torsdag 23 februari 2012

About protons and atoms.

The highest resolution image of a single atom ever taken.

Ben Norton and supervisor David Kielpinski, of Griffith University's Centre for Quantum Dynamics in Australia, developed new techniques to capture an atom, which is about 400th billionths of a meter across. The research areas of the Centre include attosecond science, atomic and molecular dynamics in ultrafast fields, atom optics, quantum optics, quantum information and computation, and fundamental quantum mechanics. The team also took the first-ever image of the shadow of a single atom, at their smallest, are only one tenth of a millionth of a millimetre across.

Norton has captured the highest resolution images ever of a single atom with the help of a special lens. The atoms were imaged in an ion trap that was built from the ground up. By cooling the atoms down to absolute zero (-273.25 degrees Celsisus), he is able to reduce any movement. Norton then traps the atoms in an ultra-high vacuum that contains less air than outer space and holds them in place for a 'photo opp' using electric fields. These lenses can be made so small and light that they can be put inside the vacuum chamber with the atoms, allowing to collect as much light as possible.

Extreme imaging wins science praise
Ben Norton's remarkable image of an atom was runner up in the CiSRA Extreme Imaging Competition. Credit: Griffith University. From
The smallest trapped ion spot size that they captured high resolution images of was 370nm. Here is the link to the paper by Ben Norton et al, Millikelvin spatial thermometry of trapped ions. New Journal of Physics 13 (2011) 113022.

The strong Coulomb coupling makes laser-cooled trapped ions attractive for sympathetic cooling at millikelvin temperatures in investigations of fundamental physics [7], dynamics of complex molecular [8, 9] and biomolecular [10] ions, nano-mechanical oscillators [11, 12], resonant electric circuits [13] and Bose–Einstein condensates [14].
Look, what pictures! They cannot be bringed here!
Images of a trapped ion for three different external heating rates.
Errors were dominated by systematic uncertainty. All images were taken with the laser detuned 15MHz from resonance. The ion signal was integrated for 2 s in all three images.

Norton's basic research is in atomic physics, using lasers to interact with atoms. The team is now hoping to use the high resolution images of single atoms to understand how atoms behave.
Precise imaging of atoms can help scientists understand physics as a whole, the new field of quantum computing, and possibly even ultra-high resolution imaging of cells in the body.
The dynamics of quantum systems are systems composed of microscopic particles such as photons, electrons, and atoms, the behaviour of which is governed by quantum mechanics, and is very different from the familiar behaviour of macroscopic systems.

A crystal of 8 ions trapped by electric fields, from the Centre for Quantum Dynamics.
Another one! The February 21, 2012 press release from the Centre for Quantum Dynamics is entitled “Single atoms talk to electric circuits at Griffith."

Scientists at Griffith University’s Centre for Quantum Dynamics have advanced quantum computing in a big way with their ability to manipulate one atom within an electrical circuit, what is called atom-based quantum communications. They have been able to use a single atom to transfer information within an electrical circuit.
The scientists involved with this research compare their ability to manipulate a single atom to communicate within an electrical circuit with the much older process of getting “voices … transmitted over radio.”
By this Australian-American team “… could have far-reaching implications for the future of secure communications and code breaking.”
Dr. Dave Kielpinski, the chief investigator on this project and a professor at Griffith University, from the press release: "Atom-based quantum communication is guaranteed to be secure by the laws of physics, so the atom-circuit interface can extend this security to electronic devices. They are also excellent sensors for acceleration, gravity, and electrical fields. But working with them requires exotic laser and vacuum technologies. The atom-circuit interface will let us plug atom-based devices into more widespread electronic technology such as computers. This is the first time that the quantum theory of a single atom has been combined with a quantum electrical model. Quantum mechanics normally manifests itself on a microscopic scale. While well isolated single atoms can readily be controlled on the quantum level, large objects such as computers normally behave classically.”
The paper highlighting this work will be published in the journal Physical Review Letters -- within the February 24, 2012 issue.
"Proton Smaller Than Thought—May Rewrite Laws of Physics." from 2010. The hydrogen atom is also seen as the “Rosetta Stone” of quantum physics.

