The h Bar

qwerty

02:15

@ACuriousMind this is what category theory tries to "make precise", right? i'm still just at the tip of the iceberg... so it's not just saying the same thing with different jargon, there's somehow more information contained? you're saying that there are specific differences for each case people talk about things like "the form of the equation being preserved"; it seems like category theory is the overall connection/generalisation though?

PM 2Ring

03:24

@qwerty You might like to check out the Babylonian Astronomical Diaries.

May 25, 2023 at 10:58, by PM 2Ring

As I said in https://astronomy.stackexchange.com/a/47584/16685 The Babylonians made ~7 centuries of daily astronomical observations from around 700 BC. That data was the basis of the astronomical tables of Hipparchus, which Ptolemy used in creating the formulae of celestial motions in the Almagest. There's more information about these diaries on https://www.livius.org/articles/concept/astronomical-diaries/

Oct 27, 2022 at 13:04, by PM 2Ring

I have to admire the people who are working on translating the Babylonian Astronomical Diaries. It's awesome that we have that data, but it's not exactly riveting reading material. ;)

naturallyInconsistent

I think it is just important; not that it is riveting or anything. It is that we want to make sure that there wasn't some sudden break in our astronomical past when some insanity happened.

It will also feel amazing when we run out simulations backwards and see that the predicted observations agree with what the diaries show

PM 2Ring

The Diaries also contain weather observations. They also mention crop yields, and important political & military events. That's because they wanted to accumulate data so they could search for astrological patterns.

naturallyInconsistent

also makes it clear that we have good translations with all the numbers all correct and all

PM 2Ring

Well, our earliest ephemerides came from that data, via Ptolemy. And our current ephemerides are certainly consistent with our older ephemerides. So, really, we've been using that data all along. Especially the stuff related to the solar & lunar cycles.

Sure, that old data isn't very high precision. But it gives us valuable info on the slowing of the Earth's rotation, especially the solar eclipse records, from Babylon & elsewhere.

Also eclipse records from various cultures help us to synchronise the historical records of those cultures.

The smallest irregularities in the Earth's rotation are essentially unpredictable: they amount to predicting the "weather" in the mantle. So any ancient data relating to the Earth's rotation is pretty important. eclipsewise.com/help/uncertainty.html

Even with all our modern technology, terrestrial & space-based observations, and massive computing power, we still can't predict Delta T to high precision very far into the future. Which is why there's no fixed schedule for leap seconds. Of course, there are large predictable components, but the small components are large enough to screw things up. ;)

qwerty

03:54

@PM2Ring thanks for the links :) I'm not really super into astro/observation stuff, but still cool how much the ancients knew and did

7 centuries of daily observations is really remarkable

you need a lot of political/cultural stability for that

PM 2Ring

@qwerty The livius.org link isn't huge, and it's a fun read. The Diaries themselves are pretty boring, but kinda hypnotic. ;)

naturallyInconsistent

@qwerty actually, the rate at which things change is, other than a function of policies encouraging or discouraging change, also a function of how many people there were, and the past didn't feed enough people for change to happen so often. It is also the case that the political structure can divest power and make for stability; people are ok with having the same powerless figurehead if they can effect political change elsewhere. Hence the millennium run of the Crysanthemum throne

PM 2Ring

@qwerty Yep. And motivation. They weren't just doing it for idle curiosity. There was some practical motivation to get good solar & lunar data, for calendar construction, and predicting Moon phases & eclipses. But the main motivation was astrology.

That's a bit embarrassing for many modern astronomers. :) OTOH, the separation of astronomy & astrology is only relatively recent. Kepler & Newton both believed in astrology, but Halley thought it was nonsense. Cassini got into astronomy because of astrology...

qwerty

@naturallyInconsistent interesting! I've never considered the population size as the underlying variable. apparently the reason why sci fi /time travel is a relatively new cultural idea is that because in the past, the rate of change was so slow

@PM2Ring interesting how the nomenclature never got "reclaimed" as well, I guess because astrology remains a popular cultural pasttime (somewhat disappointingly). even though it's just star-study, whereas -nomy implies the more boring "naming of".

