Saturday, November 21, 2015

Saturday morning physics

Quantum nonlocality has been getting a good run on the web lately (and here) due to some recent papers confirming its reality. 

Nature has had a feature up discussing in some detail the promising idea (to many physicists, apparently) that quantum entanglement is actually at the heart of space-time:
All that’s needed, he asserted, is ‘entanglement’: the phenomenon that many physicists believe to be the ultimate in quantum weirdness. Entanglement lets the measurement of one particle instantaneously determine the state of a partner particle, no matter how far away it may be — even on the other side of the Milky Way.

Einstein loathed the idea of entanglement, and famously derided it as “spooky action at a distance”. But it is central to quantum theory. And Van Raamsdonk, drawing on work by like-minded physicists going back more than a decade, argued for the ultimate irony — that, despite Einstein’s objections, entanglement might be the basis of geometry, and thus of Einstein’s geometric theory of gravity. “Space-time,” he says, “is just a geometrical picture of how stuff in the quantum system is entangled.”
The story includes some graphics which help, a little bit, but here is perhaps the key one:

Anyhow, the article explains more.

I've also noticed an interesting paper on arXiv by someone from the University of Bristol, of all places.   I think it's fair to summarise his proposal as being that quantum non locality derives from a geometry you can get by fiddling with the "time" part of space-time.   Here's his introduction:
An elementary discrepancy between quantum theory and relativity is that quantum theory is inherently nonlocal, whereas spacetime has the structure of a manifold, and is thus local by construction. The discrepancy is resolved on the level of information, since the intrinsic randomness in the measurement of a quantum state prevents instantaneous signaling (by the no-communication theorem [10, II.E]). This resolution is satisfactory if information is considered to be fundamental [10, III.C].  However, if one considers geometry to be fundamental, then the discrepancy remains.

Here we pursue a possible resolution from the perspective that geometry is fundamental, with the aim that it may shed light on the nature of quantum gravity.1 Just as simultaneity has no universal meaning in special relativity, we propose that a ‘moment of time’ has no universal meaning, and different observers will in general disagree about the ‘duration’ of a single moment of time. In particular, even clocks in the same inertial frame may  disagree. The paper is organized as follows. We first propose a new operational definition of time using the identity of  indiscernibles: we postulate that time passes if and only if a system undergoes a transformation which is not local and invertible. We then show that this postulate is compatible with the thermodynamic arrow of time in a generic example. Furthermore, the postulate results in a spacetime with positive dimensional events, thus giving rise to Bell nonlocality without requiring  retrocausality.

Finally, we examine the ontology of the wavefunction in this framework. In particular, we show that if spacetime events are topologically closed, then the wavefunction is epistemic. Moreover, we find that the preparation assumption of the PBR theorem does not hold using the worldlines of 4-photon entanglement swapping.
 Of course I don't understand all of that, but its sounds rather interesting.

And as for gravity and the detection of gravity waves:   it's good to see via a post at Sabine H's  Backreaction blog that the Parkes Radio Telescope (which I forced my family to visit last Christmas) has been doing valuable work on trying to detect gravity waves via careful pulsar watching.

The sort of sad result, though, is that they haven't found them; which, as Bee says, is "the birth of a new mystery in physics".   Not sure is that is "cool" or not....

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