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I believe I have been holding a misconception for a long time, surely dating back to some sketchy science fiction. I would like to verify what truth there is in this. Say you had person $A$ floating in a vacuum, and an exact copy of person $A$ but made out of antimatter, call them person $B$, was at an arbitrary distance $r$ from $A$. Assume they have zero relative velocity at first. The belief I have held and have not doubted until now was that person $A$ and person $B$ would attract each other and eventually mutually annihilate, and that this attraction would be quite powerful (much stronger than gravitation).

I know this would be (more or less) true in the case of an electron and a positron, say, simply because they have opposite charge, and the electromagnetic force would take care of the rest. But in the above scenario, persons $A$ and $B$ have no charge. Part of me assumes they would simply stay stationary (ignoring gravity), but another part of me naively believes they will attract each other, because "matter and antimatter want to annihilate". Or that the electrons in person $A$ are somehow more attracted to all the positrons in person $B$ than by what binds them to the protons in their respective atoms, depending on $r$.

What's the correct way to think about this? And is there a distance $r$ where attraction is guaranteed? What would that be?

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    $\begingroup$ Neutral atoms can attract each other stronger than gravity by the van der Waals interaction. It will be applicable here too. There is no excess attraction just because they are antimatter. $\endgroup$
    – naturallyInconsistent
    Commented Apr 23 at 7:16
  • $\begingroup$ @naturallyInconsistent So at distances in the scale of meters, $A$ and $B$ would not move? $\endgroup$
    – Paul Cusson
    Commented Apr 23 at 7:18
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    $\begingroup$ Good question! The attractive force is the London dispersion force which is calculated by treating the perturbation of one atom's wavefunction by the other atom. I have never seen this calculated for an atom and an anti-atom, so I cannot answer definitively, however I think an attractive force would still exist. As you say, it would be negligibly weak at everyday distances. $\endgroup$
    – John Rennie
    Commented Apr 23 at 7:27
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    $\begingroup$ I thought the usual reason for seeing antimatter "attracted" to normal matter is that most of the time we are talking individual particles, not neutral atoms. A lone positron in a sea of normal matter is going to find an electron somewhere. $\endgroup$
    – SoronelHaetir
    Commented Apr 23 at 15:53
  • $\begingroup$ A matter and an anti-matter atom could attract each other also by van der waals force. VdW is the residual force of the EM interaction between the charged parts of the generally neutral composite objects. This would be essentially different in a matter - antimatter relation, as in matter-matter or antimatter-antimatter relations. Probably not stronger but very different. Of course, this effect would last very short, because the million or billion times stronger electron-positron annihillation of the outer electron shells would explode the whole system. $\endgroup$
    – peterh
    Commented Apr 24 at 2:06

3 Answers 3

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As far as we know, antimatter is affected by the same four fundamental forces as matter - the strong, weak, electromagnetic and gravitational forces. There is no mysterious fifth force that attracts matter to antimatter or vice versa. So if we have two electrically neutral objects, one made of matter and one made of antimatter, and they are sufficiently far apart that we can ignore the strong and weak forces, then the only force that would cause them to be attracted to one another is gravity.

The experimental evidence we have so far strongly suggests that gravity acts on matter and antimatter in exactly the same way. However, this is an ongoing area of research - see this Wikipedia article for more details.

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  • $\begingroup$ I think this answer could be improved by explaining/estimating what we should expect if the distance between the objects is not that large. Having clouds of electrons around one object and of positrons around the other should intuitively lead to some net attraction, which (to me) would be the interesting part. $\endgroup$
    – Seb
    Commented Apr 23 at 23:27
  • $\begingroup$ Interestingly, Wikipedia doesn't yet seem to have results from September 2023 that almost without doubt prove the last paragraph in the answer above is correct at least for anti-hydrogen. Citation: Anderson, E.K., Baker, C.J., Bertsche, W. et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023). doi.org/10.1038/s41586-023-06527-1 $\endgroup$
    – Stax
    Commented Apr 24 at 1:45
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    $\begingroup$ No, the objects have to be also sufficiently apart so that we can ignore dipole-dipole interactions arising from uneven electric charge distributions, and dispersion interactions due to fluctuating dipoles. What you said does hold for spherically symmetric elementary particles, but not for composite particles like atoms, even if they are spherically symmetric. $\endgroup$
    – wzkchem5
    Commented Apr 24 at 11:54
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Antimatter isn't attracted to normal matter by any extra special forces (as far as we currently know), it still obeys the four fundamental forces. The attraction would be the same as if they were both regular matter or both antimatter. Keep in mind that this is only as far as we know though, it is completely possible that we discover contrary evidence at some point in the future. This has been verified for anti-hydrogen if I remember correctly, and I don't see why it'd change based on the element.

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To add to the existing answers:

Scientists are still seriously exploring whether other stars, galaxies, etc. are composed of matter or antimatter. See for example this paper or this article (about that paper). If there were some relevant attraction between anti-stars and stars, this question would arguably have been long answered. (Mind that nobody seems to strongly favour anti-stars since it’s difficult to explain where they are coming from, but that doesn’t make for direct evidence.)

As for your hypothetical scenario, the people floating in space would rather experience a small repelling force. The reason for this is both would constantly evaporate particles into space and if those meet each other, they would annihilate and release energy as photons in more or less random directions. If those photon hit the people, they would push them people slightly away from the annihilation point. As the annihilation points are statistically more likely to be between the two people, they are pushed away from each other. This also holds in a more extreme variant if they touch each other: You have a considerable explosion at the point of contact pushing the people away from each other.

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  • $\begingroup$ Dropping an anti-star into a common or garden variety black hole might be interesting :-). Or just combining two stars of opposing anti-ness. $\endgroup$
    – Russell McMahon
    Commented Apr 24 at 16:44
  • $\begingroup$ @RussellMcMahon: All of this made me wonder whether anybody has ever computed to what extent a an anti-star and a star annihilate before the resulting explosion pushes them apart. $\endgroup$
    – Wrzlprmft
    Commented Apr 24 at 17:00
  • $\begingroup$ @Wrzlprmft I think that a drop of water on a very hot skillet would be a pretty good analogy. $\endgroup$
    – hobbs
    Commented Apr 24 at 21:20
  • $\begingroup$ @Wrzlprmfto As small as possible a black hole and ingesting an antimatter star should deal with the Leidenfrost effect. Difference in Hawking radiation, if any, compared to ordinary matter, may be interesting. $\endgroup$
    – Russell McMahon
    Commented Apr 25 at 10:00

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