Quantum information theorists use Einstein’s Principle to solve “Einstein’s quantum riddle”

Quantum information theorists use Einstein’s Principle to solve “Einstein’s quantum riddle”

Albert Einstein, Boris Podolsky, and Nathan Rosen introduced the mystery of quantum entanglement (entanglement) in 1935 and it has been called “Einstein’s quantum riddle.” Many physicists and philosophers in foundations of quantum mechanics (foundations) have proposed solutions to Einstein’s quantum riddle, but no solution has received consensus support, which has led some to call entanglement “the greatest mystery in physics.” There is good reason for this 90-year morass, but there is also good reason to believe that a recent solution using quantum information theory will end it in ironic fashion.

Simply put, entanglement is one way that quantum particles produce correlated measurement outcomes. For example, when you measure an electron’s spin in any direction of space you get one of two outcomes, i.e. spin “up” or spin “down” relative to that direction. When two electrons are entangled with respect to spin and you measure those spins in the same direction, you get correlated outcomes, e.g. if one electron has spin “up” in that direction, then the other electron will have spin “down” in that direction. Einstein believed this was simply the result of the electrons having opposite spins when they were emitted from the same source, so this was not mysterious. For example, if I put two gloves from the same pair into two boxes and have two different people open the boxes to “measure” their handedness, one person will find a left-hand glove and the other person will find a right-hand glove. No mystery there. The alternative (which some in foundations believe) is that the electron spin is not determined until it is measured. That would be like saying each glove isn’t a right-hand or left-hand glove until its box is opened. No one believes that about gloves! So, Einstein argued, if you believe that about electron spin, then explain how each electron of the entangled pair produces a spin outcome at measurement such that the electrons always give opposite results in the same direction. What if those electrons were millions of miles apart? How would they signal each other instantly over such a great distance to coordinate their outcomes? Einstein derided that as “spooky actions at a distance” and instead believed the spin of an electron is an objective fact like the handedness of a glove. No one knew how to test Einstein’s belief until nine years after his death, when John Bell showed how it could be done.

In 1964, Bell published a paper that tells us if you measure the entangled electron spins in the same direction, you can’t discern if Einstein was right or “spooky actions” was right. But if you measure the spins in certain different directions, then quantum mechanics predicts correlation rates that differ from Einstein’s prediction. In 1972, John Clauser (with Stuart Freedman) carried out Bell’s proposed experiment and discovered that quantum mechanics was right. Apparently, “spooky actions at a distance” is a fact about reality. Later, Alain Aspect and Anton Zeilinger produced improved versions of the experiment and, in 2022, the three shared the Nobel Prize in Physics for their work.

Given these facts, you might think that the issue is settled—quantum mechanics is simply telling us that reality is “nonlocal” (contains “spooky actions at a distance”), so what’s the problem? The problem is that if instantaneous signaling (nonlocality) exists, then you can show that reality harbors a preferred reference frame. This is at odds with the relativity principle, i.e. the laws of physics are the same in all inertial reference frames (no preferred reference frame), which lies at the heart of Einstein’s theory of special relativity. In 1600, Galileo used the relativity principle to argue against the reigning belief that Earth is the center of the universe, thereby occupying a preferred reference frame, and, in 1687, Newton used Galileo’s argument to produce his laws of motion.

Physicists loathe the idea of abandoning the relativity principle and returning to a view of reality like that of geocentricism. So in order to save locality, some in foundations have proposed violations of statistical independence instead, e.g. causes from the future with effects in the present (retrocausality) or causal mechanisms that control how experimentalists choose measurement settings (superdeterminism). But most physicists believe that giving up statistical independence means giving up empirical science as we know it; consequently, there is no consensus solution to Einstein’s quantum riddle. Do we simply have to accept that reality is nonlocal or retrocausal or superdeterministic? Contrary to what appears to be the case, the answer is “no” and the alternative is quite ironic.

The solutions that violate locality or statistical independence assume that reality must be understood via causal mechanisms (“constructive efforts,” per Einstein). This is the exact same bias that led physicists to propose the preferred reference frame of the luminiferous ether in the late nineteenth century to explain the shocking fact that everyone measures the same value for the speed of light c, regardless of their different motions relative to the source. Trying to explain that experimental fact constructively led to a morass, much like today, in foundations and here is where the irony begins—Einstein abandoned his “constructive efforts” to solve that mystery in “principle” fashion. That is, instead of abandoning the relativity principle to explain the observer-independence of c constructively with the ether, he doubled down on the relativity principle. He said the observer-independence of c must be true because of the relativity principle! The argument is simple: Maxwell’s equations predict the value of c, so the relativity principle says c must have the same value in all inertial reference frames to include those in uniform relative motion. He then used the observer-independence of c to derive his theory of special relativity. Today, we still have no constructive alternative to this principle solution to the mystery of the observer-independence of c.

The next step in the ironic solution occurred when quantum information theorists abandoned “constructive efforts” in the exact same way to produce a principle account of quantum mechanics. In the quantum reconstruction program, quantum information theorists showed how quantum mechanics can be derived from an empirical fact called Information Invariance and Continuity, just like Einstein showed that special relativity can be derived from the empirical fact of the observer-independence of c. The ironic solution was completed when we showed how Information Invariance and Continuity entails the observer-independence of h (another constant of nature called Planck’s constant), regardless of the measurement direction relative to the source. Since h is a constant of nature per Planck’s radiation law, the relativity principle says it must be the same in all inertial reference frames to include those related by rotations in space. So, quantum information theorists have solved Einstein’s quantum riddle without invoking nonlocality, retrocausality, or superdeterminism by using Einstein’s beloved relativity principle to justify the observer-independence of h, just as Einstein did for the observer-independence of c.

Feature image credit: Jian Fan on iStock.

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