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> The idea that our experiments might be detecting a fourth neutrino remains controversial, however, because the Standard Model of particle physics is one of th

> The idea that our experiments might be detecting a fourth neutrino remains controversial, however, because the Standard Model of particle physics is one of the most tested and thoroughly confirmed theoretical frameworks in history—and it allows for only three neutrinos. I am frustrated with how the "Standard Model" has become a veil with which a priestly particle physicist class hides sacred knowledge of how the universe works from plebeians like myself. Why does the Standard Model only allow for three neutrinos? Is it because the math says so? Is it because some other maths only work if this number is three? Does it come from some fascinating group-theory or symmetry groups that nobody teaches at a highschool math level? I can't be the only one who wants to learn more about this stuff. I don't believe that the Particle Physicist or the broader Physics community is at fault here. I blame our approach to school education. If we accept that it's ok for a 14 year to be bewildered at quadratic equations and is expected to accept the "Don't worry, it'll all make sense later" line, it should also be possible for the same student to learn about Heisenberg's uncertainty principle, the basics of non-Euclidean geometry and symmetry groups. Our society today has several cohorts of adults who do not have the necessary foundations to understand the Standard Model. We don't plan to educate children in schools today to be able to understand this stuff. It will take a highly motivated adult several years of self study to reach a point where the Standard Model becomes approachable. Do we really want to live in the world where an attempt to understand fundamental truths about the universe can only be an extremely ambitious goal? History shows us how the world changed when the first translations of the Bible became widely available. We need to think hard about what lessons we can learn and how to apply it to the comprehension-inequality that plagues our world today. I wonder if the stagnation in Particle Physics after 2012 needs a second renaissance led by a generation who grew up never hearing the phrase "Nobody understands Quantum Mechanics".

