In essence I want to know if GW can affect the momentum of baryonic matter. I know that massive objects like planets, stars, galaxies, etc will not be "ripped apart" or even disturbed by even the most massive GW from a SMBH major merger. But what about gas in the ISM or IGM? Is there any way a particle, subatomic or larger, can be "carried" away by GW as they radiate outward? Any sources or resources that show the mathematical description would also be appreciated.
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1What a great question. – Fattie Aug 10 '22 at 18:15
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This is a big plot element in Schlock Mercenary :) – Mad Physicist Sep 02 '22 at 20:48
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The classic thought experiment showing that GW carries real energy is the "sticky bead argument" due to Feynman. Two beads on a rigid rod will be moved relative to each other and the rod by the wave as it passes, and if there is friction some energy will be released.
The question of how much energy there is in a wave is a bit tricky, since energy is not well-defined in spacetimes lacking time translation symmetry, and the weak field limit is approximate anyway. See Carroll p.160- for a discussion and math.
The field doesn't give particles net acceleration, but rather makes them oscillate relative to each other. That means gas and dust will not absorb any energy (since they do not touch each other), but extended rigid bodies can turn some of that energy into mechanical vibrations. They still don't get any momentum, though.
Justin T
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Anders Sandberg
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3Great answer! If one wants to read further about sticky beads, there is in Physics SE Transfer of energy from gravity back to other "more familiar" forms of energy? and also potentially of interest there's Does shortening the path length of an excited etalon do work? What about LIGO? – uhoh Aug 09 '22 at 22:20
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1This makes me wonder where the "transition point" is between a long-wave GW and "DC" gravitational force as seen in, say, a neutron star. – Carl Witthoft Aug 10 '22 at 12:17
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3Gas touches other gas particles sometimes; the mean free path is (hopefully) finite. When the gravity wave goes through the gas, it will be compressed and stretched, changing the mean free path, causing more or less collisions, no? Or does it compress/stretch in a way that somehow avoids that? – Yakk - Adam Nevraumont Aug 10 '22 at 13:27
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A paper for reference on the matter. In it, it says, "a test particle initially at rest acquires no energy from a linearized gravitational wave.", which notably excludes non-linear effects. – Justin T Aug 11 '22 at 00:57