![]() For example, the theory does not yet have a satisfactory definition outside of perturbation theory the quantum mechanics of branes (higher dimensional objects than strings) is not understood the behaviour of string theory in cosmological settings (time-dependent backgrounds) is still being worked out finally, the principle by which string theory selects its vacuum state is a hotly contested topic (see string theory landscape). In addition, the full theory is not yet understood. String theory as a whole has not yet made falsifiable predictions that would allow it to be experimentally tested, though various planned observations and experiments could confirm some essential aspects of the theory, such as supersymmetry and extra dimensions. It is not yet known whether string theory will be able to describe a universe with the precise collection of forces and particles that is observed, nor how much freedom the theory allows to choose those details. ![]() Superstring theories include fermions, the building blocks of matter, and incorporate supersymmetry, a conjectured (but unobserved) symmetry of nature. It can also naturally describe interactions similar to electromagnetism and the other forces of nature. Interest in string theory is driven largely by the hope that it will prove to be a consistent theory of quantum gravity or even a theory of everything. String theory is thought to include some 10, 11 or 26 dimensions, depending on the specific theory and on the point of view. Furthermore, all string theories predict the existence of degrees of freedom which are usually described as extra dimensions. In addition to strings, string theories also include objects of higher dimensions, such as D-branes and NS-branes. Strings can split and combine, which would appear as particles emitting and absorbing other particles, presumably giving rise to the known interactions between particles. This object can vibrate in different modes (just as a guitar string can produce different notes), with every mode appearing as a different particle ( electron, photon etc.). Thus, any particle should be thought of as a tiny vibrating object, rather than as a point. The basic idea behind all string theories is that the fundamental constituents of reality are strings of extremely small scale (possibly Planck length, about 10 -35 m) which vibrate at specific resonant frequencies. String theory strongly suggests the existence of ten or eleven (in M-theory) spacetime dimensions, as opposed to the relativistic four (three spatial and one time). Studies of string theory have revealed that it predicts not just strings, but also higher-dimensional objects ( branes). However, with the construction of the Large Hadron Collider near Geneva, Switzerland scientists may produce relevant data. No experimental verification or falsification of the theory has yet been possible, thus leading many experts to turn to the alternate model, Loop quantum gravity. ![]() Moreover, string theory appears to be able to "unify" the known natural forces ( gravitational, electromagnetic, weak and strong) by describing them with the same set of equations. By removing this assumption and replacing the point-like particles with strings, it is hoped that string theory will develop into a sensible quantum theory of gravity. String theorists are attempting to adjust the Standard Model by removing the assumption in quantum mechanics that particles are point-like. ![]() String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects ( strings) rather than the zero-dimensional points ( particles) that are the basis of the Standard Model of particle physics. Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory ![]()
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