×

Welcome to the Slashdot Beta site -- learn more here. Use the link in the footer or click here to return to the Classic version of Slashdot.

Thank you!

Before you choose to head back to the Classic look of the site, we'd appreciate it if you share your thoughts on the Beta; your feedback is what drives our ongoing development.

Beta is different and we value you taking the time to try it out. Please take a look at the changes we've made in Beta and  learn more about it. Thanks for reading, and for making the site better!

Mystery of the Shrunken Proton

Soulskill posted about a year ago | from the neutron-did-it-in-the-nucleus-with-the-strong-nuclear-force dept.

Science 171

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."

cancel ×
This is a preview of your comment

No Comment Title Entered

Anonymous Coward 1 minute ago

No Comment Entered

171 comments

Global warming (-1)

Anonymous Coward | about a year ago | (#42690831)

must be...

ACA is 3.6% (-1)

Anonymous Coward | about a year ago | (#42690857)

It's the new tax.

Re:Global warming (0)

Anonymous Coward | about a year ago | (#42691845)

No, this one is global shrinking.

But maybe it is connected with the kilogram prototype losing weight. :-)

Re:Global warming (3, Insightful)

FatdogHaiku (978357) | about a year ago | (#42693049)

You'd shrink too... it's cold out there in the aether.
Well, that's what he's telling the other particles...

I read this as Shuriken proton. (-1, Offtopic)

Anonymous Coward | about a year ago | (#42690919)

So I assumed it was about Atomic Ninja.

That would have been awesome.

Take that Ghost Pirates!

Usual suspect (0)

gmuslera (3436) | about a year ago | (#42690931)

Maybe they mixed imperial with metric units in some part of the process. At least they could had enjoyed to get back home and say "Honey, i shrunk the proton"

Re:Usual suspect (-1)

Anonymous Coward | about a year ago | (#42690971)

This isn't NASA we're talking about.

Let's use a British Foot to measure them! The King died yesterday, how wide is a proton now?? I don't know,we have to put it up to the new King's foot!

They could always use the British Stone for weighing protons too. How much does a British Stone weigh? Somewhere between 5 & 40 pounds! Yea, that'll never cause any confusion!

Re:Usual suspect (0)

Anonymous Coward | about a year ago | (#42691097)

The best part is when you have a British Imperial Gallon and an American Customary Gallon that differ slightly for no good goddamn reason, and then British internet posters wonder why American cars get so few miles per gallon of gasoline.

Re:Usual suspect (2)

Nexus7 (2919) | about a year ago | (#42691231)

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.

Re:Usual suspect (4, Funny)

Razgorov Prikazka (1699498) | about a year ago | (#42692993)

I call it a very VERY slight difference. This is why:
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 /everyone/ would just use 1 system...

Dr. George Costanza theorizes (4, Funny)

idontgno (624372) | about a year ago | (#42690933)

that it was the cold water.

Re:Dr. George Costanza theorizes (1)

camelrider (46141) | about a year ago | (#42691105)

that it was the cold water.

Or old age.

Re:Dr. George Costanza theorizes (0)

Anonymous Coward | about a year ago | (#42692555)

I think you're confusing shrinkage with lower hanging ;)

Re:Dr. George Costanza theorizes (0)

Anonymous Coward | about a year ago | (#42691175)

How can such an apt analogy be deemed offtopic?!? Shrinkage!

Re:Dr. George Costanza theorizes (0)

Anonymous Coward | about a year ago | (#42691225)

That's why there are more male physicists than female physicists. Women don't know about shrinkage.

Re:Dr. George Costanza theorizes (0)

Runaway1956 (1322357) | about a year ago | (#42691983)

1. Get married.
2. Wait for the new bride to wash you wool.
3. Blow up when you find that you can no longer wear hundreds, even thousands, of dollars worth of perfectly good clothing.
4. Stay in the dog house for a week, or until she feel horny.
5. Explain to her, very gently, that it was the HOT WATER that destroyed your wardrobe.

To be fair, my wife only destroyed about $750 worth of clothing. I should have known that a southern girl wouldn't know how to launder wool . . . .

Re:Dr. George Costanza theorizes (2)

ColdWetDog (752185) | about a year ago | (#42693057)

Wool?

Please turn in your Geek card at the turnstile.

It's polyester. The fabric of the future. For formal occasions (weddings, funerals, job interviews), cotton is acceptable.

