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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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Registered: 12/7/04
Acoustic Metrics, and the OPERA neutrino result
Posted: Feb 12, 2012 6:41 PM
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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"

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.

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")

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