View Electronic Edition

The Multiverse

What is conventionally called "the universe" could be just one member of an ensemble. Countless other universes may exist in which the laws are different. This new concept of a "multiverse" is, potentially, as drastic an enlargement of our cosmic perspective as the shift from pre-Copernican ideas to the realization that the Earth is orbiting a typical star on the edge of the Milky Way, itself just one galaxy among countless others.

The multiverse could encompass all possible values of fundamental constants. I think cosmologists are starting, by better observations, to pin down the key numbers describing our universe. At the same time, physicists will soon be able to tell us which of these numbers are fixed by some underlying theory of the multiverse, and which are, in some sense, "arbitrary" outcomes of how our particular universe cooled down. The latter numbers would not be the same in other universes. The universe in which we've emerged belongs to the "unusual" subset that permits complexity and consciousness to develop. Once we accept this, various apparently special features of our universe—those once adduced as evidence for Providence or design—occasion no surprise.

In addition, other universes may follow life cycles of very different durations: some, like ours, may expand for much more than ten billion years; others may be "stillborn" because they recollapse after a brief existence, or because the physical laws governing them aren't rich enough to permit complex consequences. In some there could be no gravity; or gravity could be overwhelmed by the repulsive effect of a cosmological constant. Some could always be so dense that everything stayed close to equilibrium, with the same temperature everywhere. Some could even have a different number of dimensions from our own.

By mapping and exploring our universe, using all the techniques of astronomy, we are coming to understand—to a degree that even a decade ago would have seemed astonishing—our cosmic habitat. We have intimations of other universes, and perhaps . . . we can infer the scope and limits of a final theory even if we are still far from reaching it—even if, indeed, it eludes our intellectual grasp forever.

If the universe isn't everything, what is it?

We need to specify carefully (especially when talking to philosophers) what we mean by "the universe." We must distinguish three things:

(i) What we can actually observe—i.e., what lies within the horizon of present telescopes.

(ii) The larger domain that may, one day in the future, be observable, or at least in causal contact with us (or our remote descendants).

(iii) "Everything there is"—which includes all other possible universes, though these may never be observable, even in principle (and have different laws, dimensions, etc.). This is what I've called the "multiverse."

There are two ways we can gain circumstantial evidence about whether other universes exist:

(i) When we understand the early stages of the "big bang" better, it may turn out (indeed I strongly suspect it will) that the theory would predict a whole succession (or even an infinite ensemble) of separate bangs, each sprouting into a separate universe.

(ii) The features of our universe that make it seem specially "tuned" for life can, I believe, be best explained by supposing that there is a whole ensemble of universes, governed by different laws. It is then of course not surprising to find one member of the ensemble that fits any requirement.

As our universe cooled, its specific mix of energy and radiation, even perhaps the number of dimensions in its space, may have arisen as "accidentally" as the patterns in the ice when a lake freezes. The physical laws were themselves "laid down" in the big bang.

Our universe, and the laws governing it, had to be (in a well-defined sense) rather special to allow our emergence. Stars had to form; the nuclear furnaces that keep them shining had to transmute pristine hydrogen into carbon, oxygen, and iron atoms; a stable environment and vast spans of space and time were prerequisites for the complexities of life on Earth.

Life in other universes?

It's hard enough to speculate about what life might exist elsewhere in our own universe, let alone in others where the physical laws are different! The basic laws allow an immense range of complex structures, which we can't necessarily yet envisage. Science fiction authors are the best at this, of course, and we can get more stimulus from them than from most scientists. Our universe is still near the beginning of its cycle—we are certainly not the culmination of evolution. (I also make the point, which is unpopular with some people, that, though SETI [Search for Extra-Terrestrial Intelligence] projects are worthwhile, I rather hope that life is now unique to our Earth. This may seem at first sight to make life an irrelevant fluke in our vast universe. But life "seeded" from Earth could spread through the galaxy and beyond, making our tiny Earth central to the cosmic scheme of things.) I emphasize that biology is a harder subject than astronomy—astronomers deal with things that are big, but not specially complex. That's why it isn't presumptuous to aspire to understand them. A frog poses a more daunting scientific challenge than a star.

In the first and last chapters of my book where I briefly allude to the implications of a SETI detection, I speculate about whether there might be "superintelligences" who could visualise the Mandelbrot set as easily as we visualise a circle, and who could perhaps infer the existence of universes (e.g., with extra dimensions) too complex even for us to imagine. I like the analogy with the observer in a rowboat in the middle of an ocean. The sea may seem to stretch uniformly to the horizon, but there may be continents, "dragons," etc., far beyond.

Is cosmology relevant?

I think the concept of the multiverse erodes any basis there might have been for the theological "argument from design" [that the Universe is so perfect and complex that it must have a creator]. The concept of the multiverse could allow multiple universes of various levels of perfection or complexity. However, the argument from design is not itself taken seriously by many theologians. While science raises problems for certain "literal" beliefs, I don't think it has any further relevance to one's religious attitudes. I mention briefly in the book that, just as Newton's contemporaries reacted differently to nature, so do present-day scientists. If being a scientist teaches me anything, it is that even a single atom is tough to understand. That makes me skeptical of anyone who claims more than an incomplete and metaphorical understanding of any deep aspect of reality.

Cosmology is of no day-to-day significance. But people have always been fascinated by "origins." Dinosaurs have been high

in the popularity charts ever since Richard Owen discovered them in 1841. So, of course, has Darwinism ever since the 1860s. Cosmology fascinates people now, just as Darwinism has for more than a century. (I'd certainly get less satisfaction from my own researches if they only interested a few other specialists.)

I don't think modern cosmology carries any real message relevant to ethics. Cosmologists can hold almost any faith, or none. They may, however, have a special "ecological" perspective. The intricate biosphere, of which we're part, has taken several billion years to evolve. Our Earth is a speck in the universe. Planets are common, but those that harbor such complexity could be surpassingly rare, or even unique in those vast spaces. In terms of cosmic timespans, we're not yet at the half-way stage—we're still nearer to Darwin's "simple beginning" than to the endpoint of the evolutionary process. But it is collective human actions that will determine how, or even if, that process unfolds. Being mindful of these potentialities should deepen our commitment to understand our world, and conserve its web of life.