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There are some theories of gravity which explain it as emerging from other fundamental forces. To better understand the evidence for and against these theories, I would like to have a better understanding of evidence for the mainstream assumption that gravity affects all particles to a degree which is proportional to their mass.

Obviously, most of our observations of gravity measure its influence in attractions between large objects consisting of atoms.

Now, in an answer to the question of whether subatomic particles are affected by gravity, Alan Rominger describes experiments which appear to have isolated the influence of gravitaty on ultracold neutrons. That is very interesting.

But assuming these results are correct, it seems consistent with current evidence that gravity might be a weak attraction between hadrons (protons and neutrons), not affecting other particles.

Do we, for example, have direct evidence that gravity affects electrons? (Please don't say "the Millikan oil drop experiment"!)

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    $\begingroup$ Surely the percentage of electrons differs enough between carbon and gold that feathers would fall measurably slower in a vacuum if gravity didn't attract electrons. $\endgroup$ – Peter Shor Jun 2 '16 at 20:13
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    $\begingroup$ journals.aps.org/pr/abstract/10.1103/PhysRev.168.737 $\endgroup$ – Count Iblis Jun 2 '16 at 20:19
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    $\begingroup$ In fact, Wikipedia says this experiment was done using aluminum and platinum, which is clearly a big enough difference to account for electrons.. $\endgroup$ – Peter Shor Jun 2 '16 at 20:20
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    $\begingroup$ @PeterShor: I'm not so sure about that. By my (admittedly quick) calculation 1 kg of Be-9 contains 0.00024 kg of electrons. 1 kg of U-238: 0.00021 kg of electrons. The difference is small. $\endgroup$ – Gert Jun 2 '16 at 22:18
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    $\begingroup$ Braginski and Panov. The Wikipedia table says they tested aluminum and platinum (which give electron concentrations comparable to Be and U), and the experiment was accurate to one part in 10^12. This accuracy has apparently been disputed by other experimenters, but you only need one part in 10^5 to show electrons are affected by gravity. $\endgroup$ – Peter Shor Jun 2 '16 at 23:58

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