If Sean Carroll is right, this interview has happened before. Not just once, but infinitely many times. In an earlier cycle of the cosmos, we met in Dublin, asked and answered these same questions and discussed the unsettling possibility that the universe is destined to repeat itself exactly.
Carroll, a quantum cosmologist and philosopher at Johns Hopkins University in Baltimore, Maryland, pursues one of physics’ deepest questions: how gravity and the familiar classical universe emerge from the strange rules of quantum mechanics. But unlike most cosmologists, who begin with the universe we observe and then try to work backwards, Carroll starts from the vast landscape of all quantum possibilities – essentially taking quantum theory itself as his guide. Against his better judgement, this approach has led him towards a cosmos that continually expands then contracts before a new cycle begins.
Cyclic universes are nothing new, but Carroll’s proposal has a peculiar twist. In most such models, each cycle differs from the last. But in contrast, his cycles are exactly the same: every galaxy reforms, every life unfolds again and every conversation is repeated word for word.
No space, no time, no particles: A radical vision of quantum reality This bizarre picture also offers a possible escape from an uncomfortable puzzle involving so-called Boltzmann brains; in many models of universes that persist for long enough, self-aware brains with false memories should pop into existence, to the point that they outnumber ordinary minds. New Scientist caught up with Carroll at the Dealing with Time and Chance philosophy conference at Trinity College Dublin, Ireland, to ask how he landed on such a strange cosmology, how it solves the Boltzmann brain problem and what it tells us about the nature of time. Jacklin Kwan: Your approach begins with quantum mechanics rather than the universe described by general relativity.
Sean Carroll: Quantum mechanics is the single most foundational view that we have of the world. However, when you take physics classes, you are first taught classical mechanics. Then, when you’re taught quantum mechanics, you start with a classical system and learn how to turn it into a quantum system.
In other words, you learn to “quantise” the theory. That approach has worked really well for forces like electromagnetism, but it hasn’t seemed to work in any normal, straightforward way for gravity. So, my perspective is that we got lucky with the other forces of nature.
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