Slashdot has gone so low these days.....

easy (1, Interesting)

AbrasiveCat (999190) | about a year ago | (#42690941)

The universe it growing (including our meter sticks) and the proton is staying the same size.

Re:easy (3, Interesting)

michelcolman (1208008) | about a year ago | (#42691147)

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?

Re:easy (3, Informative)

Anonymous Coward | about a year ago | (#42691271)

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.

Re:easy (1)

michelcolman (1208008) | about a year ago | (#42691423)

But if protons are the same size and the apparent shrinkage is due to our meter sticks expanding, why would they be expanding? Is it the forces between particles that are changing? Energy levels? Why would particles be further apart now than before?

Anyway, I don't think the universe, or protons, have changed size by 4% in a few decades so this discussion is a bit pointless. They just used two different methods of measuring the size, and one or both of those methods are wrong. Which could yield interesting new physics, but nothing radical like protons shrinking 4% in the blink of a cosmological eye.

Re:easy (1)

RoccamOccam (953524) | about a year ago | (#42691743)

There's that word again. "Shrinkage." Why are things shrinking in the future? Is there a problem with the Universe expanding?

Re:easy (1)

Changa_MC (827317) | about a year ago | (#42692739)

option 1: the entire universe has expanded in the last couple decades, including the distance between every particle, but not the size of protons.
option 2: protons have shrunk in the last couple decades
option 3: one of our measuring methods is wrong for a reason we don't yet understand.
Hint: those are sorted in reverse order by ridiculous over-complication.
If you hadn't misplaced your razor, Mr. Occam, you'd have already eliminated the first one on your own.

Re:easy (1)

RoccamOccam (953524) | about a year ago | (#42692859)

My comment was in jest, referencing a well-known quote from the movie "Back to the Future". Thanks for making me explain it!!

Re:easy (3, Interesting)

WillgasM (1646719) | about a year ago | (#42691505)

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.

Re:easy (5, Informative)

Anonymous Coward | about a year ago | (#42692437)

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.

Re:easy (1)

PKFC (580410) | about a year ago | (#42693083)

So if this new measurement is related to charge in the proton, could it be an impact of how close or perhaps more likely how fast the muon orbited the proton? As muons (and taus) are heavier than electrons, I would expect them to move slower around the proton which could cause a shift in how the quarks are positioned or interact within the proton. Mind you the muon is huge compared to the up or down quarks. And this is where my brain stops processing once I read that quantum chromodynamics is responsible for the majority of the mass-energy of the proton.. :P Oh well :P I tried!

Re:easy (0)

Anonymous Coward | about a year ago | (#42692659)

The whole point of the experiment was that the size of the proton does have an effect on the orbital energies. It is a very subtle effect, but still there. In a (bad) analogy, it would be like including tidal forces in orbital dynamics of planets and stars, in which case the size and composition of the bodies do matter and have subtle influences.

Re:easy (0)

Anonymous Coward | about a year ago | (#42691307)

The protons are sub-atomic particles. The meter sticks are made of molecules of atoms, so when the empty space between the sub-atomic particles increases then the meter stick grows while the proton stays the same.

Re:easy (0)

Anonymous Coward | about a year ago | (#42691673)

Protons are not fundamental particles, and are composed of a collection of quarks and gluons with space between them too..

Re:easy (1, Funny)

ve3oat (884827) | about a year ago | (#42691149)

Exactly. To their great credit, the protons have never raised their debt ceiling. So while the universe around them inflates out of control, the protons stay the same and still prosper. (There's a lesson here somewhere but the type on my screen is now so small that I can't read it anymore.)

Re:easy (5, Insightful)

Anonymous Coward | about a year ago | (#42691191)

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

Re:easy (1)

cupantae (1304123) | about a year ago | (#42691437)

Will mods please raise this comment to +5? The GP clearly didn't RTFA or didn't understand the situation, FFS.
If there actually were a 4% shrinkage over less than a hundred years, then how big was the proton hundreds of millions of years ago? How about 13.7 billion years ago? Use your heads, please!

Re:easy (1)

bondsbw (888959) | about a year ago | (#42691683)

But what exactly do you mean by "simultaneous"?

Obviously, the measurements are being done from different inertial reference frames.

Re:easy (2)

sabt-pestnu (967671) | about a year ago | (#42691837)

> 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 [pbs.org] 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?