Smaller Proton Size Revealed by Lasers.
In a ten-year experiment, a team led by Randolf Pohl of the Max-Planck Institute of Quantum Optics in Garching, Germany, used a specialized particle accelerator to alter hydrogen atoms into 'muonic hydrogen'. The team replaced the atom's electron with a particle called a muon, which is 200 times more massive than an electron.
"Because the muon is so much heavier, it orbits very close to the proton, so it is sensitive to the proton's size," said team member Aldo Antognini, of the Paul-Scherrer Institute in Switzerland.
Muons are unstable, and they decay into other particles in just 2.2 microseconds. 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.
Errors possible:
- the Rydberg constant hasn't been correctly measured. This value describes the way light gets emitted from various elements—a key component of spectroscopy.
- the smaller size of a proton could mean the equations in QED theory will fail
The spectroscopic determination of the Lamb shift of the energy levels in muonic hydrogen in 2010 yielded a value for the proton radius which was significantly smaller than deduced from previous measurements – a fact that still puzzles the scientific community. Present knowledge of the (root-mean-square) proton charge radius which does not depend on hydrogen spectroscopy comes from electron scattering experiments. A recent reanalysis of all electron-proton scattering data [1], accounting for Coulomb distortion and using a parametrization that makes it possible to include data at higher q2, finds a radius of 0.895(18) fm, i.e. the uncertainty is as large as 2%. A measurement of the muonic Lamb shift with 30 ppm precision will determine the proton radius with 0.1% precision.
For years the accepted value for the radius of a proton has been 0.8768 femtometers, where a femtometer equals one quadrillionth of a meter. Now the scientists detected x-rays at an assumed proton radius of 0.8418 femtometers—4 percent smaller than expected. If it proves correct, it means something fundamental is wrong in particle physics.
The size of a proton is an essential value in equations that make up the 60-year-old theory of quantum electrodynamics, a cornerstone of the Standard Model of particle physics. The Standard Model describes how all forces, except gravity, affect subatomic particles. But the proton's current value is accurate only by plus or minus one percent—which isn't accurate enough for quantum electrodynamics, QED, to work perfectly.

Awards: “European Research Council Starting Grant” in August 2011.
Spectroscopy of muonic hydrogen in 2010 yielded a tenfold precision in the measurement of the protonic radius. However, the result showed a large discrepancy with previous measurements. The new project “Charge Radius Experiment with Muonic Atoms” is meant to solve this puzzle. By extending the spectroscopy of muonic hydrogen to muonic helium it will be possible to measure the size of its nucleus with tenfold precision. This will also shed some light on the proton size puzzle. “I have spent twelve years on measuring the charge radius of the proton”, Randolf Pohl says. “It may again take a long time to find the resonance of muonic helium. On the other hand, when we are successful, the gain will be huge, as progress in fundamental physical constants is notoriously slow.”

Professor Stefan Kuhr: “Single-atom-resolved detection and manipulation of strongly correlated fermions in an optical lattice” was the other reciever.
Because the interaction of fermions with each other is quite different from the interaction of bosons, the new project requires a completely new experimental setup. Among others, this work will lead to a deeper understanding of the mechanisms that give rise to macroscopic properties of matter such as magnetism or superconductivity.

...and Gustav Hertz Award of the German Physical Society, DPG, in nov 2011. with Antognini, will be given Stuttgart in March 2012.
From the homepage: in order to improve the determination of
  • the proton charge radius by a factor of 20 (to 1x10-3 relative accuracy)
  • the deuteron charge radius by a factor of 20 (to 1x10-3 relative accuracy)
  • the Rydberg constant by a factor of 6 (to 1x10-12 relative accuracy)
  • the proton magnetic radius (Zemach radius) to 1x10-2 relative accuracy
  • the deuteron polarizability
and extend the test of bound-state quantum electrodynamics (QED) theories in hydrogen and deuterium to a level of 3x10-7.
Latest publications:
Randolf Pohl, Aldo Antognini, The size of the proton, Nature, vol. 466, issue 7303, pp. 213-216 (2010).
Randolf Pohl, Aldo Antognini, et al. The Lamb shift in muonic hydrogen, Can. J. Phys. vol. 89, pp. 37–45 (2011).
Aldo Antognini, et al. Illuminating the proton radius conundrum: the muonic helium Lamb shift, Can. J. Phys. vol. 89, pp. 47–57 (2011).

