
Quantum entanglement shows that reality can't be local - suprgeek
http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/
======
ChuckMcM
This is for me the most interesting part of the whole teleportation /
entanglement research.

When these papers started coming out we debated the notion of 'faster than
light' communication. The counter argument was that you had to move the
particles apart and that was constrained by the speed of light. Then the
question of "when" the state was resolved was pondered. There were two thought
experiments proposed at this meetup.

In the first, two highly accurate clocks, separated by enough distance that
that speed of light issues were unambiguously resolved by the clocks, which
where triggered by the resolution of the quantum state of an entangled
particle. Capturing the exact time when the state was known (detangled) on
each clock, and then comparing the times on the clocks to determine whether or
not it was truly 'instant'.

The second experiment was to set up a 'set' of entangled particles at a
distance and resolve only 'some' of them. The set representing 'bits' and the
resolved set representing the 'information.'

The question of course is whether or not you can move information this way.
Seems like we are getting closer to answering those questions in a definitive
way.

I always thought it would be fun to write a short story about someone doing
this and discovering that it was the galactic equivalent of 'shortwave' radio
and have the alien equivalent of the FCC come out to the planet to arrest the
scientists for transmitting without a license :-)

~~~
rorrr
The most interesting question to me is - what happens when one of the
entangled particles get annihilated. If the other one does too, instantly,
then we can use that to communicate faster than light.

~~~
gus_massa
No. If you annihilate one of the particles, the other particle continues
"unaffected". Whatever you do to one of the particles, the other particle
continues "unaffected". Quantum mechanic is a little more complex, so it is
difficult to explain what "unaffected" really means.

It's clearer with an example. Let's suppose that you and I have each one a
particle that is a half of an entanglement particle pair:

* If you don't do anything with your particle, for each measurement that I can do there are some possible results with some probabilities.

* If you do any measurement to your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities.

* If you do annihilate your particle :( , for each measurement that I can do I have exactly the same possible results with the same probabilities.

* Whatever you do with your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities.

In that sense my particle is "unaffected". I can't do any measurement to know
what you have done with your particle. So you can't use your particle to
transmit information to me.

The strange effect is that if we later meet and compare notes, we will see
that the results you get with your particle and the results I get with my
particle are related. Some measurements always are equal, some have the
opposite value and some are related in more complex ways.

Each measurement (yours and mine) alone are perfectly normal. The strange
effect appears only when the result are compared.

~~~
chii
Hmm, this somehow defies common sense (but of course, its quantim mechanics,
so thats a given!).

You are saying that, if theres an entangled pair, they cannot be used to
transmit information that couldn't be otherwise transmitted via "conventional"
means. However, if you performed the measurements, and then compared notes you
will find that somehow, it looks like the measurements were "synced"?

How does it work this way? that is, how could it be the case that those
measurements can be related, but you can't use that relation to transmit
informatino?

~~~
gus_massa
> How does it work this way? That is, how could it be the case that those
> measurements can be related, but you can't use that relation to transmit
> information?

In a simple example, both people measure for example the spin in the x axis of
the particles that travel in the z direction. Each has a 50% probability of
obtaining "up" and a 50% probability of obtaining "down". But in every case
the get the opposite result, so each one has a random number generator that is
synchronized with the other. But each one alone has only a random number
generator.

Lest suppose that someone try to use that to create an intergalactic first
person shooter for two players :). Using the "synchronized random number
generator" it is possible to make the bots move exactly in the same way in
both players computers, but they are only "synchronized random number
generator" so it's impossible for one player to know what the other player has
done (unless they wait until the information arrive in a conventional way,
with a speed <=c, but that would be a very big laaaag).

But this is only part of the story, because this process can be simulated with
a central "random number generator" that sends the signals to both players.

The strange property is that if both players "magically" decide begin to
measure the spin in the y direction, they will have the same 50% chances and
always get the opposite result. Another possibility that avoids magic is that
one of the players continues measure spin in the x direction and the other
players begins to measure the spin y direction. Now each one has a "random
number generator" that is independent of the other "random number generator",
so each player has no clue about what the other player result. It's not useful
for a IFPS, but it's useful as a physic experiment. (And one of the problems
with quantum mechanics is that you can't measure the spin in both directions,
you must choose one. It's a little more complicate, but I don't' want to enter
into the technical details.)

