Once Before Time - Martin Bojowald [105]
Since oscillations of a single atom are quantized, as described by quantum mechanics, so are collective oscillations. Their intensity cannot change continuously but only in discrete steps by creating more phonons through atomic vibrations. Here we have an atomic, quantized picture of sound, much as photons give us the quantized picture of light. For gravitational waves one expects a similar quantized view, but would have to employ the difficult quantum gravity for a complete derivation. While their structure remains incompletely known, the name graviton for these atomic excitations of space-time already exists. In loop quantum gravity, the final picture might be very close to that of condensed-matter physics; there is indeed a discrete atomic structure of space-time. Its excitations, once they are mathematically understood, should result in gravitons. Initial investigations have been done in quite some detail by Madhavan Varadarajan.
Waves arising from the propagation of an oscillation can be used to transmit signals, provided there are methods for a targeted creation and a precise perception. For light and sound, we use seeing and hearing. On a microscopic level, individual photons of light or phonons in the air are perceived and interpreted in their totality as messages. These exchange particles also have a fundamental meaning in the understanding of physics, for they form the elementary picture of a force. In the case of phonons, this force is the elastic one: A deformation in a solid or an increase of pressure at one place in a liquid propagates as a wave from its starting place and eventually influences distant regions. Deformations and pressure increases occurring there can be traced back, in the macroscopic interpretation disregarding elementary processes, to the action of a force.
Similarly, photons are the elementary constituents of light, which in enormous quantities form the complex signals we often use to communicate. Light propagates even in empty space; no material force is transmitted by photons, only the electromagnetic one. Gravitons have been postulated as the elementary exchange particles of gravity. Despite the incompleteness of its theoretical formulation in quantum gravity, this notion, in contrast to Newton’s efforts, nevertheless shows a clear advantage: Gravitons as messengers must first travel through empty space between two places; the force they transmit cannot act instantaneously. A consistent formulation of elementary gravitons as quanta of gravity would automatically resolve Newton’s reservations about his own formula.
26. A waterfall illustrates a horizon where waves, as sound signals in water, cannot propagate upward against the falls. One can see this by the placid run of the water at the top, remaining undisturbed by the turbulence that builds up at the bottom. Analogously, there is a horizon around a black hole from behind which light cannot escape. (Leura Falls, Blue Mountains, Australia.)
A consistent formulation of the force picture, as it is given in its classical description by general relativity, is very complex and often weighed down by apparent paradoxa. Again, the existence of black holes clarifies the problems: A black hole is the source of a strong gravitational force by which it influences masses in its neighborhood. The force is so strong that most galaxies are bound into rotating discs by gigantic black holes in their centers. Only by the gravitational influence on