Sunday 1 September 2013

Communicating information faster than light

The New Scientist has been agonising about reality again. The cover of the issue of 3 August 2013 advertises the article thus:

Free Will
One of them is wrong. But which?

Michael Brooks, "Reality Checked" New Scientist Volume 219 No 2928.
Online version: Quantum weirdness: The battle for the basis of reality accessed 1/9/13

It comes down to entanglement in quantum mechanics requiring the violation of causality because it requires the communication of information faster than the speed of light. I've discussed this before (see my posts labelled causality) and my speculation was that whatever it is that is sent faster than the speed of light is not really information, and therefore there wasn't a problem.

Well this latest article argues quite explicitly that that get-out has been proved to be wrong.
Relativity only forbids an influence propagating above light speed when it carries information. So what if some weird phenomenon unknown to physicists could break relativity, connect two entangled particles, while being information-free?

We have even less idea what that sort of influence might look like. Chances are it doesn't matter: since last year, this escape route back to normality has also been blocked off. Together with Gisin and others, Jean-Daniel Bancal at the University of Geneva worked through what would happen within a network of four senders and receivers that could synchronise their measurements of entangled photons. In this theoretical set-up, influences could travel through space-time at whatever speed they liked, just as long as they contained no information.

And it failed to reproduce reality. There was no way any physical mechanism of any stamp could produce the quantum correlations seen in experiments unless hidden influences within the network could also send information at above light speed (Nature Physics, vol 8, p 867). 
I remember the moment at school when I first got really excited by physics. It was during an A-level physics lesson when I was introduced to relativity and the idea that time itself was relative, that time in a moving object passed at a different 'rate' than time in a stationary object (so to speak). It was the discovery that our every day common-sense interpretation of reality could be wrong - and that physics knew better. Of course that aspect of relativity is no big deal any more. That's to say, I've got used to it and realised that the world still makes perfect sense with the speed of light rather than time as the universal reference.

I'm back there now, though, losing my footing in reality. Now that physicists are closing off the escape routes.  Can I comprehend a universe that doesn't respect causality? Or one without free-will?

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