What is Quantum Non-Locality?

In order to properly review Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics by Tim Maudlin, I need to introduce you to a concept called nonlocality.  

Nonlocality is one of the three most mind-blowing concepts I have ever contemplated. It frequently keeps me up at night, wondering what it really means.

In a previous discussion about the theory of relativity, I noted how difficult it would be for a group of intelligent snails living in a jungle to understand centrifugal force, which is the force we feel as we slide sideways in our seats when we take curved highway exits at high speeds. 

At the slow speeds that snails walk, they simply cannot generate enough momentum to experience centrifugal force. And thus it would be very difficult for them to consider such a force “real.”

Snail, Shell, Mollusk, Probe, Mucus, Crawl, Slowly

Humans face a similar problem when it comes to the concepts of relativity and nonlocality, which manifest themselves only incredibly fast speeds (relativity) and tiny microscopic levels (nonlocality).

Just like snails are too slow to experience centrifugal force, we are too big to experience nonlocality in ways that are readily noticeable.

A good way to explain nonlocality is to draw a contrast with its mirror opposite. The concept of locality means that everything we interact with has a cause and effect that involves a force transmitted through physical space according to the known laws of nature. 

In other words, when locality is at work, any cause you can think of must be physically connected to whatever it affects, be that by physical particle or electromagnetic wave.

For example, if you roll a bowling ball and it hits some pins we can clearly trace the causal chain of events: You used your muscles and your hand to propel a ball that rolled along the floor until it hit the pins. If your bowling alley was one mile long and sloped slightly downhill it would take your ball a few minutes to roll all the way there. But so long as you rolled the ball straight enough to avoid the gutters, it would eventually knock over the pins. 

In such a situation your bowling ball cannot physically cause the pins to fall over until the ball arrives and touches the pins. The force that knocks over the pins takes the form of the momentum that is carried by the ball from you to the pins. There needs to be physical contact between the ball and the pins to transfer that force. (We calculate the force using your old friend, the formula of force = mass x acceleration.)

Bowling, Bowler, Pins, Ball, Alley, Sport, Fun

So, how could we possibly explain what happened if you rolled the ball and then half a second later the pins fell over — several minutes before the rolling ball arrived? What if the pins fell down while the ball was still rolling?  The answer would surely have to be that some other force acted locally on the pins, such as a gust of wind or vibrations in the floor.  

Perhaps the pins were precariously balanced and it was the hard bang of the ball hitting the floor that caused a vibration wave to propagate through the wood and knock the pins over. Whatever explanation we come up with must include some type of force or particle traveling through space. All of this is another way of saying that everything we have ever observed in our lives has obeyed the principle of locality.

If we shift to using beams of light in a thought experiment, the same principle applies. If we shine an unimaginably powerful laser from Earth onto the surface of the Moon it would take 2.54 seconds for the laser light to reach the Moon, and bounce back into our eyeballs. We are causing a spot on the moon to illuminate via the mechanism of light physically traveling from here to there. 

But, how could we possibly explain what happened if a red dot appears on the moon less than a half a second after we shined our laser, instead of more than two seconds?  By definition light cannot travel faster than the speed of light. 

Illustration of Moon Showing during Sunset

The answer is that we would hunt for alternative explanations.

A possibility could be that someone is playing a trick on us by shining their own powerful laser at the same spot right before we turn on ours.  But there would be ways to test whether that is happening. For example, we could turn our laser on at random intervals, which would be hard for a trickster to anticipate. Or we could modulate our laser to send short pulses of light that could be translated into the dots and dashes of Morse Code.  

But what if our message seemed to reflect back from the Moon’s surface faster than allowed by the speed of light?

At that point we should probably not assume that we had somehow violated the speed of light which is a known constant, scientifically validated over and over and over. The speed of light is the well known C in the famous equation E=MC2 that makes possible our understanding of nuclear power and weapons and the structure of space-time itself.

As we searched for explanations we might suspect that a trickster guessed the Morse Code message we would send, and he would send it first. Perhaps he hacked our computer so that as soon as we put in our message, the computer would transmit our message to him first which would allow him to transmit two seconds before we did.  

If any of these explanations were true they would be called locality-based explanations because they all expose how the trickster found a way to send his message first. 

At some point, however, if we eliminated every possible explanation we would need to start considering seemingly impossible explanations. Maybe our trickster built a tiny satellite that looked exactly like the Moon, and was in orbit only 1,200 miles above the Earth as opposed to 240,000 miles away? That could partially explain things.

