
Bell inequality violation finally done right - rvern
http://www.scottaaronson.com/blog/?p=2464
======
weinzierl
Worth reading because it has an interesting, new (to me, at least) and
approachable explanation of _local realism_.

Money quote:

    
    
        Perhaps the best way to explain local 
        realism is that it’s the thing you believe in, 
        if you believe all the physicists babbling 
        about “quantum entanglement” just 
        missed something completely obvious.

~~~
hasenj
Forget the realism part. Can we at least get the locality part?

In my mind, all the quantum observables are not real properties, therefor it's
obvious/trivial that they don't exist until measured. That is, I have no
problem accepting they are not "real".

What gets to me is the locality aspect of it.

Of course I'm not a physicist (read: I am layman).

~~~
sciolizer
Not a physicist either, but I think "local realism" is a conjunction of
"local" and "realism". Bell inequality experiments show that "local realism"
is false, and so we have to reject either "local" or "realism", but we don't
necessarily have to reject both.

~~~
digi_owl
Best i can tell local realism basically means that if something blows up at
the other side of the universe, it should not affect whatever is happening on
this side until we can observe said event via the reception of
energy/particles from said event.

~~~
norea-armozel
Well that's the problem with local realism. All sorts of things do affect
local conditions at a distance. If entanglement between particle pairs is not
affected by any of the physical forces as described then whatever happens
between them is just a matter of fact, but no where have I seen this mean
we'll ever get ansibles or teleportation. It just means the universe as a
substructure we're not fully able to grasp and that it may be just one of
those things that connects everything despite the appearance of separation of
objects and states.

------
Ono-Sendai
Einstein et. al.'s original paper on the EPR paradox is very clear and
understandable, I recommend it:
[http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777](http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777)

------
amelius
Locality (as opposed to non-locality) is a strange concept if you think about
it. Because how would different points in space "know" to run the same set of
equations? Hence all points have something in common, giving rise to the
possibility (or non-weirdness) of non-locality. But of course, this is
completely non-scientific :)

~~~
dmvaldman
Actually it's the other way around. _Because_ of translation invariance we
have the law of conservation of momentum. _Because_ of temporal invariance we
have the law of conservation of energy. Every symmetry gives rise to a
physical conservation law, from which forces can be derived.

See Noether's theorem.

~~~
Xcelerate
* every _differentiable_ symmetry _of an action_

(Sorry to be nitpicky, but the asterisk is very important haha)

------
renox
Thanks for this article, the previous description of the experiment I read
omitted the 'discard the result unless a measurement show that the
entanglement was successful' step, leaving me very confused.. I didn't know
that this was possible, that said I still don't have any clue about HOW this
is possible.

------
fitzwatermellow
Money quote: "if people had just understood and believed Bohr and Heisenberg
back in 1925, there would’ve been no need for this whole tiresome discussion"

Prof. Aaronson is teaching a grad seminar this Fall at MIT on Computation and
Physics. Here's hoping he makes the lecture notes public ;)

------
ryandamm
This is a really good explanation... but I can personally only square most of
this entanglement non-locality with a many-worlds interpretation. Which is
itself hugely problematic.

~~~
gizmo686
What is problematic about the many-worlds interpretation. Even without non-
locality, it solves the non-determinism problem, which seems far more
problematic to me.

~~~
ploxiln
Well, the only one of the many-worlds that actually matters is this particular
one, right here. What can we say/predict about this exact one. The others
don't matter to me in the slightest...

EDIT: well that's not a great explanation of why I dislike the many-worlds
hypothesis, let me try again. Like some other ideas, including the existence
of God, it's not (yet?) falsifiable. Which doesn't make it false, of course.
But it's a really extraordinary idea. It's not just many worlds, it's a fuck-
ton. And what can we do with all of them? Are they for nothing? Does nothing
that happens matter because it happened differently in a separate but equally
real "world"?

I guess, I _can_ believe that "god plays dice with the universe", but I can
_not_ believe that "god makes infinite copies of the whole universe and tries
all possible quantum outcomes".

~~~
sciolizer
Do you believe god exists and has a psychology? I'm trying to understand if
you have trouble believing because you don't think god would create such a
universe, or if you have trouble believing because such a universe is innately
unbelievable.

If the latter, then I would ask, "Do you believe the ocean is deep, even
though you can only see the surface?"

~~~
ploxiln
The latter.

I don't really understand the ocean metaphor, since humans have seen below the
surface, quite a distance with remote-controlled submersibles.

~~~
sciolizer
Perhaps a better analogy would be, do you believe that matter exists beyond
the observable universe?

Because of the expansion of the universe, it is theoretically possible for me
to get in a space ship and travel away from you at such a high speed that we
would never be able to meet again, even if at some point we both decided to
head back toward each other as fast as possible, and even if we were both
immortal. Our light cones would no longer intersect.

Probably you would assume that I continue to exist, despite that for all
intents and purposes I cannot affect your life in anyway and therefore "don't"
exist.

Similarly if I crossed the event horizon of a black hole.

I believe that a qubit is in superposition (i.e. exists in multiple miniature
"universes"), because that theory produces results that are consistent with
experiment. When the qubit's universes de-cohere with mine, such that for all
intents and purposes I can now only ever observe the qubit in one particular
state, why should I assume that the other universes have ceased to exist?
Decoherence is a continuous process, just like traveling outside of a light
cone, so why treat them any differently. It's rather arbitrary to say that
increased distance in space preserves existence, while increased distance in
phase space does not.

------
snarfy
I don't have a problem with locality being violated. According to relativity,
distance is relative and depends on the reference frame. There will always
been some reference frame where their is no distance between points.

~~~
Manishearth
Noooooooooooooooo. No.

"distance is relative and depends on the reference frame." does not imply "
There will always been some reference frame where their is no distance between
points.". It just means that distances may be different.

Special relativity is a linear transformation, so a nonzero distance cannot
become zero.

Relativity doesn't help locality violation, far from it. Relativity has issues
(EPR) with locality violation.

~~~
snarfy
What's the distance between two points when measured from a third reference
frame moving at the speed C?

~~~
ryandamm
I don't think you can have a reference point moving at speed C, the universe
throws a 'divide by zero' exception.

~~~
snarfy
While true, I thought in QM you had to evaluate all probabilities, even those
where the speed is > C.

~~~
Manishearth
Nope. In QM you have probabilities where energy conservation is violated for a
little bit of time, or where a particle appeared to jump in a distance longer
than allowed. This doesn't mean you can have reference frames moving at the
speed of light.