See also TGD, 2010, The incredibly shrinking proton and a more detailed article here.

Interesting things! Keep the eyes open!

AddendumFirst photo of shadow of single atom July 3, 2012

First photo of shadow of single atom

In an international scientific breakthrough, a Griffith University research team has been able to photograph the shadow of a single atom for the first time.
"We have reached the extreme limit of microscopy; you can not see anything smaller than an atom using ," Professor Dave Kielpinski of Griffith University's Centre for in Brisbane, Australia.
"We wanted to investigate how few are required to cast a shadow and we proved it takes just one," Professor Kielpinski said.
Published this week in Nature Communications, " imaging of a single atom "is the result of work over the last 5 years by the Kielpinski/Streed research team.
At the heart of this Griffith University achievement is a super high-resolution microscope, which makes the shadow dark enough to see.
 First photo of shadow of single atom
Holding an atom still long enough to take its photo, while remarkable in itself, is not new technology; the atom is isolated within a chamber and held in by electrical forces.
Professor Kielpinski and his colleagues trapped single atomic ions of the element ytterbium and exposed them to a specific frequency of light. Under this light the atom's shadow was cast onto a detector, and a digital camera was then able to capture the image.
"By using the ultra hi-res microscope we were able to concentrate the image down to a smaller area than has been achieved before, creating a darker image which is easier to see", Professor Kielpinski said.
The precision involved in this process is almost beyond imagining.
"If we change the frequency of the light we shine on the atom by just one part in a billion, the image can no longer be seen," Professor Kielpinski said.

Journal reference: Nature Communications
Provided by Griffith University

Scientists make quantum breakthrough
 April 20, 2011

“We have shown that when in a vacuum chamber are guided inside a laser light beam, they too can create a speckle pattern - an image of which we have captured for the first time”.
The team trapped a cloud of cold helium atoms at the focus of an intense pointed downwards at the imaging system, and then gradually turned down the laser intensity until the speckled image appeared. The work was done with PhD students Sean Hodgman and Andrew Manning.
“We then made the atoms even colder,” says team leader Dr Andrew Truscott, “until they behaved more like waves than particles, forming a single quantum wave called a Bose-Einstein condensate (BEC).  When the BEC was loaded into the guide, the speckle pattern disappeared, showing that just one mode was being transmitted – the single quantum wave.”
The physicists demonstrated that by measuring the arrival time of the atoms on the imaging system, they were able to distinguish between the multimode (speckled image) guiding, and the single-mode (smooth image) guiding.
“Measurements for the multi-mode beam showed the atoms arriving in groups as a result of their interference – so-called atom bunching,” said team member Dr Robert Dall. “However, the BEC represents just a single quantum mode with no interference, so when we guided the BEC - we saw no bunching.”
The guiding behaviour agreed with a theoretical model developed by team member Mattias Johnsson.  “We have shown that atoms can be guided in a laser beam of light, with the same properties as light guided in an optical fibre made of glass,”
More information: Observation of atomic speckle and Hanbury Brown–Twiss correlations in guided matter waves, Nature Communications 2, Article number: 291 doi:10.1038/ncomms1292
Speckle patterns produced by multiple independent light sources are a manifestation of the coherence of the light field. Second-order correlations exhibited in phenomena such as photon bunching, termed the Hanbury Brown–Twiss effect, are a measure of quantum coherence. Here we observe for the first time atomic speckle produced by atoms transmitted through an optical waveguide, and link this to second-order correlations of the atomic arrival times. We show that multimode matter-wave guiding, which is directly analogous to multimode light guiding in optical fibres, produces a speckled transverse intensity pattern and atom bunching, whereas single-mode guiding of atoms that are output-coupled from a Bose–Einstein condensate yields a smooth intensity profile and a second-order correlation value of unity. Both first- and second-order coherence are important for applications requiring a fully coherent atomic source, such as squeezed-atom interferometry.
Provided by Australian National Universit

= Flux tubes, massless extremals with Kähler language.

