A "sector" of the Standard Model---like the QCD sector, or the electroweak sector, or the Higgs sector---is just shorthand for a part of the the Standard Model that consists of particles and forces.
It's possible that dark matter is just like Standard Model matter, in the sense that it only talks to a force that we already know in the Standard Model, like the weak force. But it's also possible that there are other forces. The "dark sector" is short hand for 'whatever stuff is making up the dark matter, and however they interact'.
It’s just shorthand for stuff that we cannot detect but ordinary (electromagnetic) means. In theory everything ‘feels’ gravity and anything with mass contributes to it. The left-handed neutrino is unusual in that it experiences only gravity and the extremely weak (though far stronger than gravity) “weak nuclear force” (and their right-handed brethren, if they even exist, might not exist en experience that).
The “dark sector” is that 95% or so of the entire universe we infer must exist for either gravitational reasons (stronger gravity leads us to infer unseen mass, which we term “dark matter” though “transparent and apparently frictionless” would be a better term, or to infer unseen energy (dark energy that seems to be driving the accelerating rate of the universe).
It's worth noting that there is an explanation for dark energy, namely quantum fluctuations of the vacuum. It's also worth noting that this explanation is off by over 100 orders of magnitude, which lead to it being called "the worst prediction in physics". Oddly enough we haven't come up with a better explanation as far as I know, the current assumption is just that the mass somehow almost cancels out.
> 95% or so of the entire universe we infer must exist
Isn’t the reason we infer dark matter exists, is our observations of the spin of galaxies? In other words there must be a lot more gravity produced by matter we aren’t observing (dark matter) to explain large scale observations?
Aren’t there competing theories to dark matter, which haven’t been ruled out, from Einstein’s theories of gravity being wrong; or in the same way Einstein’s theory of gravity breaks down/is incomplete on the quantum scale his theories may breakdown at the scale of galaxies; and even more simply our observations not be accurate?
Contrarian theories have outsized popularity here on hnews, and this tends to get fed by garbage articles from the usual sources (glances at quanta with mild disdain). Modified gravity theories are not some alternate path we just didn't explore. We explored them. They fail to explain the observational evidence. In particular the way stuff moves in galactic collisions makes very clear there's dark matter distinct from some modified version of gravity interacting with visible matter.
It ends up that the consensus view of thousands of Phds that study this stuff is actually not trivially mistaken.
I wasn’t implying mistaken by any means, I understand is the leading theory (not the only theory), I didn’t realize everything else had been ruled out or considered junk science (I just understood no one wants to be the person saying Einstein’s theories/equations for gravity need modification...even though that happens to be the consensus at the quantum level.
But the galaxy collision observations is something I want to read more about (I’ve obviously seen simulations), thanks!
To build on peer replies: the leading dark matter theories from a particle physics perspective imply "particles which do not feel electromagnetism." This ought not to be weird! After all, some particles do not feel the strong force (leptons) and others do not feel the weak force (right-handed); why should every particle feel EM?
It feels weird because almost everything we see is EM in nature: chemistry, electricity, light, life itself! But there is no a priori reason to demand that everything in the universe interacts through EM. There ought to be some stuff that does not. And if it were to exist, it would be like a ghost: particles which could not shed energy through emitting photons.
And the cosmological perspective is that "there's a lot of this stuff, huge amounts" and why not?
So it seems like this may have a link to dark matter but not energy. I have no expertise in physics but it seems like dark is a bad term as it only reflects our ignorance and inability to detect through normal electromagnetic means.
Astronomers work through photons obtained and scientific inference on those photons. Lots and lots of inference. "Light" matter simply means "we can see it through a telescope because it gives off light," which means stars at a great distance, or closer up, light from a star reflected off of a surface.
Dark, therefore, means "we know it is there but it isn't directly observable by gathering photons with a telescope."
The tl;dr is that distant light sources (distance obtained by among other things comparing the angular diameter and brightness on the sky of spiral galaxies containing common bright sources within them such as supernovae or molecular clouds lit by quasars) are redshifted in a way that is very hard to explain without an accelerating expansion of space (which in the equations of the Standard Cosmology we represent as dark energy), and are lensed by luminous galaxy clusters in such a way that is very hard to explain without mass that doesn't emit or absorb light.
"Hard to explain" because (just as one example) the Lyman-alpha line and its relativistic behaviour is extremely well studied in controlled laboratory conditions in various places in our solar system (notably sensitive experiments carried in various spacecraft probing other bodies in our solar system). In a number of spacecraft, Lyman-alpha photometry testing is used to determine the Hydrogen-Deuterium ratio of planetary atmospheres, since cells of those two gasses form practically ideal narrow-band rejection filters, and over time a mixture of cells reveals the atomic ratio of those isotopes in the atmosphere. People who observe Mars have used this technique to study the rate at which Mars is outgassing water vapour, in particular. As it is useful to planetary scientists to look through varying depths of a given planetary atmosphere backlit by distant luminous objects, in practice the cosmological Lyman-alpha forest can be sampled by orbiting Ly-a photometers "for free". You can't really reject the resulting Ly-a data without also rejecting the Deuterium/Hydrogen ratio obtained from probes on the surface of Mars, which happen to match. And the data typically also encodes other spectral lines principally from the intergalactic medium, and those lines are similarly redshifted and amplified by relativistic -- including general relativistic -- effects. Both dark matter and dark energy induce matching general relativistic effects, and nobody has figured out how to produce the same spectral features without including dark matter and dark energy.
> 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".
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.
> 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.
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.
