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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

#91

Post by godelski »

pdonis wrote: post #89
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…

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.

What the parent is saying is that the field is a continuum. What matters is a certain strength of the field that matches measurable effects.

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

#92

Post by fsh »

The article doesn't mention that since 2010 two more measurements on atomic hydrogen that determine the proton size with similar accuracy have been published. The first one (https://science.sciencemag.org/content/358/6359/79) agrees well with the muonic value, while the second one (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...) disagrees with the muonic value and agrees with the old, larger value.

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

#93

Post by davrosthedalek »

fsh wrote: post #90
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).

Almost, but not quite. The rms radius is defined via the slope of the proton electric form factor G_E. Via a Fourier transform you can connect the form factor to the distribution of charge in some space-like coordinate. That works out to be (very close) to the rest frame space for heavier atoms, because relativistic effects are small. For the proton, this is not true -- the Fourier transform gives a spacial distribution in the so called Brick-wall or Breit-frame, not in the rest frame of the proton. (There are additional complications about waveforms not being the same before and after).

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

#94

Post by pdonis »

godelski wrote: post #91
pdonis wrote: post #89

Earlier quoted context omitted.

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.

What the parent is saying is that the field is a continuum. What matters is a certain strength of the field that matches measurable effects.

> What the parent is saying is that the field is a continuum

Yes, I know that. I am simply giving a link to more detailed information about what lisper was describing.

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

#95

Post by davrosthedalek »

fsh wrote: post #92

The article doesn't mention that since 2010 two more measurements on atomic hydrogen that determine the proton size with similar accuracy have been published. The first one ( https://science.sciencemag.org/content/358/6359/79 ) agrees well with the muonic value, while the second one ( https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12... ) disagrees with the muonic value and agrees with the old, larger valu…

Yes, very true. I think it's a shame that some people just declare victory and ignore the discrepancy in these modern experiments. At the same time, a similar split seems to happen using the scattering method, but the data are not published yet.

The results from Hessels were announced as preliminary results more than a year ago -- it's great that they are finally out peer-reviewed. But the community at large didn't stop back then when the results came out, with more experiments planned all over the world, especially but not only in the scattering sector. We made progress, but the puzzle isn't dead yet. There is actually a conference starting next week on this topic: http://ecsac.ictp.it/ecsac19/

It's a rare occasion where nuclear physics and AMO overlap. Another one is the Zemach radius, which also requires knowledge of the proton magnetic form factor.

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

#96

Post by davrosthedalek »

BurningFrog wrote: post #36
qws wrote: post #10

Earlier quoted context omitted.

Someone please help correct my intuition here: the harder it is to find flaws in the Standard Model, the harder it would be to use such new physics in engineering. Basically I'm curious whether continuing failures to find new physics can be taken as evidence that, if and when we find the new physics, it will be very difficult to apply. I'm not against science for its own sake, however. Just more of an engineer than a…

If applying the new physics requires gigantic particle accelerators, sure. But maybe it's under a completely different rock, that we haven't thought of turning over.

The nice thing about precision nuclear physics is that often lower energies are enough -- most accelerators actually have too much energy to measure the proton radius for example. And energy recovering linacs for these energies can be build quite small. Think Basketball court.

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

#97

Post by Ididntdothis »

skrebbel wrote: post #23
qws wrote: post #10

Earlier quoted context omitted.

Someone please help correct my intuition here: the harder it is to find flaws in the Standard Model, the harder it would be to use such new physics in engineering. Basically I'm curious whether continuing failures to find new physics can be taken as evidence that, if and when we find the new physics, it will be very difficult to apply. I'm not against science for its own sake, however. Just more of an engineer than a…

Don't forget that our engineering ability grows as well. I guess that when Einstein proposed his relativity theories, it seemed totally inapplicable. But fast forward a few decades, and we got GPS which wouldn't work without them.

I remember that when they discovered electrons they thought that it’s also totally useless.

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

#98

Post by Ididntdothis »

analog31 wrote: post #47
adrusi wrote: post #26

Earlier quoted context omitted.

While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measureme…

One thing that's always interested me is that the recent history of breakthroughs doesn't tell you when the next one will arrive. For instance reconciling quantum mechanics and gravity, we don't know if we're within 11 years, 110 years, or even 1100 years!

It seems people like newton or Einstein come around every few hundred years. So another huge leap may take a while.

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

#99

Post by marcosdumay »

willis936 wrote: post #60
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…

Does the magnitude of the field wave drop at some fixed shape (like an exponential)? Is the wavelength fixed? Is it in meters? Is it the field of a proton made up of multiple frequencies/modes? Is a proton’s influence on fields extend to all space or is there a point (within the hubble sphere) at which a proton does not influence fields at all?

- Yes, it's fixed. It's also a complicated function.

- A quantum mechanics wave function does not necessarily have a wave length or a frequency. It is a different concept from waves, although classical waves are a consequence of it.

- In QM all particles influence all of space. It is also too small an influence to make any difference.

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

#100

Post by fastaguy88 »

One has to wonder why the original "muon" paper, which produced such controversial results, did not include the control of measuring with the same methods conventional ("electron") hydrogen. Doing the control would have cleared things up a lot sooner.
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