Protons are smaller than we thought...could the entire universe be wrong?

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Protons are 0.00000000000000003 meters smaller than we thought. That sounds like nothing, but it means one of these things must be true: Undiscovered particles are lurking, quantum mechanics needs recalculating...or the universe is impossible. (Here's hoping it's the first two.)

The measurements find that the proton is actually about 4% smaller than what decades of previous experiments had found. This new data, the result of years of testing by a team of European physicists, has left scientists baffled. The mathematics of quantum mechanics rely on some fairly finely tuned values for the measurements of various particles, and unless this discrepancy can be explained the entire discipline is in danger of falling apart. Ingo Sick, a physicist at Switzerland's University of Basel, puts the matter in no uncertain terms:

"It's a very serious discrepancy. There is really something seriously wrong someplace."


Measuring the size of protons can't be done directly, but the laws of quantum mechanics provide a very effective indirect approach. The energy levels that electrons occupy in the atom are partially described by the size of the proton, and physicists have keyed in on this fact to arrive at incredibly accurate measurements of the proton radius. Before this experiment, that figure was thought to be about 0.8768 femtometers, a little less than one quadrillionth of a meter.

The new experiment substituted a different subatomic particles for the electrons. Instead, the physicists used the muon, which is very similar to the electron except about 200 times heavier. This much greater relative weight makes them much more sensitive to the effects of protons on their energy level. The research team fired muons at a cloud of hydrogen, displacing the electrons from the atoms and leaving the muons in their place. They then used lasers to figure out the muonic energy levels, and that's how they arrived at their impossible new figure.


How impossible? The measurement falls so far outside what the scientists expected that it actually took them six years to even notice it. The first two times they ran the experiment, in 2003 and 2007, they didn't find anything in the tiny range in which they thought the proton radius would be founding. They figured there was something wrong with their laser systems, but on the third try in 2009 they decided to expand the range of where they looked. They quickly found the energy signature far, far away from where they figured it would be.

These findings might mean some of the constant values of quantum mechanics are incorrect, which would entail some fairly major recalculating. It's the Rydberg Constant that links together proton size and the energy levels of orbiting particles; if it's not what we thought it was, then a lot of other numbers are wrong as well. And that's the straightforward explanation.


The more exciting possibility is that an as yet undiscovered, undetected particle is messing with the interaction between the muon and the proton, skewing the result. These mystery particles might be wonderfully named "sparticles", hypothetical particles predicted by the quantum theory of supersymmetry. Sparticles (short for supersymmetry particles) are partners of the various known subatomic particles that are thought to be 1,000 times more massive than their known counterparts and existed for only trillionths of a trillionth of a second after the Big Bang.

Of course, there is a more mundane possibility: maybe the measurements are just wrong. There might have been some problem with the researchers' experimental procedure, or maybe the approximations used in quantum calculation just aren't as precise as we thought. Or - as long as we're keeping all possibilities on the table - perhaps we've discovered the one inherent cosmic flaw that makes the universe impossible, bringing all of existence crashing down around us. Thanks a lot, quantum physicists.


[Nature via Scientific American]