* 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.)
>> 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?
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.
>> 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. ;-)
> 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?
Fun fact: if you’ve ever had a PET scan, you’ve had antimatter generated and annihilated within your own body, and you most likely survived the process.
The letters PET stand for Positron Emission Tomography. It works by you ingesting a material that emits positrons. Positrons are antimatter electrons, they annihilate with electrons in your body and produce EM radiation which is detected by the scanner machine.
More specifically, the material emits high energy photons, that almost immediately "decay" into positron/electron pairs that travel at almost exactly opposite directions. Then detectors at opposite ends of the machine detect the electron and the positron, match the event pairs and can then reconstruct the line of travel and the emission depth (from the detection timestamp delta).
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.
I am a physicist and you have it pretty close to correct, except reversed. The atom emits a positron (and in the process turns one of its protons into a neutron), the positron annihilates with an electron and this produces opposite photon pairs which are detected.
In general yes. In this specific case not entirely, because once the radiating nucleus has undergone positron emission it has turned one of its protons into a neutron, which means it now needs one less electron in order to form a neutral atom. The "spare" electron can wander off and replace (eventually, on average) the electron that gets annihilated by the positron.
Yes, had to correct myself after checking Wikipedia. In retrospect there was no way a positron would make it out of the body into to the photodetector unscathed (or, better, unscattered).
Absorption of light by stuff is frequency dependent. At the right frequency, stuff can be almost entirely transparent.
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).
A photon from an electron-positron anihilation has frequency around 10^20 Hz, somewhat higher frequency than what we usually call x-rays. Some of them do get absorbed by the body, but enough pass through to be detected usefully.
For those of you, like myself, who only know about antimatter from science fiction and space opera, here's the key statement:
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 :'-)
I think it is fair to say that physicists in general are eager for results that are counter to expectations, though not so much if they rule out a hypothesis they personally have been pursuing for some time.
The experiment is very cool and it is satisfying to see a clear demonstration of the effect.
Experimentally, however, the effect itself, that antimatter falls, has been demonstrated at much higher precision by weak equivalence principle tests for decades.
This is so strange. I took a tour of CERN more than a decade ago, and during the tour we were told about an experiment done there to prove antimatter responded to gravity. They described building and antimatter “fountain” where antimatter was released from an upward directed device and a detector was placed on the floor. They detected annihilations when the particles hit the detector. It sounded like a really elegant setup. I would assume the energy profile of antimatter annihilation would be quite specific and mitigate detecting other random particles. So did I dream this very detailed discovery or is this really old news?
No hatred involved here. This was supposed to be a light joke referring to the use of Comic Sans in the Higgs boson presentation by CERN scientists (that later led to a Nobel Prize). No matter how you see it, Comic Sans was an unusual choice. I thought is was simply hilarious and I actually hope someone will do it again.
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.
- "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 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? }"
Create several thousand anti hydrogen atoms. Put them in a magnet trap. Open top and bottom of the trap. Detect collisions with wall. If more down. It's affected by gravity.
The answer you gave is like the meme of how to draw an owl in two steps (1. draw a perfect circle, 2. draw the rest of the owl).
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.
A friend in college believed in the concept of a matter-repelling antimatter drive that generates propulsion by one repelling another. However, this experiment demonstrates that it may not be possible.
Could this experiment enforce theories that propose gravity operates differently from quantum theory?
Not really, as the current dominant theory already predicted that gravity affected antimatter in the same way as normal matter. This experiment affirms the standard model rather than disproving it.
You're probably thinking of exotic matter with negative mass, not antimatter.
Antimatter is matter made of (elementary) antiparticles, which are identical to the usual particles expect they have opposite electric charge.
So, we didn't expect there would be any difference as far as gravity is concerned.
Antimatter has opposite charges for all charges, not just electric charge. Anti-electrons have opposite lepton number, anti-quarks have opposite flavour charges etc. This is true for all the charges we understand in the standard model.
That was my first thought too: all those sleek (or less sleek, as in The Fifth Element) anti-gravity flying cars from sci-fi films just became a little less probable...
So this seems to mean that antimatter is not negative matter, it is normal matter with different characteristics. Seems like a huge clue as to what matter actually is made of and what holds it together.
This is what physicists expected. Both part of gravity as warping space expects this, and that gravity fields are based on energy content. Anti particles still have positive energy, so should generate and interact with gravity like normal.
I couldn't understand this headline. Antimatter only differs in charge so of course it's affected the same way by gravity. So this is just a confirmation?
Science is not theology: ideas must be tested against reality.
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!
Sure. It's just not really much of a headline, or an idea. There are lots of interactions between combinations of things that haven't been studied. But fine, I guess it's not exactly on the Guardian's front-page.
That's right, it's usually been theorized that gravity would affect antimatter the same way as regular matter, but it's been hard to confirm as it's a serious challenge to obtain a significant mass of antimatter
The reason you're reading this on some nerd website and not every single news outlet is that this was the strongly expected result - we see photons affected normally by gravity all over the place, we see CPT symmetry hold up everywhere we look, and negative gravity would be hard to square in with a general relativity way of thinking.
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.
As far as we know, there likely isn't any, at least not in a persistent large-volume amount. Antimatter particles are continually created by high-energy collisions of regular matter particles, but annihilate quickly. Cosmology speculates some miniscule imbalance in the ratio of baryons to antibaryons shortly after the Big Bang, the cause of which we don't know, is the only reason anything material exists in the modern universe at all. Most particles annihilated with their corresponding antiparticles, but there were somewhat more baryons that survived because no antibaryons remained to annihilate with, and those became all of the galactic structure we're familiar with, see, and are made of. No antimatter should have remained. More is always produced, but it annihilates well before having any opportunity to become macro-scale structure of its own.
* 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.)