rimunroe 6y ago
I get what you're saying. I studied physics in undergrad but decided not to go to grad school. I kept hearing stuff like that well into my junior and senior years. I ended up doing internships on two HEP experiments, one at Brookhaven National Lab and the other at Jefferson Lab. It was enormously frustrating to hear that something was allowed/not allowed because "the Great and Powerful Standard Model says so". If there's one thing I picked up from my program and being on experiment teams, it's that effectively communicating science is extremely hard. It's not fair to characterize it as malicious ("priestly particle physicist class hides sacred knowledge") when it's really just people not knowing how to simplify concepts accurately. I don't know a single physicist who wouldn't dearly have loved to be able to explain this stuff to laypeople more easily. People are scared to use metaphors or other rough answers. In the least bad case they can result in misunderstanding or confusion. In the worst case they can lead to perpetual motion machine people. This is extremely niche knowledge and there's a lot more stuff that should be explained first. I went to grade school in Virginia in the 90s and early 2000s. We were taught that the American Civil War was about states rights, and that it was the War of Northern Aggression. I'd much rather we focus broad educational efforts on making people understand their fellow humans and try to prevent repeating the atrocities of the past than get people to understand quantum flavordynamics or chromodynamics.
- dilippkumar 6y ago
  > It's not fair to characterize it as malicious ("priestly particle physicist class hides sacred knowledge") when it's really just people not knowing how to simplify concepts accurately. I don't know a single physicist who wouldn't dearly have loved to be able to explain this stuff to laypeople more easily. You are right - I spoke out of frustration. I understand that one of the greatest joys of pure science is being able to share it with the world. I’m sure the Physics community is just as frustrated at how difficult it is to spread this knowledge accurately to a broader audience.
  - rimunroe 6y ago
    I appreciate it. You're absolutely right about how exciting it is to be able to share explanations with people. This is a video of Feynman's answer to an interviewer asking about what and why you feel something when you put two magnets together https://www.youtube.com/watch?v=MO0r930Sn_8 https://www.youtube.com/watch?v=MO0r930Sn_8. It's a great description of the problems inherent to explaining this sort of thing to people. (Also, if anyone knows a video/essay/talk along these lines by a less sexist person, I'd love a link.)
  - knzhou 6y ago
    Explaining the Standard Model clearly and correctly is just really hard! I've tried, a lot. Costless simplification is impossible, because the mathematical form it's usually written in is already the simplest, clearest, and most concise way we know to describe it. (If there were a better way, we'd be using that instead!) Anything simpler necessarily sacrifices correctness. Metaphors are great, but it's clunky to explain when the metaphor stops working or becomes actively harmful to understanding. For example, adding a 4th neutrino that is otherwise identical to the other 3 would break the Standard Model would violate anomaly cancellation, making the model mathematically inconsistent. But explaining from scratch what an anomaly is or why they need to cancel would require a ton of vague, fragile metaphors layered on top of each other, at which point I'm not sure what value it would provide. Imagine explaining how a CPU works on the silicon level to somebody from the 18th century. It's the same problem -- it's hard to say anything that isn't the length of a full textbook, or completely misleading, or bordering on woo. I've written and then thrown away a lot of attempted explanations because of this.
deleted 6y ago
[deleted]
pdonis 6y ago
> Why does the Standard Model only allow for three neutrinos? The short answer is, it doesn't. The Standard Model, AFAIK, does not rule out the existence of more neutrinos (or more generally, more "generations" of leptons and quarks than the three we know); there is no hard and fast limit in the math of the Standard Model on the number of generations. So I think the article's statement is, if taken literally, false. (Unfortunately, this kind of thing is common in pop science articles, even in magazines like Scientific American that used to be better at avoiding such things.) I think a better way of stating our best current knowledge would be that we have only observed three generations, and if a fourth generation does exist, all of the particles in it would have to be very massive--massive enough to have not been detected in any of our current experiments up to and including the LHC. Most physicists appear to think it's unlikely that a fourth generation exists given those limits, but I'm not aware of any theoretical argument that definitely rules it out.
zaarn 6y ago
I heard an interesting comparison yesterday: An average child can easily be beaten in chess by the average adult. An average adult is easily beaten by the average professional chessplayer. The average professional chessplayer is beaten by the grand master easily. Now the grand master physicists are the grand masters and professionals, playing against a child. The standard model is a grand-master level of chess play and we're the children moving the figures somewhat randomly. It is likely that the average person is simply not on the level of expert knowledge in the field to even begin understanding the strategy.
oyoman 6y ago
I hope to provide some perspective : The Standard Model (SM) is based on a specific number of free parameters, such as the masses of elementary particles, the number of generation of families (quarks, leptons/neutrinos) etc. Those parameters are not fixed by the theoritical model, in the SM they are free (I hope it is clear enough). They are inferred from the experimental data so that we have defined what they are now. The number of family of neutrinos was deduced for the first time in LEP experiments (in the 90s, the predecessor of LHC at CERN) : It was not known before whether there were 2, 3, or 4 family of neutrinos. If you want to learn more : http://pdg.lbl.gov/2020/reviews/rpp2020-rev-light-neutrino-types.pdf http://pdg.lbl.gov/2020/reviews/rpp2020-rev-light-neutrino-t... a review about this subject. In summary, the combined result from LEP experiments is N_neutrinos = 2.984 +- 0.008. If you are interested, you can see the experimental plot that shows the fit and the difference between of a SM with 2, 3, and 4 neutrinos : https://arxiv.org/pdf/hep-ex/0509008.pdf https://arxiv.org/pdf/hep-ex/0509008.pdf Page 36 Figure 1.13, the data points are in red with their error bars (extremely important to pay attention to them and their size !) and the curves are the SM prediction for 2, 3, and 4 neutrinos. In my opinion, this is a very nice plot that shows how different is the SM with 4 neutrino. This is why the SM is with 3 neutrinos and not otherwise, it is because the experimental data that were used to infer all the free parameters of the SM. The last ones were related the Higgs boson, and now everything is fixed. To accommodate a 4th neutrino, then we would need to go beyond SM.
