
Physicists demonstrate new way to violate local causality - dnetesn
https://phys.org/news/2017-04-physicists-violate-local-causality.html
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deepnotderp
Haven't local hidden variables as an explanation for quantum phenomena been
discounted a while ago with Bell's Theorem ?

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Ceezy
They demonstrate nothing, very misleading title

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leblancfg
I would be really grateful if someone could provide a paragraph or two as to
how this would be understood in terms of the Bohmian interpretation.

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ttctciyf
The article seems particularly hermetic, especially wrt "bilocal causality"
which doesn't seem to occur as a phrase before Dec 2016[1].

The piece tells us "bilocal causality" is

> a concept that is related to the more standard local causality, except that
> it accounts for the precise way in which physical systems are initially
> generated.

and later references

> a new type of Bell inequality that accounts for the fact that the two
> sources of states used in the experiment are independent, the so-called
> bilocality assumption.

If, like me, you find this non-obvious, you may like to get some background
from an earlier paper[2] (different authors) which has a seemingly less opaque
description of bilocality than the phys.org article:

> Nowadays, fast progress towards advanced demonstrations of quantum
> communication networks, involving quantum repeaters [10] based on
> entanglement swappings [11] and quantum memories [12], are underway in many
> labs around the world. In these future quantum networks, several independent
> sources of entangled qubit pairs will distribute entanglement to partners
> who will then connect their neighbours by performing joint measurements on
> two (or more) qubits, each entangled with one neighbouring qubit, as
> illustrated for the simple case of three partners in Fig. 1. Such
> experiments have an interesting feature that has so far received little
> attention in previous works on nonlocality: the multipartite correlations
> between the measurement results at each site do not originate from a single
> multipartite entangled state, but from a series of bipartite entangled
> states that are initially independent and uncorrelated from each other;
> i.e., there is not a unique initial joint state (the analogue of λ in a
> locally causal model) that is responsible for the observed correlations, but
> these are instead created from smaller systems through joint measurements.
> [...]

1:
[https://www.google.com/search?q=%22bilocal+causality%22+-%22...](https://www.google.com/search?q=%22bilocal+causality%22+-%22phys.org%22&biw=1280&bih=656&tbs=cdr%3A1%2Ccd_min%3A%2Ccd_max%3A12%2F1%2F2016)

2: [https://arxiv.org/pdf/1112.4502.pdf](https://arxiv.org/pdf/1112.4502.pdf)

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erikpukinskis
It's shocking to me no one has written a physics game engine based on
observation and waveform collapse.

I believe there's a 5000 line genre buster just waiting to be written, using a
simple SDF renderer and the wave equation. Please, someone beat me to it I
have other things to write first.

Polygons are not allowed, only fields.

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philipov
So in laymen's terms, it sounds like they found a way to send information
through an entangled pair of particles. Is that accurate?

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johncolanduoni
No, there were no no-communication theorem violation.

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mrcactu5
a lot at stake this year with a discussion of the foundations.

Leonard Susskind, Adam R. Brown, Ying Zhao

Quantum Complexity and Negative Curvature
[https://arxiv.org/abs/1608.02612](https://arxiv.org/abs/1608.02612)

The Second Law of Quantum Complexity
[https://arxiv.org/abs/1701.01107](https://arxiv.org/abs/1701.01107)

~~~
rayuela
These are some really cool papers. Thanks for sharing them.

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bvod
Legitimate scientists have rejected local causality as the mechanism to
explain spooky quantum actions at a distance for some time now. Of course when
something is false there will be multiple ways to prove it, but giving a new
way doesn't really teach us anything new

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x1798DE
> explain spooky quantum actions

Ugh. Please don't give more life to the meme of "spooky" quantum mechanics.
It's mostly propagated by popular science outlets trying to make physics seem
weirder than it is.

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mirimir
Failure of local causality does seem rather "spooky". How is it not weird?

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joeberon
Wow the comments here are weak. Honestly surprised that the physics knowledge
is so low here...

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pavement
Software developers are not physicists. Software developers aren't even
mathematicians, although many are enamoured with related concepts.

To grasp the difference between the hard academic sciences and software
development community, consider the differences between a telephone company
and an astronomical observatory with a large telescope on a mountain top.

Many people here might be amateur enthusiasts, mixed with a handful of
professional academic participants. Any award winning physicists hanging out
here would be surprising.

That said, feel free to distinguish yourself by outshining everyone else. It
would be a welcome addition.

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amoruso
If anyone wants to learn more about Bell's inequality, the simplest (real, no
bullshit) explanation I've ever seen is by David Griffiths. You can follow it
if you know the basics of calculus:

[http://www.physics.umd.edu/courses/Phys270/Jenkins/Griffiths...](http://www.physics.umd.edu/courses/Phys270/Jenkins/Griffiths_EPR_BellInequality_Excerpt.pdf)

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lisper
And if you don't know calculus, there's Mermin's version:

[http://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/f...](http://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf)