This is the crux of the problem. Mathematicians, and by extension, theoretical physicists are under the impression that the most terse, most technical, most "correct" (by some abitrary definition!) definition is the only one that matters, and that nothing else should ever stain their lips.
Take a tour of any abstract mathematical topic on Wikipedia. It's a travesty.
Nothing at all can ever be understood from those pages by even an extremely intelligent person who doesn't already know the topic.
Do you see the issue? You have to already understand to gain understanding... err... of a thing you don't understand, hence the need for a Wikipedia page. If you understood it, then as a "quick reminder reference" the Wikipedia page might be useful, but then the audience is a few hundred people.
There is absolutely no difference between this idiotic behaviour of mathematicians, and the insistence of English priests in the middle ages of sticking to Latin for theology. NONE.
If you argue this point, first explain to me why physics is done with Greek letters.
I bet whatever you say will be indistinguishable from the arguments made by those priests.
> Imagine explaining how a CPU works on the silicon level to somebody from the 18th century
I could do this, no problem. It would take about 20-30 hours of talking and something to scribble on, but I could do it.
I would use analogies with things they already understand, such as the hydraulic control lines of steam-powered machinery, and explain that electronic transistors (called 'valves' back in the day!) are just like the hydraulically or pneumatically controlled valves they use every day in industry. Just more of them, faster, and smaller.
See, it's not that hard.
> I've written and then thrown away a lot of attempted explanations because of this.
Try using geometric algebra instead of the mishmash that is vector algebra. Try avoiding complex numbers. Try to avoid Greek letters. Try to avoid mixing ten branches of mathematics just to shave off a few letters in an equation somewhere. Try using an analogy with things the audience might already be familiar with.
And then you end up with explanations like "Gravity is like a deformation of a rubber sheet". Completely misleading. In a similar case (not physics), I literally wasted years on trying to understand something correctly because I wriggled from analogy to analogy instead of putting effort into learning it correctly.
> I would use analogies with things they already understand, such as the hydraulic control lines of steam-powered machinery
How many people from the 18th century knew how those worked? You would be talking to an elite engineer. And even then you would first have to explain to them how you can represent numbers and numerical operations with those valves. I am pretty sure that if you take a today's elite engineer and take the time to explain him the basics correctly, he would be able to understand theoretical physics. That's what's happening in universities every day.
> If you argue this point, first explain to me why physics is done with Greek letters.
There are probably some historical reasons for that, but today it's just because you are quickly running out of latin letters in complex equations, at least that's my opinion :)
> And then you end up with explanations like "Gravity is like a deformation of a rubber sheet".
If you haven't read Eistein's "Relativity : the Special and General Theory", I strongly recommend you do.
It's a book aimed at laypeople, that requires only a basic (1st undergrad semester) understanding of calculus and vectors (not vector calculus, not linear algebra, just vectors). Yet, it's precise and correct, sacrificing conciseness to get easiness to understand - like every teaching book should.
So, no. It's perfectly possible to explain it without the bad analogies.
Special relativity is well-known for having the least prerequisites of any modern physics subject. Yes, you can explain it with just calculus and vectors. That's precisely what we already do in university courses.
But the complaint that started this whole thread is "why can't physicists explain why it's hard to add in extra neutrinos to the Standard Model?" Understanding the reason requires 2-3 semesters of relativistic quantum field theory, a subject which in turn requires relativity, quantum mechanics, and classical field theory.
Seriously, any resource that legitimately explains all of this, without bad analogies, and assuming no prerequisites, would end up precisely as hard to read (or likely harder) than the ~10,000 pages of textbook people currently study. The textbooks have already been optimized for ease of learning, there is no magic way to make a hard thing easy.
If you argue this point, first explain to me why physics is done with Greek letters.
Because there are not enough letters in the Latin alphabet? You could of course use words instead of single letters, but I think most people that do numeric computing would agree that the transliteration of formulas into programming language syntax using ASCII makes things less readable.
Using different alphabets (we don't just use Greek, there's also Fraktur, bold letters, etc, though physicists tend to prefer markers like vector arrows, dots, hats, ...) allows for a compact way to namespace symbols.
If I do linear algebra and see an expression like
A u = λ v + μ w
I'll know immediately what type of quantities the various symbols conventionally denote.
Conventions where the same letter means the same thing in different pieces of work, are useful. There aren't enough latin letters to go around. Hell, there aren't even enough greek ones. Maybe we should start using unicode emoticons next?
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. 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.)
>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.
Feynman managed to do this with quantum electrodynamics, in "QED: The Strange Theory of Light and Matter." That layperson book shows an inefficient yet accurate way to calculate path integrals of Feynman diagrams without any calculus, and uses the exact analogy of a clock spinning for phase. I understood it without any background in the math.
There's also four lectures on QED for grad students, which is the "formal" version of pretty much the same content, featuring math.
Why can't someone take the QED approach for the whole standard model? Symmetry groups can be explained visually. I think the physicists just can't be bothered, and/or they underestimate the intelligence of uninformed but curious laypeople.
Roger Penrose is another example of a physicist who does this right, in Cycles of Time for instance.
I think many people think soft skills are not real -- that if you are good at a hard skill such as theoretical calculations, you are automatically good at soft skills such as explaining it to a lay audience.
Feynman had a rare talent for explanation, which few physicists have. This talent is just as real as his talent for physics.
It's also not really part of your job description: You're supposed to advance the state of the art of theoretical physics, not "waste time" trying to come up with better explanations for the layperson.
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-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 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.
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?
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".
> 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.
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.
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.
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.
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.
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.
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 :) )
