This thread will likely contain spoilers for the second two books, so if you've only read the first you may want to look away for now.
To summarize quickly, they are four types of parallel universes scientists believe may exist;
Type 1 Parallel Universes:
If our universe of space and time is infinite, then mathematically in the infinite reaches of space there must exist an infinite number of near exact replicas of Earth with very slight differences. The immense distances between these replica earths prevent them from interacting.
Type 2 Parallel Universes:
These also exist in our space and time, and are giant "bubbles" floating in a sea amongst countless other universes in hyperspace. Each Type 2 universe can contain in infinite number of Type 1 universes. These are constantly branching out into new universes and also vanishing randomly
Type 3 Parallel Universes:
Type 3 universes occupy the exact same space in time, overlapping each other, but invisible and unreachable to each other due to them existing in different dimensions. Every possible outcome of everything from subatomic molecule collisions to wars being won and lost by different factions, or different forms of technology emerging. There are in infinite number of Type 3 universes in every Type 1 and Type 2 universe. Essentially anything a human being can imagine is a valid possibility within the infinite range of Type 3 parallel universes.
The universes of His Dark Materials may most closely resemble Type 3 parallel universes.
Type 4 Parallel Universes:
Universes where the most fundamental laws of physics and nature are radically different. The very ideas of matter, time, and energy in our universe may not apply, (gravity for example, may not exist) and life emerging from these universes may bear no similarity whatsoever to our universe. The Chariot may be a Type 4 parallel universe.
Parallel Universe Video Links (enjoy these while they last):
Part 1: http://www.youtube.com/watch?v=pxjUHEmm1PE&NR=1&fmt=18
Part 2: http://www.youtube.com/watch?v=L691jXYUj5M&feature=related&fmt=18
Part 3: http://www.youtube.com/watch?v=YlR1xUhVIds&feature=related&fmt=18
Part 4: http://www.youtube.com/watch?v=7ni-TcL5LWk&feature=related&fmt=18 (especially relevant to HDM)
Part 5: http://www.youtube.com/watch?v=xbTtgo4g ... ted&fmt=18
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Well, it seems to me that the universes Will travels between with the Subtle Knife are Type 3 because the worlds are essentially overlapping and don't seem to involve traveling over distances to get between, and it is understood that there is no distance between the worlds HDM deals with, so odds are these are Type 3 worlds.
Perhaps Lyra's universe is a Type 3 universe in which some very minor quirk in the laws of physics and nature causes the component of human brain activity that constitutes the daemon to be translated outside the body and take a physical form emulating an animal. I could imagine this working sort of like the demonstration in the show of a man shining a laser through glass apple and the light is translated into two places at once, only instead of a refraction of light, a physical daemon would be sort of a refraction of the human's emotions and thought processes.
Wow. If Type 3 universes are proven to exist that means that physical daemons might be a valid scientific possibility. Mommy, I'm scared.