PM 2Ring

04:10

There are some old stories of people going to other realms where time passes at a different rate, though. So when they return to our realm many years have passed, even though they were in the other realm for a few days. Or vice versa.

@qwerty Right. Astronomy is "just the facts", with no interpretation. But I suppose we've bypassed the need to reclaim the nomenclature, since we now have more specific terms like astrophysics, and orbital mechanics.

qwerty

true

PM 2Ring

When Brian May started his degree, his field was just known as astronomy. When he finally returned to uni, it had become astrophysics.

He discovered evidence in the zodiacal dust that the Solar System is moving through an interstellar dust cloud. But that info didn't get published for a few decades because Bri was busy being a rock star. Other observers discovered that dust & published papers abour it in the mean time.

That dust is pretty low density, and our local dust dominates it in the inner system. But out past Uranus or so, the interstellar dust becomes significant.

naturallyInconsistent

@qwerty im not sure that is the case. one first needs a complicated conception of time in order to start playing more than just "time passes differently" as different constants or some such. That's a pretty modern phenomenon and it is difficult to even disentangle which contributed what

PM 2Ring

04:27

Fair point. Stories involving different time rates are found in various cultures. But stories of traveling to the past or future (and returning to the present) seem to be a fairly modern concept.

qwerty

04:38

@PM2Ring "zodiacal dust"! that's new to me

@naturallyInconsistent mhmm, it was just one thing I read. but "I wonder what it would be like if I could visit Camelot/Queen Elizabeth I" etc seem like such basic, even child-like, as a imaginative scenario that is seriously strange to the modern sensibility that it is in fact such a recent idea

PM 2Ring

@qwerty There's a lot of it, but it's not easy to see. You need good eyes, no light pollution, and no visible Moon. en.wikipedia.org/wiki/Zodiacal_light

Asteroid sizes roughly follow a power law, so there's a lot of really teeny ones. astronomy.stackexchange.com/a/49425/16685 I like to call the really small ones dusteroids. ;)

qwerty

@PM2Ring may I ask what's your physics/professional background? is astro your specialisation?

PM 2Ring

I'm just an enthusiastic amateur. :)

qwerty

any areas in particular? :)

PM 2Ring

One of my high school science yeachers was a former president of the Royal Astronomical Society ras.ac.uk He was also a co-author of our science text book. He worked in industrial chemistry before immigrating and becoming a textbook author & teacher.

@qwerty I like celestial mechanics. I'm also intrigued by stellar nuclear processes, but I don't have the skills to understand it deeply.

I'm interested in cosmology, too. I'm not motivated enough to study it deeply, but I like to know about it at a bit deeper than a pop-sci level.

qwerty

04:58

did you do undergrad physics?

PM 2Ring

No, I'm self-taught.

7

qwerty

if you were in fact motivated to learn deeply, there's plenty of textbooks/resources you could follow

PM 2Ring

Oh, I've read (and worked through) lots of good textbooks. Before the internet, I always had access to good libraries. But I'm getting too old to learn new stuff. I can learn stuff that connects to stuff I already know, but it's really hard to retain new stuff.

Sir Cumference

@PM2Ring That's hella impressive

PM 2Ring

Thanks

Sir Cumference

05:06

Most of my time is spent reading textbooks too, but I usually rely on my curriculum to pick them

I guess you look for suggestions then?

PM 2Ring

These days, I mostly just read stuff from arXiv on topics that catch my interest. Mostly mathematics, rather than physics.

