
Physicists finally nail the proton’s size, eliminating an anomaly - theafh
https://www.quantamagazine.org/physicists-finally-nail-the-protons-size-and-hope-dies-20190911/
======
ur-whale
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?

~~~
lisper
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 proton". What matters is not really the number that you get out of
this, but the fact that the experimental results of measuring this value
appeared to _change_ in the presence of muons. This was a phenomenon that was
not predicted by present theory, and if it had held up, would have been a
major breakthrough. 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. That makes it hard to improve the model!

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

[https://en.wikipedia.org/wiki/Charge_radius](https://en.wikipedia.org/wiki/Charge_radius)

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

~~~
godelski
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.

~~~
pdonis
_> 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.

------
conistonwater
> _If the discrepancy was real, meaning protons really shrink in the presence
> of muons, this would imply unknown physical interactions between protons and
> muons — a fundamental discovery. Hundreds of papers speculating about the
> possibility have been written in the near-decade since._

This reminds me of a joke. An experimental physicist walks into a theoretical
physicist's office with a really cool experimental result, shows the printed
out graph to the theoretician. He thinks for a while and says, "this is
perfectly in line with theory, let me explain how". The experimentalist looks
at the graph, scratches his head, says "uh, this is upside down", and rotates
the paper 180 degrees. The theoretician thinks a while more and says, "well
this can also be explained".

~~~
WhitneyLand
The clever thing about these kinds of jokes as when it happens in politics is,
they manage to mock two groups of people such that each group reads the joke
and tends to laugh because they infer it as poking fun at the other group.

~~~
AnthonBerg
We laugh even if it is rotated by 180 degrees.

~~~
charles_f
Reminds me of a person on HN that was telling a joke on a specific group of
people, and another person came and said "that can be laughed about", and the
initial person said "actually it's about all groups of people", and the other
person said "that can also be laughed about".

------
jpmattia
Probably worth noting: The author writes the article as though the latest
experiment "solves" the issue.

A more nuanced description (which is not exactly quantamagazine's forte) would
note there is a conundrum about the proton size. The article describes a
measurement that falls under "spectroscopic methods" in the wiki [1].

Why eg scattering measurements should yield a different value is not at all
clear.

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

------
raldi
This was a great writeup. Accessible, yet I feel like I learned some genuine
not-dumbed-down science.

~~~
FuckButtons
Quanta has a lot of good articles like this one, definitely worth looking at
every now and then.

------
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](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...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.183001))
disagrees with the muonic value and agrees with the old, larger value.

~~~
davrosthedalek
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/](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.

------
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.

~~~
fsh
It took Eric Hessel's group eight years of hard work to do the measurement in
electronic hydrogen. The experiments are completely different.

------
dschuetz
I don't understand how physicists cannot see the obvious: the size of a proton
appears not to have a fixed/natural value. It seems to depend on the quantum
state of their system, energy levels, and the coupling partner's quantum
state, energy levels and mass. "Physicists finally nail..." sounds like a part
of a joke: "Physicists finally nail the coffin with the research funds shut"

------
vectorEQ
so, a failure in measurement sparked a mystery which went on for decades. i
wonder how often that happens in science ;P

~~~
chrisamiller
All the time! People propose hypotheses, supported by some preliminary data,
and then they need to be tested. Some problems are easy, some take years or
decades to be overturned. It's messy sometimes, but that's how science is
supposed to work!

~~~
cpsempek
I think the parent was being sarcastic, hence the ;P emoji at the end of the
post.

However, I wanted to commend your reply. It seeks to educate without
judgement. Cheers.

~~~
vectorEQ
op here, i was sarcastic. the whole of science is based on best hypothesis is
accepted as the ruling one. not as a fact of nature or reality ,but the best
hypothesis. people take it often as fact though ,which led me to post a
sarcastic comment just to point that out. sorry if it confused or offended
someone :')

------
pavel_lishin
What a headline. Should someone talk to the author, make sure they're doing
ok?

~~~
colechristensen
There is a certain philosophical dissatisfaction with the standard model and
it just keeps getting confirmed over and over, nobody can seem to find any
significant new physics because possible discrepancies keep getting eliminated
one by one. _Hope dies_ is maybe a bit florid, but it's a bit real if you
don't take it too seriously.

~~~
ChuckMcM
Yup, pretty much. Now if they could just use the standard model to explain
dark matter/energy or the inconsistencies in the cosmic age evidence, or what
ever the non-fundamental physics but really interesting problem is.

I wonder if we figure out how to regrow a damaged heart into a healthy heart
with a simple injection and 10 weeks of physical therapy if hope will die in
all those cardiologists out there who no longer are able to push the
boundaries in heart transplant surgery or heart substitutions.

What is the effect on people who have spent their lives becoming an expert in
a field or subject which now seems to be "done." I could imagine they might
lose hope.

------
Bostonian
I think in some areas of physics, such as high-energy physics, there is not
much left to discover. The Standard Model works. What branches of physics are
likely to see progress?

~~~
qws
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 scientist, myself.

~~~
skrebbel
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.

~~~
adrianN
Much more impressive are the gravitational wave detectors.

