A large fraction of the physics communi- ty has abandoned trying to explain our world as unique, as mathematically the only possible world. Right now the multi- verse is the only game in town. Not every- body is working on it, but there is no co- herent, sharp argument against it.
In 1974 I had an interesting experience about how scientific consensus forms. People were working on the as yet untest- ed theory of hadrons [subatomic particles such as protons and neutrons], which is called quantum chromodynamics, or QCD. At a physics conference I asked, “You people, I want to know your belief about the probability that QCD is the right theory of hadrons.” I took a poll. No- body gave it more than 5 percent. Then I asked, “What are you working on?” QCD, QCD, QCD. They were all working on QCD. The consensus was formed, but for some odd reason, people wanted to show their skeptical side. They wanted to be hard-nosed. There’s an element of the same thing around the multiverse idea. A lot of physicists don’t want to simply fess up and say, “Look, we don’t know any other alternative.”
The universe is very, very big. Empiri- cally we know it’s at least 1,000 times big- ger in volume than the portion that we can ever see. The success of the concept of cosmic inflation opens the possibility that the universe is varied on big-enough scales. String theory provides Tinkertoy elements that can be put together in an enormous number of ways. So there’s no point in looking for explanations of why our piece of the world is exactly the way it is because there are other pieces of the world that are not exactly the same as ours. There can’t be a universal explana- tion of everything that it is any more than there can be a theorem that says the aver- age temperature of a planet is 60 degrees Fahrenheit. Anyone who tried to make a calculation to prove that planets have a temperature of 60 degrees would be fool- ish because there are lots of planets out there that don’t have that temperature.
But nobody knows the underlying rules for multiverses. It’s a picture.