Elizabeth Pearson answered on 22 Mar 2012:
There are many theories but here are 2 as I understand them:
1 is because of quantum every choice is not predetermined, but has a probability of happening. By choice I don’t just mean someone picking between pizza and a burger for lunch, but which way an electron will go. The theory of multiverses is that at every choice has happened in a universe somewhere, every combination of possible things. People think this means that somewhere there’s a world where humans can fly or soemthing, but that’s not necessarily so. It’s not what you can imagine, but what is possible given the laws of physics which are the same across all universes
2 is a bit more complicated. Basically it states that there are many different universe but that the basic laws of physics are different between them.
Sam Vinko answered on 22 Mar 2012:
A multiverse is just a system of many (possibly infinitely many) universes which don’t communicate (if they did they would arguably be the same universe). Since they don’t communicate by definition, you cannot ever observe there parallel universes, and so they’re not something we’re particularly interested in, because there is no way to prove something you define as completely invisible doesn’t exist.
If it cannot be falsified, its not science. Makes for good science fiction reading though.
Ben Smart answered on 22 Mar 2012:
There are several multiverse theories, and the general idea behind them is that there could be other universes like the one we’re in right now (or perhaps very different!) existing somewhere (different theories have different explanations of where that ‘somewhere’ would be). The main problem is that with many of the theories there is no way to test if the theory is right or not, because there would be no way for us to communicate with the other universes.
One theory that does allow our universe to communicate with other universes is ‘M-theory’ or ‘Brane-theory’ which is based on string theory. In this theory, there are many dimensions (many more than the 4 dimensions we’re currently aware of), and each universe is confined to a different set of dimensions. So our universe lives on the 4 dimensions we’re aware of, and another universe would live of a different 4 dimensions. Stuff that exists in our 4 dimensions can’t interact with other dimensions of other universes… except gravity. In M-theory gravity can interact with all the dimensions in all the universes, so gravity could be used to perhaps communicate with these other universes. Since gravity would be ‘spread out’ over all the different universes this would explain why gravity is so much weaker than all the other forces we’re aware of.
Unfortunately we still have no way to find out whether M-theory is real or not. We may never be able to test whether it’s real or not, but you never know what the future might hold…