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According to Newtonian gravity (in the absence of other forces or obstacles) a free electron (not moving at high velocity) should (because it has mass) fall towards the Earth's centre with the same acceleration as a hammer, feather, atom, neutron, proton, or any other massive object. (I think that General Relativity predicts a similar behavior).

I would like to know what evidence (experiemental or observational) there is, if any, to demonstrate that electrons do conform to Newtonian gravity in the same way that more massive objects do.

I suspect that it is difficult to obtain such evidence at Earth's surface because of the presence of electric and magnetic fields which would be difficult to compensate for at the required accuracy.

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  • $\begingroup$ Would the famous oil drop experiment qualify? en.m.wikipedia.org/wiki/Oil_drop_experiment $\endgroup$
    – KF Gauss
    Sep 23, 2020 at 16:01
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    $\begingroup$ @KF Gauss I assume that (given the mass of the electron is so small compared to the mass of a Milikan oil droplet) the gravitational force ($F = m*a$) on the electron is negligible compared to the gravitational force on the droplet. $\endgroup$
    – steveOw
    Sep 23, 2020 at 16:08
  • $\begingroup$ That's fair. Do you limit your question to electrons, or would another charged particle (proton, muon, etc.) be OK? $\endgroup$
    – KF Gauss
    Sep 23, 2020 at 16:43
  • $\begingroup$ @KF Gauss - mainly electron but also interested in positron. $\endgroup$
    – steveOw
    Sep 23, 2020 at 16:55

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There are several experiments and some have the similar approach: In the electron experiment the gravitational potential difference $mgh$ is cancelled by the gravitationally induced electric potential from the electrons on the walls and therefore in pure gravity one would measure 0 acceleration. Basically the system consists of a hollow tube and electron source. For more experimental details I suggest the article Experiments to determine the Force of Gravity on Single Electrons and Positrons by Fred C. Witteborn & William M. Fairbank.

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    $\begingroup$ Thanks for the source (which I sadly cannot access at present). [Elsewhere](www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/fairbank-william.pdf) I found this:- "the effort to measure the force of gravity on electrons and positrons was never completed although four Ph.d. theses were generated by the work. the measurement of the force of gravity on falling, singly ionized helium atoms was convincing; the force on electrons and positrons less so...(contd) $\endgroup$
    – steveOw
    Sep 23, 2020 at 16:16
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    $\begingroup$ (contd)..."in order to understand how it might have been possible to measure the force of gravity on such light particles, surface states inside copper tubeswere invoked to shield the interior of the tube from the known patch effect electric fields. follow-up work by the last two graduate students did not confirm the earlier results; however, there were indications of a transition occurring within the copper drift tube at low temperatures"" $\endgroup$
    – steveOw
    Sep 23, 2020 at 16:18
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    $\begingroup$ Many thanks, got it :) Also see answer & comments by Lewis Miller "Fairbanks and his group continued to tweak this experiment until shortly before his death in 1989. The original claim was that they established normal gravitational behavior for both the electron and the positron. After his death his team reached the conclusion that their result was not definitive for the positron but was got the electron" in this (closed) post physics.stackexchange.com/questions/222998/… $\endgroup$
    – steveOw
    Sep 23, 2020 at 16:37
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    $\begingroup$ A recent historical review by Michael Holzscheiter: https://indico.cern.ch/event/361413/contributions/1776296/attachments/1137816/1628821/WAG2015.pdf $\endgroup$
    – steveOw
    Sep 23, 2020 at 17:09

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