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A new Approach against String theory
:: 25 October, 2007
Ernest Rutherford used to tell his physics students that if they couldn't explain a concept to a barmaid, they didn't really understand the concept. With regard to the cosmological implications of string theory, the barmaids and physicists are both struggling—a predicament that SLAC string theorist Shamit Kachru hopes to soon resolve.
String theory is currently the most popular candidate for a unified theory of the fundamental forces, but it is not completely understood—and experimental evidence is notoriously elusive. Physicists can, however, gain crucial insight into the theory by evaluating how accurately its models can predict the observed universe.
Using this indirect approach, Kachru, in collaboration with theorists at Rutgers University and the Massachusetts Institute of Technology, sought models that could reproduce inflation—the prevailing cosmological paradigm in which the nascent universe experienced a fleeting period of exponential expansion. Although there is already a substantial body of literature presenting such models—spawned in part by publications of Kachru and his Stanford and SLAC colleagues Renata Kallosh, Andrei Linde and Eva Silverstein in 2003—the complexity of the models leaves room for doubt.
"They incorporate inflation, and they're the most realistic models of string theory," Kachru said, "but they're complicated. They're fancy. They have a lot of 'moving parts,' and we need to fine-tune all of them, so we can't verify anything to a high degree of accuracy. It forces us to ask—are we confident that we really understand what's going on?"
To achieve a comprehensive understanding of how inflation can be embedded in string theory, Kachru and his collaborators employed a pedagogical tactic. "What we wanted was an explicit 'toy' model," Kachru explained. "The goal wasn't to have something realistic, but to allow us to understand everything to every detail."
"There are deep conceptual questions about how inflation is supposed to work," Kachru continued. "In order to understand these issues, it's best to have a simple model. There's so much clutter in the complicated examples, you can't disentangle the conceptual issues from the clutter."
The group investigated three versions of the simplest formulation of string theory, and found that they were incompatible with inflation. "This means we're going to have to consider slightly more complicated scenarios," said Kachru. "There are a lot of levels between this and the fancier working models, so we'll find one eventually."
Kachru and his colleagues published their work in Physical Review Letters D, providing a framework for others in search of simple inflationary models of string theory. "There are so many successful models out there that incorporate string theory and inflation, so we'll undoubtedly find a simpler version. When we find that 'toy model,' where all the moving parts are obvious, we can address deep conceptual questions without getting lost in the details."
Note for String theory
String theory is a model of fundamental physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point particles that form the basis for the standard model of particle physics. The phrase is often used as shorthand for Superstring theory, as well as related theories such as M-theory. By replacing the point-like particles with strings, an apparently consistent quantum theory of gravity emerges. Moreover, it may be possible to "unify" the known natural forces (gravitational, electromagnetic, weak nuclear and strong nuclear) by describing them with the same set of equations, as described in the Theory of everything.
For a scientific theory to be valid it must be verified experimentally. Few avenues for such contact with experiment have been claimed.
With the construction of the Large Hadron Collider in CERN some scientists hope to produce relevant data, though it is widely believed that any theory of quantum gravity would require much higher energies to probe directly. Moreover, string theory as it is currently understood has a huge number of equally possible solutions.[2] Thus it has been claimed by some scientists that string theory may not be falsifiable and may have no predictive power.
Studies of string theory have revealed that it predicts higher-dimensional objects called branes. String theory strongly suggests the existence of ten or eleven (in M-theory)[7] spacetime dimensions, as opposed to the usual four (three spatial and one temporal) used in relativity theory; however, the theory can describe universes with four effective (observable) spacetime dimensions by a variety of methods.
An important branch of the field deals with a conjectured duality between string theory as a theory of gravity and gauge theory. It is hoped that research in this direction will lead to new insights on quantum chromodynamics, the fundamental theory of strong nuclear force.
Note for Toy model
In physics, a toy model is a simplified set of objects and equations relating them that can nevertheless be used to understand a mechanism that is also useful in the full, non-simplified theory.
In "toy" mathematical models, this is usually done by reducing the number of dimensions or reducing the number of fields/variables or restricting them to a particular symmetric form.
In "toy" physical descriptions, an everyday example of an analogous mechanism is often used to illustrate an effect in order to make the phenomen easier to visualise.
Some examples of "toy models" in physics might be: orbital mechanics described by assuming that the Earth is attached to the Sun by a large elastic band; Hawking radiation around a black hole described as conventional radiation from a fictitious membrane at radius r=2M (the black hole membrane paradigm); frame-dragging around a rotating star considered as the effect space being a conventional "draggable" fluid.
The phrase "Tinker-toy model" is also sometimes used in this context, and refers to a particular children's construction toy that allows objects to be built easily but somewhat unrealistically.
Release link: http://web.mit.edu/
Tags: Ernest Rutherford , barmaid , cosmological implications , SLAC , Shamit Kachru , Massachusetts Institute of Technology , Renata Kallosh , Andrei Linde , Eva Silverstein , Physical Review Letters D , toy model , String theory. ,
