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New Finding: Protons 4% Smaller than Previously Measured

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Description: Established particle physics may require significant revisions
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Mefiante
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« on: July 08, 2010, 11:49:55 AM »

The finding means that either the theory governing how light and matter interact (called quantum electrodynamics, or QED) must be revised, or that a constant used in many fundamental calculations is wrong, the researchers said.



If the new value is confirmed, it could mean some rewriting of basic physics is in order.

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Peter Grant
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« Reply #1 on: July 08, 2010, 13:01:58 PM »

Exciting stuff. Grin
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Brian
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« Reply #2 on: July 08, 2010, 13:17:16 PM »

what astounds me as a total layman, is that 4% of something so infinitesimally small can have what seems to be a significant impact  Huh?
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Mefiante
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« Reply #3 on: July 08, 2010, 15:05:16 PM »

As a point of order, a proton isn’t strictly speaking infinitesimally small.

It has to do with the so-called “cross-section” of particles.  This quantity describes the probabilistic target area that a particle presents to the world when it interacts physically with others.  Remember that atomic and subatomic particles are not distinct objects in the same way that we usually think about, say, a marble; instead, they are fuzzily smeared out in spacetime around a focus point where the probability of finding it is at a maximum.

If the reported results are borne out by independent verification, then many other measured and calculated results that depend immediately on the proton’s cross-section (and there are a great many of them) will be in error by at least that 4% margin.  Moreover, because it is a systematic error, the cumulative knock-on effect of those second-tier errors into yet other results is that the errors will tend to grow more and more rapidly until the errors exceed the values of the results, at which juncture the latter become effectively meaningless.  A billiard ball analogy helps illustrate the point: it’s not too hard to play the cue ball so that it knocks another ball directly into a pocket.  It is much harder to play the cue ball onto another ball so that the second ball then knocks a third one into a pocket.  Even harder is doing this with a fourth ball.  The difficulty increases exponentially with the number of balls involved in such a chain because any error in despatching the cue ball, however slight, is amplified at each successive collision in the chain.

Assuming that the reported result is correct, it may even be that the accuracy issues discussed above are a lesser concern.  If it cannot be reconciled with existing theory, the finding may lead to the discovery of a presently unknown flaw in the Standard Model, and thence precipitate a slew of new revelations concerning the natural world.  It may well be the beginnings of the shakeup modern physics has been looking for since at least thirty years ago.

That, in a nutshell, is potentially what’s at stake.

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« Reply #4 on: July 08, 2010, 15:21:22 PM »

Interesting. Let's hope they got the mass of these one right as well Shocked.
Physicists unlock mystery of subatomic particle

Interesting how particles morph from one type to another. Do you think it is possible that electrons can also morph into other particles say... muons?
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Brian
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« Reply #5 on: July 08, 2010, 15:22:21 PM »

Thanks Mefiante, it's as clear as mud to me but your explanation of the knock-on effect is clear.
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rwenzori
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« Reply #6 on: July 10, 2010, 05:12:34 AM »

Admittedly I am a quantum ignoramus, but could the finding not be evidence that the "probabilistic target area" of the proton varies depending upon what is "orbiting" it ( for want of the correct word )? In other words, is the fatty muon not squashing the proton in the exotic atom, more so than the normal lithe and slim electron?
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Mefiante
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« Reply #7 on: July 10, 2010, 09:36:23 AM »

It’s possible that some form of contraction effect is at play.  However, if it is then it would still require sizeable revisions to current theory.  It may be possible to test this idea using the tau particle from the lepton family, an even heavier cousin of the electron by a factor of about 3,500, but its very short half-life of around 3×10-13 seconds (cf. muon ~2×10-6 seconds) will complicate this tremendously.

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rwenzori
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« Reply #8 on: July 11, 2010, 08:12:32 AM »

Could they not use their fancy laser to blast plain old common-or-garden stable hydrogen atoms to see what size they find in there? Or is it too small?
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Rigil Kent
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« Reply #9 on: July 11, 2010, 08:34:14 AM »

If examining a proton with a laser is anything like what limits resolution in microscopy, my guess would be that a laser would be too coarse a tool to apply to a proton. A laser is still light and as such has a wavelength in the same order as light, around 400-700nm. I don't think it will interfere sufficiently with a sub-Angstrom proton to have measurable effect on the laser.

For the same reason you will need a cathode ray (a stream of electrons), not light, to resolve something as small as a virus or DNA strands. The wavelength of the electron is in the same order as the size of a large atom. But unfortunately, still much higher than the size of a proton.

To get a useful signal, you would probably have to smash something of comparable size onto the hydrogen nucleus, like another  proton, or maybe an alpha particle.

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Mefiante
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« Reply #10 on: July 12, 2010, 08:23:06 AM »

Could they not use their fancy laser to blast plain old common-or-garden stable hydrogen atoms to see what size they find in there? Or is it too small?
Oops, replying to this almost fell out of my head.  (With smaller proton size comes reduced memory retention capacity, it seems… Tongue)  The observed 4% discrepancy in proton size is relative to the value measured in a variety of experiments, including some that used standard hydrogen.  As the article says, the muon’s excitation levels didn’t occur where they were expected to be found, as predicted by current quantum theory when the hitherto accepted values are applied in the calculations.  These predicted values are correct in the case of standard hydrogen so the only likely benefit of repeating the experiment with standard hydrogen is further confirmation (and there’s already stacks of that) for the model being correct in the case of a standard hydrogen atom.

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rwenzori
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« Reply #11 on: July 13, 2010, 07:27:10 AM »

Thank you for your patient reply.

Surely, though, the implication is then that the size of the proton is variable depending on what else is in the atom ( muon: electron: whatever ), rather than that the "proton IS smaller" by 4%? Or that not all quantums are created equal?  Wink
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Mefiante
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« Reply #12 on: July 13, 2010, 08:23:34 AM »

Well, the phrase was “4% smaller than previously thought” in regards to the proton’s size.  The experiment with the muon-flavoured hydrogen atom needs to stand up to peer review first in terms of its technicalities and analysis, followed by validation from independent laboratories.  If everything turns out to be above board and the results are successfully replicated, then it will be time to have a serious rethink about some of the currently-accepted theory, including your suggestion that the muon’s greater mass may play a role in shrinking the proton’s cross-section.  The latter would have profound implications for QED theory.  In any and all of the possible cases, there is much to investigate.

If the above doesn’t answer your question, then I’m not entirely sure what you are driving at.

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rwenzori
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« Reply #13 on: July 15, 2010, 04:33:18 AM »

If the above doesn’t answer your question, then I’m not entirely sure what you are driving at.


It does. Thanks very much.
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