- cjfd 6y ago
  This seems to be assuming that the neutrinos are massless, or at least have a mass quite a bit smaller than the Z particle. It could be that there is a fourth generation where both lepton and neutrino are heavier than the Z particle, no?
  - evanb 6y ago
    In the Standard Model as written in the 1970s the neutrinos had to be exactly massless. We now have evidence that neutrinos aren't exactly massless, but they're very VERY light. Like, absurdly light. We still don't know the absolute scale of the neutrino masses, we know from mixing that they're damn small and from cosmology that the sum of all three masses is less than the mass of the electron / a million. It is logically possible that the neutrino of the fourth generation is very heavy. In that case - the constraint on the number of neutrinos from collider experiments is relaxed because the ultra-heavy neutrino's contribution to the observable would be extremely suppressed (because the collision energy was too low to be sensitive). - the cosmological constraints are relaxed because the heavy neutrinos are already frozen out by the time of the electroweak phase transition in the early universe. - the mixing constraints... well, right now it seems that the mixing matrix between the generations is unitary---there's no "leak" into a fourth generation. But our experimental precision is mediocre because precisely measuring the mixing is difficult (though there are experiments under way). It is also logically possible for there to be a fourth generation but that the neutrino doesn't mix at all---its mixing with the lighter neutrinos is precisely 0. While it's perfectly possible logically, we physicists do not like this kind of "fine tuning" without some explanation of how it could happen. In the SM the neutrino masses/mixings are input parameters, not things determined dynamically---they are axioms, so to speak. So any explanation of the mixing being really small would need to invoke more beyond-the-Standard-Model physics than "it's the same but there's a fourth generation".
sideshowb 6y ago
"Nobody understands quantum mechanics" In the process of learning it myself I tried to do my bit towards that by writing a little game. Sorry for the Java http://tropic.org.uk/~crispin/quantum/ http://tropic.org.uk/~crispin/quantum/ I'm not the only one https://quantumgame.io/ https://quantumgame.io/ https://github.com/stared/science-based-games-list https://github.com/stared/science-based-games-list I do share your frustration with the mysteries of the Standard Model (and hope to understand them one day)
- evanb 6y ago
  :wave: https://news.ycombinator.com/item?id=11813473 https://news.ycombinator.com/item?id=11813473 Still not going to install anything Java, but I hope my comments were helpful!
  - sideshowb 6y ago
    Absolutely they were! I eventually wrote a reply addressing your points on that thread, not sure if you saw it (though this isn't a chase for further replies, alas I haven't had time for QM in a few years). Thanks again for your help there. One of those threads that really shows the value lurking around HN (alongside its anti-java bias :) )
    - evanb 6y ago
      By the time I saw it the replies were closed, unfortunately.
dboreham 6y ago
I'll guess the answer is: Because Yang-Mills Guage Theory
voldacar 6y ago
It is totally possible to teach group theory at the high-school level. Instead, literally multiple years are spent teaching the quadratic equation over and over, and a year on "pre-calculus" which is just more quadratic equation plus memorizing some points on the unit circle. Group theory in high school would be sooo much better than the status quo. It would, however, require high school math teachers to actually learn some math.
jfengel 6y ago
Physicists already understand quantum mechanics. There are open questions, but it's no longer the case that the physicists themselves just stand slack-jawed at it. Every field has open questions, and nobody says "Nobody understands DNA" or "Nobody understands polymer chemistry". Quantum mechanics attracted a lot of mystical woo early in its development, because it was so deeply unfamiliar from classical physics, but we've been at it for over a century now. We've moved way, way past the weirdness of quantum erasers and other examples that get promoted in pop science books. As I said, that doesn't mean all of the questions are answered, but a lot of questions are answered. You just can't understand the answers by expressing them in purely conventional terms, as pop science books try to do. So physicists don't come out of school wearing pointy hats and feeling like the keepers of arcane knowledge. They can do quantum mechanics and apply it. That's not forbidden knowledge. It just takes work. Nobody ever promised you an understanding of physics from comfortably reading about it at a middle-school level, any more than you'd expect to learn dishwasher repair or glassblowing just from breezy introductions to it. The information is accessible, and lots of "Quantum mechanics for dummies" books can actually give you a good handle on it. Yes, it takes math, but not very advanced math. Yes, you have to learn a new language, but that's to be expected. Novel ways of viewing the universe require new ways of talking about it. It's right there, publicly accessible.
- dschuetz 6y ago
  > Physicists already understand quantum mechanics. No, they don't in fact. Most of them even admit that they don't understand it at all, yet it works for them so far, except when it comes to the precise origin of gravity. Some say that the reason why quantum mechanics and gravity couldn't be reconciled until today, because nobody on earth truly knows how exactly quantum mechanics works yet.
throwaw4y-plate 6y ago
I am frustrated with how the "internal combustion engine" has become a veil with which a priestly mechanic class hides sacred knowledge of how cars works from plebeians like myself.

