String Theory: Disagreements about the Anthropic Principle
At Stanford, Leonard Susskind and his colleagues seem to be embracing the anthropic principle. To hear (or read) Susskind on the subject, the string theory community is quickly jumping on board. It’s unclear whether the movement is spreading quite as intently as this rhetoric implies, though.
Since its introduction in 1974, the anthropic principle has invoked passion among scientists. It’s safe to say that most physicists don’t consider invoking the anthropic principle to be the best scientific tactic. Many physicists see it as giving up on an explanation, and just saying “it is what it is.”
One barometer of the popularity of the movement could be the literature. Out of 13 string theory books (written after 2003 — 8 popular books, 5 textbooks) randomly selected for perusal, here are the statistics:
5 make no mention of the anthropic principle in the index
2 discuss the anthropic principle for precisely one paragraph
2 contain more general discussions of the anthropic principle, lasting about two pages
2 attack the landscape and anthropic principle as major failures of the theory, devoting roughly an entire chapter to the concept
2 argue that the anthropic principle is crucial to understanding our universe (and one of those is written by Susskind himself)
On the other hand, a search of the arXiv.org theoretical physics database shows 218 hits on a search of the phrase “anthropic.” Searching on “anthropic principle” obtains 104 hits, and adding words such as “string” and “brane” only causes it to drop from there.
For comparison, searching on “string theory,” “cosmological constant,” or even the far less popular “loop quantum gravity” result in so many hits that the search cuts off at only 1,000 papers. So the jury is certainly still out on how well the string theory community has adopted the anthropic principle.
Some string theorists, such as David Gross, appear to be strongly opposed to anything that even hints at the anthropic principle. A large number of string theorists bought into it based on the idea — championed by Witten’s promise of M-theory in 1995 — that there would be a single theory at the end of the rainbow.
String theorists seem to be turning to the anthropic principle mostly out of a lack of other options. This certainly seems to be the case for Edward Witten, who has made public statements indicating he might be unenthusiastically turning toward anthropic thinking.
Instead of five distinct string theory solutions, we have 10500 or so. It’s unclear what the fundamental physical properties of string theory are in a field of so many options. The only hope is that new observations or experiments will provide some sort of clue about which aspect of the string theory landscape to explore next.