Posted: Jun 11, 2011 2:02 am
jamest wrote:... Firstly, assuming that there are many worlds like this that exist at the same space & time, it should be acknowledged that these worlds cannot be comprised of the same matter, for if one (the same) set of matter exists at one (the same) set of space & time, then we only have one world, not many. That is, the MWI requires that different matter/bodies exist at the same space & time. But, how can more than one body exist at the same space & time? That is illogical. (Problem 1)
MWI does not propose that that "more than one body exists at the same time" in any fundamental way. To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence. Parts of the quantum wave function become "decoherent" from other parts, meaning that they cease to interact in any way worth talking about. This gives the appearance of separate worlds, with separate objects. Suppose a ball exists in my world, and I can interact with it. Another ball exists in someone else's world, and I can't interact with that. The two balls don't interact with each other - even though that might be in the same space. The underlying explanation of this isn't that "two bodies occupy the same space". It is that a single wave function occupies the same space, and parts of it don't interact with other parts - and yes, the two parts can overlap. However, the idea that waves can overlap is hardly radical.
A (very crude and open to misuse) analogy might be radio transmissions. Someone might ask how radio transmissions of two separate radio shows can pass through the same space at the same time. Doesn't this require two things to occupy the same space? This is dealt with when we see that nothing really passes through the space except radio waves - and a single description can be made of the radio waves passing through the system. Likewise, you could step back from the many-worlds of MWI and describe everything in terms of just "one world" with the wave function - and parts of that wave function ceasing to interact with other parts explain the appearance of separate worlds. The different worlds in MWI aren't really separate in any ontologically "profound" way but merely by lack of interaction and limited human perception due to our status as observers embedded in the system.
In a way, the concept of matter as we know it is no longer fundamental in MWI anyway. It makes little sense to object to a ball in one world being "the same matter" as the ball in another world. Looked at closely enough it is all just waves anyway.
Even thinking of this in terms of "bodies" existing in different worlds suggests a problem to me, so I have doubts about whether it is worthwhile getting into the other questions right now. I will say that MWI makes no claim that the results of quantum mechanics experiments should be predictable by humans.