This is a good question, Clint, and an excellent and interesting interview between Sean Carroll and science writer John Horgan. Professional theoretical physicists work out at the edges of current knowledge. As Carroll admits, these various multiverse theories are speculative, and like Horgan, I am rather surprised that many of the top scientists have incorporated such things into their research programs that are unlikely to be empirically verified even in the near future.

But I think Sean Carroll gives an answer to this question of why these scientists seem to be leaving observational science behind and speculating about such things as multiverses. His answer? "We are forced to." What we do observe in our universe is forcing us to seek answers that explain our observations within a broader framework.

I happen to be reading Paul Davies The Cosmic Jackpot. I'm about in the middle of the book, and I'm not sure where he's going with this, but he is reporting on the long-standing question about the physical constants and why they seem to be so finely tuned to allow stars and galaxies and observers such as us to exist. If the relative masses of the proton and neutron were slightly different, our universe would be nothing like what we observe, and indeed, observers could not exist.

Yes, this is the Anthropic question, and Davies points out that historically scientists have considered this as tautological, unproductive, and unworthy of any scientific consideration. But in the last few years, several respected and very knowledgeable theoretical physicists and cosmologists have said, "Wait a minute. These cosmic coincidences are significant and too coincidental to ignore any longer." And the odd comparative strengths of the various constants? Why the heck is the electromagnetic force 10⁴⁰ times stronger than the gravitational force? (But as it happens, that's a good thing.) And the biggest miscalculation of all time, why does the vacuum energy appear to be 10¹¹⁹ times weaker than quantum mechanical calculations imply it should be? (Again, a darn good thing it is, too!)

I'm not sure if the relevance of the Anthropic question was laid bare by the recent discovery of the accelerating expansion or what, but obviously several top scientists feel this is one of those questions that they are being forced to confront, as Sean Carroll explains. And of course an appeal to the supernatural is not an option in science. Scientists seek natural explanations. And so several versions of multiverse theories have been proposed by theorists such as Susskind, Smolin, Vilenkin, Linde, Steinhardt&Turok, and I'm sure others I'm not aware of. But these theories are not "accepted" as you imply. They are proposed. The authors aren't saying, "This is how it is." They are saying, "This might be how it is, and this would explain why we observe what we do."

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