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Big problems sometimes come in small packages. The problem with which physicists must now concern themselves measures a mere 0.0350 millionth of a millionth of a millimetre. This is precisely the difference between the new, smaller, dimension of the proton, the nucleus of the hydrogen atom, and the value which has been assumed so far. Instead of 0.8768 femtometres it measures only 0.8418 femtometres.
At the Paul Scherrer Institute in Switzerland, an international team of researchers including physicists from the Max Planck Institute of Quantum Optics has now measured this in experiments which are ten times more accurate than all previous ones. They thus present physics with some tough problems: at least one fundamental constant now changes. And physicists have also to check the calculations of quantum electrodynamics. This theory is assumed to be very well proven, but its predictions do not agree with these latest measurements "The size of the proton".
| For many years, Randolf Pohl and his colleagues believed their measuring instrument was not accurate enough: they first performed an experiment to determine the size of the proton back in 2003, but they had not discovered the signal which would provide them with the relevant insight. "This was not down to the accuracy of our method, but to the fact that we did not expect such a large deviation," says Randolf Pohl. The researchers had therefore chosen too small a window for their measurements. "It is good, nevertheless, that we have significantly improved our method yet again, otherwise people might not believe us now," continues Pohl. | |
| Randolf Pohl and his colleagues in an international collaboration have measured the charge radius of the proton with an accuracy of better than one thousandth of a femtometre. This is the radius which the charge of the positive hydrogen nucleus assumes. To this end, they have investigated tiny details in the atomic structure, using muonic hydrogen, where it is not an electron but a heavier muon which orbits the nucleus (see 'Background: a ruler for a proton'). Their measurements show that the hydrogen nucleus measures 0.8418 femtometre. A result which is outside the margin of error which physicists had applied to the previous measurements for the proton radius by a factor of five. | |
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Even if the deviation is negligible on a day-to-day scale, it possibly has significant consequences. Researchers are unable to say precisely what these may be, however. What is certain is that this changes the Rydberg constant. Quantum physicists use this constant to calculate which energy packets atoms and molecules absorb and emit when they change their states. These energy packets correspond to the spectral lines of the elements. The calculations for the spectral lines now shift noticeably and no longer match the experimental findings.
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| The theoreticians are now searching for the error in the calculation | |
| "Since the Rydberg constant is the most accurately determined fundamental constant so far, it is as solid as a rock," says Randolf Pohl. If physicists draw a self-consistent picture of all fundamental constants, the other fundamental constants such as Planck's constant or the mass of the electron can only move around the Rydberg constant. The fact that this rock has been moved slightly will hardly impress the other fundamental constants: they have been determined just as exactly as the Rydberg constant so they will probably not feel the jerk at all. The test for this is still pending, however. | |
| "We also have to be very careful with more far-reaching consequences," says Pohl. Many theoreticians all over the world are now recalculating the predictions of quantum electrodynamics with the new proton radius. This quantum theory describes how atoms, electrons, elementary particles and other players move in this diminutive world and which electromagnetic fields are created in the process. It also provides a value for the proton radius for comparison with experimental data - but this is significantly higher than the one measured now. "I assume that an error has been made somewhere in the calculation, because the theory of quantum electrodynamics is very consistent and has been rigorously proven," says Pohl. If this is not the case, the slightly shifted proton radius would trigger an earthquake in physics, which would at least result in considerable 'fault lines' in this theory. | |
| While theoreticians are now trying to get to the bottom of the mystery of the erroneous proton radius in their models, the Garching researchers and their colleagues are checking the new measurement result with further experiments on the hydrogen atom. They also want to redesign their experimental set-up so that they can also measure the charge radius of the helium nucleus. These investigations are also intended to tell them something about how atomic nuclei are deformed when they interact with a negative charge. In this way the physicists want to discover the exact structure of matter step-by-step - and hope, of course, to come across more mysteries of physics. |
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