Because I seem to have this habit for going off on weird tangents mildly related to someone's original, deep philosophy entry.
The Science of Realities
The Best (and Worst and Mediocre) of all Worlds
Quantum mechanics is confusing. So confusing that scientists are still trying to figure out how to interpret all that happens in the hyper-microscopic realm of reality. One interpretation of quantum mechanics is known as the Many Worlds Interpretation (MWI). This interpretation is based on the fact that when you try to observe a particle such as an electron or photon, its wavefunction (a representation of all the probabilities of where a particle could be) spontaneously collapses into the whimsical point particle we all see in pictures of atoms. The particle could have appeared anywhere on the wavefunction, but we only see one. MWI says the particle appeared in different places on the wavefunction in different universes that split off from the original one. Each possibility happens in its own universe. Theoretically, this also happens on macroscopic scales. If you flip a coin and you see it lands tails, another universe will have split off where it landed heads. And theoretically, there's another you in that universe that saw the coin flip that way. Universes split off for every possibility of every action ever. In one universe, you won the lotto 5 years ago. In another universe, Al Gore is president right now. In a third universe, the dinosaurs never got wiped out and evolved to become reptilian humanoids and started creating space colonies.
But what does this have to do with reality? Not much really. Just that if MWI is right, there is an infinite number of realities and an infinite number of futures with an infinite number of possibilities.
It's All in Your Head
Another popular interpretation of quantum mechanics is the Copenhagen interpretation, which leaves out constantly branching universes. I'll leave the explanation for this one to Bob Berman:
"They believed the inability to know both an electron's motion and position simultaneously was not just a measurement problem. Instead, the issue was far more fundamental: An electron does not have a position or a motion unless someone looks at it!
The idea goes like this: Every solid object is made of energy and has a wave nature. Small-particle waves are well understood, and their wave function collapses when they are disturbed. Only then can that particle actually exist in a specific spot. What makes a particle's wave function collapse? To measure it, you have to hit it with something like light, and that instantly does the job. But these guys went further and said merely learning about the electron triggers the collapse. Knowledge alone collapses a wave function.
Wait, it gets worse. From there, it was only a short mad stroll to the increasingly accepted (but still minority) notion that no objective universe exists. Observations are the cosmos. Without your mind, things don't exist — certainly not in their present forms."
Certainly a weird consequence of physics if there ever was one. Here, reality wouldn't exist were it not for conscious minds to observe it.
Now for some non-quantum reality warping theories.
You all have heard about the Big Bang. Well after the Big Bang created our universe, there was this spiffy thing called inflation when the atom-sized universe went from that size to the size of a tennis ball in 10-43 seconds (way faster than it would have without inflation). In recent years, a lot of scientists says inflation leads to multiple universes, though there are several variants.
Flavor 1: Our universe is big. Reaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaal big. So big that we can't see it all actually. In fact, we live in just one pocket universe in thi scenario. We can only see so far as light lets us. There are many other pocket universes too (thousands and thousands) in this version. The exact number depends on the details, some people say the thing is finite in size so there aren't infinite universes but still a hefty lot. Other versions say it's infinite in size and there's an infinite number of pocket universes, but there's a finite number of histories for the set. That means there are copies of universes. For example, there is an exact copy of you meters away. And yous in different universes might have different life stories.
Flavor 2 (Eternal Inflation): The way inflation works, only part of the fledgling universe inflated. The rest was surrounded by false vaccuum. Bits and pits of the false vacuum constantly decay and inflate into other universes, but the false vaccuum expands more than the universes expand within it, meaning new universes will be created forever. This one says that the universes that inflate into existence can have different physical constants (thought Flavor 1 may allow that too, I can't remember).
Give Me Some Branes
This one comes at you from string theory, the current wunderkind of physics. Some variants of string theory that predict higher dimensions say our universe could be a brane, a realm of space with fewer dimensions than the "bulk" (seriously, it's called that). There could be multiple branes in the bulk that move through the higher-dimensional space. Some think the Big Bang could be the result of our brane hitting another brane. String theory and many other particle physics models allow for the branes to have different physical laws again.