## Wednesday, May 02, 2012

### Lisa Randall and string theory

Five years ago, Peter, Larry, and a few other folks established bigthink.com, an online knowledge forum, which has featured interviews with 2,000 or so people so far.

A week ago, Lisa Randall recorded another interview.

She says that string theory is addressing the very short-distance phenomena and postulates that particles are actually extended objects, strings. Well, at least, this is the perturbative description.

Her comments are very concise but let me assume that you, a TRF reader, knows these 2-minute introductions to string theory. More specific comments are about Lisa's relationship to string theory. There are contexts in which is would be appropriate to say that Lisa is one of the "applied string theorists". In other contexts, it's more accurate to highlight the dichotomy and say that Lisa, a phenomenologist, is not a string theorist. She doesn't really use the two-dimensional world sheet CFTs in an essential way – and other traditionally stringy methods – so she can't be a string theorist.

The very fact the a similar question, "Is XY a string theorist?", may lead to legitimate ambiguities is a sign of some enormous transformation of high-energy physics that took place in recent two decades, and – while it may not be obvious – it's a very positive one. The point is that the overall structure of the ideas in high-energy physics has been so well understood and so meaningfully mapped that people understand the relationship of their ideas to other people's ideas and the established theories – as long as they are doing meaningful physics.

While people may focus on different aspects of the physical phenomena, different scales, different computational methods, or different possibilities in the still open landscape of possibilities, they may agree – and present evidence – that there is this landscape of possibilities, a body of knowledge, and its pieces are related in particular ways that are pretty well understood.

It feels like America a century after its discovery. You may already walk across the continent and find people who are familiar with its overlapping regions. The questions whether a theory is conceivable from a theoretical vantage point may be answered. One either studies some phenomena correctly or she or he doesn't. We still don't know where our Universe is exactly located in the space of possibilities, the stringy landscape, but we do know that this is the most accurate available framework to parameterize the space of possibilities.

The Randall-Sundrum example is a pretty characteristic story. While they had discovered some models with extra dimensions and extra dimensions have always been a typical feature of string theory, the detailed realization of their extra-dimensional scenario looked very non-stringy right after the papers were published. However, people invented heuristic stringy constructions and some years later, it became clear that string theory, even when studied with the usual stringy rigor, actually does contain vacua with warped geometries that realize the Randall-Sundrum ideas.

We still don't know whether the warping paradigm is realized in Nature – at most, we know from the LHC that the characteristic scale of the warping has to exceed 5 TeV or so – but we do know that the Randall-Sundrum constructions are a previously neglected possibility that does result from string theory. If string theorists had studied some compactifications more carefully – and without prejudices and omissions – than they did, they would have realized that string theory forces the warping upon us in some situations.

This is far from being the first example of a similar phenomenon, "string theory eating adjacent good-tasting disciplines". In the early 1980s, people would believe that supergravity was a viable candidate for a theory of everything. It used similar concepts as string theory – including supersymmetry applied to a gravitating theory – but it was believed to be decidedly inequivalent to string theory.

However, as people were learning additional things, it became more clear that nice supergravity theories are low-energy limits of string theory and the string theory completion is needed to cure non-renormalizable diagrams (or, to say the very least, non-perturbative inconsistencies of pure supergravity). Even more shockingly, an "exceptional example of supergravity" that used to look non-stringy because it had more than (the stringy figure of) 10 dimensions – namely 11-dimensional supergravity – is actually an essential component of the "full string theory". Today, we know that the 11-dimensional supergravity is the low-energy limit of M-theory which is the strongly coupled limit of type IIA or $$E_8\times E_8$$ heterotic string theory (aside from other ways how it may be connected to the 10-dimensional string theories). Realistic compactifications may be obtained by putting the 11-dimensional M-theory on more complex manifolds.

While the old-fashioned supergravity researchers could invent models that haven't been found in string/M-theory, it has become pretty clear that the interesting ones do appear somewhere in the string landscape. And frankly speaking, those compactifications that are "manifestly stringy" in origin look not only more consistent in average but also more realistic as theories of particle physics in our Universe.

This experience has transformed people's understanding of what they're actually doing. Even if one uses the same low-energy methods of supergravity to study some questions, he already realizes that what he's really doing is to study a certain compactification of string/M-theory by the long-distance approximate methods, i.e. by the effective field theory. It's a significant advantage if a proposed idea about beyond-the-Standard-Model physics may be realized in string theory.

I could tell you many more examples of this sort. Deconstruction, a template for quantum field theories with many factors in the gauge group and lots of bifundamental representations, may have been randomly invented by the particle phenomenologists as a clever idea to construct new models in which a new integer – the number of factors in the gauge group (the size of the "theory space") – may be sent to infinity. And make no doubts about it, such limits are always interesting. However, it also became clear that some research that was pursued for purely stringy, formal methods – the quiver diagrams describing D-branes – produces the same class of gauge theories and the same trick by which the multiple gauge groups emulate new dimensions of space. In fact, string theory makes it very clear that these seemingly discrete dimensions are "real" if looked at properly (and if we use some dualities).

