Guest editor, Paul Davies, notes that "Einstein once remarked that the thing that most interested him was whether God had any choice in the nature of his creation. What he was referring to is whether the physical universe necessarily has the properties it does, or whether it might have been otherwise. Obviously some of the fine details could be different, such as the location of this or that atom. But what about the underlying laws? For example, could there be a universe in which gravity is a bit stronger, or the electron a bit heavier?"

Strangely enough, centuries before Einstein, another thinker, Gottfried Leibniz, had posed a similar question or, rather, had confronted a similar dilemma. His solution was the notion of "possible worlds" any of which could have been created (actualized?) by God, but, as Leibniz observes in his 1686 Discourse on Metaphysics, section 6, "God has chosen the most perfect world, that is, the one which is at the same time the simplest in hypotheses and the richest in phenomena, as might be a line in geometry whose construction is easy and whose properties and effects are extremely remarkable and widespread." Moreover, "when a rule is extremely complex, what is in conformity with it passes for irregular" (ibid.). Furthermore, in a letter to Arnauld, also from 1686, "since there is an infinity of possible worlds, there is also an infinity of possible laws, some proper to one world, others proper to another, and each possible individual of a world includes the laws of its world in its notion." In other words, more modern ones, there are many universes; or, as they say, there is a multiverse.

And, yet, as Davies observes:

"The discovery that we live in a remarkably bio-friendly universe has prompted some scientists and philosophers to conclude that there is something contrived about the way our physical world is put together. Others have opted instead for the 'multiverse' explanation. According to this theory, our universe is but one among an infinity of actually existing universes. In other words, the alternative possibilities are not failed worlds, but real worlds that exist alongside ours. It is, of course, no surprise that we find ourselves inhabiting one of those relatively rare life-permitting universes; we would not be able to exist in a bio-hostile alternative. The multiverse theory is an old one, and often goes under the name 'the anthropic principle.'"

For Leibniz, what we call the anthropic principle (in its various instantiations) reveals something different, for given "the interconnection of things, the whole universe with all its parts would be quite different and would have been different from the beginning, if the least thing in it had happened differently than it did. [Yet it] does not follow form this that events are necessary, but rather that they are certain" (Letter to Arnauld, ibid.).

Indeed.

Moreover, in his 1697 "On the Ultimate Origination of Things", Leibniz states "all possibles strive with equal right for existence in proportion to their reality." And that is something that today's author, Max Tegmark, might agree with, for he says that people "have speculated about such things since the dawn of time, but now it's gradually becoming experimentally testable." And that is something not merely certainly interesting, but necessarily so! Read on to explore his ideas.

Tegmark's paper is part of a special series in anticipation of The Science &
Ultimate Reality Symposium in Princeton, a symposium in honor of the 90th year of John Archibald Wheeler, a great physicist and teacher of physicists. It runs from March 15-18, 2002. To get more information or to register for the Science & Ultimate Reality Symposium at Princeton, go to<http://www.templeton.org/ultimate_reality>. We hope to see many of you there. You can also subscribe to this list independently of VIEWS by going to <http://listserv.metanexus.net/metanexus/archives/wheeler.html>.

-- Stacey Ake

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Dear Colleagues,

Science & Ultimate Reality

Einstein once remarked that the thing that most interested him was whether God had any choice in the nature of his creation. What he was referring to is whether the physical universe necessarily has the properties it does, or whether it might have been otherwise. Obviously some of the fine details could be different, such as the location of this or that atom. But what about the underlying laws? For example, could there be a universe in which gravity is a bit stronger, or the electron a bit heavier?

Physicists remain divided on the issue. Some have flirted with the idea that if we knew enough about the laws of physics, we would find that they are logically and mathematically unique. Then to use Einstein's terminology, God would have had no choice: there is only one possible universe. However, most scientists expect that there are many - probably and infinite number - of possible alternative universes in which the laws and conditions are self-consistent. For example, the universe could have been Newtonian, consisting of flat space populated by hard spheres that fly about and sometimes collide. It would probably be a boring world, but it's hard to say why it is impossible.

If the universe could have been otherwise, then two questions follow. First, is there anything special about the actual universe that we inhabit as opposed to the alternatives? Second, why has this particular universe been selected for actuality from among a limitless number of different possibilities?

One feature that might be special about our universe is that there are conscious beings inhabiting it, beings that can look in awe at their world and reflect on the nature of existence. Mathematical studies suggest that some key features of our universe are rather sensitive to the precise form of the laws of physics, so that if we could play God and tinker with, say, the masses of the elementary particles, or the relative strengths of the fundamental forces, those key features would be compromised. Probably life would not arise in a universe with even slightly different laws.

The discovery that we live in a remarkably bio-friendly universe has prompted some scientists and philosophers to conclude that there is something contrived about the way our physical world is put together. Others have opted instead for the "multiverse" explanation. According to this theory, our universe is but one among an infinity of actually existing universes. In other words, the alternative possibilities are not failed worlds, but real worlds that exist alongside ours. It is, of course, no surprise that we find ourselves inhabiting one of those relatively rare life-permitting universes; we would not be able to exist in a bio-hostile alternative.

The multiverse theory is an old one, and often goes under the name "the anthropic principle." What is new is that physicists now have some halfway reasonable conjectures about how all these other universes might come to be. John Wheeler, together with Hugh Everett, suggested an early possibility, in which the alternative worlds are different branches of the wave function in a quantum superposition. (In later years Wheeler moved on from this interpretation of quantum mechanics.) Another idea is that, what we normally call "the universe," is but a small component in a much larger system, in which other spatial regions have different laws and properties. Perhaps the most extreme version of a multiverse has been suggested by Max Tegmark, a theoretical physicist at the University of Pennsylvania. Tegmark proposes that all mathematically self-consistent world descriptions enjoy real existence. There is thus a sliding scale of extravagance, ranging from multiplying worlds with the mathematical laws fixed, to multiplying laws within an overall mathematical scheme, to multiplying the mathematical possibilities too.

Some people conclude that invoking an infinity of unseen worlds merely to explain some oddities about the one we do see is the antithesis of Occam's Razor. Others believe the multiverse is a natural extension of modern theoretical cosmology. (It's worth noting that Max's website lists his multiverse theory under the "Crazy Stuff" category! Go to<http://www.hep.upenn.edu/~max/main.html> to explore!) Whatever you think, Tegmark is the ideal guide to tell you about the various multiverses on offer. A brief summary of his paper follows.

Paul Davies

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Title:: Parallel Universes

Author: Max Tegmark

Summary:

The notion that our observable Universe is merely a small part of a larger "multiverse" is attracting increasing attention among physicists, particularly those working on cosmology (motivated by inflation theory and apparent fine-tuning) and string theory (motivated by multiple "vacuum" states). The goal of the article is to survey the various types of parallel Universes proposed, focusing on - the evidence and physical motivation for each type - the extent to which the ideas are experimentally testable - what each type would be like to inhabit - philosophical implications and the structure of the multiverse. It is not widely appreciated that there are in fact numerous different types of multiverses being proposed. The key goal of this article is to describe the different kinds and the relationship between them. People have speculated about such things since the dawn of time, but now it's gradually becoming experimentally testable.

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Published   2002.03.03