# What is entanglement

What is entanglement?

Introduction
Quantum entanglement is the process that particles in the universe can naturally and experimentally influence another particle. This effect is instantaneous; when two or more particles are entangled, they can change the state that the other particle was in, meaning that the particles are dependent on each other. The most interesting proposal of quantum entanglement is that because the effects are instantaneous, the particles can be separated by a massive distance and as one particle has a state changed, the information sent to the other is faster than the speed of light.

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Body

Einstein-Podolsky-Rosen Experiment

The copenhagen interpretation brought forward by Niels Bohr and Werner Heisenberg says that we can only interpret and measure reality when looking at it, Einstein hated this idea and famously said “I like to think the moon is there even if I am not looking at it.” (Vacariu 2015) and so with Boris Podolsky and Nathan Rosen they developed a thought experiment which dictated that physical reality according to quantum physics is incomplete. This theorem was based on locality and realism, which at the time seemed acceptable, quantum mechanics seemed to reject this theorem, as Einstein viewed the copenhagen interpretation as unreasonable and impossible as this would reject special relativity, as it could mean that “spooky” action at a distance would be possible, breaking the constant speed limit of the universe (speed of light).

Bell’s Theorem

John Stewart Bell devised this theorem, which states that “No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics” (Parker 1994), this theory is described as a “no-go theorem”, meaning that this particular situation is not physically possible. The hidden variable theory was devised to show that there was an underlying variable that would need to be required so that it could explain objects that changed through their surroundings, and would disprove the “spooky” action at a distance concept.

Experimental demonstrations

“Two-channel” Bell test

Many experiments were used to prove Bell’s theorem, one is the “Two-channel” Bell test which used a source to emit two photons in opposite directions which would be polarised by the two channel polarizers through which the observer could change angle, and then the resulting split photons will be detected and sent to see how many coincidences can be found. Coincidences happen when two photons are in the same state, so for this experiment if they are polarised in the same direction then we call this a coincidence. If two photons are entangled then there will be coincidences 100% of the time, and if the photons are not entangled then coincidences occur only 50% of the time.

After taking multiple readings from different angles, you can use the following equation to check if it proves Bell’s Theorem wrong.

Where,

And N is the number of coincidences from each polarizer angle ? and ?. If the value of S;2 then it disproves einstein and local realism.

Conclusion
So what is Entanglement? Entanglement seemed to be magic in the eyes of some physicists, the idea that some information from one particle could be sent to another instantly if they were entangled seemed ridiculous. Einstein thought that entanglement must have an underlying hidden variable, this would for einstein and many other scientists make quantum entanglement acceptable, but as seen by bell’s experiments it disproves the need for a hidden variable. In modern physics, physicists such as Niels Bohr and David Bohm believe that what we see or pursue something to be separated, in reality they are not, this is called ‘non-separability’. Quantum Entanglement has shown that the world of physics is indeed stranger from what we just observe in the universe and that there is indeed a lot more we still don’t know about the fundamental parts of our universe.