Once Before Time - Martin Bojowald [106]
Back to analog gravity: As already mentioned in the description of horizons, quantum theory of matter near a black hole horizon leads to a creation process—Hawking radiation. Correspondingly, at a horizon in a medium, one should expect a similar creation process of phonons. Since one can easily build an analog horizon, and since phonons are nothing but detectable sound, one can test the general process in this way. Ralf Schützhold, for instance, has recently proposed experiments. The field is currently a vibrant one, with major contributions also from Carlos Barceló, Stefano Liberati, Matt Visser, and Silke Weinfurtner.
Although it may seem easy, one cannot simply hear the sound of analog Hawking radiation, for it is not intense enough. The medium must be isolated from other external sources, a challenge complicated by the production of a horizon whose existence requires high velocities in some parts of the medium. Hopes rest on exotic forms of matter, called Bose-Einstein condensates, that in some materials arise at very low temperatures. They have the advantage of a very small sound speed; tiny velocities of the medium are then sufficient to confine sound waves to a given region. Moreover, this kind of matter exists at low temperatures, easily lower than that of the phononic Hawking radiation. In contrast to black holes in the universe, Hawking radiation is bright (or, rather, loud) in comparison with the background radiation caused by the ambient temperature and thus should be more easily detectable.
These experiments are only in the planning stage. But they definitely provide the possibility to probe properties of horizons in a secure laboratory before undertaking risky tests of black holes in space. No dangers arise, for there is no singularity and the horizon automatically disappears when the liquid comes to rest. If analog Hawking radiation can be confirmed and precisely measured in this way, it will increase confidence in mathematical methods used to compute it. In addition, there are many foundational problems in the case of Hawking radiation that are not yet fully understood. Instead of using mathematics, which has been attempted for several decades with slow progress, successful measurements could allow us to directly ask Nature herself.
QUANTUM THEORY OF BLACK HOLES:
MAKING THE MUTE SPEAK
Black holes provide a treasure trove of questions for which the quantum theory of matter or gravity is of crucial importance. This is the case for the region of the horizon as well as for the final singularity of the collapse. In particular, we must pose the question of what a black hole really is, for general relativity, due to the existence of a singularity, does not give us a complete picture.
COSMIC EVOLUTION? BIRTHING WORLDS
It is clear that black holes exist in the universe, making it all the more shocking that we do not fully know them. The question is not only what kind of astrophysical object we have here, just as the nature of gamma ray bursts is not completely clarified. Black holes pose an entirely different problem: It remains to be determined whether they are just dense and highly curved regions in our universe, like many other heavy objects, or splitting-off processes of space-time branching out into a daughter universe. The decisive question is then: Does the horizon mark a transition region into a different world accessible only through it, or is it merely a transient stage for a region within our universe?
These two possibilities can be illustrated once more by their Penrose diagrams if one graphically extends