Antiproton Orbiting Helium Ion | Hacker News Reader

Antiproton Orbiting Helium Ion

arstechnica.com

179 pointshexo68 comments

This seems like something that wouldn't even be written about in sci-fi books because it just doesn't make sense to ever be possible. It seems impossible to even imagine the implications.

I also can't imagine future chemistry students needing to memorize two separate sets of electron and antiproton orbitals.

Note that this isn’t the first time these kind of “exotic” atoms have been created.

https://en.m.wikipedia.org/wiki/Exotic_atom https://en.m.wikipedia.org/wiki/Antiprotonic_helium https://en.m.wikipedia.org/wiki/Antihydrogen

Also of note is the fact that this is the first time they were able to get data granular enough to be of use.

From Wikipedia: https://en.wikipedia.org/wiki/Antiprotonic_helium

> The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and annihilating.

From the research article: https://www.nature.com/articles/s41586-022-04440-7

> The resonance parent states (37, 35) and (39, 35) have microsecond-scale lifetimes, whereas the daughter state (38, 34) has an Auger width ΓA ≈ 21 MHz (ref. 7; Fig. 1b)

The life of the antiproton here is too short to make an interesting chemistry experiment, but it's long enough to measure the spectral lines.

It's been a long time since chemistry and physics class, but isn't "orbital" in this context not the same thing as gravitational orbits? I was told the concept of orbitals represented where an electron could exist around a nucleus and that it wasn't actually traveling in a path bound by the gravity of the atom. So in this context, how is it actually falling into it's surface? Is it just tunneling into contact?

> how is it actually falling into it's surface? Is it just tunneling into contact?

Kinda sorta. As fsh responded elsewhere in the thread, the difference between the antiproton and an electron is that the antiproton can annihilate with one of the protons in the helium nucleus (since it's an exact antiparticle of the proton, whereas an electron is not). The probability of this annihilation happening per unit time depends, roughly speaking, on how much the antiproton's wave function overlaps with the wave function of the protons in the nucleus, which in turn depends on which orbital the antiproton is in. But the overlap will be nonzero in any orbital, which means that this configuration is kind of like a radioactive atom: the question is not whether it will decay--eventually it will--but how long it will take on average. The "half-life" of the antiproton configuration in this experiment was on the order of microseconds, which is quite long for an antiproton.

The important force is the electromagnetic, gravity is very weak and you can ignore it in these systems.

For only one electron or one antiproton, the orbitals have the same shape and classification. The only difference is that the size depends on the mass, so the orbital to put the antiproton are much smaller than the orbitals to put the electrons. Note that something similar happens with muons that have an intermediate mass and the orbital to put them have an intermediate size. https://en.wikipedia.org/wiki/Bohr_radius

It's more difficult when you have many electrons or muons or antiprotons. (I guess nobody had measured a system with many muons or antiprotons.) If you have many electrons, the problem is that you must calculate the attraction of the nuclei and the repulsion of the other electrons, so the orbitals change. In particular, the filling rules https://en.wikipedia.org/wiki/Electron_configuration#Atoms:_... don't follow the energies of the orbitals of an isolated electron.

In a system with many muons or antiprotons, all of them will be closer and the repulsion will be bigger, and I expect weird filling rules.

Electrons: 1s_up, 1s_down, 2s_up, 2s_down, ...

¿¿¿Antiprotons: 1s_up, 2s_up, 1s_down, 2s_down, ... ???

It is possible to calculate the filling order for muons and antiprotons numerically, but I'm too lazy do do the calculation now and also the standard programs [1] [2] have a lot of hidden assumptions to make the calculations with electrons more efficient and I'm not sure how difficult is to tweak the entry files to calculate this fast enough. [3]

[1] https://en.wikipedia.org/wiki/Gaussian_(software)

[2] https://en.wikipedia.org/wiki/PSI_(computational_chemistry)

[3] Perhaps it's implemented and I just need to RTFM, but otherwise it looks too straightforward for a PhD thesis, but it may be a nice undergraduate thesis.

Disclaimer: Not a physicist, but have been watching many youtube videos. So please correct me if I'm thinking of this wrong.

An electron is bound to the nucleus by the electromagnetic force, but is not a round ball, but a wave distribution of probabilities.

