Uncertainty Sets Limits On Quantum Nonlocality

An anonymous reader writes "Research in today's issue of the journal Science helps explain why quantum theory is as weird as it is, but not weirder. Ex-hacker Stephanie Wehner and physicist Jonathan Oppenheim showed that the Heisenberg Uncertainty Principle sets limits on Einstein's 'spooky action at a distance.' Wired reports that the discovery was made by 'thinking of things in the way a hacker might' to uncover a fundamental link between the two defining properties of quantum physics (abstract, supplement). Oppenheim describes how uncertainty and nonlocality are like coding problems, enabling us to make a quantitative link between two of the cornerstones of quantum theory."

Comments

for the lulz

[Pojut]

I want to believe in quantum physics, but I'm not sure.

Re:for the lulz

[Anonymous Coward]

That makes sense. Quantum physics is needed to describe the behaviour of extremely small objects.

Re:for the lulz

[idontgno]

And delicious golden Brownian motion.

Re:for the lulz

[camperdave]

But if you ask most pilots, belief is the only thing that keeps helicopters in the air!

Nonsense! Helicopters fly because they are so ugly that the ground repels them.

Re:for the lulz

[tom17]

He only ruined it if you read the post. Until you observed it, it was both ruined and not ruined.

You just shouldn't have read it.

Re:for the lulz

[maxwell demon]

Yes, that's called the collapse of the whoosh function.

There's no link to the full article here in pDF

[Anonymous Coward]

Definitely not a link to the full article here [Not-PDF warning].

Re:hmmm

[mrjb]

That's just because a rock accidentally gets misplaced here and there.

Einstein, Heisenberg...

[srussia]

Heck, they even hinted at Gödel. Why not throw in Monty Hall too... wait, they did.

a coding problem?

[phantomfive]

describes how uncertainty and nonlocality are like coding problems,

In that case, I guarantee there is a bug.

Re:a coding problem?

[Monkeedude1212]

It's because when God was whipping things up he had just switched to Dvorak - and he couldn't find the semicolon because it was under his left hand. To remedy this - he ported the universe to VB.

Re:a coding problem?

[maxwell demon]

Yes, the universe is full of Heisenbugs.

Re:Locality == Free Will?

[adonoman]

No, it's the absolute determinism of the universe that is stopping from concluding that the universe is deterministic. Neither you, nor locality had any choice in the matter.

Re:Locality == Free Will?

[somersault]

Even if there were external forces acting to control your will in this universe, how do you know they're non-deterministic themselves?

Individuals certainly are responsible for their own choices anyway, even if you can accurately simulate 100% beforehand what they're going to choose.

Define 'observe'

[Twinbee]

Okay, rant time.

Whenever I see a beginner's guide to quantum theory, I always invariably see a phrase similar to:
"Stranger still, the electron doesn't even have properties like position and momentum until an observer measures them. "

And every time, I always think "define 'observe'", because that word is incredibly fluffy, vague as well as being immensely irritating. If a bat miles away happens to look in that direction with nothing in the way, is that counted as an observation? Are there a trillion different ways to observe it, and have they all been tried out to see the phenomenon stands? None, I repeat NONE of the articles I have ever read actually even remotely begins to touch upon that subject.

Re:Define 'observe'

[Anonymous Coward]

You are asking a great question, the problem is that no-one knows the answer. This is the "measurement problem", one of the biggest conceptual problems in Quantum Mechanics.

Re:Define 'observe'

[MozeeToby]

I think the closest plain English definition would be: has an interaction with something. More accurate, but more confusing might be: things are undefined until something happens that requires them to be defined in order for that something to happen. An electron doesn't have a position or a momentum until something occurs which require the electrons position and momentum to be known in order to determine the outcome. That might be a human being with an incredibly complex apparatus measuring the properties of an individual electron, or it might be a chemical reaction that is sweeping through the entire sample of whatever the electron is a part of.

Re:Define 'observe'

[guybrush3pwood]

None, I repeat NONE of the articles I have ever read actually even remotely begins to touch upon that subject.

Perhaps they don't touch it because you read them. Don't read them, and there's a 50/50 chance they will...

