I’ve heard this phrase referenced numerous times and all seemingly to describe a variety of occurrences. What is the real definition and who came up with it? Is it more of a philosophical idea or a belief that another actual universe exists somewhere?
Okay, imagine you flip a coin. Tails or heads? Now, according to a theory put forward by a man name Everett (damn, what WAS his first name?), the universe effectively “splits” at that point. In one version of the universe, the coin landed “heads”. In the other, the coin landed “tails”. The two universes are exactly the same, except they’ve branched off from each other- one in which the coin landed head, one in which it landed tails.
This is known as the Everett Many Worlds Theorem.
Note that the branching point can be ANY situation where there’s more than one possible outcome. There are many, MANY quantum decisions being made, all the time, so there are obviously many “parallel universes” out there.
According to Everett, there’s a universe in which Dole is president. There’s one in which life never even evolved on Earth. I’d like to say that there’s one in which Carrot Top is funny, but I’m fairly certain Everett ruled that one out.
And actually, there are an infinite number of those universes, too- remember, each one branches off from itself, constantly.
I like this theory a lot… It solves the whole Grandfather paradox quite nicely, I think.
What’s the difference?
Lightnin’ wrote:
Actually, it’s known as the Everett Many Worlds Interpretation.
Big difference.
If you really want to be driven crazy, this is a good place to start: A Lazy Layman’s Guide to Quantum Physics. I’ve always been a believer in the fact that if a tree falls in the woods it makes a sound, and a cat is either alive or dead, whether or not it’s in a box. However, this is not merely a philosophical question as far as quantum physics goes.
Incidentally, I’ve never been comfortable with describing Hugh Everett’s idea as a theory.
Einstein would have called it a “witch’s brew,” because it’s untestable.
I like the idea, though, because it has explanatory power and offers an elegant solution to some of the most perplexing puzzles in the quantum world.
You don’t have to believe a philosophical idea.
Ok, here’s my hacked-together explanation:
It all comes back to Schrödinger’s cat. If you were to throw a cat in a box, and seal it off with a device made of a radioactive atom, a Geiger counter, and a vial of cyanide, you’d have a kitty death-trap set up to trigger upon radioactive decay, and modern science wouldn’t be unable to predict the moment which it would trigger (one radioactive isotope decays at some time between now and forever, which we can’t predict). So at any given moment, the cat in the box is either alive or dead. The whole parallel universe thing tries to explain how the cat is both alive and dead at the same time, and the true condition cannot be determined until the box is opened and checked. At the moment of observation the cat’s ‘waveform’ settles into one state, either alive or dead.
While this sounds like a bunch of philosophical mumbo-jumbo, in the quantum world things like this happen all the time. Single photons passed through a slit in an interference test interact with apparently nothing to produce perfect interference patterns on a photographic plate; the same effect which happens when many photons are passed through two slits (and interfere with each other). Because we don’t know why this happens (there’s only one photon, how can it interfere with itself) the possibility arises that perhaps that photon is passing through the other slit in a parallel universe, and interferes with these ghost copies of itself before striking the plate.
Here’s what I dislike about the many-worlds interpretation: Whenever the many-worlds interpretation is described, it’s always in terms of two (or possibly more) discrete choices. Many interactions in quantum mechanics have a continuum of possible outcomes. Think of the double-slit experiment. The fringes are fat, not discrete points, and the electron could hit anywahere. Another example is a photon scattering off an electron. The range of possible photon scattering angles is a continuum.
So you don’t have many-worlds, but a continuum of worlds.
David Deutsch’s web site has some interesting links on the many-worlds Interpretation of quantum mechanics. The subject pretty touchy stuff for physicists, so I think I’ll refrain from expressing any ill conceived opinions, in the hope that at least a few of me will get past this moment in time without having my head handed to us. 
Strictly speaking, “parallel universe” refers to another four-space imbedded in a five (or more) space along with ours. The spaces don’t intersect so they are “parallel”.
This is kind of hard to picture, of course, but imagine a two dimensional flat plane in a three dimensional space. You can stack an infinite number of planes parrallel to it. If you lived in a two dimensional space, you would say these other “universes” were parallel to yours.
The “many worlds” conjecture explains what might be in those parallel universes, if they exist.
[tangent]
What is it with Schrodinger and his cat?!
[/tangent]
Me, I agree with Lightnin’… [sub]Damn I gotta get to these threads faster…[/sub]
Does this mean that as I sit here repeatedly flipping a coin I can state:
[homer]
Woo Hoo, I’m creating Universes!!
[/homer]
Um…is it just me or does anyone else see a very simple way to test if the cat is still alive or not?
Why not just shake the box?
Hehehe, Ok so I know that isn’t the point.
The above quantum philosophy is all well and good, but I think that most people are familiar with the idea of “parallel universes” from pulp sf and fantasy stories. There might be some sort of pedigree going back to Schroedinger and his damned cat (this is science fiction after all. Not Everett – I think he was later than the fictional stuff), but a story-strapped author with a looming deadline and no cash wouldn’t need any more inspiration to come up with the idea of a “parallel universe”.
I don’t know who was the first to use the theme – and I’m too far from my copy of Nichols and Clute to check up on it. But Fredric Brown certainly used the idea in his wonderful (and Wayyyyy too long out-of-print) fantasy What Mad Universe?, which I think dates from the early 1950s. It’s the earliest use I can think of right now, but I feel pretty certain it was not the first.
Without looking? I couldn’t tell ya’.
–Cliffy
Slight Hi-jack.
If Parallel Universes/Alternate Realities were a verefiable
fact, and we could access those worlds(like SLIDERS), imagine the possibilities for trade.
Want a 1984 Beatles Reunion Album?
Want a copy of The Wizard of Oz, starring Shirley Temple?
Want a Chrysler Vortex, powered by a 550 hp gas turbine?
Want a cure for AIDS or cancer?
Yeah, but why would they want the crud from your universe? (Although, maybe in a parallel universe, Ernest K. Worral is a star)
The downside is that, in a lot of those universes, you’re dead. You didn’t quite get away when that car almost hit you, or whatever.
If you could visit an alternate world where you died, I’m sure those you left behind would be overjoyed you came back.
True, you wouldn’t be ‘their’ you, but it would be nice to have you back for a while.
Wouldn’t you like a visit from a favorite grandparent who didn’t die?
Trans-Reality Vacations!
Grandpa! It’s great to have you back!
Are you Ed?
No, I’m Roy!
Roy! You’ve grown. How’s your baseball game coming?
I never played baseball!
Oh, sorry. Must be the other reality. How’s your sister Alice?
Sister?