EPR paper (Einstein-Podolsky-Rosen paper)

The 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen, commonly known as the EPR paper (Einstein-Podolsky-Rosen paper), is a seminal work in the field of quantum mechanics. The paper was titled "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" and was published in the journal Physical Review.

Article headline regarding the EPR paradox paper in the May 4, 1935, issue of The New York Times.

Background:

In the early 20th century, quantum mechanics had emerged as a revolutionary theory to describe the behavior of particles at the atomic and subatomic scale. While quantum mechanics was highly successful in making accurate predictions and explaining various phenomena, it also presented philosophical challenges and apparent paradoxes that puzzled many physicists, including Albert Einstein.

The EPR Paradox: Einstein, along with his colleagues Boris Podolsky and Nathan Rosen, used the EPR paper to present a thought experiment, now known as the EPR paradox, that raised fundamental questions about the completeness of quantum mechanics. The paradox was centered on the concept of entanglement, where two particles could become correlated in such a way that the measurement of one particle's properties would instantaneously reveal information about the other, regardless of the distance separating them.

In the EPR thought experiment, they considered two entangled particles with correlated properties such as position and momentum. According to quantum mechanics, measuring one particle's position, for example, would instantaneously determine the other's momentum, even if the second particle were located far away. This seemed to imply faster-than-light information transfer, which Einstein found problematic, as it would violate the theory of relativity's prohibition against superluminal information transmission.

Einstein's Argument: Einstein's main contention in the EPR paper was that quantum mechanics was an incomplete theory. He argued that there must be "hidden variables" that determine the properties of particles independently of measurements, and that the apparent randomness and non-local connections of quantum mechanics could be explained by these hidden variables. In other words, he believed that there was an underlying, deterministic reality that quantum mechanics failed to describe completely.

Albert_Einstein, by Doris Ulmann

The Impact:

The EPR paper initiated a long-standing debate about the foundations of quantum mechanics and the nature of reality. Einstein's arguments sparked discussions among physicists about whether quantum mechanics could be considered a "complete" theory, meaning it described the full nature of physical reality, or whether there might be hidden variables or other factors at play.

In the following years, physicist John Bell formulated Bell's theorem, which provided a way to test the validity of hidden variables theories against quantum mechanics. Experiments inspired by Bell's theorem, such as those conducted by Alain Aspect in the 1980s, demonstrated that the predictions of quantum mechanics were incompatible with the idea of hidden variables. This experimental evidence supported the non-local, probabilistic nature of quantum mechanics, seemingly putting to rest the debate initiated by the EPR paper.

The EPR paper remains a historical landmark in the philosophy of science and the foundations of quantum mechanics. It challenged the scientific community to explore the deep-seated mysteries of the quantum world and continues to be a subject of discussion and exploration in the field of physics.