Once Before Time - Martin Bojowald [154]
4. QUANTUM GRAVITY
1. In some crucial investigations, they were joined by Ted Jacobson.
2. These approximations are called lattice gauge theories. When he started to work on loop quantum gravity, Smolin already had extensive experience in a version called Hamiltonian lattice gauge theory, which undoubtedly helped him make the application to gravity. A similar nonapproximate framework, but not for gravity, had earlier been developed by Rodolfo Gambini and Antoni Trias—initially unbeknownst to Rovelli and Smolin.
3. The first important step was performed with another senior mathematical physicist, Chris Isham. Soon afterward, several results were obtained with Jerzy Lewandowski, Don Marolf, José Mourão, and Thomas Thiemann. Independently, Rodolfo Gambini and Jorge Pullin worked out alternative mathematical methods.
4. It also turned out that a reformulation of Ashtekar’s original reformulation of general relativity was necessary, introduced by Fernando Barbero in 1995.
5. Several anecdotes illustrate this observation, best of all Lee Smolin’s after-dinner recollection voiced at AbhayFest, a celebration of Ashtekar’s sixtieth birthday in 2009, saying that his contact with Ashtekar taught him the difference between ideas and results.
6. Wilson loops are named after Kenneth Wilson, who introduced them for more general purposes in quantum field theories unrelated to gravity.
7. For instance, in work by Thiemann with Kristina Giesel.
8. There was, in fact, a good reason for his initial hesitation: He thought he could do better and had developed his own way of averaging symmetrical states. When he gave a seminar on this method, I pointed out that it would always give completely vanishing states: The symmetry would not only wash out structures, in this case it would completely remove everything. Ashtekar immediately realized that I was right, and after this experience he began to support loop quantum cosmology. In this whole process, he showed greatness, combined with a rare character trait that I have learned to respect highly, despite some often annoying side effects: the formation of and receptivity to opposition and criticism are essential in the pursuit of truth, and the more vigorous the criticism the better, unless they are empty and vain. Scientific striving can give rise to fierce, sometimes emotional combat. But this is usually constructive, for it quickly builds up theories, weeds out mistakes, and suggests new ideas. And after the final debate is over, one is left with a piece of knowledge agreed upon by all involved.
9. Pawlowski’s newly developed computer programs were especially crucial to this research.
10. Everyone can participate in the data analysis thanks to the program Einstein@Home (http://einstein.phys.uwm.edu). This is a screen saver using a computer’s idle time for an analysis of observational data. Currently, about 100,000 computers in many countries participate, providing gravitational research with welcome computational capacities.
11. Friedrich Schiller, Was heisst und zu welchem Ende studiert man Universalgeschichte? published in Der Teutsche Merkur, 1789 (pp. 127–128).
12. Not all descriptions of the universe follow the top-down view. As we will see in a later discussion of the uniqueness of solutions (chapter 9), loop quantum cosmology, for instance, can provide fundamental conditions for the form of its solutions without referring to the present state of affairs. It thus has more predictive power, but is also more daring: Predictions about the current state are made, and then must hold their own compared to observations.
13. Everyone loves a superpower that swoops in swiftly and with overwhelming force to save the day. Such a neoconservative view—ignoring many quantum aspects in spite of better knowledge—is often voiced in cosmology, suggesting that a single repulsive force would be sufficient to exorcise the threatening singularity and liberate the whole universe. Unfortunately, proclaiming solutions based on the consideration of only a single mechanism too often