The Incredible Shrinking Proton That Could Rattle the Physics World

By Andrew Moseman | July 8, 2010 10:41 am

LaserProtonIt wasn’t supposed to be like this. The Higgs boson, dark matter, neutrinos—weird or poorly understood phenomena like these seemed the likely candidates to provide a surprise that changes particle physics. Not an old standby like the proton.

But the big story this week in Nature is that we might have been wrong all along in estimating something very basic about the humble proton: its size. A team from the Paul-Scherrer Institute in Switzerland that’s been tackling this for a decade says its arduous measurements of the proton show it is 4 percent smaller than the previous best estimate. For something as simple as the size of a proton, one of the basic measurements upon with the standard model of particle physics is built, 4 percent is a vast expanse that could shake up quantum electrodynamics if it’s true.

If the [standard model] turns out to be wrong, “it would be quite revolutionary. It would mean that we know a lot less than we thought we knew,” said physicist Peter J. Mohr of the National Institute of Standards and Technology in Gaithersburg, Md., who was not involved in the research. “If it is a fundamental problem, we don’t know what the consequences are yet” [Los Angeles Times].

Simply, the long-standing value used for a proton’s radius is 0.8768 femtometers, (a femtometer equals one quadrillionth of a meter). But the study team found it to be 0.84184 femtometers. How’d they make their measurement? First, think of the standard picture of electrons orbiting around a proton:

According to quantum mechanics, an electron can orbit only at certain specific distances, called energy levels, from its proton. The electron can jump up to a higher energy level if a particle of light hits it, or drop down to a lower one if it lets some light go. Physicists measure the energy of the absorbed or released light to determine how far one energy level is from another, and use calculations based on quantum electrodynamics to transform that energy difference into a number for the size of the proton [].

That was how physicists derived their previous estimate, using simple hydrogen atoms. But this team relied on muons instead of electrons. Muons are 200 times heavier than electrons; they orbit closer to protons and are more sensitive to the proton’s size. However, they don’t last long and there aren’t many of them, so the team had to be quick:

The team knew that firing a laser at the atom before the muon decays should excite the muon, causing it to move to a higher energy level—a higher orbit around the proton. The muon should then release the extra energy as x-rays and move to a lower energy level. The distance between these energy levels is determined by the size of the proton, which in turn dictates the frequency of the emitted x-rays [National Geographic].

Thus, they should have seen the specific frequency related to the accepted size of a proton. Just one problem: The scientists didn’t see that frequency. Instead, their x-ray readings corresponded to the 4-percent-smaller size.

Now the task at hand is to check whether this study is somehow flawed, or is in fact a finding that will shake up physics.

In an editorial accompanying the report in the journal Nature, physicist Jeff Flowers of the National Physical Laboratory in Teddington, England, said there were three possibilities: Either the experimenters have made a mistake, the calculations used in determining the size of the proton are wrong or, potentially most exciting and disturbing, the standard model has some kind of problem [Los Angeles Times].

Related Content:
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Image: F. REISER & A. ANTOGNINI, PAUL-SCHERRER-INST (the laser used  in the research)

  • Herschel Neumann

    In a hydrogen atom, electrons do not “orbit” the proton. They do not orbit at any specific distances in any energy state. Energy levels are not distances. Muons don’t orbit, either.

    Electrons do not “orbit” the nuclei of any atoms.

    The orbiting model of an atom has been known to be incorrect for over 80 years.

    The orbiting model is NOT the “standard picture.”

    Herschel Neumann
    Professor Emeritus and former Chair
    Department of Physics and Astronomy
    University of Denver

  • JimN


  • CW

    This isn’t passing the smell test for me.

  • John Moore

    Thank you Prof. Neumann. I have tried several years to explain the “cloud” dispersment of electrons to friends and get the deer in the head lights look. I think people simply relate to this “picture” for ease of understanding but it really confuses the real picture.

  • umm

    the cloud is not that hard of a concept to understand. if these results are true, the thing i dont understand is:

    is this article saying they are able to somehow manipulate the energy of a subatomic particle (muon or electron) so the effect is a higher probablity that particule will appear a closer/further distance away from the nucleus?

  • tz


    Yes, different energy levels correspond to different spatial wavefunctions (i.e. clouds); in general, the higher the energy level, the greater the particle’s average distance from the nucleus – for example, 1s vs. 2s vs. 2p states for the hydrogen atom. Here, by exciting muonic hydrogen with laser light, the muon jumps to a higher energy state with a higher probability of being found a greater distance from the nucleus.