But this is only part of the story, because this process can be simulated with
a central "random number generator" that generate _two_ random numbers and
then sends the signals to both players, one for the x direction and one for
the y direction. This is a simplification of the "hidden variable theory",
that says that the particles "know" in advance what to do if they are measured
in for the x direction and in the y direction, in spite of that you can't
measure both.

In the experiments the idea is that the measurements/player decide which
direction to use while the particles are flying, so they don't have enough
time to communicate (at <= light speed) and agree what the result would be.
They have to be in accordance from the start (hidden variable theory) OR they
have to communicate faster than light OR something even more strange.

Really, to do the measurements it's possible to choose not only the x or y
axe, but any direction in that plane. So for every direction each player has a
"random number generator", but they are not independent. If one measure in the
x direction and the other at 45º the probability that the results agree in
some number in between 50% and 100%. The 50% is for orthogonal directions that
have independent results, the 100% is for the same direction that has ever the
same/opposite result, and for the other angles there is a formula to calculate
the value. To simulate this you need a lot of hidden variables, or at least a
few an a formula to calculate the result for each direction, or any other
variation of this idea. There are many possible proposal, some are more simple
and some are more complicated, so the idea is to put all of them in the
"hidden variable theory" bag and forget for a moment the details. The problem
is that Bell proved that for _any_ "hidden (local) variable theory" some
inequality holds. This inequality ignores the details of the specific theory,
so it's not possible to invent a more complex "hidden (local) variable theory"
that breaks the inequality. When the same calculations are evaluated using the
quantum mechanics the result is a value that for some angles is allowed by the
Bell inequality but for other angles the value is forbidden by the Bell
inequality. So there is a different prediction of some measurement using the
quantum mechanics and _any_ "hidden (local) variable theory".

And it is possible to do this experiment and calculate a value for every
angle. And the result is that for the problematic angles the result agrees
with the quantum mechanics predictions and don't holds the inequality that is
predicted from _any_ "hidden (local) variable theory". So we must eliminate
all the "hidden (local) variable theory". (For the non problematic angles the
result agrees with the quantum mechanics predictions again, and holds the
inequality as expected because they are not problematic).

More details: <http://en.wikipedia.org/wiki/Bells_theorem>

------
joe_the_user
Ok, as far as I can tell, this all just a rehash of EPR paradox and the
weirdness of the Copenhagen _interpretation_ of QM. The many worlds
interpretation resolves everything with full locality.
<http://en.wikipedia.org/wiki/Many-worlds_interpretation>

Just consider, if I take a black marble and a white in hand, shake them up and
put them in two boxes so I don't know which marble is in which box and put the
two boxes on two sides of the world. Then, when I open one box, I instantly
learn the state of the marble in the other. The information is "transmitted"
to me "faster than the speed of light" but all because the actual switch event
happened in the past. Now, as far as I can, QM entanglement is pretty much the
same thing as these boxes. The only weird thing is that the standard
_interpretation_ (not the standard theory since it is not necessary for any
meaningful measurement), the standard _interpretation_ is that in QM, the
equivalent of our marble switch happens when the final measurement happens.

There's nothing in any of this research that involves any concrete evidence
whatsoever that faster-than-light information transmittal happens, (feel free
to counter-references that), but as far as I've read, all the "transmittal" is
"transmittal" the sense of above.

~~~
ealloc
What you are describing is a "hidden variable theory", but it has been shown
(See "Bell's Theorem") such a theory cannot be true. QM is stranger than that.

~~~
joe_the_user
I'm not saying QM's mechanisms are the same boxing marbles. I'm just saying
that separating entangle particles isn't just _more interesting_ than boxing
marbles, IE, all the modifications that are going interfere with each can be
seen as already having happened - which is why QM doesn't allow you to
transmit information faster than light.

~~~
ealloc
Fair enough, if it's just an analogy I agree it gives the right intuition.

To be clear, what really happened under MWI is that the wavefunction
"branched" into two around the time you created the two particles, and the two
branches spearated from each other in configuration space just as the two
marbles in your analogy separated in physical space. When you did the
measurement, you merely realized which branch you are in.

Where the analogy fails is that the probabilities in QM are inconsistent with
there actually having been some hidden choice of marbles made beforehand.

------
fixermark
One thing I've never understood about interpretation of quantum entanglement
experiments:

Based on what I've learned of these experiments, it seems to me that the least
mind-bending interpretation is that the entanglement event results in two
particles that have some complementary property k (i.e. one particle has k and
the other has ~k) and which one of the two, k or ~k, is had by one of the
particles is unknowable without collapsing the quantum state. However, both
particles have the property from their inception, and so no faster-than-light
or non-local interpretation should be needed. When the particles are separated
to great distance and one is observed to collapse to k, the other must be
observed to collapse to ~k because it was always a ~k particle.

Clearly, there's some aspect of the experiments that I don't understand that
invalidates this simple interpretation, but I do not know what it is.

