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local realism

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[This article is under construction.]

Local realism is a quick way of saying two principles: 1) Principle of locality: the cause of a physical change must be local. That is, a thing is changed only if it is touched, and 2) Principle of realism: Properties of objects are real and exist in our physical universe independent of our minds. In other words, we live in an objective reality, not one which exists only in our minds or which takes form only upon our looking at it. These two principles are described more fully below.

While classical physics operates in accordance with the principles of local realism, quantum mechanics appears not to. Experimental evidence increasingly indicates that we do not live in a universe characterized by local realism.


In everyday life, we assume the principle of locality. We assume that for an object or energy to cause an effect on another, the two must touch. In physicist’s terms, “touching” means “interacting.” We assume that a force causes an effect solely by traveling through spacetime and, then, interacting with it. Most of us don’t think that, in the physical universe, a fairy godmother can wave a magic wand in the air and transform a pumpkin standing ten feet away into a magnificent golden coach. Instead, it is necessary

Cinderella's coach
When a pumpkin was transformed into Cinderella’s coach, local realism  was violated. [Image source: By Source, Fair use, https://en.wikipedia.org/w/index.php?curid=10025782]
to create the metal frame of the coach by applying heat directly to gold ingots, melting the ingots, and pouring the molten gold into molds. Then, after they’ve cooled, the gold pieces of the frame must be bolted together with… I guess, bolts. At each step, a great deal of touching or interacting occurs while building the coach.

smelting gold
Melting gold to form an ingot. [Image source: By Allen Drebert – Allen Drebert Family Photos 1957-1961; Public Domain, https://commons.wikimedia.org/w/index.php?curid=4749873]
Even magnets and electronics, which can operate across great distances, operate locally. Magnetic and electrical signals radiate outward from a source, for example, a radio broadcasting station. The two, the magnetic signals and the electrical signals, travel together as electromagnetic waves.* They must reach and interact with a receiver, let’s say a radio antenna, before they can create their effects. Then, we can listen—on a radio wired to the antenna—to the effects created on the antenna. The image below shows this system.

According to Einstein’s Theory of Special Relativity, electromagnetic waves travel through the vacuum of space at the speed of light. This is about 186,000 miles per second. In other media, electromagnetic waves may travel slower than the speed of light, but never faster. Never, for example, instantaneously.



radio broadcasting
Radio broadcasting. Sound is transformed into electromagnetic waves (EM). EM travels and is received by a radio receiver, which transforms the EM back into sound. [Image source: http://mscteched.weebly.com/radio-broadcasting.html]
Nor, according to Special Relativity, can any matter or energy travel faster than the speed of light. The implication for the principle of locality is that no effect can be created on something faster than the speed of light. The fairy godmother cannot wave her magic wand and, instantaneously, the pumpkin turns into a coach.

Sometimes, people shorten up the principle of locality by saying that it means that no energy or matter can cross a distance to another energy or matter and affect it at a speed faster than the speed of light. This is also sometimes called “relativistic locality.”

Action-at-a-Distance Discarded (Other Than in Quantum Physics)

Gravity as understood in General Relativity. Objects with mass curve spacetime. Curved spacetime spacetime tells objects how to move. Curved spacetime makes it look as if objects with mass are attracted to each other when, actually, they are moving along the shortest path in curved spacetime. This is a 2-dimensional image, but gravity operates in 4 dimensions–3 of space and 1 of time. So, the image is a metaphor, not a realistic representation. [Image source: By NASA – http://www.nasa.gov/mission_pages/gpb/gpb_012.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4072432]
Wait a minute—what about gravity? Doesn’t gravity work as action-at-a-distance? According to Isaac Newton and probably even your grade school science teacher, yes. According to these sources, gravity is the attraction through empty space of one body having mass on another. Newton wasn’t happy with the theory that he had created—action-at-a-distance. He called it an absurd idea.

Newton wrote in a letter in 1692/93 **: “That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.

But Newton’s mathematical laws seemed to support this “Absurdity.” And the theory is close enough to empirical experience that it has very practical uses in engineering. So, the mass-attracts-mass theory stuck. It is this Newtonian understanding of gravity that is relied on for most modern engineering projects, including the moon shots of the 20th Century.