15 kommentarer:

    Albert Einstein: The Size and Existence of Atoms

    Viewpoint: Photons and Atoms Cooperate, Virtually, Ralf Röhlsberger,
    Shifts in atomic energy levels caused by electron interaction with virtual photons can also occur in an ensemble of atoms acting cooperatively.

    The interaction of light with ensembles of many identical resonant atoms becomes increasingly relevant these days as more and more of such systems can be experimentally prepared in a controlled fashion, ranging from atoms in the gas phase, like Bose-Einstein condensates, to solid-state systems, like quantum dots. The existence of many identical resonators in close proximity modifies their properties to produce radiative and optical effects that do not occur in single atoms. A new cornerstone in the exploration of these effects has been laid

    Cooperative Lamb Shift in an Atomic Vapor Layer of Nanometer Thickness

    J. Keaveney, A. Sargsyan, U. Krohn, I. G. Hughes, D. Sarkisyan, and C. S. Adams
    Phys. Rev. Lett. 108, 173601 (2012)
    Published April 23, 2012 | PDF (free)


    Scientists in Sweden film the sub-atomic particle, the electron, for the first time. An electron is approximately 1867 times smaller than a proton and is constantly moving.
    An exotic type of symmetry - suggested by string theory and theories of high-energy particle physics, and also conjectured for electrons in solids under certain conditions - has been observed experimentally for the first time. Spectrum of magnetic resonances observed by neutron scattering in cobalt niobate in zero magnetic field, data (left) and calculation (right).


    Fine Structure of the Hydrogen Atom by a Microwave Method

    Willis E. Lamb, Jr. and Robert C. Retherford
    Phys. Rev. 72, 241 (1947)
    Published August 1, 1947

    Lamb and graduate student Retherford wanted to measure the hydrogen fine structure by investigating two specific electron states. One was a relatively long-lived S-state, with a spherically symmetric orbital, and the other was a shorter-lived P-state, with less symmetry. Standard theory predicted that the two states should have equal energy but that applying a magnetic field should influence the states in different ways and induce an energy difference between them.

    The team sent a stream of electrons at right angles into a beam of hydrogen atoms, exciting a few of them into the S-state and also deflecting them slightly from the main beam direction. The excited atoms passed through a region containing both microwave radiation and an adjustable magnetic field, and then hit a metal target. The excited atoms would then drop back to the ground state, emitting electrons that the team could detect as a current. The key to the experiment was that if the magnetic-field-induced energy difference between the two states was equal to the energy of the microwave photons, then the long-lived S-state would absorb a photon and turn into to the short-lived P-state. These atoms would drop back to their ground state before reaching the target, and the current in the detector would essentially vanish.

    By plotting the critical magnetic field strength for a variety of microwave frequencies, Lamb and Retherford could determine the energy difference between the two states in the absence of a magnetic field. Contrary to expectation, the difference was not zero.

    This departure from theory became known as the Lamb shift
    It was Hans Bethe, on the train ride home, who wrote a short paper giving a somewhat sketchy but fairly accurate calculation of the shift [1]. The solution to the self-energy problem, proposed by others, was to think of the “bare” electron as having infinite energy that is mostly cancelled out by the infinitely negative energy of its interaction with its own electric field. This so-called renormalization approach leads to a correction to the classical energy that depends on distance. A P-state electron spends a different amount of time close to the nucleus than an S-state electron, so they require different corrections. Bethe’s estimate for the resulting Lamb shift fit the experimental result remarkably well and demonstrated that renormalization—which is at the core of today’s quantum mechanics—could be verified in experiments.

    Accepting a share of the 1955 Nobel Prize in Physics for his discovery, Lamb remarked that some previous experiments [2] a decade before his and Retherford’s “indicated a discrepancy which should have been taken seriously.”

    Physicists at the Max Planck Institute of Quantum Optics in Garching near Munich, have now gained fundamental insights into a particular kind of atomic ensemble – a so-called Rydberg gas – that might play a role in the future design of a quantum computer. They observed how "super atoms" formed in the gas and ordered themselves in geometric shapes such as triangles and squares. In future, the researchers intend to control the number and geometric configuration of these super atoms. That would be an important step towards a scalable system for quantum information processing.


