The Cosmic Landscape
by Leonard Susskind
Boy, when you finish this one you'll know you've been in a fight. The Cosmic Landscape is the easiest possible book about, I think, the hardest possible science. No, it doesn't have equations aside from one quick E equals m c-squared, and it does have illustrations, but you will get your reward from reading the book only if you pay close attention and consult the author's glossary from time to time. I'm sorry if that seems discouraging--this is truly worth the effort--but it would be dishonest to suggest Susskind will be as easy a read as Steven Jay Gould or even A Brief History of Time.
In the last part of the 20th century and what we've seen of the 21st, the story of physics and cosmology has been a story of turmoil. Hardly anyone will admit to being enemies with anyone else, but good friends take every opportunity to recount decades-long campaigns against the ideas of other good friends. Scientists believe you get the best outcome when every new theory has to meet a vigorous challenge. (The best outcome but not always the quickest: Some of Einstein's work had to wait in limbo several years until an experiment could confirm his prediction and make a superstar of him, and certain claims by Darwin couldn't be checked for over a century.) Susskind now tells the story of a big idea, or rather a complex of big ideas, that received a provisional OK from physicists only after a long time of suspicion and resistance.
String theory--don't think for a moment I am going to try to explain it--describes subatomic particles and their interactions as if they were little mechanical systems, infinitesimal strings whose ends follow certain rules as they move about.
What, invisible electrons and gamma rays didn't do the job of explaining the world? In fact they didn't, not so as to accord with the past 40 years of experiments. Physicists wanted to know the same things everyone else wants to know--what is stuff made of, how did the universe happen, did somebody, um, dispose all this?--but instead of getting closer to simple answers, they just kept discovering new stuff. Things didn't fit together. Gravity was too weak to be compared with electromagnetic forces, the oldest galaxies appeared to predate the beginning of the universe, and someone set the cat among the pigeons by suggesting the laws of physics must have been designed to make the cosmos hospitable to life.
Susskind and his colleagues found that some processes were simple to describe in string-theoretic terms. Well, simpler. You could account for the fact that, for example, there is no such thing as 1/3 of the charge of the electron. At the same time, you might see a way to fit gravity and other forces into a single frame, and you could make some inferences about what the universe was like just a few years after the Big Bang.
The concept Susskind keeps returning to, however, is the anthropic principle, the hypothesis that the universe was made for us. See, a cosmos with laws just a smidgin different from what we observe would have collapsed, or wouldn't have had life-generating stars and galaxies, or would have had its mass hidden in black holes. In other words, a little difference in the laws--and there would have been no us to deduce the laws. So what's the deal: Did the laws get a tweak from an unseen hand, so that we owe our existence to a designer, or did they just happen to fall out this way in a cosmic game of craps?
You can imagine that physicists were uneasy with the anthropic principle, Susskind as much so as any. But it would be laughable to rely on pure chance to produce a universe with us in it. String theory to the rescue! The string interpretation of matter and its interactions allows you to choose among forms of the laws of physics. The theory also predicts how a universe forms and expands. So choose your value for the fine-structure constant, and out pops a universe. You can compute its history--whether it lasts a tiny fraction of a second or survives to old age, whether it expands or not, whether its matter is clumped into black holes or dispersed in a haze of fog, and so forth.
Now the trick. There are 10 to the 500th power (i.e., a gazillion) possible sets of laws, hence that many possible universes, and all of them are real. They are regions in a super-universe that Susskind calls the landscape. The notion of a landscape of possibilities is not too hard to comprehend; it's been used to describe outcomes of biological evolution, for example (by Richard Dawkins). Hilltops are regions of low probability and high energy, and systems tend to travel down to valleys of stability and persist there until bumped. Now of the gazillion real universes, only a tiny fraction are capable of containing us. But a tiny fraction of a gazillion is a lot of universes, which means it is not an outlandish notion that some of them should have just this combination of laws. And indeed we do have an example of a cosmos with just this combination.
Susskind's conclusion, for which he claims growing support, is that a large number (a tiny fraction of a gazillion) of universes can have life, and one of them has us. No designer, no tweak, no special providence is necessary. Chance is involved, but as for a creating and guiding deity, we're with Laplace: We have "no need of that hypothesis."
The easiest possible book, I said. Susskind writes gracefully but doesn't shy from explaining at length when his points are hard to understand. He enriches the work with stories about colleagues and controversies. The Cosmic Landscape is the best source you're going to find concerning string theory, and maybe the best on the anthropic controversy. It's well worth reading, but you have been warned: It's about the hardest possible science.
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The Cosmic Landscape