Quantum Gravity
Donald Marolf, University of California, Santa Barbara

In the early 1900s, quantum mechanics revolutionized physics, and indeed all of science. Though its implications are most obvious for very small systems (like atoms and molecules), quantum mechanics plays also critical roles in our understanding of macroscopic solids, liquids, and gases.
This new physics taught us many fundamental lessons: that matter is both a particle and a wave, that light comes in (quantized) photons, and that materials have characteristic spectra, which in turn led the way to new technologies like the transistor, superconductivity, nuclear magnetic resonance devices, etc.

The goal of quantum gravity is to understand the quantum nature of the gravitational field. To the outsider, this may seem to be a narrow, technical subject. But one must recall that the modern understanding of gravity is through Einstein's theory of General Relativity. Einstein taught us that gravity is intimately connected with space and time.

The goal of quantum gravity research is thus to understand the behavior of space and time near the big bang, at black hole singularities, on very short distance scales. General relativity and led to the discovery of the big bang and black holes. In particular, it tells us that the universe itself was of microscopic size at the beginning of big bang and that huge stars collapse to microscopic size in the formation of black holes. Only quantum gravity can explain what happens in such regimes.

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