The EPR Paradox Failed to Discredit the Copenhagen Interpretation

FROM THE LECTURE SERIES: What Einstein Got Wrong

By Dan Hooper, Ph.D., University of Chicago

The consensus formed around the Copenhagen interpretation of quantum mechanics in the late-1920s, which claimed that quantum mechanics was a strictly probabilistic theory and not deterministic, deeply bothered Albert Einstein. He kept trying to poke holes in the Copenhagen interpretation of quantum mechanics. The philosophical position known as scientific realism was one such attempt, and the EPR paradox was another.

An illustration of an experiment being carried out to demonstrate the EPR paradox.
An illustration of the EPR paradox thought experiment, which is being performed here using pairs of electron-positron. (Image: Krishnavedala/Public domain)

What Is Scientific Realism?

Einstein was uncomfortable with the way a given particle could be in multiple places at once or how it could be moving with multiple speeds, all simultaneously, according to the Copenhagen interpretation. After years of debate and consideration, Einstein ultimately came to take and insist upon a philosophical position known as scientific realism to counter this.

As Einstein saw it, one is a scientific realist if they believe in the existence of a real and well-defined state of the world, and that the world exists independently of any observations that you might make of it. In other words, the world is a real and well-defined thing that exists independently of us. By observing it, we can learn things about the world but our observations don’t make the world what it is.

Einstein’s insistence on scientific realism fell in stark contrast to the Copenhagen interpretation of quantum mechanics. According to the Copenhagen interpretation, an electron could be in multiple places at one time but when an observation is made of an electron its wave function collapses, and it transforms into no longer being in multiple locations but instead being in only one. This interpretation was not compatible with Einstein’s ideas about the world or his adherence to scientific realism.

Learn more about Einstein’s Rejection of Black Holes.

Is the Theory of Quantum Mechanics Incomplete?

Even though Einstein didn’t agree with the Copenhagen interpretation, he had to accept that the predictions being made with the equations of quantum mechanics were in agreement with any number of laboratory measurements and tests. Quantum mechanics did not seem to be simply wrong.

Portrait of Albert Einstein in 1931.
A portrait of Albert Einstein from 1931, about four years before he published the EPR paper with Podolsky and Rosen. (Image: Doris Ulmann/Library of Congress Prints and Photographs Division)

So, he focused his efforts on trying to demonstrate that the theory of quantum mechanics was somehow incomplete. Einstein hoped that he would be able to find a more complete version of quantum mechanics that was deterministic, and that was compatible with scientific realism.

However, all the philosophical objections Einstein managed to raise against the Copenhagen interpretations were subjective at best and failed to persuade other physicists that the consensus view of quantum theory was incorrect or incomplete.

Einstein needed to expose a logical inconsistency or identify a major problem in the Copenhagen interpretation that could be recognized as a fatal flaw in order to convince his colleagues.

Learn more about the Quantum Entanglement.

Quantum Entanglement and the EPR Paradox

New York Times headline which says Einstein attacks Quantum Theory.
A headline in The New York Times issue dated 4 May, 1935, after the publication of the EPR paper. (Image: New York Times/Public domain)

For a number of years, Einstein had been thinking about groups of particles with wave functions that depend directly on one another. Today we refer to such wave functions as ‘entangled’, but this terminology hadn’t yet been coined in the late-1920s.

While Einstein hadn’t yet fully explored or understood the implications of quantum entanglement, he did recognize that quantum entanglement was an inevitable consequence of the Copenhagen interpretation of quantum mechanics. He also recognized that some particularly strange behavior could result from quantum entanglement.

In 1933, Einstein took up a position at Princeton’s Institute for Advanced Study, after fleeing Nazi Germany. There he worked with two other physicists, Boris Podolsky and Nathan Rosen. Over the next two years, they wrote an influential article entitled, “Can A Quantum-Mechanical Description of Physical Reality Be Considered Complete?” This paper contained the first description of what would become known as the EPR paradox or the Einstein-Podolsky-Rosen paradox.

This is a transcript from the video series What Einstein Got Wrong. Watch it now, Wondrium.

The EPR paper described a hypothetical experiment, intended to demonstrate what Einstein saw as the paradoxical consequences of the Copenhagen interpretation. The EPR experiment was one of Einstein’s most famous thought experiments.

A number of different versions of the EPR thought experiment have been discussed and proposed over the years. They all have the same basic elements, including a pair of particles that start out near each other and interact, and then travel far away from each other in different directions.

One of the better later versions describes an atom that is about to decay. It produces two particles with the same mass. Since the system starts with no momentum, the law of conservation of momentum says that the total momentum of the two particles will have to add up to zero. This means that these two particles must travel away from the atom in opposite directions, and with equal speeds.