However, many decades ago, among physicists, Einstein’s Theory of General Relativity replaced Newton’s action-at-a-distance explanation of gravity. General Relativity was accepted by theoretical physicists soon after Einstein proposed it in 1915. This is not due to Einstein’s reputation but because his theory predicts empirical results more accurately than does Newton’s action-at-a-distance. For example, the equations of General Relativity describe the orbit of Mercury around the sun accurately, whereas Newton’s gravitational equations to not.

However, the mathematical difference between calculations using Newton’s action-at-a-distance theory of gravity and using General Relativity diverge significantly only in very limited situations. These include, for example, when dealing with extremely large masses such as the masses of the planet Mercury and the sun. Nevertheless, according to physicists, objects do not exert an instantaneous gravitic force on each other through empty space. There must be touching—that is, interaction. In the case of General Relativity, objects with mass touch spacetime, itself.

In quantum mechanics, locality is problematic. 

Describing quantum mechanics is complicated by there being so many interpretations of it. The description which follows sees quantum mechanics through the eyes of the Transactional Interpretation.****

While macroscopic objects like tables and chairs are subject to the principle of locality, quantum physics raises severe issues regarding locality. The experimental evidence for quantum entanglement requires that we reject the assumptions of, at least one of the following:

  • Locality,
  • Realism,
  • Free will on the part of the experimenter,
  • Or a combination of these.

quantum entanglement
[Image Source: NASA/JPL/Cal Tech]
Given experimental results demonstrating quantum entanglement, it’s not possible that our universe is characterized by all three: locality, realism, and the freedom of the experimenter to choose which experiment to conduct. This is because two entangled particles correlate their behavior instantaneously even if they might be located across the universe from each other. This coordination seems to be a violation of the speed of light, which is the maximum speed possible for matter or energy according to Einstein’s Theory of Special Relativity. So, one of the above three assumptions, all of which underlie Special Relativity, must be inapplicable in quantum mechanics.

When, in the 1930’s, this violation became apparent in the mathematics of quantum mechanics, Albert Einstein concluded that quantum mechanics must be incomplete. As he saw it, a principle that would reconcile quantum mechanics with Special Relativity was missing. In 1935, he, along with colleagues, published the famous EPR paper, outlining the “paradox” of the non-locality of quantum mechanics. This paper is among the most cited physics journal articles of all time.

However, experiments conducted by physicist, John Clauser, and later by Alain Aspect and other physicists, have shown that the coordination between entangled particles is real. It occurs faster than light and, possibly, instantaneously. Mathematically, entangled particles are described as parts of the same equation, not as if they are two different particles interacting. And this is how they seem to behave in experiments, regardless of their distance from each other. They seem to act as though they are part of a single system, one described by a single equation. For more information, see the article on entanglement.

However, there are possible loopholes regarding locality as described below. For a time, physicists thought quantum physics might squeeze through one of these loopholes: a lack of realism or the experimenter’s lack of freedom to choose his/her experiment. The issue of realism is addressed below as is, briefly, the issue of Free Will.

Quantum mechanics may violate realism. 

The physics of macroscopic objects, such as tables and chairs, assumes an objective reality exists external to us. This is the principle of realism. As Einstein put it, “I like to think that the moon is there even if I’m not looking at it.” Objects which accord with the principle of realism have defined properties independent of our measurements. A macroscopic object, for example, a chair sitting in Carnegie Hall, has an exact position. We can confirm that position by measuring its distance from the walls and the floor. But we assume that before we measured it, its position was definite and completely defined. The chair’s position is not a range of numbers, each having a certain probability of actualizing at the moment of measurement. No—leave that kind of behavior to atomic and subatomic particles.

The position of an atomic or subatomic particle is a cloud of possibilities until the particle interacts with an aspect of the physical universe. For example, it might hit the screen of a particle detector. Hitting the screen, an interaction with a part of the physical universe, is what physicists mean by “observation, also called “measurement.” Some physicists and philosophers have held that measurement or observation must involve human consciousness,*** but at this time, this is not a widely-held view.

Upon measurement, the particle instantaneously adopts a particular position. Physicists can use the math of quantum mechanics to predict the likelihood of any particular position but they cannot predict a definite position. Nor did a definite position even exist prior to measurement. In other words, atomic and subatomic particles do not behave in accordance with the principle of realism. Experiments such as the Double Slit Experiment provide empirical evidence of the lack of realism of atomic and subatomic particles. For more on this topic, see the definition of measurement.

