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2021-04-10, 04:58 AM (ISO 8601)
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- Apr 2009
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- Germany
What's the big deal with the Muon g-2 experiment?
I see people everywhere getting super excited about the Muon g-2 experiment, making such claims that there's been a major discovery, that new fields of particle physics are opening up, and that we might have discovered a fifth fundamental force.
As I understand it, 20 years ago an experiment was done to measure the g value of muons and got a result of ~2.002.
Now the experiment was repeated and they also got a result of ~2.002.
So what? What has changed in our understanding of anything since last week? Why is this news?We are not standing on the shoulders of giants, but on very tall tower of other dwarves.
Spriggan's Den Heroic Fantasy Roleplaying
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2021-04-10, 06:04 AM (ISO 8601)
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- Dec 2005
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Re: What's the big deal with the Muon g-2 experiment?
The error bars are smaller now.
The measured value does not quite match the value predicted by the Standard Model, but with the previous experiment that difference could have been an error.
With the more accurate measurements now, that's unlikely enough to assume the difference is real.
That means something is wrong with the existing theory - thus, "new physics."
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2021-04-10, 06:54 AM (ISO 8601)
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- Jun 2018
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- Belgium
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Re: What's the big deal with the Muon g-2 experiment?
The guy from Piled Higher and Deeper (PHD) has done a comic on it: https://physics.aps.org/articles/v14/47
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2021-04-10, 07:15 AM (ISO 8601)
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- Apr 2009
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- Germany
Re: What's the big deal with the Muon g-2 experiment?
So the previously measured value was not too far off from the theoretical value to be statistically relevant? But the new measurement gives more confidence that the two are actually different?
The comic doesn't say that, but there seems to be some implication of that on one of the pictures.We are not standing on the shoulders of giants, but on very tall tower of other dwarves.
Spriggan's Den Heroic Fantasy Roleplaying
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2021-04-10, 01:59 PM (ISO 8601)
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- Jul 2017
Re: What's the big deal with the Muon g-2 experiment?
The brookhaven experiment had a 3.7 sigma confidence, the fermilab one raised that to 4.2. Still not the 5 sigma to call it a done deal, but closer and a sign that this isn't just random noise confounding things. So it's exciting, but not quite time to break out the cake and party hats just yet.
As for what we know now that we didn't know last week? Nothing, really. Even if we assume this to be true (which, again, is likely but we're still not as confident as we'd like to be), we don't know what caused the discrepancy. However, if this does happen to be a place where experiment diverges from theory, that'll be the first time in a good, long while that our major theories have failed to accurately predict experiments. It'll be a place for new ideas to sprout up and see which ones best match the new evidence. So it's less that we know something new today, and more that we've found more space to discover new things.
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2021-04-10, 04:33 PM (ISO 8601)
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- Jul 2008
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- Sweden
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Re: What's the big deal with the Muon g-2 experiment?
I can't say I fully understand it but here are a few interesting resources.
The TL;DR is that there may be new physics at play. New particles, forces. Or maybe experimental/math error, but probably not.
https://imgur.com/gallery/3xUSoK3 (info-graph)
https://www.youtube.com/watch?v=O4Ko7NW2yQo (pbs spacetime)Black text is for sarcasm, also sincerity. You'll just have to read between the lines and infer from context like an animal
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2021-04-11, 04:56 PM (ISO 8601)
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- Aug 2011
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- Sharangar's Revenge
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Re: What's the big deal with the Muon g-2 experiment?
Wobbling muons hint strongly at the existence of bizarre new physics
Originally Posted by Bad Astronomy
0.001 165 918 10 (±0.000 000 000 43) Predicted Value
0.001 165 920 61 (±0.000 000 000 41) Measured Value
The prediction is in the range of ...1767 to ...1853
The measured value is in the range of ...2020 to ...2102
That is a minimum discrepancy of 4 times the error bars on both the measurement and the prediction.
Here's what I've taken away from what I've read/heard
Have they found a 5th force? No. Not yet.
Have they found new particles? No. Not yet.
But they've found something interesting that doesn't agree with our current model, and could possibly be explained by a new force and/or particle.Last edited by Lord Torath; 2021-04-11 at 09:13 PM.
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2021-04-12, 07:27 AM (ISO 8601)
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- Jan 2007
Re: What's the big deal with the Muon g-2 experiment?
Very much to the point. Essentially something opposite to the discovery of Higgs boson: that experimental result fits the current models very well, so it was a happy moment to confirm the theory but a bit of a let down at the same time as it did not lead to anything new.
Progress can be made only when we discover where our theories fail: special relativity and quantum mechanics were built, when it became evident that Newtonian mechanics fails when connected with electrodynamics and at atomic scales. Planck's results on black body radiation have also shown that electrodynamics itself does not work properly, which has driven people to build quantum electrodynamics right after the basics of quantum mechanics have been figured out. Through falsification of old theories we make room for better ones.
