The proton shrinks in size
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By Geoff Brumfiel
The proton seems to be 0.00000000000003 millimeters smaller than researchers previously thought, according to work published in the July 8 issue of Nature.
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."
Protons are among the most common particles out there. Together with their neutral counterparts, neutrons, they form the nuclei of every atom in the Universe. But despite its everday appearance, the proton remains something of a mystery to nuclear physicists, says Randolf Pohl, a researcher at the Max Planck Institute of Quantum Optics in Garching, Germany, and an author on the Nature paper. "We don't understand a lot of its internal structure," he says.
From afar, the proton looks like a small point of positive charge, but on much closer inspection, the particle is more complex. Each proton is made of smaller fundamental particles called quarks, and that means its charge is roughly spread throughout a spherical area.
Physicists can measure the size of the proton by watching as an electron interacts with a proton. A single electron orbiting a proton can occupy only certain, discrete energy levels, which are described by the laws of quantum mechanics. Some of these energy levels depend in part on the size of the proton, and since the 1960s physicists have made hundreds of measurements of the proton's size with staggering accuracy. The most recent estimates, made by Sick using previous data, put the radius of the proton at around 0.8768 femtometers (1 femtometer = 10-15 meters).
Small wonder
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 percent smaller than previously thought.
That might not sound like much, but the difference is so far from previous measurements that the researchers actually missed it the first two times they ran the experiment in 2003 and 2007. "We thought that our laser system was not good enough," Pohl says. In 2009, they looked beyond the narrow range in which they expected to see the proton radius and saw an unmistakable signal.
"What gives? I don't know," says Sick. He says he believes the new result, but that there is no obvious way to make it compatible with years of earlier measurements.
"Something is missing, this is very clear," agrees Carl Carlson, a theoretical physicist at the College of William & Mary in Williamsburg, Va. The most intriguing possibility is that previously undetected particles are changing the interaction of the muon and the proton. Such particles could be the "superpartners" of existing particles, as predicted by a theory known as supersymmetry, which seeks to unite all of the fundamental forces of physics, except gravity.
But, Carlson says, "the first thing is to go through the existing calculations with a fine tooth comb." It could be that an error was made, or that approximations made in existing quantum calculation simply aren't good enough. "Right now, I'd put my money on some other correction," he says. "It's also where my research time will be going over the next month."
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32 Comments
Add CommentWow! One femtometer is a heck of a lot bigger than I thought! About 10 to 15 meters, huh?
Reply | Report Abuse | Link to thisSeems someone doesn't know how to properly indicate powers of ten when superscripted text isn't available. For the record that should read "1 femtometer = 10↑-15 meter".
These things happen all the time when text is converted from one format to another. The superscript tag wasn't picked up by the software used. Maybe it was just a cut&paste.
Reply | Report Abuse | Link to thisSo, just attribute the discrepancy to 'Dark Space' and move along... Nothing to see here.
Reply | Report Abuse | Link to thisThese aren't the droids you're looking for....
Reply | Report Abuse | Link to this"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... Using lasers, the team measured relevant muonic energy levels with extremely high accuracy and found that the proton was around 4 percent smaller than previously thought."
Reply | Report Abuse | Link to thisImagine you were instead using gravitation to measure the size of the moon by launching projectiles at it the size of meteors and observing their trajectory. Once these objects reached the size of asteroids, they would start attracting the moon itself.
Have these researchers accounted for the fact that the muon's relatively ponderous mass might be affecting the readings somehow?
san francisco onion - Excellent suggestion. Alternatively, perhaps the spatial displacement of the proton is physically affected by the mass of the muon.
Reply | Report Abuse | Link to thisSo little is known about the internal structure of protons: they are composed of quarks, they appear only in pairs; each proton is thought to contain three pairs, and they each have an assigned characteristic property of fractional charge.
Perhaps the higher energy 'orbit' of the muon alters the gap between quarks, for example.
The munchkin particle.
Reply | Report Abuse | Link to thisSee what happens if you try a second cousin of an electron
Reply | Report Abuse | Link to thisThis brings up an issue that isn't seen as an issue because it likely cannot be seen: what if atomic mass is slowly shrinking over time? Would that account for the apparent acceleration we see in the universe?
Reply | Report Abuse | Link to thisPerhaps it can be inferred? The moon is slowly moving away from the earth. Could that be accounted for with math to help form part of a proof?
This is pure nonsense ,a proton has no definite limits,it is a standing wave ,its field strength is function of distance
Reply | Report Abuse | Link to thismorp - I agree for all material energy, except when it is detected by some interaction with matter which causes their manifestation as a spatially localized particle.
