If you've ever had a phenomenally bad day and wished that maybe there were a version of reality where all of your wrong turns, missteps and poor judgment calls righted themselves, congratulations! You've joined a community of scientists, philosophers and theologians who have long speculated that the universe we know isn't the only game in town.
To most people, the concept of multiple universes - or the "multiverse," as it has come to be known - conjures images of better places where prettier versions of themselves amass the wealth, power and fame that eludes them in this world. And who can blame them? Some of the most entertaining science fiction is built on this idea. But fans may be disappointed to learn that the idea that proved such an effective device in television series like Star Trek and Sliders is actually a misrepresented oversimplification of an issue scientists have been wrestling with for decades.
But take heart, stalwart Trekkers. The real thinking behind the concept of multiple universes is boldly taking physics, cosmology, religion and philosophy where no one has gone before.
The big bang's bigger meaning
British author Michael Moorcock introduced the general public to the term "multiverse" in a short story called "The Sundered Worlds," printed in a 1962 issue of Science Fiction Adventures. But the idea of multiple universes had been bandied about, in one form or another, as early as the 16th century.
It was 1576 when British astronomer Thomas Digges modified Copernicus' idea of the universe. What Copernicus thought was a clearly defined "outer rim," said Digges, was really unbounded space filled with stars stretching infinitely in every direction. Though Digges couldn't have known it at the time, a big-bang explosion started our universe in motion and, in doing so, produced cosmic microwaves. As Digges peered into the sky, these rays were traveling through that unbounded space; their detection confirmed the big-bang theory and earned Arno Penzias and Robert Wilson the 1978 Nobel Prize in Physics.
The radiation led physicists to believe that space is expanding and that matter is spread more or less randomly through it. In the 1980s, Alan Guth at the Massachusetts Institute of Technology and Andrei Linde at Lebedev Physical Institute in Moscow came up with the idea that the universe ballooned in a rapid burst soon after the big bang, and that burst exponentially increased the size of the universe. This new theory of "inflation" set the stage for modern multiverse theory.
Inflation creates what physicists call a "false vacuum," a fancy name for "bubble." If inflation created the bubble we know as our universe, who's to say it didn't create others in the same way? There may be an infinite number of universes co-existing with the bubble that is our universe.
It's easy to get tripped up by the terminology; most dictionaries simply define "universe" as "all matter and energy, including Earth, galaxies and intergalactic space." So even if there is more out there than we previously thought, it makes sense that "universe" would cover it all. But when we say "universe," we really mean "the universe as we understand it" - our concept of the unchanging laws of nature, the pull of gravity, the dimensions in which we move. The places beyond our realm, where these constants may not be so constant, are all parts of the multiverse.
We can't see these places, even with the best lenses on the biggest telescopes. These other universes, if they exist at all, are inaccessible. Events that happen there won't affect us. As it turns out, the possibility that these other worlds might be hovering somewhere in the great beyond may provide an explanation of how we came to be in the first place.
That explanation, though, is taking awhile. Until recently, some physicists had lost hope that it would come during their lifetimes.
"What happened during the last two years is quite amazing," said Linde, who now teaches at Stanford University. During that time, Linde and other researchers at Stanford were able to reconcile our universe's continued expansion with a popular, still-evolving theory that may eventually explain our universe, the multiverse and all that lies in between. Physicists refer to this still elusive, all-encompassing crystal ball of an idea as, aptly, the Theory of Everything.
"We see right now that even if we confine ourselves with one particular theory [of how the universe began], it may have 10 to the degree of 1,000 different versions of it," he said. "This idea of the multiverse became much more mathematically developed."
Here's where science-fiction scribes' imaginations go into warp drive. With that many possible combinations of matter, there's also that many possible ways a universe similar to ours could develop. The mundane view is that there's also that many possible ways a universe exactly like ours could develop, but it's much more fun to think of your interstellar alter ego basking in the adoration of a crowd of cheering fans than it is to imagine him or her making your monthly mortgage payment.
