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초끈이론과 장론

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quantum field theory and Einstein's classical theory

By late 1970s, quantum field theory and Einstein's classical theory of gravity proved to be suitable theoretical frameworks to address most of observed features of our universe, from elementary particles like electrons and protons to evolution of the universe in the cosmological scale.

However, there were also many fundamental problems that remained to be solved. Elevating gravity to quantum level had been one grand unsolved problem since the days of Einstein, while other smaller but equally mysterious problems, such as how to solve quantum chromo-dynamics (QCD) why the cosmological constant of our universe is so small (thought to vanish at some point but later proven otherwise by observation), and whether properties of black holes are consistent with quantum principle, were abundant as well.

Now, 30 years since then, many theoretical physicists seem to believe that string theory did or will offer answers to many such questions. The original idea of string theory that everything in nature originates from loops or segments of strings moving in the relativistic way, seemed ludicrous at first. Yet, its unique ability to define a quantum mechanically consistent gravity is not something that theorists could easily resist. Existence of gravity in string theory was recognized as early as 1975, which was then elevated to a realistic computational framework in 1980's, but putting it to actual use was another problem.

Better understanding and use of string theory became possible through the realization in the 1990's that there are hidden symmetries, known as "duality." Recently, it has been shown that a strongly coupled regime of one superstring theory can sometimes be understood as a weakly coupled regime of another, "dual" superstring theory. Such relations demonstrated that different models of superstrings are actually different perturbative realizations of one and the same theory. One ultimate theory, which was conjectured to contain all superstring theory as special cases, has been named M theory. Another lesson from these developments in the 1990's is that string theory is made up of not only strings but all kinds of other extended objects, known as D-branes and M-branes, as well.

Probably the most celebrated example of dualities, found in 1997 and has been exploited and generalized widely since then, is AdS/CFT. In its most general reincarnation, it asserts equivalence between certain pairs of open string theory and closed string theory. In practice, one actually considers the limiting cases where the open string side reduces to a strongly coupled gauge field theory and/or the closed string side reduces to classical gravitational theory. The equivalence offers completely new methods for solving many strongly interacting theories, most notably QCD.

The very acute issue of black hole in quantum gravity was also addressed through these developments, resulting in a consensus among many theoretical physicists that quantum principle is probably not destroyed by existence of quantum black holes in string theory. A complete resolution of the problem, applicable to all type of black holes is, however, still unavailable.

Pioneers of string theory hoped that they might be able to "derive" a unique theory of universe where every fundamental quantity of nature can be predicted unambiguously and accurately. With better understanding over the last twenty years, we now begin to realize that this hope was probably mislaid. String theory is far more than a single theory for a single universe. It proved to be more of a new paradigm and framework, even more so than the ubiquitous quantum field theory was. Whether and how we can realize our own universe within this framework is a very highly constrained and difficult problem, which still suggests a vast predictability when compared to conventional model building method in particle physics and in cosmology.

Members of KIAS are active participants in current development of string theory in its most general sense. Topics include D-branes and M-branes, AdS/CFT and its application to QCD, non-perturbative nature of supersymmetric gauge theories, and physics of BPS states and wall-crossing, and building string theoretical models for elementary particles and cosmology.