Once Before Time - Martin Bojowald [88]
Initially, these bursts were thought to be events in our Milky Way; it was difficult to imagine how their great brightness could be caused by a very distant object. But when more and more of them were found over time, they showed a largely isotropic distribution over the whole sky, in no way following the structures of the Milky Way. Gamma ray bursts must thus be objects outside and far from our galaxy.
Moreover, these gamma ray explosions are so energetic that they emit the majority of their radiation not as visible light but as gamma particles—known also from a form of radioactivity. Just like light, these are electromagnetic waves, but of extremely high frequency and short wavelength, endowing them with another advantage for scrutinizing space-time structures: The wavelength is closer to the expected scale of discrete space-time than it would be for visible light, even though the difference from the Planck length remains extreme, and thus corrections to its propagation by the crystalline structure are more pronounced and more easily detectable. (Then again, the frequency should not be too high owing to quantum theoretical effects of the electromagnetic field. Radiation is composed of photons—energy packets carrying the radiation’s total energy. With high energies of single packets, and thus high frequency, the total radiation is distributed over few packets. Fewer quanta then hit a detector, and the measurement result is subject to stronger statistical fluctuations, from which the precision suffers.)
In June 2008, the satellite GLAST (the Gamma-ray Large Area Space Telescope, now officially called the Fermi Gamma-ray Space Telescope, but still the old acronym) was launched, dedicated to a precise detection of such eruptions. It has already provided many new insights into the behavior and distribution of these stellar explosions. Whether this satellite’s results will be significant for quantum gravity remains to be seen, but it shows good potential. The main purpose of the satellite is, foremost, to provide a better understanding of the origin of these bursts that bring us knowledge from the depth of space. In the meantime, a lack of knowledge of their origin certainly does not prevent us from using their radiation for other purposes (such as testing quantum gravity). One possible explanation for the slightly longer eruptions is a star collapsing to a black hole. Peter Meszaros and Martin Rees, in the early 1990s, analyzed how the collapse energy can be converted into gamma rays, in agreement with many observations. (An alternative explanation for shorter eruptions is the merger of two neutron stars, whose end result could again be a black hole.) Irrespective of whether this is the case, black holes provide a further stage for explorations of quantum gravity.
BLACK HOLES: WORDS OF BLACKNESS
Black holes are plagued by the same singularity problem as the entire cosmos. It isn’t the whole universe that collapses here, but only some part of its matter content gathered in a bounded region. Still, in the end waits the breakdown of general relativity in the form of a singularity, grimly reaping not just all of matter but space and time themselves. The repulsive forces of quantum gravity can prevent the total collapse and the singularity, as we will see in more detail in the next chapter. For now, we are interested primarily in the observability of key effects arising in the process.
Black holes are extremely uncommon objects, and so their existence in the universe was long doubted (and occasionally still is). By now, however, they have been clearly confirmed by many observations. Some astronomical objects can, given their compactness, be understood only as black holes. They appear as highly compressed distributions of mass in a small spatial region, with an enormous density compared to all other stellar objects (and yet much smaller than the Planck density). As already seen, gamma ray bursts can possibly be traced back to black holes, but this explanation is not completely confirmed.
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