Quantum rules allow connections prohibited by classical physics. They showed that two particles, in this case, electrons, could influence each other instantaneously, even if they were a mile apart. Quantum mechanics incorporates a wave-particle duality and explains all the above phenomena. In doing so, quantum mechanics changes our understanding of nature in fundamental ways.
While the classical laws of physics are deterministic, QM is probabilistic. We can only predict the probability that a particle will be found in some region of space. Einstein saw quantum theory as a means of describing nature at the atomic level, but he doubted that it would be a useful basis for all of physics. He thought that describing reality required firm predictions followed by direct observations.
However, individual quantum interactions cannot be directly observed, so quantum physicists have no other option but to predict the probability of events occurring. Challenging Einstein, physicist Niels Bohr defended quantum theory. He argued that the mere fact of indirectly observing the atomic kingdom changes the outcome of quantum interactions. According to Bohr, quantum predictions based on probability accurately describe reality.
Rather than pointing to a problem with quantum physics, the EPR paradox ended up opening the door to an entirely new type of quantum weirdness. However, the EPR article introduced topics that form the basis of much of current physical research. The surprising thing about this story is that the best physicist of the 20th century tried to prove that quantum mechanics was wrong, or at least incomplete, and ended up doing exactly the opposite. Although the Stern-Gerlach experiment with macroscopic particles is purely theoretical, the team believes that in the near future it will be possible to carry out a real empirical study using particles heavy enough to expect them to behave classically.
To demonstrate why such hidden variables should exist, Einstein wrote an article in 1935 with Boris Podolsky and Nathan Rosen in which he presented a thought experiment that used the structure of quantum mechanics. Einstein and Niels Bohr began discussing quantum theory at the prestigious Solvay conference in 1927, which was attended by the best physicists of the time. However, when we leave the subatomic world, objects are better described according to the laws of classical physics, which means that at some point the quantum description of large bodies must be somehow reduced to Newtonian concepts. The first was Einstein's General Theory of Relativity, which dealt with the universal scope of physics.
Niels Bohr and Max Planck, two of the founding fathers of quantum theory, each received a Nobel Prize in Physics for their work on quantums. This is known as the rise of classicality, which states that a quantum mechanical description of large objects must be the same as their classical description.