The idea of uncertainty is fundamental to the world of quantum mechanics. We cannot measure all the features of a system simultaneously, no matter how perfect the experiment. Niels Bohr's Copenhagen interpretation effectively tells us that the very act of measurement selects the characteristics that are observed.

Entanglement is quite a peculiar property of quantum mechanics. If two electrons, for example, are ejected from a quantum system, then the conservation of momentum laws tell us that the momentum is equal and opposite to that of the other. However, according to the Copenhagen interpretation, neither particle will have a definite state until it is measured. When you measure the momentum of one, it will determine the state and momentum of the other particle, regardless of the distance between them.

So,

When any two subatomic particles interact with each other, their states become interdependent â€“ they are entangled.

They remain connected even when physically separated (even by enormous distances like different galaxies).

Manipulation of one particle instantaneously alters the other.

Measuring the properties of one particle gives us data about the other.

This is known as non-local behaviour though Einstein called it "spooky action at a distance". In 1935, Einstein claimed that there are hidden variables at work make it unnecessary. He argued that for one particle to affect another, a faster-than-light signal between them would be required. This was forbidden according to Einstein's theory of special relativity.

Bell's Theorem:

In 1964, physicist John Stewart Bell proposed an experiment that tackled the question of whether or not entangled particles actually did communicate with each other faster than the speed of light. For this he imagined a case of paired electrons, one with spin up and one with spin down. (Spin refers to the angular momentum of electrons). According to quantum theory, the two electrons are in a superposition of states until they are measured. Either one of them could be spin up or spin down. However, as you measure the spin of one, you know the other has to be the opposite.

Bell derived formulas called the Bell inequalities, which determine how often the spin of a particle should correlate with that of the other particle. This was if normal probability was involved which is actually opposed by quantum entanglement. The statistical distribution of this proved mathematically that Einstein was incorrect and that there is an instantaneous connection between entangled particles. According to physicist Fritof Capra, Bell's theorem portrays how the Universe is "fundamentally interconnected".

To conclude, the quantum realm is not bound by rules of locality. When two particles experience entanglement, they are effectively a single system that has a single quantum function.