Local Realism

In everyday life, most of us assume:

  • Locality: Causes must travel through spacetime at the speed of light or slower to affect other energies or objects, and
  • Realism: The universe exists external to our minds and exists whether or not we observe or measure it.

That is, we assume local realism. We, like Einstein, would like to think that the moon exists whether or not we’re looking at it. And we like to think that a photon cannot instantaneously coordinate its behavior with that of another photon that is across universe from it. However, quantum mechanics experiments do not support the assumption of local realism.

Experimental Evidence in Quantum Mechanics Regarding Local Realism

When Albert Einstein wrote the EPR paper, the debate about local realism was philosophical. It was not possible to test for local realism experimentally. However, in 1964, an Irish physicist at CERN, John Bell, published a paper showing that such an experimental test is possible. He proposed what is now called “Bell’s Theorem.” This is a mathematical theorem that the results of quantum entanglement experiments will vary depending on whether or not local realism holds. In other words, if local realism is true, quantum entanglement experiments will yield one specified set of results. If local realism is not true, the same experiments will yield a different specified set of results.

In 1972, John Clauser, a physicist at the University of California at Berkeley along with colleagues, ran experiments on Bell’s Theorem. These experiments were done on entangled photons. Clauser found, as did later experimenters, notably French physicist, Alain Aspect, and Viennese physicist, Anton Zelinger, that local realism does not hold for entangled photons. That is, either locality, realism, or both are false assumptions. Clauser, Aspect, and Zeilinger won the 2010 Wolf Prize in physics for this work.

Bell’s Theorem and Free Will

However, there is a loophole through which Nature could preserve local realism. Local realism could still be a true assumption if physicists are not free to choose their experiments, that is, if they don’t have Free Will. Let’s say that the universe is governed by Superdeterminism. Under the assumption of superdeterminism, scientists and everyone else and everything else are no more than puppets whose every move move is determined. In this situation, they might think that they are deciding to set a light polarizer to measure vertical polarization rather than horizontal polarization. However, they are deceived by Nature, who has actually determined in advance that the vertical setting should be selected. And, importantly, Nature also determines the results of the experiment. Possibly, the laws of Nature work such that local realism is true but other laws of Nature obscure this when doing certain types of experiments. If Nature is pulling all the strings, scientists would have no hope of learning the truth—all their actions when conducting experiments are being choreographed. This is superdeterminism. Superdeterminism would defeat the scientific enterprise. We could never know if we are moving closer to the truth or only going through pre-programmed motions with pre-programmed outcomes.

John Bell described the Superdeterminism loophole this way:

“There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the “decision” by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster than light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already “knows” what that measurement, and its outcome, will be.” *****

Were superdeterminism to hold, no experiment could tell us if our universe is characterized by local realism or not. Superdeterminism may seem like an extreme assumption to propose, but throwing out local realism is also an extreme step.


* Isaac Newton, Letters to Bentley, 1692/93 as quoted in the Wikipedia.org article on action at a distance.

*This understanding of electromagnetism as a wave traveling through a field was first developed by Michael Faraday in the mid-1800’s.

** Known loopholes for the violation of local realism in quantum mechanics entanglement were ruled out by an experiment performed in 2015 at the Colorado National Institute of Standards and Technology in coordination with the Jet Propulsion Laboratory, https://www.nist.gov/news-events/news/2015/11/nist-team-proves-spooky-action-distance-really-real. The experiment found that entangled pairs of photons correlated their polarization and were able to rule out the “freedom of choice” loophole, hidden variables, and signaling between the two particles at light speed or slower. Later experiments have confirmed these results.

***One of the founding fathers of quantum mechanics, Eugene Wigner, held the view that human consciousness was required for a measurement to occur. But he later rejected it. Interpretations such as Information Theory and the Transactional Interpretation explain quantum mechanical phenomena without invoking human consciousness. But quantum mechanical phenomena draw many towards the view that we live in a simulation and that consciousness is required for the appreciation of the simulation.

**** Ruth E. Kastner, Understanding Our Unseen Reality, Solving Quantum Riddles, Imperial College Press, London, 2015.

*****Interview with John Bell by Paul Davies, BBC Radio, 1985 as quoted at https://en.wikipedia.org/wiki/Superdeterminism#cite_note-2.