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October 19, 2004

Ad Hominid Arguments

Sean Carroll and Luboš Motl have both written blog posts recently on the anthropic principle. I’ve touched on the subject in the past and I don’t have that much that I wish to add to my previous discussions.

But there is one pitfall of anthropic reasoning that — I believe — is behind a lot of people’s unease with the subject. It’s the propensity, if one is not very careful and self-critical, to lapse into telling anthropic “just-so” stories of the sort that permeate, say, the “discipline” of Evolutionary Psychology.

Take, for instance, the problem of baryon number violation. The observed lower limit on the proton lifetime is 20 orders of magnitude longer than the anthropic bound. The generic supersymmetric theory has dimension-4 baryon number violating operators and the coefficient, λ\lambda, of these operators must be highly suppressed, to satisfy the anthropic bound. To satisfy the observational bound, λ\lambda must be 4 or 5 orders of magnitude smaller, still. In a theory with low-energy SUSY breaking, this means we need to explain why the vacuum has a (very nearly) exact R-parity symmetry (λ<10 13\lambda\lt 10^{-13}), even though the anthropic bound is only λ<10 8\lambda\lt 10^{-8}. Split supersymmetry ameliorates the puzzle by making both observational and anthropic bounds on λ\lambda much weaker, but the 5 orders of magnitude discrepancy between them remains.

It was in this context that I hungrily fell on a suggestion by Nima Arkani-Hamed that the smallness of λ\lambda might have a different anthropic explanation. An approximate R-parity means that the lightest superpartner is approximately stable. To furnish a dark matter candidate, it should have a lifetime comparable to the age of the universe. Plug in the numbers, and you get something surprisingly close to the observational bound on λ\lambda in models of split supersymmetry.

Tada! The smallness of λ\lambda, and hence the observed proton longevity, is explained by the anthropic need for dark matter. Or it would be, if structure formation were utterly impossible in the absence of dark matter, and if an LSP were the only dark matter candidate available…

Posted by distler at October 19, 2004 12:20 AM

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Re: Ad Hominid Arguments

It seems to me (as I’m sure I’ve mentioned) that anthropic reasoning is too often predicated on a failure of imagination.

After all, who’s to say that a lack of galaxy formation precludes intelligence?

Posted by: Aaron on October 19, 2004 12:50 AM | Permalink | Reply to this

Shall We Play A Game?

Don’t pooh-pooh evolutionary psychology, Jacques, it’s actually really fun! Here’s a neat game to play on a rainy afternoon:

Consider any individual human behavior or social trait X: romantic love, paying taxes, foot fetishes, stamp collecting, whatever. In two minutes or less, construct an argument that explains why X confers an obvious evolutionary advantage over not-X.

Now consider not-X. In two minutes or less, construct a similar evolutionary argument in favor of not-X.

Posted by: Evan on October 19, 2004 9:44 AM | Permalink | Reply to this

Re: Shall We Play A Game?

Mockery is the poorest form of debate. OH, I know it can be entertaining - and may even result in a false victory (as those of less social strength bow out to avoid your sarcasm) but a good argument? Never!

In fact, this is the type of “reasoning” style that maintains the bizarre set of beliefs surrounding romantic love, and other unsuccessful forms of human mating endeavors.

On the other hand, animal research into mate guarding behavior offers insights into human mating (and why it so often fails) that only an evolutionary psychologist would be even consider thinking about.

Hows your relationship, by the way?

Posted by: Angela on July 6, 2007 5:02 AM | Permalink | Reply to this

Re: Ad Hominid Arguments

Since apes could perform the same reasoning, we could also call it the “simian principle”.

Not to speak about other animals.

Posted by: François on October 19, 2004 1:29 PM | Permalink | Reply to this

Re: Ad Hominid Arguments

Despite my anti-humanity viewpoints, I think that evolutionary psychology - represented e.g. by Steve Pinker - is still more scientific than the anthropic principle as we know it on the boundary of current physics.

I liked Pinker’s lecture about the evolution of God. Well, it’s not a real testable science, but I think that one can consider the different explanations and look which of them are broadly compatible with the general principles how we think about the brain and about the evolution.

Your points about the insufficience of the anthropic arguments for the proton decay etc. are good.

Concerning the dark matter in split supersymmetry - right. It is only convincing once you accept that dark matter exists - which is an observational fact (unless there’s MOND etc., see my blog), but anthropic principle would like to claim that it can only work with the assumption of *any* life, which does not seem to depend on dark matter either.

