[samhuk]

Key take aways (for layman):

* Previous studies have used charged antimatter like positrons and antiprotons, which they imply is kind of silly because the electromagnetic force is 10^42 times stronger than gravity, so you have to set up absolutely impossibly precise electromagnetic confinement apparatus to measure the relatively tiny gravitational force

* So instead, they formed anti-hydrogen (which is neutral), and shot them (10^6 at a time, as my understanding of the text goes?) into a vertical magnetic trap

* They waited for the anti-hydrogen to either "rise up" or "sink down" to the top or bottom walls of the apparatus and measured the frequency of annihilations

* They biased the vertical magnetic field to various values, to see, at what magnetic field bias, the "top" and "bottom" annihilations were exactly 50/50.

* If anti-matter is repulsive, they would expect to need a magnetic field bias that would "help the atoms stay down" to get to the 50/50 "top" and "bottom" rate.

* They measured that they needed a magnetic field bias applied to the anti-hydrogen equivalent to "pushing them up" with 0.75g (+/- 0.25g or so), so anti-matter is attractive. No new physics.

* 10^-13 % chance that anti-matter is repulsive

* Rules out quite a lot of cosmological work that used repulsive antimatter to explain various troublesome cosmology roadblocks (dark energy, etc.)

[phkahler]

>> They measured that they needed a magnetic field bias applied to the anti-hydrogen equivalent to "pushing them up" with 0.75g (+/- 0.25g or so), so anti-matter is attractive. No new physics.

Antimatter is attracted to matter. Isn't it still an open question if matter is attracted to antimatter, and if antimatter is attracted to antimatter? What if antimatter is gravitationally repulsive? This experiment wouldn't show that.

Not that I think it's likely, but it hasn't been ruled out by this experiment has it?

[wyager]

> Isn't it still an open question if matter is attracted to antimatter

Conservation of momentum (force*time) means they both experience the same force. The attraction is symmetric

[ben_w]

A mass with negative effect on local curvature, would I think still follow the same geodesics (i.e. fall down).

Same in Newton, though there it would be GMm/r^2 = F = ma but both m have the same sign so acceleration is the same regardless of value (including -ve), though if M was negative then both +ve and -ve valued m would accelerate away rather than towards.

Conservation of momentum and energy is conserved because they're mv and 1/2mv^2, so an isolated equal and opposite +- pair co-accelerating has a total of zero of both all times.

[phkahler]

>> Conservation of momentum (force*time) means they both experience the same force.

That's right. I'm just saying it hasn't been confirmed. Wouldn't that be some exciting new physics though? It could explain why there isn't any around, why galaxies apparently aren't made of it, and why there is annihilation radiation sourced from the edge of galaxies. ;-)

[Georgelemental]

Violating conservation of momentum like that would allow generating arbitrarily large amounts of energy for free.

[phkahler]

Yes it would. Maybe that's what's accelerating the expansion of the universe ;-)

[gmadsen]

wouldn't energy conservation require matter to experience the same force to anti-matter?

[AnimalMuppet]

> They measured that they needed a magnetic field bias applied to the anti-hydrogen equivalent to "pushing them up" with 0.75g (+/- 0.25g or so), so anti-matter is attractive. No new physics.

That part bugged me a bit. Why 0.75g? Shouldn't we expect 1.0g? (Yeah, I know, +/- 0.25g...)

Did I miss something, or is antimatter attracted less than regular matter?

[eigenket]

The expectation is that the true value is 1.0g, and the difference is due to errors in their experiment.