~~~
Osmium
"However, both particles have the property from their inception, and so no
faster-than-light or non-local interpretation should be needed."

This is incorrect, both particles have a superposition of k and ~k (to use
your terminology). It is not the case that when the wavefunction collapses, we
just suddenly find out the "internal state" of the particle. This "internal
state" simply does not exist until the wavefunction collapse has occurred.

The "no faster than light" doctrine refers specifically to the transfer of
information. And no information, in the strictest sense, has been transferred.
Think of it like this: try to imagine how you could communicate with someone
using quantum entanglement. If you can communicate something, then information
has been transferred. But you can't. All you know is that if your particle is
k, then theirs is ~k.

Now, you could make up a rule book that says "if my particle is k, then I'll
do x, and you'll do y" but that rule book will have to have been shared
between both parties before hand, at less than the speed of light.

Edit: should just mention I'm not an expert here, and articles like this are
always interesting. Finding loopholes in our conventional understanding. But
it's important to know just /what/ our conventional understanding is first to
realise why things like this are important.

~~~
iliis
Even with such a preshared rule book you wouldn't get any FTL communication.
You could crate a pair of random number generators this way which will always
give the same result. Certainly useful for cryptographic purposes but sadly
not direct communication.

~~~
Osmium
Agreed; that's what I was trying to say. Perhaps it's disingenuous to frame
the problem in this way (that is, talking about rule books and the like), but
I was just trying to make it more relatable to normal human experience.

------
jere
Can someone provide or point to a layman's definition of "realism" in quantum
mechanics?

Recently on a call-in podcast I listen to, a caller has claimed that the
following article shows that a) realism has been disproven and b) that proves
we are all in a computer simulation:
[http://physicsworld.com/cws/article/news/2007/apr/20/quantum...](http://physicsworld.com/cws/article/news/2007/apr/20/quantum-
physics-says-goodbye-to-reality)

>Now physicists from Austria claim to have performed an experiment that rules
out a broad class of hidden-variables theories that focus on realism -- giving
the uneasy consequence that _reality does not exist when we are not observing
it_

Sure. Anyway, I'm certain this guy is equivocating, but I don't even know
where to read up on the relevant topics. And I read articles like these and
they keep using the word realism without defining it.

~~~
toufka
Philosophically (as written by Einstein, Heisenberg, etc.) there are three
assumptions about the world - of which one is broken by the quantum world. We
do not know which one. Depending on your interpretation of the Schrodinger
equation you can interpret the breaking in any of the three ways. This article
says its realism - but as far as I know there is no way to prove its realism
and not one of the others. One of these cannot hold:

\- Realism, Locality, or Separability

Realism - Einstein's idea of 'elements of reality' - the idea that things
actually are material and unchanging. Their properties are constant unless
acted upon (ie a Hydrogen atom at time1 and point1 has a constancy of
properties at time2 and point2). You could imagine a universe that had, as a
property of the universe, atoms change mass every other day - just for the
hell of it.

Locality - The idea that a system can only be affected by things within its
lightcone - no effect can travel faster than the speed of light. And two
objects must be 'local' before they can interact.

Separability - any system separated by non-trivial space/time can be
considered independent of each other. The idea that 1/2mv^2 does not include
every other part of the universe in trying to figure out what m or v are. Not
quite the same as locality - as it would still require local interactions, but
that net of interactions could be so thick and pervasive as to prevent any
meaningful isolation of any system.

As is stated above, these concepts fall out of the EPR experiment.

