The EPR paradox (or Einstein–Podolsky–Rosen paradox) is a topic in quantum physics and the philosophy of science regarding measurements of microscopic systems (such as individual photons, electrons or atoms) and the description of those systems by the methods of quantum physics. It refers to a dichotomy, where either the measurement of a physical quantity in one system affects the measurement of a physical quantity in another, spatially separated system or the description of reality given by a wave function is not complete.
This challenge to the Copenhagen interpretation originated from the consequences of a thought experiment introduced in 1935 by Einstein, Podolsky, and Rosen and resulted in what seemed to be a contradiction in the interpretation. The thought experiment involves two systems that interact with each other and are then separated so that they presumably interact no longer. Then, the position or momentum of one of the systems is measured, and due to the known relationship between the measured value of the first particle and the value of the second particle, the observer is aware of the value in the second particle. A measurement of the second value is made on the second particle, and again, due to the relationship between the two particles, this value can then be known in the first particle. This outcome seems to violate the uncertainty principle, since both the position and momentum of a single particle would be known with certainty.
Einstein never accepted quantum mechanics as a "real" and complete theory as he could not believe that measurement of the second particle invalidates the measurement of the first as this requires "information" to travel faster than light between the two (although it is not actually possible to transfer information in this manner as the results would be uncontrollable). Einstein struggled to the end of his life for a theory that could better comply with causality, protesting against the view that there exists no objective physical reality other than that which is revealed through measurement as it is interpreted in terms of the quantum mechanical formalism.
Here's a simple video on EPR Paradox brought to you by DrPhysicsA on YouTube -https://www.youtube.com/watch?v=0x9AgZASQ4k
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