But even if we can’t figure out how it is happening, we continue to be confident that there has to be some reasonable explanation.  Because there is just no way that we could be violating the speed of light. 


This image has an empty alt attribute; its file name is road-4088226_1280.jpg

Well. This is where we get to the idea of nonlocality.

Scientists have discovered that if we take a pair of tiny particles, such as photons, and we entangle them by shining them through a certain type of crystal, those two particles can develop a connection that transcends space and time in ways that allows the particules to communicate instantaneously no matter how far apart they are.

When I say instantaneously, I mean instantaneously in ways that are clearly in conflict with the speed of light, and are unexplainable by classical physics. For the first time, we seemed to have evidence of truly nonlocal behavior in which cause and effect were not connected by a known force traveling through known space. 

When this concept first came to light, Albert Einstein could not accept that quantum physics could involve nonlocality. Einstein derisively called this phenomenon “spooky action at a distance” and set out to prove that there must be some yet-undiscovered local explanation for a seemingly nonlocal phenomenon. 

Albert Einstein, Portrait, Theoretician Physician

For example, if you and I are criminal collaborators who get get separated, we could agree in advance to stick to a certain story such that if we were concurrently interrogated by different detectives we would give the same answers. 

in 1935, Einstein and two of his peers (Podolsky and Rosen) published a thought experiment titled “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” In this famous paper they argued that the claim by quantum physicists of nonlocality being a real thing . . . . must be incomplete. Einstein and his pals used numerous fancy words but their point was unmistakably simple: We may not know what is missing, but you people are definitely missing something important. Your nonlocality theory is bananas.

For decades, physicists and philosophers argued about what the quantum physicists could possibly be missing. Nobody could think of anything. Many physicists felt in their gut that Einstein was right, even as experimental evidence mounted that he was wrong.  

After all, nothing travels faster than light, and it takes at least three minutes for light to travel from Earth to Mars. So, it can’t possibly be true that if you do push a button on Earth it could cause anything to happen instantaneously on Mars. Because there is no known physical mechanism known that would allow for the transmission of that signal.

In 1964, John Stewart Bell wrote a famous paper that addressed these issues with more rigor than anyone before. He set out to investigate whether there was any way to solve the nonlocality problem by finding hidden variables such as particles somehow “agreeing” in advance to show certain behaviors under questioning by scientist detectives in laboratories distant from each other.

Bell showed that it was possible to create situations in which no possible “hidden variables” could resolve the paradox. The explanation is technical but Bell’s logic was airtight: Nonlocality is somehow real, even if it makes us uncomfortable and we do not understand how it works. 

Cube, Escher, Gradient, Mc Escher, Optical Illusion

Over the following decades physicists and philosophers have continued to ask themselves what the hell is going on and how can we possibly understand it?  They have performed thousands of experiments with increasingly powerful and sophisticated equipment. They have discussed and debated and hypothesized. And still they are not sure what the hell is going on, though they are getting closer.

In Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics, author Tim Maudlin takes us through an investigation that is unbelievably rigorous. He takes us through an incredibly careful re-telling of history, re-explaining relativity, the structure of space-time, Euclidian space, invariant quantities, Newtonian Space-time, Galilean Space-time, special relativity, and the fascinating concept of Lorentz Transformations that explain how objects physically change shape as they approach the speed of light.  

Maudlin then spends chapter after chapter exploring in great detail every possible mechanism that could explain away nonlocality. Superliminal matter transport? Tachyons? Secret messages? Lorentz Invariant Collapse Theories? Preferred sets of hyperplanes? Non-Minkowski Space-time?  Check, check, check, check, check, check, and no, no, no, no, no no. 

Interestingly, Maudlin is not a physicist, but rather a philosopher. His book is exceedingly difficult and there is plenty of math. But you needn’t actually understand vector spaces, eigenvalues, and orthnormal bases in order to follow most of his reasoning.

Quantum Non-Locality and Relativity: Metaphysical ...

Across his chapters Maudlin presents one theory after another and discusses the strengths and weaknesses of each theory. One thing that he makes clear, however, is that you cannot have your cake and eat it too. He explores both determinate theories and stochastic theories that each appear to have something going for them — and also appear to have fatal flaws. He leaves it up to you to decide which fundamental truths you will cling to, while making it clear that at least one thing you believe is almost certainly completely wrong. He just does not know which one.