~~~
Anderkent
Do you have any sources for the separability part? From what I remember, the
three assumptions should be realism (the universe would exist even if no one
would 'observe' it), locality or counter-factual definiteness - the ability to
talk and reason on results of measurements that have not been performed.

~~~
toufka
Search this article - it explains much of the original rationale:

<http://plato.stanford.edu/entries/qt-epr/>

------
Osmium
As an aside, congrats to Ars for including a link to the paper (and a DOI) in
the article. They always do this, and it's really something that needs to be
encouraged elsewhere, especially in newspapers. I think it's fair to say if
you don't link to the original paper, what you do shouldn't be counted as
"science journalism."

------
nessus42
This article completely ignored the Many Worlds Interpretation, which
preserves all of realism, locality, and Relativity.

~~~
MBlume
Yeah. It shouldn't be that surprising that when you arbitrarily assume that
part of the wavefunction magically disappears sometimes, you get weird results
like non-locality.

~~~
Osmium
Could you expand on this a little more? I'm not quite sure I follow.

~~~
btilly
The many world's interpretation is based on the assumption that both the
observed system and the observer is modeled by a wavefunction. In that case
observation is entanglement, which seems weird to us, but which makes perfect
logical sense.

Other interpretations of quantum mechanics start with the assumption that the
world is fundamentally like our naive experience leads us to think of it as.
Therefore when we observe something, we couldn't really be thrown into a
superposition of states, each of which observed a different thing and which
cannot meaningfully interact due to quantum mechanical principles. If we can't
be thrown into such a superposition, then there must be some sort of
underlying reality, or quantum collapse, or other weirdness that is not
explained by QM.

All of the weirdness of interpretations other than many worlds can be
understood as the conflict between how they would like to understand the world
(there is a reality that happened), and how quantum mechanics described
things.

All of the weirdness of the many worlds interpretation can be understood as,
"We can't perceive the process of quantum superposition, so it seems really,
really weird to us."

------
tripzilch
BTW, while I'm not entirely sure what this article is saying, here's one of
the best explanations of "Quantum" I've ever seen. Unlike most, it doesn't
start out from physics, but from a mathematical/probability framework ... that
allows for negative probabilities. Yeah.

<http://www.scottaaronson.com/democritus/lec9.html>

I'll quote a bit:

    
    
        -----------
    

So, what is quantum mechanics? Even though it was discovered by physicists,
it's not a physical theory in the same sense as electromagnetism or general
relativity. In the usual "hierarchy of sciences" -- with biology at the top,
then chemistry, then physics, then math -- quantum mechanics sits at a level
between math and physics that I don't know a good name for. Basically, quantum
mechanics is the operating system that other physical theories run on as
application software (with the exception of general relativity, which hasn't
yet been successfully ported to this particular OS). There's even a word for
taking a physical theory and porting it to this OS: "to quantize."

But if quantum mechanics isn't physics in the usual sense -- if it's not about
matter, or energy, or waves, or particles -- then what is it about? From my
perspective, it's about information and probabilities and observables, and how
they relate to each other.

    
    
        Ray Laflamme: That's very much a computer-science point of view.
    
        Scott: Yes, it is.
    

My contention in this lecture is the following: _Quantum mechanics is what you
would inevitably come up with if you started from probability theory, and then
said, let's try to generalize it so that the numbers we used to call
"probabilities" can be negative numbers._ As such, the theory could have been
invented by mathematicians in the 19th century without any input from
experiment. It wasn't, but it could have been.

    
    
        -----------
    

Greatly recommended. I can't say I understand it all, but the concept of a
"qubit" and why it's so different from a regular bit is a lot clearer to me.

------
Eliezer
It can be local in many-worlds.

<http://lesswrong.com/lw/q1/bells_theorem_no_epr_reality/>
[http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_n...](http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_nocommunication/)

~~~
wamatt
Looks like you're a firm believer, so I ask, is MWI by your definition
falsifiable?

Do any experiments past, current or future purport to test predictions made
from MWI?

While MWI may make sense in a universal reading of QM, I'm not sure we can
simply extrapolate this onto the macro level, despite it's pragmatic successes
over the last century. IMHO QM itself, still feels like a temporary hack
(shim), when compared alongside ER.

Until we have a TOE, it may be too presumptuous to be making positive
assertive claims on the correlation between the MWI model and reality.

