# How seriously do string theorists take the “landscape”

The string theory landscape seems to this outside observer to be an intermediate step in the intellectual progress toward a more robust theory that explains why our one universe has the particular properties that it has. is this the majority opinion or do most string theorists view the landscape as a plausibly being included in the final form of the theory?

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This is going to depend on the string theorist. There is no "official" string theory answer to this question. –  Peter Shor Sep 4 '12 at 12:58

I don't know about other people, but the notion that there is a unique 4d solution for our universe I find a little preposterous--- the situation is exactly the same as for Newtonian mechanics and the solar system--- the solar system has many different possible configurations, you can't predict that Jupiter will be this and such size at this and such a distance, but you can predict general features, and find the general laws.

It is not possible that string theory will be shown to uniquely predict our universe, since it allows for 11 dimensional stable vacuum, along with 10 dimensional ones, and so on. So the landscape isn't going away. In point of fact, the landscape was already appreciated by Scherk and Schwarz in the 1970s, when they did toroidal compactifications, but it was politically inconvenient, since it suggested that it will be difficult to predict experimental results from string theory. Because of this, some people kept on saying that the theory has a unique vacuum well into the 1990s, although this was already clear in 1977.

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This is clearly right. But "landscape" also gets used, perhaps sloppily, as a shorthand for the picture of eternal inflation populating all of these vacua. And I think that's, rightly, a lot more controversial. –  Matt Reece Sep 4 '12 at 4:36
@MattReece: Eternal inflation is not only controversial, I consider it dead, since it is holographically ridiculous. I think this is now commonly said by all the usual suspects in the holographic mafia (Fischler/Susskind/Banks/Shenker). –  Ron Maimon Sep 4 '12 at 4:41
But why should we believe in holography rather than eternal inflation? There's no actual experimental evidence for either one. –  Peter Shor Sep 4 '12 at 13:00
@RonMaimon: Given that Susskind and Shenker have written papers trying to build a holographic theory of eternal inflation (by taking the perspective of a late-time FRW observer), I don't think they agree that it's dead. E.g. Sekino/Susskind in arxiv.org/abs/0908.3844: "In this paper a holographic description of eternal inflation is developed." So I think "controversial" is the status, even among people who take holography seriously.... –  Matt Reece Sep 4 '12 at 15:03
@RonMaimon If a string theory wont ever be able to predict it, how do you account for our vacua without resorting to the anthropic principle (which is pretty obviously giving up)? I'm curious what your thoughts on this are Ron because I respect your opinion. Will we have to look for a yet deeper theory to explain their values? ;) –  user11841 Sep 4 '12 at 15:43

That there are multiple possible solutions isn't seriously questioned, but I note this recent paper The Top $10^{500}$ Reasons Not to Believe in the Landscape. This is not questioning the existance of $10^{500}$ solutions, but it is questioning the interpretation as a landscape. Specifically the paper claims the solutions are entirely separate and you cannot have transitions between solutions within it.

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This is Bank's old schtick, and it gets annoying--- what he is saying is that different Hamitlonians means different theories. But one just knows that Matrix theory and AdS/CFT are linked by compactification and adding branes, and these cannot be disconnected, although they are in Banks' technical sense of not being able to move from one to the other with a finite number of excitations. The points he make about EI being incompatible with holography are well taken (and clear), although you don't need Bousso, there is no failure in computing fluctuations in local coordinates. –  Ron Maimon Sep 4 '12 at 8:02
It's impossible for us outsiders to judge the merit of papers like this, though I note it hasn't made it to a refereed journal (yet). Sean Carroll appears to be taking it seriously (blogs.discovermagazine.com/cosmicvariance). –  John Rennie Sep 4 '12 at 8:34
Forget about refereeing, it is useless for radical stuff like this. Of course one has to take what Banks says seriously--- he understands holography. –  Ron Maimon Sep 4 '12 at 16:02