rimunroe 6y ago

I get what you're saying. I studied physics in undergrad but decided not to go to grad school. I kept hearing stuff like that well into my junior and senior years. I ended up doing internships on two HEP experiments, one at Brookhaven National Lab and the other at Jefferson Lab. It was enormously frustrating to hear that something was allowed/not allowed because "the Great and Powerful Standard Model says so". If there's one thing I picked up from my program and being on experiment teams, it's that effectively communicating science is extremely hard. It's not fair to characterize it as malicious ("priestly particle physicist class hides sacred knowledge") when it's really just people not knowing how to simplify concepts accurately. I don't know a single physicist who wouldn't dearly have loved to be able to explain this stuff to laypeople more easily. People are scared to use metaphors or other rough answers. In the least bad case they can result in misunderstanding or confusion. In the worst case they can lead to perpetual motion machine people. This is extremely niche knowledge and there's a lot more stuff that should be explained first. I went to grade school in Virginia in the 90s and early 2000s. We were taught that the American Civil War was about states rights, and that it was the War of Northern Aggression. I'd much rather we focus broad educational efforts on making people understand their fellow humans and try to prevent repeating the atrocities of the past than get people to understand quantum flavordynamics or chromodynamics.

dilippkumar 6y ago
> It's not fair to characterize it as malicious ("priestly particle physicist class hides sacred knowledge") when it's really just people not knowing how to simplify concepts accurately. I don't know a single physicist who wouldn't dearly have loved to be able to explain this stuff to laypeople more easily. You are right - I spoke out of frustration. I understand that one of the greatest joys of pure science is being able to share it with the world. I’m sure the Physics community is just as frustrated at how difficult it is to spread this knowledge accurately to a broader audience.
- rimunroe 6y ago
  I appreciate it. You're absolutely right about how exciting it is to be able to share explanations with people. This is a video of Feynman's answer to an interviewer asking about what and why you feel something when you put two magnets together https://www.youtube.com/watch?v=MO0r930Sn_8 https://www.youtube.com/watch?v=MO0r930Sn_8. It's a great description of the problems inherent to explaining this sort of thing to people. (Also, if anyone knows a video/essay/talk along these lines by a less sexist person, I'd love a link.)
- knzhou 6y ago
  Explaining the Standard Model clearly and correctly is just really hard! I've tried, a lot. Costless simplification is impossible, because the mathematical form it's usually written in is already the simplest, clearest, and most concise way we know to describe it. (If there were a better way, we'd be using that instead!) Anything simpler necessarily sacrifices correctness. Metaphors are great, but it's clunky to explain when the metaphor stops working or becomes actively harmful to understanding. For example, adding a 4th neutrino that is otherwise identical to the other 3 would break the Standard Model would violate anomaly cancellation, making the model mathematically inconsistent. But explaining from scratch what an anomaly is or why they need to cancel would require a ton of vague, fragile metaphors layered on top of each other, at which point I'm not sure what value it would provide. Imagine explaining how a CPU works on the silicon level to somebody from the 18th century. It's the same problem -- it's hard to say anything that isn't the length of a full textbook, or completely misleading, or bordering on woo. I've written and then thrown away a lot of attempted explanations because of this.