~~~
Eliezer
[http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_te...](http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_testable/)

~~~
pfedor
_If decoherence is "untestable" relative to collapse, then so too, collapse is
"untestable" relative to decoherence._

Of course! Nobody questions that. The two are equivalent. That means, neither
is more true than the other. You just happen to be in the minority which feels
that thinking about multiple worlds makes their head ache less than thinking
about the state vector reduction.

 _What if the history of physics had transpired differently—what if Hugh
Everett and John Wheeler had stood in the place of Bohr and Heisenberg, and
vice versa?_

My guess is, we wouldn't have had to wait until 1957 for someone to say: "Fuck
that noise, let's just forget about the multiple worlds and pretend the wave
function collapses when I make a measurement." They would be like, "All right
buddy, I'm sure you're totally right that there are really multiple worlds and
all that, I'll just act like it's merely a wave function collapse for a
moment, just to get on with my work of actually doing something as a
physicist." My guess is that would happen within the year.

------
cs702
I've always found classical Newtonian physics, the Special Theory of
Relativity, and (to a lesser extent) the General Theory of Relativity to be
understandable at an intuitive level, but quantum phenomena just baffles me,
even when I think I "understand" it.

PS. IvoDankolov: I find it difficult or impossible to think _intuitively_
about quantum phenomena.

~~~
btilly
Let me make it worse.

Ever heard of chaotic systems? Any dynamic system that shows extreme
sensitivity to initial conditions is chaotic. For instance smoke curling from
a cigarette, planetary orbits over time, weather, water dripping from a tap -
all of these show extreme sensitivity to initial conditions.

Here is the fun thing. In quantum mechanics everything evolves linearly.
Therefore extreme sensitivity to initial conditions is entirely impossible. We
only think that we observe that. Yet the world is full of cases where we can
demonstrate such sensitivity!

See <http://www.iqc.ca/publications/tutorials/chaos.pdf> for some of the
attempts to reconcile observed classical facts with what we think are true
quantum truths.

~~~
cs702
Thanks. A perfect example of the outright weirdness of quantum phenomena.

I read the first section of that paper ("Why quantum chaos?"), and its
proposition makes no sense to me. None other than Poincare PROVED (proved!)
ages ago that the motion and position of just three little points attracted to
each other according to Newton's formulas are extremely sensitive to
infinitesimal changes in their initial motions and positions (i.e., the system
is chaotic in the classical sense). And as you point out, the world is full of
cases that demonstrate such hyper-sensitivity. Yet, according to this paper,
such a system is impossible in nature. I don't get it.

Instinctively, I have to believe there must be a more fundamental underlying
explanation for this and other apparent contradictions... it's just that at
the moment no one knows what this explanation might be.

------
pjscott
Here's the paper in question:

<http://arxiv.org/pdf/1110.3795.pdf>

(Man, I love arxiv.org. It's so nice.)

------
scythe
Here's the original paper:

<http://arxiv.org/abs/1110.3795>

[pdf] <http://arxiv.org/pdf/1110.3795v1>

So let's clear up some misconceptions: First, this is not a problem specific
to Copenhagen or any other interpretation of QM. Second, it is not just the
EPR paradox. Third, it is not the first time someone has proposed a system
utilizing quantum entanglement for superluminal communication; that title goes
to Karl Popper:

<http://en.wikipedia.org/wiki/Poppers_experiment>

Popper's experiment was a very clever way to get around the no-communication
theorem. It was eventually performed, but no FLT communication could be
observed [some suggest we need to keep looking]. I presume that this new paper
is another clever way to avoid the NCT, but I don't have time to read it at
the moment.

------
Xcelerate
If you all want a good, intuitive explanation of quantum mechanics that really
captures the "deep" aspects of the subject (as opposed to the shallow, rote
memorization style of McQuarrie, et al.), see:

<http://lesswrong.com/lw/r5/the_quantum_physics_sequence/>

It's kind of weird and has some asides about consciousness and what not but
other than that, it gives you a really good mindset for thinking about the
subject.

------
tehwalrus
Having read this very carefully, I can't see how quantum mechanics as I always
understood it can ever be either non-local or break relativity. The following
was probably written down by someone else a long time ago, but I don't know by
who or when.

When you entangle two systems, they have to actually be in the same place/with
slower-than-light communication distance. Thus, they each store "secret"
information in some _deterministic_ way which is impossible to measure
directly, and which we can probe only using repeated experiments and
probability distributions. The idea is that while quantum systems are
deterministic just like mechanical ones, they hide their determinism in black
boxes whose mechanisms it is impossible to discover. The "uncollapsed
waveform" has in fact been collapsed ahead of time, you just don't know what
the answer is until you measure.

Back to our entangled particles, when you separate them the determined
decision they made when they were together is revealed. They do not need to
communicate, as they already have their story straight.

It seems this entire area of research is a misnomer - if you let the prisoners
talk to each other just after arrest, of course their stories will be
complimentary later, even if one is in Alcatraz and the other in Siberia. You
just have to assume that _particles so small we can only "see" them in the way
they interact with other particles have some internal structure/state that we
also can't see,_ and thus like the prisoners, can remember the story.

(of course, declaring an entire area of research irrelevant is usually a sign
that I've missed the point wildly - if anyone would like to correct my mistake
I'd be interested to hear!)

------
ZenoArrow
Usual disclaimers: I am not a scientist and my knowledge of quantum mechanics
is very minimal.

However... "Repeated experiments have verified that this works even when the
measurements are performed more quickly than light could travel between the
sites of measurement"

What interests me at this stage is not 'why' entanglement works, but rather
how it can be used. The article indicates that repeated experiments have
verified it appears to be a real process, where actions on one particle can
influence another particle (seemingly) no matter how far away, so why can't we
use it for communication? To use a simple example, if I encode data as
movement in the 'sender' particle, and the movement can be monitored in the
'receiver' particle, then you can have a system for sending messages.

Seems to me the need to control the internal state of the particles to
communicate may be unnecessary. Perhaps to explore further, can anyone here
the types of measurable phenomena that have been seen to be mirrored in
entangled particles (the ones you know of, anyway)? Would particle spin be one
example? Thanks!

------
mixedbit
Are you aware of any attempt to explain quantum superposition (and
entanglement) on the information theory basis? Is it possible that
superposition is an optimization that allows the Universe not to compute stuff
that does not have any observable effect? Maybe classical Universe would be
much more computationally expensive?