I don't know if it's the actual case, but I've been thinking of all "particles" like "waves" in a pool. Small disturbances in the fields (e.g. electromagnetic), like jumping in a pool. The electron IS the disturbance, because to us that is what we can measure (and we can only measure one place at a time). So the electron doesn't rotate around the nucleus - instead its a wave in the same area where the nucleus is. The electron's "probability wave" disturbs the space around the nucleus - including inside it, and 1000 miles away from it. It's just has way more disturbance in certain places - AKA "the shell" its in. Just like any wave, it "fades" with distance but who's to say where the wave ends.

In fact, that wave isn't exactly the same for every electron. Each "shell" is waving at a different amplitude (is amplitude the right analogy?) - and you can only have so many "waves" add up in a shell simply because there's no more room because of the length of each wave (like a spiral-o-graph). You can only put another wave in the gaps of the previous wave (if you did put two in the same place, it would double and be part of the next shell, right?). And when an electron's amplitude changes, e.g. is lowered, the total energy can't be destroyed, so the difference is emitted as another wave of the difference - that wave is a photon. And the same applies when a photon is added to that wave. Like rowing in the water - the exiting wave and the rowed wave is added together. I think of Photons are partial Electrons. They are like the "wake" of an electrons wave change.

More or less the right idea. There are actually two fields at play here. The electromagnetic field and the electron field. The electron field gets quantised into electrons and the electromagnetic field quantised to photons. The electron field and the electromagnetic field are fundamentally separate fields, but they do interact. Any particle field with a charge can interact with the electromagnetic field. It's the charge that the electron field carries that "leaves a wake" in the electromagnetic field. In this sense photons are no more partial electrons than they are partial protons.

In an atom, the electron state energy comes from the electric potential energy due to the electron's charge sitting in the electric field produced by the nucleus. The further out the charge is, the higher the potential energy (and hence the lower the binding energy). This is distinct from the electron amplitude. The electron amplitude is essentially how much electron is present at a point, and lowering it violates conservation laws (lepton number, mass, charge).

Dumb question: Why does the antiproton fall to the nucleus' surface, after orbiting?

I think this is a great question because not too long ago we struggled with the opposite question: How can electron-based molecules exist at all? (Funny that now I have to differentiate it from antiproton-based molecules...) The old model of atoms where electrons 'orbited' classically/astronomically interpret the electron as literally circling around the nucleus, and because a moving charge radiates energy electromagnetically it should shed it's orbital energy until it reaches the nucleus. This was a big conundrum way back before we settled on quantum mechanics, which solves this by recognizing that electrons 'orbit' is more like an acoustic standing wave and that there are a small number/quanta of available states that the electron can possibly exist in. Electron orbitals are not continuous or near-continuous, they are very much discrete, and there is some minimum activation energy required to shift between the various orbital states.

This quora answer actually looks like a reasonable description of the issue past physicists faced: https://www.quora.com/Why-do-electrons-in-an-atom-keep-a-dis...

So I'd guess that the reason why the antiproton's orbit decays is because its orbital energy levels are "continuous enough" that its orbital energy can decay smoothly down to zero. Maybe this is related to the fact that the electron is relatively massless compared to the antiproton.

I think it's cool that we found a state of matter where the old model of atomic orbital motion that we interpreted as a paradox might be a physically accurate description in this circumstance.

It doesn't. The ground state orbital of the antiproton is in principle stable, just like the electron orbitals in a regular atom. The difference is that antiprotons can annihilate with protons from the helium nucleus when they get too close. This is why the experiments observe orbitals with high quantum numbers where the overlap between the antiproton wavefunction and the nucleus is small.

The idea is that electrons, muons and antiprotons have stable orbitals, where they can stay forever. For one electrons, muons or antiprotons, they have the same shape and classification, but they don't have the same size. They are all stable.

It's a bad idea to imagine them like a planet orbiting around the Sun, or that they suddenly make a turn and decide to head to the nuclei, or that they are going in spirals until they colide.

In some case, electrons can interact with the nuclei, and the nuclei absorbs them, one proton changes into a neutron and the process releases a neutrino https://en.wikipedia.org/wiki/Electron_capture But the electron is in a stable orbital and suddenly it interact with the nuclei.

Something similar happens with the antiproton. The antiproton is in a stable orbital and suddenly it interact with the nuclei and is annihilated.

The stranger case is really the electron, which just sits there and never falls to the nucleus (that's the original conundrum of the model of the atom as negative electrons orbiting a positive nuclear center). Physicists applying QM to the hydrogen electron found that the electron could only exist in certain energy levels, and drew the analogy of standing waves to explain it, hence 'wave equations'. The electron is no longer a point orbiting another point, but a wave function delocalized over the entire orbital.

