Uncertainty Sets Limits On Quantum Nonlocality 223
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."
There's no link to the full article here in pDF (Score:3, Informative)
Re:Define 'observe' (Score:2, Informative)
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' (Score:5, Informative)
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.
Paywall (Score:3, Informative)
Re:Define 'observe' (Score:1, Informative)
Contrary to other commenters in this thread and your own perspective of it - an "observation" in the quantum mechanical sense is far from vague and very easily defined (and yes, people know what it means). Observation = an interaction of a quanta with another quanta, once it has interacted, whether considering a particle wave or composite nature, it ceases to be what it was in terms of position and momentum - passing along only one of the two to a measuring device within our means to construct at present. Though I do agree, quantum mechanics is a horrible branch of physics in all regards - its essentially a hack-job meant to encompass all we knew about 60-80 years ago and its account for *enough* new tech to keep it kicking without building a new branch. In reality, it will probably persist as long as Newtonian physics and relativity - kept in context of the context in which it applies.
Re:Define 'observe' (Score:2, Informative)
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' (Score:5, Informative)
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.
Wired article completely misleading (Score:5, Informative)
The actual paper [cam.ac.uk] 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.
Captain Obvious (Score:2, Informative)
Re:Define 'observe' (Score:3, Informative)
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.