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The term “entanglement,” used to refer to the correlation of dynamical properties among two or more quantum particles, was coined by physicist Erwin Schrödinger nearly a century ago. Yet the first Nobel Prize awarded to scientists who studied the phenomenon was only awarded in 2022. Why did it take so long for progress on this phenomenon to be made? It has to do with a long-running argument between Albert Einstein and Neils Bohr, which Einstein ultimately lost, as reported by a new article in Nature.
The discovery that microscopic particles (such as atoms, electrons, protons, and photons) do not follow the same physical laws as macroscopic objects (such as baseballs, cars and planets) required the creation of a whole new theory to describe such particles, which we call quantum mechanics. From the point of view of experimental physics, the fundamental difference between microscopic particles and ordinary-sized objects is that, while it’s possible to make measurements on ordinary objects throughout a particular experiment—for example, measuring changes in velocity for the trajectory of a baseball—experiments on microscopic particles are necessarily statistical. It’s necessary to make many measurements over a large number of identically prepared particles, and to analyze the distribution of outcomes that arise from such measurements, to be able to make some amount of sense regarding the process being studied. It’s not possible to make measurements on the particles along the way because the process of interim measurements will destroy the common initial state in which all the particles have been prepared. Because the particles are so small and light, any method of measurement will necessarily irreversibly disturb the particle from its original path.
As such, the theory of quantum mechanics allows the calculation of average values and distributions of potential measured values for particular dynamical quantities (such as energy or momentum), but there is no way to predict with any certainty, say, the velocity of a single particle at a particular time or place during the experiment. One can only infer from the distribution a probability that a particle had this or that value for its velocity.
It’s important to understand that quantum probability distributions are not like probability distributions that come out of systems of particles governed by classical mechanics. An example of the latter sort of system is the probability distributions of molecules in a sample of gas at equilibrium. (Though the gas molecules are indeed all microscopic particles, the kinetic energies of the molecules are high enough, relative to the energies of the corresponding quantum states, that the classical description of their motion correctly describes such a system.) Each molecule in the gas sample has a particular and precise kinetic energy and velocity at any particular time, and techniques exist by which they can be measured. The probability distribution simply arises by applying statistics to the system of classical particles (the molecules) in how the total energy available within this system at a particular temperature is distributed among the particles.
In a quantum system, the probability distribution does not arise by randomly distributing energy among an ensemble of particles. The distribution arises from the wavefunction of a single particle (more precisely, the absolute square of the wavefunction), and the wavefunction is the solution for the Schrödinger equation of the particle. Put another way, the wave function arises from the particular dynamics of a single particle, which means that the probability distributions that arise from quantum mechanics apply to each particle individually, not to the collection of particles with a distribution of energies (as is the case with a classical distribution). If we were to try to assign particular, precise values for energy and momentum to each quantum particle, that would radically change the dynamics, resulting in a quantum distribution that differs radically from what is observed in experiment.
Other than knowing that a series of measurements of a dynamical property of a series of identical particles prepared in an identical fashion will reflect a probability distribution predicted by quantum mechanics, there is no way in the theory to even conceive that an individual particle has one particular value for, say, its momentum as it approaches the detector.
Danish theoretical physicist Niels Bohr seized on this behavior as a foundation to his interpretation of quantum mechanics, called the Copenhagen interpretation. What answer you get from an experiment depends on what question you ask. For example, regarding wave-particle duality, if you performed an experiment on an electron to test it’s particle properties, it would behave like a particle; but if you performed an experiment to test the electron’s wave properties, it would behave like a wave. Bohr went so far as to say that it was a meaningless question to ask what the value of, say, the velocity of a quantum particle is before actual measurement, because it could literally be anything. Bohr had such authority in the realm of quantum physics that his interpretation held sway in the field for many decades.
Albert Einstein, who himself made crucial contributions to quantum theory, had no patience for Bohr’s interpretation. He believed that if a particle exists, regardless of its size or mass, if it was moving from one place to another, it had to have a velocity, and that velocity had to be measurable, even within the restrictions of the uncertainty principle and wave-particle duality. Further, Einstein recognized that Bohr’s Copenhagen Interpretation was a dogma that forbid asking certain questions. Bohr did so because the quantum theory as it had been devised couldn’t answer them, yet he believed that the theory as it existed was complete. Under the Copenhagen interpretation, any uncomfortable questions would be met with the response “Shut up and calculate!” The answers that came out of the calculations matched observations, so the theory must be right. Einstein, in contrast, felt that quantum theory’s inability to answer such questions meant that the theory was not complete, and that an effort had to be made to find a theory that could do so. Bohr and Einstein would have conversations regarding their disagreement, but they mostly talked past each other.