Reading this book made me realize that at least one of several mind-blowing possibilities is highly like to be true:

First, some microscopic particles may be able to move briefly backwards in time via “closed timelike curves” that allow them to interfere with themselves. If this is true it could beautifully explain the mysterious wave-particle duality experiments in which particles appear to interfere with themselves to create interference patterns. 

Second, spacetime could be far more complex than it appears. There may be additional physical dimensions that we have yet to discover, and it may be possible that particles that seem physically distant from each other are in fact “touching” via a dimension that we cannot yet see or understand.

Third, the “many-worlds interpretation” may be correct. I hate this idea so much that I will not even describe it.

Fourth, we could be living in a simulation, and the concept of nonlocality could be programmed into our operating system.

Fifth, the “flashy GRW” theory described near the end of the book could be correct. Unfortunately I do not understand it well enough to summarize it here.

Wave, Particles, Physics, Abstract

As Maudlin says at the end of his book:

We are in a methodological quandary. It is hard to imagine a neutral methodological principle that could militate in favor of retaining a pre-existing theory of space-time at all costs, while allowing for the abandonment of an equally entrenched pre-existing theory of space-time at all costs, while allowing for the abandonment of an equally entrenched pre-existing account of the local distribution of matter at the scale which we think is probed by microscopes. The microscopic distribution of matter is not open to our direct inspection, but neither is the structure of space-time at any scale. And if you accept a many-worlds picture, then even the local macroscopic distribution of matter is not open to immediate inspection: most of the matter in any given region is invisible to us. So all of our options — adding a foliation, flashy GRW, many-worlds interpretation that denies unique outcomes — one way or another postulates a physical world that shields itself from our view. Each one asserts that the natural conclusions of what seem to be straightforward scientific investigations somehow go radically awry.  One way or another, the world is not at all what it appears to be.

Overall, this book is as difficult as it is interesting. The concepts it discusses are fascinatingly thought-provoking. I expect to read it several more times and still not understand parts. Until then I will hope that Maudlin eventually writes a fourth edition that includes new chapters to describe what our theoretical and experimental physicists are learning right now.

Until then, I will not rest easy, because I am burning with desire to understand how it can be that a particle on Mars can communicate instantly with a particle on Earth.

  • Bart Epstein, June 2020

3 thoughts on “What is Quantum Non-Locality?

Add yours

  1. Dear Bart, I think your description of the situation of entanglement is incorrect. There is *no* faster than light communication, as no information is passed between the two parties at the moment of measurement. When you measure your side here on Earth, you only know what *you* have measured. Your friends on Mars also only know what *they* have measured. The information about the results of any of the two measurements can arrive to the other side only at the speed of light or slower. In other words, you and your Martian friends cannot compare results
    before you have communicated. And it is only after you compred results that the information of the correlation between the parties arrives at both ends. It is important to clarify that the state of correlation between the two parties is *not* created when their state is measured on Earth or on Mars. It was created *before* that, when the two parties were close enough to interact (for example, in that nonlinear crystal you mentioned).
    So there is no instantaneous or faster-than-light communications here. Do you agree?


    1. Dear Eilon, my description is correct but you are right as well to the extent you are pointing out that we cannot use entanglement to communicate faster than light travels. It is important to understand that when two particles become entangled, and are then separated by a great distance, they somehow retain an ability to *influence* each other instantaneously — in ways that are yet unknown to us. In the example of an entangled particle being measured on Mars, at the moment that someone causes that particle to have a certain orientation the other particle back on Earth instantly responds as well. It is crucial to understand that the particles did not “agree” how they would behave when they parted ways. The author of this book does a great job explaining every possible “hidden variable” that could explain how the two particles are “not really” communicating instantaneously. This is what Bell’s inequality is all about. He showed that “the only way that hidden variables could explain the predictions of quantum physics is if they are “nonlocal” and later explained that “If [a hidden-variable theory] is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local.” There is currently no known way for humans or machines to send any information through whatever mechanism allows entangled particles to seemingly communicate with each other. And I say seemingly because while one possibility is that there is communication happening (such as via another dimension that we can’t access), other possibilities include the many worlds interpretation and various wave function theories.


      1. After the particles have been entangled (by interaction, when they were close enough), there is no communication between them at all. There is no need for that, as they are already a part if a single quantum state. All the infornation is already there. However, we can access it only by nonlocal correlation measurements. This is where the particles *seem* to influence one another. But they don’t. It is just the way we get the information about their common quantum state. So no need for extra dimensions here.


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