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• Albert Einstein played a key role in launching quantum mechanics to his theory of photoelectric effect.

• But remains deeply bothered by its philosophical implications.

• And though most of us still still remember him for deriving e equal m c square.

• His last contribution to physics was actually a 1935 paper.

• Co-author with his young colleague - Boris Podolsky and Nathan Rosen.

• Regarded as an art philosophical foot note well into the 1980s.

• This EPR paper has recently become central to a new understanding of quantum physics.

• With its description of a strange phenomenon now known as entangled states.

• The paper begins by considering a source that splits out the pair of particles

• Each with two measurable properties.

• Each of these measurements has two possible results of equal probability.

• Let’s say zero or one for the first probability and A or B for the second.

• Once a measurements is performed, subsequent measurement of the same properties in the same particle will yelled the same result.

• The strange application of this scenario is not only the state of the single particle

• Is indeterminate until it’s measured.

• But that the measurement then determine the state

• What’s more the measurements affect each others.

• If you measure a particle as being in state one and followed it up with the second type of measurement

• Youll have a fifty percent chance of getting either A or B.

• But if you then repeat the first measurement,

• Youll have a fifty percent chance of getting zero

• Even that the particle have already been measured one.

• So switching the properties been measured scramble the original results.

• Allowing for a new random value.

• Things get even stranger when you look at both particles.

• Each of the particle will produce random results, but if you compare the two

• Youll find that theyre always perfect league correlated.

• For example, if both particle are measured at zero.

• The relationship will always hold.

• The states of the two are entangled.

• Measuring one will tell you the other with absolute certainty.

• But this entanglement seems to defy Einstein’s famous theory of relativity

• Because there is nothing to limit the distance between particles.

• If you measure one in New York at noon, and the other in San Fransinco and then the second later

• They still give the exactly same result.

• But if the measurement does the terminate value then this will require one particle sending some sort of signal to the other

• At thirteen million time the speed of light which according to relativity is impossible.

• For this reason, Einstein dismiss entanglement asspuckhafte ferwirklung

• Orspooky action at a distance

• He decided that the quantum mechanics must be incomplete a mere approximation of a deeper reality.

• In which all particles have pre-determine states that are hidden from us.

• So porter of orthodox quantum theory led by Neil Bohr maintain that quantum state really are fundamentally indeterminate

• And entanglement allows the states of one particle to depend on that the distance partner

• For thirty years, physics remained at in past until John Bell

• Figured it out that the key to testing the EPR argument was to look in cases involving different measurements on the two particles

• The local hidden variable theories favored by Einstein Podolsky and Rosen

• Strictly limited how often you can get results like 1A or B0

• Because the outcome would have to be defined in advance

• Bell showed that the purely quantum approach where the state is truly indeterminate until measured has different limits

• And predicts measurement results that are impossible in the pre-determine scenario

• Once Bell had worked out how to test the EPR argument physicists went out and did it.

• Beginning with John Clauster in the seventies and Alain Aspect in the early 80s

• Dozens of experiments has tested the EPR prediction and all have found the same thing

• Quantum mechanics is correct.

• The correlations between the indeterminate states of entangle particles are real

• And cannot be explained by any deeper variable

• The EPR paper turned out to be wrong but brilliantly sell

• By leading physicists to think deeply about the foundations of quantum physics

• It led to further elaborations of the theory and help launch research into subjects like quantum information

• Now a thriving field with the potential to develop computers of unparallel power

• Unfortunately, the random of measure results prevent science fiction scenario like using entangle particles

• To send messages faster than light.

• So relativities is save for now but the quantum universe is far stranger than Einstein wanted to believe

Albert Einstein played a key role in launching quantum mechanics to his theory of photoelectric effect.

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# 【TED-Ed】Einstein's brilliant mistake: Entangled states - Chad Orzel

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Go Tutor posted on 2014/11/17
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