Norm Geras comes across a discussion of the multiverse hypothesis, and plaints:

Come on, help me out, someone who understands the science. It’s a question I’ve posed before, but I’m still none the wiser more than five years later. The context of my question is the hypothesis that ‘our universe may be just one in a vast collection of universes known as the multiverse’; and the question itself is prompted by this sort of thing… There has to be a way of explaining this that saves it from its prima facie air of total unreasonableness.

The problem is, Norm has it backwards. (This happens a lot.) The anthropic principle, which is what is being invoked at the “this sort of thing” link and is causing him puzzlement, doesn’t specify a cause, much less declare humans the cause of anything. Where a causal relationship exists, it goes the other way.

There are a lot of things in physics that are (currently at least) unexplainable — not just “we don’t know”, with the implication that maybe we could find out with the right experiment, but inexplicable a priori. For instance, there’s a quantity called the “Fine Structure Constant“. Its value is so close to 137 that for years it was thought to be an integer, and because it’s calculated as mass/mass it has no units, so it’s the same in the metric, traditional, and furlong-footnight-stone systems. There’s no discernable reason it should have that value; scientists have been looking for the “why” ever since it was first defined, with no tiniest glimmer of a way to find a clue, much less actual evidence — but if it were different we wouldn’t be here, because many processes go the way they go because of that value.

The anthropic principle doesn’t say the fine structure constant has that value because we’re here; it says we’re here because the fine structure constant is what it is — if it were different it would still be what it was, but there wouldn’t be any physicists to calculate it. It answers the “many worlds” and “multiverse” hypotheses by saying, in effect, “So the f* what? Pay attention, people!” As such, it’s philosophical guidance for scientists rather than a physical law.

The principle is contentious because it is mainly aimed by physicists at mathematicians. Mathematics is perhaps the ultimate anthropopism; it’s not at all clear why it’s possible to use math to support descriptions of the Universe, no causal relationship that means the math operators must have physical meaning. It is and they do, but it’s purely a matter of pragmatism. At the higher levels, mathematics is a way for people who find card games and World of Warcraft boring because they’re too simple to entertain themselves and one another. The fact that it often turns out to be useful is merely a delightful coincidence, made more emotionally satisfying (to the mathematicians) by putting the nerds who couldn’t play dodgeball in school at the top of the “scientific” heap.

Mathematicians delight in contriving new, complex, and abstruse gimmicks and pursuing them to their conclusions, sharing them with other mathematicians in a genius-version of “can you top this?” Oddly enough, in many (perhaps most) cases the puzzles they solve for their own amusement turn out to have physical meaning, and physicists are accustomed to finding some effect and going to the mathematicians looking for a way to describe it usefully. Meanwhile the pure mathematicians are off in their usual flights of fancy, formulating mathematical systems that don’t have real-world application for their own edification and entertainment. The anthropic principle is physicists saying to mathematicians (and physics theorists, who are today mostly mathematicians in lab coats) that such pursuits are the equivalent of playing Solitaire instead of finishing up that proposal, and could you get back to some useful work, please?