The proton is one of the fundamental components of the atom. For a long time scientists have believed it to be 0.8768 femtometers in size (a femtometer is one quadrillionth of a meter). Now, it looks like they may have been wrong, the size is 0.84184 femtometers. In a way, the discrepancy is very small…as in anything measured in quadrillionths of a meter is already incredibly small…and this discrepancy (an error by another name) is but a tiny fraction of a femtometer. In other ways, this is a big deal: One, because so many have been wrong for so long, and two, because the size of a proton is so fundamental to so many other aspects of nuclear and quantum physics that even so small an error could lead to massive changes in models of atomic dynamics.

But wait. This is the reported result of one paper, based on one type of (novel) experiment. True, the scientists representing a large collegium of institutions and publishing in the journal Nature put the size uncertainty at 0.00067 femtometers, whereas the old uncertainty size was 0.0069. The new one is an order of magnitude better. Nevertheless, whenever something this, well, the word is shocking, comes along the reaction of most scientists is to ask questions: What is the integrity of the experimental setup? Is it possible that mistakes in measurement or calculation were made? Is it possible that while this may seem to contradict or invalidate many theoretical constructs, a small tweak here or there might explain this result? In short, the basic reaction is not “This must be wrong!” It is, “We must question, test, and validate (or invalidate) this result as thoroughly as possible because it is extremely important.” That it is.

Not as important for me and thee as discovering the sun is going to explode, but for the many theories in quantum electrodynamics (QED) and nuclear particle science that are based on the way a proton of a specific size interacts with the orbits of electrons – a fundamental notion of how the physical world is constructed – it’s very important indeed. (To trivialize for an example, what would happen if you discovered your name was Smit instead of Smith? If you wanted to be correct, how many documents would need to change?)

How is it that something so fundamental as the size of a proton could be wrongly measured? Simple, scientists didn’t have the right tools to make such an accurate measurement. The idea for the experiment was conceived over forty years ago, but until recently the equipment and the techniques were not available. The essential technique was to use pulsed laser spectroscopy (the new equipment) to look at an atom of hydrogen where the usual single electron has been replaced by a muon, which acts like an electron but is 200 times heavier. Because they’re bigger, muons are more sensitive to the size and pull (electromagnetic) of the proton and this sensitivity is large enough to be measured. By calculating backward (so to speak) from the behavior of the muon, the size of the proton is revealed – and was revealed to be 4% smaller than expected.

At this point, the real impact of the experimental results finding a smaller proton is not to change the world or even the world of physics – it is to excite the activity of scientists who find the possibility of a ‘game changer’ a great challenge or a dreadful possibility. In either case, the chase is on to either confirm or deny the findings through further investigation, experimentation, and theoretical noodling.

For a good backgrounder and commentary, check out the blog Uncertain Principles:

So, protons are way smaller than we thought? Yes and no. This measurement suggests that the size is smaller than that measured in previous experiments, but the difference isn’t all that big in absolute terms. And it’s possible that there’s some effect they haven’t accounted for properly in making this measurement.

What sort of effect? Well, if they knew that, they would’ve accounted for it. There’s a fearsome amount of theory going into the conversion from Lamb shift to proton size, so it’s possible that something there is a little bit off. It’s also possible that there’s something wrong experimentally– this is the first time anybody has ever done laser spectroscopy of muonic hydrogen, so they might’ve overlooked something.

Or it could be completely new physics.

[Source: Uncertain Principles]

Physics this complex and important doesn’t usually come gift-wrapped from the mind of one scientist; it takes a massively collective effort:

Randolf Pohl1, Aldo Antognini1, François Nez2, Fernando D. Amaro3, François Biraben2, João M. R. Cardoso3, Daniel S. Covita3,4, Andreas Dax5, Satish Dhawan5, Luis M. P. Fernandes3, Adolf Giesen6,13, Thomas Graf6, Theodor W. Hänsch1, Paul Indelicato2, Lucile Julien2, Cheng-Yang Kao7, Paul Knowles8, Eric-Olivier Le Bigot2, Yi-Wei Liu7, José A. M. Lopes3, Livia Ludhova8, Cristina M. B. Monteiro3, Françoise Mulhauser8,13, Tobias Nebel1, Paul Rabinowitz9, Joaquim M. F. dos Santos3, Lukas A. Schaller8, Karsten Schuhmann10, Catherine Schwob2, David Taqqu11, João F. C. A. Veloso4 & Franz Kottmann12

1. 1 Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
2. 2 Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
3. 3 Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
4. 4 I3N, Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal
5. 5 Physics Department, Yale University, New Haven, Connecticut 06520-8121, USA
6. 6 Institut für Strahlwerkzeuge, Universität Stuttgart, 70569 Stuttgart, Germany
7. 7 Physics Department, National Tsing Hua University, Hsinchu 300, Taiwan
8. 8 Département de Physique, Université de Fribourg, 1700 Fribourg, Switzerland
9. 9 Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA
10. 10 Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
11. 11 Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
12. 12 Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland
13. 13 Present addresses: Deutsches Zentrum für Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany (A.G.); International Atomic Energy Agency, A-1400 Vienna, Austria (F.M.).