

Physicists spooked by faster-than-light information transfer - bk
http://www.nature.com/news/2008/080813/full/news.2008.1038.html

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hugh
Nothing new here, just a demonstration of stuff we've known for years on a
somewhat more spectacular length scale.

The editors of Nature should know better than to put a headline like "faster-
than-light information transfer" on there, since that is exactly what _isn't_
going on.

It's a shame that the article didn't make any mention of the "many-worlds"
interpretation of QM (I don't like that term myself) which as far as I know is
the only explanation for these results which makes any sense.

~~~
glymor
How is many-worlds a scientific theory ie how is it testable?

~~~
gaika
Many worlds is testable, though I wouldn't recommend it:

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

Edit: :)

~~~
hugh
I don't buy quantum immortality as a necessary consequence of many-worlds.
There are some rather dodgy steps in the logic -- in particular the one that
says that "you" can't find your personal experiences going down a branch where
your existence gets terminated. Surely _somebody_ has to be there to
subjectively experience having a bullet blast a hole in their skull?

Personally I think if you tried that, even if many-worlds is true, you'd
almost certainly wind up dead.

~~~
gaika
That "you" can be you in other people's mind as described by Hofstadter in
<http://en.wikipedia.org/wiki/I_Am_a_Strange_Loop>

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bentoner
The linked article is a little confusing. Here's a better description:

 _Pairs of quantum-mechanically entangled particles seem to know at once what
is happening to each other. Experiments show that even if this signalling is
not instantaneous, it must be really, really fast._

This is from Terry Rudolph's commentary:

[http://www.nature.com/nature/journal/v454/n7206/full/454831a...](http://www.nature.com/nature/journal/v454/n7206/full/454831a.html)

The Editor's summary is also good:

[http://www.nature.com/nature/journal/v454/n7206/edsumm/e0808...](http://www.nature.com/nature/journal/v454/n7206/edsumm/e080814-10.html)

~~~
michael_nielsen
Ben - All of those are sitting behind a publisher paywall :-( Do you have a
quick summary of how this goes beyond previous tests of the Bell inequality?

~~~
bentoner
Whoops, sorry.

Suppose we try to explain quantum correlations by postulating that as soon as
one member of an entangled pair of particles is measured, a very fast signal
travels to its partner, updating its state. If the signal is instantaneous,
then such a model is indistinguishable from quantum theory. But what if the
signal is fast, but not infinitely fast?

Salart and collaborators establish a lower bound on the speed of any such
signal (it must be > 10,000 times the speed of light). But we have to be
careful about which reference frame is used to define the velocity. Natural
choices, such as the rest frame of the experiment or the rest frame of the
cosmic microwave background, were ruled out by an earlier experiment, also by
Gisin's group [1]. The new experiment makes use of the rotation of the Earth
to establish a lower bound that holds for _any_ reference frame.

1\. <http://arxiv.org/abs/quant-ph/0002031>

------
glymor
Better reporting of the result:

[http://arstechnica.com/news.ars/post/20080813-bohms-
bummed-w...](http://arstechnica.com/news.ars/post/20080813-bohms-bummed-wave-
theory-needs-10000x-light-speed-to-work.html)

~~~
johntabet
Much better reporting over at ars. The articles are completely different, and
it seems that the ars writer actually knew what he was talking about.

------
davidmathers
From: <http://www.signandsight.com/features/614.html>

We've learnt in the natural sciences that the key to understanding can often
be found if we lift certain dividing lines in our minds. Newton showed that
the apple falls to the ground according to the same laws that govern the
Moon's orbit of the Earth. And with this he made the old differentiation
between earthly and heavenly phenomena obsolete. Darwin showed that there is
no dividing line between man and animal. And Einstein lifted the line dividing
space and time. But in our heads, we still draw a dividing line between
"reality" and "knowledge about reality", in other words between reality and
information. And you cannot draw this line. There is no recipe, no process for
distinguishing between reality and information. All this thinking and talking
about reality is about information, which is why one should not make a
distinction in the formulation of laws of nature. Quantum theory, correctly
interpreted, is information theory.

And can you explain all these strange quantum phenomena conclusively with your
information concept?

Not all of them yet, but we're working on it. With limitation it works
excellently.

How?

I imagine that a quantum system can carry only a limited amount of
information, which is sufficient only for a single measurement. Let's come
back to the situation of two particles colliding like billiard balls, and in
so doing entering a state of limitation. In terms of information theory that
means that after the collision the entire information is smeared over both
particles, rather than the individual particles carrying the information. And
that means the entire information we have pertains to the relationship between
both particles. For that reason, by measuring the first particle I can
anticipate the speed of the second. But the speed of the first particle is
entirely random.

Because the information isn't sufficient.

Exactly. Its randomness is ultimately a consequence of the finiteness of the
information.

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Eliezer
Here's what's actually going on:

<http://www.overcomingbias.com/2008/05/bells-theorem-n.html>

<http://www.overcomingbias.com/2008/05/spooky-action-a.html>

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slackerIII
So if you have a pair of entangled particles A1 & A2, can the observer close
to A2 quickly determine if A1 has been measured? I understand that you can't
transmit information using a single pair of particles, but what if you had a
pair A1 & A2 and a second pair B1 & B2. If one side can immediately tell that
A1 was measured before B1 or vice versa, it seems like that transmits a bit.

~~~
ars
No you can't tell if it's been measured. Assume you are measuring angle on the
photons: the angle of both sets always adds up to 0. So if you measure 10,
he'll measure -10.

So by measuring yours you know what he got, but that's it. You can't unmeasure
it, and you can't tell what he did.

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mattmaroon
"In both these cases, the information is communicated at or below the speed of
light, in keeping with Einstein’s axiom that nothing in the Universe can go
faster."

Isn't it that nothing that has mass can go faster? (And technically, if I
remember correctly, it's that nothing that has mass could accelerate beyond
that.) That's a pretty significant distinction.

~~~
pfedor
You don't need the notion of mass here, it's purely kinematics. From the
Lorenz transformation you see that if something traveled from A to B faster
than light, then there is a reference frame in which the arrival to B happened
earlier than the departure from A. If you want to do the math yourself, which
is actually quite illuminating and only requires high school math, then
consider a reference frame moving very fast (but slower than light) in the
direction from A to B. So, traveling faster than light would lead to the
problems with causality similar to time travel.

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robg
_The researchers found that when each photon reached its destination, it could
instantly sense its twin’s behaviour without any direct communication_

Isn't the problem explaining, scientifically, _sense_ and _direct_ in that
context? Without that explanation, I don't see how _without_ carries any
explanatory weight.

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jmilton
Ansible?

~~~
olefoo
No. At least as described in the article Alice and Bob both learn the same
string of random numbers at the same time, but they aren't able to pass
messages to each other through the channel.

