I have been reading A Short History Of Nearly Everything by Bill Bryson and he touched on the Many Worlds Theory, but gave it only a line or two. The concept is fascinating, but most of the information I have found online is about a mile over my head. Can anyone out there give me the Cliffs Notes version of this??
I can try.
OK, in quantum mechanics, a particle is described by a wavefunction, which essentially, is a measure of the probability of finding the particle in a given state, say with a given spatial position and momentum. When an observation is made, we do not see this probabilisitic distribution, but rather we see the particle in a certain state. There are two interpretations as to what happens here - the Copenhagen interpretation, and the Many Worlds interpretation.
The Copenhagen interpretation basically states that the wavefunction “collapses”. That is to say, that yes, before the measurement was made, the particle could have been in any state, but as soon as the measurement was made, the particle is essentially forced to adopt one state or another, and there is no longer anything probabilistic about it - the probability distribution has collapsed - this is the wavefunction collapsing.
The Many Worlds interpretation however states that when a measurement is made, the universe essentially splits into a universe where the particle has the properties we measure, and another universe in which the particle will have a different set of properties.
That’s it, I think, in a nutshell.
I’m sure if I’ve got anything wrong (this is not my area of expertise, although I do teach basic quantum mechanics on occasion), someone will jump in and say so.
Man, that is interesting. Does the mainstream science community buy into this theory? I told my wife about it and she just shook her head.
Quantum physics has some pretty damn weird implications. I failed the course this year 
but I’m going to take it again next year when I’m a part-time student, so I can devote all my study time to it… it’s really bizarre and fascinating stuff.
As Angua says in quantum physics you have a quantity now as the wavefunction (Ψ) which is the solution to a second order differential equation (the Schroedinger equation) and it gives you (or more precisely the square modulus of the normalized wavefunction as the wavefunction is an essentially complex quantity), for example the probabilty of finding a particle at a certain point at a certain time if you were to make a measurment.
Again as Angua says when a mesurement is made the wavefunction ‘collapses’ froms this ‘superposed’ states into one of these states (i.e. a mesuremnt produces a reult equal to one of the eigenvalues of the opertaor of the observable measured) completely at random, dpending only on the probailty of each state. This is a basic description of the Copenhagen interpreation, the convential interpretion of non-relativistic quantum mechanics.
In the many worlds interpretaion collapse does not occur, what happens when a measuremnt occurs is that the appartus gets entangled with the wavefunction and the hole system evolves into different branches that may not interfere with each other. In other words for every possible result of the measurement there exists a parallel world.
Many worlds theory can’t be rejected on observational evidence as it produces the same results as the Copenhagen interpretaion, but without the troublesome collapse of the wavefunction, which makes it particularly attractive to those involved in quantum cosomology when tlaking about the wavefunction of the universe. Howver it’s not the conventional intepretaion as it has such a high ontological cost and it has the problem that it can’t attach meaning to probabilty as individual branches do not necessarily have the same probailty attached to them.
Not really - the Copenhagen interpretation is the most commonly accepted, and with obvious cause; it just doesn’t seem plausible that the universe splits over the position of an electron. I personally think that its a load of hooey, but that’s just an astophysicist’s view, and I’m sure that someone who’s far more knowledgeable on the subject will be along shortly to explain why it seems so implausible.
I am sure this is a dumb question, but if no measurement is taken does the “split” still happen? If the system is not interfered with then it is still in a sort of static state where all possibilities still exist?
No. Without a measurement, the particle continues to follow the wavefunction (can’t get Greek fonts on this computer! ;)). It is the act of measurement that causes wavefunction collapse (or world splitting).
is the measurement truely random? what i mean is if the wave function and everything that affects it were ether copied simulated perfectly, would it happen the same gauranteed, or is there a chance it could be different?
You’ve pretty much hit it on the head, if we were to perform the same measuremnt twice in exactly the same way and we have excactly the same superposed state before we make our measurement we can still get two different results (which result we get are only dependent on the probailties given by the wavefunction of the superposed state). This is the heart of the uncertainty principle.
Does the split occur at the point of the event and spread owtward to the rest of the universe at the speed of light, or does the whole universe split instantaneously and simultaneously (something forbidden by Relativity)?
What "split’? In each universe, the universe after measurement is almost exactly the same as the universe before, so the change would not need to proceed at light speed unless the effects of the measurement necessarily proceeded at light speed.
In fact the mechanics of the situation are rather difficult for me to visualize, so I dont exactly know.