~~~
Confusion
IAAP, but that is still a great article to drive home how fundamental and
simple this property of our universe is.

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toxicFork
Going meta here, speaking from the perspective of a software engineer with a
great interest for physics.

There are a lot of terms I do not fully understand in this article.

Can you recommend an application that would allow me to select for example the
words "distant particles can be correlated in ways that are impossible for
classical objects" and find out what lead to that discovery? Or what does
"Bell inequality" really mean?

If not, are there recommended methods of understanding articles better?

I am thinking of this process:

\- copy the article to a document

\- find any words or sentences that you would like to understand better,
highlight them in some colour e.g. red

\- do some googling / wiki surfing or ask in HN or some kind of forums about
them

\- add sections after the paragraphs with highlights to explain the words or
sentences

\- recursion

\- once satisfied, highlight the words in a different colour?

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ENGNR
I'm in the same boat as you, but started looking a while ago when quantum
computers started showing promise.

I found:

\- PBS Spacetime on YouTube is awesome

\- Check out the double split experiment. It seems simple, but when you think
about what getting interference patterns when only sending one particle at a
time really means..

\- Then when ready, the quantum eraser

\- Bells inequality is a simple idea that proved the strange interpretation of
the double split experiment was true, rather than a more mundane
interpretation

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semi-extrinsic
If you like the double slit experiment, you'll probably like Feynmann's
"motivation" of the path integral in quantum field theory:

Suppose you take the double slit (plate with two holes in it), and you start
adding more holes. Of course the particle will go through all of them, but
with less probability the further they are off-axis. We can also add more than
one plate, and the particle will go through all possible combinations of holes
with some probability. Now suppose we let the number of plates go to infinity,
and then let the number of holes in each plate go to infinity. The particle
still goes through all of the infinite combinations of holes, with less
probability if the path is unlikely. But really, there is nothing left anymore
between the particle source and the screen! There is just free space! So this
path integral with the particle going sort-of-everyhere weighted by
probability actually describes propagation through free space.

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zeroer
That connection between slits in the infinite limit and path integrals is
really fascinating. I've read of lot of Feynman, but never that. Where does he
talk about it?

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semi-extrinsic
I'm actually not sure, off the top of my head, but I think it's from his book
" _Quantum Mechanics and Path Integrals_ "; he also used it in lectures IIRC.

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sirclueless
To someone who doesn't actually know many of the terms here, it sounds like
they didn't accomplish much. They defined a stronger form of locality called
"bi-locality" and found a way to violate it.

The structure of the argument is, "We stood up a strawman and knocked it
down," which sounds pretty weak. Is bi-locality an inherently interesting
property? Does all classical computation follow it? I don't know enough to
know if I care, but press releases tend to oversell so my default stance has
to be "No, I don't really care."

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coldtea
> _The structure of the argument is, "We stood up a strawman and knocked it
> down," which sounds pretty weak._

Weaker than "I don't understand what the paper says, only read a high level
layman summary, not sure even what what the experiment was for, but I'm gonna
piss on it anyway"?

Even assuming what you already describe, the ability to show bi-locality as a
new lower bound above which locality constraints apply, is very interesting,
and nothing like a strawman.

It would only be a strawman if they claimed they violated locality in general.

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RushAndAPush
It's amazing how so many people on the internet feel so confident in their own
intellectual abilities that they can read something that they don't understand
and dismiss it as if it's non-important.

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mgarfias
That's funny, I read his comment as I don't understand this, it sounds lame,
can someone please explain the import for me as I trust this source wouldn't
be pushing lame crap.

That is, he was giving credit as respect to the researchers.

