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L E S AGE G RAVITY by Robert Nemiroff Michigan Tech

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Physics X: About This Course Officially "Extraordinary Concepts in Physics" Being taught for credit at Michigan Tech o Light on math, heavy on concepts o Anyone anywhere is welcome No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at: "Starship Asterisk" then "Physics X" o

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G RAVITY Is the mechanism for gravity known? 1. Yes, it is the changing geometry of spacetime. 2. No, gravity still requires "action at a distance". 3. Maybe and sometimes.

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L E S AGE G RAVITY "There is no model of the theory of gravitation today, other than the mathematical form." - Richard Feynman (The Character of Physical Law 1965) Le Sage gravity was an early attempt to provide a mechanism for gravity. was discussed in detail in the mid-1700s by Le Sage. was preceded by several similar theories, most notably one by Newton's contemporary Fatio. shares some attributes with virtual quantum forces.

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L E S AGE G RAVITY has problems -- no classical formulation works. was never accepted as a serious mechanism for gravity. is occasionally reinvented today by people who don't know its history or flaws. remains useful today as o a toy model. o "mock gravity" in some physical situations.

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L E S AGE G RAVITY Basics: A sea of unseen particles moves in all directions. A single body experiences no net force as it is impinged from all directions. o Interestingly, this does not depend on the object's shape. Two bodies will create "shadows" in this sea of particles that will cause mutual attraction.

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L E S AGE G RAVITY Basics: Objects are mostly empty space, so interaction cross-section scales not as area but volume (assuming constant density). o Correctly shows proportionality to object mass. Scattering is somehow unimportant. Le Sage gravity correctly yields an inverse square law in these approximations.

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L E S AGE G RAVITY L E S AGE G RAVITY : P ROBLEMS Basics problems: Masses must be (almost) completely transparent to these "ultramundane corpuscles". o or Le Sage gravity will not be proportional to mass. Reflected corpuscles cannot create force. o or Le Sage gravitational force will not drop as inverse square. Absorbed corpuscles would heat any object to explosion quickly. Drag force expected for motion through the sea. No external evidence for these corpuscles.

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L E S AGE G RAVITY L E S AGE G RAVITY : P ROBLEMS Basics problems (continued): Range: corpuscular interactions would limit the range of this force. Aberration: orbital aberration of Le Sage gravity would cause spiraling out, unless corpuscles moved much faster than light. Lunar Theory: doesn't match Moon's motion. o Relativistic effects unaccounted for. o Binding energy effects unaccounted for.

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L E S AGE G RAVITY L E S AGE G RAVITY : U SEFUL V ISUALIZATION T OOL Sometimes useful for visualizing force directionality. For example, using Le Sage gravity, it is easy to picture that everywhere inside a massive spherical shell there is no net gravitational acceleration. This is because the shell blocks Le Sage particles the same in all directions.

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L E S AGE G RAVITY L E S AGE G RAVITY : M OCK G RAVITY Mock gravity, a distant cousin of Le Sage gravity is actually useful in some astrophysical situations. One such situation is two opaque dust grains in a photon field. The dust grains attract because they create shadows in this field. The force is only proportional to the area of the grain, however, and not the mass of the grain.

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G EORGES -L OUIS L E S AGE Developed an early version of the electric telegraph in 1774 o predecessor of the Internet o had 26 wires strung between two rooms one for every letter

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