Thursday, 17 July 2014

Multiverses And Anthropic Reasoning

I liked this article summarizing the current state of science regarding multiverse theories. It's very clear and well-illustrated, and, as far as I know, accurate.

This quote is particularly interesting:

So as appealing as the idea is that there are other Level 1 Multiverses out there with different constants than our own, we have good physical reasons based on observable evidence to think it’s unlikely, and zero good reasons (because wanting it to be so is not a good reason) to think it’s likely.

He doesn't mention why anyone would "want it to be so", i.e. believe that other universes of a "Level 1 Multiverse" could have different constants to our own. However, I'm pretty sure he had in mind the selection-bias explanation for the anthropic coincidences. That is, if we accept that only a narrow range of possible values for the fundamental physical constants are compatible with the existence of intelligent life (and most scientists do, I think), then we would like to be able to explain why our universe's constants are in that range. If there are an abundance of different universes, each with different values for the physical constants, then most of them would be dead but a lucky few would sustain intelligent life, and naturally we can only observe one of those latter.

This reasoning relies on the assumption that there are abundance of different universes with different values for the physical constants. Scientists obviously would prefer to be able to deduce this from observations rather than pull it out of thin air. As discussed in the above article, theories of chaotic inflation --- which are reasonably well-grounded in observations of our own universe --- predict the existence of alternate universes. If those universes could have different values for physical constants (or even different physical laws), we'd have an observationally-grounded theory that predicts exactly the kind of multiverse needed to power the selection-bias explanation for the anthropic coincidences. Unfortunately for proponents of that explanation, the science isn't working out.

Of course, the selection-bias explanation could still be valid, either because new information shows that chaotic-inflation universes can get different constants after all, or because we assume another level of multiverse, whose existence is not due to chaotic inflation. However, many scientists (such as in the article above) find the assumption of a higher-level multiverse quite unsatisfactory.

Unsurprisingly, I'm comfortable with the explanation that our universe was intentionally created for intelligent life to live in. Incidentally, you don't need to be a classical theist to adopt this explanation; some atheist philosophers argue with varying degrees of seriousness that we are (probably) living in a simulation.


  1. Le Dahu Lévogyre17 July 2014 04:00

    Obviously, if the constants could be different in other multiverses, the anthropic coincidence wouldn't be a valid argument in favor of creationist theories.
    But the anthropic "coincidence" is not a very good term anymore if the constants are the same in every other multiverses. Because it means they cannot be different than what they are here (for some reason). Then it can't be called a coincidence.
    A coincidence only makes sense if there are multiple possibilities.

    So, I always fail to understand that anthropic coincidence as an argument in favor of creationism. (Maybe you can help me understand ?)

    A second point : even if this universe was created, the questions remains. Who created the creator ? What is the simulation container ?
    Don't the religious people ask themselves these questions ?

    1. People who use the word "coincidences" here are comparing to hypothetical alternative universes that are much like our own but don't support life. Even if some deeper theory of physics constraints parameters to have our values, it would still be surprising that those values should be precisely the values that support life ... to me, and I expect also to other thinkers of all kinds.

      I'm not sure anthropic coincidences point directly to a creator, but if you think they demand an explanation then a creator is one explanation (and multiverses are another). I prefer the creator explanation because I believe it explains other evidence as well.

      Religious people certainly do ask questions like yours! The classical answer is that God is the only uncreated entity. In fact, that could almost be the definition of God. People complain that's special pleading, but I think that's somewhat subjective.

      Even if we ignore theism for a moment, I think it might be possible to explain the anthropic coincidences via a simulation argument that doesn't trigger infinite regress. For example the creator(s) might live in a universe that is nothing like ours and has no tunable parameters.

      Sorry about the delayed response, I had written a comment earlier but Google ate it.