Brian Greene, The Hidden Reality

If you like science because you like to have your mind blown by the possibilities of our stupendous universe, you would probably enjoy Brian Greene's The Hidden Reality: Parallel Universes and the Deep Law of the Cosmos (2011). If, on the other hand, you value things about science like the explanatory power of simple theories or equations, the amazing precision of our instruments, the detailed understanding we now have of things like protein synthesis and plate tectonics, or just knowing the truth, you should probably read something else.

The Hidden Reality is a tour of scientific theories that postulate a multiverse, or a world that contains many separate universes. There area now a number of such theories; Greene counts nine different versions of the multiverse being tossed around in contemporary physics. I listened to this book because I am curious about such notions, and because my library doesn't have many recorded science books. My reaction was mostly neutral. Greene is a good writer and he goes over some interesting science, but I don't find any of these notions remotely convincing. In fact I am a little troubled by the vast amount of intellectual energy being spent on things like string theory or the detailed modeling of cosmic inflation, without any good evidence that these things exist. This sort of physics is more like composing academic music than it is like the applied physics of solid state labs, or even the theoretical physics of 1650 to 1970, which kept in close touch with laboratory results.

I would say that one reason for the explosion of interest in multiverse theories is that physicists have hit a dead end in trying to explain our own universe. Consider the string landscape multiverse. One of the reasons that physicists got interested in string theory in the 1980s was that it seemed to offer a way to predict, from theoretical fundamentals, the properties of all our fundamental particles. Standard quantum mechanics has nothing to say about why electrons or quarks have the mass they do; those properties have to be measured. String theorists thought they could predict them. But then in the 1990s they realized that they could not. String theory requires that our universe have ten spatial dimensions, seven of which are curled up into tiny shapes so we never see them. The properties of a string universe depend on the exact way the extra dimensions are curled up. Recent calculations show that there are about 10⁵⁰⁰ possible ways to do this, so string theory can give rise to 10⁵⁰⁰ different sorts of physics. The theory cannot even in principle determine why we have one spatial geometry rather than another. Enter the multiverse! If there are an infinite number of parallel universes, each of which has its own geometry and therefore its own physics, then there is no particular reason why we live in a universe with certain properties. It was just chance.

Or was it? The really controversial thing about multiverse theories has to do with something called the anthropic principle. This is a way of answering the question, why do we live in a universe where intelligent life is possible? Physicists have shown that even very minor fiddling with physical laws, such as small changes to some of the key constants, leads to a universe in which stars cannot form and life would be pretty much impossible. So why are the laws of physics exquisitely balanced to create our cozy universe? Because if the laws were different, we wouldn't be here. We could only have evolved in a universe with certain laws. Therefore, says the anthropic principle, we find ourselves in a universe with such laws.  In a multiverse scenario this makes a certain kind of sense; if there are infinitely many universes, then of course we find ourselves in one of those where life is possible and there is no particular reason to ask why.

This reasoning drives some scientists crazy. It is, they think, the very definition of non-science. Don't ask why, it just is! And isn't that grand? No, say some physicists, it is just an excuse for ignorance. So the anthropic principle has produced furious arguments among physicists and shed no particular light on the world we happen to live in.

Brian Greene loves this stuff. But then, here is a man who has happily devoted his career to minor points in the geometry of 10-dimensional Calibi-Yau shapes, with no real evidence that his work has any relationship to a world outside the imagination of string theorists. Whatever, it's his life. I think I'll go read about trilobites.