"The space of all possible universes is not an easily delimited or unique concept," wrote George Ellis, a South African cosmologist. "Are we prepared to consider: universes with quite different physics; . universes with different kinds of logic and perhaps alternative forms of mathematics; universes allowing magic, such as envisaged in the Harry Potter series of novels by J.K. Rowling?"
Some cosmologists call our universe the "Goldilocks universe," because all of its settings are exactly right for us to exist and flourish. It's not too hot or too cold. It's expanding at a rate that allows cosmic materials to gather into stars without collapsing into black holes. A very small change in even one of those factors, such as the mass of the electron, would mean we're kaput. But for some reason, we're exactly where we're supposed to be, doing exactly what we're supposed to be doing.
That reason, though, is what's up for grabs. So science and religion, each lacking hard evidence and yearning for ultimate answers, begin to circle each other in a dance full of leaps of faith.
"We can dismiss it as happenstance. We can acclaim it as providence, or we can conjecture that our universe is a specially favored domain in a still vaster universe," Sir Martin Rees, the United Kingdom's astronomer royal, wrote in God and Design: The Teleological Argument and Modern Science.
Scientists who are fans of the multiverse theory like the theory because it gets at our existence through the law of averages: If there are an infinite number of universes, it makes mathematical sense that some would have conditions that would support life as we know it. Most would not. Our ability to stand on the planet and wonder about how we came to be is no more than a byproduct of the favorable circumstances that allowed us to exist in the first place.
Philosophers and theologians look at it differently. If our universe is so specifically geared toward our existence, they say, maybe it's because God made it so. This line of reasoning grows out of something known as the anthropic principle.
The anthropic principle stradles the border between science and metaphysics. Many physicists and cosmologists hate the anthropic principle on purely scientific terms; the response to knowledge gaps should be "We don't know yet," they say, not "God did it."
"The anthropic principle is always, at best, a guesstimate," said Lawrence Krauss, Ambrose Swasey Professor of Physics, a professor of astronomy and chairman of the physics department at Case Western Reserve University in Cleveland. "It says, `Hey, look. There's an interesting coincidence.' That's interesting. But it can never say more than that."
Until the ambiguity is cleared up - if ever - scientists like Linde and Krauss prefer to err on the side of reason.
"If you think that there are many universes and I just live in one that is consistent with my properties - consistent, but not built for me - it's as though we are compatible with the universe just like dolphins are compatible with the ocean and flies are compatible with the air," said Linde.
"The other dangerous aspect of the anthropic principle is that it's often misinterpreted by religious people that there's somehow some evidence for God or purpose, and it's precisely the opposite," Krauss said. "Life is a random occurrence and randomly occurs in the universes that allow it."
"It seems to me that we have very little idea of what God's full purposes are," McMullin said, adding that the scientific side is equally murky. "We don't have a unified science of relativity and quantum theory," he said. "Since that's the theory one would need to discuss the big bang in any full way, we have to go very cautiously."
The scientific and theological explanations don't refute each other because they come from such different fields, he said. Therefore, believers of all stripes are left to weigh the theories themselves.
"They're both extravagant," McMullin said. "You have to decide which kind of extravagance you want to believe."
Taking it on faith
For 20 years, physicists and cosmologists have been playing cat's cradle with string theory and its possible meaning for the multiverse. Each conference, each publication, each time two scientists sit down for coffee could mean a possible reshaping of the multiverse and our lives and beliefs in it.
Some choose to hedge their bets.
"We don't know the nature of the laws at the deepest level - how permissive they are," said Rees. "Until we do, we don't know whether multiverse and anthropic reasoning are inevitable or irrelevant. Either could be the outcome, and it seems absurd to have strong bets or prejudices either way at the moment."
Others, like South Africa's Ellis, handle the indistinct nature of the multiverse by embracing it.
"We simply do not know," he said. "In the end, belief in a multiverse will always be just that - a matter of belief, based in faith that logical arguments proposed give the correct answer in a situation where direct observational proof is unattainable and the supposed underlying physics is untestable."