    Shrunken proton baffles scientists
    Geoff Brumfiel, 24 January 2013

    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, says Aldo Antognini, a physicist at the Swiss Federal Institute of Technology Zurich in Switzerland and an author of both muonic papers.

    Arrington and Sick both have their doubts. Given the power of existing theories, Sick says, the idea of fundamental differences between muons and electrons is "sort of hard to imagine".

    But equally hard to imagine is what might have gone wrong. There could be a problem with the models used to estimate the proton size from the measurements, but so far, none has been identified. "Many of the ideas that have been stated have all been looked at in more detail," Sick says. "Nobody has come up with a clear result."
    Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the Formula transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the Formula transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, rZ = 1.082(37) femtometers, and the magnetic radius, rM = 0.87(6) femtometer, of the proton. We also extracted the charge radius, rE = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.

  8. Artificial hydrogen tests quantum theory

    27 January 2011
    A normal hydrogen atom contains a single negatively charged electron orbiting a nucleus made of a single positively charged proton. About 0.015% of natural hydrogen consists of the heavy isotope deuterium, in which the nucleus contains a proton and an electrically neutral neutron, and which has a mass twice that of normal hydrogen. And there is a third isotope with a proton and two neutrons: tritium, three times as massive as hydrogen, which is produced in trace quantities by cosmic rays interacting with the atmosphere, but is too dangerously radioactive for use in such experiments.

    The chemical behaviour of atoms depends on the number of electrons they have rather than their masses, so the three hydrogen isotopes are chemically almost identical. But the greater mass of the heavy isotopes means that they vibrate at different frequencies, and quantum theory suggests that this will produce a small difference in the rates of their chemical reactions.

    Helium has two electrons, two protons and two neutrons. But because it is more massive than an electron, the negative muon orbits the nucleus much more closely, masking the positive charge of one of the protons. In effect, the atom becomes a hydrogen-like composite: a 'nucleus' made of the existing two-proton, two-neutron nucleus and the muon, orbited by the remaining electron. It has a mass of a little over four times that of hydrogen.

    Fleming and colleagues found that the reaction rates for hydrogen exchange involving these analogues that were calculated from quantum theory were close to those measured experimentally. "This gives confidence in similar theoretical methods applied to more complex systems," says Fleming.

    quantum calculations use a simplification called the Born–Oppenheimer approximation, which assumes that the electrons adapt their trajectories instantly to any movement of the nuclei. This is generally true for electrons, which are nearly 2,000 times lighter than protons. But it wasn't obvious that it would hold for muons, which have a tenth of the proton's mass. But Fleming and his colleagues propose now to look at the 'hydrogen' exchange reaction between the super-heavy 'hydrogen' and methane (CH4).

    The neutral muonic helium atom may be regarded as the heaviest isotope of the hydrogen atom, with a mass of ~4.1 atomic mass units (4.1H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of 4.1H with 1H2 to produce 4.1H1H + 1H at 295 to 500 kelvin. The experimental rate constants are compared with the predictions of accurate quantum-mechanical dynamics calculations carried out on an accurate Born-Huang potential energy surface and with previously measured rate constants of 0.11H (where 0.11H is shorthand for muonium). Kinetic isotope effects can be compared for the unprecedentedly large mass ratio of 36. The agreement with accurate quantum dynamics is quantitative at 500 kelvin, and variational transition-state theory is used to interpret the extremely low (large inverse) kinetic isotope effects in the 10−4 to 10−2 range.

  9. If the smaller size is correct, then there's something missing in physicists' understanding of quantum electrodynamics, which governs how light and matter interact.
    Chad Orzel, an associate professor of physics and astronomy at Union College and author of "How to Teach Physics to Your Dog" (Scribner, 2010), said the results are good for physics generally, because of the questions they raise. "It's really boring when all the measurements and theory agree with each other. This kind of disagreement gives us something to talk about that isn't the Higgs boson."

