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Our world is ruled by four fundamental forces: the gravitational pull of massive objects, the electromagnetic interaction between electric charges, the strong nuclear interaction holding atomic nuclei together and the weak nuclear force causing unstable ones to fall apart.

Physicists have quantum theories for the last three of them that allow very precise calculations of phenomena on the smallest, subatomic scales. However, does not fit into this scheme. Despite decades of research, there is no generally accepted quantum theory of gravity, which is needed to better understand fundamental aspects of our universe.

In this Q&A, Particle Physics and Astrophysics Professor Lance Dixon of Stanford University and the Department of Energy's SLAC National Accelerator Laboratory explains one approach to developing such a theory, called .

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