# Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?

As the title suggests, Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?

I am aware any mass produces and effects gravity but couldn't find anything in my searching if it is possible or theoretically possible to detect this effect remotely.

The gravitational field of small objects can be measured. In fact as far back as 1797 Henry Cavendish measured the gravitational field from lead spheres. He used pairs of spheres of mass 158kg and 0.73kg, so for a person weighing say 70kg the same method is in principle possible, although in practice people are an inconvenient shape for doing the experiment and you'd probably have to kill them to get them still enough.

However you'd need to get pretty close for the field to be measurable. In the Cavendish experiment the distances used were around 0.2m. Once you get more than a few metres away I doubt any existing instrument would be sensitive enough to measure the gravitational field from a person.

• I believe it is also important that Cavendish arranged the measurement so that the measured effect was perpendicular to the terrestrial background – dmckee Apr 24 '13 at 16:02
• @dmckee: in practice, yes. In principle, of course, you could do something like measure the difference in force between a lead weight when placed a fixed distance above and below a second lead weight. – Jerry Schirmer Apr 24 '13 at 16:06
• @dmckee: Cavendish used a torsion balance so his forces had to be acting horizontally. However I don't think there's a fundamental reason for this. It was a purely practical requirement. – John Rennie Apr 24 '13 at 16:21
• @JohnRennie, John, not fundamental but necessary for experimental reasons. The vertical measurement would require about $10^{-6}$--$10^{-8}$ relative precision, while the torsion measurement simply requires sensitivity to a very small force. These days you could probably manage the necessary precision vertically, but the measurement would be expensive, persnickity and get a low precision result despite having a fantastically precise instrument. Waste of resources. – dmckee Apr 24 '13 at 16:28
• Come to think of it, my father's employer had a balance that was good for almost $10^{-6}$ on 2-ton measurement at one point, and that was a COTS instrument (albeit an extremely expensive one). – dmckee Apr 24 '13 at 16:35

According to this site http://en.wikipedia.org/wiki/Gravimetry Gravimeters can have a accuracy of up to 0.002 mGal (= 2*10^-8 m/s^2) The gravity of a person of lets say 100kg at a distance of one meter is approximately G * m/r^2 = 6.7*10^-9 m/s^2 So that person would need to weigh at least 300kg to be detectable.

• The type of gravimeter described in the Wiki article are far from the last word in sensitivity. Even the torsion balance Cavendish used in 1797 was more sensitive! – John Rennie Apr 24 '13 at 18:28