~~~
svedlin
Nice observation. I believe there was a recent paper that touches on this
topic:

[http://phys.org/news/2012-03-quantum-physics-matrix-
efficien...](http://phys.org/news/2012-03-quantum-physics-matrix-
efficient.html)

[http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.h...](http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.html)

------
chiph
Not a physicist (theoretical or otherwise), but to me the introduction of the
concept of "hidden variables" reminds me of the addition of Phlogiston to the
body of knowledge.

Unless... they're using it as a placeholder for as-yet undiscovered ways that
entangled particles can interact, and it's known that it's a placeholder.

~~~
Osmium
"Hidden variables" have been proven to not be a satisfactory explanation for
quantum mechanical effects for some time now.

<http://en.wikipedia.org/wiki/Bell%27s_theorem>

Though, I should add, what makes this paper interesting is that they try and
find loophole around Bell's theorem. This would be a big deal if true.

~~~
Anderkent
Local hidden variables. Bell's theorem does not address non-local variables.

~~~
numbsafari
So Satan was the angel that made reality work by using global variables. He
was cast out by god for violating the coding standard. All the evil you see in
the world is the result of errors caused by the use of global variables and a
lack of sufficient testing prior to release that was the result of an
unreasonable schedule that was pulled out of God's ass simply because he
wanted to prove he could make the angels ship something in six days.

We need an update to Time Bandits.

------
ffk
This reminds me of the Ansible, a communication device allowing instant or
near instant faster-than-light communication. It is seen in many science
fiction works and often justified as quantum entanglement in practice.

<https://en.wikipedia.org/wiki/Ansible>

~~~
jmaygarden
I didn't realize that Ursula Le Guin coined that term. My first recollection
of it was from Orson Scott Card and later, Dan Simmons. In any event,
entanglement certainly doesn't look like it will lead to an ansible yet.

~~~
ffk
Yea, my first encounter was in Ender's Game. :)

~~~
jmaygarden
I loved that book as a kid. It looks like they are finally going to release
the movie version in a year from now. They've been teasing us with movie
announcements for the better part of a decade.

I wonder if they'll directly mention quantum entanglement?

<http://www.imdb.com/title/tt1731141/>

------
Anderkent
The paper is behind a paywall, so I cannot see if they researches addressed
this already, but doesn't Everett interpretation preserve locality and not
need faster than light propagation? Are the results incompatible, or was it
simply disregarded to make the paper look more impactful?

