I would like to start off by saying I know nothing about physics apart from the excess of Sci-Fi I watch when I was younger and the random conversations I have with my physics friend Bob. Nevertheless I do like it despite it never making any sense. As a biologist there is a reason behind just about everything (ie evolution has shaped it all). It unsettles me when I don’t know why something is the way it is (so most of immunology then). Before reading the rest of this you need to assume a couple of things about the universe for what I am going to say to make sense, they are apparently correct but what do I know. Firstly space is infinite or extremely large (according to Prof Max Tegmark evidence for a small universe or donut shaped one is weak and he believes the universe is infinite) and secondly that matter is evenly spread out throughout space (not just clumped around us).
For the first part we will say our universe is the part of space we can see (the outer edges are determined by the age of the universe and how fast light it, as it gets older we can see more because more light has had time to travel to us). This is also called our hubble. Level I multiverse or parallel universes are simply hubbles out there that are like ours (and many more not like ours). If space is infinite all arrangements of matter that are possible exist! I like to think star wars and LOTRs obey the laws of physics so they could exist. Everything in level I has the same laws of physics as our universe because it is only an extension of ours. Out there, there are more hubbles just like ours but very far away. One estimate says one just like ours in every way could be 10 to 10118 metres away! Closer will probably be some hubbles that are very similar to ours but slightly different. Their will be an infinite number of all hubbles because space is infinite (from my understanding). I think these estimates are far to low and don’t take into account some things but it gives you an idea that even a low estimate is amazingly far away.
That was the easy stuff. Level II multiverse (or parallel universes) is less accepted than level I but still apparently explains a lot of things in physics. It helps biologists in their fight against pro intelligent design arguments. It is where many multiverses (same as in level I) exist but each multiverse is separate and has different physical constants or number of dimensions. This explains why our universe just so happens to be able to support life – it isn’t custom built to support life but is just one possibility of how the universe works. We have 3 spatial dimensions and one time dimension but if we had more time dimensions events would be completely unpredictable and if we had more spatial dimensions atoms would be unstable. If the mass of an proton was slightly larger it would decay to fast for molecules to be made so obviously nothing for evolution to act on and life to form. Each multiverse is still infinite within a sea of inflating infinite empty space (don’t ask me how you can have infinite space inside infinite space, because I just don’t understand). We can never travel between these multiverses even at the speed of light because they are moving away from each other faster! That sounds like science-fiction to me.
Level III is the one that interested me the most as a child (yes I have always been this way, Egon Spengler was my favourite Ghostbuster…need I say more). Every choice means you have to choose a path to follow. But in level III all outcomes exist but the other choices exist in another universe and not in space as we know it but ‘elsewhere’. However the choices I am talking about take place on the quantum scale! This goes back to Schrödinger’s cat (if you don’t know what it is, it would be best to look it up before continuing). From what I understand all it means is all states or positions exist until someone observes what state it is in. So something is both on and off or dead and alive until someone checks. This is called a superposition. An alternative theory is that both do exist but new universes are created to accommodate the other possibilities. It is like rolling a die that is only governed by the rules of the quantum world and not the overall rules of the universe. Because it is only on the quantum scale the outcome is completely random (from what I understand). According to level III multiverse it will not land on a 1 or a 2 or a 3… but will land on all six values at once. How; each one exists in a different universe, easy! And we thought biology was screwed up, at least it usually makes some sense. The outcome of level III is the same as for level I and II, more universes most slightly different from ours. The difference is how it is made.
Level IV is the one I feel least confidence about explaining. It is there to explain why our universe works under a set of specific mathematics and not controlled by other models. Universes that work using different mathematical models may exist outside our spacetime and work in completely different ways. They work using different laws of physics even more different to ours than by multiverses in level II. Level I, II and III were created by the same big bang. Level IV exists outside spacetime and will have had there own starting events but more level I and II (and III) could have been created by other big bangs as well. In level I and III you will have a Doppelgänger but in II and IV space will be so radically different you will not. Well I think you might if a parallel universe created in level II or IV is very similar and works off the same rules as ours.
All of these theories do make some predictions so they no longer lay in the realms of metaphysics but of real science. So in the next few years we may see some of these confirmed or rejected as fact. Whether they exist will have little impact on our lives. Will it comfort you to know someone out there in the infiniteness of space is in exactly the same situation as you or better of, or in a worse situation! I don’t think it will keep me awake at night. Nothing we can do will have an affect on them. I only discuss this because I find it interesting. But I find most things about our world its place in the universe interesting. We didn’t evolve to understand things like this but we did evolve to ask question about our environment.
For more information see Scientific American special report on Parallel Universe by Max Tegmark or his website:
this has more references.
Please ask me questions, no promises I can start to answer but I have thought about the topic for a while and would like to hear your thoughts and see of you have the same questions as me.