6 ms·

It does not predict the fine structure constant, that is an input into the model.

T-A 8y ago
https://en.wikipedia.org/wiki/Fine-structure_constant#Measurement https://en.wikipedia.org/wiki/Fine-structure_constant#Measur...
- jsweojtj 8y ago
  Exactly. Its value comes from being measured.
  - T-A 8y ago
    It comes from the magnetic moment of the electron being measured and then run through more than 10k Feynman diagrams. The second part wouldn't work if QED didn't.
    - jsweojtj 8y ago
      I can't tell whether you are agreeing or disagreeing with me. Let's make it easier. Do you agree or disagree with the following statement: The value of the fine-structure constant is not predicted by any physics theory.
      - T-A 8y ago
        Given what? If a theory gives you a well-defined relation between the magnetic moment of the electron and the fine structure constant, you can measure either one and then compute the other one. Which one is "predicted" is just a convention. Eq. (13) in [1] is a prediction of the electron's magnetic moment given the fine structure constant. Eq. (15) in [1] is a prediction of the fine structure constant given the electron's magnetic moment. For the purposes of that paper (testing QED) it turns out to be more convenient to use Eq. (15). [1] https://arxiv.org/abs/1205.5368 https://arxiv.org/abs/1205.5368
        jsweojtj 8y ago
        I'll leave this quote here as my response: > There is a most profound and beautiful question associated with the observed coupling constant, e – the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out, without putting it in secretly! — Richard Feynman, Richard P. Feynman (1985). QED: The Strange Theory of Light and Matter. Princeton University Press. p. 129. ISBN 978-0-691-08388-9. [my emphasis added]
        T-A 8y ago
        I have never seen a physicist put that particular number on a wall (these days, the cosmological constant would be a better bet). I am vaguely aware of numerological attempts (some collected in [1]) to "explain" why 137 is "special", none of which has ever led anywhere. As far as I can tell, the fascination with it got started by the number being close to an integer, and maybe the remark in [1] that "In ancient Hebraic language letters where used for numbers, and Cabbala is the word corresponding to 137" played a role. But we know that it isn't an integer, and that it runs [2] like any coupling constant in QFT, so at best you could marvel about it taking on some particular value at some particular interaction energy, which would mean... what? I dunno. As Feynman also said [3], You know, the most amazing thing happened to me tonight... I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing! [1] https://arxiv.org/abs/1009.1711 https://arxiv.org/abs/1009.1711 [2] https://en.wikipedia.org/wiki/Coupling_constant#Running_coupling https://en.wikipedia.org/wiki/Coupling_constant#Running_coup... [3] https://www.goodreads.com/quotes/649893-you-know-the-most-amazing-thing-happened-to-me-tonight https://www.goodreads.com/quotes/649893-you-know-the-most-am...
walrus1066 8y ago
Yes, you're right. I was getting it jumbled up with the magnetic moment of the electron, which is predicted by the SM (well, the QED part anyway), to be slightly different to the 'classical' prediction. Experiment measurement of this is accurate to one part per billion, and is consistent with the QED prediction. The electroweak bits of the SM predict the W & Z bosons, along with their masses, which have also been measured, to around 1 part per 10,000, and match SM predictions. EDIT: last but not least, the Higgs Boson was also predicted by SM, with a ballpark figure for it's mass, and other properties (how often it decays into photons, W bosons, quarks etc). So far all measurements of these properties are consistent with SM predictions.
- muhbags 8y ago
  Correct me if I'm wrong, but the standard model makes no prediction for the masses of any of the fundamental particles. To my knowledge, they are all input parameters. It might put certain bounds on them, but in the end they are all not predicted by the standard model itself. This is seen as one of the big issues with the standard model, that it does not actually explain a lot of the characteristics of the fundamental particles like their couplings and masses.
  - walrus1066 8y ago
    For the W & Z bosons, their masses are derived from 3 other 'free parameters' of the SM. The Higgs mass is indeed a free parameter, but the SM wouldn't work of it's mass was greater than 200 GeV or so. The Higgs interacts with other particles of mass, and the strength of interaction is proportional to the Higgs mass, it influences certain processes (like W boson scattering), the rate at which these happen would deviate from experimental observation if Mhiggs was over 200 GeV. That's why the LHC was such a big deal, it reached the energies required for direct observation of sub-200 GeV Higgs, so it would either find the SM Higgs, or rule it out and invalidate the SM. Unfortunately the former seems to have happened. SM free parameters: https://en.m.wikipedia.org/wiki/Standard_Model#Construction_of_the_Standard_Model_Lagrangian https://en.m.wikipedia.org/wiki/Standard_Model#Construction_... Indirect constraints on Higgs mass in SM (a bit technical, slide 6 chart is the key one, strongly influenced specs & mission of the LHC) https://indico.lal.in2p3.fr https://indico.lal.in2p3.fr

