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Sunday, October 15, 2006

Reading List: Many Worlds in One

Vilenkin, Alexander. Many Worlds in One. New York: Hill and Wang, 2006. ISBN 0-8090-9523-8.
From the dawn of the human species until a time within the memory of many people younger than I, the origin of the universe was the subject of myth and a topic, if discussed at all within the academy, among doctors of divinity, not professors of physics. The advent of precision cosmology has changed that: the ultimate questions of origin are not only legitimate areas of research, but something probed by satellites in space, balloons circling the South Pole, and mega-projects of Big Science. The results of these experiments have, in the last few decades, converged upon a consensus from which few professional cosmologists would dissent:
  1. At the largest scale, the geometry of the universe is indistinguishable from Euclidean (flat), and the distribution of matter and energy within it is homogeneous and isotropic.
  2. The universe evolved from an extremely hot, dense, phase starting about 13.7 billion years ago from our point of observation, which resulted in the abundances of light elements observed today.
  3. The evidence of this event is imprinted on the cosmic background radiation which can presently be observed in the microwave frequency band. All large-scale structures in the universe grew from gravitational amplification of scale-independent quantum fluctuations in density.
  4. The flatness, homogeneity, and isotropy of the universe is best explained by a period of inflation shortly after the origin of the universe, which expanded a tiny region of space, smaller than a subatomic particle, to a volume much greater than the presently observable universe.
  5. Consequently, the universe we can observe today is bounded by a horizon, about forty billion light years away in every direction (greater than the 13.7 billion light years you might expect since the universe has been expanding since its origin), but the universe is much, much larger than what we can see; every year another light year comes into view in every direction.
Now, this may seem mind-boggling enough, but from these premises, which it must be understood are accepted by most experts who study the origin of the universe, one can deduce some disturbing consequences which seem to be logically unavoidable.

Let me walk you through it here. We assume the universe is infinite and unbounded, which is the best estimate from precision cosmology. Then, within that universe, there will be an infinite number of observable regions, which we'll call O-regions, each defined by the volume from which an observer at the centre can have received light since the origin of the universe. Now, each O-region has a finite volume, and quantum mechanics tells us that within a finite volume there are a finite number of possible quantum states. This number, although huge (on the order of 1010123 for a region the size of the one we presently inhabit), is not infinite, so consequently, with an infinite number of O-regions, even if quantum mechanics specifies the initial conditions of every O-region completely at random and they evolve randomly with every quantum event thereafter, there are only a finite number of histories they can experience (around 1010150). Which means that, at this moment, in this universe (albeit not within our current observational horizon), invoking nothing as fuzzy, weird, or speculative as the multiple world interpretation of quantum mechanics, there are an infinite number of you reading these words scribbled by an infinite number of me. In the vast majority of our shared universes things continue much the same, but from time to time they d1v3r93 r4ndtx#e~—…. 

Reset . . .
Snap back to universe of origin . . .
Reloading initial vacuum parameters . . .
Restoring simulation . . .
Resuming from checkpoint.
What was that? Nothing, I guess. Still, odd, that blip you feel occasionally. Anyway, here is a completely fascinating book by a physicist and cosmologist who is pioneering the ragged edge of what the hard evidence from the cosmos seems to be telling us about the apparently boundless universe we inhabit. What is remarkable about this model is how generic it is. If you accept the best currently available evidence for the geometry and composition of the universe in the large, and agree with the majority of scientists who study such matters how it came to be that way, then an infinite cosmos filled with observable regions of finite size and consequently limited diversity more or less follows inevitably, however weird it may seem to think of an infinity of yourself experiencing every possible history somewhere. Further, in an infinite universe, there are an infinite number of O-regions which contain every possible history consistent with the laws of quantum mechanics and the symmetries of our spacetime including those in which, as the author noted, perhaps using the phrase for the first time in the august pages of the Physical Review, “Elvis is still alive”.

So generic is the prediction, there's no need to assume the correctness of speculative ideas in physics. The author provides a lukewarm endorsement of string theory and the “anthropic landscape” model, but is clear to distinguish its “multiverse” of distinct vacua with different moduli from our infinite universe with (as far as we know) a single, possibly evolving, vacuum state. But string theory could be completely wrong and the deductions from observational cosmology would still stand. For that matter, they are independent of the “eternal inflation” model the book describes in detail, since they rely only upon observables within the horizon of our single “pocket universe”.

Although the evolution of the universe from shortly after the end of inflation (the moment we call the “big bang”) seems to be well understood, there are still deep mysteries associated with the moment of origin, and the ultimate fate of the universe remains an enigma. These questions are discussed in detail, and the author makes clear how speculative and tentative any discussion of such matters must be given our present state of knowledge. But we are uniquely fortunate to be living in the first time in all of history when these profound questions upon which humans have mused since antiquity have become topics of observational and experimental science, and a number of experiments now underway and expected in the next few years which bear upon them are described.

Curiously, the author consistently uses the word “google” for the number 10100. The correct name for this quantity, coined in 1938 by nine-year-old Milton Sirotta, is “googol”. Edward Kasner, young Milton's uncle, then defined “googolplex” as 1010100. “Google™” is an Internet search engine created by megalomaniac collectivists bent on monetising, without compensation, content created by others. The text is complemented by a number of delightful cartoons reminiscent of those penned by George Gamow, a physicist the author (and this reader) much admires.

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