------
svedlin
Bohmian mechanics [1] is a deterministic interpretation of QM that's
empirically equivalent to traditional quantum theory. However, it requires
non-locality (instantaneous action across vast distances).

From a recent paper [2]:

"Bohmian mechanics reproduces all statistical predictions of quantum
mechanics, which ensures that entanglement cannot be used for superluminal
signaling. However, individual Bohmian particles can experience superluminal
inﬂuences."

This paper [3] referenced in the Ars Technica article shows that finite
superluminal velocities (c < v < inf) can be exploited to achieve superluminal
signalling.

Very interesting result. I assume BM is still consistent with this. The paper
does mention BM:

"Bohmian mechanics and the collapse theory of Ghirardi, Rimini, and Weber
[...] reproduce all tested quantum predictions, [however] they violate the
principle of continuity mentioned above (otherwise they would not be
compatible with no-signalling as our results imply)."

The principle of continuity described in the paper:

"In both cases, we expect the chain of events to satisfy a principle of
continuity: that is, the idea that the physical carriers of causal influences
propagate continuously through space."

"Clearly, one may ask whether infinite speed is a necessary ingredient to
account for the correlations observed in Nature or whether a fi nite speed v,
recovering a principle of continuity, is sufficient."

Actions can happen instantaneously under BM ("infinite speed"), so BM is still
consistent with QM and doesn't allow FTL communication.

[1] <http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory>

[2] <http://arxiv.org/pdf/1207.2794.pdf>

[3]
[http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys...](http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2460.html)

------
teeja
My grandma puts a donut in one of two boxes, closes both, gives one to each of
us. You leave for Mars and, at an agreed-upon instant, I open my box. Now I
know what is in your box, and you know what's in mine. No faster-than-light,
no mystery.

------
Liron
I hate this article, because locality is an important property that we think
reality _does_ have.

It's like saying "Gravity violates conservation of energy!!!" No it doesn't;
quit playing fast and loose with well-understood principles.

~~~
xanmas
Locality in this sense refers to the presence of "hidden variables" as first
put forth by bohm and later suggested by other people. It was believed that
the various CHSH, HOM, and other bell-inequality inspired experiments would
have put a nail in the coffin but critics always came back and say "but you
didn't control for X." This experiment is just another in the long line of
experiments started by Aspect and continued by others to put bounds on this.

------
mindstab
:/ the more I learn and read about quantum mechanics I'm starting to realize
the FTL quantum communication seems to be an SF pipe dream hoped and imagined
on some possible theories but they seem to be less than likely. What a PITA.
Ah well, maybe we can still use singularities or something to punch holes in
space/time and shoot info through singularity routers :) Though that does seem
even farther away then making quantum mech doing communication for us. ah well

------
Xcelerate
The way that I best understand quantum entanglement (and maybe someone like
jessriedel can correct me if I'm wrong) is thus:

You may be familiar with the Schrodinger equation (HΨ = ih' dΨ/dt) or the more
accurate time-dependent Dirac equation. In each of these equations is a
function called the wavefunction (denoted with Ψ). This function represents
the "quantum state" of your system -- in other words, all the information that
exists within a system. Ψ evolves deterministically in time. You can apply an
operation to this function, and when you do, you get the original function
back multiplied by some value. There are different operators, each
corresponding to an "observable" (the thing you measure in the lab). For
example, you can measure momentum, position, energy, spin etc... and each of
these observables has a different corresponding mathematical operation that
you perform on Ψ to get XΨ, where X is the mean value of the observable.

Now in QM textbooks, you'll frequently see Ψ written as Ψ(r, t), where r is a
position vector and t is time. r denotes the position of whatever particle
constitutes your system. So what if you have a two particle system like
hydrogen (proton and electron) or positronium (positron and electron)? Well, a
QM textbook will write the state of your quantum system as Ψ1 * Ψ2 and
_completely_ gloss over the fact that this approximation _does not apply_ in
all situations. It is physically inaccurate to say that Ψ of two particles is
two individual Ψ's multiplied together. For some systems, it is a good
approximation and makes things easy to calculate, but in reality, there is
really only one wavefunction Ψ and it is a function of all the particles in
the universe.

So now you can see where entanglement comes in. If Ψ is a wavefunction of all
extant particles, then surely there will be correlations between every
measurement of Ψ that you take! Now most of the time, you can't find
correlations -- too many particles are affecting too many other particles (
_decoherence_ ). But if you prepare two of them together and keep outside
particles from interfering with them, then you can observe the correlations
between two particles no matter what the distance!

So now for the whole "information transfer" business. I said earlier that
applying an operator to Ψ gives you the value of an observable -- what you
measure. The weird thing though is that what you measure isn't always exactly
this value. Instead, the _mean_ value of many measurements will be this value.
You can also compute the standard deviation of these measurements using Ψ, but
that's about it. Nobody knows where the random "noise" in measurements comes
from. So far, it seems as though our universe just has some randomness
inherent to it (and we're quickly ruling out all remaining superdeterministic
theories; Gerard t'Hooft seems to be a hold-out:
[http://physics.stackexchange.com/questions/34217/why-do-
peop...](http://physics.stackexchange.com/questions/34217/why-do-people-
categorically-dismiss-some-simple-quantum-models))

So you can't control or predict the individual measurements to as much
precision as you'd like. Sucks, huh?

Anyway, you can plot and analyze this data, and what you'll notice for two
entangled particles separated by thousands of miles or more is that there are
_statistical correlations_ between the two sets of data (again, data you can't
control -- if you can't control it, you can't send information with it).

Obviously, you need both sets of data to notice that there are correlations.