T-A 8y ago

https://en.wikipedia.org/wiki/Fine-structure_constant#Measurement https://en.wikipedia.org/wiki/Fine-structure_constant#Measur...

jsweojtj 8y ago
Exactly. Its value comes from being measured.
- T-A 8y ago
  It comes from the magnetic moment of the electron being measured and then run through more than 10k Feynman diagrams. The second part wouldn't work if QED didn't.
  - jsweojtj 8y ago
    I can't tell whether you are agreeing or disagreeing with me. Let's make it easier. Do you agree or disagree with the following statement: The value of the fine-structure constant is not predicted by any physics theory.
    - T-A 8y ago
      Given what? If a theory gives you a well-defined relation between the magnetic moment of the electron and the fine structure constant, you can measure either one and then compute the other one. Which one is "predicted" is just a convention. Eq. (13) in [1] is a prediction of the electron's magnetic moment given the fine structure constant. Eq. (15) in [1] is a prediction of the fine structure constant given the electron's magnetic moment. For the purposes of that paper (testing QED) it turns out to be more convenient to use Eq. (15). [1] https://arxiv.org/abs/1205.5368 https://arxiv.org/abs/1205.5368
      - jsweojtj 8y ago
        I'll leave this quote here as my response: > There is a most profound and beautiful question associated with the observed coupling constant, e – the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out, without putting it in secretly! — Richard Feynman, Richard P. Feynman (1985). QED: The Strange Theory of Light and Matter. Princeton University Press. p. 129. ISBN 978-0-691-08388-9. [my emphasis added]
        T-A 8y ago
        I have never seen a physicist put that particular number on a wall (these days, the cosmological constant would be a better bet). I am vaguely aware of numerological attempts (some collected in [1]) to "explain" why 137 is "special", none of which has ever led anywhere. As far as I can tell, the fascination with it got started by the number being close to an integer, and maybe the remark in [1] that "In ancient Hebraic language letters where used for numbers, and Cabbala is the word corresponding to 137" played a role. But we know that it isn't an integer, and that it runs [2] like any coupling constant in QFT, so at best you could marvel about it taking on some particular value at some particular interaction energy, which would mean... what? I dunno. As Feynman also said [3], You know, the most amazing thing happened to me tonight... I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing! [1] https://arxiv.org/abs/1009.1711 https://arxiv.org/abs/1009.1711 [2] https://en.wikipedia.org/wiki/Coupling_constant#Running_coupling https://en.wikipedia.org/wiki/Coupling_constant#Running_coup... [3] https://www.goodreads.com/quotes/649893-you-know-the-most-amazing-thing-happened-to-me-tonight https://www.goodreads.com/quotes/649893-you-know-the-most-am...

walrus1066 8y ago

Yes, you're right. I was getting it jumbled up with the magnetic moment of the electron, which is predicted by the SM (well, the QED part anyway), to be slightly different to the 'classical' prediction. Experiment measurement of this is accurate to one part per billion, and is consistent with the QED prediction. The electroweak bits of the SM predict the W & Z bosons, along with their masses, which have also been measured, to around 1 part per 10,000, and match SM predictions. EDIT: last but not least, the Higgs Boson was also predicted by SM, with a ballpark figure for it's mass, and other properties (how often it decays into photons, W bosons, quarks etc). So far all measurements of these properties are consistent with SM predictions.

muhbags 8y ago
Correct me if I'm wrong, but the standard model makes no prediction for the masses of any of the fundamental particles. To my knowledge, they are all input parameters. It might put certain bounds on them, but in the end they are all not predicted by the standard model itself. This is seen as one of the big issues with the standard model, that it does not actually explain a lot of the characteristics of the fundamental particles like their couplings and masses.
- walrus1066 8y ago
  For the W & Z bosons, their masses are derived from 3 other 'free parameters' of the SM. The Higgs mass is indeed a free parameter, but the SM wouldn't work of it's mass was greater than 200 GeV or so. The Higgs interacts with other particles of mass, and the strength of interaction is proportional to the Higgs mass, it influences certain processes (like W boson scattering), the rate at which these happen would deviate from experimental observation if Mhiggs was over 200 GeV. That's why the LHC was such a big deal, it reached the energies required for direct observation of sub-200 GeV Higgs, so it would either find the SM Higgs, or rule it out and invalidate the SM. Unfortunately the former seems to have happened. SM free parameters: https://en.m.wikipedia.org/wiki/Standard_Model#Construction_of_the_Standard_Model_Lagrangian https://en.m.wikipedia.org/wiki/Standard_Model#Construction_... Indirect constraints on Higgs mass in SM (a bit technical, slide 6 chart is the key one, strongly influenced specs & mission of the LHC) https://indico.lal.in2p3.fr https://indico.lal.in2p3.fr