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Currently, the only ways to measure gravity is on quite large scale. So these experiments measure gravity averaged between vast systems of particles. Of course, such an averaging results¹⁾ in a spherically-symmetric field.

¹⁾ This neglects interaction between particles which may have had a chance to “align” them.

So how much (if any) the current measurements restrict the possibilities of asymmetry²⁾ on the level of individual particles?

²⁾ I never saw such a mechanism for antigravity “invented in SF”. If the asymmetry is so large that the interaction is repelling in some directions, then “suitably aligning³⁾ the particles of a car” could make it fly.

³⁾ Of course, this may require applying a lot of energy.

(Having small artificial satellites orbiting other planets allows to clock the solar system quite precisely. — However, the huge number of particles in Sun may make this approach useless. I cannot estimate quickly whether existing gravity experiments on the Earth orbit would be more productive.)

Update: one should distinguish two types of asymmetry. In general, the interaction may depend on “orientation” of two particles (relative to the line connecting the particles). Call this orientation as grin (for what follows it is not relevant whether this is a vector/direction/tensor/whatever). Note that we can take an average of the interaction over all possible grins of one particle.

In one possible case, this would make the interaction independent of the grin of the other particle. In the other case, a certain “residual asymmetry” still remains after such averaging.

Anyway, it seems that the first case should be much harder to refute than the second one…

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    $\begingroup$ Remember that the motion of celestial bodies were predicted looong before they were acknowledged to be made up of atoms. $\endgroup$
    – Marius Ladegård Meyer
    Commented Apr 22 at 5:01
  • $\begingroup$ … gravity averaged between vast systems of particles. … Of course, such an averaging results in a spherically-symmetric field. Not in general. Adding up the gravity of all the particles in a cube doesn’t produce a spherically symmetric field. Adding up the gravity of a spherically symmetric mass distribution does produce a spherically symmetric field, but is that surprising? $\endgroup$
    – Ghoster
    Commented Apr 22 at 5:06
  • $\begingroup$ @Ghoster: indeed, gravity created by the particles in the cube is not symmetrical — so I should have stated it more clear. What I meant is that the result from “particles creating asymmetric fields” is the same as from “particles creating spherically symmetric field” — provided their orientations are sufficiently mixed up (e.g., by the thermal noise). $\endgroup$
    – Ilya Zakharevich
    Commented Apr 22 at 5:28
  • $\begingroup$ It seems that my initial mental picture was defective: somehow, I was thinking of tidal forces as what would make the (hypothetical) particles “orient w.r.t. gravity” However, such asymmetry seems to lead to a breakage of the equivalence principle, so one should look instead at “the whole gravitational potential”. And the escape velocity on Earth’s orbit being about 48 km/s, even a 0.005% asymmetry would not be hidden by the thermalization… (Wait: what about 300 km/s escape velocity from the Milky Way?!…) $\endgroup$
    – Ilya Zakharevich
    Commented Apr 22 at 9:18

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We can measure gravitational force on an individual atom, so does that confirm that it doesn't need any special alignment?

Here's an experiment measuring the gravitational force between individual atoms and a centimeter-scale piece of metal, but not between one atom and another so it might not be strong enough evidence for what you're asking. https://www.nature.com/articles/nphys4189

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I can find a very coarse argument which may show that “too strong” an asymmetry is not possible (here I discuss “the residual” asymmetry in the language of the Update to my question). It is probably enough to reject the case of the residual asymmetry “as strong as causing antigravity in some directions”. But it might also lead to much finer estimates too. Below is a sketch.

First of all, such an asymmetry seems to break the equivalence principle — hence it affects not only tidal forces, but also the global gravitational potential. The escape velocity from our galaxy is ∼500 km/s; adding to this the gravity of the Local Group leads to numbers closer to 1,000 km/s. CONCLUSION if the grins of the Local Group are not (strongly) randomized, then the asymmetry of the interaction leads to enormous energy-penalty for a particle to be oriented “not in the best possible way”. This penalty does not allow randomization of the grin of a particular particle even when thermalized to inner star temperatures.

With residual asymmetry, the same holds even if grins are strongly randomized. CONCLUSION: then the grins of all the particles in the Local Group are “polarized” according to the global gravitational potential.

But if the grins of particles in both bodies have a very strong directional preference, there is going to be no masking of the asymmetry of interaction (due to randomization of the grin). So the fact that the “macroscopic” observations are spherically symmetric implies that (under our assumptions) the same hold for the microscopic interaction.

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