The reasons for this are obvious: On the one hand, there are no independently known mass distributions on length scales larger than the solar system, and on the other hand, it is difficult to get enough matter in close enough proximity to obtain a background-free gravitational signal at length scales smaller than 1 mm. We review these speculations in Section 2.Īlthough it is conventionally assumed that the ISL should be valid for separations from infinity to roughly the Planck length ( m), until a few years ago this assumption had only been precisely tested for separations ranging from the scale of the solar system down to a few millimeters. But the remaining theoretical problems have focused attention on possible new phenomena that could show up as deviations from the familiar inverse-square law (ISL) of gravity, generally at length scales less than a few millimeters, but sometimes also at astronomical or even cosmological distances. Because quantum field theories cannot describe gravitation and General Relativity predicts an infinite spacetime curvature at the center of a black hole, neither of these two standard models is likely to be truly fundamental.Ĭonnecting gravity with the rest of physics is clearly the central challenge of fundamental physics, and for the first time we have a candidate theory (string or M-theory) that may unify gravitation with particle physics. The strong, weak, and electromagnetic interactions are explained as results of the quantum exchange of virtual bosons, whereas the gravitational interaction is explained as a classical consequence of matter and energy curving spacetime. There is a broad consensus that the two standard models are incompatible. Furthermore, gravitation is not included, and in fact not includable, in the imposing quantum field theory that constitutes the standard model of particle physics. Recently, a completely unexpected and fundamentally new gravitational property was discovered using distant Type Ia supernovae: the apparent acceleration of the Hubble expansion ( 1, 2), which is as yet unexplained. Yet some three centuries after Newton, gravitation remains one of the most puzzling topics in physics. Since then, General Relativity has passed all experimental tests and is today the standard model of gravitation. This theory stood virtually unchallenged until Albert Einstein developed his relativistic theory of gravitation in 1917. Isaac Newton's Theory of Universal Gravitation connected terrestrial phenomena (the “falling apple”) with astronomical observations (the “falling Moon” and Kepler's Laws). Gravitation was the first of the four known fundamental interactions to be understood quantitatively and the first “grand unification” in physics.
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