Some of the insights that the model builders and string theorists found were overlapping, others were only found on one side (and some additional ones haven't been found by no one, perhaps). But many of those that have been found by one group only may have been imported to the other side because people appreciate that they study the same thing. Or to say the least, they study theories with the same clever idea that may be produced by another fundamental construction in string theory. Much like science is a connected entity that has no "sharp borders" in it, high-energy physics that includes string theory has become similarly connected. One can't really deny the existence of some other parts of string theory if one studies high-energy physics; one may only prefer to focus on some questions or certain methods.

Of course, there are also theories proposed by various people in the past (and the present) that haven't become a part of this club – a club of ideas that are connected to the observable physics and its known consistent extensions. For almost all of them, it seems obvious that the reason of their exclusion is that they're simply wrong.

We still face the question what is our vacuum. The possible theories may be parameterized in various ways. In renormalizable quantum field theory, one has to decide about the field content and the value of the finite number of renormalizable mass parameters and couplings. However, quantum field theories that have a different field content had to be thought of as different islands that have nothing to do with each other. So quantum field theory forced us to think about the space of possible theories as a huge archipelago of islands, each of which has a certain multi-dimensional parameter space. That was quite a messy situation because there could exist reasons why some islands are much more likely than others. But because of the disconnectedness of the islands, they couldn't communicate with each other. And without such communication, one can't objectively compare the qualitatively different quantum field theories.

String theory has changed the picture. In a sense, it has given us the navy. The ships may sail from one island to another. The archipelago has become connected. The stringy version of quantum gravity guarantees that the topology change is possible; you may say that this insight is analogous to the quantum tunneling in quantum mechanics that has also and literally "demolished many walls". To simplify the discussion, we may realize that string theory has also given us the stringy bomb that may be used to evaporate the ocean and find out that all the islands are actually hills on the same continent.

What string theory hasn't changed is that there are still many possibilities and we don't really know which possibility is the right one. Whatever arrangement of the possibilities you use, the observed constraints coming from accelerators or cosmological observations restrict the set of conceivable vacua and/or at least make some of them more likely than others. So even when it comes to the ignorance, string theory leads to a similarly uncertain picture as quantum field theory, pragmatically speaking.

However, it's clear that the "more fundamental" way to organize the space of possibilities simply has to be the stringy one because the laws of string theory are the fundamental ones while the low-energy effective field theories are just approximations, derived consequences of the short-distance fundamental laws. If you really want to get to the "primary questions" that Nature was deciding when it was choosing the right vacuum where we should live, it's clear that you must parameterize the space of possibilities in terms of the parameters that are natural at very short distances. And once you get to distances that are approximately Planckian, effective quantum field theories are no longer enough. You need to use the full string theory.

This changes the preferred "grouping" of possibilities. For example, two low-energy effective field theories may be very similar, lead to very similar long-distance phenomena, but they may differ by a subtle, hardly observable detail. A quantum field theorist could say that they're almost the same. However, from the viewpoint of string theory, they may be "islands" that sit in totally different oceans or continents of the landscape and that just randomly look alike at long distances. And on the contrary, two effective field theories may look very different – lead to very different observable phenomena – but they may have a very similar or the same stringy origin, a stringy vacuum or a pair of nearby stringy vacua that only start to behave very differently if you begin to derive some long-distance physics out of them. (String theory is also full of dualities in which seemingly very different vacua are actually exactly equivalent which can only be seen if all the non-perturbative and other corrections are taken into account and if the observables on both sides are carefully and nontrivially mapped onto each other.)

The parameterization and the "notion of proximity" that could be natural from the effective quantum field theory viewpoint may be useful to do particular long-distance predictions. At the same moment, it's obvious that it isn't the right "notion of proximity" to address the deepest questions about the Universe. To get to Nature's bedroom or at least bathroom, you must use Her fundamental measures quantifying the proximity of models and possibilities, the stringy ones.

Much like quantum field theory turned out to be another layer in our ever deepening understanding of the world – a well-defined incarnation of theories that respect both the postulates of quantum mechanics as well as special relativity – string theory is yet another level. Quantum field theories may have to be generalized to describe quantum gravity but they still contain lots of the truth which will never go away again. The same is true for string theory as we know it today. However, we may still see some conceptually new discoveries in string theory that may have almost nothing to do with the strings themselves – which gave the theory its name – but which will complete our current understanding in similar ways as string theory itself has completed and clarified our insights from quantum field theory. In fact, we already know that the deeper essence of "string theory" isn't quite about the strings.

Some good ideas that are still considered independent of string theory may be proven to be a previously overlooked part of the stringy empire sometimes in the future. But don't expect that this means that all ideas will be a part of string theory; many ideas invented by the people – as opposed to Nature – are just bad or wrong.

#### 1 comment:

1. Lubos, I know you don't pay much attention to Woit's blog, so maybe you haven't noticed this yet. Woit never mentions quantum computers and he always deletes those comments that do, especially when they dare to suggest that quantum computers might someday be useful for doing string theory simulations. I think he is terrified that string theorists will ally themselves with quantum computerists, because then he would find it much harder to criticize string theorists as impractical. It could put him out of business!