Forgot your password?
typodupeerror
Science

The Proton Just Got Smaller 289

Posted by samzenpus
from the size-does-matter dept.
inflame writes "A new paper published in Nature has said that the proton may be smaller than we previously thought. The article states 'The difference is so infinitesimal that it might defy belief that anyone, even physicists, would care. But the new measurements could mean that there is a gap in existing theories of quantum mechanics. "It's a very serious discrepancy," says Ingo Sick, a physicist at the University of Basel in Switzerland, who has tried to reconcile the finding with four decades of previous measurements. "There is really something seriously wrong someplace."' Would this indicate new physics if proven?"
This discussion has been archived. No new comments can be posted.

The Proton Just Got Smaller

Comments Filter:
  • by RobertB-DC (622190) * on Thursday July 08, 2010 @12:10PM (#32841098) Homepage Journal

    This paragraph from TFA has the most salient information:

    Pohl and his team have a come up with a smaller number by using a cousin of the electron, known as the muon. Muons are about 200 times heavier than electrons, making them more sensitive to the proton's size. To measure the proton radius using the muon, Pohl and his colleagues fired muons from a particle accelerator at a cloud of hydrogen. Hydrogen nuclei each consist of a single proton, orbited by an electron. Sometimes a muon replaces an electron and orbits around a proton. Using lasers, the team measured relevant muonic energy levels with extremely high accuracy and found that the proton was around 4% smaller than previously thought.

    4% sure does seem significant. But more interesting is that the measurement is thought to be much more precise because of the method of measurement. Doesn't it seem more likely that it's just not possible to get an accurate measurement with the electron -- like measuring a grape with a yardstick instead of a micrometer?

    And of course, there's that stupid cat-in-a-box thing... you can't measure something without affecting it, so maybe muons interact in some strange (lol) way with protons that doesn't happen (or happens differently) with electrons. But as a non-physicist, even throwing those terms out there puts me far outside my league.

    Of course, these more prosaic explanations don't lead to nearly as many cool sci-fi plot threads. FTL drive powered by a process that squeezes protons to black hole density, perhaps? That would be awesome. Or, perhaps the expansion of the universe is actually reducing the size of subatomic particles -- so in a few billion years, all matter will simply wink out of existence. Or, there's a time dilation effect as well, so that time drags longer and longer, especially on Mondays.

  • by meringuoid (568297) on Thursday July 08, 2010 @12:28PM (#32841348)
    4% sure does seem significant. But more interesting is that the measurement is thought to be much more precise because of the method of measurement. Doesn't it seem more likely that it's just not possible to get an accurate measurement with the electron -- like measuring a grape with a yardstick instead of a micrometer?

    Maybe, but this is still surprising. Measure a grape with a metre rule, you should still be able to say 'it's between a centimetre and a centimetre and a half.' Measure it with a micrometer, and you'd expect to see a result like 'It's 1.2144 centimetres.' If the micrometer instead measured the grape at 0.7218 centimetres, well, you'd be puzzled. First of all, of course, you'd check you were doing it right. You'd examine your micrometer and make sure you were operating it correctly. You'd recheck how you measured it with the metre rule - is it zero from where the number is printed, or from the edge of the ruler, is the ruler maybe worn down at the edge?

    But if all that checked out and you still had this discrepancy, you'd start to wonder if your ruler and your micrometer were really measuring the same units.

    Hence the suggestion of new physics. Theoretically the muon should act like a heavy electron - interacting with the proton in just the same way, so that it can be used as a more precise probe on the size of the proton. It would be the micrometer to the electron's metre rule. If it doesn't - if the muon interacts with the proton in some unexpected way so as to throw the measure off - then we've discovered something beyond the standard model.

    There are quite a few indications that there is physics beyond the standard model - heavy neutrinos, the abundance of matter over antimatter, the dark matter - and so if we can add this to the list then maybe it can help pin down just what sort of a new theory we're looking for. We've got to have something to do once the good people of Geneva finally hammer us out a Higgs, after all :-)

  • Re:Ummm... (Score:1, Interesting)

    by Anonymous Coward on Thursday July 08, 2010 @12:52PM (#32841742)

    As a physicist, yes it most certainly does. Errors above 1% always matter, even if we cannot do anything about them, (for errors below, it depends on expected error ranges). 4% is a big deal to pretty much everybody.

