Imprints of black hole area quantization in gravitational waves

Tuesday, Jan 25th Adrián del Río, PennState

Imprints of black hole area quantization in gravitational waves
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By Jorge Pullin, LSU

One of the distinct places where quantum mechanics has an imprint on our everyday life is in atomic spectra. The theory says that atoms can only absorb and emit certain discrete quantities of energy (quanta) when they interact, for instance, with an electromagnetic field. Since the energy of photons is directly related with their frequency, this means that atoms emit light at specific frequencies. An example of this is easily seen by sprinkling table salt on an open flame. It will turn yellow, the color associated with the typical quanta of sodium. 

Quantum gravity theories predict that the mass (and therefore the area) of black holes is quantized. This means that when black holes interact with gravitational waves, the frequencies involved will be quantized. The situation is analogous to  that of atoms and electromagnetic radiation. This talk shows that this quantization can leave imprints in the types of gravitational waves that are currently being detected by interferometric gravitational wave observatories like LIGO or VIRGO. These waves come from collisions of black holes. The collisions perturb the holes and the latter "ring down" emitting gravitational waves, much like a bell rings down emitting sound waves (in fact, for Solar sized black holes the frequencies are similar, of the order of kiloHertz). This ring down would have imprints from the quantization of the area that appear as "echos", repetitive patterns in the waves emitted. 

The quantization of areas has long been expected in theories of quantum gravity. Usually people assumed that the quantization would be given by some integer multiple of the "fundamental" area given by the Planck length squared. The Planck length is the fundamental length one can construct using the gravitational constant G, the speed of light c and Planck's constant of the quantum theory known as "hbar". This talk showed that if areas are quantized in this way, then there are potentially observable consequences in gravitational waves detected from black hole collisions. Loop quantum gravity, on the other hand, has a more sophisticated prediction about the quantization of areas. In it, the quanta are not equally spaced, but quickly "bunch up" as one considers larger areas. This implies that for large black holes like the one LIGO can observe, the effects of the quantization are incredibly small. In a sense this can be viewed a positive aspect of the theory, since no deviations from the predictions of the classical theory have been actually observed in gravitational wave detections.