[HN Gopher] Protons are probably smaller than long thought
___________________________________________________________________
Protons are probably smaller than long thought
Author : hhs
Score : 131 points
Date : 2022-02-04 18:36 UTC (4 hours ago)
(HTM) web link (www.uni-bonn.de)
(TXT) w3m dump (www.uni-bonn.de)
| justinator wrote:
| The difference is .04 femtometers and a femtometer is one
| quadrillionth of a meter.
|
| So, I'll be sitting down for a little while.
| munk-a wrote:
| Did you happen to know that the difference between 1 lightyear
| and 1.000001 light years is 9.4605284 x 10^24 femtometers? That
| means they're not at all similar distances on an astronomic
| scale!
| YaBomm wrote:
| justinator wrote:
| Not sure why my og comment was downvoted. In absolute terms,
| the difference in size of the proton is quite large. In
| relative, the difference in size is quite small. And that
| blows my little sapien mind.
|
| And that was all.
| pomian wrote:
| I thought that someone had a measure for a thought and that a
| long thought was measurably shorter than a short(?) thought, by
| the length of a proton. This then went off into a consideration
| of neural message length, etc. Cosmic black hole of thought in a
| spilt second.
| ffhhj wrote:
| What if the size of the proton is variable?
| YaBomm wrote:
| miika wrote:
| https://hiup.org/wp-content/uploads/2017/05/Addendum_QGHM_2....
| fsh wrote:
| This is not even wrong [1].
|
| [1] https://en.wikipedia.org/wiki/Not_even_wrong
| mrlonglong wrote:
| What would it mean if they were actually 0.88 when first measured
| and at 0.84 now?
| ur-whale wrote:
| What does it even mean?
|
| We're at quantum scales and the very notion of "size" is rather
| ill-defined.
|
| The _position_ of a proton, is - as a matter of fact - a
| probabilistic affair, so when you talk about size ... if you don
| 't even know where the darn thing is, how are you going to
| measure where it starts and where it ends ...
| fsh wrote:
| It is a misconception that things are ill-defined just because
| they are at a quantum scale. The proton has an electric charge
| distribution. The definition of the proton size used here is
| the root mean square charge radius of that charge distribution.
| davrosthedalek wrote:
| Indeed, but it's actually a little bit hard to define what
| that charge distribution is (or in which frame...). The way
| it shows up, both in the cross section for elastic lepton
| scattering as well as in spectroscopy, is via the related
| quantity "form factor", as the slope at zero four-momentum
| transfer. While the form factor can be thought as the Fourier
| transform of the charge distribution for heavy objects (say,
| iron nuclei), for the proton, this becomes dicey.
| moron4hire wrote:
| Is this going to mean going back and looking at a bunch of old
| experiments and slapping your forehead and saying, "god dammit,
| THAT'S why it didn't work" like I do with my code when I find a
| wrong sign or off by one error? Or even worse, saying "how the
| hell did that ever work?" That one... ooooh, that one.
| kspacewalk2 wrote:
| > Or even worse, saying "how the hell did that ever work?"
|
| The most unsettling feeling. I often have that feeling when
| doing the devops and infrastructure part of my job, e.g.
| encountering paradoxes in the dark dreary bowels of systemd.
|
| The idealist and perfectionist in you wants to keep digging
| deeper to arrive at a proper understanding. The realist and
| lazy SOB in you wants to slowly back away and pretend this
| never happened. This dialectic hopefully guides you toward a
| happy compromise, somewhere down the middle.
| Karunamon wrote:
| Worse, you dig at it for too long and collapse the wave
| function, and it _never works again_ despite you having made
| no changes.
| mensetmanusman wrote:
| Because the code you are looking at isn't what is compiled
| and deployed, doh!
| zw123456 wrote:
| OK, another dumb question from a neophyte here, doesn't the
| Heisenberg uncertainty principle make it impossible to know the
| size of a proton for sure? It seems like if you cannot not know
| the position and momentum of a particle then measuring its size
| exactly would be impossible?
