Does the idea of parallel universes really describe reality? (Image: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech)

In Our Mathematical Universe Max Tegmark tries hard to make the seemingly outlandish theories of multiverses sound almost obvious and unavoidable

SOME years ago, the philosopher David Hull wrote a book entitled Science as a Process, in which he argued that science works through an evolutionary process. Imaginative scientists toss out ideas and hypotheses, creating and maintaining the equivalent of natural variation in biological populations. Then other scientists test those ideas, using evidence and logic to select out and eliminate the ones that don’t measure up. Variation and selection, repeated: that’s a form of evolution.

But there is a condition. This only works properly with a diversity of personalities and specialisms among scientists. Research would get nowhere if it were driven solely by the dour, hard-boiled sceptics who only believe on the basis of solid evidence. The sceptics feed off the raw creative material of the speculators, who imagine what might be possible and never stop dreaming about “what if”. The speculators produce the diversity of ideas on which selection can act, and they require, in turn, the discipline of sceptics to stop them from running away into fantasy.


And yet fantasy is the very word that occurs to many – including some physicists – when they hear some of the ideas popular in cosmology, a discipline which aims to answer the big questions about the origins of the universe.

The fantasy trajectory started off gently enough when physicist Alan Guth proposed that many puzzling features of the observable universe – such as the extremely homogeneous distribution of matter within it – would be explained if the universe had undergone a short, early period of rapid expansion, termed inflation. Extremely rapid, as in expanding in volume by a factor of 1078 in a time of 10-30 seconds.

Since then, other inflationary cosmologists have opened the speculative throttle so fully that physicists now talk routinely of such things as an infinitude of parallel universes, or a “multiverse”. In the multiverse, every conceivable world exists, and individuals identical to you and I live out parallel lives in places we cannot have access to.

Is this still science? Or has inflationary cosmology veered towards something akin to religion? Some physicists wonder. The enthusiasts, of course, see it very differently. Max Tegmark, a physicist at the Massachusetts Institute of Technology, certainly does. His new book, Our Mathematical Universe, is an impassioned defence of the theory, especially its implications for parallel universes.

The book is an excellent guide to recent developments in quantum cosmology and the ongoing debate over theories of parallel universes. Tegmark tries hard to make the seemingly outlandish sound almost obvious and unavoidable, and offers a taxonomy to help organise a zoo of imagined parallel universes.

Max Tegmark tries hard to make the seemingly outlandish sound almost obvious and unavoidable

As it turns out, the terms parallel universes and multiverse mean many things to different people. But Tegmark’s taxonomy of parallel universes are all, he argues, implied by observed evidence and the laws of physics.

His first set, the Level I Multiverse, refers to an idea that many cosmologists already accept. Rapid early inflation would have created what Tegmark describes as “universe-sized parts of space so far away from us that light from them hasn’t had time to reach us”. These other domains – or “universes” – could well exist, although we currently have no observational evidence for them.

Tegmark’s Level II Multiverse refers to a bolder idea, championed by physicist Alexander Vilenkin and others. There may be other domains of space also created by inflation that are too far away to see. These will forever remain out of our reach because continuing inflation drives them from us faster than the speed of light. This idea refers to real, distinct, physical universes that cannot ever be observed.

At this point in the taxonomy, however, Tegmark leaves cosmology behind. In reading, I began to feel that his aim is to see parallel universes in as many places as he can. Enter the Level III Multiverse. This turns out to be a language for talking about the mathematics of quantum theory using the many worlds interpretation of that theory, first proposed by physicist Hugh Everett in the 1950s.

This interpretation describes all physical processes as part of an ongoing, perpetual branching of the universe into many other universes. It is indeed possible to interpret quantum theory this way, but readers should know that many other interpretations, equally in tune with observations, don’t invoke the idea of parallel universes at all.

Then there is the Level IV Multiverse. Again, this has nothing to do with cosmology, but is an ambitious thought about mathematics. Tegmark argues that reality isn’t simply described by mathematics, as most physicists readily accept, but that it is, in fact, mathematical.

Reality isn’t simply described by mathematics, as physicists accept, but is, in fact, mathematical

Furthermore, he believes that the mathematics of our universe is just one of an infinity of conceivable mathematical structures. He goes on to wonder: if this mathematical structure is a universe, why not all the others? And so he makes a bold claim – that all other mathematical structures should also exist physically as further parallel universes.

Of course, we don’t really know. The history of science ought to have taught us that just because something sounds unbelievable, it doesn’t mean it is. Human history, after all, is one long progression of people being surprised by what they previously thought was impossible. Isolated tribes learned of other islands and continents, and of the other peoples living there, for example. In modern times we learned of other planets, galaxies, clusters of galaxies and so on. Why not universes? It might even feel quite natural for our universe to just be one of many, especially in the sense of the Level I Multiverse.

Even so, there does seem to be something a little questionable with this vast multiplication of multiverses. While the notion of the Level I Multiverse at least makes contact with real physics and possible evidence, it isn’t clear that any of these other ideas ever could. Multiverse champions seem quite happy, even eager, to invoke infinite numbers of other universes as mechanisms for explaining things we see in our own universe. In a sense, multiverse enthusiasts take a “leap of faith” every bit as big as the leap to believing in a creator, as physicist Paul Davies put it in an article in The New York Times.

In the end, this isn’t science so much as philosophy using the language of science. “Inflation”, Tegmark notes, “is the gift that keeps on giving, because every time you think it can’t possibly predict something more radical than it already has, it does.”

This quote is a good example of Tegmark as a creative, speculating scientist, churning out radical ideas as rapidly as possible. It suggests that prediction alone is the point and measure of science, whether or not those predictions turn out to be true.

But all writers overstate their position on occasion, and uninhibited speculation is only one side of Tegmark’s brand of science. Much of his early work, which built his reputation as a physicist, wasn’t of this kind at all. It was hard, empirical stuff, developing methods for analysing data from large-scale telescope projects to measuring fluctuations in the cosmic microwave background.

Perhaps this book is proof that the two personalities needed for science – the speculative and sceptic – can readily exist in one individual.

Our Mathematical Universe: My quest for the ultimate nature of reality Max Tegmark Allen Lane