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Mystery of the Shrunken Proton
ananyo writes "The proton, a fundamental constituent of the atomic nucleus, seems to be smaller than was previously thought. And despite three years of careful analysis and reanalysis of numerous experiments, nobody can figure out why. An new experiment published in Science only deepens the mystery. The proton's problems started in 2010, when research using hydrogen made with muons seemed to show that the particle was 4% smaller than originally thought. The measurement, published in Nature, differed from those obtained by two other methods by 4%, or 0.03 femtometers. That's a tiny amount but is still significantly larger than the error bars on either of the other measurements. The latest experiment also used muonic hydrogen, but probed a different set of energy levels in the atom. It yielded the same result as the Nature paper — a proton radius of 0.84 fm — but is still in disagreement with the earlier two measurements. So what's the problem? There could be a problem with the models used to estimate the proton size from the measurements, but so far, none has been identified. The unlikely but tantalizing alternative is that this is a hint of new physics."
that it was the cold water.
Welcome to the Panopticon. Used to be a prison, now it's your home.
Subway Corporate announces that their foot-long measurements were unfortunately based on accepted assumptions of larger protons.
"He's using a quantum encryption scheme! That'll take hours to break!"
It's old physics that we haven't figured out yet, but thought we had.
But what are our meter sticks made of? Why would they grow with the universe if they are made of particles that stay the same size?
uh sure, if the prior method for measuring was also showing the reduced size, but it's not ... so how does "the universe expanding" explain two simultaneously different measurements? besides, if the universe were expanding and protons weren't, i don't think our meter sticks would be expanding.
how the hell did this get +5 anyway ... brainless mods
Not slightly.
Imperial gallon = 4.55 l
US gallon =- 3.79 l
Enough to cause a difference of several hogsheads to the gallon and a half.
The vast majority of the space inside atoms is empty, determined by the size of the orbits of the electrons around the nuclei, which are essentially unaffected by the proton size. It would be like saying the sun doubled in size, but stayed the same mass: all the planets would still orbit at the same range (orbital distance is determined by mass and attractive force). The quantum case is a tiny bit more complicated, but this classical example illustrates the point.
Duplicate with another similar post, but I'll bite on this one anyway.
The simplest counter is that the old methods still get the old values.
The more complicated answer has to do with the abundant consequences of expanding inter-atomic distances in a universe where attractive forces decrease in strength by the cube of the distance. A universal 4% increase in interatomic size should result in a ~12% decrease in magnetic and gravitic attraction. This would be very noticeable.
There are even more complicated answers, but I don't feel like even doing the basic estimate math for those.
This doesn't appear to be a case where the measurement is changing over time. That is, it seems many here are misinterpreting the summary to suggest that things are different NOW relative to THEN.
Instead, things are different if we measure THIS WAY vs. THAT WAY. But we can still go back and measure both ways. If we use the old method(s), we get the old result.
That's what's creating the angst. Theorists cannot see why the two methods would differ. And they've checked and rechecked their work. Experimentalists have also checked and rechecked their work.
This is one of those "that's funny" things that becomes rather interesting.
It does not work that way. Things like metre sticks are held together by the electromagnetic force, which is decoupled from the expansion of the Universe. This means that objects in the Universe do not expand, they just move along with the expansion. If everything in the Universe expanded with the Hubble flow then we would never be able to detect the Hubble flow. Only spacetime expands, not what is sitting around in spacetime.
The explanation for the unexpected small size of the proton is probably something to do with the way that muons interact with protons. We assume that electrons and muons interact with protons in exactly the same way, but this is a hypothesis. There is very little observational evidence supporting the idea that electrons and muons behave in exactly the same way when they are bound to an atomic nucleus. The problem with this idea is that it requires that particle physics be extended beyond the standard model. It is also possible that the problem is something much more mundane, like a faulty connection somewhere in the experimental setup. We need an independent verification of this result before we start rewriting the textbooks.
Just because you are paranoid does not mean that no-one is out to get you.
Wait. I thought the whole point of this experiment was that they extrapolate the size of the proton by actually measuring the size of the orbital. So it's not necessarily that the proton has gotten smaller, just that muons orbit closer than electrons. If anything, they've taken some of the empty space out of the atom.