Lucky Chouhan

05:35

@PM2Ring I really feel discouraged when I can't write down the proper solution of exercises/problems in the textbook. Would you advise me on this? (PS: Even after reading the textbook)

PM 2Ring

05:53

@LuckyChouhan Sorry, I don't know you (or your knowledge level) well enough to give you much practical advice. But one thing that I found helpful is to work methodically through a textbook. Do not skip over the easy exercises at the start. They help you "warm up" for the later more complicated exercises. And they introduce you to the way the author structures their exercises.

A good author structures their exercises so that you gradually gain the skills you need to do those exercises. Sadly, not all textbook authors work like that, and you get the feeling that they just threw a random bunches of exercises together as an afterthought...

qwerty

06:10

@SirCumference seconded

naturallyInconsistent

06:49

@qwerty Changes used to be so slow that people thought that antiquity was as advanced / primitive as they lived. When there is no visible sign of progress, people thought that it didn't exist, and that Ramsees lived in a world similar to Elizabeth, just that politics changed.

@qwerty thirded

qwerty

it's funny because some things we might now think of as modern are in fact kinda ancient too. I went into a deeeeep rabbit hole about the history of toy dogs and dog genetics research and ended up digging up like ming(?iirc or even earlier) ancient Chinese texts about little dogs imported from the Roman Empire all the way to China(!!!). I'm a heritage Chinese language speaker (but poorly and not mandarin) and illiterate so that was a SLOG to try to read and translate for myself but soooo fun

naturallyInconsistent

@qwerty oooo~

It is already pretty difficult to present physics in mandarin; if they want meow meow to do it in Hokkein, miao miao would dieeee

qwerty

basically there were little white fluffy dogs from Malta aka Maltese (don't let the kennel clubs tell you it's not well defined yada yada... I mean aka in phenotype sense)

recorded by Aristotle and others in ancient Rome as well

the context was basically people bashing some nationality or group or other for being effeminate for taking little handbag dogs to the gym

like REAL WARRIORS don't carry little dogs to the gym

very funny... I'll dig a link up later

naturallyInconsistent

lol

insecure men being insecure for millennia

qwerty

07:05

I find the idea of an ancient Roman Paris Hilton with a handbag dog somehow extremely amusing

naturallyInconsistent

hahahahhahhaha

Roman male Paris Hilton?

qwerty

Athenaeus, in his voluminous early 3rd century CE Deipnosophistae (12:518–519), states that it was a characteristic of the Sicilian Sybarites, notorious for the extreme punctiliousness of their refined tastes, to delight in the company of owl-faced jester-dwarfs and Melite lap-dogs (rather than in their fellow human beings), with the latter accompanying them even when they went to exercise in the gymnasia.[n]

2

wiki had it

Ryder Rude

hi

qwerty

this is from a Victorian era book. The Emperor of the Turkoman country visited the Honan Emperor in 609 and accompanied the Emperor Yang Ti on an expedition to Korea. He married a Chinese princess on his return. His successor sent an envoy with two dogs, one male and one female, to the Emperor Kou Tzu (6 18-629). Their height was about 6 tsun (inches), and their length i ch'ih (foot) and a little.* These dogs were of great intelligence.

2

naturallyInconsistent

Note that we dont even have to go that far. King Louis XIV made it fashionable for noblemen to wear lace and keep coming to his parties. IIRC he was also the one who wrote advice to his son that such parties are just keeping up appearances, and that the state affairs still had to be done properly; the point is to infect the rest of the courts with one-uppance and waste their finances and time.

qwerty

07:19

They could lead horses by the reins, and each was trained to light its master's path at night by carrying a torch in its mouth. These dogs were born in the Fu Lin (拂菻 en.wikipedia.org/wiki/…) country."

