The idea of unifying all the known fundamental interactions in nature into a "theory of everything" has been pursued by several notable scientists beginning with Albert Einstein. These four fundamental forces are gravity, electromagnetism, strong nuclear force and weak nuclear force. String theory is an attempt to reconcile quantum mechanics with gravity, and is considered a candidate for a theory of quantum gravity, which in turn is seen as a contender for the "theory of everything."
In a press release posted at the Carnegie Mellon website, Ira Rothstein, professor of physics at Carnegie Mellon and co-author of the paper, explained that their research "shows that, in principle, string theory can be tested in a non-trivial way." Grinstein, professor of physics at UCSD, and another contributor claimed that "the beauty of [their] test is the simplicity of its assumptions." "The canonical forms of string theory include three mathematical assumptions-Lorentz invariance (the laws of physics are the same for all uniformly moving observers), analyticity (smoothness criteria for the scattering of high-energy particles after a collision) and unitarity (all probabilities always add up to one)," they explained. "Our test sets bounds on these assumptions."
The test involves measuring the scattering of high-energy particles called W bosons when they collide. Many scientists, including the authors of the paper, anticipate these collisions to be first seen in the Large Hadron Collider (LHC), a particle accelerator located near Geneva. The LHC, scheduled to begin operation sometime late 2007, is expected to become the world's largest particle accelerator and the center for pursuit of some of the most tantalizing questions in high-energy physics.
Speaking about the relevance of the tests, co-author Jacques Distler, professor of physics at the University of Texas at Austin, said, "If the bounds (set by the tests) are satisfied, we would still not know that string theory is correct. But, if the bounds are violated, we would know that string theory, as it is currently understood, could not be correct. At the very least, the theory would have to be reshaped in a highly nontrivial way."
String theory rests on formulating tiny strings, which have extremely small dimensions of about a billion trillion trillion times smaller than a meter ruler. String theory, when initially devised, was seen as particularly attractive because of its elegance and simplicity. It also did away with the problem of infinities that arose when dealing with the graviton, a particle hypothesized to carry the gravitational force. When closed, like strings on a guitar these strings can represent certain notes, which have tensions that relate to fundamental particles.
In fact, there are many different, interconnected, string theories. For example, string theories for particles such as the electron vary depending on the nature of strings considered. Specifically, they depend on whether or not the strings are closed/constrained (like the strings in a guitar), and whether or not they are orientable (one can determine which direction one is traveling along the string). Among other things, string theory proposes the existence of more than four dimensions of space-time, with ten considered the minimum number required to represent the "real" physical world. These extra dimensions could exist in minute spaces, or alternatively be extremely big and somehow constrain all gravity and known matter to propagate in the three dimensions of space and one dimension of time that we have learned to know and love.
Many professors in the Amherst College physics department conduct research that ties in with one or more aspects pertaining to the principal assumptions of string theory. For example, Professor Larry Hunter carries out research to test the assumption of Lorentz invariance. Professor Kannan Jagannathan recently wrote an important review, in the December 2006 edition of Physics Today, of two books that cast serious doubts on string theory. Professor William Loinaz, an enthusiastic theoretical physicist, found the research paper "especially interesting for the generality of its conclusions."
"The work isn't about string theory per se although, as the authors point out, it certainly has ramifications for string theory," he said. "Rather, Distler et al. address very general features of a broad class of physical theories. They find that certain assumptions often considered sacrosanct (such as causality and Lorentz invariance) when constructing models of subatomic particles and interactions imply surprisingly strong bounds on the interactions of certain particles (the scattering of W bosons)."
Alex Bridges '07, a thesis student working with Professor Hunter, states, "Our experiment provides insight into physics beyond the Standard Model, but let's be realistic: No experiment in the world could prove or disprove string theory at this young stage. These 'string theory tests' aren't that at all-they're Standard Model tests, just like everything else. It will be many years and embittered arguments before anything relevant emerges."
Professor Loinaz summarized the importance of the test devised as follows: "The features of physical theories that will be tested in the W boson scattering experiments appear to be on solid footing when tested in other experiments, so there's no reason to anticipate that they'll be overthrown by the LHC measurements … But, even a small violation of (say) Lorentz invariance or microcausality would be a revolution that would force us to reevaluate much of the physics we've come to take for granted over the past century, and physicists would cheer the revolution. High-energy physics has been at something of an impasse over the past few decades. The Standard Model, a theory formulated in the 1970s to describe the low-energy side of high-energy physics … is known to have such serious problems that it cannot be the final word in fundamental theories … String theories, by contrast, are formulated at the highest energy scales; they are designed to cure many of the problems of the Standard Model, but they make notoriously few general predictions. An experimental result which invalidates both of these at once would be fabulously liberating, and that's the hope Distler et al. offer."
There is no telling how long it will take to ultimately answer the questions this experiment aims to examine. But don't get strung up over it.