You've got to believe you can be a stand-up before you can be a stand-up. You've got to believe you can act before you can act. You've got to believe you can be an astronaut before you can be an astronaut. But you've got to believe.
Multiverse theory is a funny old thing. It's one of those theories that might solve a bunch of problems, like the way our Universe seems to be incredibly finely tuned to allow complex life to develop, or some of the oddities of quantum theory like particles being in two different places at once. It's a theory that has been developed and delved into by some of the most eminent physicists ever to have lived...and it's not even science.
I'm not suggesting, of course, that Martin "Astronomer Royal" Rees, Stephen Hawking, Max Tegmark and their colleagues are cheating in any sense, or wasting their time researching the subject, because it's possible that they're laying the groundwork for a game-changing new theory. It's still not really science though...yet.
Suggesting that there is such a thing as "a Multiverse theory" is a little over simplistic and disingenuous. There are a great many Multiverse theories. Tegmark's work alone suggests there are at least four different possible "layers" of Multiverse, any combination of which could be correct.
There's the simplest kind, level one, which suggests that our normal, everyday Universe is in fact infinite, or at least far, far bigger than we presently observe. If you're sat in a boat on a calm sea with your eyes around two metres from the surface of the water then you can only see about five kilometres in any direction...the Earth curves away, limiting how far you can see. We have the same horizon problem with the Universe, except it curves away in time.
|The CMB: The furthest we can see.
But the horizon problem still exists. Currently we have no way whatsoever of exchanging information with bits of the Universe beyond our cosmic horizon, other than (possibly) very short range glitches in the CMB. The main feature of a level one multiverse is that it's all part of the same Big Bang - everything is based on the same laws of physics as our little "local" bit.
The second level is where things start getting a bit weird. It suggests there are other "post-inflation bubbles". In essence, other Big Bangs which took place somewhere else. The laws of physics are the same as ours, but the physical constants will be different. Gravity might be stronger, probably resulting in a rather spectacular, short lived universe full of black holes. Or weaker, meaning few, if any, stars and very little other than Hydrogen and Helium. Atoms themselves may behave differently, or never form at all. A level two Multiverse is an attractive idea because it naturally solves the fine tuning problem. There are around forty constants in physics, numbers that are the same everywhere we look. The charge on an electron is the same anywhere in our Universe, but there's no good reason why it has the charge it does. If it was different then the laws of physics would still mesh together perfectly well, the Universe would still exist, it would just be different. Life as we know it almost certainly wouldn't exist, but that doesn't really matter. In fact, change any of the forty-odd constants by just a little bit and the chances are that any kind of complex structure, including life, couldn't exist. So why is our Universe so delicately balanced? So subtly "designed"?
The level two Multiverse solves this problem. If there are an enormous number of universes, each with its own set of physical constants, then there's bound to be a few that by sheer fluke hit the right combination for life. Life eventually evolves in this small subset of universes and sits there wondering why its universe is so well designed...
Level three is based on the weird results of quantum theory. One of the founding experiments is called Young's Double Slit. I'll let Mark Everett (of rock band Eels) explain:
Why is a rock star talking about a two hundred year old physics experiment? It's because his father, Hugh Everett III, was a physicist who suggested that this experiment shows two parallel universes overlapping. The only difference between "our" Universe and the parallel one is that in ours the photon went through the left slit, and in the other universe it went through the right. Because the two universes are otherwise identical there is a certain amount of leakage between the two, they are able to very subtly influence each other, resulting in the interference pattern.
Everett's "Many Worlds" interpretation was fairly roundly rejected by the rest of the physics community when he first suggested it, and you can see why. It really does sound like something from a science fiction novel. The idea is undergoing a little bit of a renaissance however. The modern interpretation is that all possible moments in time, in all possible universes, actually exist, and ours Universe is simply a "most likely" path through this higher level of Multiverse. It explains the probabilistic nature of quantum mechanics, the slightly fuzzy nature of the Universe when we take a very close look at it...we're looking at a collection of possible universes, not our Universe at all.
Again, there's no real way to test this satisfactorily. There is the quantum suicide experiment for example. The experimenter stands in front of a machine gun which is connected to a device which measures the spin of a subatomic particle. The spin can be either up or down - if it's up then the gun fires, if it's down the gun doesn't. If Everett's idea is right, and does actually represent a Multiverse, then there will always be a universe where the experimenter survives. If the measurement is done ten times then there will be 1023 universes where the newspaper headline is "Idiot Scientist Shot In Face" and one where the experimenter survives. Unfortunately (for the theory, if not the experimenter), there is always exactly the same small chance of survival (even if the Multiverse idea is wrong, so you can never have a definitive answer.
Tegmark's last level, the fourth, is the most philosophical in nature. All of the previous levels share one thing in common, the laws of mathematics. Even if the laws of physics change, they can still be described with equations and mathematical expressions that would be recognisable to us. Changing the universal gravitational constant, G, changes the universe. A level four parallel universe changes the equation that G appears in. Currently there are no known ways, even theoretically, that this idea can be physically tested or explored in any way.
Tegmark's ideas are by no means the only ones, but they are a popular basis for investigating the nature of a Multiverse if it exists. No, none of it makes testable and new predictions at the moment, so it's not science yet. The implications, on the other hand, are fascinating.
If an infinite Multiverse of any kind exists then we can have some fun with it. Let's take a lottery for example. It's intuitively obvious that you have to buy a ticket to win, but if we're in a Multiverse then that statement changes slightly...buying a ticket guarantees that you win. Or at least one version of you, somewhere. You're also immortal. Whatever happens that might kill you, there's always a version that survived against the odds, and as you're able to read this, you are that immortal version...so far at least. The flipside is also true. Every time you cross the road there's a version of you that is killed. Gerry, my flatmate, has taken this principle to a humorously logical conclusion: he delights in setting small traps for me that have a tiny, miniscule, theoretical chance of killing me. He reasons that every time he does this he gains the satisfaction of knowing he's killed me in a parallel universe, without all the drawbacks of being arrested and later murdered in prison. He's an odd man in many ways.
So within reason anything is possible in a Multiverse. But there's no short cut, you can't just sit back and expect to win the lottery or become an astronaut or even a stand-up comedian. The universe where that happens is one where you bought a lottery ticket, or studied physics, or died at umpteen comedy clubs first. You have to make sure you're in the right universe, which means you have to put the work in.
But first, you have to believe.