These electron quantum rules don't work the same for an antiproton, which is 1800X more massive than an electron. There's probably some other factors, like the antiproton can get close enough to the nucleus for strong force effects etc.

An electron orbital overlaps with the nucleus, though; any bound electron can in a very real sense be said to exist partly inside the nucleus all the time. But electron–nucleon physics don't care. In particular the electron does not feel the residual strong force. The situation is different with a bound antiproton, however. If the antiproton wavefunction ends up sufficiently overlapping with the wavefunction of one of the nuclear protons, they will annihilate.

I am not a physicist, but I suppose it is because the s-orbital has an amplitude peak at r=0?

In other words, it is not so much the antiproton falling to the nuclear surface as much as the antiproton finding itself at the nuclear surface.

EDIT: The context is that the antiproton was in an orbital with large principal and azimuthal quantum numbers. Still, there would be some non-zero probability of the antiproton finding itself close to the nucleus, no?

Could we keep it perpetually in an excited state that has no amplitude at the centre?

It reaches the nucleus by a process of tunneling. You can never completely stop it, but it reacts exponentially to you increasing a barrier.

An excited state isn't a barrier, it's a state with a higher energy than ground state. Several non-ground-state orbitals have a wavefunction whose amplitude drops to zero toward the centre.

An electron won't tunnel to a location where its wavefunction amplitude is zero.

Practical applications, a laser that explodes if you turn it off.

My rudimentary guess is either due to the strong and/or weak forces combined with lack of orbital/shell velocity.

The antiproton is negative.

The protons with neutrons are collectively positive.

The orbital velocity of the antiproton is slower than that of an electron (being something like 180x more massive).

And along with above, the - and + charges attract strongly. So the antiproton orbit rapidly decays to the nucleus.

No. The anti-proton is 1836x as massive as an electron. This makes its wavelength very short, so its lowest orbit very close to the nucleus, close enough that it has non-negligible amplitude to interact directly with a nuclear proton.

Also, its higher-orbital energy levels are very close to one another, so it is easy for it to spit out a very low-energy photon and drop to the next orbit down.

Not too literate in this, but I'd say that the mass of a proton is large enough so that quantum effects (such as the quantization of of energy) become less relevant than classical mechanics (which lets the antiproton get pulled toward the proton).

Correct me if I'm wrong though.

You're wrong. Mass has nothing to do (as much as is known at this time) with quantum effects being important or not. Quantum mechanics would (as far as we know) work the same for a 1kg particle as they do for a 1eV particle.

Instead, the difference is that the antiproton can interact with a proton and annihilate, while an electron can barely interact with a proton or neutron outside of the EM attraction.

Muons can do something similar, and it has the effect of reducing the effective radius of the atom, which can catalyze fusion.

Which makes me wonder… is antiproton catalyzed fusion a thing? Does the antiproton last long enough? Muons are inefficient to produce. Can antiprotons be made significantly more efficiently?

It is much more difficult to produce antiprotons than muons.

Furthermore, my guess is that the proton-antiproton annihilation rate is much faster than the rate of antiproton-catalyzed fusion. Muon catalysis doesn't have the annihilation channel (the heavy negatively-charged particle is always close to one H/D/T nucleus), so it will catalyze fusions all day until it decays. The antiproton can simply annihilate.

That said, antiprotons probably would catalyze fusion at some rate. Whether it is higher or lower than muon catalysis, I'm not sure. If the antiproton orbital radius is too small, it may actually lower the capture cross-section for a neighboring hydrogen, even if the post-capture fusion cross-section is (almost certainly) higher.

> is antiproton catalyzed fusion a thing?

I imagine on any geometry you can create the anti-proton will be absorbed by the nucleus much quicker than another nucleus can.

On what is easier to produce, currently muons are much easier.

I’m not sure about antiproton catalyzed fusion, but I have heard of antiproton induced fission induced fusion: https://space.nss.org/wp-content/uploads/Space-Manufacturing...

A muon kind of makes sense. It has the same spin and charge as an electron. Kind of mind blowing to think a particle that isn't even a lepton kind of works. I only have a hazy recollection of some engineering level physics but I wouldn't have guessed that.

Seems it'd have to be ultra-hot and ultra-cool at the same time

So a free neutron will decay into a proton and an electron. This already confuses me because the only difference between the nucleons is an up quark (uud vs ddd) and an electron an a lepton is a fundamental particle.

