Mystery of the Shrunken Proton 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."
Usual suspect (Score:1)
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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 (Score:5, Funny)
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
Dr. George Costanza theorizes (Score:4, Funny)
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that it was the cold water.
Or old age.
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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 . . . .
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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.....
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Not so sure ... most of my kilts are wool, my winter hats and scarves are wool, my suits and ties are all silk.
Polyester has its place, but nor for "good" clothes.
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Odd, it's usually the wife educating the husband about such shrinkage. As you also mention, perhaps this is a latitude issue.
easy (Score:1, Interesting)
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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?
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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.
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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 n
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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.
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My comment was in jest, referencing a well-known quote from the movie "Back to the Future". Thanks for making me explain it!!
Sorry, in my last time travel I've passed by the film studios where they made that movie. Well, I did some mistake. Fortunately it did not do very much damage, but one effect is that this sentence is now missing from the movie. Sorry about that.
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Sorry for ruining the moment! I didn't get the reference because none of the most important words are the same, so I think we can classify it as "obscure."
Perhaps next time you can help a nerd out by citing Doc brown or something. That's probably the least quotable line in the whole trilogy to start with, because who even said, "heavy" beside McFly? I barely got the humor when I was watching the movie, let alone out of context.
And there are people who have propose shrinking matter as a valid hypothesis
Re:easy (Score:4, Interesting)
Re:easy (Score:5, Informative)
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.
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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 fo
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Re:easy (Score:5, Insightful)
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
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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!
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But what exactly do you mean by "simultaneous"?
Obviously, the measurements are being done from different inertial reference frames.
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> 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.
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We don't. They are all red shifted ...
Yes, and we know they are red shifted because they are (by hypothesis, actually, we can't prove this yet) the same structure there as they are here. Otherwise, the "red-shifting" would be essentially meaningless.
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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 comput
Re:easy (Score:5, Insightful)
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.
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Dude, you're probably right but it's almost inhumane to offhandedly discount any new idea presented by physics students. Sure it's a rough world out there but there are new fields of study which still are looking for answers. See new physics.
Re:easy (Score:5, Insightful)
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.
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"This means that objects in the Universe do not expand, they just move along with the expansion... Only spacetime expands, not what is sitting around in spacetime."
I'm not sure I understand this. Maybe you can help me with my confusion? It sounds like you are saying this:
If a proton and electron are seperated by 100 ly, then the distance between them will expand by X% because new space is "appearing" in between them.
If they are seperated by 1 ly, it is the same.
If they are seperated by 10^6 m, it is the sam
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The expansion of the Universe does not create new spacetime, it just stretches the existing spacetime.
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The difference is whether the objects are bound. The distance between far-away galaxies is arbitrary. The size of an atom is determined by quantum mechanics and quantum electrodynamics.
Imagine two objects lying on the floor. If you drag them away from each other, their distance grows. However if they are connected with a string, the distance won't grow. The string holds them together.
expansion of space (Score:3)
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. G
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For a ruler to be expanding, the ends would have to be drifting apart under its own inertia. If the ends were in free-fall and moving apart, then the ruler would expand. But, the ruler is held together by molecular forces, so it isn't able to drift apart. Since the ruler is bonded together, the effect of any subtle tidal forces (not sufficient to tear it apart) might slightly perturb the arrangement, but they do not integrate over time. They'll snap right back if you turn off the forces. On the other hand,
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The AC you replied to seems to have gotten things a bit wrong. While correct in the sense that metric expansion of space is even everywhere, it is not correct to think of it as a force like a conveyor belt. The metric expansion of space is more analogous to inertia. Stuff created around the Big Bang, in a sense was thrown out with what triggered the expansion of space, and it all kind of streams outward from inertia. But once you start pulling things together, there is no force pulling things apart (on
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.
Citation please.
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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.
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This is true overall of the universe, such that its scale factor is growing and taking galaxy clusters apart from one another. However the metric surrounding gravitationally bound objects is not dialating the same way, because the local matter density is high enough to stop it and because the FRW eq
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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).
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Protons are growing as well... along with everything else.
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Subway's explanation now makes sense (Score:4, Funny)
It's not New Physics (Score:4, Insightful)
It's old physics that we haven't figured out yet, but thought we had.
It's not smaller, everything else is bigger! (Score:1)
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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 a
Re:It's not smaller, everything else is bigger! (Score:5, Informative)
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.
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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?
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Except that, contrary to Slashdot belief, Michelson-Morley type interferometer experiments (there were a lot, not just one) were done to help choose between a whole bunch of theories, some of which predicted a difference and some of which didn't.
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No, no, no.
The new physics model is that when you have data that just absolutely PROVES that neutrinos go faster than light, you publish first in the popular press and then in a journal like NATURE. Only later do you mumble something about how you knew all along it was too good to be true.
My brother (in physics) forecast a major discovery followed by a major scandle 12-31-11, but he's repeating the forecast this year, and it looks like he'll repeat every year on that one.
Vetting is so passe.
Welcome to the
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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.