Posted by: Lubos Motl on October 22, 2004 1:17 PM | Permalink | Reply to this

Dark Matter

Concerning the dark matter in split supersymmetry - right. It is only convincing once you accept that dark matter exists - which is an observational fact …

But it’s not an observational fact that the dark matter is an LSP. Maybe it’s axions, or something even more exotic.

Even if we were to accept that dark matter were somehow anthropically necessary, that still would not mean that it has to be an LSP. So this is not a (correct) anthropic argument for a very long-lived LSP.

Posted by: Jacques Distler on October 22, 2004 1:29 PM | Permalink | PGP Sig | Reply to this

Axion Dark Matter

Maybe it’s axions

I was recently engaged in some discussion related to axions.

I expect that an axion ϕ\phi that comes from dualizing the 4-d components of the string’s KR field induces a torsion effects on the dynamics of 4d fermions ψ\psi by giving rise to a minimal coupling of the form

(1)ψ¯(dϕ) abcγ abcψ. \bar \psi (\star d\phi)_{abc}\gamma^{abc}\psi \,.

I would therefore expect that axion dark matter could in principle be demonstrated by detecting anomalous spin precession of fermions.

I have found a couple of articles concerned with roughly this idea, but none of them seems to give a satisfactory discussion.

What would you think?

Posted by: Urs Schreiber on October 24, 2004 11:04 AM | Permalink | Reply to this

Re: Ad Hominid Arguments

I am writing this comment for the fifth time already. The page always jumps to a different page and erases everything. The comments on this blog seem unusable to me. I have Linux and Mozilla right now.

Let me continue later separately

Posted by: Lubos Motl on October 22, 2004 5:54 PM | Permalink | Reply to this

Popup Window Trouble?

If the trouble you were having was that a keypress on the blog page would re-open the comment entry window, then thanks for reminding me about it! I had been meaning to get around to fixing that petty annoyance.

The solution was to add a


to the javascript code that pops up the comment-entry window.

If it was something else, my apologies. Perhaps a more detailed description of the problem might be in order…

Posted by: Jacques Distler on October 23, 2004 1:18 AM | Permalink | PGP Sig | Reply to this

Re: Ad Hominid Arguments

You must first decide what assumptions your model has, how likely they are, and then calculate what good predictions it makes and how nontrivial they are.

If I talk about split SUSY, the assumption “there must be dark matter” is not independent from the assumption “there must be dark matter made of LSPs” simply because split SUSY naturally predicts LSPs as dark matter.

Posted by: Lubos Motl on October 22, 2004 5:56 PM | Permalink | Reply to this

Re: Ad Hominid Arguments

One more comment: well, if you say that Nima et al. don’t have an argument for LSP dark matter that is based *only* on the anthropic principle, then it’s certainly the case. It’s also based on some other physical considerations and calculations. I hope that we’re not far enough that the anthropic arguments are the *only* arguments that can be used, as opposed to calculations, for example. ;-)

Posted by: Lubos Motl on October 22, 2004 6:00 PM | Permalink | Reply to this


Most String Compactifications (on the Landscape or otherwise) have lots of extra junk (in hidden sectors, etc.) which, for an appropriate range of parameters, could serve as dark matter candidates.

It then becomes a detailed and model-dependent question whether further fine-tuning λ\lambda (over its already fine-tuned value required by the anthropic bound on proton decay) to obtain a long-lived superpartner of a Standard Model field is more probable than whatever tuning of parameters gives rise to all other possible dark matter candidates.

While it’s certainly attractive that the same fine-tuning that makes the proton lifetime compatible with observations also makes the LSP into a dark matter candidate, it’s not clear that either of these is required, either anthropically or for other reasons.

Posted by: Jacques Distler on October 23, 2004 1:36 AM | Permalink | PGP Sig | Reply to this


Hi Jacques,

I think that we can agree that your arguments make it clear that it is very unclear what features of the models are really useful/necessary for life and which of them are not.

It seems that you/we are already studying these anthropic questions with a good enough resolution that makes it obvious that some rules are missing.

I hope that I can now safely thank you for fixing the reloading keypress bug and the text won’t disapper before the thanks arrives to you haha.

All the best

Posted by: Lubos Motl on October 23, 2004 11:06 AM | Permalink | Reply to this

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