My first idea would be that the muon does indeed shrink the proton. After all, the proton is not some solid body, but consists of interacting charged quarks. The muon has a higher probability to be inside the proton (that's exactly why it is useful for measuring its size), and thus lowers the charge density there (it adds some negative charge density to the proton's positive charge density). The electrostatic repulsion inside the positively charged proton should certainly affect its size; decreasing that repulsion due to the partial screening by the muon should therefore allow the proton to shrink a bit. Not much, but maybe enough to explain the difference.
You are exactly the opposite of right. They don't actually measure the proton, they measure an orbital and do some math to determine the size of a proton. They would expect a muon to orbit at the same distance since it has the same charge as an electron, but they're getting a smaller sized orbital and therefore determining that the proton has shrunk. In reality, protons are the same size, and we're stumped as to why muons are behaving differently than electrons. If anything, muonic hydrogen has less empty space than regular hydrogen. Nothing expanded. The overall size of the atom shrunk even though the components stayed the same size.
It reminds me of the Michelson-Morley experiment. Back then no one understood why an experiment that should have given different results for the speed of a ray of light failed to do so. As we know today, the constant speed of light is the basis for Einstein's relativity theory and has been proved right many times.
Could this be one of those moments?
> besides, if the universe were expanding and protons weren't, i don't think our meter sticks would be expanding.
Perhaps you are missing the fact that the meter sticks are made of atoms, not protons. An expanding electron shell, and resulting inter-atomic distance might go some way towards explaining the meter stick phenomenon.
But just to argue the other side as well, astronomic evidence suggests that the universe is expanding. That we can tell this means that we have some metric that is NOT expanding. In this case, the speed of light.
So perhaps we can use the speed of light to determine if atoms are changing size relative to the speed of light ... traversing them, perhaps?
Short answer is that I suspect the physics is not new, but something related to something we think we qualitatively know, but we don't really know how to bound the computational errors correctly in a complicated system.
AFAIK, the QED computation techniques that are used to compute bound state of a proton (often modified ordered pertubation methods) aren't particularly convergent so many shortcuts are taken (e.g., use orders of different quantities like non-relativistic velocity, etc). By using a muon and a proton (instead of an electron and a proton), we are essentially replacing something we know more about (the electron) with something we know less about (muon), to try and compute something about something we don't know much about (the proton). Since we don't know much about protons yet, I believe most computations of the bound state are currently just assuming things about them (charge is a point source, nothing about quarks). I haven't read the paper yet, so it's hard to know what they are doing in the QED corrections.
Maybe there is a slight chance that this simplistic system (muon+proton) can macroscopically exhibit something that hints that QCD confinement inside a proton or muon isn't perfect (e.g, the heavy quarks sortof show themselves in a way that we can measure) which would be some interesting new gluon physics that is currently beyond our particle collider reach. But in some ways this might just show us that the QED based adjustments we are making aren't good enough for the real system and we need some even harder to dream up QCD adjustments and it's hard to say that this would definitly be new physics, but perhaps just new math on old QCD physics....
They would expect a muon to orbit at the same distance since it has the same charge as an electron
Actually, no they don't. The whole point of using muons is that their orbitals would be much closer to the proton due to the muon's mass. The size of the orbitals and structure of the orbitals depends on the mass ratio between the two parts, and since the muon is much more massive than the electron, it was expected to have smaller orbitals, much smaller than 4%. And hence, it was expected the smaller orbitals would be more sensitive to structure of the proton. The discrepancy comes from the effects of the proton on the orbital not being quite what they expected from electron based measurements, not from just a change in the size of the orbital.
Now, they tell me.
Please do not read this sig. Thank you.
It is more complicated than that. The measurements using the muon yielded two different sizes, a size related to the distribution of charge within the proton, and a size related to the magnetic structure of the proton. The latter is in agreement with electron and spectroscopic measurements. It is only the first one related to the charge distribution of the proton that disagrees. This heavily points toward a slight discrepancy in the structure of the proton. This points toward improving work with computational QCD, which while having made some great strides, still has a lot of room for improvement.
It is not so simple as a change in size of the orbital structure. First off, the point of the experiment was that the muon orbital would be much smaller. Second, they measured two different atomic transitions in the system, involving four different orbitals. It wasn't the over all size/energy of the orbitals that was under consideration, it was the relative energies involved in these transitions.
