Scientific American magazine is kind of like Men's Health these days, and as seen on its August 2011 cover, SciAm's version of "Get Rock Hard Abs" is anything having to do with the multiverse. The real existence of an ensemble of very different universes, much too far away to ever be seen, has become a popular speculation. But it's nothing more than an imaginative idea invented by well-meaning humans who seek explanations without having enough information — not terribly unlike the idea of "God."
First, some background: The idea of a multiverse originated in the 1950s, when a graduate student named Hugh Everett wrote one of the most influential Ph.D. dissertations ever. He argued that just as the electron cloud surrounding an atomic nucleus represents all possible positions for that electron (were we to pin down its location), so the entire universe, obeying the same laws of physics, must have a "universal wave function" that represents all possible configurations and courses of events within the universe. The idea took root in the form of a world that is constantly splitting or branching into different possibilities; when you decided to turn right at that intersection and collided with a bicyclist, there's another "branch" of the universe in which you turned left and got a traffic ticket. We can't observe how the other branches turn out, but some physicists believe that those branches are every bit as real as the branch you and I are currently experiencing.
Over the decades, the multiverse idea has evolved; currently there are four definitions of "multiverse" under consideration. A Level I multiverse proposes simply that our universe is infinitely large, and we can observe only a tiny region. Other regions have the same laws of physics but different distributions of matter. Therefore, with an infinite number of these regions, some of them must be similar to ours — including perhaps a world where everything is the same as our world, except you turned left at that intersection. In a Level II multiverse, the laws of physics vary from place to place; only our local "neighborhood" operates on the physical laws familiar to us. A Level III multiverse is the kind originally envisioned by Everett, where there is really only one local universe, but within that universe are an infinite number of branches — including a branch where you turn left, and some branches where the laws of physics started out completely different and matter never formed. Finally, a Level IV multiverse comprises the sum of any and all possible mathematical structures that may represent universes, with the mathematical structures themselves being fundamental or irreducible entities, not the universes they represent. In a Level IV multiverse, "self-aware substructures" (which are also fundamentally mathematical) arise — conscious beings like you and me.
Granted, Level III and Level IV multiverses are abstract and subtle. Maybe that's why, in popular science media, you hear so much more about Level I and Level II. For example, in the History Channel variety of science "edutainment," the Level II multiverse, with its vast array of distant "bubble" universes, all with different physical constants, has become the default go-to explanation for why our universe appears to be fine-tuned for the existence of matter. Fine-tuning has become a problem in physics in the last couple of decades, but the Level II multiverse offers an easy way out. If there are an infinite number of bubble universes, each with different physical laws, surely some of those would have laws that support the formation of matter, and eventually life. So, finding ourselves in such a "fine-tuned universe" should not be surprising at all.
Here is where I bristle. Yes, we do need an explanation for our peculiar and benevolent set of physical constants that's better than "a loving intelligent creator designed the universe to be that way." But let's not be ridiculous about it. Proponents of the Level II multiverse insist that other bubble universes must exist for no other reason than we find ourselves living in one such bubble. To me, that's an arrogant and small-minded conclusion to draw. As I've argued previously, it's like seeing MTV playing on your television, and then based on that one observation, concluding that there must be hundreds of invisible TV cables somehow entering your home, each carrying a different channel.
In The Trouble With Physics, Lee Smolin writes that one of the greatest powers of science is to protect humans from their own imaginations. We are very good at noticing patterns in the world and generating possible explanations for those patterns. But when we aren't given enough information — as is always the case — our creative imagination fills in the gaps. Whether it's the Earth supported on the back of a turtle, or a Judeo-Christian Yahweh dividing light from the darkness, our naive explanations tend to be fanciful. Medieval astronomers imagined the Sun and planets moving on rotating crystal spheres, but then Copernicus, Galileo, and Kepler showed that the crystal spheres weren't necessary. The same happened to the aether, the invisible medium of space that was believed to carry light waves, and which was vanquished by Einstein. This steady correction of fanciful human fabrications is the legacy of science. "More than anything else," Smolin writes, "[science] is a collection of crafts and practices that, over time, have been shown to be effective in unmasking error. It is our best tool in the constant struggle to overcome our built-in tendency to fool ourselves and fool others."
So we have the bubble universes, which are posited to "really" exist, despite being completely unobservable. If science is our best hope for unmasking wrong explanations, it can't help us here. Unchallenged, the bubble universe theory could persist for hundreds of years, as untested as Judeo-Christian creation before it, simply because it is an explanation, even if it isn't a scientific one. This is why it irks me to hear an authority in physics just lay it out, as verified truth, that distant, very different bubble universes really do exist out there — that this view explains everything. No need to think about the issue anymore; it's been solved. You've just gotta have faith!
I am reminded once again of a telling moment from Carl Sagan's Cosmos. Long ago, people wondered what was on the surface of Venus. Through a telescope, the planet was a white blur. It must be covered with clouds. If it's covered with clouds, it must rain a lot, and the surface must be wet and swampy. If it's swampy, there must be swamp creatures, maybe even dinosaurs. As Sagan put it, "Observation: Can't see a thing. Conclusion: Dinosaurs." The crazy collection of unobservable bubble universes thought to be out there are today's Venus dinosaurs. In 100 years, we will laugh at the naivete and arrogance of this colorful, if incredibly small-minded, conception. Science moves on, forever banishing the errors of the human imagination. In this case, it's a sure thing.