Re:easy (0)

Anonymous Coward | about a year ago | (#42692099)

If there was a change in speed of light relative to atomic structure over time, we would not have been able to spectroscopically observe the same atomic and molecular structure in distant galaxies as seen locally.

Re:easy (0)

Anonymous Coward | about a year ago | (#42692221)

If there was a change in speed of light relative to atomic structure over time, we would not have been able to spectroscopically observe the same atomic and molecular structure in distant galaxies as seen locally.

We don't. They are all red shifted ...

Re:easy (0)

Anonymous Coward | about a year ago | (#42692503)

A lot of the work on atomic structure, even the one in the paper referenced here, looks at the ratio of energies between transitions and orbitals. There are a lot of non-linear effects, and additional effects that are proportional or quadratic with magnetic and electric fields. If you simply changed the size, you would not get something anywhere near as clean and simple as a bulk red shift affecting all the lines the same way. Additionally, there are many cases of studying red/blue-shifted atomic transitions in the lab...

Re:easy (0)

Anonymous Coward | about a year ago | (#42691895)

The original post didn't fully flesh out his idea to explain that. Let me fill in the details. The universe is expanding by ~4% on the time scale of seconds. It shrinks back down when researchers switch back to the old method. What they really need to do, is to run both the old and new method at the exact same time. Fortunate for the universe trying to hide its prank, there is no consistent definition for "exact same time" in relativity.

Re:easy (0)

Anonymous Coward | about a year ago | (#42692279)

Quantum expansion.

Re:easy (2, Informative)

Anonymous Coward | about a year ago | (#42692293)

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.

Re:easy (0)

Anonymous Coward | about a year ago | (#42691197)

Ha! Extremely unlikely! It is not that the universe increases in size by 4% over a period of a few years!

The simplest explanation is someone is wrong[1], including the theory. For centuries no one could explain why Mercury wasn't returning to same position in its orbit every revolution, until Einstein explained it. [2]

[1] - http://hermiene.net/essays-trans/relativity_of_wrong.html [hermiene.net]
http://www.youtube.com/watch?v=2tcOi9a3-B0 [youtube.com]

[2] - http://en.wikipedia.org/wiki/Tests_of_General_Relativity#Perihelion_precession_of_Mercury [wikipedia.org]

Re:easy (0)

Anonymous Coward | about a year ago | (#42691275)

Is gravity also increasing? Something to do with the extition of the dinosaurs? Less gravity helped building the pyramids?

Re:easy (5, Insightful)

mrsquid0 (1335303) | about a year ago | (#42691493)

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.

Re:easy (1)

backslashdot (95548) | about a year ago | (#42691641)

Unless it is happening randomly (shrinks at a certain rate for a few years, then stops or expands for time etc) I think there would be detectable changes in the Sun's output over time if that was the case because the rate of nuclear fusion would change (reduce, I think).

Re:easy (0)

Anonymous Coward | about a year ago | (#42692741)

Or why assume that the particle is spherical? If it's squashed or oblong, then it would be possible to have different radii measurments which all happen to be correct.

Re:easy (0)

Anonymous Coward | about a year ago | (#42692869)

There are experiments and measurements to see if the particles are not spherically symmetrical in structure, and other measurements examining the exact change in structure in magnetic and electric fields. Some of the former have places some incredible bounds on such structures, e.g. the electric dipole moment of the neutron has found its charge distribution to be incredible spherically symmetric. If the neutron were scaled up to the size of the Earth, the latest measurements would be equivalent there was not an electron worth or charge misplaced by more 50 micrometers within that Earth sized neutron.

Misread the title as Shoryuken (0, Offtopic)

Anonymous Coward | about a year ago | (#42690963)

I was moderately excited.

Cold (-1)

Anonymous Coward | about a year ago | (#42691013)

Probably coming out of a cold pool

Subway's explanation now makes sense (4, Funny)

GovCheese (1062648) | about a year ago | (#42691085)

Subway Corporate announces that their foot-long measurements were unfortunately based on accepted assumptions of larger protons.

mod 0P (-1)

Anonymous Coward | about a year ago | (#42691093)

Practical purposes, followed. Obviously website Third, you people's faces at haplees *BSD achieve any of the

It's not New Physics (3, Insightful)

whoda (569082) | about a year ago | (#42691107)

It's old physics that we haven't figured out yet, but thought we had.

"An new experiment"... (-1)

Anonymous Coward | about a year ago | (#42691117)

Stupid Americans...