The results of comparing the transition energies were done two different ways, one sensitive to the magnetic structure of the proton, the other sensitive to the charge structure of the proton. The former was in agreement with previous measurements of the magnetic size of the proton. The latter is the one that is off by 4% from older measurements. There wasn't some singular, overall change in the size of everything involved. Instead, this points to there being something wrong with the understanding of the charge structure of the proton, and hence that structure's predicted impact on the muon orbitals.
Just changing sizes or talking about expansion wouldn't account for the second half of their results where they found agreement with past, electron based measurements.
I think a proton is shaped like a dodecahedron, so of course it's going to measure up to 4% less depending on it's orientation.
That was the turning point of my life--I went from negative zero to positive zero.
The weird thing seems to be that it's not a single measurement that differs. Measured a couple of different ways gives one size, if you measure a couple of other ways you come up with another size. Consistently. It's as though you measured a board with a meter stick and it was 90 cm long, but when you measure with a tape rule it's 86 cm long.
"Think about how stupid the average person is. Now, realise that half of them are dumber than that." - George Carlin
because if *everyone* does the experiment and the results don't match up with the theory, then there's something missing in the theory.
In this case, taking the same measurement two different ways results in two different numbers, and the theory says they should match.
Do these muons make my ass look fat?
No, your ass makes your ass look fat; the muons actually make you look 4% slimmer!
I call it a very VERY slight difference. This is why:
/everyone/ would just use 1 system...
1 Gallon is 8 pints in both systems.
US pt. = 0.47375 L
IM pt. = 0.56875 L
The difference between the two gallons is 0.76 L which is 1 2/3 of a US pint or 1 1/3 of an imperial pint.
So the difference is actually just 1/3 of a pint, that is 0.16L (1/3 US pt.) or 0.19L (1/3 IM pt.) So the difference (0.19L - 0.16L) in Litres is ACTUALLY only 0.03L.
And since 0.03 litre is only about 1 Fl.Oz and a Fl.Oz is 1/160 imperial Gallon as anyone knows, and 1/128 US Gallon for that matter the difference all the sudden is only 1/32 Fl.Oz which in turn is hardly a teaspoon full...
erhm...
Hmmm... maybe I made a boo-boo somewhere along the road...
Maybe things would be much more clear if
rm -rf --no-preserve-root /
You'd shrink too... it's cold out there in the aether.
Well, that's what he's telling the other particles...
You have the right to remain sentient. If you give up the right to remain sentient, you will be elected to public office
how the hell did this get +5 anyway ... brainless mods
Anyone's who's read an amateur physics forum knows that the expanding scale universe "model" is reinvented several times a year by isolated eager guys armed with high school diplomas, apocryphal tales about Einstein and quotes by Galileo. It's one of those ideas that seem obviously true for several seconds until you actually think about it.
Here's a tip: The age of simple discoveries in mature sciences is over. That's why they're called mature. Unless you've spent years studying physics intensely while getting frequent feedback from experienced physicists, your chances of making minor contributions to physics are infinitesimally close to zero. Any idea that you quickly stumble upon based on your high school or college Physics 101 understanding has literally been thought, tried and discarded a thousand times before by physicists.
I did not say this very well. A better way of putting it is that molecular bonds (in fact, any of the four fundamental forces) are stronger than the expansion of the Universe over short distances. This is why you and I and my pint of beer do not expand along with spacetime. We are sitting in spacetime and are held together by the four fundamental forces. It is a bit like the way that a marble sitting on a rubber sheet does not expand when one stretches the rubber sheet.
Just because you are paranoid does not mean that no-one is out to get you.
I don't think anyone said anything about the proton's mass, just the radius.
A difference of 4% in the previously measured mass would be a much bigger story.
The radius, on the other hand, has much less significance -- it even depends heavily on an arbitrary definition, since a proton doesn't have a definite boundary.
It's not correct to think of objects as being passive points attached to an actively expanding grid which carries them along. Objects (masses) are primary participants in the shaping of spacetime and not simply being dragged along. In other words, we shouldn't think of the expansion of the universe as causing faraway galaxies to move away from us. Rather, the fact that faraway galaxies are moving away from us is the expansion of the universe (and not a symptom). Everything is moving apart from each other. Galaxies are coasting away from us due to inertia, causing the expansion to continue. (Dark energy, or cosmological constant, is causing the expansion to accelerate by altering the geodesics that these galaxies are following.)
With this picture in mind, it should be more clear that the expansion of the universe won't cause a ruler or the Sun to expand.