~~~
jessriedel
Being requested by name is too ego-boosting to pass up, so let me take a crack
at clarifying at least one apparent confusion.

> For example, you can measure momentum, position, energy, spin etc... and
> each of these observables has a different corresponding mathematical
> operation that you perform on Ψ to get XΨ, where X is the mean value of the
> observable...

>The weird thing though is that what you measure isn't always exactly this
value. Instead, the mean value of many measurements will be this value. You
can also compute the standard deviation of these measurements using Ψ, but
that's about it.

A good QM textbook will actually say something much more precise. It says that
(a) the set of possible outcomes of the measurement is equal to the spectrum
of the observable being measured and (b) the chance of getting a particular
outcome is given by the squared inner product of the wavefunction with the
appropriate eigenvalue. (The whole business of calculating means and standard
deviations is confusing unless you understand that; unfortunately, this is
allowed to happen often in into QM courses.) This means that QM doesn't just
predict _some_ statistical properties of the outcome distribution, it
completely specifies the distribution.

Also, I figure you know this, but I want to mention that when you say

> you can plot and analyze this data, and what you'll notice for two entangled
> particles separated by thousands of miles or more is that there are
> _statistical correlations_ between the two sets of data

it's important to emphasize that these are _non-local_ correlations (in the
Bell sense). You can generate mere local correlations using everyday classical
systems.

~~~
Xcelerate
Great, thanks for the clarifications!

------
powertower
I'm starting to believe that we confuse the mathematics of quantum mechanics
_with being the actual underlining physical phenomena_ , to the point where we
start to believe that "wave function collapse" are real, rather than just a
way of looking at something via probabilities; or being the models, or
abstractions, or the artifacts of the maths involved.

~~~
btilly
Take that one step farther and you'll arrive at the conclusion that if we take
QM seriously, then there is never a collapse, and the many worlds
interpretation is a logical consequence.

------
teresko
It would be interesting to see the result of experiment where one electron in
entangled pair collides with a positron.

I suspect that this might give some interesting results.

------
LokiSnake
That very adorable kitten is way too distracting, and made it very hard to
focus on the article.

------
pauldirac137
Better not tell this to Mitt Romney.

------
ecliptic
If you were able to construct a rigid beam 25 million miles long wouldn't you
be able to transmit data (push a button) on the other end faster than the
speed of light?

~~~
MBlume
Nope. Pushing on the end of the beam sends a pressure wave through the beam.
The button gets pushed when the wave reaches the other end. Wave travels
significantly slower than light.

~~~
ecliptic
If you push on the beam and the other end does not move right away doesn't
that imply that the beam has compressed and is not rigid?

~~~
ars
> doesn't that imply that the beam has compressed and is not rigid?

Yes it does. And in fact that is the case: The beam has compressed. A fully
rigid beam is impossible.

------
pebb
There is another explaination: <http://discovermagazine.com/2010/apr/01-back-
from-the-future>

The future causing the past.

------
jcknight
What?