Mr. Feynman

@PM2Ring For some reason I thought you worked in academia. I'm impressed. Of course, getting a formal education is not the only way to gather knowledge in a field, but self-teaching is tenfold as difficult. You did great

@naturallyInconsistent is physics easier in English for you?

naturallyInconsistent

@Mr.Feynman by farrrrrr

Mr. Feynman

Which makes me ask the following question to the chat: what comes more natural to you when doing physics? Your native language or English (if English is your native language reply "HONK")? For me it's about the same level, but after QFT I need to borrow many English words

Ryder Rude

@PM2Ring i am self taught too

qwerty

og citation for the dog spam is [ T'ung K'ao, by Ma Tuan Lin (lived about A.D. 960, in Sung Dynasty)en.wikipedia.org/wiki/Wenxian_Tongkao , and T'ung Tien by Tu Yo (en.wikipedia.org/wiki/Tongdian , Tongdian 通典 ). also Schafer has alt translation p77., suggests they are Roman dogs i.e. Maltese.

/end dogspam

@Mr.Feynman HONK

Mr. Feynman

07:32

Yes

Sorry, I can't honk back

qwerty

Woof?

meow?

(this chat is a menagerie)

ACuriousMind

@qwerty there are different views on this, but to me the primary use of category theory is not that it contains more information but that it gives me a unifying language to talk about the similarities and differences between different fields with different objects and different notions of what a "good" map between the objects of study is

Mr. Feynman

@qwerty if I didn't have sore throat I would try to go for a crow's call

And I would fail nonetheless

ACuriousMind

Its focus on the morphisms as just as relevant as if not more than the objects pulls into focus that often we are less interested in the objects in isolation but the relationships between them

@Mr.Feynman English, but that's because it has been the primary language of my study and work for over a decade. At this point German is just what I speak with most of my friends :P

naturallyInconsistent

@Mr.Feynman H O N K ~

Claudio

07:49

@Mr.Feynman English

Mr. Feynman

@ACuriousMind I relate a lot to this; especially over the last 2 years, speaking of physics in Italian started feeling like translation :P

"Should I keep this word in English or adapt it?"

@naturallyInconsistent Is English your native language?!?!

I thought you were Asian (although I can't pinpoint where :P)

naturallyInconsistent

08:07

@Mr.Feynman im actually more fluent and comfortable in mandarin but I cannot fully work in that language since miao miao only knows physics in English

@Mr.Feynman The vast majority of English speakers are Indian, followed by Chinese, if myow understanding of the demographics is correct, lol

Slereah

Annoyed by universities who delete a professor's page with all their papers when they move out

Mr. Feynman

08:31

@naturallyInconsistent In India English is a native language in India. Is it in China?

@Slereah a bit bitchy

Slereah

@Mr.Feynman Maybe in the Shangai Municipality

qwerty

08:58

@Mr.Feynman there are also places such a singapore, which have english as the lingua franca

naturallyInconsistent

@Mr.Feynman It is not. But never underestimate the irrationality of an entire peoples. For example, the Korean pre-university examinations feature an English exam so difficult that it will shock Brits

qwerty

@Mr.Feynman are you sick? get well soon :)

Mr. Feynman

@qwerty Thanks :)

@naturallyInconsistent Oh, sure. I'm not talking about the English level. It was more about the label of native vs non-native

naturallyInconsistent

@Mr.Feynman sure, that wont be at the native level, but look at how miao miao wrote that; it was English speakers only miahahahaha. Or better, writers. One can write without being able to pronounce.

@Mr.Feynman gws too~~

Slereah

@naturallyInconsistent What about people in Alabama though

They can't write or pronounce it

qwerty

09:09

@ACuriousMind hm, but you were saying that "preserving the form of an equation" or "preserve the form of Hamilton's equations" isn't a precise statement, whereas something like a "symplectomorphism, leaving the symplectic form of phase space invariant" is. the definition of a morphism you were happy with was "a function that preserves some kind of structure", which to me seems kind of the same level of vagueness until you specific the category and the morphism?