But an electron in an atom won’t merge with a proton to form a neutron. There seems to be some hand waving here around the strong nuclear force and zero energy state that I don’t really understand.

But in a neutron star the forces are so great that I believe this can happen (or how else does neutronium form?).

And now we see an antiproton can annihilate a proton. Doesn’t this have the same zero energy problem?

Can someone ELI5?

Worth noting that a neutron decays into a proton, an electron, AND an electron anti-neutrino, so that lepton number is conserved (electron is +1 and anti-neutrino is -1). This interaction conserves charge and baryon number as well.

Electrons in an atom do merge with a proton to form a neutron. https://en.wikipedia.org/wiki/Electron_capture

> So a free neutron will decay into a proton and an electron. This already confuses me because the only difference between the nucleons is an up quark (uud vs udd) and an electron an a lepton is a fundamental particle

Conversion between different Quarks are mediated ny the weak force.

The associated particles of the weak force are three bosons: W+, W- , and Z0. (Positively, negatively and neutrally charged).

To convert a neutron into a proton, you convert an up into a down quark.

This is done by emitting a W-, carrying away the negative charge. The W- then decays into an electron and an anti-electron neutrino.

As noted by parent commenter, the same process can and does happen in reverse, just not for free protons. It happens in Proton rich nuclei and for some is there predominant decay channel, see an table of nuclides:

https://en.m.wikipedia.org/wiki/Table_of_nuclides

The nuclides above the stable line are Proton rich and thus decay by either electron capture or positron emission.

ITYM ELI27....

ELITTCCCAHEM

Explain-like-I-took-three-college-calc-courses-and-hated-every-minute

This makes me wonder, if we had a significant amount of anti-hydrogen (but not enough to make a star), what would be the most complicated thing we could build out of it, and how would we go about doing so? (I also asked on: https://physics.stackexchange.com/questions/699258/building-...)

As fuel for a reactor? And just colliding it with normal matter, driving a steam engine would be enough.

I’m not sure what you mean by enough to make a star. Stars don’t have antimatter in them.

Positive beta decay can and does happen, so while this assertion is correct for all intents and purposes, it is not strictly accurate.

One of the potassium-40 decay paths is the emission of a positron, and this does happen in the human body. Stars with potassium-40 metalicity would see this beta decay as well.

"Very rarely (0.001% of events), it decays to 40Ar by emitting a positron (β+) and a neutrino."

https://en.wikipedia.org/wiki/Potassium-40

I just mean that most elements besides hydrogen are made in stars, theoretically with a very large amount of hydrogen you could make an antimatter star, but without doing that, what elements, molecules etc would we be able to make, if we had access to a large amount of anti-hydrogen that was contained so that it would not annihilate with our matter.

One would expect a sufficiently large amount of antihydrogen to form an antistar.

There was an old science fiction story where the first interstellar expedition discovers that the entire universe except for the solar system is antimatter. The hard way, when they try to land on an alien planet.

Only it was written at some point in time between the concept being invented and the current terminology so they didn't call it antimatter.

(I think it may be "Minority Report", written in 1949 by Theodore Sturgeon and not related to the Tom Cruise movie which seems to be based on a Phillip K Dick story of the same name)

Question: would antiproton with antielectron orbiting around it be stable as normal hydrogen atom?

Did somebody made an experiment like that?

AFAIK, anti-hydrogen is very stable on its own and only gets annihilated because it quickly comes in contact with matter.

> In November 2010, the ALPHA collaboration announced that they had trapped 38 antihydrogen atoms for a sixth of a second, the first confinement of neutral antimatter. In June 2011, they trapped 309 antihydrogen atoms, up to 3 simultaneously, for up to 1,000 seconds.

https://en.wikipedia.org/wiki/Antihydrogen

Yes. It should be indefinitely stable as long as it does not come into contact with regular matter.

https://en.m.wikipedia.org/wiki/Antihydrogen

If antimatter was created during the Big Bang (and it was, we actually just don't know why there is more matter than antimatter [1]) it could exist in issolated-enough patches of the Universe. Photons, gravitational waves or neutrinos coming from those regions would not differ in any way, so we would not be able to identify those regions, up until they merge with normal-matter-filled ones (which is unlikely due to accelerated expansion of the Universe [2]).

1. https://home.cern/science/physics/matter-antimatter-asymmetr...

2. https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_th...

Edit: as a matter of fact experiments, like the one described in the posted paper, allows to shed some light on the properties of antimatter and why there is more matter in the Universe (matter-antimatter anisotropy).

