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That classic xkcd string theory post (updated version)



* Uh, wait a minute... if string theory is correct, then as Len Susskind says there are many ways to compactify 11-dimensional space and we'll have a landscape containing a vast number of possible instantiations of string theory, right? Probably around, I don't know, maybe 10⁶⁰�� of them? Now let's think about what happens at the beginning of the universe when it's still at the Planck scale, its dynamics will be determined by the Wheeler-DeWitt equation and you'll have waveform protouniverses wandering across the landscape. Some of them will settle down into local minima and start inflation processes to create macroscopic universes, won't they? But which ones? Hm, hm... need to calculate here... some of those condensed matter methods will come in handy... looks like the high-energy ones! That's interesting, as good old Roger Penrose keeps saying it's always been a bit hard to understand why our own universe started off in this apparently very unlikely state! The explanation in terms of the Anthropic Principle just isn't very convincing, is it? And... hold on, of course you're going to have quantum interactions between all those different alternatives before they decohere, won't you? Is it possible that that could produce observable effects in our own universe? Well, I suppose it might, but they'll obviously be so faint that you'll never be able to see them with today's technology, probably you'll have to wait a few centuries if it's possible at all... anyway, let's do another calculation... wait, I don't believe it, this predicts a massive void that should be easily visible in the CBR... a few more things too... let me just call the satellite guys and see if they found anything like that... incredible! They did!! Five-sigma result!!! I should write a book about this and get famous... what? You say Laura Mersini-Houghton already has? How come no one told me? Damn, scooped again.
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But seriously...

I would very much like to thank the author, who not only sent me an advance copy of the book but took time out of her insanely busy schedule to point me to background reading and correct some of my misconceptions about her work. Here are my revised thoughts.

(view spoiler)[Alas, it seems I was a little too optimistic. According to the author, this work doesn't yet provide a strong experimental test of string theory. It is indeed the case that she and her colleagues developed a theory which starts with the string theory "Landscape", uses it to model the evolution of the very early universe, and makes verifiable empirical predictions. It is also the case that some of those predictions have now been verified to a high level of statistical significance.

However (at least if I am understanding this correctly) the problem is that a fairly broad class of theories would make similar predictions. The important thing is that the landscape of possible vacua is sufficiently disordered. The string theory landscape has this property, but other theories could have it too. So the data does provide good empirical support for the claim that, at some very stage in the universe's evolution, there were multiple quantum-entangled universes, but it doesn't provide good support for the claim that the string theory landscape exists. Of course, even the first part of this is already quite a lot.

The key publications for the book are the following papers, all freely available from arXiv:

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They are far from being an easy read, and there were large parts of them that I found totally incomprehensible. A great deal of advanced physics is presupposed. But some of it made good sense, in particular the passages at the end of the third paper that talk about empirical verification. It seems that there are a lot of angles here, and (again, if I understand correctly), many of them have not yet been thoroughly investigated.

If you're into fundamental physics and cosmology, this is real edge-of-your-seat stuff. (hide spoiler)]