In short, the muon experiment has shown us, where to dig for new discoveries.In a war it doesn't matter who's right, only who's left.
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2021-05-03, 01:08 AM (ISO 8601)
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- Feb 2016
Re: What's the big deal with the Muon g-2 experiment?
If we plug in the experimentally measured value and work backwards does it break the standard model's predictive ability for other things?
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2021-05-04, 10:54 AM (ISO 8601)
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- Apr 2009
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- Germany
Re: What's the big deal with the Muon g-2 experiment?
The much less reported story was that at the same time as the g-2 experiment was being done, a different research time had fed the original equations into a shiny new super computer to make a new, more accurate calculation of the predicted value.
And that new calculated value matches up quite well with the new measured value.
So it looks again like the original equations were correct. It's just that the old measurement and the old predicted value where not very good, but coincidentally happened to agree with each other 20 years ago.We are not standing on the shoulders of giants, but on very tall tower of other dwarves.
Spriggan's Den Heroic Fantasy Roleplaying
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2021-05-04, 04:37 PM (ISO 8601)
- Join Date
- Jan 2007
Re: What's the big deal with the Muon g-2 experiment?
That's actually quite disappointing (a bit similar to the experimental finding of Higgs boson). It has been a really long time since we had an actual breakthrough to some completely new physics. There has been a huge progress in many fields including reconciliation of general relativity with quantum physics, but the basic theories are still standing tall despite the effort of so many people.
In a war it doesn't matter who's right, only who's left.
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2021-05-05, 01:11 AM (ISO 8601)
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- Aug 2020
Re: What's the big deal with the Muon g-2 experiment?
"By Google's own reckoning, 60% of the ads that are charged for are never seen by any human being – literally the majority of the industry's product is a figment of feverish machine imaginations." Pluralistic
The bots are selling ads to bots which mostly bots are viewing, We really are living in XXI century.
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2021-05-05, 04:09 AM (ISO 8601)
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- Apr 2009
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- Germany
Re: What's the big deal with the Muon g-2 experiment?
The paper in question is called "Leading hadronic contribution to the muon magnetic moment from lattice QCD" by Borsanyi, Fodor, and others. Didn't find a good article that spells out the details in simple language, but from what I picked up last month it comes down to the new more accurate measurement agreeing with the new more accurate prediction.
We are not standing on the shoulders of giants, but on very tall tower of other dwarves.
Spriggan's Den Heroic Fantasy Roleplaying
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2021-05-05, 08:23 AM (ISO 8601)
- Join Date
- Aug 2020
Re: What's the big deal with the Muon g-2 experiment?
Heh, it is complicated, but I think I pinpointed main issue. I think what they claim is that previous measurement hadn't included proper "leading-order, hadronic vacuum polarization (LO-HVP) contribution", but I don't have a clue what that is : P
SpoilerOn the theory side, the dominant source of error is the leading-order, hadronic
vacuum polarization (LO-HVP) contribution. To fully leverage the upcoming measurements,
it is critical to check the prediction for this contribution with independent methods and to
reduce its uncertainties. The most precise, model-independent determinations currently rely on
dispersive techniques, combined with measurements of the cross-section for electron-positron
annihilation into hadrons [3–6]. Here we use "ab initio" simulations in quantum chromodynamics
and quantum electrodynamics to compute the LO-HVP contribution with sufficient precision
to discriminate between the measurement of (gµ−2) and the dispersive predictions. Our result,
[(gµ − 2)/2]LO−HVP = 707.5[5.5] × 10−10, favors the experimentally measured value of (gµ − 2)
over the results based on the dispersion relation. Moreover, the methods used and developed
here will allow further increases in precision, as more powerful computers become available.
(...)
At this level of precision, all of the interactions of the standard model contribute. The leading
contributions are electromagnetic and described by quantum electrodynamics (QED), but the one that
dominates the theory error is induced by the strong interaction and requires solving the highly non-linear
equations of quantum chromodynamics (QCD) at low energies. This contribution is determined by the
leading-order, hadronic vacuum polarization (LO-HVP), which describes how the propagation of a virtual
photon is modified by the presence of quark and gluon fluctuations in the vacuum. Here we compute this
LO-HVP contribution to (gµ − 2)/2, denoted by a LO−HVP µ, using ab initio simulations in QCD and QED.
"By Google's own reckoning, 60% of the ads that are charged for are never seen by any human being – literally the majority of the industry's product is a figment of feverish machine imaginations." Pluralistic
The bots are selling ads to bots which mostly bots are viewing, We really are living in XXI century.