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Physicists finally nail the proton’s size, eliminating an anomaly

quantamagazine.org

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Re: Physicists finally nail the proton’s size, eliminating an anomaly

#81

Post by wnoise »

jarfil wrote: post #40
ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

"Radius" is a distance at which something stops happening, defining a boundary between some "inside" and some "outside". Interactions between particles, waves, fields, etc. are quantized, which means there is a set of distances at which a given interaction can happen, and a set of distances at which it can't. This creates a boundary, often spherical, which can be described as having some "radius", and thus some "diam…

> Interactions between particles, waves, fields, etc. are quantized,

Yes.

> which means there is a set of distances at which a given interaction can happen, and a set of distances at which it can't.

... that's not how quantization works. The exact ways in which quantum effects are actually discrete is much subtler than in most popularizations. In fact, sharp effects with distance are more likely to be seen in classical models than in quantum ones because the quantum models often allow for classically forbidden effects to happen with small probability, which decreases as the distances increase. You only really see sharp transitions for "bound states"; for everything else (e.g. scattering experiments) there are wider or narrower peaks of more likely to occur depending on both spatial and other parameters.

The radius really is measuring "over about how much space is this particle spread", and while the exact details of how you define that can give different numbers, they are all measuring interaction widths -- how close something has to be to feel its direct effect. I say direct effect, because obviously the indirect effects such as through the EM field can be felt at great distances.

Note that this measure of spread is distinct from the how the wavefunction of the center of a particle is spread, which in the right states can be highly delocalized, even though the particle hasn't gotten any wider. Electron orbitals, for instance, can have different radii in different states (or topology, as you note, if you pick a cutoff that splits high-density regions in two), but the electron still has the same negligible (usually modeled as zero) radius in comparison to any orbital.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#82

Post by cthalupa »

ufmace wrote: post #73
cthalupa wrote: post #42

Earlier quoted context omitted.

>For all we know, we might need a solar system size collider to attain the required energies. We just need to observe the collisions happening at these energies - we don't necessarily need to produce them ourselves. Some physicists are starting to think about how we might be able to observe naturally occurring collisions at these energy levels rather than producing them ourselves.

AIUI, the trick with colliders isn't so much creating high-energy collisions, but creating them exactly inside gigantic detectors that can measure what happens in those collisions, as well as the collisions having a known amount of energy and contents.

Well, to the best of my understanding, both are an issue with current colliders. I've not read any of the papers around the idea of observing collisions caused by high energy events elsewhere in the galaxy/universe, but the articles I read made it sound like this would solve a portion of the problem that we otherwise might not be able to without a solar system sized collider. They didn't make it sound like this would be anything we could do anytime soon, however. But potentially sooner than we'd be able to generate the energy needed to cause collisions at those energy levels.

I'm having trouble finding the articles I've read on the subject - I'll follow back up if I do manage to find 'em.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#83

Post by jxdxbx »

ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

One point that is so basic that physicists often forget to explain it: leptons and quarks don't really have a "size," they are considered point particles. A proton is a configuration of three point particles though whether it makes sense to say that this configuration has some spatial structure is beyond me.

Also, mass has nothing to do with size, or "stuff"-ness. A proton has about 100 times more mass than its quarks (from binding energy), but an atom has less mass than its protons and electrons added up (from the loss of potential energy). A top quark has about as much mass as an atom of gold, but again is just a point particle. And photons have no mass but the energy of photons bouncing around in a confined space does have mass.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#84

Post by timthorn »

DoctorOetker wrote: post #69
lisper wrote: post #27

Earlier quoted context omitted.

That is an excellent question! The definition is somewhat arbitrary, but still has some real physical significance. In actual fact, a proton is a field, so it doesn't have sharp boundaries. But the amplitude of the field still dies off very rapidly with distance from the center, so you can pick some arbitrary small value and say "the point at which the amplitude becomes less than this value is the radius of the proto…

> One of the biggest problems in physics right now is that there are no experiments (except possibly this one) whose results are at odds with the Standard Model. If we say that new physics comes from inconsistencies (either between theory and experiment, or simply within a theory), then there is plenty of new physics to be done, for example the pretense that an electron is a fundamental particle results in the incons…

I assume that your solution is too big to fit in the margin of a Web page?