Why do you keep writing "an" instead of "a"?

It's not smaller, everything else is bigger! (1)

Agent0013 (828350) | about a year ago | (#42691127)

If the universe is expanding everywhere, and if this included the space between protons and atomic particles, then this result would be due to the length we use to measure being larger than it was before. So it's possible that the proton isn't getting smaller, but that everything else in the universe is expanding with the expansion of the universe. Is there anything that precludes this as a possibility?

Re:It's not smaller, everything else is bigger! (2, Informative)

Anonymous Coward | about a year ago | (#42691315)

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.

Re:It's not smaller, everything else is bigger! (5, Informative)

ChromaticDragon (1034458) | about a year ago | (#42691323)

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.

Re:It's not smaller, everything else is bigger! (3, Insightful)

jalvarez13 (1321457) | about a year ago | (#42691821)

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?

Re:It's not smaller, everything else is bigger! (1)

cupantae (1304123) | about a year ago | (#42691349)

Is there anything that precludes this as a possibility?

Yes. That would mean a continuous growth, not a sudden change when a different method is used. Not to mention that a 4% change in a few years would mean that the proton was enormous around the time of the dinosaurs, even. If the proton was shrinking that quickly relative to collections of atoms, we would need an overhaul of a great deal of the current body of science.

Re:It's not smaller, everything else is bigger! (2, Interesting)

WillgasM (1646719) | about a year ago | (#42691749)

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.

Re:It's not smaller, everything else is bigger! (5, Informative)

Anonymous Coward | about a year ago | (#42692005)

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.

Re:It's not smaller, everything else is bigger! (1)

locofungus (179280) | about a year ago | (#42692161)

Are you sure?

I thought the ground state orbital radius was proportional to the reciprocal of the mass of the orbiting particle?

But it's so long since I did this that I might just be talking rubbish and I certainly cannot remember how to derive it from the Schroedinger equation.

But it also presumably relies on the approximation that the orbiting particle is essentially massless when compared to the proton which also may not hold for the proton-muon ratio.

Twenty years ago I could have answered this definitively. Now, I'd need to go back to my books and do a lot of revision... :-(

Tim.

Re:It's not smaller, everything else is bigger! (1)

WillgasM (1646719) | about a year ago | (#42692811)

lol, no actually, I'm not sure. I'm just going off what I read in TFA and my limited understanding of particle physics. However, I'm willing to bet that protons aren't actually shrinking when orbitted by muons, but rather our current understanding of muon electrodynamics is less than complete.

Global Recession (0)

Anonymous Coward | about a year ago | (#42691161)

I hadn't realised the recession was this bad!

Precision vs. Accuray (0)

Anonymous Coward | about a year ago | (#42691169)

Might we not just be mistaking precision (low variance in repeated measures) for accuracy (distance from ground truth)?

This appears all the more likely given that indirect methods of measure are needed for such extreem cases!

Quantum flux. (1)

expending (2823627) | about a year ago | (#42691185)

It's possible that we are all smaller now than at any specific point in the past. Just as space-time may speed up, slow down or, go up or down, unbeknownst to us, it can also shrink or expand.

It is probable that the mystery of Quantum Mechanics lies in the fact that sub-atomic particles are in-between two worlds. In that they are abiding by two separate laws, two separate realities, at the same time.

I assume though that as these particle are tied to you, if they are in another universe, the occupants there observe that they display odd properties (like dark matter possibly?)

Horses vs Zerbras (0)

Anonymous Coward | about a year ago | (#42691189)

Extraordinary results require extraordinary proof. How many hair-brained theories were generated to explain the Pioneer trajectory aberration.

so (0)

Anonymous Coward | about a year ago | (#42691203)

that's why old people shrink !

Rear view mirror (0)

Anonymous Coward | about a year ago | (#42691257)

everthing seems smaller that you are going away from, perhaps the signal is fading fast

gravity (1)

WillgasM (1646719) | about a year ago | (#42691385)

The extra mass of the muon is holding it in a tighter orbit around the nucleus. We've been kidding ourselves with all this Higgs Boson crap.

Creeping up on an accurate value (1)

fermion (181285) | about a year ago | (#42691697)

It has not been unheard of in science that a fundamental measurement is initially in error. It has also been known for that error to not be correctly quickly, but rather slowly over time. It seems that many researchers will accept a value as accurate. However, when new measurements do not agree, sometimes the new more accurate value is not reported, but rather a value that is close to the original measurement, but shifted in the direction of the new better measurement. As no one really knows what the most accurate measurement, this is a rational way to approach the discrepancy. Over time a new consensus value is established.

