particle is instantaneously dependent on the state of the other, regardless of the distance separating them. The video script uses entanglement to illustrate the non-local aspects of quantum mechanics, such as the conservation of total spin in a particle decay process, where measuring the spin of one particle instantly determines the spin of the other.

💡Spooky Action at a Distance

The term 'spooky action at a distance' was famously used by Albert Einstein to describe his discomfort with the non-local implications of quantum mechanics, particularly entanglement. In the video, the concept is discussed in the context of Einstein's belief that there must be a local hidden variable theory that could explain quantum phenomena without the need for non-local interactions. The script points out that the term has been misapplied to entanglement, which does not involve 'action' in the traditional sense.

💡Hidden Variables

Hidden variables are hypothetical properties of particles that are not described by quantum mechanics but could potentially explain the outcomes of quantum measurements. The video script discusses hidden variables in the context of Einstein's local realism, suggesting that there might be underlying variables that determine the outcomes of quantum events without the need for non-locality. However, the script also points out that quantum mechanics, as it stands, does not require hidden variables.

💡No-Signalling Theorem

The no-signalling theorem is a principle in quantum mechanics that states that no information can be transmitted faster than light using entangled particles. The video script explains that despite the non-local correlations in entanglement, one cannot use this phenomenon to send signals, thus preserving the principle of locality in the transmission of information.

💡Bell's Theorem

Bell's theorem is a result in quantum mechanics that shows that certain predictions of quantum mechanics cannot be replicated by any local hidden variable theory, which obeys an inequality known as Bell's inequality. The video script discusses Bell's theorem in the context of experimental evidence that violates this inequality, suggesting that quantum mechanics is inherently non-local. However, the script also points out that Bell's original assumption of measurement independence has been overlooked in many interpretations of his theorem.

💡Measurement Independence

Measurement independence is the assumption that the outcome of a measurement on one system does not depend on the measurements made on another system. The video script explains that in order for a local hidden variable model to be consistent with quantum mechanics, it must violate measurement independence, as shown by Bell's theorem. The script uses the example of a Mach-Zehnder interferometer to illustrate how the outcome of a measurement can depend on what is measured, thus violating this assumption.

💡Superdeterminism

Superdeterminism is a philosophical position that suggests that the appearance of randomness in quantum mechanics is an illusion, and that all events, including measurements, are determined by underlying variables that are not yet known. The video script briefly mentions superdeterminism as a concept that has been used to explain away the non-locality implied by quantum mechanics, although it is not the focus of the video.

Highlights

The Nobel Prize in physics was awarded for demonstrating the universe's non-locality, challenging the concept of locality in physics.

Locality traditionally means that interactions and actions occur only between immediate neighbors in space and time.

Non-locality suggests the possibility of instantaneous effects over distance, akin to a portal for matter or information.

Einstein's theory of space and time does not allow for non-locality or faster-than-light travel.

The concept of 'spooky action at a distance' was Einstein's critique of quantum mechanics' non-local implications.

Quantum mechanics describes probabilities of outcomes rather than deterministic certainties, challenging classical physics.

The wave-function 'collapse' is an instantaneous update of quantum states, suggesting non-locality in quantum mechanics.

Entanglement in quantum mechanics creates non-local correlations between particles, regardless of distance.

Bell's theorem explores the concept of local causality and its violation in quantum mechanics, indicating non-locality.

Experiments have shown violations of Bell's inequalities, supporting the non-local nature of quantum mechanics.

Hidden variables theories propose that underlying, yet unknown, variables determine quantum outcomes, potentially restoring locality.

Measurement independence is a key assumption in hidden variables theories, the violation of which could explain quantum non-locality.

The concept of 'superdeterminism' emerges as a local hidden variables model that violates measurement independence.

Quantum mechanics does not allow for faster-than-light communication or signaling, as dictated by the no-signalling theorem.

The video discusses the philosophical implications of quantum mechanics, including debates on free will and determinism.

The presenter offers a course on quantum mechanics for deeper understanding, suggesting the complexity and depth of the subject.

Transcripts

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Related Tags

Quantum Mechanics Non-Locality Einstein Entanglement Wave-Function Bell's Theorem Physics Nobel Space-Time Local Realism Superdeterminism