|
| Thanks in advance to anyone willing to take the time to give me
| an ELI5 on this :)
| whatshisface wrote:
| How big is the smell of a baking pie? The concentration of pie
| molecules in the air is not a binary yes/no function of
| position, but at the same time the volume is definitely smaller
| than the county, and larger than the kitchen. Fuzzy-edged
| objects have some sense of size but there is some freedom in
| where to define their edge.
|
| For the proton, the "size" is defined as a length-valued
| parameter in the function that expresses the charge
| distribution through space. The parameter is objective - but
| its association with the word "size" is imprecise. It is
| _necessarily_ imprecise, not because of anything quantum or
| even unfamiliar, but because there 's not another English word
| for the scale of objects without hard edges.
| coliveira wrote:
| The Heisenberg principle only makes it impossible to know the
| exact size of a single proton in a particular moment in time.
| But it doesn't say that you can't calculate the average of
| millions of protons, for example.
| ninkendo wrote:
| I'm an ignorant layman here, but the uncertainty principal only
| says that if you're sure about the size/location, then you're
| very unsure about the momentum. It's possible that measuring
| its size does not need much certainty about its momentum, so
| they can get the accuracy they want.
| Bootvis wrote:
| If I'm blindly shooting a lot of bullets at a lot of ducks, I
| should be hitting some ducks. If I then count the number of
| shots fired and the number of ducks dead on the ground I should
| be able to calculate the size of ducks with high precision.
| J5892 wrote:
| A better analogy would be to subtract the amount of bullets
| embedded in the wall behind the ducks from the total bullets
| fired.
|
| Assuming ducks stop the bullets.
|
| And assuming it's not just one immortal duck flying around
| super fast.
| ummonk wrote:
| So to summarize, standard model theory predicted that the radius
| of the proton would be 0.84 femtometers. High energy electron
| scattering experiments from the 90s and 2000s suggested it was
| more like 0.88 femtometers, which was a large discrepancy that
| causes some consideration that the standard model would have to
| be revised. These researchers performed a reanalysis of the data
| correcting for some confounding phenomena (might have been
| neutron formation) to suggest that the old experimental data was
| consistent with the 0.84 femtometers predicted by both theory and
| by newer lower energy scattering experiments.
|
| I'll leave it as an exercise for the reader to decide whether the
| title of the article is clickbait.
| ISL wrote:
| It's not clickbait.
|
| I've been to oodles of talks on the problem. It has had special
| sessions at APS meetings and I've watched acquaintances and
| colleagues expend meaningful fractions of their careers
| attempting to resolve the Proton Radius Puzzle.
|
| If an appeal to authority is required -- Nature gave it the
| cover. https://www.psi.ch/en/media/our-research/protons-
| smaller-tha...
|
| Pohl's measurements were incontrovertible, yet they disagreed
| with decades of work. It has been a big problem in a quiet
| community for some time.
|
| If someone has a reliable angle to resolve the problem, it is
| big news. Enough really good physicists have tried and failed
| to resolve the conundrum that one should wait to see if this
| new approach holds up, too.
| ummonk wrote:
| The headline gives the impression that the researchers found
| a new discrepancy where protons are found to be smaller than
| theory would predict. In actuality, the researchers are
| proposing a resolution to a preexisting discrepancy.
|
| Additionally, I'm not sure why you're saying we should wait
| to see whether the results hold up, if you don't think
| writing "probably" in the headline is clickbait.
| [deleted]
| asdfman123 wrote:
| I misread this as "pronouns."
|
| I need to spend less time on Twitter.
| Victerius wrote:
| What are quarks made of?
| labster wrote:
| Snips, snails, and gold-pressed latinum.
|
| Or possibly strings. If that's not true, the best answer is the
| tautology: quarks are made of quarks.
| 323 wrote:
| Quarks are excitations in the quark fields. Thinking about them
| as particles is a useful simplification/model.
|
| Analogy: there are no "red pixels", there are just red
| excitations in the display RGB field.
| davrosthedalek wrote:
| As far as we know, quarks are not composite particles. They are
| fundamental (and point-like). Same as electrons, photons, etc.
| brabel wrote:
| I wish they explained how exactly the fact that protons are 5%
| smaller than previously thought actually affects our current
| knolwedge. Or it just doesn't?