As in all hard science, the results are not nearly as fascinating as the methods. It could be that we are measuring two different quantities. Or that some mistake was in the calculations, which is what many seem to expect. If the accepted value is not accurate, we may expect to see the measured size of the proton slowly shrinking over time until a new consensus value is reached.

As always a single measurement is simply that. A single measurement, a guess, a data point.

Re:Creeping up on an accurate value (4, Informative)

cusco (717999) | about a year ago | (#42692863)

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.

Maybe a muon does make a proton shrink (3, Interesting)

Anonymous Coward | about a year ago | (#42691729)

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.

Re:Maybe a muon does make a proton shrink (1)

WillgasM (1646719) | about a year ago | (#42691885)

is it more likely that placing a muon in orbit causes the nucleus to shrink, or that we're flawed in our assumption that muons will orbit at the same distance as electrons?

Re:Maybe a muon does make a proton shrink (0)

Anonymous Coward | about a year ago | (#42692057)

There was no such assumption that the muons would orbit the same distance, as the scientist already knew there would be a substantial difference in the size of the muon orbits, just apparently their predictions of how much smaller was not as exact as expected.

Re:Maybe a muon does make a proton shrink (4, Informative)

reverseengineer (580922) | about a year ago | (#42692359)

It's actually well known that muons do not orbit at the same distance at electrons (orbit in the quantum atomic orbital sense, of course, but since we're talking about hydrogen-like atoms, they can be described with the Bohr model). The calculations of energy levels do include the rest mass of the electron or muon as appropriate. The very reason to use muons in an experiment like this is their greater mass amplifies certain quantum electrodynamic interactions, allowing scientists to take experimental measurements of these interactions and plug them into QED calculations to determine basic physical properties (like the sizes of particles).

In this case, they used a phenomenon known as the Lamb shift. [gsu.edu]Essentially, two energy levels that should be identical have a slight difference due to a self-interaction effect. This difference can be measured by spectroscopy.

As they are both the same sort of particle (leptons), electrons and muons should behave identically in this experiment except for the 207 times greater rest mass of the muon, which is accounted for in the calculations. What this result suggests is either the Lamb shift of the electron and of the muon work the same and the experimental setup measures them differently somehow, or that they work differently and there is some sort of new interaction not being accounted for.

bound state QED and QCD (4, Informative)

slew (2918) | about a year ago | (#42691955)

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....

Re:bound state QED and QCD (1)

Anonymous Coward | about a year ago | (#42692163)

My understanding of the QED calculations, is they are more "straightforward," just a really bitch in terms of effort to actually take beyond the first order or two of expansion. In principle, with an army of grad students, could carry out the calculation to more orders and double check it is converging as fast as expected. I've seen this done in some other QED calculations, carried out to some insane number of orders along with some computer assistance to generate the exhaustive list of loop diagrams needed.

The assumptions and approximations made by QCD calculations on the other hand, could be sweeping a lot under the rug. There is still a lot of room for work and improvement to computational QCD.

Re:bound state QED and QCD (1)

inputdev (1252080) | about a year ago | (#42692513)

I believe most computations of the bound state are currently just assuming things about them (charge is a point source, nothing about quarks).

This is my suspicion as well, specifically about the charge distribution. I think a 4% effect could easily be explained using a model with distributed charge.

Re:bound state QED and QCD (0)

Anonymous Coward | about a year ago | (#42692699)

Considering the 4% discrepancy is in reference to the RMS radius of the charge distribution of the proton, I am pretty sure they are already using a distributed charge model. The issue is with the exact distribution of charge within that model.

Is the mass unchanged? (0)

Anonymous Coward | about a year ago | (#42692157)

If the mass is unchanged then the previously measured radius was more than likely the result of less than perfect calibration of the equipment, much like erroneously reported FTL neutrinos.

Were the previous measurements done in Texas? (0)

Anonymous Coward | about a year ago | (#42692295)

Were the previous measurements done in Texas? If so, everyone knows that it has been scientifically proven that "everything is bigger in Texas"

muonic hydrogen? (0)

Anonymous Coward | about a year ago | (#42692365)

I have no idea what muonic hydrogen is, but I have decided that I like the word "muonic" and all its derivatives.

