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Comment by zbanks | Hacker News Reader

Has anyone heard if they managed to observe the effect of gravity on the particles? The Nature article was vague...

I remember hearing that this was one of the reasons why they needed to trap antimatter for observable lengths of time. To my understanding, they aren't completely sure yet which way antimatter "falls."

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It will still be a while before this is directly observed.

We're fairly sure that it "falls" downwards for a wide range of theoretical reasons and also due to the 1987 observation of gravitational lensing in neutrinos and antineutrinos from a supernova. If we observe the opposite in a lab then it will be incredible because we'll have to reevaluate a lot of what we think we know about physics. If we observe it "falling down" then it will be an important result but not at all revolutionary.

Thanks. I guess it's similar to the whole P != NP issue.

"falling down" - does that mean gravity is an attractive force between matter-antimatter or antimatter-antimatter (or both)?

Antimatter has opposite electrical charge (and magnetic moment) from normal matter. It should have the exact same mass and interaction with gravity as normal matter.

An anti-electron is called a positron because it has a positive charge instead of negative. Antiproton (sometimes called negatron, but rarely) is just a proton with opposite charge. You can even make a hydrogen atom out of an antiproton and a positron, called antihydrogen. https://secure.wikimedia.org/wikipedia/en/wiki/Antihydrogen

I meant falling downwards towards the surface of the earth which would experimentally show that there is an attractive force between matter and antimatter. Experimentally showing that antimatter gravitationally attracts antimatter is next to impossible for technical reasons but there is even less doubt about this than about matter attracting antimatter. No matter how sure we think we are about something we can also be wrong though. Experiment gives us the final word (albeit with statistical limitations) and there have been numerous times in history where established theories have been flipped upside down by experiments.

This is actually somewhat of a complicated topic. The overwhelming expectation is that anti-matter has the same inertial and gravitational mass as ordinary matter does. However, there are some intriguing possibilities for matter that doesn't.

Consider what happens if an object has "negative mass" though. It would be repelled by ordinary matter. OK, simple enough though, right. But then what happens? Well, F = ma right? So the repulsive force would repel ordinary matter, but it would result in an attractive acceleration of the negative mass. Now, if it turned out that gravitational and inertial mass could be different, that would be an incredible result as well.

(only a very rough grounding in particle physics here)

it offers an explanation for the abundance of matter - if large enough quantities of matter and anti-matter are created, and can gain enough separation, then although the attractive forces of the electromagnetic attraction are strong, the result of gravity repulsing opposite matter types would separate them. so matter/anti-matter creation would be symmetric, but all the anti matter would be beyond the edge of the observable universe.

lets just hope the LHC doesn't create a mini black hole which eats up the planet, before we find out..!

To further venture down the rabbit hole: it's unlikely that anti-matter actually has negative inertial mass though. Since that would result in an electrostatic repulsion from normal matter (which would, for example, prevent an anti-proton from annihilating with a nucleus). The observational evidence is pretty strong that anti-matter has at least a positive inertial mass. There is a remote possibility that inertial and gravitational masses can be different, which would be a very unexpected result.

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