U1L6a.txt

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Today, I'll be talking about quantum weirdness.
And I think to talk about this, I first
want to go back to a paper by Einstein, Podolski, and Rosen--
that's EPR 1935.
And what Einstein argued was that quantum mechanics
is incomplete.

So what Einstein used in this paper
was position and momentum, but we're
working with discrete quantum mechanics, so we'll just use--

well, up and down, or 0 and 1.

So I want to give Einstein's argument--
so suppose you have 0 minus 1.

So suppose Alice and Bob share a pair of entangled particles
like this.
If Alice measures in the 0,1 basis--

Bob will have the opposite particle
no matter what Alice measures.
So if she measures a 0, he gets a 1.
And if she measures a 1, he gets to 0.
And if she measures in the plus or minus basis--
if Alice gets a plus, Bob will always get a minus.
And if Alice gets a minus, Bob will always get a plus.

So because Alice and Bob can be reasonably far apart and can
measure at the same time with relativistic speeds,
Bob's particle cannot know whether Alice measured a 0 or 1
when Bob measures it.
So what this says is if Bob--
well classically, you would think
that if Alice can predict what Bob gets,
which she can because she can predict
in the plus minus basis, if she measures
in the plus minus basis, and she can predict in the 1,0 basis
if Bob measures in the 1,0 basis.
So if Alice can always predict 100% what Bob will do,
then there must be something in the quantum state
that tells what Bob will get when he measures it.
Yes?
Should the state [INAUDIBLE] on top be 0,1 minus 1,0?
It should be.
I'm sorry, I wrote down the wrong state.

I'm going to switch to 0,0 plus 1,1 later in this lecture
because then--
I'll tell you why when I get there.

So you would think that there's something in this quantum state
that tells whether it's going to be a 0 or 1
when Bob measures it, but we know according to quantum
mechanics that there is nothing in this state that
tells whether it's going to be a 0 or 1.
So what that means is that there must
be a more complete theory of reality than quantum mechanics.
So this is what Einstein argued.
And Schrodinger wrote a paper that answered that.
And Schrodinger's paper said that qubits are--
or the particles are verschrankung--
which is a German word that means crossed arms, or crossed
legs, or intertwined fingers--
and that's why this happens, which really is not
that great of an explanation.
And then Schrodinger later translated
verschrankung into English as entangled
because crossed would really have not worked.

Well, nobody paid much attention to Einstein's argument
for the next 25 or 30 years or so.
And Einstein was never satisfied with quantum mechanics
completely, but quantum mechanics
worked so they ignored this objection to it.
And then in 1964, Bell published a paper that said, well,
Einstein was both right--
there's something very strange going on here,
but wrong in that there's no way to complete
this theory to a real theory because the arguments