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Comment by gus_massa | Hacker News Reader

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.

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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?

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).

> 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.

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

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.

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.

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.

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.

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.

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.

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.

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