*14 March, 2011* • **Issue 15.5** • Science

## Universes of Possibilities

**Brian Greene**

*The Hidden Reality*

Allen Lane, 2011

384 Pages

¬£25.00

ISBN 978-0713999785

…

…

** **

Einstein once famously wrote that the whole of science is nothing more than a refinement of everyday thinking. This can seem like false modesty or wishful thinking on Einstein’s part when one considers some of the outlandish ideas that have come to populate the conceptual landscape of theoretical physics during the 20th and early 21st centuries. Indeed, the central thesis of Brian Greene’s new book, that our universe may be just one among many parallel universes in a much grander “multiverse”, seems about as remote from everyday thinking as one can possibly get. Yet, with a slew of clever analogies, Greene communicates with uncommon clarity, intuition, and honesty the essential elements of the reasoning that has carried researchers over the centuries from the realm of everyday experience to the most obscure corners of reality—and, for the fact that he does not invoke a single line of mathematics in the whole text, with impressive faithfulness to many of the original ideas.

There are two central and recurring philosophical themes throughout *The Hidden Reality*. The first is what Greene calls the Copernican Pattern, the idea that successive developments in science have displaced humanity progressively farther from the central position that we thought we occupied before Copernicus first proposed that the sun and not the Earth lay at the center of the universe. To Greene, the possibility that our universe is one among an infinite number may turn out to be the culmination of this progression. He writes “Some people recoil at the notion of parallel worlds; as they see it, if we are part of a multiverse, our place and importance in the cosmos are marginalized. My take is different…For me, it is the depth of our understanding, acquired from our lonely vantage point in the inky black stillness of a cold and forbidding cosmos, that reverberates across the expanse of reality and marks our arrival.”

Yet others recoil at the notion of parallel worlds because the idea simply sounds too strange or extravagant to be believable, and more like science fiction than science. To which Greene responds with the second refrain of *The Hidden Reality*: take the math seriously. In this book, as in his two previous best-selling popular physics books, Greene repeatedly emphasizes the power of mathematics to extend the reach of our knowledge into extremely remote realms of existence that are as yet inaccessible to experimental observation. Often, he suggests, our only foothold into understanding what goes on far beneath the scale of the atom or well beyond our cosmic horizon comes from the assumption that nature is governed by a unified set of laws. Over the history of physics, the search for unification, particularly in the mathematical formulation of physical theories, has proven remarkably successful, often sending theory leaps and bounds ahead of experiment. And, as Greene is often at pains to bring home, if we take the math of many of our current best physical theories seriously, we are lead naturally to the idea that what we conventionally conceive of as our universe is in fact a miniscule speck in an inconceivably vast expanse.

The proliferation of parallel universes that Greene describes begins with the simplest. Greene asks us to consider the possibility that space is infinite—whether it in fact is turns out still to be an open question. What we would be forced to conclude in such a case, given that there is only a finite number of ways in which matter can arrange itself (because of the fundamental discreteness imposed by quantum mechanics), and given the infinite expanse in which it can do this, is that if you travel out far enough, you’ll find places where their arrangement of matter, and therefore everything of physical interest, is identical in every respect. The infinite set of copies of our world which tile the endless expanse of space is what Greene calls the “Quilted multiverse”.

The second kind of parallel universe, called the “inflationary multiverse”, arises from a careful analysis of the equations of Einstein’s general theory of relativity. It has long been known that these equations in their original form imply that the universe is expanding, suggesting that it began at a single point in an event now known as the Big Bang. But astronomical data strongly implies that this expansion occurred at speeds different from those predicted by Einstein’s original equations; when these equations are tweaked to accommodate these results by incorporating varying concentrations of “dark energy” at different points in space, the result is not only that space is expanding, but that some parts of it are expanding much faster than others. In this case, our universe emerges as only one among many bubbles of relatively slow expansion which are separated by regions of ultra-fast, “inflationary” expansion.

