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Comment by saagarjha | Hacker News Reader

Forgive me if this sounds presumptuous, but is this experiment literally trying to answer "does antimatter attract normal matter"? To me, this seems like a pretty simple thing to test–why hasn't this been done before?

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The strength of the gravity between a proton and an (anti-)proton is 30+ orders of magnitude weaker than electromagnetism. The only reason we are able to notice gravity at all is because the electromagnetic forces between macroscopic amounts of matter (~10^23 protons) cancel out essentially perfectly, leaving only the gravitational force remaining. But the amounts of antimatter we can create and collect is extremely microscopic, so we can't use this canceling trick. Furthermore, we can't keep antimatter from annihilating for long enough to take long detailed measurements.

Why not use neutrons and antineutrons, or any other neutral particle that has mass?

Antimatter is generated at extremely high (relativistic) speeds and generally has to be slowed down ("cooled") in order to make careful measurements or to even collect multiple particles together. And that can't be done unless you can steer the particle with electromagnetism.

If you just generated individual relativistic anti-neutrons and tried to watch them follow ballistic trajectories in gravity, I suspect they'd just look like perfectly straight lines up to the limits of your detector resolution. Not sure though.

(The only other quasi-stable neutral particles are neutrinos, and those are essentially non-interacting with any detector. They leave relativistically and you'll never see them again.)

Good point; I forgot about the process of actually creating the antimatter.

what I get from the article is that all 3 experiments are doing something new with antimatter in some way. So I imagine the extend to which we can actually make, handle and measure antimatter to be a very determining factor in what sort of experiments can be done

Neutrons can be contained in a magnetic bottle, even if they don't last long in isolation. Presumably the same is true for antineutrons, although I don't know how hard it would be to slow them with magnetic fields...

Yea, thanks for mentioning. I believe those bottles only work when the neutrons are already at very slow speeds. They exploit the strong force and the residual electromagnetic "handle" neutrons have by virtue of being built from charged components (quarks), which leave them with a magnetic dipole moment.

If you could produce antineutrons, how would you contain them?

Place your container in freefall and then watch the antimatter collide with the walls of your (shaped) container due to the attraction between the container and the antimatter?

A single neutrino passing through a light year of solid lead has only about a 50% chance of actually interacting with it.

Also, you'll need the neutrinos to start out at rest; they tend to travel near the speed of light.

I’m talking about neutrons, not neutrinos.

In that case you'll only have to figure out how to cool the antineutrons down enough to ensure that they aren't colliding with the walls simply due to random thermal motion (remember, neutrons don't stick together on their own, so there's no vibrating bonds to dump potential energy into, all that thermal energy in 1.5kT will basically get dumped into kinetic energy, and they don't have much mass; at room tempature they're whizzing around at about 2 km/s). You'll also run into the issue that regular free neutrons (and also anti-neutrons) are stable for only 15 minutes, after which point they'll decay. Note you'll also need to cool them off in that 15 minutes too (though you might be able to be generate them in a way that they start off cool).

So a fun problem that comes up is how long will it take for the neutrons to collide assuming they're only being accelerated by the mass of the container.

Maybe if you couldn’t contain them you could focus them?

How do you propose focussing something that is uncharged and thus insensitive to both electric and magnetic fields?

Fresnel lens made of neutron reflectors

Yes. And it's a difficult question to ask because the effects of gravity on subatomic particles is incredibly hard to measure. Thats why they need to make anti-hydrogen to remove the effects of other forces on the particles because it's electrically neutral. If it touches matter of any kind it will instantly annihilate so you need near perfect vacuums, without electric/magnetic fields.

Lots of stuff you'd think is "pretty simple" is still an area of active experimental research. For example, ongoing experiments to test whether inertial mass is the same as gravitational mass.

Containing antimatter, let alone doing stuff with it and measuring effects e.g. Just gravity is pretty fucking difficult.

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