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1) The Anthropic Principle: Leonard Susskind's Reply #1 to Lee Smolin #1

Originally posted on sciy.org by Ron Anastasia on Thu 28 Feb 2008 01:23 PM PST  

Here's Leonard Susskind's answer to Lee Smolin's first email.



"Answer to Smolin"], distrib
https://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.htmluted by email to his mail list of physicistshttps://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

Answer to Smolin

https://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

L. Susskind
Department of Physics
Stanford University
Stanford, CA 94305-4060

Abstract
Smolin's recent criticisms on the anththropic principle are answered.

Yesterday I received an email message from Lee Smolin asking for my comments about a paper he had posted the previous day [1]. Having not had a chance to read the paper I asked if he would summarize the arguments. He was kind enough to do so in an admirably concise and clear summary. (See above).

I took a quick look at the paper just to make sure that there was not more that I might have missed in the summary

I noticed that in the paper Smolin quotes the philosopher, Karl Popper. Personally I don't think these are deep philosophical issues requiring a heavyweight like Popper. Weinberg was just using good old fashioned common sense.

Weinberg's argument, which is clearly stated in his general audience book Dreams of a Final Theory is not that the cosmological constant has to be smaller than the limit from galaxy formation. If it were, then Smolin would be correct: the Anthropic Principle doesn't add much to the observed fact that galaxies exist. What Weinberg argued was that if the Anthropic Principle is correct, then the cosmological constant will probably not be much smaller than the galaxy formation limit. Weinberg was just expressing the common sense opinion that if anthropics is the only reason the cosmological constant is small, then it is unlikely that it will be orders of magnitude smaller than the anthropic limit.

Was it a prediction that could be proved wrong? Most people thought so. Just about everyone I know was certain that the cosmological constant was exactly zero.

Smolin's next argument involves what he calls a scientific alternative to the Anthropic Principle. He argues for a Darwinian natural selection principle. Smolin believes, as I do, that universes can reproduce and give rise to mutated offspring that differ in the values of the constants of nature. He believes the mechanism involves black holes while I believe it involves eternal inflation and Coleman de Luccia bubble nucleation. But either will do.

Smolin further believes that the constants of nature are determined by survival of the fittest: the fittest to reproduce that is. Those properties which lead to the largest rate of reproduction will dominate the population of universes and the overwhelming likelihood is that we live in such a universe. At least that's the argument.

But this logic can lead to ridiculous conclusions. In the case of eternal inflation it would lead to the prediction that our universe has the maximum possible cosmological constant, since the reproduction rate is nothing but the inflation rate.

It can also lead to ridiculous conclusions in a more zoological context. Imagine what a fool Gregor Samsa would have been if he woke up one morning wondering what sort of creature he was: "When I woke this morning, I realized that I must be a bacterium. They are the fastest reproducers and they outnumber everything else by a factor of trillions."

Samsa's error is obvious but I will spell it out anyway. Just the fact that he could even think to ask the question implies that that he is a rare exception and not an average organism. Of course, given that he was very far from average, he might have successfully used a similar logic to conclude that he was unlikely to be the queen of England.

In the same way Smolin incorrectly concludes that we are in the sort of universe which maximizes the rate of reproduction. But the conditions for life are fantastically exceptional and the fact that we are here at all means we are certainly not in an average universe.

Samsa's common sense observation, that he is probably not Queen Victoria, is an example of Vilenkin's Principle of Mediocrity. It doesn't always work but it is good common sense.

References

[1] Lee Smolin, "Scientific alternatives to the anthropic principle", hep-th/0407213


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