Banks, The Top 10^{500} Reasons Not to Believe in the Landscape

[bcrowell]

Banks, "The Top 10^{500} Reasons Not to Believe in the Landscape"

Does anyone have any opinions about this paper? http://arxiv.org/abs/1208.5715 It's way over my head technically. The paper is relatively recent, but apparently Banks has been saying this for a while.

If I'm understanding correctly, some minority of string theorists, including Banks and Lubos Motl, don't believe in the landscape, or don't believe in the anthropic principle as a way of dealing with the issues it raises. What is not clear to me is how string theory can be viable if it's really some huge number of separate theories, which are computationally intractable to sort through and match up with observation (Denef and Douglas, http://arxiv.org/abs/hep-th/0602072 ).

 

[atyy]

I think there are 2 issues: the landscape and the anthropic principle. I think most believe in the landscape, but think the anthropic principle is too nebulous. Banks appears to believe in neither.

At the very least, string theory is the only known candidate for quantum gravity. So it provides an example of quantum gravity - just as Nordstrom's theory provided an example of relativistic gravity before GR. If string turns out to be in fact the correct theory, but there is a landscape, then the answer has to be too bad for us - we have to deal with it.

 

[bcrowell]

At the very least, string theory is the only known candidate for quantum gravity.

Loop quantum gravity?

 

[atyy]

Loop quantum gravity?

I should have said that reduces to GR and solves the renormalizability problem. LQG probably solves at least the UV finite part of the renormalizability problem, but isn't yet known to give nice geometries - although LQC does - but it's not a full theory of quantum gravity. Asymptotic Safety does reduce to GR, but I don't think the UV fixed point is proven. String theory in full is undefined, but there is AdS/CFT which provides a non-perturbative definition of full quantum gravity - it appears not to give cosmologies consistent with what we observe, so I would say the value of AdS/CFT so far is to tell us what quantum gravity could be like - just as Nordstrom's theory and its geometric reformulation by Fokker and Einstein was a valuable first relativistic theory of gravity that was inconsistent with observation.

There's an interesting comment on universality in AdS/CFT in McGreevy's notes:

"... for the plasma made from any CFT that has an Einstein gravity dual; the answer is always $1/4\pi$. Each such CFT is what we usually think of as a universality class, since it will have some basin of attraction in the space of nearby QFT couplings. Here we are saying that a whole class of universality classes exhibits the same behavior. What’s special about these theories from the QFT point of view? Our understanding of this ‘bulk universality’ is obscured by our ignorance about quantum mechanics in the bulk. Physicists with what could be called a monovacuist inclination may say that what’s special about them is that they exist. The issue, however, arises for interactions in the bulk which are quite a bit less contentious than gravity, so this seems unlikely to me to be the answer."

He defines "Monovacuist (n): One who believes that a theory of quantum gravity should have a unique groundstate (in spite of the fact that we know many examples of much simpler systems which have many groundstates, and in spite of all the evidence to the contrary (e.g. [26, 27]))."

There's also ideas such as Acharya, Kane and Kumar's, which I think are agnostic to the landscape, but realize there's a problem with finding the right theory either way, and try to find generic predictions. "In recent years it has been realized that in string/M theories compactified to four dimensions which satisfy cosmological constraints, it is possible to make some generic predictions for particle physics and dark matter: a non-thermal cosmological history before primordial nucleosynthesis, a scale of supersymmetry breaking which is "high" as in gravity mediation, scalar superpartners too heavy to be produced at the LHC (although gluino production is expected in many cases), and a significant fraction of dark matter in the form of axions."

 

[mitchell porter]

I wrote about it here.

 

[bcrowell]

I wrote about it here.

Thanks, very helpful!