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Zenith Grant Awardee

Sean Carroll

California Institute of Technology

Project Title

What Happens Inside the Quantum State?

Project Summary

The quantum world is a strange one, where observation seems to change reality and predictions are a matter of probabilities rather than certainties. But for working physicists, that strangeness doesn\'t really get in the way; we know what the rules are for situations we encounter in the lab, and those rules have been as well-tested as any in the history of science. Where we run into trouble is when we try to extend our understanding far outside the lab, into the universe and multiverse. Modern cosmology frequently imagines situations that are far removed from the safe confines of our local environment — places where there are no observers, no discernible difference between past and future, perhaps no notion of space and time at all. This research proposal seeks to investigate what quantum theory has to say about those realms. Are there people, planets, and civilizations fluctuating in and out of existence? Does time stretch forever? Is there just one \"world”, or are there many, or perhaps an infinite number? Modern physics has reached the point where such profound questions need to be answered if we hope to make further progress in understanding the universe.

Technical Abstract

While the empirical success of quantum mechanics (QM) is indisputable, its ultimate formulation and interpretation remain elusive. The textbook \"Copenhagen\" version of QM works very well, but only under very specific circumstances: when there is a clean separation between system, measuring apparatus, and environment, and the initial state is sufficiently unentangled (low entropy) that unitary evolution increases the entropy and brings about decoherence. In this proposal, I will investigate how to address important questions about what actually “happens\" inside the quantum wave function in much more general circumstances, especially those applicable to cosmology. This question is important for a variety of reasons. There may be a nonzero amplitude for an intelligent observer to appear in the wavefunction, one who could potentially be counted in anthropic calculations — but is an amplitude sufficient, or does the observer\'s branch need to decohere? Under what circumstances does a classical spacetime emerge from the quantum state? These and other issues will be studied in the context of the Everett formulation of QM. This work will serve both to illuminate outstanding problems within the Everett approach, and to help apply it to quantum gravity and cosmology.

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