Two perspectives on the "Landscape"
Here are two perspectives on the String "Landscape" exerpted from Woit's blog "Not Even Wrong". They are by people who have sometimes posted on PF----
Urs Schreiber and Peter Woit----and were prompted by a a lay question from an artist by the name of Pyracantha.
Here is Pyracantha's original question:
"Pyracantha from "Electron Blue" here, the artist who is trying to learn math/physics in middle age. I read your site in the hope that someday I'll understand what you and your colleagues are talking about. But I have heard one phrase many times and it intrigues me. What is the "landscape?" Could you explain it in terms that a beginner like me could understand?"
---Urs quote from Comments on "Witten in Crete"-----
Hi Pyracantha -
in so-called perturbative quantum theories one chooses a solutiuon of the classical equations of motion, the so-called 'background' and then studies quantum corrections to that background order by order.
For instance in ordinary quantum field theory the background might be flat Minkwoski spacetime and in that background we can imagine photons and electrons to propagate and interact in Feynman-diagram fashion. The 'vacuum' background together with all these particle whizzing around would then be a full perturbative state of the theory.
(One problem is that not all aspects of the full quantum theory are captured by such a perturbative procedure.)
Now, in string theory the idea is pretty much the same, only that here the particles are not pointlike but a have a small linear extension. This seemingly simple modification has drastic consequences. While in field theory there are many possible choices of fundamental particles, their interactions, and choices of background, the consistency of string interaction very much constrains all three of these. The big open question is: How much exactly?
When people talk about the 'string theory landscape' they are thinking of the abstract space in which each point is one consistent perturbative string theory background, i.e. one consistent choice of particle content, particle interaction and classical spacetime that they propagate in. In principle the number and position of points in this 'theory space' is determined by the background equations of motion of string theory (or equivalently, if you want to hear the technical terms, by the requirement that there is a supercfonformal field theory with central charge 15 on the worldsheet of the string).
There has been some recent progress in better understanding this space - but it is still immensely ill understood. Still, the progress that has been made has appeared significant enough to some people to base some more far reaching speculation on it. That's because a good understanding of which background solutions string theory admits is the key to be able to apply string theory to pheonomenological considerations. When a string theory background is found which is consistent with the observed particles of nature, then studying the stringy quantum corrections to it would allow to deduce what this background predicts as corrections to the currently known physics.
Peter Woit here has pointed out repeatedly that some of the speculations concerning the landscape that have been published are not at all based on results that have really been calculated.
On the other hand, the mere fact that a discussion of such a 'theory landscape' is possible (even though not easy) is important. It is not possible in field theory of point particles. There we also have some restrictions on the Lagrangians (i.e. the particle content and interaction) that we are allowed to consider as a consistent field theory, but they are far less severe than those found in string theory.
As has been pointed out very nicely by Jacques Distler in his weblog, the points in the landscape which are consistent with the experiments that we have made are probably very rare. In any case, none has been found so far. If there is none at all, then string theory is wrong as a theory of nature. If there is a single such point, then string theory, based on the currently known data, could make predictions about for instance new particles that could be found in future colliders. (These predictions could still be disporved by experiments, of course.). If however there are very many such points then predictions for new particles etc. would be very difficult. One might, in this case, still try to make some statistical predictions. Such statistics about properties of the 'landscape' are currently what some people are trying to do. But it seems fair to say that this is, while an intersting idea, quite premature.
Finally, there is the theoretical possibility that the world we live in cannot be understood as a small perturbation of some background. The success of perturbative field theory suggests otherwise, but nobody can know this for sure. So one possibility is that none of the points in the 'landscape' correspond to the world we live in, but some nonperturbative description of string theory is necessary to describe our world. Nonperturbative description of string theory tend to be described not by full classical backgrounds, but by asymptotical backgrounds, this means roughly that at spatial infinity the background is fixed, while 'in between' physics is described fully nonperturbatively. Nonperturbative discriptions of string theory are known for instance for universes which asymptotically have the geometry of what is called 'anti-deSitter Space'. This is, roughly, the shape of a universe with a negative cosmological constant.
Now, unfortunately for string theorists, recent very exciting measurements of various cosmological parameters have shown that instead we observe a cosmological constant which is positive. This means that the particular anti-deSitter non-perturbative deswcription of string theory appears not to be applicable to describe the universe that we live in.
This is probably the main reason for the current excitement about landscape discussions. Namely people are trying to find out if in the landscape admits universes which only temporarily have a positive cosmological constant, while asymptotically this constant goes negative. If this were the case then there would still be hope that the nonperturbative string theory description which involves asymptotically anti-deSitter space could be used to describe the world we observe.
So that's what all this landscape talk is about. Unfortunately, since there is so little known for sure about the 'landscape' (even though the landscape is a well defined mathematical object (space of all superconformal 2d theories with c=15) which can in principle be understood exactly), some of the discussion concerned with it recently has tended to be more philosophical than scientific.
Posted by
Urs Schreiber at July 13, 2004 04:38 AM
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