Anyway, FTFS (didn't' RTFA, obviously) it appears that the "shrunken" measurements were all made on protons in muonic hydrogen, and those measurements disagree with measurements/calculations based on unmuonised protons. Could it be that the muonic state or muonising process somehow causes the shrunken result? Shouldn't the next step be to repeat the experiment, but using some other unmuonate medium and compare those results?

Muonic. Mmmmm-uuuuuuu-onic. Mu.

Re:muonic hydrogen? (2)

QQBoss (2527196) | about a year ago | (#42692937)

Do these muons make my ass look fat?

No, your ass makes your ass look fat; the muons actually make you look 4% slimmer!

universal expansion (0)

Anonymous Coward | about a year ago | (#42692415)

Well the universe is constantly expanding, its an illusion of that.

just like we learned in school (1)

slashmydots (2189826) | about a year ago | (#42692469)

"The unlikely but tantalizing alternative is that this is a hint of new physics."
It's just like we learned in math and science class in school. If your experiment or equation doesn't result in what you were predicting, claim it was accurate and make some shit up. Like dark matter for example. Some guys sitting here on Earth with computers and telescopes didn't measure the mass of the ENTIRE UNIVERSE quite right so...must be magical invisible matter we just made up on the spot! Protons shrunk? Must be new laws of physics.

Re:just like we learned in school (0)

Anonymous Coward | about a year ago | (#42692771)

Damn, I had science wrong all this time. I used to think when a prediction was shown to be wrong by measurement, that meant you had to come up with a new theory, followed by new experiments to check the new theory. I guess scientists are instead supposed to just supposed to close their eyes and stick their fingers in their ears, ignoring the new measurements? Sounds easier and quicker at least than the detailed analysis of error sources in the theoretical computations and measurements they did, and much quicker than repeating the experiment like they did, the second time with muons getting the same thing they did as before with smaller error bars.

Yes...exactly like that (2)

Chirs (87576) | about a year ago | (#42692911)

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.

Re:just like we learned in school (0)

Anonymous Coward | about a year ago | (#42693053)

There's more evidence for dark matter than a discrepancy in some measurements of the mass of the universe. Gravitational lensing and galaxy rotation come to mind. While some properties of gravitational lensing might be explained by undetectable black holes or some other single-point gravitational source we can't find, the way galaxies rotate and stay coherent isn't so easily explained.

Please educate yourself before you sound too much like somebody arguing against evolution or global warming (not to start a troll fest if this gets modded up, so let's just throw AGW vs. natural warming out of the picture here and now, since there are still people who amaze me by calling temperature measurements into question to argue that no warming, not even natural warming, has taken place in the last 100 or so years).

And as others have pointed out, the title and summary are misleading. Nothing has magically shrunk. There are two methods of measuring a proton's size, they're in disagreement, and it's uncertain why they should disagree.

Finally, science isn't religion.

If your experiment or equation doesn't result in what you were predicting, claim it was accurate and make some shit up.

That's exactly how it works except that the "make some shit up" stage is called trying to figure out why the experiment or equation worked all those other times except for during your experiment and finding a way to account for your results AND everyone else's results AND in a way that some different experiment could falsify or confirm a prediction you're making.

Let me guess from your tone, though. <sarcasm>This must be part of some kind of left-wing agenda to undermine the American Way and punish job creators for their success by redefining the atomic family to be 2 mommies who let their sons wear dresses, and I'm sure it's all Obama's fault, too. These scientists are paving the way for the anti-Christ by changing the protons that our Great Nation was built on to fit their left-wing agenda!</sarcasm>

More evidence we're living in a simulation? (1)

runeghost (2509522) | about a year ago | (#42692655)

Obviously, the beings running the simulation just changed a constant. Or maybe the computer our universe is currently running on has a manufacturing flaw in some of its hardware.

Load More Comments
Slashdot Account

Need an Account?

Forgot your password?

Don't worry, we never post anything without your permission.

Submission Text Formatting Tips

We support a small subset of HTML, namely these tags:

  • b
  • i
  • p
  • br
  • a
  • ol
  • ul
  • li
  • dl
  • dt
  • dd
  • em
  • strong
  • tt
  • blockquote
  • div
  • quote
  • ecode

"ecode" can be used for code snippets, for example:

<ecode>    while(1) { do_something(); } </ecode>
Sign up for Slashdot Newsletters
Create a Slashdot Account

Loading...