    I am wondering what this will do to the standard model/gluon theory?

  • by radtea (464814) on Thursday July 08, 2010 @12:55PM (#32841766)

    Which means that the theory is wrong

    At best it means either the theory or the experiment is wrong, and the "wrong" can vary from mundane to really interesting, with the vast weight of probability on the side of mundane.

    The structure function of the proton is not simple, and calculating it depends on QCD approximations that are even less simple. The notion that it can be characterized by a single parameter is questionable.

    Muons probe a very different part of the proton structure function than electrons. Muon orbitals are much smaller than electron orbitals, so protons look even less like a point mass to them. As such it is not surprising that they would result in a significantly different value for a single parameter in a particular model of the proton, even if the experiment is not in error somehow. By far the hardest part of the structure function of nucleons to model in QCD are the tails, and that is exactly what muons will be most sensitive too.

    This is how experimentalists react to anomalous results: the most probable explanation, always, is that the people doing the work screwed up. We then set out to prove how they screwed up. If we can't, we start to think about other corrections seriously.

    Theorists will of course have no difficulty explaining this result, even if it later turns out to be incorrect. But even if the results are correct, they will almost certainly be accounted for by relatively insignificant tweaking of QCD estimates of the proton structure function, which is good solid science, but not the kind of great big deal that TFA seems to want to make of it.

  • by mbkennel (97636) on Thursday July 08, 2010 @01:24PM (#32842202)

    As far as our particle accelerators & theory can tell, electrons, muons and quarks are all elementary particles with no internal structure. Internal structure is not necessary for particle decay---particle decay isn't really inside-parts spewing out, it is energy in one form of matter being allowed by laws of physics & quantum mechanics to transform into another state.

    It would be extremely unlikely if muons had internal structure and electrons didn't.

    The most likely scenario is (unfortunately) that there are some effects which actually are part of Standard Model physics, but they weren't included in the theoretical calculations. The theoretical calculations can get quite hairy and complex; perhaps something was approximated in a way that isn't actually as valid as originally believed or some other interaction which is hard to compute was ignored.

  • Re:Ummm... (Score:5, Interesting)

    by smackenzie (912024) on Thursday July 08, 2010 @01:27PM (#32842240)
    "Oh, welcome back to Citibank, Mr. Smith. Your portfolio indicates that all of your investments are 4% down, but we think the difference is so infinitesimal small that it might defy belief that you cared."

    "Hi Ms. Smith. Your cancer cell growth has increased 4%, but we think the difference is so infinitesimal that it might defy belief that you cared."

    "Little Timmy scored an 86.6 (grade B) instead of 90 (grade A), but we think the difference is so infinitesimal small that it might defy belief that he cared."
  • Re:Anonymous Coward. (Score:3, Interesting)

    by poopdeville (841677) on Thursday July 08, 2010 @01:30PM (#32842308)

    How exactly are you encoding arithmetic in "physics", as a formal theory? If the universe is finite, Godel's theorem doesn't apply.

  • Re:Ummm... (Score:5, Interesting)

    by blair1q (305137) on Thursday July 08, 2010 @01:44PM (#32842488) Journal

    If it were grains of sand packed under the foundation of your house, it might be important.

    But protons only aggregate in small groups that don't get close to each other very often. In fact, they emit a force-field that prevents it, so the size of the proton rarely if ever comes into play even in interatomic interactions.

    Which brings up a rather glaring point: SLAC, Fermilab, CERN, et al have been colliding protons together for decades. You'd think they would have noted something funny in the statistics by now to indicate that their colliding objects were consistenly not colliding with the predicted probabilities. If "size" means anything, it means the most when you try to make objects bash each other head-on.

    I'll be rightly surprised if the re-review of past data confirms that the 4% discrepancy was there and they simply ignored it.

    And maybe I missed this yesterday when I read the story (linked from Twitter; /. is about as timely as the Wall Street Journal any more), but is the 4% volume, cross-sectional area, or radius? A 4% volume difference would be trivially easy to miss; a 4% radius error would be one hell of an oversight.

