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Nov 16
2009
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Physicist frequently search for unifying principles that hopeful lead to deeper, more fundamental  laws of Nature. The unification of the theory of electricity with the theory of magnetism led to an understanding of light as electromagnetic radiation. One obvious unification is between quantum mechanics and general relativity, the so-called theory of quantum gravity.
Quantum gravity is a type of quantum theory of elementary particles and their interactions that is based on the particle symmetry known as supersymmetry and that naturally includes gravity along with the other fundamental forces (the electromagnetic The electromagnetic and the weak forces are now understood to be different facets of a single underlying force that is described by the electroweak theory. Further unification of all four fundamental forces in a single quantum theory is a major goal of theoretical physics. Gravity, however, has proved difficult to treat with any quantum theory that describes the other forces in terms of messenger particles that are exchanged between interacting particles of matter. General relativity, which relates the gravitational force to the curvature of space-time, provides a respectable theory of gravity on a larger scale. To be consistent with general relativity, gravity at the quantum level must be carried by a particle, called the graviton.
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Unifying theories usually begin by exploring new realms of experience. For example, particle theories (what are the fundamental particles that matter is made of) find deeper meaning by exploring high energies (i.e. high mass ranges) using large particle accelerators.
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Quantum gravity asks the question, ``what is the behavior of gravity on extremely small scales?'' What are the properties of mini black holes and how does the force of gravity compared to other subatomic forces? These questions are particularly crucial to cosmology since the very early Universe was an environment dominated by extremely high pressures and temperatures, and the folding of spacetime on quantum scales.
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There is currently no complete theory for combined quantum and gravity, as the process of unification proved to have many more mathematical difficulties than expected. Many believe that the problems indicate that a new, much deeper theory must exist out of which quantum mechanics and general relativity emerge. However, some partial elements of a working composite of quantum mechanics and general relativity have predicted gravitational waves and Hawking radiation.
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