
Bounding the speed of ‘spooky action at a distance’ - espinchi
http://arxiv.org/abs/1303.0614
======
scythe
So what they actually measured is something that appears to go faster than
light in the reference frame of measurement. This does _not_ mean they
measured the actual speed of the interaction. Instead, the point of this
experiment is to lose a loophole in Bell test experiments: simply put, Bell
experiments had previously proven that this seemingly-"non local" (bear with
me) state exists and preserves dntanglement over long distances. The loophole
was that it previously wasn't possible to time the ecperiment well enough to
make sure that the whole thing didn't occur in some unexpected, _subluminal_
way. Now we know it couldn't have occurred by some sort of classical
interaction between the entangled qubits.

So how fast is the interaction, really? Well, it turns out that if something
moves even 0.0001% faster than light, we can always find a reference frame in
which it moves a million times faster than light, or infinitely fast, or even
in which it appears to move at some huge negative velocity: back in time. See
"tachyonic antitelephone". In other words, once something is faster than
light, it is, essentially, instantaneous. This is in accordance with standard
predictions of quantum mechanics.

Source: I'm a physics grad student.

~~~
lisper
You might enjoy these then:

[http://blog.rongarret.info/2011/09/faster-than-light-
probabl...](http://blog.rongarret.info/2011/09/faster-than-light-probably-not-
but-you.html)

<http://www.flownet.com/ron/QM.pdf>

~~~
scythe
My favorite interpretation so far:

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

It's sort of like many worlds with a whole lot of unnecessary baggage stripped
away.

------
tedsanders
Suppose you and a distant friend both agree to wave a flag at 1200 GST.

To an alien observing Earth, it looks as if the "human flag-raising
interaction" is an instant effect that "travels" faster than the speed of
light, since the flag raising happens at the same time in two places.

But really, nothing is travelling. You and your friend just pre-agreed to do
something at the same time at different places.

If energy or information is not transmitted, then what is? I'd argue nothing.

(-A physicist)

~~~
didgeoridoo
Almost, but spooky action is quite a bit spookier. Suppose you and your friend
both agree to MAYBE wave a flag every day at 1200 GST, then communicate no
further about it. The alien would always observe that either both flags are
being waved, or both are not.

~~~
GuiA
(My physics education is limited to some undergrad level classes, so sorry if
my interpretation is erroneous)

Could it be that spooky action has underlying causes that we can't tell yet?
For example, reusing the flag metaphor- you and your friend decide to MAYBE
wave a flag every day at 1200 GST, and then in practice it happens that every
sunny day you both wave it (because hey it's sunny, everybody likes to go
outside) and on rainy days no one does (because it's warm and dry inside in
your house).

An alien with no knowledge of meteorology and our reaction to it observing
this would always observe that either both flags are waved, or both are not,
in a seeming random matter, where in fact it is not random at all.

~~~
andrewflnr
I believe that would be labeled a hidden-variable theory, which have
apparently been mathematically eliminated from consideration.

~~~
tedsanders
Only local hidden variables has been eliminated (as opposed to nonlocal), and
even local hidden variables can logically exist with some tortured reasoning.

------
eoinmurray92
Everyone here is getting confused by the fact that you cannot control the
polarity/polarization of the qubits before/as you send them.

This means that one __cannot __send information faster than light.

What is useful however is the fact that when you send entangled qubits very
far away, and measure them is the fact that the results of the measurements
are always correlated/anti-correlated.

As an example.

Alice has a pair of entangled qubits, they are in a superposition of 1 and 0
(lets say they are an correlated pair, such that when measured both quibits
will always agree).

She sends one to Bob.

Bob and Alice now hold one qubit each out of the entangled pair. They both
measure their qubits. The measurement is probabilistic, you have a 50/50
chance of getting a 1 or a 0, but Bobs quibits and Alices quibit will agree
everytime.

Since you can't control the outcome of the experiment you cannot decide which
bit to send and hence cannot send information.

The useful result is that Alice and Bob have a string of bits which __only the
two of them __know the arrangement of. They can use the string of bits as a
one-time pad for encryption.

I have simplified this immensely but I think the point gets across.

Source : I currently undertake research into generating qubits for quantum
computation and communication purposes.

~~~
icambron
Can you explain the one-time pad application a bit more? It seems to me that
if Alice and Bob can share a bunch of entangled qbits, they could just as
easily share a classical one-time pad. What am I missing?

