EPR paradox

The EPR paradox describes a thought experiment jointly developed by Albert Einstein, Boris Podolsky, and Nathan Rosen and published in 1935. It was initially devised to highlight what Einstein, Podolsky, and Rosen (EPR) considered to be a fundamental flaw in quantum mechanics theory, with the EPR paradox intending to show that quantum theory could not accurately describe real phenomena. However, subsequent experiments have demonstrated that the apparent paradox described by the EPR consortium is not actually a paradox at all, but instead an accurate but vexing description of the characteristics of physical systems.

Einstein, Podolsky, and Rosen initially published their thought experiment in Physical Review, a peer-reviewed scientific journal. In the twenty-first century, the paper continued to rank among the ten most widely read papers ever published in Physical Review, as rated by impact on the scientific community.

Background

Einstein was a German-American physicist who is widely considered the greatest scientific genius of the twentieth century. Born in Ulm, Germany, in 1879, Einstein is most famous for developing the general and special theories of relativity, with the general theory of relativity attempting to explain gravity’s effects on the space-time continuum and the special theory of relativity endeavoring to define and describe the interrelationship between space and time. Einstein also earned fame and accolades, including the 1921 Nobel Prize in physics for his explanation of the photoelectric effect, which occurs when light or another form of electromagnetic radiation strikes a solid surface to cause an ejection of electrons. He is also considered the founding figure of cosmology, a specialized branch of physics and metaphysics that seeks to understand and explain the origins of the universe. Einstein spent the latter decades of his career in the United States, where he died in 1955.

Podolsky was an accomplished Russian-born physicist who emerged as a leading figure in the evolving and then-largely theoretical field of quantum mechanics during the early 1930s. As a scientist, Podolsky is best-known for his 1935 collaborations with Einstein and Rosen that yielded the famous EPR paradox. Einstein first encountered Podolsky when Einstein was working at Princeton University’s Institute for Advanced Study, with Podolsky impressing Einstein with his advanced investigative and mathematical abilities. Podolsky was later discovered to be working as a spy for the Soviet Union, supplying Soviet intelligence with valuable secrets related to US atomic bomb technology during the World War II (1939–1945) era.

Born in Brooklyn, New York, in 1909, Rosen attended the Massachusetts Institute of Technology (MIT) and had just completed his doctorate in physics when he collaborated with Einstein and Podolsky on the EPR paradox. As an MIT student, Rosen had earned recognition in the scientific community for developing the first reliable quantum methods for calculating the physical properties of hydrogen-based molecules. Later in his career, Rosen pioneered the theoretical concept of “wormholes,” known in physics as Einstein–Rosen bridges, which solve field equations contained in Einstein’s general theory of relativity by suggesting the possible formation of tunnel-like structures that “pinch” spacetime and bridge gaps normally separated by enormous distances.

Quantum mechanics is a specialized sub-branch of physics concerned with the physical properties and behaviors of matter on the atomic and subatomic scales. Physics historians recognize the earliest theories of quantum mechanics as appearing in the mid-nineteenth century, but the field largely developed during the early decades of the twentieth century. Einstein and the German theoretical physicist Max Planck are jointly recognized as shaping forces in the initial development of quantum mechanics. Matter and particles can behave in strange and unexpected ways when observed on the quantum scale, an observation that formed the basis of the famous EPR paradox.

Overview

Quantum mechanics developed at a rapid rate during the early years of the twentieth century. By the 1920s, the mainstream scientific community had come to accept the radical principles proposed by quantum theory while recognizing that legacy explanations rooted in classical mechanics were incapable of accurately explaining the characteristics and behaviors of atomic matter. For example, Planck proposed the concept of “quanta,” which describes the discontinuous bursts of energy that occur during physical interactions. Einstein later applied Planck’s concept of quanta to advance a fully formed explanation of the photoelectric effect, earning a Nobel Prize in physics for his efforts.

As quantum theory evolved, scientists realized that their entire physics models required complete reconception and revision to accommodate emerging findings. A new physics model emerged around quantum theory and was strongly shaped by the work of pioneering theoretical physicists including Werner Heisenberg and Niels Bohr. Heisenberg and Bohr stressed that quantum theory was incapable of describing every observable feature of atomic behavior. For example, quantum mechanics could not be used to foretell the exact moment at which an atom would release a burst of light following a physical interaction. Einstein, Podolsky, and Rosen believed this shortcoming represented a flaw in quantum theory, which they endeavored to illustrate by developing the EPR paradox.

The EPR paradox is built on a thought experiment, which focuses on a core principle of quantum mechanics. This principle states that not all the observable physical characteristics of a given system can be completely and precisely defined at any given point in time. Instead, these observable characteristics occur in ranges that combine to create mathematically complete solutions to quantum mechanics problems. With respect to subatomic particles, the concepts of position and momenta function as commonly cited examples: when a subatomic particle has achieved a state of momentum, its position at any given point in time becomes functionally impossible to specify with precision. That is, the precise quantitative values associated with a subatomic particle’s position and momentum cannot be known simultaneously.

Einstein, Podolsky, and Rosen believed that the inability of quantum mechanics to define both the position and momentum values of a subatomic particle at any given point in time indicated a fundamental flaw with the underlying theory. Prior to the publication of the EPR paradox, Einstein had engaged Bohr in a series of highly publicized and high-profile disagreements on technical points of quantum theory, with Einstein and his partners developing the paradox as part of a continued bid to illustrate their position in the ongoing debate.

The EPR paradox used complex mathematical modeling to show that, according to the established principles of quantum theory, measurements taken on a particle in one part of an entangled quantum system can immediately affect the measurements of particles in different parts of the system, even if those particles are separated by long distances. In quantum theory, an entangled system is any system in which the individual parts cannot be functionally separated from the whole. Entanglements make it impossible to describe connected particles independently of each other, even if those particles are separated by distances measured in light-years. They rank among the most strange and counterintuitive phenomena described in quantum mechanics.

Einstein, Podolsky, and Rosen believed the EPR paradox supported their shared theory that unidentified physical properties they called “hidden variables” governed the otherwise inexplicable discrepancies suggested by their mathematical solutions. In their view, quantum theory as it existed in 1935 was incomplete and required further development in order to close the logical gaps presented by the paradox. Prior to the publication of the EPR paradox, Bohr had passionately argued that no such hidden variables existed.

The divergent views of Einstein (and his colleagues) and Bohr continued to function as a defining feature of quantum theory until the 1960s. In 1964, the Northern Irish physicist John Bell devised an experiment designed to prove or disprove the existence of these theorized hidden variables. His experiments, which yielded what is known in quantum physics as Bell’s theorem or Bell’s inequality, fundamentally showed that the hidden variable theory could not possibly apply to particles in certain physical states, thus demonstrating that Einstein, Podolsky, and Rosen had been fundamentally incorrect in insisting that hidden variables were the only possible explanation for the observational discrepancies of quantum mechanics.

Despite its historical significance, Bell’s inequality did not definitively solve certain technicalities of quantum mechanics, and the underlying questions posed by the EPR paradox remain without a complete and universally accepted resolution. Rather than demonstrating fundamental flaws with quantum theory, the EPR paradox is today viewed as having illustrated and confirmed the precise ways in which quantum mechanics fundamentally departs from the physics principles of classical mechanics.

Bibliography

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