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#85

Post by fdavison »

timthorn wrote: post #84
DoctorOetker wrote: post #69

Earlier quoted context omitted.

> One of the biggest problems in physics right now is that there are no experiments (except possibly this one) whose results are at odds with the Standard Model. If we say that new physics comes from inconsistencies (either between theory and experiment, or simply within a theory), then there is plenty of new physics to be done, for example the pretense that an electron is a fundamental particle results in the incons…

I assume that your solution is too big to fit in the margin of a Web page?

DoctorOetker's most recent theorem?

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#86

Post by dkersten »

WhitneyLand wrote: post #45
ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

It may be simpler to start our not thinking of particles as matter, but instead thinking of them as little energy fields. Like a magnetic field, a particle has properties and interacts with some things and not others. This is why it's a "field theory". Say you had ten different light bulbs. Could it make sense to talk about their differing light output as "diameters"? It wouldn't be defined in terms of a physically m…

> It may be simpler to start our not thinking of particles as matter, but instead thinking of them as little energy fields.

I dunno about simpler, but it certainly helps explain many of the weird effects we’ve observed in the universe. I recently went down the Quantum Field Theory rabbit hole (thanks to PBS Space Time on YouTube) and it’s an absolutely fascinating topic (even if the maths is beyond me) and QFT especially both makes sense to me and explains a lot of the “problems” with particle physics as I learned it in school.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#87

Post by red75prime »

Filligree wrote: post #72
Symmetry wrote: post #19

Earlier quoted context omitted.

That may require waiting to become a Kardashev III[1] civilization so we can do experiments where we know General Relativity and the Standard Model give different results. The energies required for the disagreements to show up are really insane. [1] https://en.wikipedia.org/wiki/Kardashev_scale

More or less. We can hope that something more clever would be possible, like in the novel Schild's Ladder... Though we'd have to hope that, unlike that novel, we don't accidentally set off a vacuum collapse. That would be unfortunate.

It will also be a civilization-scale experiment on quantum immortality, where we can't get negative result.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#88

Post by fdavison »

thatcherc wrote: post #28
ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

Depends on the particle! Electrons are considered to be point particles with an infinitesimal radius. A proton, however, is made of three bound quarks, which all whiz around each other within a small but finite region. By bouncing things off the proton, as is done in scattering experiments, a notion of how large that region occupied by the quarks is can be determined.

That reminded me of this video of NDT describing the size of the electron: https://www.youtube.com/watch?v=w6kjY6ppJRE&t=71

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#89

Post by pdonis »

lisper wrote: post #27
ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

That is an excellent question! The definition is somewhat arbitrary, but still has some real physical significance. In actual fact, a proton is a field, so it doesn't have sharp boundaries. But the amplitude of the field still dies off very rapidly with distance from the center, so you can pick some arbitrary small value and say "the point at which the amplitude becomes less than this value is the radius of the proto…

The notion of "size" being referred to here is the charge radius of the proton:

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

It is one of several distinct possible notions of "size" for a particle.

Re: Physicists finally nail the proton’s size, eliminating an anomaly

#90

Post by fsh »

ur-whale wrote: post #21

I wish a physicist could explain here how the very notion of "diameter" has any meaning for an object whose size (IIUC) belong entirely to the quantum realm. Is the hydrogen atom two hard little balls of matter orbiting one another, as we were taught in primary school, or are they a probabilistic soup with various, vaguely localized extrema? If the latter, how do you even define the notion of diameter?

As far as atomic physics is concerned (except for hyperfine structure), the proton is simply a blob of charge which can be to first order described by a position-dependent charge density \rho(r). The RMS charge radius r_p is defined by r_p^2 = (\int r² \rho(r) d³r) / (\int \rho(r) d³r).
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