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Me and my brother were watching television tonight, and he asked me a question that was somewhat along the lines of this:

What would happen if the Earth was made of matter, and the moon was made of antimatter?

I tried to answer this, saying a few things about how atoms attract, and such, but I then realized I didn't actually know the answer to this.

I'm now curious as to how matter and antimatter interact gravitationally. Do they attract? Repel? I'm specifically wondering what would happen on a planetary scale, but the nitty-gritty particle interactions would be nice too.

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    $\begingroup$ Possible duplicate of physics.stackexchange.com/q/83307. That focuses on the matter-matter and antimatter-antimatter cases, but the result is the same. $\endgroup$ – HDE 226868 Oct 7 '15 at 1:31
  • $\begingroup$ @HDE226868 Maybe. I feel like that question is asking specifically about interactions between the same type of matter, where as I'm asking about the interactions between the two different types. $\endgroup$ – Ethan Bierlein Oct 7 '15 at 1:35
  • $\begingroup$ To be clear, what do you mean by anti-matter? Nowadays, it means matter composed of particles with the opposite charges. It doesn't have anything to do with negative masses in modern parlance. $\endgroup$ – innisfree Oct 7 '15 at 3:06
  • $\begingroup$ Some things to ponder: we have antimatter; it's easy to collect, at least in small quantities. If antimatter repelled gravitationally, general relativity lets you build a wormhole out of it. A wormhole allows time travel. And time travel utterly destroys the causal structure of the universe as we know it: all of science and predictability, and indeed the entire notion of time, would suddenly become meaningless. All this, if antimatter repelled gravitationally. Good thing it doesn't :) $\endgroup$ – user10851 Oct 7 '15 at 6:01
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/9371/2451 , physics.stackexchange.com/q/9375/2451 , physics.stackexchange.com/q/83307/2451 and links therein. $\endgroup$ – Qmechanic Oct 7 '15 at 6:16
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What would happen if the Earth was made of matter, and the moon was made of antimatter

To start with, our elementary particle physics standard model has antiparticles which have the same mass as particles and all the quantum numbers, charge, baryon number ..., the opposite. What happens when a particle meets an antiparticle is that the quantum numbers annihilate (add up to zero) and the energy of the two masses is freed to become other particles/antiparticles , from photons to electron positron etc, as long as the total quantum numbers add to zero.

This model is validated by innumerable data. As an antiparticla has the same type of mass , i.e. positive, the hypothesis is that antiparticles behave under the force of gravity the same as particles.

Wnen antimatter ( like antihydrogen, which has been created in the lab) meets matter (hydrogen) the same thing happens. Annihilation of quantum numbers and release of energy.

We know that the moon is composed of matter from the gross experiment of having landed on it. No explosion resulted.

Any dust, meteorite small or large, of matter hitting the moon would create explosions and thus we know it is not made of antimatter. This argument extends up to galaxies , because we would be detecting the specific annihilation radiations if an antigalaxy or even a cluster of antigalaxies existed.

Now the same gravitational interaction is a hypothesis and is being tested in the AEGIS experiment at CERN:

A system of gratings in the deflectometer splits the antihydrogen beam into parallel rays, forming a periodic pattern. From this pattern, the physicists can measure how much the antihydrogen beam drops during its horizontal flight. Combining this shift with the time each atom takes to fly and fall, the AEGIS team can then determine the strength of the gravitational force between the Earth and the antihydrogen atoms.

The AEGIS experiment will represent the first direct measurement of a gravitational effect on an antimatter system.

So current knowledge tells us that an antimatter moon would behave the same as a matter moon as far as gravity goes. Maybe AEGIS will find something different.

One should add that an antimatter moon in a matter planetary system, or where there exists matter and antimatter would not last long (in cosmological times) due to the explosions from matter meteorites falling on antimatter.

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  • $\begingroup$ Anti-matter is matter composed of anti-particles (nothing to do with negative mass). Do we know the moon isn't made of anti-particles i.e. anti-matter? $\endgroup$ – innisfree Oct 7 '15 at 8:02
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    $\begingroup$ @innisfree if the AEGIS experiment finds antihydrogen going up instead of down, then this could be interpreted as a gravitational charge with negative sign. Do you have a link for your "negative mass"? Antiparticles in the standard physics have the same mass as the particles, there is nothing negative about it. It is only the quantum numbers that have the opposite sign for antiparticles, and they keep the particles' individuality. $\endgroup$ – anna v Oct 7 '15 at 9:25
  • $\begingroup$ continued. I found a wiki article en.wikipedia.org/wiki/Negative_mass and it is a weird hypothesis in classical general relativity. According to "Such a couple of objects would accelerate without limit (except relativistic one); however, the total mass, momentum and energy of the system would remain 0." the moon is not behaving in this way, rather the classical attractive newtonian fit its trajectory perfectly, so it is not even this bizzare negative mass. $\endgroup$ – anna v Oct 7 '15 at 9:33
  • $\begingroup$ Sorry, I misread your answer. My bad. $\endgroup$ – innisfree Oct 7 '15 at 10:54

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