If we move down to billions of meters as compared to the size of an atom, where entirely new sets of rules are followed having a superposition. A state in which subatomic particles have half up spin and half down spin until it is measured.
In early 1980’s when Einstein-Podolsky-Nathan EPR paper was published, it gave birth to the trickiest crazy ridiculous concept of modern physics, Quantum Entanglement. In this phenomenon, two particle interact physically in such a way sharing all the information although if we separate them billions of light years away without any linking source, they will still show entanglement property. Seems they were talking. Einstein named it Spuckhafte Frewirkli / Spooky Action At a Distance. Total angular momentum has to be constant if one particle measured, show up spin then it is 100% sure other would show down spin.
Einstein was unsatisfied due to the reasons….
1. Entanglement allows the exchange of information faster than the speed of light which violates his theory of relativity.
2. No local cause required but according to Einstein everything in this universe is affected by local cause.
3. Quantum mechanics can not predict the sure outcome because we are unaware of its hidden information.
John Bell and John Clauses machine measured thousands of entangled particle and showed information exchange occurs simultaneously from miles away, proving Niels Bohr right. Locality is not permanent and entangled particles have the same properties. Light striking on one particle collapses its superposition state and measurement of one particle instantaneously effect other having randomness probabilities i.e co-related. Therefore, does not violates the theory of relativity. The scientists are trying to implement this phenomenon in the form of quantum computers, quantum cryptography, quantum teleportation.