One of the most unexpected and disturbing discoveries of string theory is that instead of one single theory, it turns out there may be a huge number of possible theories (or, more precisely, possible solutions to the theory) — possibly as many as 10500 different solutions! (That’s a 1 followed by 500 zeroes!)
While this huge number has prompted a crisis among some string theorists, others have embraced this as a virtue, claiming that this means that string theory is very rich. In order to wrap their minds around so many possible theories, some string theorists have turned toward the anthropic principle, which tries to explain properties of our universe as a result of our presence in it.
Still others have no problem with this vast number, actually having expected it and, instead of trying to explain it, just trying to measure the solution that applies to our universe.
With such a large number of theories available, the anthropic principle allows a physicist to use the fact that we’re here to choose among only those theories that have physical parameters that allow us to be here. In other words, our very presence dictates the choice of physical law — or is it merely that our presence is an observable piece of data, like the speed of light?
The use of the anthropic principle is one of the most controversial aspects of modern string theory. Even some of the strongest string theory supporters have expressed concern over its application, because of the sordid (and somewhat unscientific) applications to which it has been used in the past and their feeling that all that is needed is an observation of our universe, without anything anthropic applied at all.
As anthropic principle skeptics are quick to point out, physicists only adopt the anthropic principle when they have no other options, and they abandon it if something better comes along. It remains to be seen if string theorists will find another way to maneuver through the string theory landscape.