| davrosthedalek wrote:
| (Disclaimer: I work in this field, having done of the original
| measurements. I do not believe this is a settled case at all.)
| If the proton is indeed smaller, it changes the Rydberg
| constant by several sigma. The Rydberg constant is one if not
| _the_ best determined constant of nature. This has implications
| for precision tests of QED, for example.
| H8crilA wrote:
| What is even the _size_ of a proton? I mean it 's not like
| it's a nice spherical ball [made of something else].
|
| From reading the article it seems like there's a hard
| distance boundary beyond which it will not "collide" with the
| electron?
| davrosthedalek wrote:
| In this context, it's the root-mean-square of the electric
| charge distribution. (Or more precisely, it's related to
| the slope of the electric form factor at Q^2=0.) There is
| also at least a magnetic radius (similar to the electric),
| and a gravitational radius.
| nobrains wrote:
| Does the electrical charge ever reach 0 or does it keep
| getting less and less and for practical purposes, we have
| to set a cut-off limit and count the boundary of the
| proton from there?
| dataflow wrote:
| > What is even the size of a proton?
|
| Just a layman but I think they usually define size via the
| halfway distance between the centers of two identical bound
| particles. Not entirely sure what that would be in this
| case though, given helium-2 is unstable and helium-3 might
| give a different result. (?)
| contravariant wrote:
| As far as I know the size in these discussions is some
| kind of scattering cross-section. So roughly it's just
| how likely you are to hit it / how precise you need to
| aim.
|
| I don't know the exact details of the definition though.
| davrosthedalek wrote:
| That's one way to determine the size. But the same thing
| pops up in spectroscopy.
| gowld wrote:
| Protons don't bind without neutrons mixed in.
| dataflow wrote:
| That's why I wrote He-2 is unstable, yes.
| vikingerik wrote:
| They do for a very short duration (10^-9 s.) Nuclear
| fusion happens by protons binding via the strong force,
| then one beta-decays into a neutron before the
| electromagnetic repulsion pushes them beyond the range of
| the strong force. The beta decay happens fast enough in
| 0.01% of proton-proton collisions.
| MrLeap wrote:
| Is there.. any reason why all protons must be the same size?
| db48x wrote:
| Yes. All protons are indistinguishable from each other. No
| matter what aspect of a proton you measure, it will be
| identical to any other proton you could measure instead.
|
| This has important implications.
|
| Consider a quantum mechanics experiment where you emit a
| proton at A and try to detect it at A'. You will find that
| that there is some quantifiable chance of detecting the
| particle at A' some time T. Call that probability P.
|
| Now consider a second experiment where you emit a proton at
| both A and B, and try to detect them at A' and B'. What is
| the probability of finding a proton at A' at time T? You
| will find that you get a different number! This is because
| the proton at A could travel to A' and be detected, but
| also the proton at B could travel to A' and be detected
| too. Since you cannot distinguish the two protons, you
| won't be able to distinguish between these two outcomes,
| and so the probability must be different from P.
| busyant wrote:
| How does your experiment demonstrate that all protons are
| indistinguishable?
|
| Couldn't it be that your measurement for a proton being
| at A' is simply measuring the wrong feature of the
| protons?
|
| edit: I have no idea if protons are indistinguishable
| from one another, but this experiment doesn't seem
| compelling.
| xwolfi wrote:
| Turn it the other way around: if it has a different level
| of energy in its quantum field, it doesnt expose the
| properties of what we call "a proton".
| MrLeap wrote:
| I was thinking the same thing. If you read the experiment
| described but replace the word "proton" with "cat" -- I
| would just assume that the scientists in question were
| from a society with very coarse senses and measurements,
| not that all cats are indistinguishable.
| scott_s wrote:
| Think of it this way: we can experimentally figure out
| what the probabilities are; they are an observed thing.
| The only way to make sense of these probabilities is if
| all protons are indistinguishable.
|
| In my statistical mechanics course, we went through an
| illuminating exercise where we started by trying to take
| account of every atom in a gas cloud. We started taking
| limits and making assumptions. One of them was that all
| atoms are indistinguishable from each other. This
| decreases the possible states of the system by N!
