There are few parallels in human endeavor than the very deep and profound argumentative disagreement between Neils Bohr and Albert Einstein on the very nature of Quantum Mechanics. While Bohr was in the Copenhagen Interpretation gang, Einstein was looking for the hidden variables.
Einstein wrote a paper with Podolsky and Rosen enunciating the EPR paradox. It was a thought experiment. This is how it is described in wikipedia:
The EPR paradox draws on a phenomenon predicted by quantum mechanics, known as quantum entanglement, to show that measurements performed on spatially separated parts of a quantum system can apparently have an instantaneous influence on one another. This effect is now known as "nonlocal behavior" (or colloquially as "quantum weirdness" or "spooky action at a distance").
Broadly, the thought experiment in very simple terms can be described thus:
Hiesenberg’s uncertainty principle says motion and position cannot be measured with accuracy at the same time. So, you cannot know the exact location of the electron. However, if two electrons were brought together and went towards each other, they will be repelled – while adhering to the conservation of momentum – both together will have the same momentum combined after the "collision" as they had before the collision. After the collision, if one were to measure the position and momentum of electron 1 at different times (in keeping with Hiesenberg’s principle), one could due to correlation from conservation of momentum, calculate the exact measurement of electron 2 without disturbing it!
Interestingly, Einstein and co. devised EPR paradox to refute QM by bringing in classical bearings on it. However the opposite happened. It helped establish how actually QM violated the classical laws. Here is the explanation:
Although originally devised as a thought experiment that would demonstrate the incompleteness of quantum mechanics, actual experimental results refute the principle of locality, invalidating the EPR trio’s original purpose. The "spooky action at a distance" that so disturbed the authors of EPR consistently occurs in numerous and widely replicated experiments, though the validity of these experiments does remain in debate. Einstein never accepted quantum mechanics as a "real" and complete theory, struggling to the end of his life for an interpretation that could comply with relativity without implying "God playing dice", as he condensed his dissatisfaction with quantum mechanics’s intrinsic randomness and counter-intuitivity.
The EPR paradox is a paradox in the following sense: if one takes quantum mechanics and adds some seemingly reasonable conditions (referred to as locality, realism, counter factual definiteness, and completeness), then one obtains a contradiction. However, quantum mechanics by itself does not appear to be internally inconsistent, nor — as it turns out — does it contradict relativity. As a result of further theoretical and experimental developments since the original EPR paper, most physicists today regard the EPR paradox as an illustration of how quantum mechanics violates classical intuitions.
The most basic dichotomy was thus between principle of locality itself within the context of causality.