  10. Also read Matt Strassler post on protons. Very good.

  11. And so the proton, which has a diameter about 60,000 times smaller than ordinary hydrogen, is only 300 times smaller than muonic hydrogen. That makes the details of muonic hydrogen more sensitive to the proton’s size, and thus allows for a more precise measurement.

    On the experimental side, the experimenters use a laser whose energy per photon can be adjusted, and they measure the energy of photons needed to make muonic hydrogen transition from one state to another (similar to what is shown in Figure 3). [In quantum mechanics, atoms and other similar systems can only exist in very particular states, each one with a very particular mass, shape, size, and other properties.] Only photons of precisely the right energy will do the job for a particular transition. On the theoretical side, one uses known properties of protons, muons, and electromagnetic forces, etc., and (treating the proton’s size as completely unknown) one calculates carefully what the energy of the required laser photons is expected to be. The answer depends on the proton’s unknown size, so by requiring the calculation agrees with the measurement, one learns what the proton’s size is! a recent useful review article on the subject by Pohl, Gilman, Miller and Pachucki. The proton radius is extracted from many different measurements (blue and violet data points) of state transitions in ordinary hydrogen; the blue solid band gives their weighted average, and the orange band gives the 2010 result from muonic hydrogen. All uncertainty bars show one standard deviation. The most recent muonic hydrogen measurement is not shown; the discrepancy is now even more significant.

  12. Speaking today (April 13) at the April meeting of the American Physical Society, researchers said they need more data to understand why new measurements of proton size don't match old ones.

    "The discrepancy is rather severe," said Randolf Pohl,The question, Pohl and his colleagues said, is whether the explanation is a boring one — someone messed up the measurements — or something that will generate new physics theories.

    One possibility is that photons aren't the only particles that carry forces between particles — perhaps an unknown particle is in the mix, causing the proton-measurement discrepancies.

    Next steps

    To find out what's going on, physicists are launching a new set of experiments across multiple laboratories. One major line of research involves testing electron-scattering experiments to be sure they've been done correctly and that all the facets are understood, Arrington said.

    Another goal is to repeat the scattering experiments, but instead of shooting electrons at protons they'll shoot muons at protons. This project, the Muon Scattering Experiment, or MUSE, is set to take place at the Paul Scherrer Institute in Switzerland.

    the first direct observation of an atom’s electron orbital — an atom's actual wave function! To capture the image, researchers utilized a new quantum microscope — an incredible new device that literally allows scientists to gaze into the quantum realm.

    An orbital structure is the space in an atom that’s occupied by an electron. But when describing these super-microscopic properties of matter, scientists have had to rely on wave functions — a mathematical way of describing the fuzzy quantum states of particles, namely how they behave in both space and time.
    Up until this point, scientists have never been able to actually observe the wave function. Trying to catch a glimpse of an atom’s exact position or the momentum of its lone electron has been like trying to catch a swarm of flies with one hand; direct observations have this nasty way of disrupting quantum coherence. What’s been required to capture a full quantum state is a tool that can statistically average many measurements over time. Image: Examples of four atomic hydrogen states. The middle column shows the experimental measurements, while the column at right shows the time-dependent Schrödinger equation calculations — and they match up rather nicely.

    You can read the entire study at Physical Review Letters: "Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States."

    Physicists have been searching for this dipole moment for 50 years. Now a group called the ACME collaboration, led by David DeMille of Yale University and John Doyle and Gerald Gabrielse of Harvard University, has performed a test 10 times more sensitive than previous experiments, and still found no signs of an electric dipole moment in the electron. The electron appears to be spherical to within 0.00000000000000000000000000001 centimeter, according to ACME’s results, which were posted on the preprint site arXiv. “It’s a surprise,” says Ed Hinds, also of Imperial College London, who worked with Hudson on the previous best limit, set in 2011. “Why on Earth is it still zero?”

    "Now in a new paper published in EPL, physicist Roberto Onofrio at the University of Padova in Padova, Italy, and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, has suggested that the muonic hydrogen experiment may be providing a hint of quantum gravity. He has proposed that the proton radius puzzle can be solved by considering a new theory of quantum gravity that is based on the unification of gravity and the weak force, also called "gravitoweak unification.""