Black holes

So far, none of the predictions from either string theory or loop quantum gravity have been observed, although this is a situation that could change at any time. So one may ask why scientists are so enthusiastic about these theories. There are two reasons. The first is that each approach seems to unify general relativity and quantum theory, at least to the point that there are no (mathematical) inconsistencies so far discovered and, at least as importantly, genuine new experimental predictions have been made. Another reason is that each approach has gone some distance towards explaining some of the mysteries that arose from earlier attempts to understand quantum gravity.

Some of these older mysteries concern the nature of black holes. In the 1970s a group of physicists including Jacob Bekenstein, Stephen Hawking and William Unruh, discovered some remarkable consequences of combining quantum theory with relativity. One of them, now known as the Unruh effect, is that accelerating any object necessarily heats it up. This is a deep and rather unexpected discovery about the physical nature of our universe. Another consequence, discovered by Hawking, is that black holes are not actually black. Instead, they radiate as if they were hot bodies with a finite temperature, like a hot glowing coal. Before them, Bekenstein had also found that black holes must have a property called entropy (a measure of intrinsic randomness), a property common to all other objects in our universe.

Both loop quantum gravity and string theory were shown in the 1990s to explain the temperature and entropy of black holes. This has led, in each case, to further predictions concerning the details of what would be seen were a real radiating black hole discovered. The study of black holes continues to be a central part of the search for a quantum theory of gravity, here at PI as elsewhere.

Quantum gravity and cosmology

Quantum gravity is also closely connected with the study of cosmology. Theory and observation put the explanation for the observed structures in the universe—such as stars and galaxies—on the structure of space and time at very early times, a very small fraction of a second after the universe began expanding. Thus, another task of a quantum theory of gravity is to explain how the universe began and to answer questions such as whether the Big Bang was really the beginning of existence and time, or whether there was something earlier than the Big Bang. Related to this are the problems of explaining the nature of the dark matter and dark energy that are observed by astronomers to make up as much as 95% of the “stuff” in the universe. String theory and loop quantum gravity are among the theories that make predictions about these questions, and these are also among the questions under investigation at PI.

Finally, the extension of quantum theory to cosmology raises new questions concerning the meaning and interpretation of quantum theory. This is because the conventional uses of quantum theory presume that the observer is outside the system studied. But the universe contains everything, including all its possible observers. This means that quantum theory must be extended (or re-invented) to allow observers to be part of the system they are observing. This is presently a lively area of research, to which scientists at PI have contributed important ideas and results.

Summary

Quantum gravity is a difficult, foundational problem that has at last begun to yield real results and predictions, due to the development of new theories that appear to solve at least part of the puzzle of unifying quantum theory and relativity. Scientists at PI are making important contributions to the search for a quantum theory of gravity and, even in its first year, PI has become a major centre for work in this area. The unique situation of PI makes it possible to offer a home for research in which people working on different approaches to this key problem are brought together, in close proximity to each other as well as to people working on the related fields of cosmology, the foundations of quantum theory, quantum information theory and elementary particle physics. We believe that this, together with the cultivation of an atmosphere that fosters risk-taking and the pursuit of new, original ideas, will make PI a leading international centre for work in this area.