deleted 6y ago

[deleted]

pdonis 6y ago

> Why does the Standard Model only allow for three neutrinos? The short answer is, it doesn't. The Standard Model, AFAIK, does not rule out the existence of more neutrinos (or more generally, more "generations" of leptons and quarks than the three we know); there is no hard and fast limit in the math of the Standard Model on the number of generations. So I think the article's statement is, if taken literally, false. (Unfortunately, this kind of thing is common in pop science articles, even in magazines like Scientific American that used to be better at avoiding such things.) I think a better way of stating our best current knowledge would be that we have only observed three generations, and if a fourth generation does exist, all of the particles in it would have to be very massive--massive enough to have not been detected in any of our current experiments up to and including the LHC. Most physicists appear to think it's unlikely that a fourth generation exists given those limits, but I'm not aware of any theoretical argument that definitely rules it out.

zaarn 6y ago

I heard an interesting comparison yesterday: An average child can easily be beaten in chess by the average adult. An average adult is easily beaten by the average professional chessplayer. The average professional chessplayer is beaten by the grand master easily. Now the grand master physicists are the grand masters and professionals, playing against a child. The standard model is a grand-master level of chess play and we're the children moving the figures somewhat randomly. It is likely that the average person is simply not on the level of expert knowledge in the field to even begin understanding the strategy.

oyoman 6y ago

I hope to provide some perspective : The Standard Model (SM) is based on a specific number of free parameters, such as the masses of elementary particles, the number of generation of families (quarks, leptons/neutrinos) etc. Those parameters are not fixed by the theoritical model, in the SM they are free (I hope it is clear enough). They are inferred from the experimental data so that we have defined what they are now. The number of family of neutrinos was deduced for the first time in LEP experiments (in the 90s, the predecessor of LHC at CERN) : It was not known before whether there were 2, 3, or 4 family of neutrinos. If you want to learn more : http://pdg.lbl.gov/2020/reviews/rpp2020-rev-light-neutrino-types.pdf http://pdg.lbl.gov/2020/reviews/rpp2020-rev-light-neutrino-t... a review about this subject. In summary, the combined result from LEP experiments is N_neutrinos = 2.984 +- 0.008. If you are interested, you can see the experimental plot that shows the fit and the difference between of a SM with 2, 3, and 4 neutrinos : https://arxiv.org/pdf/hep-ex/0509008.pdf https://arxiv.org/pdf/hep-ex/0509008.pdf Page 36 Figure 1.13, the data points are in red with their error bars (extremely important to pay attention to them and their size !) and the curves are the SM prediction for 2, 3, and 4 neutrinos. In my opinion, this is a very nice plot that shows how different is the SM with 4 neutrino. This is why the SM is with 3 neutrinos and not otherwise, it is because the experimental data that were used to infer all the free parameters of the SM. The last ones were related the Higgs boson, and now everything is fixed. To accommodate a 4th neutrino, then we would need to go beyond SM.

cjfd 6y ago
This seems to be assuming that the neutrinos are massless, or at least have a mass quite a bit smaller than the Z particle. It could be that there is a fourth generation where both lepton and neutrino are heavier than the Z particle, no?
- evanb 6y ago
  In the Standard Model as written in the 1970s the neutrinos had to be exactly massless. We now have evidence that neutrinos aren't exactly massless, but they're very VERY light. Like, absurdly light. We still don't know the absolute scale of the neutrino masses, we know from mixing that they're damn small and from cosmology that the sum of all three masses is less than the mass of the electron / a million. It is logically possible that the neutrino of the fourth generation is very heavy. In that case - the constraint on the number of neutrinos from collider experiments is relaxed because the ultra-heavy neutrino's contribution to the observable would be extremely suppressed (because the collision energy was too low to be sensitive). - the cosmological constraints are relaxed because the heavy neutrinos are already frozen out by the time of the electroweak phase transition in the early universe. - the mixing constraints... well, right now it seems that the mixing matrix between the generations is unitary---there's no "leak" into a fourth generation. But our experimental precision is mediocre because precisely measuring the mixing is difficult (though there are experiments under way). It is also logically possible for there to be a fourth generation but that the neutrino doesn't mix at all---its mixing with the lighter neutrinos is precisely 0. While it's perfectly possible logically, we physicists do not like this kind of "fine tuning" without some explanation of how it could happen. In the SM the neutrino masses/mixings are input parameters, not things determined dynamically---they are axioms, so to speak. So any explanation of the mixing being really small would need to invoke more beyond-the-Standard-Model physics than "it's the same but there's a fourth generation".