Reply | Report Abuse | Link to thisThe article states:
"Physicists can measure the size of the proton by watching as an electron interacts with a proton. A single electron orbiting a proton can occupy only certain, discrete energy levels, which are described by the laws of quantum mechanics. Some of these energy levels depend in part on the size of the proton, and since the 1960s physicists have made hundreds of measurements of the proton's size with staggering accuracy."
It goes on to state of the muon measurement:
"Using lasers, the team measured relevant muonic energy levels with extremely high accuracy and found that the proton was around 4 percent smaller than previously thought."
The method of measurement is unfortunately not explained and there is no reference other than some unnamed Nature article.
As best as I can understand, the orbital energy of an electron in a single electron atom is determined by its probability of being detected within a given range of spatial dimensions. Using the overly simplistic planetary system analogy, the higher electron energy states equate to increasingly distant orbits from the central mass. I guess that the size of the proton is ‘accurately’ guesstimated from the detected spatial dimensions of the lowest energy orbital and specifications of quantum mechanics theory.
However, the use of muons rather than electrons in this measurement attempt most likely invalidate some aspects of the method employed, as it is in essence a higher mass electron produced in higher velocity particle collisions. If I am generally correct in my guess that the estimated proton size depends largely on orbital spatial dimensions, they most likely vary for the higher mass muons, producing different results. I would expect that the more massive muons would ‘orbit’ nearer the proton than electrons and may in fact change the spatial dimensions of the proton. However, these measurements would not be relevant to atoms composed of protons and electrons.
Geoff Brumfiel - Please seriously consider the above very loose description of the imposed procedural error and pass along to Randolf Pohl, or at least Ingo Sick.
Reply | Report Abuse | Link to thisWhile standard procedures have generally been followed, they a being applied inappropriately to new conditions. I have spent >30 successful years as an information systems analyst: I am highly confident that this generally describes the source of error.
Please do not allow a repeat of the 'Dark Matter' debacle.
LOL. so a proton is actually 15 meters in diameter...lol... that is a large discrepency from previous calculations...lol.. yeah, SciAm doesn't seem to know how to format text.
Reply | Report Abuse | Link to thisIt sounds absurd to believe that any particular particle even a proton would always have to have an exactly precice measurable mass or diameter that cannot change under different circumstances. In this test they used laser light, and a more massive orbiting muon. It makes no sense that a particle made up of infinite smaller particles would have a constant mass or size. The smaller particles can change much like atoms can have radioactice isotopes that decay. This changes the size of the nucleus. Many much briefer changes occur when particles get smaller and smaller.
Reply | Report Abuse | Link to this@jimhenson, "It sounds absurd...," what always seems absurd to me is when armchair scientists claim the results of decades of scientific work into such technically complex fields of research as quantum mechanics are wrong and their hunches are right. Why don't you go to MIT and demand a Phd. Just tell them you have a hunch you understand physics better than those know-it-all scientists and their fancy book learnin.
Reply | Report Abuse | Link to thisin this case I did as other writers agree that the proton is a standing wave with a magnetic field that weakens with the square of the distance. Such cannot allow for a fixed permanent size or mass because of e=mc2. What has happened is the discovery that the mass of the proton varies far more widely then previously believed.
Reply | Report Abuse | Link to thisMaybe the proton is increasing due to global warming. You know, cuz heat expands.
Reply | Report Abuse | Link to thismitroc,
Reply | Report Abuse | Link to thisYes that is very likely is the reason.
Oh-oh, on checking I find that the proton has shrunk.
"a spherical area."
Reply | Report Abuse | Link to thisDo we now get 3D areas?
Would it be possible for someone to explain, (in simple language) what are the consequences of this 4% reduction in size? I would guess it doesn't mean that the proton just rattles about a bit.
Reply | Report Abuse | Link to thisAnybody that didnt like this article might light national geographics version.
Reply | Report Abuse | Link to thishttp://news.nationalgeographic.com/news/2010/07/100707-science-proton-smaller-standard-model-quantum-physics/
QuantumGuy: Thanks for the NatGeo link. That explains the situation much better. I was wondering how the experiment compensated for the decay of the muon. Now I know.
Reply | Report Abuse | Link to thisSince the muon is unstable, does it spontaneously decay when the laser knicks it up an energy level or does the intact muon survive long enough to return to the base state as it emits the x-rays. As one of the products of typical muon decay is an electron, any decay prior to x-ray emission would seem to alter the measurement.
"So little is known about the internal structure of protons: they are composed of quarks, they appear only in pairs; each proton is thought to contain three pairs, and they each have an assigned characteristic property of fractional charge."