Slereah

Is category helpful rly

a "morphism" can be anything as long as it can be composed

qwerty

@Slereah boxes

Slereah

A better way to consider morphism as "preserving structure" is model theory

4

A: mathematical logic: definition of a homomorphism, isomorphism and related notions

You're right that these definitions are confusing. Is it possible that they've been translated from another language? It seems to me that Definition 1 is defining what I would call an embedding (though missing the condition of reflecting relations - unless that's contained in Definition 2). Defin...

$$h(f^A(a_1,\dots,a_n)) = f^B(h(a_1),\dots,h(a_n))$$

naturallyInconsistent

@Slereah You might be amused that, one fine day, a friend of mine wanted meow meow to deal with his smug Alabaman uncle. That was a quick conversation; there wasn't much that people can recognise as a nice recent impression of Alabama.

Slereah

De facto that is how morphisms work on a practical level

naturallyInconsistent

09:14

@qwerty The issue is that, once pressed for it, ACM can always supply the necessary ingredients to make the statements precise. That is not true for how we physicists usually put it.

Slereah

for instance a morphism in vector spaces with the constant $0$ and the functions $+, \cdot$ is a function $f : V \to W$ such that $f(0_V) = 0_W$, $f(+(v_1, v_2)) = +(f(v_1), f(v_2))$ and $f(\cdot(x, v)) = \cdot(x, f(v))$

For symplectic spaces it's the same but you also have to preserve the function $\omega$

in all formulas

Mr. Feynman

@naturallyInconsistent I think we're having some issue getting through to each other. I think that a non-native speaker can speak and write better than a native, no doubts about that. I was really just discussing the political side of it, i.e. the official language of the place where one was born :P

And now: linear response theory!

Slereah

@qwerty The Greeks were very impressed by it and also a bit naive

Some would say that the data stretched back as far as a million years

Which I assume was some Sumerian priest messing with them

ACuriousMind

@qwerty Well, when you say what the morphisms in a specific category are, you don't wave your hands and say "they preserve some structure I guess" but you make a formal definition like the one for symplectomorphisms

Slereah

I think you can make the "it preserves structures" argument rigorous in category theory but you'd probably have to mess with the category's internal logic or something :p

ACuriousMind

09:31

Category theory is not the thing that makes the notion of preserving structure precise - what you need for that is just the proper mathematical formulation of a subject instead of the handwaving of physicists :P

Slereah

Model theory seems pretty clear to do it rly

simple enough

and that's pretty much how people mean it when they do it in practice

Ryder Rude

are higher dimensional branes fixed hypersurfaces or are they quantum objects whose state has to be path integrated

people say that these branes r similar to strings, so r these branes quantum objects which r dynamical?

but it also says that branes give the boundary conditions for the string propagation. this gives the vibes that the brane is just a fixed hypersurface

Slereah

By "branes" do you mean D-branes specifically

Ryder Rude

yes

are these dynamical quantum objects like strings or just fixed hypersurfaces

ooh wiki says they can scatter off each other

so they're dynamical and quantum

so they're like strings, except higher dimensional and they also give the boundary conditions for string propagation

naturallyInconsistent

09:47

@Mr.Feynman miao miao can always just quibble~

@Mr.Feynman by what method? Kubo?

ACuriousMind

@Slereah I'll take your word for it, the "internal logic" part of categories always remained somewhat opaque to me :P

Slereah

Maclane's book on the topic is a pretty good introduction :

books.google.fr/books?id=LZWLBAAAQBAJ

Slereah

10:03

I wonder if any of the predictions of babylonian astrologers were true just by sheer correlation

The sky's appearance is strongly dependent on the time of the year, many events are also dependent on the time of the year

also the "astrology" included the weather back then which is also probably a good predictor of things?

Ryder Rude

do we path integrate over D brane propagation like we do over string propagation

such that, in the worldvolume diagram, the boundary of the open string stays on the D- brane

is this the correct understanding of dynamical d brane

naturallyInconsistent

10:20

@qwerty miao miao was cut off by work and couldn't finish the sentence in time to piece them together, so sorry for 2nd ping: The situation is the same as maths at the professorial level; there must be use of some estoeric or vague language, but the crucial difference is that a mathematician can always, upon pressed, produce the rigorous version. It is just an exercise. It is not "Just learn category theory.".