sideshowb 6y ago

"Nobody understands quantum mechanics" In the process of learning it myself I tried to do my bit towards that by writing a little game. Sorry for the Java http://tropic.org.uk/~crispin/quantum/ http://tropic.org.uk/~crispin/quantum/ I'm not the only one https://quantumgame.io/ https://quantumgame.io/ https://github.com/stared/science-based-games-list https://github.com/stared/science-based-games-list I do share your frustration with the mysteries of the Standard Model (and hope to understand them one day)

evanb 6y ago
:wave: https://news.ycombinator.com/item?id=11813473 https://news.ycombinator.com/item?id=11813473 Still not going to install anything Java, but I hope my comments were helpful!
- sideshowb 6y ago
  Absolutely they were! I eventually wrote a reply addressing your points on that thread, not sure if you saw it (though this isn't a chase for further replies, alas I haven't had time for QM in a few years). Thanks again for your help there. One of those threads that really shows the value lurking around HN (alongside its anti-java bias :) )
  - evanb 6y ago
    By the time I saw it the replies were closed, unfortunately.

dboreham 6y ago

I'll guess the answer is: Because Yang-Mills Guage Theory

voldacar 6y ago

It is totally possible to teach group theory at the high-school level. Instead, literally multiple years are spent teaching the quadratic equation over and over, and a year on "pre-calculus" which is just more quadratic equation plus memorizing some points on the unit circle. Group theory in high school would be sooo much better than the status quo. It would, however, require high school math teachers to actually learn some math.

jfengel 6y ago

Physicists already understand quantum mechanics. There are open questions, but it's no longer the case that the physicists themselves just stand slack-jawed at it. Every field has open questions, and nobody says "Nobody understands DNA" or "Nobody understands polymer chemistry". Quantum mechanics attracted a lot of mystical woo early in its development, because it was so deeply unfamiliar from classical physics, but we've been at it for over a century now. We've moved way, way past the weirdness of quantum erasers and other examples that get promoted in pop science books. As I said, that doesn't mean all of the questions are answered, but a lot of questions are answered. You just can't understand the answers by expressing them in purely conventional terms, as pop science books try to do. So physicists don't come out of school wearing pointy hats and feeling like the keepers of arcane knowledge. They can do quantum mechanics and apply it. That's not forbidden knowledge. It just takes work. Nobody ever promised you an understanding of physics from comfortably reading about it at a middle-school level, any more than you'd expect to learn dishwasher repair or glassblowing just from breezy introductions to it. The information is accessible, and lots of "Quantum mechanics for dummies" books can actually give you a good handle on it. Yes, it takes math, but not very advanced math. Yes, you have to learn a new language, but that's to be expected. Novel ways of viewing the universe require new ways of talking about it. It's right there, publicly accessible.

dschuetz 6y ago
> Physicists already understand quantum mechanics. No, they don't in fact. Most of them even admit that they don't understand it at all, yet it works for them so far, except when it comes to the precise origin of gravity. Some say that the reason why quantum mechanics and gravity couldn't be reconciled until today, because nobody on earth truly knows how exactly quantum mechanics works yet.

throwaw4y-plate 6y ago

I am frustrated with how the "internal combustion engine" has become a veil with which a priestly mechanic class hides sacred knowledge of how cars works from plebeians like myself.