~~~
eoinmurray92
Im not explaining it very well and its difficult to condense, perhaps I'll
write a blog post that goes into more detail for a layman reader.

~~~
icambron
Let me know if you do; I'm certainly interested.

------
tedsanders
It's like you and a friend have a deck of cards.

You each take a card from the deck, and then travel a million miles away.

Then, at the crack of noon (synchronized to GST, of course), you both look at
your cards. Your card is the 3 of Clubs.

You travel back to meet your friend, and discover that his card (which he
measured at noon, just like you) was the 7 of Diamonds.

You repeat this experiment a gazillion times and are amazed!! No matter how
many times you run it, you and your friend never get the same card!!!

It's as if the cards are sending spooky signals to each other!

And after you double check that you and your friend are both looking only at
1200 GST, you realize that this spooky signal travels faster than the speed of
light (since the it takes no time to "travel" a million miles).

It's like magic, eh?

Not really. The fact there is one of each card means it's impossible for you
and your friend to have the same card. No magical spooky signals are being
sent.

(The real question is whether the quantum mechanical case is analogous to this
contrived example. I think it is. The important difference in quantum
mechanics it that you really don't know what card you have until you look at
it. However, that doesn't mean that your card is sending a faster-than-light
spooky signal to tell the other card what to be. And you certainly can't use
it for signalling or anything useful.)

~~~
Filligree
What you're describing is a hidden-variable theory, which has been ruled out
by Bell's Theorem. Sorry.

~~~
tedsanders
No need to be sorry. :)

The point of my post is to convey the sense that this article headline has
when it says that "spooky action" has a speed.

Then, in the last paragraph, I ask whether the example I gave is actually
analogous to quantum mechanics. I think it is, particularly if you see things
through the many-worlds interpretation.

In the many-worlds interpretation, you and your card exist in a superposition
of 52 states, 1 for each possible card. When you look at your card, your
wavefunction separates into 52 different experiences and that effect
propagates out into the universe at a speed less than the speed of light.
Nonetheless, when you compare your results with your friend, you find that you
never got the same card, even though you looked at the same time.

Nitpick: Bell's Theorem doesn't rule out hidden variable theories. Only local
hidden variable theories.

------
tedsanders
Physics works as we already thought it did. Move along, folks. Nothing to see
here. :)

------
ryanatkn
This is ignorant speculation, but what if the interaction doesn't depend on
distance, and instead always takes the same fixed amount of time? What if that
delta t is equal to 1 plank time, one tick in the cosmic 3D simulation? Of
course that's an advanced experiment to run, but the implications would be
fun.

------
_of
Watch out for arxiv.org, these papers may or may not have passed peer review.
A lot of junk is deposited there.

~~~
jessriedel
This same group research group, including the same first author, published in
Nature last year.

<http://arxiv.org/abs/1205.2024>

Since it's common practice to first post to the ArXiv before submitting to a
journal (note, for instance, that the first version of their Nature paper was
posted to the ArXiv 4 month before being published), I'm confident this paper
is or soon will be submitted for peer review. It's always possible they made a
mistake or are overstating their findings, but I don't think these guys are a
bunch of hacks.

------
digz
I sent this to my friend who has a phd in physics and is a post-doc at
Cambridge and he said it's bogus.

~~~
ChrisClark
Then he didn't understand what you were asking. It can easily be faster than
the speed of light because no information can be transmitted through this
phenomenon.

~~~
n3rdy
> Then he didn't understand what you were asking. It can easily be faster than
> the speed of light because no information can be transmitted through this
> phenomenon.

If its observable then how could information not be transmitted?

I'm the furthest thing from a physicist but if quantum entanglement causes an
atoms polarity to change because millions of miles away another atom it is
entangled with did the same, that would seem like an excellent way to encode
information into bits, supposing you can change the atoms polarity at a whim.

~~~
gambiting
Because just observing the polarity changes it, so you can never be sure what
position it was in originally. So you can affect an object that is light years
away from you instantly, but it's useless for transmitting information,
because while you know which state it is in, the party on the other end does
not,and has no way of finding out without affecting the information and making
it useless.

~~~
n3rdy
> Because just observing the polarity changes it, so you can never be sure
> what position it was in originally.

That does put a crimp in my universal router plans, but that's really
interesting by itself that just looking at the atom affects its polarity.

~~~
gambiting
Well it does not _really_ change the polarity.

Think about it this way:

You have two cards(A and B) - you and your friend pick one randomly. Until you
look at yours, it's both A and B at the same time. It's the same with your
friend's card. So you both own cards that are in a "mixed" A/B state. And
then, when you and your friend look at them, the state is "decided".

Therefore, you could send your friend and his card on a rocket million years
away from Earth, and then look at the card. Then,you can tell with 100%
guarantee which card your friend has. Is that information useful? No,not
really. You were able to instantaneously find out the state of an object
millions of light years away,but you can't use it to communicate.

------
loceng
This would be helpful for intergalactic domination. Kidding.

