Once Before Time - Martin Bojowald [44]
In quantum gravity, we must apply quantum theory directly to space and time and their complicated games of change as they are realized in general relativity. According to loop quantum gravity, space is then also atomic. Spatial atoms are abstract rather than visual images. Some of their properties can best be described by attributing to them an extended, one-dimensional looplike rather than spherical shape, thus the name loop quantum gravity. In this atomic picture, the volume of any spatial region can only grow discretely in specific steps, by adding or exciting spatial atoms. As with energy, two different versions of growth are realized. But the process follows a different dynamic compared to material atoms since volume, unlike energy, is not conserved. The total volume can grow without having to be provided from an external source, with the ultimate consequence that the whole universe can grow in and of itself. In general terms, the interplay of space, time, and possibly matter in an expanding universe causes spatial atoms to be excited and occasionally, once they have become sufficiently large, to subdivide. Then a new spatial atom is formed.
A solid theoretical description needs a base to start from. One may view this quantum gravitational base as analogous to the matter vacuum, but it is much more ominous. In the absence of spatial atoms, no space exists at all, but it is still a physical state rather than nothingness. What is this state? It is distinct from empty space or the material vacuum, as is realized to a high degree in outer space. For an empty space, though devoid of matter, still has space: an extension and volume. There is not much to be found in the universe between the galaxies, but there is still something—an immense space. In loop quantum gravity, all volume, be it small or large, must come from spatial atoms; when we remove all of them, no volume remains. In an empty state of quantum gravity, not even space and volume exist. The vacuum of loop quantum gravity is of an inconceivable emptiness: no sound, no light, no stuff, no space; only time as a faintly glimmering hope to leave behind this wasteland.
One may view the spatial vacuum of loop quantum gravity in analogy to certain states in quantum theories of matter, but it does not correspond to the matter vacuum. Rather, even though such a state is devoid of anything, its more precise analog would be a state of infinite temperature. This observation led the quantum field theorist Klaus Fredenhagen to dub this state the State of Hell.
Any theory of quantum gravity fully addressing space and time must grapple with this State of Hell. As we will see later, this state is particularly relevant for the singularity problem, where space has collapsed completely. It is then not surprising that loop quantum gravity, as the most comprehensive theory yet of this utterly empty state, has made significant progress toward an understanding of singularities. It also shows that loop quantum gravity is much darker, and perhaps deeper, than the harmonious, playful string theory. Even large regions with many atoms of space keep a memory of the absolute vacuum, the State of Hell, they came from. There is a considerable theoretical risk: If the base state turns out to be incorrect, this original sin may bring down the theory.
COMPARISON: APPLES AND ORANGES
String theory and loop quantum gravity are unlikely allies that share an aim but have different ambitions and beliefs. Although they are scientific theories, beliefs enter in the form of principles. String theory is based on cherished concepts that have been derived through many decades of particle physics research, most important that of abstract symmetries as hidden relationships among different types of particles and forces. For instance, the electroweak interaction is based on a symmetry connecting electrons with neutrinos, two particles that at first glance are entirely different. Similarly, string theory proposes symmetries