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The anthropic principle has become a very popular explanation among theoretical physicists lately. Life is unlikely, but only life can observe, so we find ourselves in an unlikely position, so to speak. What I would like to ask is just how unlikely is life? Is the evolution of life generic for most laws of physics, or extremely unlikely with extreme fine-tuning? What is so controversial about the anthropic principle? No one is surprised to find themselves on an Earth-like planet orbiting a long lived star at just about the right distance from the star with plenty of liquid water, a thick atmosphere and plenty of carbon amidst the sterile antiseptic reaches of space. What are the major pros and cons? String theory conveniently provides a multiverse and a landscape.

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Is this homework? – Georg Nov 13 '11 at 14:04
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Dear @Wilsor, I wouldn't be able to guess it's a homework. Well, one may surely design laws of physics qualitatively similar to ours but laws which wouldn't allow any kind of life we may think of. It's enough to change some constants of Nature by 10 percent or so and key atoms, stars, nuclei will become prohibited, making it impossible for sensible structure to evolve. Indeed, it's not surprising that we must find ourselves in a Universe and on a planet that admits life: it's a tautology. The controversial part is the implicit claim that no "better" explanation beyond this tautology exists. – Luboš Motl Nov 13 '11 at 18:28
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Indeed, in the whole history of science, we were ultimately able to find better and more accurate explanations of various things. We could have said that the energy levels of atoms etc. were a priori random and Nature just chose a collection of random numbers and the only constraint was that these numbers - energy levels - allowed life to evolve. Except that it was proven that the levels were not random: today we're able to calculate all (thousands) of them just from a few starting parameters. There are things in subnuclear physics we can't precisely calculate today; but we shouldn't give up. – Luboš Motl Nov 13 '11 at 18:30
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So the controversial part of the anthropic principle is that it makes the assumption that some things are random - and urges physicists not to ask the question "why" any longer - which means that it discourages further research. It invites the people to be satisfied with a plausible explanation that doesn't really explain or predict much and that could be fundamentally wrong. It's an explanation that, if adopted at any earlier stage of the history of science, would slow down the further scientific progress. It's similar to religion: don't calculate and just admire God's greateness – Luboš Motl Nov 13 '11 at 18:33
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@LubošMotl: any chance you might consider posting that as an answer? – David Zaslavsky♦ Nov 13 '11 at 21:55
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i don't think the cons are meaningful, at least in two ways.

1) the anthropic principle does not stop people in the spot to think about these issues. If people discover good reasons why the universe is special (or seems to be), no amount of anthropic principle will make those reason vanish (assuming they are really good)

2) as a fan of modal realism* i think the anthropic principle is an unavoidable fact of existence. We are by construction on a special frame of reference; for instance, the odds of life evolving near the big bang are negligible. The combined viewpoint of anthropic principle and modal realism is akin to the copernican revolution coming from a ptolemaic dogma: Although in this case, there is no way that we can elucidate these questions with observations.

*(the idea that there is no substantial difference between possible universes (in a mathematical sense) and "real" universes; the former will also have entities that might ask themselves big questions.)

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Any string theorist who is not in massive denial and hiding their heads in the sand knows that string theory predicts an exponentially large landscape of stable and metastable compactifications. That much is an objective fact of string theory. Any string theorist worth their salt also knows a generic inflationary state leads to chaotic inflation, and given some likely conditions eternal inflation. Some parts decay to a lower energy compactification, but those which don't inflate exponentially faster to more than compensate. Combine eternal inflation starting from some metastable state with a Planck sized vacuum energy with the landscape, and a multiverse is an inevitable result. Most bubbles are not hospitable to life but it is tautological that observers will always find themselves in a bubble which is just right for life.

The multiverse explanation of the anthropic principle is actually a prediction of string theory, and any string theorist not too chicken to face reality will realize this simple truth.

