Prompted by some of my recent entries, which were in turn prompted by some recent discussions, which in turn were prompted by my slate article, I decided to read Bernard Carr’s Universe or Multiverse, which came out in paperback earlier this year. It’s a collection of talks from some VERY eminent people from several different conferences focusing on how the multiverse scenario plays out in physics. I’ve seen a lot of these arguments before, but it’s nice to have them all in one place. A warning to non-physicists: while Carr tries to keep equations to a minimum, most of the talks assume a detailed knowledge of inflationary cosmology and particle physics, and at least passing familiarity with string theory.
Since I’ve been writing so much about parallel universes lately, I thought I’d touch on something very closely related — the anthropic principle. For those of you who’ve never heard of this before, I a) wanted to give you a quick primer, and b) wanted to warn you to be very sure about a physicist’s sensibilities before bringing it up in his/her company.
The anthropic principle stems from a big question in physics. Essentially, physicists have done a great job of figuring out how things work: the four fundamental forces, the collection of fundamental particles, and the like. We’ve even done a pretty good job combining some of those forces and particles into coherent theories. For example, electromagnetism has been combined with the theory of weak nuclear reactions to form electro-weak theory which predicts, among much else, the existence of the Higgs boson (reminder: that’s one of the main motivations for the potentially world destroying Large Hadron Collider).
The complication is that within our standard model, there’s no fundamental reason why many of the numbers are the values that they are. Why is electromagnetism so weak? What would happen if the strong force were slightly weaker (or stronger) compared to electromagnetism? Why 3 spatial dimensions?
Or, why is the content of “Dark Energy” in the universe so small compared to the natural value suggested by naive vacuum energy calculations. Dark energy is about 10^100 times less dense than it naively ought to be.
The answer, simply, is that if the Dark Energy were much larger then galaxies couldn’t form, and you and I wouldn’t be here talking about it. In fact, it’s not just Dark Energy. It turns out that lots of these seemingly arbitrary parameters needed to be in a very particular range in order for complex life (human or otherwise) to come about.
In the left-hand diagram, the tiny horizontal grey line is the only combination of electromagnetic and strong coupling constants that could allow for our existence, or anything like it. We are very, very lucky to be alive. Surely, the universe (or some higher power) must have conspired to make it so. Naturally, we wonder whether there’s some “anthropic principle” at work to allow for the utter improbability of us. In fact, there are two:
- The Strong Anthropic Principle
This basically says that these seemingly arbitrary parameters are set to values that allow intelligent life (such as us) to exist. This is anathema to most physicists, and most natural scientists.
- The Weak Anthropic Principle
This is far less controversial. Basically, the WAP says that intelligent life arises in universes, or in times and places in the universe, in which conditions allow it to arise. This is basically a truism! Of course life
arises where it can. Of course, nothing prevents the entire universe from being inhospitable. However,
we get lots and lots of universes, in the standard model of inflationary cosmology.
Around 10^-35 seconds or so after the big bang, the universe undergoes enormous expansion (by a factor of a googol or so). However, different patches undergo expansion at slightly different times, and we’re left with an infinite web of disconnected universes. This is the “Level 2 multiverse” (in Tegmark’s scheme) that I alluded to last week. These different inflationary bubbles might have (for example) very different dark energy densities, and only those where people and galaxies can form are ones where there are people — and hence physicists — to talk about them.
It may also be that string theory is correct, in which case, we may an enormous multiverse filled with many universes, each with very different physical constants. Likewise, only those with conditions right for life are filled with intelligent creatures to study them, however.
I’d be remiss if I didn’t point out that it may be that the anthropic principle (even the weak one), may be completely off base. It may be that eventually physics finds that all of the seemingly arbitrary parameters can, in fact, be predicted from first principles. Even then, it’s hard not to feel lucky that those parameters just happened to be conducive to life.