Smolin Vs. Susskind: The Anthropic Principle

Smolin Vs. Susskind: The Anthropic Principle

Lee Smolin, Leonard Susskind [8.18.04]


Leonard Susskind          Lee Smolin      


Recently, I received a copy of an email sent by Leonard Susskind to a group of physicists which included an attached file entitled "Answer to Smolin". This was the opening salvo of an intense email exchange between Susskind and Smolin concerning Smolin's argument that "the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science".

After reading several postings by each of the physicists, I asked each if (a) they would consider posting the comments on Edge, and (b) if they would write a new, and final "letter".

Both agreed, but only after a negotiation: (1) No more than 1 letter each; (2) Neither sees the other's letter in advance; (3) No changes after the fact. A physics shoot-out.

While this is a conversation written by physicists for physicists, it should nonetheless be of interest for Edge readers as it's in the context of previous Edge features with the authors, it's instructive as to how science is done, and it's a debate that clarifies, not detracts. And finally it's a good example of what Edge is all about, where contributors share the boundaries of their knowledge and experience with each other and respond to challenges, comments, criticisms, and insights. The constant shifting of metaphors, the intensity with which we advance our ideas to each other — this is what intellectuals do. Edge draws attention to the larger context of intellectual life.

Below are the original email pieces, followed by the final letters presented side-by-side.


LEE SMOLIN, a theoretical physicist, is concerned with quantum gravity, "the name we give to the theory that unifies all the physics now under construction." More specifically, he is a co-inventor of an approach called loop quantum gravity. In 2001, he became a founding member and research physicist of the Perimeter Institute for Theoretical Physics, in Waterloo, Canada. He is the author of The Life of The Cosmos and Three Roads to Quantum Gravity. Lee Smolin's Edge Bio Page

LEONARD SUSSKIND, the discoverer of string theory, is the Felix Bloch Professor in theoretical physics at Stanford University. His contributions to physics include the discovery of string theory, the string theory of black hole entropy, the principle of "black hole complementarity," the holographic principle, the matrix description of M-theory, the introduction of holographic entropy bounds in cosmology, the idea of an anthropic string theory "landscape." Leonard Susskind's Edge Bio Page


July 26, 2004

Smolin's publication of "Scientific alternatives to the anthropic principle" and his summary of the arguments, written at Susskind's request.

Dear Lenny,

Thanks. Glad to. I'll start with one of the main arguments I make.

I show that the argument of Weinberg and others [Garriga and Vilenkin] are incorrect. The subtle point is that their arguments have embedded in them correct arguments having to do only with what we observe. To an already correct argument is then added mention of the anthropic principle. As it is added to an already correct argument, the anthropic principle plays no role in the actual scientific argument.

Here is how it goes: We start with a theory of structure formation that tells us

"Too large positive Lambda interferes with galaxy formation.

We do observe that galaxies have formed. Therefore we predict that the cosmological constant could not have been too large.

This is correct reasoning, and it agrees with observation.

What Weinberg and others have done is to make the error of embedding this argument into one that first mentions some version of the antrhopic principle. The mistake is not to notice that because it has been added to an argument that is already correct, the mention of the anthropic principle [or the principle of mediocrity, or life] plays absolutely no role in the argument.

The logic of their arguments is

A implies B B is observed B, together with theory C implies D.


A is any form of the Anthropic Principle of Principle of Mediocrity, together with assumptions about priors, proabability distrbutions on universes etc, plus our own existence, that leads to the conclusion that we should observe B.

B is that galaxies have formed.

C is the theory of structure formation,

D is that the cosmological constant is not too large.

The fallacy is not to recognize that the first line plays no role in the argument, and the prediction of D is equally strong if it is dropped. One can prove this by noting that if D were not seen, one would have to question the theory C [assuming the observation is correct, as it certainly is here.] One would have no reason to question either A or the assertion that A implies B.

This is the same fallacy involved in Hoyle's argument about carbon. He reasoned simply from an observation that carbon is plentiful in our universe to a prediction that, as it must have been formed in stars, there must be a resonance at a particular energy. This was correct and the resonance was observed. But he fallaciously attributed the argument to the existence of life, which was a non-sequitor.

In the paper I show that every use of the anthropic principle claimed in physics and cosmology is either an example of this fallacy, or is so vague that one can get any conclusion one wants, and match any observation, by manipulating the assumptions made.

I then go on to explain what a multiverse theory would have to do to yield genuine falsifiable predictions that actually depend on the existence of the multiverse. I give conditions for this to work. I then show that there exist real theories that satisfy these conditions, one of them being my old natural selection idea.

Therefore, the anthropic principle cannot help us to do science. But there are ways to do science if we are faced with a multiverse.