Physicists spooked by faster-than-light information transfer | Hacker News Reader

Physicists spooked by faster-than-light information transfer

nature.com

41 pointsbk51 comments

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.

There is definitely something new here, although the linked article does obscure what this is---read the actual paper or see my other comment.

Why do you think there is an important connection to many-worlds? Many-worlds is an interpretation of quantum theory: the experiment tests quantum theory itself, independent of your interpretation.

You're right, now I come to read more carefully there is something new here -- it's a more convincing test than has been done before.

The experimental setup is neat -- by setting up the detectors along an east-west axis and using the Earth's rotation, they can show that there isn't some "special" frame of reference in which the particles are communicating. Very neat experiment.

"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."

Totally. The article even contains this sentence:

"In other words, these photons cannot know about each other through any sort of normal exchange of information."

Very very disappointing.

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.

Um, this part seems to imply so... "the researchers found that when each photon reached its destination [some 18 kilometres apart], it could instantly sense its twin’s behaviour without any direct communication."

The key is, this "sensing its twin's behavior" only works if you interfere with the twin in a randomized way. You can't set one particle to a 0 bit and have the other one sense it. So you can't use this for sending information.

It's hard to explain this in the space of a comment. You might try Wikipedia - http://en.wikipedia.org/wiki/Quantum_entanglement

Think of ‘entangled’ photons as sharing a secret. If either one gives up that secret than you also know what the other one is doing. The only spooky part they don't behave like they have a specific value until you measure it and you can randomly change it's value.

PS: You can't send information from a to b using two ‘entangled’ photons sent from c and read at a and b.

Couldn't you measure the state of the entangled pair and record it? Then take one particle and walk it over the next village. Assume Bob and Alice have both agreed to look at their particles every 3 seconds. If the state changes relative to the last state, that's a 1. If you see the same state twice, that's a 0.

So would that be 1 bit of information every 6 seconds, or am I totally screwing up the quantum reverse tachion stream?

No. Once you measure it, it is no longer entangled. Also, you cannot set the state to whatever you want. You can only measure different properties of the particles.

It doesn't really make sense. Think of it as, there's probably something really intuitive happening, but physicists haven't figured that out yet. Instead we only have really complicated formulas and unintuitive metaphors. So don't think that necessarily things "really change" when you measure them. That's just the simplest assumption we have right now that makes the formulas workable.

> Once you measure it, it is no longer entangled.

This is not true. If it were then FTL communication would be possible. See http://www.flownet.com/ron/QM.pdf

>> Once you measure it, it is no longer entangled.

I was struggling to figure out how this wouldn't be useful for communication. That was the piece I was missing. Thank-you.

>which makes any sense.

Luttgeinstein/Chomsky blah blah blah. Who says nature is supposed to make any sense? Our "sense" is an artificial construct of the thoughts we are capable of thinking, limited by our inadequate brains and even more inadequate language.

Having said that my money is also on many-worlds, because it is the only one that makes sense.

Well, seems to make more sense then separating observers from the universe.

I agree there is nothing new here except the length.

The many worlds explanation does not help for this. That explanation is only useful for dealing with quantum interference. Many worlds does not explain why the two photons give related results.

A theory that can explain it is the hidden variables theory (which makes a lot of sense to me, because I can't understand the bell inequality). It has still not been ruled out, although the gut feeling is that it's not happening.

I have another option: the two photons are hard linked in the matrix simulator of the universe. That would explain it pretty nicely no?

The planck length is the size of each cell in the simulation. All the various quanta (and there are some really weird ones, like direction) are because that's the smallest increment in the variables that keep track of each cell.

Anyone know a good hacker? I want root access :)

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

It's more like, right now we have two rather bad explanations for basic quantum mechanics. There is the "particles change when a human measures them" explanation, and there is the "there are many worlds and we can usually only observe one" explanation. They both explain the same set of formulas that have been verified many times by experiment. Depending on what you want to do with the theory, it may be more intuitive to use one formulation than the other. Hopefully at some point in the future it becomes obvious that one of the formulations lacks the explanatory power of the other, and then scientists can cohere on one.

Like epicycles as a scientific explanation of how the sun revolves around the earth. Somewhat off topic but interesting reading - http://en.wikipedia.org/wiki/Deferent_and_epicycle

You're thinking about it wrong. Start from Bell's theorem - it tells us we have to sacrifice locality or realism. MWI weasels out of it by making worlds be observer dependent, basically splitting the observer up. Bell's theorem is the root. It's not an interpretation & all interpretations come from it.

I don't think it's testable -- that's why I'd call it an interpretation rather than a theory.

Then how is it different from counting angels on pinheads?

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

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

Edit: :)

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.

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

Can't this experiment just be reduced to a succession of observations that are extraordinarily improbable? Why sex it up with the suicide attempts?

The key is to link up observer with the experiment outcomes so you can only observe unlikely possibilities (and be dead in the rest).

Understood, but you could also set up an apparatus that simply observes a large region for a long period of time without killing anyone, watching for highly improbable events. Since each observer also lives their total lifespan, you get a much higher probability of a given observer observing a succession of improbable occurences.

Just create a machine that reads bits from a random data source and stops forever upon reading a 0 bit. Will the device run forever in some universe?

If you built the device, and it didn't run forever, what could you learn from this? Nothing, except maybe how to build cool counting machines. :)

That what reporters are for :) If you read the news you will see an endless stream of highly unusual events, but unfortunately that doesn't prove anything.

Touche. :) But in the same vein, what would a series of failed suicide attempts prove that the mentioned apparatus could not?

Any machine that you build will produce expected results, up to 10^-15 precision with current hardware.

Failed suicide attempt is unlikely and provides evidence towards many worlds as a theory and not just interpretation.

Sorry, by apparatus I mean any method of observing a large body of matter or space that exhibit quantum behaviour (i.e. anything), not necessarily a physical machine. We are just looking for a series of highly improbable quantum state transitions.

Doesn't matter, any method of observation you choose will produce expected results.

Unless you observe yourself, where you will not be able to observe expected results because you're dead.

Think of each observer attempting to push a rock into their palm and having it appear on the other side, without enough force to push it through. Think of everyone on the planet attempting this simultaneously, thousands of times. If the rock disappears and reappears on the other side for one observer, doesn't this accomplish the same thing, at least for that observer, that the suicide attempt does?

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

>The conclusion was that the minimal speed of hypothetical spooky action at a distance, under plausible assumptions for this experiment, is at least 10,000 times greater than the speed of light.

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...

The Editor's summary is also good:

http://www.nature.com/nature/journal/v454/n7206/edsumm/e0808...

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?

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

Can you upload Rudolph's commentary to scribd?

I can't without violating copyright, but I emailed Terry and asked him to.

Rudolph's commentary is reproduced at the bottom of this page:

http://www.physicsnerd.com/qi.htm

Better reporting of the result:

http://arstechnica.com/news.ars/post/20080813-bohms-bummed-w...

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.

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.

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

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.

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.

"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.

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.

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.

Ansible?

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.

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