Then Einstein (with coauthors Boris Podolsky and Nathan Rosen), in his continuing effort to demonstrate that quantum mechanics was an incomplete theory, wrote a paper on the topic on quantum entanglement. The thought experiment they used to describe the conceptual problems that arise is an awkward one to describe. Some years later, the American physicist David Bohm recast the problem in a much more understandable way, so everyone uses Bohm’s example to describe the problem. I’ll try my best to present an example of entanglement here:
Imagine a diatomic chlorine molecule, Cl2. All its electrons are paired, and imagine it’s not rotating, so its angular momentum is zero. Then the molecule absorbs a passing photon with sufficient energy to break the bond between the chlorine atoms, and the atoms fly apart. Now, each of the electrons has an unpaired electron, each having a component of spin angular momentum with a magnitude of ½. Because the molecule started with no angular momentum, and angular momentum is conserved, the total spins angular momentum for the two atoms taken together must still be zero, meaning that one of the atom’s unpaired electron must have spin +½ (spin “up”), and the unpaired electron on the other atom must have spin -½ (spin “down”). If this were a process described by classical mechanics, it would be clear from the outset which of the atoms was “spin-up” and which was “spin-down.” In quantum mechanics, we can’t know which atom has which spin until one of the spins is actually measured, so the wavefunction must include both possibilities, such that one half of the wavefunction has the atom on the left with spin-up, while the one on the right has spin-down, and the other half has the atom on the left being spin-down, and the one on the right spin-up:
This is what is called a superposition state, since two different states coexist within the same system. However, when a measurement occurs, only one value is determined, meaning that only one state is observed to survive the measurement process, and it’s impossible to predict which it will be. These two atoms can travel away from each other for miles, even lightyears, and as long as nothing disturbs them, this superposition state will persist. This is an entangled system, where the spin states of two particles are correlated even though they can be separated by long distances.
What happens when the spin of just one of the chlorine atoms is measured? At that point, the superposition state is said to “collapse” to just one of the two states. Say we measure the spin of the unpaired electron on ClL, and it’s in the spin-down state. Within the Copenhagen Interpretation, what has happened is that the spins of both unpaired electrons are immediately specified, where beforehand, they were undeterminable. Conservation of angular momentum requires that the unpaired electrons have opposite spins, so if ClL is -½, that immediately specifies ClR to be +½, regardless of how far apart the two atoms are. The measurement of the spin of one of the electrons instantaneously specifies the spin of the other electron despite their potentially large separation. Einstein’s Law of Special Relativity permits information to be transferred between separated objects no faster than the speed of light, but the specification of the electron spins on the two atoms occurs instantaneously regardless of their separation. This fact best expresses why Einstein had a problem with quantum mechanics. Einstein used the word “spooky” to express his distain for how this process is depicted within the Copenhagen Interpretation.
At the time Einstein’s paper was published, it got little attention, and most physicists reacted with derision, that Einstein just wasn’t capable of understanding this new kind of physics. But Einstein’s disagreement with quantum mechanics was and remains both legitimate and relevant. A few physicists at the time (like Bohm) appreciated this fact. However, Einstein believed that there was no superposition state, and that the spins states of each electron were specified at the point the Cl2 molecule fell apart, which is also wrong. Unfortunately, the hegemony of the Copenhagen interpretation among physicists deterred a large majority of physicists to take Einstein’s objections seriously. Most physicists, lacking a philosophical bent, just didn’t care about this discussion at all. Again, it was “Shut up and calculate!”
It took another generation or two of physicists to challenge the Copenhagen interpretation. In the 1950s, American physicist Hugh Everett developed what came to be called the “many worlds” interpretation of quantum mechanics, such that all possible outcomes of a quantum phenomenon occur, but only one of these occur in our universe, while the rest occur in other universes we can’t observe. (So, in the above example, in some other universe, ClL was spin-up, and ClR was spin-updown, but because it happened in another universe, we couldn’t observe it. (Full disclosure: I’m not a fan of this interpretation for various reasons.) Louis De Broglie, one of the founders of quantum mechanics, developed what was called the “pilot wave” interpretation, later further developed by David Bohm. This interpretation leverages wave-particle duality to assert that, rather than a quantum entity behaving as either a particle or a wave, that the entity has, as components, both a particle and a wave, and that the wave guides the behavior of the particle. (Full disclosure: I think this interpretation holds promise.)
Irish physicist John Bell managed to make an experimental prediction regarding entanglement. In 1964, he managed to find conditions of measurement in Einstein’s thought experiment where classical and quantum theories made radically different predictions. The physicists Alain Aspect, John Clauser, and Anton Zellinger won the 2022 Nobel Prize in Physics by testing John Bell’s result, and finding that quantum mechanics made the correct prediction. (Bell no doubt would have shared in the prize if he had still been alive, but he died in 1990.)
(Apologies for the length.)
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