| (factorial, not surprise). After making that assumption,
| out pops the ideal gas law.
| MrLeap wrote:
| This is convincing to me, thank you.
| MrLeap wrote:
| Your hypothetical seems to describe a current inability
| to differentiate between protons, rather than convincing
| me that protons _must_ be identical. Is this like the
| monsters on old sea maps, just a gate around an unknown?
| octoberfranklin wrote:
| There is a lot of circular reasoning involved in quantum
| information and thermodynamics.
|
| It's all totally, perfectly self consistent, but it does
| not derive from first principles like set theory or
| mathematical logic do. Physics is an experimental
| science; they are not required to state their axioms.
| Oftentimes they do (QFT for example), but the most
| glaring case where they don't is anything involving
| information.
|
| The whole postulate that information is physical is
| something that was stumbled upon, and then turned out to
| explain a whole bunch of other weird things like heat and
| entropy, and some of those explanations in turn implied
| that information is physical.
|
| I suspect that our current efforts to build
| cryptographically-relevant quantum computers are a lot
| like the efforts to build perpetual motion machines in
| the 1700s. Our current understanding of things isn't
| _wrong_ , but there is some undiscovered general
| principle that we keep butting up against, so we'll keep
| trying to build these things until we figure out why
| nature keeps blocking us. That discovery -- rather than a
| computationally-useful device -- will be the most
| important result of all the quantum computing research
| going on right now.
| adgjlsfhk1 wrote:
| that seems plausible, but currently very unsupported by
| evidence. in the past decade, quantum computers have
| gotten way more powerful, and progress doesn't seem to be
| stalling out.
| MatteoFrigo wrote:
| I think that GP is trying to say the following.
|
| In quantum mechanics, probabilities are given by the
| square of the absolute value of more fundamental
| quantities called amplitudes. When something happens in
| two ways that can be distinguished, you must add the
| probabilities. When something happens in two
| indistinguishable ways, you must add the amplitudes,
| which yields a different probability after squaring. For
| example, .3^2+.4^2 != (.3+.4)^2. Thus, you can verify
| experimentally whether particles are or are not
| distinguishable.
| Iolaum wrote:
| Think of it in a simpler way. There is nothing we can
| currently measure, that will be different for one proton
| versus another.
| [deleted]
| RobotCaleb wrote:
| Does this mean that there's possibly only one proton?
| kazinator wrote:
| Two protons P0 and P1 are the same only if they match in
| every single quantum parameter.
| willis936 wrote:
| On the contrary, protons are baryons and they must each
| have a unique quantum state (they must be in a
| unique/distinct location).
| infogulch wrote:
| You mean like the "One-electron universe" postulate [1]?
| First, I don't think it's really useful beyond being a
| fun idea to consider. Also there's a lot more matter than
| antimatter, which raises some... logistical problems.
| Also, unlike electrons/positrons, protons are not
| fundamental particles, they're made up of quarks which
| throws a whole other wrench in the idea.
|
| [1]: https://en.wikipedia.org/wiki/One-electron_universe
| burnished wrote:
| Good question. I'm not sure. We treat them like they are
| all the size, which I assume is a function of all them
| being themselves composed of pieces that we treat as
| fungible. On the other hand its not like anyone is
| measuring a proton size with a pair of calipers, so them
| all being the same 'size' could simply be a function of
| them all having the same charge.
| stronglikedan wrote:
| layman, but I believe it's the "size" (defined by the wave
| function) that makes a particle a proton. at least, one of
| the factors that does, anyway
| uoaei wrote:
| Each solution to the Schrodinger equation describes a
| different kind of particle. We assume that QFT is
| deterministic and complete so each particle will have
| exactly the same properties as the others of its kind aside
| from position and momentum (as far as we can tell).
|
| As a fun consequence, the theory treats all particles the
| same -- even so-called quasi-particles! The difference
| between fundamental and other kinds of particles is that
| the fundamental particles can exist (at least for a short
| time before decaying) _in vacuo_.
| gowld wrote:
| Protons are made of 3 quarks and a bunch of tiny gluons. if
| a big proton existed, it would be made of different or more
| quarks/stuff, and be called something else.
