Quantum Gravity Detail

Perimeter Institute’s principal mandate is to conduct research on foundational problems in theoretical physics. There is no more foundational problem than that of discovering the quantum theory of gravity. This is the theory that will unify the two main theories on which all of science is now based: quantum theory and general relativity.

Modern quantum theory was developed in the 1920s as a response to the realization that Newton’s laws, while indispensable for understanding everything from the building of bridges to the motions of the planets, completely fail to explain the inner workings of the atom or the physics of subatomic particles. It was realized that the fabric of reality at such very microscopic scales is quite different from our “common sense” picture of the world—much more subtle and beautiful—and that Newton’s laws are merely a crude approximation to this deeper underlying reality. The enormous success and importance of quantum theory cannot be overstated. From unravelling the mysteries of the periodic table of the elements to its applications in a huge range of chemical and biochemical technologies and materials sciences. From understanding how light interacts with matter to applications such as the laser, used everywhere from CD and DVD players to carrying today’s high speed Internet traffic to experimental fusion technology. From magnetic resonance imaging (MRI) devices used in medicine to superconducting quantum interference devices (SQUIDs) used to search for new oil deposits or map out electrical activity in the brain. The list is endless. However, despite the fact that our universe is clearly quantum mechanical at its foundations, quantum theory contains deep mysteries that continue to puzzle theorists. Why is there inherent randomness or uncertainty at the heart of quantum theory? Is quantum theory really a complete description of reality, and, if so, how should we interpret what it says about the nature of our universe? These and other questions are being explored by researchers at Perimeter Institute working in the two overlapping fields of “foundations of quantum theory” and “quantum information theory”.

General relativity, on the other hand, is Einstein’s theory of space, time and gravity. It is also the framework for our modern understanding of cosmology—the theory of the history and organization of the whole universe. In response to deep conceptual problems with Newtonian ideas, Einstein developed an entirely new understanding of the universe in which gravity is not a force, as Newton taught us, but rather a manifestation of the geometry (the warping, or curvature) of space and time itself. (More on this here.) This deep insight has proven to be hugely successful. Beginning with the solar system, which contains only very weak gravitational fields, general relativity’s predictions concerning the motions of the planets differ only very slightly from those of Newton’s theory, but for each such discrepancy tested, general relativity has passed the test whereas Newton’s theory has failed. But it is in the realm of stronger gravitational fields that general relativity really comes into its own, predicting many new phenomena that have no analogue at all in Newton’s theory: black holes, gravitational lensing, time dilation, frame dragging, gravitational waves and so forth. Most of these predictions (i.e. all of those within reach of experiments using today’s technology) have been verified. In short, Einstein gave us a new and extremely beautiful geometrical understanding of space, time and gravity, which has superseded Newtonian theory and has passed every experimental test we are able, at present, to perform.

Both quantum theory and relativity theory were invented early in the 20^th century, and each represented a radical departure from the previous theory rooted in the ideas of Galileo and Newton. The latter theory was thus decisively overthrown by the 1920s and replaced, not with one theory, but with two—quantum theory and relativity, both of which have been phenomenally successful. This was a major scientific revolution, by any definition, but it was incomplete, because the unity of nature requires a single theory to describe it. More than that, there are deep mysteries of the universe that neither relativity theory nor quantum theory alone can explain. For example, how did the universe begin? There is an overwhelming amount of evidence that supports a Big Bang picture, but this picture implies that the entire universe, consisting presently of almost unimaginably vast volumes of space containing countless stars and galaxies was, near the beginning of the Big Bang, compressed to a size much smaller than the nucleus of a single atom! To describe such a bizarrely singular situation will require a theory that seamlessly combines our best understanding of the very small (quantum theory) with our best understanding of space, time and gravity (general relativity). At present we have only glimpses of such a unified theory. Called the quantum theory of gravity (or quantum gravity, for short), this theory is currently the subject of very active research at Perimeter Institute and several other institutions around the world, and is widely regarded as the holy grail of 21^st century theoretical physics.