ScienceNOW | 07 July 2010
The proton shrinks in sizePerhaps the recession is to blame. The most precise measurement ever made of the proton's radius shows that the subatomic particle is 4% smaller than previously thought. That's a big puzzle for physicists, because the theory used to deduce the size—quantum electrodynamics (QED)—is the most precise one in physics and has proved accurate to a few hundred millionths of a percent in certain circumstances.
"The most elegant solution would be that there's a miscalculation in there somewhere, but the theorists tell us everything is fine," says Randolf Pohl, an experimenter at the Max Planck Institute of Quantum Optics in Garching, Germany, and the first author on the study. But some theorists say that the problem may not be that the proton is smaller than had been thought, but that they do not really understand what's going on inside it.
Rather than measuring the radius of a proton directly, the international team of 32 physicists inferred it more accurately by measuring energy levels in an odd type of hydrogen. An ordinary hydrogen atom consists of a wispy electron bound to a beefy proton. The electron can occupy various cloudlike quantum states called orbitals, which give the probability of finding it at any particular place. Each orbital has a precise energy, and at first blush some orbitals should have exactly the same energy.
But in 1947, American physicists Willis Lamb and Robert Retherford discovered that two such "degenerate" orbitals actually had very slightly different energies. The difference, called the "Lamb shift," comes about because of quantum fluctuations in the electromagnetic field that binds the electron to the proton, as famed theorist Hans Bethe soon explained. Bethe's calculation was a key first step toward a fully quantum mechanical theory of the electromagnetic field, QED, still the most accurate and precise theory physicists have ever developed.
Pohl and colleagues have spent 10 years trying to test the limits of that accuracy in a new way. Using a particle accelerator at the Paul Scherrer Institute in Villigen, Switzerland, they generated hydrogen atoms in which they replaced the electron with a muon, a particle 207 times as massive that lasts for only 2 microseconds before decaying. They used a high-precision laser system to measure Lamb shift in the "muonic hydrogen" atoms.Odd Ruler: To infer the proton's radius, physicists measured the
energies of the 2s (left) and 2p (right) orbitals in hydrogen atoms in
which they replaced the electron with a particle called a muon.
Credit: Wolfgang Christian/Davidson College
Because the muon is so massive, it hovers closer to the proton than an electron does and probes different manifestations of the quantum fluctuations. Whereas the fluctuations kick the electron around and effectively smear it into a diffuse cloud of charge, the fluctuations in muonic hydrogen tend to polarize the empty space in the atom and create an even bigger Lamb shift.
When Pohl and colleagues measured the Lamb shift in muonic hydrogen, however, the result was even larger than expected from data from ordinary hydrogen combined with QED calculations, they report in tomorrow's issue of Nature. The size of the shift depends on the radius of the proton, and the team extracts a value of 0.84184 millionths of a nanometer—4% smaller than the previous estimate based on studies of ordinary hydrogen.
So is QED faulty? Not likely, says Rudolf Faustov, a theorist at the Russian Academy of Sciences in Moscow. He notes that the proton is actually a roiling mass of particles called quarks and gluons, all held together by the so-called strong force. That inner complexity, Faustov says, makes it difficult for physicists to handle precisely the electromagnetic force between the proton and muon in their calculations. "It's not quite clear how to separate these interactions," he says. In particular, he says, physicists may have to reconsider how the muon affects the proton.
The result may even point to new physics, says Krzysztof Pachucki, a theorist at the University of Warsaw. Thanks to quantum fluctuations, the proton is chock full of quark-antiquark pairs flitting into and out of existence. If the proton also contained lots of electron-positron pairs, they would increase the polarization of space with the muonic atom and resolve the discrepancy in the Lamb shift with no need to revise the textbook value of the proton's radius, Pachucki says. "That would be the first thing I would check."
Nature News | 07 July 2010
Quantum electrodynamics: A chink in the armour?Tiny change in radius has huge implications.Measurements with lasers have revealed that the proton may be a
touch smaller than predicted by current theories. (PSI/F. Reiser)
The proton seems to be 0.00000000000003 millimetres smaller than researchers previously thought, according to work published in today's issue of Nature1.
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 femtometres (1 femtometre = 10-15 metres).
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% 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, Virginia. 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."
- Nature 466 (08 July 2010) DOI: 10.1038/466195a
- Nature 466 (08 July 2010) DOI: 10.1038/nature09250
Science News | 07 July 2010
Incredible shrinking proton raises eyebrowsSubatomic particle may be smaller than theory dictates
Nothing is immune to downsizing in these tough economic times — not even subatomic particles. New measurements published in the July 8 Nature suggest that the proton has a radius about 4 percent smaller than previously thought.
The result could just be a mistake. But if confirmed, a smaller proton could have enormous implications, scientists say.
New Scientist | Physics & Math | 07 July 2010
‘Horrendously Intense’ Laser Shrinks the ProtonHow big is a proton? The most accurate measurement yet suggests it's smaller than we thought. This could be due to an error – or it might just hint at totally new particle physics.How wide is the proton in the middle?
(Image: Mehau Kulyk/Science Photo Library)
"The new experiment presents a puzzle with no obvious candidate for an explanation," says Peter Mohr of the international Committee on Data for Science and Technology (CODATA), which calculates values for fundamental constants in physics, who was not involved in the new work.
Like most quantum objects, a proton is fuzzy around the edges. Its size is defined by the extent of its positive charge rather than a crisp physical boundary. This charge radius cannot be measured directly but can be inferred from the hydrogen atom, which consists of a proton and an electron.
The electron can sit in a variety of energy "shells", each with a different distribution in space. One shell's distribution requires the electron to dive in and out of the proton, and another sits entirely outside the proton. The energies of both of these shells can be combined to deduce the proton's radius, using a theory known as quantum electrodynamics (QED).
Wired Science | 07 July 2010
Particle physics: 'Honey, I shrunk the protonNew laser-assisted measurements find that the fundamental building block of matter, the proton, is about 4 percent smaller than previously thought. The new size could poke holes in one of the pillars of the standard model of particle physics.
“It’s a big deal,” commented physicist Jeff Flowers of the National Physical Laboratory in the U.K., who was not involved in the new work. “It’s given us a glimpse of a chance that there’s a real theoretical leap forward to be made.”
The potentially threatened theory, called quantum electrodynamics or QED, describes how charged particles interact with light. Since the late 1940s, the theory has been wildly successful at predicting where electrons in atoms will spend most of their time. The calculations are especially accurate for the simplest atom, hydrogen, which consists of just one proton and one electron.
But the distance between the electron and the proton depends slightly on the proton’s size, similar to how a planet’s distance from its star depends on the star’s mass. In the last decade, the accuracy of hydrogen studies and the precision of theoretical predictions have gotten so good that physicists can no longer ignore the proton’s girth.
PhysOrg | General Physiscs | 07 July 2010
Scientists lobbed a bombshell into the world of sub-atomic theory on Wednesday by reporting that a primary building block of the visible Universe, the proton, is smaller than previously thought.
More precisely, revised measurements shave four percent off the particle's radius, according to a study in Nature that is highlighted on the journal's cover.
That may not seem like much, especially given the proton's infinitesimally tiny size.
But if borne out in further experiments, the findings could challenge fundamental precepts of quantum electrodynamics, the theory of how quantum light and matter interact.