|
| Now, is there a reason why there isn't a something else
| that acts like a proton?
|
| That gets into Elementary particle physics and what
| combinations are stable and have matching charge. Quarks
| have charge +- n/3 and generally come in triples.
|
| Could the number of gluons vary? Maybe? But they wouldn't
| affect the size measurements much? And variations wouldn't
| be stable?
| saurik wrote:
| (Disclaimer: I don't do anything even slightly related to
| this field, and I somehow managed to skip taking any physics
| in college except quantum, which I literally slept through
| and dropped because it was too early in the morning _three
| times_ before simply giving up. So, my question is probably
| super super dumb ;P.)
|
| Isn't a sigma a _lot_? Like, I think that 's a standard
| deviation? If there is even a longshot chance that we are
| might be off on that constant by multiple standard
| deviations, isn't that certainly a less-determined constant
| than, say, the acceleration of gravity? I feel like there is
| no chance in hell we could one day discover that our
| calculations are that far off for gravity.
| davrosthedalek wrote:
| Yes and no. It's a lot in the sense that we are way off
| from what we believed or knowledge to be. But on absolute
| scale, it's not a lot. The current determination of the
| Rydberg constant puts it at 10973731.568160 1/m with an
| uncertainty of 0.000021 1/m. So a relative precision of
| ~2*10^-12 (or maybe only 10^-11).
|
| The standard acceleration of gravity is, btw, defined, so
| no uncertainty. The gravitational constant G is known only
| to 10^-5 or so.
| jacksnipe wrote:
| The acceleration due to gravity isn't a constant of nature
| --- it's dependent on the mass and mutual distance of both
| objects.
|
| EDIT: distance, not size
| moron4hire wrote:
| Perhaps they meant the gravitational constant, which is
| not known to a very high degree of precision.
| saurik wrote:
| But we've got to have it down within two standard
| deviations, right? ...No? Do I just not know what a sigma
| is? :(
| penteract wrote:
| I think you have a misunderstanding about sigma. When
| describing the measurement of a particular physical
| constant, the "standard deviation" is something that
| changes as we get better at making measurements. It
| basically means "If all our assumptions (e.g. assumptions
| about how good our equipment is, uncertainties about
| other physical constants) are correct, then it is
| unlikely that we would have made the measurements we did
| if the true value is not within 2 standard deviations of
| the result we got". When a more accurate measurement is
| made, then "1 standard deviation" gets smaller, so we
| know the value better, but it's always true to say "we
| know the value to within a few standard deviations (given
| some assumptions made by experimenters)" . If it turns
| out the measurement was wrong by several standard
| deviations, then it's very likely that some assumptions
| were wrong.
| SkittyDog wrote:
| Your correction is wrong... The size of the two objects
| is irrelevant. The only variables are their respective
| masses and the DISTANCE between them.
|
| If it's worth correcting people, it's worth correcting
| them correctly... Don't you agree?
|
| Also... The previous poster was pretty obviously talking
| about the constant of Gravitation
| (https://en.m.wikipedia.org/wiki/Gravitational_constant).
| His way of phrasing it is a common English shorthand that
| I see frequently enough that it's a well understood
| usage. They may not have used precise language, but their
| phrasing was definitely less misleading than your
| (incorrect) correction.
| jacksnipe wrote:
| I assumed the gp was thinking about 9.8m/s^2 -- because
| on the English speaking non-physics-expert world, this is
| called the _acceleration due to gravity_.
|
| You're right that it's distance, of course, but I was
| coming from a place of trying to be helpful, and thought
| that if they were talking about terrestrial physics, it'd
| be easiest to imply it depended on the size of the earth
| (which determines our distance from it) and your height.
| ummonk wrote:
| That's not something that could change based on
| experiment though. We define standard gravity as 9.80665
| m/s^2 but the actual value will vary considerably based
| on location on the earth.
| ummonk wrote:
| It's only dependent on the mass of the other object, not
| the mass of the object whose acceleration we're
| measuring.
| uoaei wrote:
| I'm sure they meant the gravitational constant. Jeez.
