Parallel Universes

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 worldand 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!

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