Ryder Rude

what can we say about whether dark matter is scientific or just a way to save general relativity from falsification

the cosmological constant wouldn't be seen as a way to save general relativity. what makes dark matter different

John Rennie

2

Q: Why is "dark matter" theory accepted? Why wasn't general relativity rejected?

Dark matter was made up to account for unexplained effects such as gravitational lensing, the speed of expansion of the universe, or the rate of rotation of certain galaxies. However, as Feynman famously stated (see e.g. wikiquote), If it disagrees with experiment it is wrong. In that simple...

Slereah

10:39

@RyderRude What does "scientific" mean

Ryder Rude

10:53

@JohnRennie thanks

@JohnRennie this means dark matter is a successful model

@Slereah as in, one should aim to out more predictions out from fewer assumptions

John Rennie

Well it works and so far has yet to be disproved.

Ryder Rude

has it made predictions?

John Rennie

Though it would be nice to detect dark matter particles directly.

Mr. Feynman

@naturallyInconsistent I'll be honest man. I don't understand a thing :P

Ryder Rude

why must dark matter arise from particles when cosmological constant doesn't need to arise from particles @JohnRennie

John Rennie

11:02

The difference is the way the two scale as the universe expands.

Ryder Rude

maybe because dark matter requires a stress energy tensor

@JohnRennie oh

so we need matter in our model to fit the data

John Rennie

The density of dark matter scales as 1/a³ i.e. the same as regular matter

Ryder Rude

@JohnRennie oh

i think it's a good hypothesis. A well motivated one

John Rennie

The density of dark energy/cosmological constant is a constant and does not depend on a at all. The different scaling is why dark matter slows the expansion and dark energy accelerates it.

The term "dark matter" really just means "something with an energy density that scales as 1/a³"

And as far as we know only matter scales that way. Radiation scales as 1/a⁴.

Lucky Chouhan

@naturallyInconsistent Miao Miao~~ should write physics textbook in mandarin :P

Ryder Rude

11:05

so we are able to fit the observations when input this energy density in the equations

John Rennie

Yes, though that's not the achievement that it sounds like since the density of dark matter is an almost free variable.

Ryder Rude

thanks

@JohnRennie oh

yeah.. if we are allowed to fine tune the energy density, we could fit any observation we want

but dark matter doesn't require fine tuning. 1/a3 is a simple hypothesis

John Rennie

That is, in galaxies we can adjust the function giving the dark matter density as a function of distance from the centre of the galaxy.

Slereah

@RyderRude we can see the motion of dark matter via gravitational lensing astronomy

Ryder Rude

@JohnRennie so we do finite tune it a bit

@Slereah oh

John Rennie

11:09

The 1/a³ dependence is the average density on a large scale i.e. a scale greater than the size of galaxy clusters.

Ryder Rude

@JohnRennie is this function arrived at by some natural assumptions that any kind of matter would obey, or is this function freely fine tuned

John Rennie

There are three "observations" of dark matter:
- galaxy rotation curves
- galaxy cluster dynamics
- gravitational microlensing

@RyderRude I think it comes from modelling the WMAP data.

I'm not at my PC so I can't easily get a link to the WMAP papers.

Ryder Rude

@JohnRennie but then we are freely fine tuning it to observation. is this model even falsifiable?

@JohnRennie does this model predict any of these observations or is it fine tuned to all of these

John Rennie

But I imagine a quick Google would find them. They are on the arxiv.