However, these are quantum particles with no uniquely defined values of their velocities. Instead, they are described by a wave function that can be used to calculate the probability that they will be found to have a particular velocity when measured. In addition, before any measurement is conducted, the velocities of these particles have multiple values, and all at the same time.

Imagine that these particles travel some significant distance away from the atom, and as they do so, they become increasingly separated from each other. After they are separated, you take a measurement of the speed of one of the particles. Let’s say, for example, that you measure the particle to be traveling at a speed of 100 miles per hour.

According to the Copenhagen interpretation, by making this measurement you collapse the particle’s wave function. However, according to the EPR experiment, you seem to have done something else as well. And this is the main point of the EPR experiment.

Since momentum is always conserved, by measuring the speed of one of the particles you also learn the speed of the other particle. After all, the two particles have to be moving at equal speeds. So, by measuring the speed of one of the particles, you don’t only cause that particle’s wave function to collapse, you also collapse the other particle’s wave function. Without getting anywhere near the second particle, you have somehow forced its wave function to collapse.

Einstein believed that this kind of behavior was patently impossible. He argued that there is nothing that one can do to a particle at one location that could possibly affect a different particle at a different location. Whereas, the EPR experiment demonstrates that this kind of thing has to happen according to the Copenhagen view of quantum mechanics. This objection is the essence of the EPR paradox. Einstein thought he had finally shown why the Copenhagen view had to be incomplete. Or maybe even wrong.

Learn more about why Einstein rejected the concept of black holes.

Niels Bohr’s Response to the EPR Paradox

Danish physicist Niels Bohr, who was one of the primary proponents of the Copenhagen interpretation of quantum mechanics, felt that it was his responsibility to respond to Einstein’s attack and to clarify and perhaps correct the situation.

Bohr was convinced that quantum mechanics was a valid theory, and he feared that Einstein’s attacks would unfairly diminish its credibility. So, Bohr put everything else aside and spent six intense weeks formulating and writing a response to the EPR paper and its criticisms of the Copenhagen interpretation of quantum mechanics.

In his article responding to the EPR paper, Bohr did not try to challenge the conclusion that the Copenhagen interpretation leads to the entanglement of wave functions. It was clear that it does. Bohr argued that there was nothing logically inconsistent with entanglement. Entanglement is weird, but that doesn’t mean it is not also real.

One of the grounds on which one might object to quantum entanglement is that it seems to involve faster-than-light travel. According to relativity, nothing can move through space faster than the speed of light.

This apparent problem comes from the fact that when one measures the velocity of one of the particles in the EPR experiment, it instantaneously collapses the wave functions of both particles. Given that a significant distance separates these two particles, this seems to require instantaneous travel through space.

Einstein referred to this as “spooky action at a distance”, and it seemed to violate a central tenet of relativity.

Upon closer scrutiny, it turns out that quantum entanglement might seem to violate relativity, but in reality it doesn’t. More specifically, it doesn’t enable any particle or any other form of information to move between two locations at a speed faster than light. Two particles may be linked through their entanglement, but this could never be used to send a signal or an object, from one place to another at a speed faster than the speed of light.

Bohr had shown that a closer look at the EPR paradox revealed that there is really no paradox there at all. Although Bohr’s response did little to change the mind of Einstein, most physicists seem to have found his rebuttal to be convincing. Today, the EPR paper is widely viewed as a misstep by Einstein.

The EPR paper brought attention to the phenomena of quantum entanglement, but it did not ultimately provide a valid case against the Copenhagen interpretation of quantum mechanics. Einstein had hoped the EPR paper would deliver a fatal blow to the consensus view of quantum mechanics, but the theory survived and became stronger than ever before.

Common Questions About the EPR Paradox

Q: Why is the EPR paradox wrong?

Einstein had hoped that the EPR paradox, which seemed to suggest that the theory of quantum mechanics was incomplete, would finally deflate the consensus around the Copenhagen interpretation. The EPR paradox suggested particles traveled at speeds faster than that of light, which violated general relativity barriers. However, this was later demonstrated to be incorrect. Hence, the EPR paradox is wrong.

Q: What is the entanglement theory?

The entanglement theory says that quantum particles that are entangled remain entangled, and any action performed on one of the particles equally affects the other particles, even if the said particles are far apart.

Q: Is quantum entanglement faster than light?

No, quantum entanglement follows the rules of relativity and doesn’t allow travel faster than the speed of light. Entangled objects behave similarly, which creates the impression of travel faster than light, but no actual travel or communication faster than light takes place.

Q: What is superposition and entanglement?

In simple words, quantum entanglement refers to the back-and-forth transfer of information between a pair of quantum particles. On the other hand, quantum superposition refers to the theory that suggests quantum particles simultaneously exist in multiple states.

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