Entry 709, on 2008-02-28 at 20:57:59 (Rating 1, Science)
I have mentioned multiverse theories in several previous blog entries and I have described how these can solve the problem of explaining the fine tuning of our own universe. But multiverse theories have always seemed too convenient and too unsupported by evidence for my liking. I recently listened to a BBC, In Our Time podcast which discussed the multiverse, and in some ways I am a lot more positive about the idea now. But in other ways I'm just as skeptical!
An important message of this podcast is that a multiverse isn't just a convenient invention which can be used to explain the fine tuned universe. A multiverse is a natural prediction of various particle physics and cosmological theories (such as inflation theory). The fact that it also allows us to escape the problem of the fine tuned universe is coincidental.
For example, many string theorists think there would have been 10^500 vacuums at the Big Bang, and each of these would be a Universe with unique laws. That means there are a googol googol googol googol googol universes in the multiverse - never heard of a googol? Look it up at Wikipedia! Here's a clue: its a million quadrillion quadrillion quadrillion quadrillion. That's a lot of universes. If they all have different laws (as theory predicts) it would be unbelievable if there weren't some universes which are almost perfect for life.
But what is the chance of us existing in one of the few universes suitable for life? Well about 100% actually. Obviously the fact that there is life here means this universe is suitable for it. Its like someone being dealt a hand in cards and asking what are the chances of them getting that hand. Of course the chances of getting that hand before actually receiving it are incredibly low, but you've got to get one hand and all are equally likely.
There are at least three places the other universes could exist. Elsewhere in space, elsewhen in time or in other dimensions. A universe in different space would be beyond our universe but still in the same dimensions. A universe in different time would exist before or after our universe - maybe universes go on infinitely into the future and past. A universe in a different dimension could exist in a dimension "above" the three spatial dimensions we know about.
Unfortunately, its hard to see how we could detect any of these universes and prove the multiverse theory is true. That's where I start becoming skeptical of the whole idea. Being able to test a scientific hypothesis is so fundamental to science that I start worrying when a theory cannot be tested. Of course, that shouldn't mean that we shouldn't take the idea seriously if theory predicts it.
There might be some indirect way to detect other Universes discovered in the future so maybe we should just hope that it happens. I've mentioned this before in relation to various topics, but it gets back to the necessity for developing a quantum gravity theory. We need to understand the structure of space on smallest scale. Space is granular on a scale 10^24 times smaller than an atom. Recently an individual electron has been "photographed" but this structure is a trillion trillion trillion trillion times smaller than that, so its going to be a challenge!
Whatever the challenges it seems that there is an increasing acceptance amongst cosmologists, particle physicists, and scientific philosophers that multiple universes are an acceptable theory. Its a really interesting idea and I look forward to further details being hypothesised.
Comment 1 (1224) by SBFL on 2008-02-29 at 23:26:50:
OJB - is it possible you can add the feature to list the most recent (10?) comments on a side margin of your main blog index page? Most other blogs have this already. I tire of having to scan through 3 months of your posts to find out where I last commented (and you last replied). It seems lazy but knowing the hot debates adds an exciting dimension to your blog.
Comment 2 (1231) by OJB on 2008-03-01 at 11:28:25:
Yes, that is a feature I have been meaning to add for a while now. I will do it when I get the time.
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