Re:Define 'observe'

[Sockatume]

Indeed. It's phenomenologically pretty well-defined, inasmuch as we can set up systems and we know whether we're observing them or not, and what'll happen to them if we do observe them, but we haven't a clue as to the mechanistics of it all.

Re:Define 'observe'

[Anonymous Coward]

In the quantum mechanical sense "observe" means to measure the property. That is a particle does not have momentum until someone measures it. Thus if a bat miles away happens to measure the the property it would count as an observation.

Essentially, in order for a quantum mechanical system to be observed there must be an interaction between the system and whatever does the observing (such as a photon). Prior to observation the system is thought of to exist in a superposition of states and after observation it is said that the wave function describing the system has "collapsed". The thought experiment of Schrodinger's cat is designed to explain this issue.

Re:Define 'observe'

[Just Some Guy]

That might be a human being with an incredibly complex apparatus measuring the properties of an individual electron, or it might be a chemical reaction that is sweeping through the entire sample of whatever the electron is a part of.

Fair enough. But does that chemical reaction require an observation to define its outcome if it depends on those quantum events? At what point do you decide that the decision must be made?

Re:Define 'observe'

[Anonymous Coward]

So what you're saying the the universe uses "just in time" physics.

Re:Define 'observe'

[gsliepen]

The best definition I have heard is this: suppose we have an observer O in state A, and a system S which is in the superposition of the states 1 and 2. When the observer observers the system, the state of S does not collapse, rather the observer and system become one, say OS, and is in a superposition of the states A1 and A2.

You can interpret this in various ways; one could say that this means the observer, or even the whole universe for that matter, branches all the time, and/or all possible states of the observer/universe exist simultaneously, however that again is just a description, not what might really be the case.

Disclaimer: I am a physicist.

Re:Define 'observe'

[Anonymous Coward]

The universe lazily loads the details when we want to inspect it. In reality the algorithm has optimized these things away. Its like forcing it to load the debugging symbols when you attach to the physics.exe process.

Re:Define 'observe'

[Ignatius]

No, if the System would end up in state 1/sqrt2 * (|A1>+|A2>), then no observation has taken place as

    1/sqrt2 * (|A1>+|A2>) = |A> x 1/sqrt2 * (|1>+|2>)

with "x" being the tensor product. The post measurement state would have to be an entangled state, e.g. something like

    1/sqrt2 * (|A1>+|B2>)

with |B> being the state of the observer after having heard a click on the Geiger counter, while in |A> there has been no click.

Re:Locality == Free Will?

[delinear]

I think maybe this?

Paywall

[zrbyte]

If anybody cares to read it, a preprint of the whole article can be found here.

Wired article completely misleading

[DrJimbo]

The Wired article "explains" entanglement by talking about Bob predicting what Alice did even though Alice is far away from Bob. This is the fundamental misunderstanding of quantum entanglement and has led to all sorts of wacky (and false) speculations and "theories".

The actual paper correctly says:

Non-locality can be exhibited when performing measurements on two or more distant quantum systems – the outcomes can be correlated in way that defies any local classical description. This is why we know that quantum theory will never by superceded by a local classical theory. Nevertheless, even quantum correlations are restricted to some extent – measurement results cannot be correlated so strongly that they would allow signalling between two distant systems.

Quantum entanglement (QE) provides a correlation not a communication. What this means is that not only can't you use QE to pass signals (or any information) between Alice and Bob, you actually need some other form of after-the-fact communication between them to detect the correlation in order to determine if QE happened at all. If QE was a method of communication then you could verify it by sending Bob a "cheat cheat" of what Alice was going to do or transmit. Instead, you need to look at the outcome of a series of measurements taken by Alice's and the outcome of a series of measurments taken by Bob just to see if QE actually happened.

Correlation is not communication.

Higgs Boson particle = Null value?

[Anonymous Coward]

Let me guess... The "God" (Higgs Boson) particle is nothing more than nature's null value to assign properties of something without it actually existing. How would that be for irony?

Captain Obvious

[Vedanuzal]

From the Quantum Physics that I did at uni I would have thought that the relationship stated in the article was blindingly obvious, but then the way Scientists like to complicate Quantum Physics is one of the reasons I'm glad I never completed the degree in Applied Physics. They have the same problem lawyers do, the more complicated it sound the more important they feel.