
Neutrino Particles May Be a Link to the Dark Sector - LinuxBender
https://www.scientificamerican.com/article/hidden-neutrino-particles-may-be-a-link-to-the-dark-sector/
======
dschuetz
"Dark Sector" \- the next big thing in sciency stuff?

~~~
qubex
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).

~~~
throwaway_kufu
> 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?

~~~
jasonwatkinspdx
There are many independent lines of evidence:
[https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...](https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence)

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.

~~~
throwaway_kufu
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!

------
dumbfoundded
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.

~~~
at_a_remove
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."

~~~
raattgift
Crucially we can show it's there by examining how it affects the light from
bright background sources passing through it: there is a spectral shift
without the imposition of new emission or absorption lines, and there is
Einstein lensing [
[https://en.wikipedia.org/wiki/Gravitational_lens](https://en.wikipedia.org/wiki/Gravitational_lens)
]. We can see this in the large by observing the
[http://www.astro.ucla.edu/~wright/Lyman-alpha-
forest.html](http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html) on
lines of sight through and near galaxy clusters. For example, [presentation
abstract] [https://depts.washington.edu/astron/event/tba-liang-
dai/](https://depts.washington.edu/astron/event/tba-liang-dai/) [related paper
[https://arxiv.org/abs/1804.03149v2](https://arxiv.org/abs/1804.03149v2) ] or
[https://arxiv.org/abs/1706.07870](https://arxiv.org/abs/1706.07870)

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.

------
dilippkumar
> 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
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
> 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.

~~~
knzhou
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.

~~~
jiggawatts
> Anything simpler necessarily sacrifices correctness.

But not _clarity_.

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.

Try harder.

~~~
tralarpa
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 :)

~~~
marcosdumay
> 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.

~~~
knzhou
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.

