Name: 【TED-Ed】Einstein's brilliant mistake: Entangled states - Chad Orzel
Uploaded: 2014-11-17T09:27:45.000Z
Duration: 5 min 10 s
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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 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

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

You’ll have a fifty percent chance of getting either A or B.

But if you then repeat the first measurement,

You’ll 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.

Things get even stranger when you look at both particles.

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

You’ll find that they’re always perfect league correlated.

For example, if both particle are measured at zero.

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

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 as ‘spuckhafte ferwirklung’

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

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

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