    My money is on the possibility that the guys doing this new research bollixed the theory that predicts the frequency to use in their experiment. And then on the possibility that the theory they're using has never been confirmed very well. QED seems to be more concerned with photons and electrons and other less-massive particles (in fact, it doesn't say anything about mass, and if this 4% is real maybe it's a way to link gravity and GUT (the Grand Unified Theory of electromagnetism, the weak force, and the strong force...) to make the GUTE (Grand Unified Theory of Everything).

  • Re:Ummm... (Score:4, Interesting)

    by Jeremy Erwin (2054) on Thursday July 08, 2010 @01:48PM (#32842552) Journal

    Old radius: 0.8768(69) femtometres
    New radius: 0.84184(67)femtometres

    Our result implies that either the Rydberg constant has to be shifted by 110kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

    source [nature.com]

    It's not the absolute magnitude of the change that's so worrisome, it's the relative magnitude.

  • Re:Ummm... (Score:3, Interesting)

    by Jeremy Erwin (2054) on Thursday July 08, 2010 @01:50PM (#32842576) Journal

    Wouldn't that depend on the method of packing [wolfram.com]??

  • Re:No it didn't. (Score:5, Interesting)

    by UnknownSoldier (67820) on Thursday July 08, 2010 @02:10PM (#32842810)

    > oh well, i'm figuring that he's probably right seeing as science is just a bunch of atheistic dogma anyway...

    Considering that Max Plank said:
      "Eine neue wissenschaftliche Wahrheit pflegt sich nicht in der Weise durchzusetzen, daß ihre Gegner überzeugt werden und sich als belehrt erklären, sondern vielmehr dadurch, daß ihre Gegner allmählich aussterben und daß die heranwachsende Generation von vornherein mit der Wahrheit vertraut gemacht ist."
    which is translated as
      "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."
    or paraphrased as the common English phrase:
      "Truth never triumphs -- its opponents just die out."
      "Science advances one funeral at a time."

    You might be right on the dogma bit.

    http://en.wikiquote.org/wiki/Max_Planck [wikiquote.org]

  • by Anonymous Coward on Thursday July 08, 2010 @02:42PM (#32843186)

    There is another sentence that doesn't make sense either,

    "Would this indicate new physics if proven?"

    Physics doesn't get "proven", mathematics gets proven. It's akin to proving reality - it doesn't make sense. AFAIK, the cornerstone of physics is the experiment. If the experiment shows something, then that's it. There is no debate except maybe about the procedure employed. There is never argument if something is "real" - it's right there.

    If the proton is shown to be smaller than what QM predicts, then the universe is basically showing that QM is not complete and there *must be* new physics and that's a *huge* result. Frankly, I don't think the submitter really understands either how physics works or how science works.

    From the abstract,

    On the basis of present calculations (11, 12, 13, 14, 15) of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67)fm, which differs by 5.0 standard deviations from the CODATA value3 of 0.8768(69)fm. Our result implies that either the Rydberg constant has to be shifted by 110kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

    http://www.nature.com/nature/journal/v466/n7303/full/nature09250.html

    And that's a *huge* result. 5 standard deviations is not a little change - something doesn't add up.

  • by HasselhoffThePaladin (1191269) on Thursday July 08, 2010 @02:52PM (#32843286)

    I love science, but it always seems like I know less than yesterday.

    This is, in fact, usually true. As the saying goes, "The more you know, the more you know you don't know." If you picture the sum of all knowledge as a rectangle and the sum of your knowledge as a circle inside that rectangle, the boundary of that circle represents what you know that you don't know. As the circle grows, so does the boundary and your awareness of how little we actually understand. Sorry for the long-winded exposition on your comment; I just find that concept fascinating.

  • Re:Anonymous Coward. (Score:1, Interesting)

    by Anonymous Coward on Thursday July 08, 2010 @02:54PM (#32843316)

    Gödel's theorem does not apply to the universe (be it infinite or finite), it applies to physics which uses arithmetics and natural numbers.

    There is quite interesting Stephen Hawking lecture "Gödel and the end of physics" [cam.ac.uk].

Machines certainly can solve problems, store information, correlate, and play games -- but not with pleasure. -- Leo Rosten

Working...