Understanding the Universe requires a lot more than just knowing some advanced mathematics, and even more than knowing how to apply that math. It requires a knowledge of natural history, and an understanding of the requirements for allowing that history to happen.
In other words, the laws of nature must be such that the Universe can exist as it does. Seems like a very simple, innocuous, and self-evident statement. Yet in this simplicity, we can learn a few important things about the Universe. Reasoning, using this statement, is often referred to as the anthropic principle.
There is more matter than antimatter in the Universe, for example, and therefore there must be some physical law or process that allows us to produce more matter than antimatter. Although we don’t know what that process is exactly yet, the fact that the Universe is full of matter and relatively devoid of antimatter tells us that this process must exist. That’s one example of anthropic reasoning, and it’s valid and (marginally) useful. The anthropic principle has, exactly once in the course of human history, been used to make a scientific prediction: Fred Hoyle’s prediction of the existence of a Carbon-12 excited state that is equal to the mass of three Helium-4 nuclei. The Universe is full of carbon, and so we need some way to make it, he reasoned. The only conceivable way to make it from Hydrogen and/or Helium was to have three Helium nuclei combine into a Carbon nucleus, meaning that there had to be an excited state of Carbon-12 that allows this to happen. Fred Hoyle predicted it, and then Willie Fowler found it.
And that’s really it. That’s all the anthropic principle is useful for, and it upsets me when otherwise respectable scientists hijack it and make exaggerated claims using it. How? Instead of:
The laws of nature must be such that the Universe can exist as it does,
there are those who make the claim that the laws of nature exist as they do because human beings are here to observe it. This most recently came up because of my article on dark energy. Commenter jb paraphrased an argument often used by string theorists to validate their idea of the Landscape:
Well maybe our naive calculations for the cosmological constant give us a number too big by a factor of 10^120, but The Landscape(TM) gives us 10^500 possible universes, and at least some of those are going to have the right value, and the others don’t matter because there’s nobody there.”
This is problematic for one major reason: it isn’t a scientific prediction. It doesn’t teach us anything new. If we wanted to use the valid version of the anthropic principle, all we can learn is that the cosmological constant couldn’t be too large, or it wouldn’t allow for gravitational collapse to occur, creating stars, galaxies, heavy elements, and Scienceblogs readers. But there’s no reason, even with the anthropic principle, for dark energy or the cosmological constant to have the value that it does. It could be ten, or a hundred, or even a thousand times larger than its observed value, and the Universe would still be here, and similar to the way it is now. It could also be a tenth, or a millionth, or a googol-th of the value we observe, and the Universe (and you and I, for that matter) would be just fine. It could also be exactly zero, but it isn’t. The fact is, while we can measure the value that it has, we do not have any understanding of how it got to have that value. And the anthropic principle, no matter how you twist it, is of no help in that regard.
Any type of scientific reasoning is only useful when it tells you something you don’t already know. So beware of abuses of the anthropic principle. They may be logically consistent, and they may be possible, but unless they predict something, they’re not scientifically useful.
I await your hate mail, Lubos.