Thursday 21 May 2015

How information helps to banish quantum weirdness - the fivefold way.

One of the strands in the current excitement about information is that it can provide a narrative for explaining our experiences of the world* that does away with the mental contortions that other scientific narratives demand. An easy example - or rather the one I find most useful - is that you can stop worrying about 'spooky action at a distance' if no information is communicated faster than the speed of light (see my past posts on the EPR paradox).

* The New Scientist would spin it by saying 'explaining reality' but I think I am a reality-agnostic. Anyway, I don't like word.

More generally, a number of narratives are developing that are more of less 'grand'.
Quantum purity: How the big picture banishes weirdness

Anil Ananthaswamy

We have become accustomed to the universe blowing our minds – perhaps too accustomed. Quantum weirdness – things like particles being in two places at once, or appearing to share a telepathic link – has been baffling us for more than a century now. The physicist Richard Feynman once said that nobody really understands the quantum world. Or as others have put it: if you think you understand it, then you definitely don't. So it is tempting to throw up our hands and say human brains can never grasp it.
But maybe we shouldn't be so defeatist. Isn't it just possible that we simply haven't yet got to the bottom of how quantum mechanics really works? That's what Giacomo Mauro D'Ariano of the University of Pavia in Italy, and his colleagues Giulio Chiribella and Paolo Perinotti think – and they have been doing something about it. They have come up with a non-weird foundation from which all quantum weirdness can arise.
Ananthaswamy goes on to draw parallels from scientific history, of how we had descriptions of the world that worked but seemed arbitrary until we later found an underlying principle that help it all together.

Kepler's laws of planetary motion worked but were arbitrary until underwritten by Newton's universal law of gravity. Lorentz's equations were ad-hoc solutions to accommodate contradictory experimental observations about the speed of light until Einstein proposed some physical principles – that the speed of light in a vacuum is constant and independent of the motion of the source of light – from which he was able to derive Lorentz's transformations
"When I look at quantum theory, I see something that's akin to the ad hoc nature of Kepler's laws of planetary motion and of Lorentz transformations," says Hardy [Lucien Hardy of the Perimeter Institute in Waterloo, Canada]. "What we need is some deeper set of principles."

In the early 2000s, Hardy made his own attempt at coming up with some. It wasn't a full solution, he admits, but it inspired D'Ariano, Perinotti and Chiribella to dig deeper. "The idea was to somehow reprogram the genetic code of quantum mechanics, choose some physical properties and say, 'this is the fundamental thing', and re-derive the rest from it," says Chiribella, who is now at Tsinghua University in Beijing, China.
For both Hardy and D'Ariano's team, the "fundamental thing" has to do with information.


Eventually, they came up with five fairly common-sense ideas that worked: things like ensuring the future can't influence the past
Here are those five ideas:
Stuff in the future cannot affect a measurement you're making right now.

If a state is not too noisy, then there exists another state that can be distinguished from it.

If you know everything it is possible to know about all the stages of a process, then you know everything you can about the whole process.

There are ways to efficiently transmit all the information relevant to a measurement of a physical system without having to transmit the system itself.

When you have a system with several parts, the statistics of measurements carried out on the parts is enough to identify the state of the whole system.

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