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Topic: Acoustic Metrics, and the OPERA neutrino result
Replies: 5   Last Post: Feb 16, 2012 11:15 AM

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Posts: 19
Registered: 2/10/12
Re: Acoustic Metrics, and the OPERA neutrino result
Posted: Feb 12, 2012 6:51 PM
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"Eric Baird" <> wrote in message
| Some of the people working on quantum gravity have been studying the
| properties of acoustic metrics, because AMs can generate some effects
| that coincide with QM's statistical predictions, even though the AM
| models are essentially classical.
| Carlos Barceló, Stefano Liberati, and Matt Visser, "Analogue Gravity"
| gr-qc/0505065
| The hope was that by understanding how acoustic metrics managed to allow
| phenomenology that seemed physically indistinguishable from Hawking
| Radiation (/etc.), we might be able to pick up some clues as to how we
| might create a super-theory ("Quantum Gravity") that incorporated and
| reconciled the best bits of general relativity and quantum mechanics,
| resolving the current awkward incompatibilities between these two
| branches of theoretical physics.
| Many of the QG guys have been treating acoustic metrics strictly as "toy
| models" -- as ways of studying the phenomenology and becoming familiar
| with it in a more intuitive context, without the "toy"'s exact machinery
| necessarily having to end up in the final theory.
| Acoustic metrics are fascinating, chaotic, complex things, but it's
| normally assumed that they can't be the Genuine Article, because they
| don't contain special relativity as an exact subset. AM-based models can
| be "relativistic" (in the literal sense) ,and can generate a lot of the
| same basic physical behaviours as SR along with a whole load of
| behaviours that look very similar to their SR counterparts, but the
| underlying machinery is very different. Special relativity is built on a
| solid flat-spacetime "base", whereas acoustic metrics are distinctly
| warped and writhy creatures that tend to generate some of the more
| familiar flat-spacetime results as emergent effects on an underlying
| curved and dynamic geometry rather than as initial hand-set laws.
| Since a full reduction to SR is typically used as one of the defining
| properties that any theory is supposed to have in order to be considered
| "credible" (and worthy of passing peer review), acoustic metrics have
| pretty much had to be presented as "toys" in order to be studyable. And
| even then, they didn't really get taken seriously until the 1990's.
| Anyhow...
| One of the reasons why we knew that these acoustic-metric-based models
| couldn't be real, /literal/ physics was the way that AMs dealt with
| lightspeeds and light-velocities. AMs don't have the same sort of
| lightspeed barrier as an SR-based physics. Depending on the choice of
| equations, an AM model can produce something that looks superficially as
| if it's agreeing with SR, and has the same particle-accelerator
| "lightspeed limit" as the special theory for /directly-accelerated/
| particles ... but can allow particles to be accelerated indirectly to
| more than the background speed of light, as long as they aren't
| travelling at more than the local velocity if light, at that time and
| location, and in the relevant direction. In an AM, it's only the local
| velocity of light that can't be exceeded, and what the speed of light
| might happen to be somewhere else isn't especially important. You can
| exceed it without breaking any fundamental laws. The results can
| certainly look wierd to a bystander, and it might look as if signals
| have impossible broken though what ought to be a horizon, but that's
| where the "Hawking radiation" descriptions kick in. "Acoustic metric"
| horizons aren't smooth like their GR counterparts, they seethe and
| fluctuate according to whatever else happens to be going on in the
| region, and that fluctuation is what lets them radiate.
| In an acoustic metric, the presence of signals in a region affects the
| region's signal-transmission speeds, so these things can be desperately
| non-linear. If you have a high-mass particle with a lot of momentum,
| travelling at almost the speed of light, it can create a local
| distortion around it that means that the nearby velocity of light is
| then greater in that region, in the direction that the particle is
| moving (essentially, it drags local lightspeeds). If that heavy particle
| then throws off a cloud of lightweight daughter-particles without
| slowing down too much, those teeny daughter particles can be initially
| emitted at more than /background/ c without travelling at more than
| /local/ c. The gravitomagnetic distortions warp the geometry and the
| definitions of speed and distance.
| It might be that the daughter-particles "brake" once they've left the
| influence of their parent, but for that short time they could outrun
| "bulk" background lightsignals in the region (although not their own).
| If the particle-creation event coincided with the generation of an EM
| pulse, then any daughter-particles that had slowed back to fractionally
| less than cBACKGROUND would still have a head-start on the main body of
| the pulse, and arrive early (although not necessarily by an amount that
| scaled with the path distance).
| It seems to me that this behaviour that we "knew" couldn't really be
| right, and which we only invoked as "toy" behaviour as a way of
| generating the same basic statistical "leaky horizon" behaviour as
| Hawking radiation, may now have been seen for real at OPERA.
| What we have is very heavy parent particles smashing into a target and
| producing exceptionally lightweight daughter particles (in this case,
| neutrinos), which then arrive at our detector too early. We also have
| (in the case of naturally-occurring neutrino bursts), an example of
| lightweight daughter-particles produced in highly energetic situations
| arriving ahead of the main EM wavefront, but seemingly not by an amount
| that scales with distance.
| On the face of it, the behaviour that we've just been seeing at OPERA
| (and which confuses us so much), appears to be a pretty decent match to
| what we'd expect to see if the acoustic metric concepts that the Quantum
| Gravity guys were looking at aren't just a theoretical toy, but are
| actually the real, actual, underlying physics.
| so, I was wondering ... are any groups in the "quantum gravity" /
| "acoustic metric" research community out there looking at this and
| investigating whether, by picking on AMs as a "disposable" interim
| model, they might have accidentally hit on the final solution, perhaps
| without fully appreciating it?
| That would be awfuly cool.
| Eric Baird (often online as "ErkDemon")

What we really need is to create a super-theory ("Quantum Gravity")
that incorporates and reconciles the best bits of general relativity
and quantum mechanics, or as the tailor said to the emperor buying
an invisible new suit, "Never mind the quality, feel the width!"

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