| [deleted]
| brian-armstrong wrote:
| Most likely it indicates protons have devalued their currency
| slightly. Probably not a big deal for now but we should keep a
| close eye on it in case it happens again.
| AnimalMuppet wrote:
| Cosmic inflation?
| bonzini wrote:
| We had two methods to measure the proton and they gave
| different results. People used to trust the old value more, but
| a study in 2019 implied that it was the incorrect one and the
| correct value is the new one. See
| https://en.wikipedia.org/wiki/Proton_radius_puzzle,
| https://www.quantamagazine.org/physicists-finally-nail-
| the-p....
|
| The latter says "The new result implies that earlier attempts
| to measure the proton's radius in electronic hydrogen tended to
| overshoot the true value. It's unclear why this would be so"
| and it seems that these researchers have now shown why.
| fsh wrote:
| The value of the proton charge radius in itself is pretty much
| irrelevant, otherwise it wouldn't be so hard to measure.
|
| As scientists we want to know if our understanding of nature is
| correct. To test this, we measure the same quantity, for
| example the proton charge radius, in different ways. If the
| underlying theory is correct, the results should agree within
| the experimental uncertainties.
|
| Since 2010 there was a big disagreement between the proton
| charge radii measured by hydrogen spectroscopy and
| electron/proton scattering (which roughly agreed at the time),
| and a much more accurate measurement using muonic hydrogen
| spectroscopy. This has lead to a lot of excitement, since
| discrepancies could be a hint for new physics. Since then, more
| accurate hydrogen spectroscopy experiments have been performed
| and most agree with the muonic hydrogen value. This probably
| indicates that the discrepancy is due to underestimated error
| bars in the old measurements.
|
| In contrast to laser spectroscopy which gives relatively direct
| results, getting the charge radius out of electron scattering
| data is notoriously hard. Different groups have found different
| charge radii from the same data for a long time.
| sundarurfriend wrote:
| Ignorant outsider to the field here: I've often heard the claim
| that quantum mechanics has been verified to amazing accuracy,
| that its predictions match with reality to the maximum degree
| that our instrumental precision allows, etc. A 5% difference
| seems big enough that at least some of our experiments should
| have error limits less than that. So how is it that this is only
| now being found, and there's still uncertainty surrounding it?
|
| I feel like I'm missing something fundamental here, and I'd like
| to know what it is.
| gowld wrote:
| Well this 5% is the instrument precision.
| Yajirobe wrote:
| Quantum theory has the largest error between theory and
| prediction (120 orders of magnitude). See
| https://en.wikipedia.org/wiki/Cosmological_constant_problem
| evanb wrote:
| That's just a back-of-the-envelope calculation. Nobody knows
| how to do a bone-fide calculation because we don't have an
| accepted quantum theory of gravity.
| db48x wrote:
| There are lots of different numbers that you can use quantum
| mechanics to predict. It turns out that the size of the proton
| is both harder to predict and harder to measure than many of
| those other numbers.
|
| Meanwhile we can measure the fine structure constant to 12
| decimal places (https://en.wikipedia.org/wiki/Electromagnetic_c
| oupling_const...) and that measurement is in very close accord
| with the predictions of quantum mechanics.
| dnautics wrote:
| I thought the fine structure constant was primitive: there is
| no theory arguing what it's value should be.
| 323 wrote:
| That maximum precision had a confidence interval. The new
| smaller 5% value is within that confidence interval, which is
| now also tighter.
| not2b wrote:
| Real physicists can correct me, but here's my understanding of
| it: if only the electromagnetic force is involved, the numbers
| provided by QED are amazingly accurate (for example,
| calculating the magnetic moment of an electron). But when the
| strong force is involved, as for the radius of the proton, the
| calculations are much more difficult: you can't calculate what
| the radius should be.
| davrosthedalek wrote:
| That's correct. It's very hard to calculate the proton radius
| ab initio, even with LatticeQCD approaches.
| 77pt77 wrote:
| The size of the proton is a very poorly defined measurement.
|
| The amazing accuracy you have read about is the anomalous
| magnetic moment of the electron, which is a very clear cut
| measurement.
___________________________________________________________________
(page generated 2022-02-04 23:00 UTC)