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Is this an ironic answer making fun or is it meant serious ...? – Dilaton Nov 16 '11 at 16:48
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Hm ok, after rereading it I some kind of like this answer somehow, +1. But nevertheless, I think it would be nicer if the extent of the landscape could be reduced physically and/or mathematically to finally detect the "needle(s) in the haystack". – Dilaton Nov 16 '11 at 22:57
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This answer is assuming that eternal inflation is compatible with holography and this is not at all obvious. – Ron Maimon Nov 19 '11 at 20:51
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What exactly is it being selected for in the anthropic principle? Conscious observers, but what is consciousness? Intelligent life, but why not nonintelligent life? Humans specifically, but why not some other intelligent lifeform? Why isn't a planet of apes only but no humans enough? Anyone wishing to take the anthropic principle seriously has to explain why we humans lie on the side of selection. Evolutionarily speaking, a universe with a planet of only apes is far more likely than a universe with a planet containing humans. Why aren't neanderthals or hunter gatherers enough? According to neo-darwinism, progressive evolution is never inevitable. Is it because of the higher population growth afforded by advanced technology, but even with a factor of a million in additional population, might it be that less than every one in a million planets with neanderthal level intelligence leads to intelligence advanced enough to cause extreme population growth?

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no, it is exactly like you said at the beginning: life, intelligent or not, is what pre-conditions automatically certain stability and "well-behavedness" in the physical laws. The intelligent part is only important to us because we are studying those laws and trying to figure out the patterns that seem "unnatural", and discussing such things in forums that live in computers that work with electric power. – lurscher Nov 19 '11 at 17:41
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The often stated claim that the anthropic principle makes no concrete predictions is a BALD FACED LIE propagated by detractors hoping to smear the good name of the anthropic priciple. Hoyle predicted a carbon nuclear resonance at a particular energy level because that is what is needed for nucleosynthesis of the heavier elements, and we turned out to be right. Weinberg predicted the correct order of magnitude for the cosmological constant at a time when almost everyone thought it was exactly zero.

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very interesting. +1 – lurscher Nov 15 '11 at 15:15
""Hoyle predicted a carbon nuclear resonance at a particular energy level because that is what is needed for nucleosynthesis of the heavier elements, and we turned out to be right."" Ah, You and Hoyle made that prediction? – Georg Nov 15 '11 at 15:18
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You describe it as "Hoyle predicted carbon nuclear resonance because that is necessary for heavier elements and life, using the anthropic principle". I would say "Hoyle started from the (obvious) observation that nucleosynthesis of heavy elements occurred in our universe, and reached conclusions about the mechanism by which it must have occurred". I don't see how the anthropic principle is at all helpful for this. For example: We see that Saturn has rings, therefore we can reason about how those rings came about. Hmm, but Saturn's rings are unnecessary for life...who cares?? – Steve B Nov 15 '11 at 15:54
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The anthropic principle has many "flavors" but they have in common the basic observation that many characteristics of our universe appear to be uniquely suited - some would say "designed" for humankind. While there are indeed strong patterns and high levels of directionality to be found in nature, particularly in the fields of biology and chemistry, they are general evolutionary trends on which our species happens to be on one part of a very long pathway.

Rather than being the product of some kind of "creator" they are simply features of the way the wonderful machinery of nature happens to work.

See my recent book "The Goldilocks Effect: What Has Serendipity Ever Done For Us?" (free download in e-book formats from the "Unusual Perspectives" website) for more info on this broad evolutionary model.

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If the anthropic principle were true, we humans will be the meaning and purpose of the universe. No longer will we be accidental inessential insignificant beings in an indifferent meaningless purposeless universe. We will be at the heart of the universe itself. We will be of utmost importance. Our lives will have cosmic significance.

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this assuming existence in itself is a thing. But if you believe in modal realism, even hypothetical beings in mathematically possible universes are able to ask themselves questions about the universe – lurscher Nov 14 '11 at 18:07
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The anthropic principle, as most physicists use the term, is the idea is that there are gazillions of universes and the overwhelming majority of them are bleak, empty, uninhabitable places. A few, including ours, happen to harbor intelligent life, and here we are in one of those. If this is the correct picture, it certainly does not imply that the humans are special or that the laws of physics and/or the universe care whether or not humans exist. It implies the exact opposite. – Steve B Nov 15 '11 at 16:14

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