Lucky Chouhan

@Mr.Feynman keep them as they are. You may know "eigenvalues and eigenvector" here they word "eigen" is German, and whenever we want to discuss about it, we don't translate it in our mother tongue or other languages as we don't translate names so, it won't make sense to translate those words remember "Use Alphabets to ask questions about Dinosaur"

Ryder Rude

11:13

@JohnRennie i will check it out..

would you say that dark matter is falsifiable? as in, what could it not model? @JohnRennie @Slereah

could it model any reasonable observations

fine tuning seems to suggest it

John Rennie

Well if matching the observed galaxy rotation curves required a physically improbable dark matter distribution that would be strong evidence against it.

After all dark matter experiences the same gravitational fields as regular matter and has to respond in the same way.

Ryder Rude

that makes sense. So the distribution is not completely freely tunable

John Rennie

It's tunable within limits ...

Ryder Rude

the hypothesis that it is matter constrains the distribution

naturallyInconsistent

@LuckyChouhan never. That's wayyyy too time consuming

John Rennie

11:17

Yes

naturallyInconsistent

@Mr.Feynman it's linear response. It aint the horrible stuff yet!

Ryder Rude

let's get to predictions. Slereah said we can test some of its predictions. So it is not entirely about just fitting observations

i think predictions are another test of a hypothesis, other than falsifiability

@Slereah so the model predicts how dark matter is supposed to move, and we have verified that?

Slereah

No, that's the equation of state of dark matter, which is independent of GR :p

We can just see that there's something

Lucky Chouhan

@naturallyInconsistent so you teach in mandarin and use physics' names as they're in books? Just asking because this is what most of the people do (and makes sense), but here in India teachers don't teach in Hindi even when all students know Hindi :(

Ryder Rude

oh. so no predictions that have been verified other than that it seems to be matter there @Slereah

it makes sense. Predictions are too much to ask for from a hypothesis like this

Slereah

11:21

Sounds like a pretty good verification to me

Mr. Feynman

@naturallyInconsistent Ok, yes, it is indeed called "Kubo formula". Anyways, it's the notes, not the topic

Ryder Rude

@Slereah lol

Mr. Feynman

@LuckyChouhan Unfortunate example: in Italian we call them "autovettori"

Ryder Rude

in particle physics, we keep discovering new particles. So i think new matter is an okay hypothesis

naturallyInconsistent

@LuckyChouhan Well, if the lecture is in mandarin, then the name would be using the translated, and then in brackets, the original names.

Mr. Feynman

11:22

where "auto" is the translation of "eigen"

*adaptation

Not translation

naturallyInconsistent

@Mr.Feynman yeah, Kubo is magic.

Ryder Rude

@Slereah is this an independent equation from say, the geodesic equation generalised to fluids

Slereah

If you don't know what an equation of state is I think you should probably not worry about dark matter for now

Ryder Rude

oh

i was just wondering whether or not it is scientific

but it seems like a good hypothesis, going by this conversation

thanks @JohnRennie @Slereah

Lucky Chouhan

@Mr.Feynman My bad :( so Italians study math and physics in Italian? Here in India, we also have Hindi (and english) medium in school, where we can find science books written in Hindi with Hindi names, but those names feel so weird and somehow for me their original English names make sense, btw what is your term for "Orthogonal Projection"?

@naturallyInconsistent have you heard the name of Richard Bellman?

Slereah

11:28

If you're worried about dark matter re. the scientific method that is the kind of topic Lakatos talks about

Not specifically that one but it is about auxilliary hypothesis

naturallyInconsistent

@LuckyChouhan no, but dynamic programming was a big thing way back

Ryder Rude

oh

does he make any comments on dark matter? If not, do u think dark matter passes the criteria he specifies? @Slereah

naturallyInconsistent

ok im out to have fun yet again miahahaha bye

Slereah

Lakatos is from the 60's

He wasn't super informed about dark matter

Lucky Chouhan

@naturallyInconsistent yeah, it is still fancy term, but we all do programming problems using it without realizing we're using the concept of dynamic programming.