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Re:It's not smaller, everything else is bigger! (Score:5, Informative)
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.
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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 definitive
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You're right. The idea is that the wavelength of the orbiting particle, no matter what it is, has to fit around the lowest orbit.
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> 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.
A functioning universe is actually a very, very precariously balanced animal. The Anthropic Principle was developed essentially to explain this. (Quick and horribly inaccurate summary: the only way to get around the apparent design is by assuming that there are other "worlds," other "realms" or other universes, each with a different collection of physical laws
Was J. J. Abrams involved? (Score:2)
gravity (Score:2)
Creeping up on an accurate value (Score:2)
Re:Creeping up on an accurate value (Score:5, Informative)
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I don't know if he is, but I am, and I don't see the problem with his post. You have to learn to deal with the fact that techniques and technology get better with time, and that it is possible for previous measurements to have missed something or have some systematic error. Scientist end up skeptical of new results, and sometimes this means a bias toward old results because the new ones get unpublished, or, more likely, get extra examination for correctness than can sometimes over correct by fixing for a
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Oh god I hope you aren't a scientist.
https://en.wikipedia.org/wiki/Oil_drop_experiment#Millikan.27s_experiment_as_an_example_of_psychological_effects_in_scientific_methodology [wikipedia.org]
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Maybe a muon does make a proton shrink (Score:3, Interesting)
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.
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Re:Maybe a muon does make a proton shrink (Score:5, Informative)
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.
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Randall Mills is whispering "hydrinos" right now and bilking a new set of investors... ;)
bound state QED and QCD (Score:5, Informative)
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....
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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 need
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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.
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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.
It has been my experience that posts where the poster annouces their "suspicions" are almost alway gibberish, displaying a profound ignorance of the most basic elements of the subject they have suspicions about. It really is a useful litmus test, to the extent that I think /code should be modified to automatically down-mod any post that contains phrases like "I suspect that" and "my suspicion".
It's not that there's anything wrong with expressing doubts. It's that this specific way of putting it seems to b
What about "loose-cable" loop corrections? (Score:2)
May be someone just didn't tighten a fiber optic cable somewhere.
Why are some people so jumpy on the news of one experiement's measurement of a quanity that contradicts all earlier evidence? I'm not trying to be an asshole, but sorry, haven't we learned our lesson yet? [wikipedia.org] I will concede that they have data dating to 2003, so we could have a real thing there and it is good to give them time to review their stuff. Still, apparently no other measurement has shown anything similar, or I'd assume that they'd have m
So you're saying size *does* matter? (Score:5, Funny)
Now, they tell me.
Measure twice ... (Score:2)
just like we learned in school (Score:2)
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 ne
Yes...exactly like that (Score:3)
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.
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"New physics" is shorthand for "changes to our theories of physics and/or understanding of the universe". Not new laws of physics magically appearing into existence.
The summary shows that new observations of protons show them to bea different size to what we were expecting based on current theories and models. Now lots of scientists have checked and replicated it, and the difference is still there. That means our old theories must have been wrong, and the universe must work differently than we though. And
More evidence we're living in a simulation? (Score:2)
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.
Well duh... (Score:3)
So, that means ... (Score:1)
I'm going to demonstrate my own stupidity (and lack of willingness to RTFA) ...
What changes with this measure? I'm sure it has loads of things which it might affect, but I have no idea.
So, what does a slightly smaller photon translate into?
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Doh, I mean proton of course.
Let's sum up: (Score:2)
Protons are not the size our current model suggests.
Gravity doesn't work on large scales the way our current model suggests.
We can't observer dark matter and dark energy the way our current model suggests.
There's literally hundreds of other examples, but am I the only one who thinks the problem is our current model?
I'm willing to wager dark matter/energy don't even exist. They are just made up to make the computer model work. Ridiculous.
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Explain gravitational lensing when there's no apparent mass causing it.
Gut feeling: Centres of charge and mass (Score:1)
In large scale orbiting systems, equivalent measurements relating to size are based on centre of masses, where the masses also govern the force controlling the orbit.
In hydrogen atoms the masses still determine the orbit, but the forces are mediated by the charges rather than by the masses directly as gravity, which may not be in the same place as the masses and may be to some extent free to move in relation to the centre of mass
Wait a minute... (Score:1)
Did they check the cables...? Perhaps, it's a loose one.
Maybe the muon is squeezing the proton (Score:2)
The muon sits much closer to the nucleus than an electron, so the charge of the muon is perhaps changing the shape of the proton, "squeezing it". Since the proton is made up of charged quarks, the ground state orbitals for the quarks could be somehow modified by the nearby muon charge. I'm totally guessing of course.
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I hate it when my heroine is full of protons.
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Do these muons make my ass look fat?
No, your ass makes your ass look fat; the muons actually make you look 4% slimmer!
Re:Global warming (Score:4, Insightful)
Well, that's what he's telling the other particles...
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Re:Global warming (Score:4, Informative)
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 [wikipedia.org], since a proton doesn't have a definite boundary.