Observation of the effect of gravity on the motion of antimatter
nature.com
nature.com
* 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.)
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?
Conservation of momentum (force*time) means they both experience the same force. The attraction is symmetric
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. ;-)
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.
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?
IANAP, but did a bit of work on tomographic image reconstruction.
edit: never mind, I got that wrong (it has been a while...): the material does emit a positron that decays into opposite photon pairs.
Similar to how by detecting infrared light JWST can peer through the dust in galaxies to reveal detail that the couldn't see in optical light using Hubble[1].
Electron-positron annihilation releases photons with an energy of roughly 0.5MeV[2], and as you can see from this graph[3] the absorption is relatively low at those energy levels. Unless I got my math wrong, about 90% of those photons survive traveling through 1 cm of water or fat (which has roughly the same density as water).
[1]: https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.h...
[2]: https://en.wikipedia.org/wiki/Electron%E2%80%93positron_anni...
[3]: https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/wat...
Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case.
Not being a physicist, I am somewhat saddened by this :'-)
Experimentally, however, the effect itself, that antimatter falls, has been demonstrated at much higher precision by weak equivalence principle tests for decades.
See, for example, this 1991 paper from Adelberger et al. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.66...
I have worked closely with the authors of that paper. I think the reasoning in that and subsequent papers is correct.
The question was how this is done.
Positrons are by-products of high energy collisions. You just need to separate them by mass and charge.
Combine them with anti-protons and you get hydrogen atoms. Then make anti-hydrogen traps.
Check Wikipedia for clues. https://en.m.wikipedia.org/wiki/Antihydrogen
A more helpful answer might be something like:
--
Each particle has a corresponding field over all of space.
The substance-like particles obey certain rules, called Fermi statistics, which leads to a model called the Dirac sea; this makes all of space analogous to semiconductors, with electrons and holes, except that because this is just an analogy the holes are actually antiparticles.
The analogy is useful, in that it takes a certain minimum energy level to create a particle-antiparticle pair, just as it does an electron-hole pair.
For fundamental particles like electrons-positrons, this is fine and works as expected; the only extra step is knowing what has to absorb the energy to create the pair… but it turns out that all normal matter will do.
For composite particles like protons and neutrons, this is much harder, as the thing you make this way are quark-antiquark pairs, and to make either an antiproton or antineutron you need a specific combination of three specific "colours" (not real colours) of quark, and we have only extremely limited control in this regard.
When you have both positrons and antiprotons, "cool" them from the absurdly high energy states necessary to create them and let them combine just like electrons and normal protons recombining after getting ionised.
https://www.nature.com/articles/nature01096
As for the analysis, anti-particles are pretty much the same as their mirror particles save for some mirrored attribute(s), so charged antimatter particles carry the same charge as matter particles, but of opposite sign. An antiproton is negatively charged and an antielectron (positron) is positively charged.
Looking at particle tracks they'll see matching masses but curves in charged fields going in opposite directions.
Uncharged particles have some other mirrored attribute, so again "it's just like regular Alice only it's a mirror Alice wrt { X? }"
https://alpha.web.cern.ch/experimental-cycle
or at a more technical level
https://sci-hub.se/10.1098/rsta.2010.0026 ("Cold antihydrogen: a new frontier in fundamental physics")
- "how do they actually know/analyze that they are making antimatter atoms ?"
That's answered in the OP paper: they detect the radiation from matter-antimatter annihilation that happens when the anti-atoms escape, and hit the experiment walls.
- "Central to the observations reported here is the antihydrogen annihilation detector[13] (Fig. 1a), situated coaxially with the mixing region, between the outer radius of the trap and the magnet bore. The detector is designed to provide unambiguous evidence for antihydrogen production by detecting the temporally and spatially coincident annihilations of the antiproton and positron when a neutral antihydrogen atom escapes the electromagnetic trap and strikes the trap electrodes. An antiproton typically annihilates into a few charged or neutral pions[14]. The charged pions are detected by two layers of double-sided, position-sensitive silicon microstrips. The path of a charged particle passing through both microstrip layers can be reconstructed, and two or more intersecting tracks allow determination of the position, or vertex, of the antiproton annihilation. The uncertainty in vertex determination is approximately 4 mm (1σ) and is dominated by the unmeasured curvature of the charged pions' trajectories in the magnetic field. The temporal coincidence window is approximately 5 µs. The solid angle coverage of the interaction region is about 80% of 4π."
- "A positron annihilating with an electron yields two or three photons. The positron detector, comprising 16 rows, each row containing 12 scintillating, pure CsI crystals[15], is designed to detect the two-photon events, consisting of two 511-keV photons that are always emitted back-to-back. The energy resolution of the detector is 18% full-width at half-maximum (FWHM) at 511 keV, and the photo-peak detection efficiency for single photons is about 20%."
As for how to detect:
When antiparticles contact matter particles, they annihilate, releasing the mass as energy.
For positrons and electrons, this is a nice simple process of "two photons with a combined energy of just over 1MeV" (in the collision frame of reference they are equal in energy, 511 keV, and going in opposite directions); for protons and antiprotons, each quark does its thing separately so you get a whole mess of other things that are themselves unstable and I don't know the characteristic signatures of, only that they have one and you can look for it.
Could this experiment enforce theories that propose gravity operates differently from quantum theory?
So, we didn't expect there would be any difference as far as gravity is concerned.
We thought antimatter would fall down, but it's been very hard to test (since gravity is so weak). In particular, antimatter tends to form as fast-moving charged particles, whose gravitational behaviour is drowned-out by other forces like electromagnetism. Hence we need to form neutral particles, like atoms of anti-hydrogen; and that requires slowing the particles down. Even then, measuring the gravitational effect on such tiny scales requires incredible precision!
Surely antimatter containment is the bottleneck here?
That all said, the opposite result would have been huge. We know our understanding of gravity is flawed to a degree we don't know about flaws to the theories of (at a low level) literally everything else we're aware of in the universe.