> it could exist in issolated-enough patches of the Universe. Photons, gravitational waves or neutrinos coming from those regions would not differ in any way, so we would not be able to identify those regions, up until they merge with normal-matter-filled ones

Interesting. So how do we know there actually is more matter than antimatter in the universe? Couldn't there be a roughly equal number of sufficiently isolated pockets of each?

It is a well-known hypothesis, but space is not empty enough for it to be consistent with observations at least within our observable universe. If there really were antigalaxies or anticlusters out there, we would expect to observe characteristic gamma ray photons from the matter–antimatter boundaries where the extremely sparse but still existing intergalactic medium would interact and annihilate.

People have looked for evidence such annihilations, but the observations we have are not able to rule them out.

Antimatter superclusters (of clusters of antimatter galaxies) remain very possible. Nobody wants to talk about them because they don't want to be ostracized for speculating on what cannot be settled by evidence. But people do check, now and then, when new information comes up.

... It must be said, too, that nobody has identified any way for the antimatter to have become so far separated from the regular matter. All the processes we know of that make matter like to make both (exactly) equally.

What a bummer - to send an intergalactic probe to a far-off supercluster only to find it’s made of antimatter and you cannot interact with it (if you survive the close encounter long enough to find this out).

If you are paying attention, and on a trip of that magnitude I think you would be, you would notice the halo of gamma rays as the antimatter supercluster interacts with normal matter around it.

That said, the Larry Niven short story “Flatlander” is pretty funny. You should read it.

Yes and yes.

As far as we know anti-atoms (antihydrogen in this case) are as stable as normal atoms.

To the point where it creates interesting questions -- if antiatoms are exactly as normal atoms, why we have abundance of normal matter but not antimatter?

So when the antiproton orbits a helium nucleus, does it occupy the same states and levels an electron would? Does it do it in a more concrete way, since it's more massive? I thought the wavicleness of matter was inversely related to its mass, and an antiproton is massive enough to be more particly.

Looks like semiconductor physics is about to get more interesting. Now we have electrons, holes, and antiprotons ...

You do not want antiprotons in your semiconductors. They will orbit much more tightly than the electrons; there is nonzero overlap of their wavefunction with the protons in the nucleus. There will be interactions (annihilations), and those will not be good for your semiconductor device.

So could we make a rocket engine out of this? We would need to store antiprotons and then shoot them at helium, right?

Nothing special about helium if you want to make a rocket, any matter will do. The difficulty is in the creation and storage.

Ok, now have a bosonic anti-alpha (two neutrons and two anti-protons) orbit in a He4 Bose Condensate!

Could this lead to a storage mechanism for anti-protons?

Antiprotons can be stored pretty much indefinitely (many years) in cryogenic Penning traps. This is done by the BASE collaboration at CERN [1] who are actually neighbors of the group that the article is about.

[1] https://base.web.cern.ch/

CERN: the only place you can pop over to your neighbor to ask for a cup^H^H^Hstream of antiprotons.

I don't think they're presently operating the antiproton source, but for decades one could get them at Fermilab, too.

https://www.fnal.gov/pub/tevatron/tevatron-operation.html

Could it lead to a new storage method? Helium lasts for microseconds, perhaps other atoms can store it indefinitely. Could that enable bulk antimatter storage?

E.g. 1 mol of mercury that contains 1 mole of anti-protons and doesn't annihilate unless subjected to some extreme condition.

How close does an anti-proton have to come to a proton to become annihilated?

And, when a ball of matter and anti-matter collide, will they completely annihilate, or will the initial impact blast them apart such that parts will stay intact? Does initial speed matter? Would we be able to partially annihilate a proton?

Antiparticles have oposite quantum numbers, so it means they also have oposite electric charge. Basicaly, they would attract as much as possible and, you know, kaboom. This is a compostite particle made of quarks, which under annihilation produce their gauge particles - gluons. These quickly undrego hadronization - will pair-up to produce mesons. These are unstable af so they decay. Ultimate fate is photons, electrons, positrons and neutrinos. Even, antiproton can annihilate with neutrons, which makes sense given similar internal structure. Keep in mind that during conversions energies are preserved. So that when lighter particles are produced, they move faster. So yes, it would be ripped apart, kind of.

> Antiparticles have oposite quantum numbers, so it means they also have oposite electric charge. Basicaly, they would attract as much as possible and, you know, kaboom.

This is true, but an electron and nucleus also have opposite charge, yet the electron typically doesn't drop into the nucleus all the time.

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