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"In String Theory, the myriad of particle types is replaced by a single fundamental building block, a `string'. These strings can be closed, like loops, or open, like a hair. As the string moves through time it traces out a tube or a sheet, according to whether it is closed or open. Furthermore, the string is free to vibrate, and different vibrational modes of the string represent the different particle types, since different modes are seen as different masses or spins." (M-theory, the theory formerly known as Strings, http://www.damtp.cam.ac.uk/user/gr/public/qg_ss.html)
Superstring Theory, M Theory and Brane Theory all evolved from the original String Theory back in the mid 1980s. While mathematical physicists see M Theory as a potential Theory of Everything, the philosopher / metaphysicist would seriously question this claim that String Theory (and its derivatives) are descriptive of real things. M Theory assumes that 'strings / membranes' somehow exist in a multi dimensional universe, but only three dimensions are visible and the remaining 'strings' are 'curled up' and cannot be observed.
However, philosophy and metaphysics both require that a complete description of Reality be founded on ONE thing (that we commonly experience, not on many discrete things / strings) to explain the necessary connection between the many things. The likely reason for the partial success of String Theory is that, like Quantum Theory and the Wave Structure of Matter, it utilises Wave Theory and resonant frequencies. The strings can only vibrate at discrete frequencies which correspond to different 'particles' and their energy states.
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M-theory (the "M" stands for the mother of all theories, magic, mystery, membrane, or matrix, depending on the source) is an adaptation of superstring theory developed by Ed Witten of Princeton and Paul Townsend of Cambridge. Townsend and Witten's version could potentially be the unified field theory sought by Einstein for the last 40 years of his life: a simple equation that would reconcile incompatible aspects of his theory of relativity and quantum theory to explain the nature and behavior of all matter and energy. Applications of this knowledge could, through unlocking nature's secrets, enable future technologies that currently are only spoken of within the realm of science fiction: an inexhaustible source of clean energy, and time travel, for example.
Superstring theory (sometimes just called string theory) has as its basic premise the belief that the four fundamental forces of nature (gravity, electromagnetism, and strong and weak nuclear forces), as well as all matter are simply different manifestations of a single essence. This essence, the material making up all energy and matter, is thought to consist of tiny (a hundred billion billion times smaller than the nucleus of an atom) vibrating strings that exist in a multi-dimensional (10 or 26 dimensions) hyperspace. The extra dimensions (beyond the ones we recognize: three spatial dimensions and time) are thought to be compactified, or curled up, into tiny pockets inside observable space. The particular vibrations of the strings within this multidimensional hyperspace are thought to correspond to particles that form the basis of everything - all matter and energy - in existence. (http://whatis.techtarget.com/definition/0,,sid9_gci549055,00.html)
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As has already been mentioned, to have a chance of being realistic, the six extra space dimensions must curl up into a tiny geometrical space, whose size should be comparable to the string length Lst..
Since space-time geometry is determined dynamically (as in general relativity), only geometries that satisfy the dynamical equations are allowed.
The HE string theory, compactified on a particular kind of six-dimensional space, called a Calabi--Yau manifold, has many qualitative features at low energies that resemble the standard model. In particular, the low mass fermions (identified as quarks and leptons) occur in families, whose number is controlled by the topology of the CY manifold.
These successes have been achieved in a perturbative framework, and are necessarily qualitative at best, since non-perturbative phenomena are essential to an understanding of supersymmetry breaking and other important matters of detail.
(http://www.theory.caltech.edu/people/jhs/strings/str142.html)
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In 1984-85 there was a series of discoveries that convinced many theorists that superstring theory is a very promising approach to unification. Almost overnight, the subject was transformed from an intellectual backwater to one of the most active areas of theoretical physics, which it has remained ever since.
By the time the dust settled in 1985, it seemed clear that there are five different superstring theories, each requiring ten dimensions (nine space and one time). To have a chance of being realistic, the six extra space dimensions must curl up into a tiny geometrical space, whose size should be comparable to the string length Lst..
A string's space-time history is described by functions Xm(s,t) which describe how the string's two-dimensional 'world sheet,' represented by coordinates (s,t), is mapped into space-time Xm. There are also functions defined on the two-dimensional world-sheet that describe other degrees of freedom, such as those associated with supersymmetry and gauge symmetries.
Surprisingly, classical string theory dynamics is described by a conformally invariant 2D quantum field theory. (Roughly, conformal invariance is symmetry under a change of length scale.) What distinguishes one-dimensional strings from higher dimensional analogs is the fact that this 2D theory is renormalizable (no bad short-distance infinities). By contrast, objects with p dimensions, called 'p-branes,' have a (p+1)-dimensional world volume theory. For p > 1, those theories are non-renormalizable. This is the feature that gives strings a special status, even though, as we will discuss later, higher-dimensional p-branes do occur in superstring theory.
In the euphoria following the first superstring revolution in 1985, some of the less experienced participants in the enterprise thought that we were on the verge of constructing a complete fundamental theory of the physical world. To put it mildly, I found this naive. In this setting, the phrase "Theory of Everything" was introduced and propagated by the public media. This was very unfortunate for several reasons.
The TOE phrase is very misleading on several counts. First of all, the theory is not yet fully formulated, and when it is (which might still take decades) it is not entirely clear that it will be the last word in fundamental physics.
Furthermore, even if the theory is a complete description of quantum dynamics, it seems unlikely that it will also provide a theory of initial conditions, which is another key ingredient required to explain why we observe the particular universe that we do.
But even if a theory of initial conditions is also obtained, there will still be much about this universe that cannot be explained. Many things, such as our very existence, are a consequence of the inherent quantum indeterminacy of nature. I believe that cannot be overcome. Maybe that is just as well, because if we had old-fashioned classical determinism, the future would be fully determined, which would undermine our humanity.
There is also a more mundane sort of unpredictability that is also to be expected. Many of the things that the theory predicts unambiguously in principle could require intractable calculations. Part of the art of physics is to identify those things that can be calculated.
The other reason the TOE phrase upset me is that it alienated many of our physics colleagues, some of whom had serious doubts about the subject anyway.
Quite understandably, it gave them the impression that people who work in this field are a very arrogant bunch. Actually, we are all very charming and delightful.
(http://www.theory.caltech.edu/people/jhs/strings/)
Editor: Haselhurst
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