The universe is an odd place. Maybe not vampires, hobgoblins, and the assorted weird shit from fantasy and science fiction odd, but still pretty damn odd.
Don't believe me? According to the best minds of science, we exist in a universe that can't make up its mind on whether a "cat in a box with poison" is alive or dead. Also, since light acts as both a wave & particle, a single photon being shot through a "barrier with two slits" leaves as a particle, becomes a "wave of potentials", goes through both slits, and then interferes with itself. However, to wrap one's head around the idea mathematically is to understand the photon goes through both slits, neither of the slits, and one slit or the other slit, with all of the possibilities existing in Quantum Superposition with each other.
While the cats & other particles are in multiple states, you are... you, right? The funny thing might be that you might not be the only "you" (although, I guess it would depend on which philosopher's definition of "identity" we're using). Yes Virginia, you might have an evil goateed version of yourself out there somewhere.
The idea of parallel worlds or (evil) doubles has been a staple of fiction for a very, very long time. Like almost everything else in life, an analogy from "Star Trek" might help. Probably one of the best known examples for this topic is Trek's Mirror Universe, where the quasi-Utopian United Federation of Planets' alter-ego is the repressive Terran Empire and all of the normal Trek characters are either evil, surly, goateed, or wearing bare-midriff tops.
Over at io9, they have a weekly "Ask a Physicist" segment. About a month ago, they responded to a question from "Annalee" over whether we live in a multiverse, and if a goateed Spock was possible?
The answer is a definite maybe... with a side of probably. However, there are some caveats. The post basically goes through Cosmologist Max Tegmark's hierarchy of possible multiverses.
► Level 1: An Infinite Universe Beyond Our Cosmological Horizon.
Late last year, the American Museum of Natural History produced a video called "The Known Universe" as part of a new exhibition.
The Level 1 Multiverse postulates that the universe is really, really big, if not infinite. We have an observable "Known" universe (what we can see from the light reaching our point in space), but just because we can't see it doesn't mean there isn't anything beyond the horizon in the video above. Those other areas, which we can't see and can't see us, might be considered universes of their own. If repeated an infinite number of times, there might be some overlap.
So, as a result, if the universe is infinite we must all have doppelgängers somewhere out there due to the mathematics of infinity.
So we do live in a multiverse. There are other regions of space with the exact same laws of physics as ours, but which lie outside of our horizon just as we lie outside of theirs. For all intents and purposes, these are different universes than our own.
How big does a Level 1 Multiverse have to be before we start getting exact duplicates of everybody? Pretty damn big. Tegmark estimates it at around 1010^29 meters from here. This is the biggest number that's going to come up in this discussion outside of infinity itself. What this means is that every atom in the duplicate universe is in precisely the same spot and moving with the same speed (up to the limits of quantum uncertainty) as in our own universe. That means that even if alternate Annalee didn't have exactly the same history as our own, her brain is configured so that she thinks she did.
Of course, if the universe were infinite (and I'm not convinced that it is) then it would be plenty big enough to accommodate not only a duplicate of you, but an infinite number of them.
► Level 2: Other Universes Popped Out Of Cosmic Inflation Along With Ours, But With Different Physical Constants.
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Our bit of the universe grew out of one tiny patch of the very early multiverse, but it's possible that others did as well. What's more, some, perhaps all, of those patches may have physics just a wee bit different from our own. Electricity might be a bit stronger or weaker; the strong force (the one that holds protons and neutrons together) could be a bit different; there could be more than 3 dimensions.
Let me make a couple of things clear:
- It's not obvious that this model is correct. It may be that the fundamental forces really are hardwired in nature, and that all universes have the same underlying physics. We don't really know, but I kind of hope that this is the case, in which case there's no Level 2 Multiverse.
- If there really is a Level 2 Multiverse, with universes each with different physics, these universes aren't going to look much like our own. No... humans, and possibly no life or complex structure at all. It turns out that physics needs to be very finely tuned in order to make things like stars or heavy elements, and most universes simply don't cut mustard. This is the origin of the "anthropic principle" (the weak version for all of you pedants out there), which says that there may be lots of universes with different physics, and we're very lucky to be in one which supports life — but of course we couldn't be anywhere else and still have the conversation.
► Level 3: The Many-Worlds Interpretation
Quantum mechanics can get very strange. At quantum levels, matter exists simultaneously as particles and as waves (wave-particle duality), a particle’s position and momentum cannot be precisely known at the same time (Heisenberg uncertainty principle), and the state of two objects can be intertwined, regardless of the physical distance between them (quantum entanglement).
In 1957, a man named Hugh Everett suggested that perhaps the reason a particle’s outcome can’t be predicted is because every possible outcome does occur. This notion led to the "Many-Worlds Interpretation" (MWI) which postulates that at the quantum level, everything that can happen does happen, and that each possible outcome branches the universe into another which is at first identical aside from the alternate outcome. So the seemingly "random" outcome is actually just representative of the one possible outcome one’s current universe happens to be based upon. The overlapping universes, between which no information can pass, would then continue to develop individually, each of them branching endlessly as well.
So in the Schrödinger's cat experiment, according to MWI, there is a universe where the cat lives & a universe where the cat dies, unlike the Copenhagen interpretation that says once an observation occurs the waveform collapses into one possibility.
The parallel universes of level III exist in an abstract mathematical structure called Hilbert space, which can have infinite spatial dimensions. Each universe is real, but each one exists in different dimensions of this Hilbert space. The parallel universes are like different pages in a book, existing independently, simultaneously, and right next to each other. In a way all these infinite level III universes exist right here, right now.
However, it's unlikely that in the other universes we'd have the same people just with evil goatees. Any point of divergence would quickly create ripples that would cause big changes between our universe & the Mirror-verse (see Chaos Theory & what happens when you step on butterflies in the past).
One of the more interesting thought experiments connected to the Many-Worlds Interpretation is the idea of Quantum Immortality.
Quantum Immortality wonders what the experience would be from the perspective of Schrödinger's cat, instead of the observer. Imagine it this way, a person stands next to a nuclear bomb which is set to explode. It's probably 99.999999999% certain that he will be vaporized, but it isn't a total certainty. If Many-Worlds is correct, in that 0.000000001% or less of a chance, there should exist a tiny set of universes where the person survives, either by the bomb not going off or an even tinier subset where the person somehow survives the blast (although, probably horribly maimed). As long as someone doesn't experience an event where the probability of surviving was exactly "0", no matter how unlikely it may be, there should always exist a universe where a version should somehow survive.
► Level 4: Mathematical Universes
Some months back I wrote a diary about Pi, which touched on a very old debate between Plato & his detractors over whether Math is a discovery or an invention of humanity? In the Mathematical Universe Hypothesis, Tegmark sides with Plato, and argues for the external reality of math, and goes on to argue our external physical reality is a consequence of mathematical structure.
Therefore, as a consequence, all structures that can exist mathematically can also exist physically.
From Discover Magazine:
Galileo and Wigner and lots of other scientists would argue that abstract mathematics "describes" reality. Plato would say that mathematics exists somewhere out there as an ideal reality. I am working in between. I have this sort of crazy-sounding idea that the reason why mathematics is so effective at describing reality is that it is reality. That is the mathematical universe hypothesis: Mathematical things actually exist, and they are actually physical reality.