  • artresh

    Protons do not have size per se. They are strings which by virtue of their spin in a continuum of homogeneous particles develop tip vortices which we recognise as electron clouds. The nucleus is a volume of space void of the continuum and therefore its centre is under extreme negative pressure from the surroundings. As a result nucleons become bound together at the centre of the nucleus and continue to spin around the perimeter. Alternate nucleons loss their effect in the surrounding and thus unable to generate electron. They become distinguished as neutrons, until they decay (leave the atom) then they become protons. Mass represents the void generated in the continuum. There are therefore four types of mass. Mass generated by nucleons, mass generated by electrons, mass generated by virtue of vibrating particles in the continuum (photons), and permeability mass (space between the homogenous particles). This explains the association of gravity and electromagnetic waves with molecular matter. Protons do not have the same mass in different locations within an atom.

  • Betareach

    My proton is much bigger than anyone ever thought.

    My electrons are huge.

    if I wear a long coat, no one will notice.

  • Ryan

    artresh: your “theory” is lacking. how do you explain nucleuses without electrons, such as alpha particles. Or stray protons, or stray electrons, or stray neutrons. It sounds like you took a bunch of unrelated concepts and threw them together…

  • YouRang

    One issue the report did not address is: a muon’s mass is much closer to that of a pion than an electron’s. Now the fraction of the muon ground state described by virtual pions is not a lot, but the interaction of those virtual pions with the nucleus is given by the strong force rather than by the electormagnetic force. They didn’t mention using QCD to predict the size, and I would be surprised if the theory is good enough for that purpose.
    And artesh, just because you have Asberger syndrome doesn’t make you an idiot savant.

  • artresh

    Ryan: protons are the only particles which are not know to decay. A stray neutron would decay to a proton, which within a range of temperatures (amplitude of vibrations of the particles forming the medium) forms an electron. Stray electrons would ultimately decay to photons and subsequently become undetectable particles. Other observed particles are manifestations of similar events which we perceive as protons and electrons, but they take place at different spin speeds.
    And thank you YouRang.

  • Dennis

    What does the “size” of a proton mean at this level anyway? It’s almost automatic to think of a proton as looking like a billard ball, but a proton is made of 3 quarks, so it’s more like a hackysack. The orbital-jumping muons or electrons are interacting with electrical fields, not a “surface” per say, (not to mention there are no orbits, just clouds, which are described as regions in which one can be 99% certain of finding a muon/electron.) Is the size determined by the base of the cloud–where there is 0% chance of the muon/electron going any closer to the center of the proton?
    So much of the language used for particle physics is metaphorical, hence confusing as soon as you try to think about the meaning. (eg, an electron “orbit” is metaphorical, since an electron “cloud” behaves in some ways that we associate with orbits, not clouds.)

  • arko

    @artresh: The electron is stable. It’s decay violates the conservation of charge. And let us keep this discussion “string” free, for strings are famous now for their predictive powers.

  • Brian Too

    I don’t understand how this finding calls into question the Standard Model. Isn’t that overreaching by, I dunno, a WHOLE lot?

    Frankly this sounds like a mere refinement. “Oops, we thought we had the proton mass nailed to 99.999%, turns out it was only to 96%”.

    Unless there is something in the Standard Model that requires the proton to have the previous best measurement’s mass, something fundamental, the Standard Model is in no danger.

    I mean seriously, the Standard Model is one of the most successful quantum theories in what, 50-60 years? If you’re going to take that one down you better have your ducks in a row. It sure sounds like these people don’t. You better have multiple lines of evidence, with multiple teams producing the same or very similar results.

    An interesting result worthy of additional investigation. For now, not much more than that.

  • artresh

    @arko: The electron is no doubt “relatively” stable. However, when it does “eventually” decay, even by collision with a positron, the outcome is photons. If we assume that energy is the motion of the continuum of undetectable particles making-up the medium of the Universe, then Einstein’s equation E = mc^2, effectively means the collapse of a void in the continuum (mass). If mass can not be conserved, as can be deduced from Einstein’s equation, then surely charge can not be conserved either.

  • walter gremillion

    what a bunch of silliness, Mr hershel neuman is however right, check this post on the nature of the universe
    you are debugging your wetware to try to determine the nature of the universe, caught up in the yin/yang wars, fighting the battle of Moot Hill.

    i suspect this will be deleted

  • Zephir

    There you can find an extensive list of all QED corrections considered in calculation of the rms proton charge radius from Lamb shift measured..

    It seems, these guys just forget the most trivial / important one, which has served for spectroscopic discovery of deuterium by Harold Urey in 1932..

    Nevertheless, the supplement is not completely explicit about what has been taken into in the calculations and what might be missing, so I can still be wrong with it.

  • Bernard Asper

    My vote for most likely reason? The standard model has some kind of problem.

  • Victor Lopez Echeverria

    What about to consider if… the electron’s “cloud” could work like the clouds in our nature surrounding… I mean, if the electrons are merily like lightings during a storm and not having a specific location but an specific action. it is just a mind “voyage”. merily an image in my brain…

  • festivals

    Not long ago i came accross your site and have been reading along. I figured I might leave my 1st remark. Nice blog. I’ll keep visiting this internet site very frequently.


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