Bell's theorem again

I found this wonderful youtube video which explains Bell's theorem with the aid of simple animation.

Enjoy!

Bell's inequality

In Youtube, I found a video which brilliantly explains Bell's inequality or theorem and how it was eventually proven experimentally. Back in the 1920s and 30's physicists discovered that particles from a common source were entangled at the point of measurement.  For instance when you measure spin direction in one electron of a pair of electrons from an atom, the spin of its twin will always be opposite.  For example if one electron was spin up the other would be spin down.  This is quantum entanglement.  Before they were measured both electrons were in both spin up and spin down, this is known supposition and is best explained by the Schrodinger's cat thought experiment. This entanglement applied no matter how far the two particles were from each other, they could even be on different ends of the universe but the quantum entanglement still applied.

However Albert Einstein, along with fellow physicists Boris Podolsky and Nathan Rosen, pointed out that for this to work there must be faster than light communication between the two particles.  And this cannot be since it breaks Einstein's special relativity that nothing can go faster than the speed of light.  This became known as the EPR paradox.  Einstein and his colleagues had pointed out a fundamental flaw with quantum physics and he described entanglement as "spooky action at a distance". Insead Einstein believed that the spin states of these particles were already set before measurement and they only become known to us at the point of measurement.  These are known as 'local hidden variables' and they imply that nature is pre-set in some ways.  Einstein characterized this by having two boxes and a pair of gloves, and each box contains a glove each.  When you open a box, not only do you see which hand the glove fits but you instantly know which hand is in the other box. So which was correct, quantum physics or local hidden variables?

For decades this issue lingered on, until 1964, when a Northern Irish scientist, John Bell, published a study on how to resolve this.  He came up with an experiment which tests local hidden variables of entangled particles.  Each entangled particle is sent to a polarizer to measure the spin and if the particle's spin and detector agree then the particle is allowed through, it not, it is blocked.  These detectors can be set to varying angles so we can measure the probability of a particle getting through or not.  Bell theorized that if hidden variables exist then there is a minimum probability of the two detectors having the same result.  This probability was found to be 33%, if the result go below this probability then hidden variable theory would be false and quantum theory would be correct.  All this became known as Bell's inequality or Bell's theorem.

It was not until 1972 that Freedman and Clauser in the USA, had the right equipment to perform this test, they found the results in favour of quantum physics.  Later on in 1983, French phyisicist Alain Aspect performed a much rigorous version of this experiment and found that the results proved Bell, and quantum mechanics itself, correct and the probability of both detectors recording the same result was below 33%.

So there is some kind of faster than light communication or relationship between 2 entangled particles.  How it comes about is another matter and yet to be explained. Further John Bell had taken on a question posed by one of the greatest scientists ever, Einstein, and won.

Misnomers

In everyday life one comes across phrases which their meanings bear no resemblance to the words that make them up, common ones include American football, bombay duck and toad in the hole.  Unfortunately quantum physics has two, quantum tunneling and quantum teleportation.

With quantum tunneling one has an image of an electron drilling through a solid wall with a set of sub-atomic tools to get to the other side. The reality is not like that and no drilling is involved. Since electrons can be waves as well as particles, one is never certain of their location at any one time.  A solid object will affect the path of the electron's wave but due to this uncertainty, there is a small chance it can be either inside the wall or the other side of it.  This is down to quantum probability and as been proven in the lab already.  Naturally the thinner the wall, the higher the probability that an election will appear on the other side.  And if the wall is thick enough then this probability will shrink to zero.

With quantum teleportation one has an image of particle such as a photon or electron dematerializing and rematerializing in another place a la Star Trek ("Beam me up"). Again the reality is not like that, you are merely teleporting the quantum states of a particle not the particle itself.  This is thanks to quantum entanglement where two particles spin states are inter-linked, if one is spin up the other must be spin down.  When you measure one, you set the state of the other.

Quantum tunneling

For anyone out there who is interested, here is an everything you need to know video explaining the wonderful and mysterious world of quantum tunneling.

How to Teach Quantum Physics to Your Dog by Chad Orzel part 2

I don't usually review books until I have finished reading them but I will make an exception with How to Teach Quantum Physics to Your Dog by Chad Orzel. To me this is the best primer to quantum physics I have read so far, due mainly to Orzel's witty style and uncomplicated explanations of very complicated material.  Although he does not delve very deep into each area, he provides a good understanding before tackling more academic books on the subject.  Even subjects such as decoherence and Heisenberg's uncertainty principle are explained to help the layman understand what they are.

How to Teach Quantum Physics to Your Dog by Chad Orzel

After finishing my quantum computing book by John Gribbin, I have bought the Kindle version of How to Teach Quantum Physics to Your Dog  by Chad Orzel.  Here Orzel tries to communicate the intricacies of quantum physics through a mythical conversation with his pet dog.  I guess the dog will have a few questions for him. I will post more about this book after I have read a bit more.

Back to quantum physics

I have just finished reading my latest astronomy book and I have bought Jim Al-khalili's "Quantum: a guide for the perplexed". He is Scottish born Iraqi physicists who made a wonderful BBC series about electricity, so I have high expectations about his written work.

Quantum non-locality

I've just finished reading the chapters on classical computers in John Gribbin's Quantum Cat's book, which is quite interesting in itself as it gave a good biography on Alan Turing and Johann Von Neuman. It would have been interesting had Turing lived a bit longer.  Now I am on the chapter to do with quantum mechanics/physics itself.  Some of the stuff here, I have already come across before except for quantum non-locality.  This is where the actions two quantum matter (electrons, light photons etc) can influence each other as if the two items had the ability to communicate to each other.  Even though the concepts are quite well presented by Gribbin I had to do some background research to fully understand it and it is absolutely fascinating.

quantum computers

I am currently reading John Gribbin's "Quantum computing for cats: from qubits to colossus".

http://www.amazon.co.uk/Computing-Quantum-Cats-Colossus-Qubits/dp/0593074882

So far a very interesting read.  If these things ever go into the production the possibilities could be endless.