Acoustic metric
In
(Generally, in mathematical physics, a metric describes the arrangement of relative distances within a surface or volume, usually measured by signals passing through the region – essentially describing the intrinsic geometry of the region.)
A simple fluid example
For simplicity, we will assume that the underlying background geometry is
This restriction can also arise if we imagine that sound is like "light" moving through a spacetime described by an effective metric tensor called the acoustic metric.
The acoustic metric is
"Light" moving with a velocity of (not the 4-velocity) has to satisfy
If
Acoustic horizons
An acoustic metric can give rise to "acoustic horizons"[1] (also known as "sonic horizons"), analogous to the event horizons in the spacetime metric of general relativity. However, unlike the spacetime metric, in which the invariant speed is the absolute upper limit on the propagation of all causal effects, the invariant speed in an acoustic metric is not the upper limit on propagation speeds. For example, the speed of sound is less than the speed of light. As a result, the horizons in acoustic metrics are not perfectly analogous to those associated with the spacetime metric. It is possible for certain physical effects to propagate back across an acoustic horizon. Such propagation is sometimes considered to be analogous to Hawking radiation, although the latter arises from quantum field effects in curved spacetime.
Quantum gravity
Since acoustic metrics share some statistical behaviours with the way that we expect a future theory of quantum gravity to behave (such as Hawking radiation), these metrics have sometimes been studied in the hope that they might shed light on the statistical mechanics of actual black holes. Some people have suggested [citation needed] that analog models are more than just an analogy and that the actual gravity that we observe is actually an analog theory. But in order for this to hold, since a generic analog model depends upon both the acoustic metric and the underlying background geometry, the low energy large wavelength limit of the theory has to decouple from the background geometry.
See also
- Acoustics
- Analog models of gravity
- Gravastar
- Hawking radiation
- Quantum gravity
- Superfluid vacuum theory
References
- ISSN 2469-9950.
- Unruh, W. G. (1981). "Experimental black hole evaporation?". Phys. Rev. Lett. 46 (21): 1351–1353. . Considers information leakage through a transsonic horizon as an "analogue" of Hawking radiation in black hole problems.
- Visser, Matt (1998). "Acoustic black holes: Horizons, ergospheres, and Hawking radiation". Class. Quantum Grav. 15 (6): 1767–1791. S2CID 5526480. Indirect radiation effects in the physics of acoustic horizon explored as a case of Hawking radiation.
- Barceló, Carlos; Liberati, Stefano; Visser, Matt (2011-05-12). "Analogue Gravity". Living Reviews in Relativity. 8 (1): 12. PMID 28179871. Huge review article of "toy models" of gravitation, 2005, currently on v2, 152 pages, 435 references, alphabetical by author.