Another possible source of parallel universes is string theory’s “brane world scenario”. According to string theory, all the fundamental constituents of matter, as well as the particles that govern their interactions, consist of vibrating filaments of string whose patterns of vibration give rise to different properties such as mass and electric charge. These strings propagate in a space-time with as many as 11 dimensions—times, the three familiar dimensions of space, and seven extra spatial dimensions that we can’t directly see. It is now widely believed that string theory not only requires the existence of one-dimensional objects (strings), but also of higher dimensional generalizations of strings called “branes” (short for “membrane”). Certain string theorists have speculated that the three dimensional space in which we live may actually be one such brane—a “three brane” since it has three spatial dimensions—and that there are many such three branes existing in parallel in a “Brane Multiverse”, separated from one another like stacked slices of bread along the extra spatial dimensions allowed by string theory. Moreover, the possibility that we are on one of many three branes raises the further possibility that two three-branes may collide, wiping out whatever structures may have formed in either of the branes’ universes and resetting the clock on each back to zero. Calculations have suggested that in such a scenario, branes will tend to collide periodically, causing the brane universes to come in and out of existence over time, and bringing about the fourth type of multiverse—in which the different universes are parallel in time rather than in space—known as the “Cyclic Multiverse”.

Greene’s fifth multiverse scenario, also suggested by string theory, is known as the Landscape Multiverse, and arises from attempts to explain the value of Einstein’s famous cosmological constant, which is crucial to understanding why space is expanding at the rate revealed by cosmological data. One well-known proposal that has featured centrally in string theory research is that the theory’s extra spatial dimensions are tightly curled up so that we can’t see them—much in the way a string’s circular width, if sufficiently small, becomes invisible from afar, causing the string to appear one-dimensional. It is widely believed that if these extra dimensions exist, then they probably take a special shape described by something known as a Calabi-Yau manifold (analogous to the extra circular dimension of the string, but more complicated). However, as it happens, there exist somewhere in the neighborhood 10^500 Calabi-Yau manifolds describing the possible shapes of these curled-up dimensions. Theorists believe that different shapes for the extra dimensions give rise to different values for this constant, of which there are only a relatively puny 10^124 possible theoretical values, and that somewhere among the 10^500 Calabi-Yaus there is almost certainly a set that produces the cosmological constant value for our universe. A number of theorists have suggested that when string theory is combined with the inflationary multiverse, the different possible Calabi-Yau shapes, and therefore the different possible values of the cosmological constant, will be distributed across each of the bubble universes that arise from inflation. Our universe emerges as just one among the vast collection of bubble universes, but one that has the particular value for the cosmological constant that we happen to measure.

Greene’s next variety of multiverse comes from a relatively commonplace domain of quantum mechanics (“relatively” being the operative word here). This quantum multiverse emerges from one attempt to make sense of the math of quantum theory, which says, for example, that a fundamental particle like an electron does not generally have a definite location, but exists only in an indeterminate but mathematically precise haze of being in multiple locations at once. The conceptual problems with this description become especially acute when these laws are extrapolated to macroscopic scales of everyday objects; when taken literally, the basic equation of quantum mechanics requires that when you look to see whether a particle like an electron is here or there, the indeterminacy in its location propagates up to the macroscopic scale, yielding a copy of you who sees the electron in one place and another copy who sees it somewhere else. The quantum multiverse is thus one in which multiple universes can emerge during the process of measurement: in this case, one universe in which you find the electron over here and one in which you find it over there.

The final species of parallel universe emerging from contemporary theoretical physics research is what Greene calls holographic parallel universes. A revolutionary result in string theory known as the AdS/CFT correspondence suggests that if the occurrences that we observe are accurately accounted for by string theory, then they are mirrored, and in some sense generated, by events transpiring on the inner boundary of our spatial region.

In the midst of reviewing the plethora of possible parallel worlds suggested by the progress of theoretical physics, Greene is diligent in speaking to concerns that parallel universe theories engage in a particularly cheap kind of explanation—our universe is the way it is only because, as it happens, reality scans every possible universe including ours—and for this reason seem less like science than and more like mathematical onanism. While Greene himself reserves firm support for any of the multiverse proposals he covers, what he does suggest is that if we take the mathematics of our best theories seriously, as history suggests we ought to, then theories with parallel universes emerge naturally as the simplest account of what we see, and are in many cases harder to avoid than they are to embrace. In this particular case, less, it seems, may not be more.

**Josh Rosaler** is reading for a DPhil in Philosophy at Pembroke College, Oxford. Josh is a senior editor at the *Oxonian Review*.