Reply | Report Abuse | Link to this"they appear only in pairs" does not make any sense at all, because the antecedent of "they" is dubious. The sensible antecedent of "they", by the rules of English, is "protons" - but I know that protons do not come in pairs. Consider the case of hydrogen. Looking further, "they" could mean "quarks", but the idea that quarks occur only in pairs is also false. In protons, neutrons, etc. ("baryons"), come in trios. In protons, two of these have a charge of 2/3, and the other has a charge of -1/3, giving a total charge of +1. In a neutron, one has a charge of 2/3, and the other two have a charge of -1/3, giving them a charge of zero - chargeless.
Mesons consist of two quarks, have a total charge of three possible values: 1/3 + 2/3 = +1; -1/2 - 2/3 = -1; 1/3 - 1/3 = 0; 2/3 - 2/3 = 0.
Thus, quarks do not always occur in pairs, and much more commonly, they occur in trios.
You have the odd idea that a proton or a neutron consists of six quarks. Very strange.
DAW
"These things happen all the time when text is converted from one format to another. The superscript tag wasn't picked up by the software used. Maybe it was just a cut & paste."
Reply | Report Abuse | Link to thisDon't go around giving excuses for such yo-yos !
The ultimate tesyt is to proofread your work once you post it on the Web. That means get onto a PC, log in, and then LOOK at the result with your God-forsaken eyes! Proofread it.
Don't go around being lazy donkeys!
DAW
"As one of the products of typical muon decay is an electron".
Reply | Report Abuse | Link to thisOn of the principles of atomic particle physics is called "Conservation of Lepton Number". Thus, leptons are neither created nor destroyed. Since a muon is a negatively-charged lepton, and the next-to-lightest lepton, the only way that that a muon can decay involves emitting an electron! The decay of a muon always leaves an electron. It cannot be a positron, because that would violate the conservation of charge, and it cannot decay into something heavier than a muon - because that would violate the conservation of mass-energy.
So, just take your "typically" and replace it with an "always" !
DAW
Nobody can explain it in simple language.
Reply | Report Abuse | Link to thisSorry, but that is just the way that it is.
You have to learn something about subatomic particle physics first.
DAW
What you say is not true, and besides that, the "standing wave" of a subatomic particle had nothing to do with electromagnetic fields.
Reply | Report Abuse | Link to thisYou are just barking up the wrong tree, and you need to learn about quantum-mechanical wave functions first.
Absolutely true: there is no such thing as a "spherical area". A sphere is a three-fimensional object, and it has volume. However, there is such a thing as the surface of a sphere, which is a two-dimensional thing, and it does have area. Thus, they needed to write: the area of a spherical surface.
Reply | Report Abuse | Link to thisI agree 100%, Mr. Schmidt. People need to learn that it takes years and years of study in physics and mathematics beyond high school for anyone to make any progress in the field of subatomic particles. Those know-it-alls who don't even have a master;s degree in physics need to shut up and listed to the experts. Sit at their feet and learn all that you can learn.
Reply | Report Abuse | Link to thisThe point being, quarks are on a tighter leash than previously thought and if the area of the proton "sphere" is smaller than thought, that could mean a lot of things, one of which would be that that the massive quarks are actually statistically closer to each other more of the time than previously thought, which may be due to gravity, which can be quite strong on the sub-atomic scale when particles get very, very close to each other.
Reply | Report Abuse | Link to thiseuropamoon100: As a layperson with a deep interest in particle physics I do suffer a high degree of misunderstanding relative to a physicist such as yourself so I appreciate the information you have so generously provided.
Reply | Report Abuse | Link to thisI used the word "typical" in my post because it was the column heading "Typical Mode of Decay" from a table of particles by the Particle Data Group, Lawrence Berkeley Laboratory. I presumed, perhaps wrongly, that "typical" would refer to spontaneous decay. Since the column heading used the word "typical" then it could be infered that an "atypical" decay is possible, perhaps when the muon interacts with other particles, such as the photon that kicks the muon up an energy level causing it to emit an x-ray photon as it returns to the base energy level. Thus was the basis of my question in my post at 03:10 PM on 07/14/10. Please forgive my failure to properly proof that post as Paragraph 2, sentence 1, should have ended with a "?", and sentence 2 was a supposition put forth to induce clarification.
If my question and supposition were sufficiently naive that clarification is impossible, then I would understand. However, I would be interested to read your thoughts regarding the experiments described in this article.
Could this be the first measurement of the quantization of Gravity between particles? Perhaps the heavier muon feels a larger gravitational tug (or quantization of a bundle of space-time curves that appears as gravity), that throws everything off and this is the first indication of its strength and nature. Yes, folks you heard it here first, from a talented layman.
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