Spooky Action at a Distance - The EPR Paradox

Einstein's paper with his two postdoctoral research companions, Boris Podolsky and Nathan Rosen, was the great EPR paradox."Can Quantum Mechanical Description of Physical Reality Be Considered Complete?" asks the paper.(1935). You might be surprised to learn that Einstein was the one who laid the groundwork for quantum mechanics in its early stages, only to propose that quantum physics is incomplete. Some refer to this as Einstein's great mistake, which led to a tremendous quantum discovery. In this blog, I will discuss EPR briefly. However, it takes a fundamental understanding of quantum characteristics, which I am here to simplify.

Even now, this study is regarded as one of the top ten most influential publications ever published in the field of physics. The argument over EPR continues to this day. One common misconception is that quantum communications are faster than or close to the speed of light, which is not true. EPR proposes something similar. This contradiction is argued by EPR on the basis of Locality and Realism. Along with Quantum Entanglement, Einstein is dissatisfied Let me explain EPR's reasoning before delving into the theory.

Locality refers to the fact that when two particles are separated by a distance, they cannot interact quickly. They need some time to pass on the information. In layman's terms "the physical process occurring at one place cannot have any influence in the other process at a location outside its light cone".

Realism, if we know the properties of a certain system and can forecast its progress, for example, if we know the mass and velocity of a specific object at a specific time, we can predict the distance travelled by that thing as time progresses. So, before the quantum arrived, all physical quantities were deterministic. Simply put, realism is that "a physical object and its properties pre-exist without the influence of an observer."

Quantum Entanglement is illogical! Assume we have two distinct quantum particles named Alice and Bob. These two particles were quantum entangled and are separated by a large distance. This is now acting as a single large system. When one tries to measure one of the particles, the other knows immediately that the measurement was conducted. This appears to be a violation of Einstein's special theory of relativity, which states that nothing moves faster than light within the universe. Because of its superposition, quantum mechanics objects appear to lack locality and realism. When measured, the item remains in numerous states at the same time and is compressed to one of them. These factors combined cause Einstein to have strange feelings regarding quantum mechanics, which he labelled as spukhafte Fernwirkungen ("spooky actions at a distance").

EPR even argued that quantum theory is incomplete because there is something in the built-in system that we do not have access to. This could be due to a lack of knowledge, a lack of understanding, or simply the way the universe operates. And this strange thing tells the system how the result should be displayed when the measurement is performed on the system. This was later known as the hidden variable theory. If we know the hidden variable, quantum mechanics is complete and deterministic; the system will not collapse randomly.

During the twentieth century, there was a lot of scientific controversy around EPR. The entire physics community has been divided into advocates of EPR and critics of EPR (led by Neils Bohr). No one shall be able to argue against EPR for the next 30 years. John Stewart Bell, a physicist from Northern Ireland, offered a proposal to countering EPR in 1964. He proposed what is now known as Bell's inequality. If it is violated by any system, the system is quantum in nature, which has had a significant impact and rapid development in quantum. Later in 1972, Freedman and Clauser were experimentally violated Bell’s inequality and also awarded Noble Prize(2022) for Bell's violation. I'll write more about Bell inequality and CHSH violation in future blogs.