Andrew Liddle contemplates an accomplished explication of the multiverse.
Our Mathematical Universe: My Quest for the Ultimate Nature of Reality
- Max Tegmark
Having trouble understanding the Universe? Try this instead: imagine 10500 possible universes, all different, and consider our place within this ensemble. Not randomly chosen, because our location should satisfy some basic conditions, such as habitability for intelligent species able to ask about their place in the cosmos. Can such a multiverse help us to fathom our Universe?
Cosmologist Max Tegmark has written an engaging and accessible book, Our Mathematical Universe, that grapples with this multiverse scenario. He aims initially at the scientifically literate public, but seeks to take us to — and, indeed, beyond — the frontiers of accepted knowledge. His explication of these ideas is more ambitious and individualistic than books on this topic by Leonard Susskind and Alex Vilenkin, for instance.
Multiverse theory stands in stark opposition to the belief that there should be some reason, perhaps a Theory of Everything, that determines physical laws such as the types of particle that exist and the ways in which they interact. In the multiverse picture, it is all an accident. What we know as 'constants' of nature, such as the strength of gravity or the proton-to-neutron mass ratio, happen to have particular values here, but in distant regions beyond our sight they may take other values and produce universes with very different properties — perhaps an absence of complex atoms and molecules, and hence of life.
Once seen as a fringe interest of dubious scientific validity, the multiverse has developed a serious following. Steven Weinberg used it in 1987 to predict that our observable Universe ought to have a non-zero cosmological constant, probably of a magnitude great enough to accommodate the acceleration of the Universe's expansion. To everyone's surprise, this was verified a decade later through observations of distant supernovae by two teams of astronomers. Those who led the work, Saul Perlmutter, Adam Riess and Brian Schmidt, won the 2011 Nobel Prize in Physics. Subsequently, string theory and inflationary cosmology were recognized as providing a setting that could predict, or at least motivate, the existence of a multiverse.
Tegmark's book captures two trends in contemporary science writing: scientific autobiography and the popular book as manifesto, expressing a body of personal scientific ideas ill-suited to traditional academic journals. Accordingly, Tegmark interweaves the science with stories of personal contributions to the endeavour. Fortunately, he is an engaging host. Tegmark makes his manifesto explicit by chopping his research life into two parts. Around a quarter of the book covers the 'sensible' work on constraining cosmological models from data. The rest is the outlandish part on quantum realities and multiple universes — even including an e-mail from a (sadly unnamed) senior academic advising him to desist before he destroys his career. It is clear where Tegmark's priorities lie: this book is his statement on the multiverse as a valid model for reality.
Tegmark likes the multiverse so much that he doesn't settle for just one; instead, he offers four different levels of multiverse. In the first, we simply have our own Universe, with its physical laws, extending forever. Shockingly, this is sufficient to ensure that, somewhere far away, there are exact replicas of you reading this review, on exact replicas of Earth. It might even be enough to imply that you are more likely to exist within a simulation of reality than in reality itself (whatever that means).
In the second incarnation, perhaps the most popular among proponents, physical laws vary within the multiverse so that distant regions can be considered to be distinct universes. This version is necessary to explain, for instance, the cosmological constant and other apparent coincidences in physical laws such as the stability of neutrons within nuclei. In the third level, the parallel universes may exist only as quantum mechanical states.
The culmination that Tegmark seeks to lead us to is the “Level IV multiverse”. This level contends that the Universe is not just well described by mathematics, but, in fact, is mathematics. All possible mathematical structures have a physical existence, and collectively, give a multiverse that subsumes all others. Here, Tegmark is taking us well beyond accepted viewpoints, advocating his personal vision for explaining the Universe.
This is a valuable book, written in a deceptively simple style but not afraid to make significant demands on its readers, especially once the multiverse level gets turned up to four. It is impressive how far Tegmark can carry you until, like a cartoon character running off a cliff, you wonder whether there is anything holding you up.
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Liddle, A. Physics: Chasing universes. Nature 505, 24–25 (2014). https://doi.org/10.1038/505024a