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Mystery of the Shrunken Proton
ananyo writes "The proton, a fundamental constituent of the atomic nucleus, seems to be smaller than was previously thought. And despite three years of careful analysis and reanalysis of numerous experiments, nobody can figure out why. An new experiment published in Science only deepens the mystery. The proton's problems started in 2010, when research using hydrogen made with muons seemed to show that the particle was 4% smaller than originally thought. The measurement, published in Nature, differed from those obtained by two other methods by 4%, or 0.03 femtometers. That's a tiny amount but is still significantly larger than the error bars on either of the other measurements. 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 — but is still in disagreement with the earlier two measurements. So what's the problem? There could be a problem with the models used to estimate the proton size from the measurements, but so far, none has been identified. The unlikely but tantalizing alternative is that this is a hint of new physics."
uh sure, if the prior method for measuring was also showing the reduced size, but it's not ... so how does "the universe expanding" explain two simultaneously different measurements? besides, if the universe were expanding and protons weren't, i don't think our meter sticks would be expanding.
how the hell did this get +5 anyway ... brainless mods
This doesn't appear to be a case where the measurement is changing over time. That is, it seems many here are misinterpreting the summary to suggest that things are different NOW relative to THEN.
Instead, things are different if we measure THIS WAY vs. THAT WAY. But we can still go back and measure both ways. If we use the old method(s), we get the old result.
That's what's creating the angst. Theorists cannot see why the two methods would differ. And they've checked and rechecked their work. Experimentalists have also checked and rechecked their work.
This is one of those "that's funny" things that becomes rather interesting.
It does not work that way. Things like metre sticks are held together by the electromagnetic force, which is decoupled from the expansion of the Universe. This means that objects in the Universe do not expand, they just move along with the expansion. If everything in the Universe expanded with the Hubble flow then we would never be able to detect the Hubble flow. Only spacetime expands, not what is sitting around in spacetime.
The explanation for the unexpected small size of the proton is probably something to do with the way that muons interact with protons. We assume that electrons and muons interact with protons in exactly the same way, but this is a hypothesis. There is very little observational evidence supporting the idea that electrons and muons behave in exactly the same way when they are bound to an atomic nucleus. The problem with this idea is that it requires that particle physics be extended beyond the standard model. It is also possible that the problem is something much more mundane, like a faulty connection somewhere in the experimental setup. We need an independent verification of this result before we start rewriting the textbooks.
Just because you are paranoid does not mean that no-one is out to get you.
They would expect a muon to orbit at the same distance since it has the same charge as an electron
Actually, no they don't. The whole point of using muons is that their orbitals would be much closer to the proton due to the muon's mass. The size of the orbitals and structure of the orbitals depends on the mass ratio between the two parts, and since the muon is much more massive than the electron, it was expected to have smaller orbitals, much smaller than 4%. And hence, it was expected the smaller orbitals would be more sensitive to structure of the proton. The discrepancy comes from the effects of the proton on the orbital not being quite what they expected from electron based measurements, not from just a change in the size of the orbital.
Now, they tell me.
Please do not read this sig. Thank you.
It is not so simple as a change in size of the orbital structure. First off, the point of the experiment was that the muon orbital would be much smaller. Second, they measured two different atomic transitions in the system, involving four different orbitals. It wasn't the over all size/energy of the orbitals that was under consideration, it was the relative energies involved in these transitions.
The results of comparing the transition energies were done two different ways, one sensitive to the magnetic structure of the proton, the other sensitive to the charge structure of the proton. The former was in agreement with previous measurements of the magnetic size of the proton. The latter is the one that is off by 4% from older measurements. There wasn't some singular, overall change in the size of everything involved. Instead, this points to there being something wrong with the understanding of the charge structure of the proton, and hence that structure's predicted impact on the muon orbitals.
Just changing sizes or talking about expansion wouldn't account for the second half of their results where they found agreement with past, electron based measurements.