Ryder Rude

11:30

makes sense

Slereah

His point is that no theory by itself is scientific or unscientific in that sense

It is that it's the evolution of the research programme is

Ryder Rude

so he assigns scientific-ness not to specific theories, but to developments of theories over time

i guess that could be a viewpoint

Slereah

In a purely Popperian sense, those theories are scientific, but so are any theory that works from ad hoc hypothesis

But you can call ad hoc basically anything

Ryder Rude

yes. The definition of ad hoc ultimately just comes down to vibes

Slereah

Lakatos' point is that some scientific programmes are productive while others are degenerate

And he would consider only the productive ones to be scientific, although that doesn't mean they're wrong or right

Ryder Rude

11:35

what is the definition of productive and degenerate that he uses

Slereah

In that sense dark matter is productive since it can lead to new predictions

Productivz auxilliary hypothesis allow for new predictions, while degenerate ones do not

Of course this isn't necessarily bad. Sometimes you do need an extra hypothesis even if it's not productive

But a programme that is only kept afloat from piling more and more degenerate ad hoc hypothesis is considered suspect

Ryder Rude

so Lakatos values predictions. on the surface, i cant appreciate his criterion, because it doesn't seem too well defined to me. But he probably makes it well defined in his book

i think it needs to used in conjunction with falsifiability

Slereah

Course, although as Lakatos goes over for a while, falsifiability is not unproblematic

Ryder Rude

oh

falsifiability is not too well defined either

i usually use falsifiability and predictability in conjunction

in this discussion, I asked "what could dark matter not model". This is related to falsifiability

Slereah

Point is, dark matter's fine and we're not flush with alternatives

People are mostly having knee jerk reactions due to the fact that we can't see it

Ryder Rude

11:42

that's great news. I think GR is extremely beautiful. I would've hated if dark matter was a technique to save GR's wrong predictions

so I wanted to rule that out

@Slereah neutrinos can't be seen either. my problem was more with "scientific-ness" of it

Slereah

Well we didn't detect a neutrino for 30 years after their prediction

Ryder Rude

yes

i am just glad that GR is likely exactly true at macroscopic scales

ACuriousMind

11:57

@Mr.Feynman how disrespectful to Mr Eigen

Slereah

In french we say "propre" instead of eigen

We're not gonna let the krauts tell us how to speak

Vecteur propre, valeur propre, etc

Ryder Rude

I saw The Exorcist. it had a scary scene where she is crawling upside down

But it is slow and anti-climatic in the end. I would give it 7/10. But it is a "realistic" depiction of Exorcism compared to modern horror movies

But the characters r too stupid. the girl is able to make things levitate and the characters still wonder if she is just mentally ill

Mr. Feynman

12:25

@LuckyChouhan Yes. I think most Europeans do, before getting to more advanced stuff

@ACuriousMind I guess he was less popular than Sir Gauge

@Slereah at least that's a proper (pun not intended) translation. "autovettore" means "self-vector" literally

ACuriousMind

12:42

@Mr.Feynman that's what the German word means, too!

Slereah

one fun thing is that particle names in Japanese are not loan words

[although they are calques]

電子, small lightning, 陽子, small positive, 中性子, small neutral, 光子, small light

although that's mostly just for the big ones

more obscure particles are just loans

Ironically not even naming the pion after Yukawa

Mr. Feynman

13:23

@ACuriousMind Wait doesn't it mean "proper"?

Ah, wait, maybe it's the other nuance of "proper"

@Slereah The process of combining kanji to create words that do not have a Japanese version is a complex task

Apparently, even if Japanese borrowed kanji from Old Chinese, since they were closer to West, they were the first to introduce some words as a combination of already existing kanji; thereafter, China borrowed these words :P

Slereah

one thing that surprised me a little is that Japanese for glass is a loanword from dutch because they had not discovered it apparently

I guess making glass is less obvious than you'd think

Mr. Feynman

The way they are formed is cute, though. For example. Take the very first kanji you wrote, 電. Semantically, this is related to electric stuff (lightning). If you combine it with 話 (speak), you get 電話 (phone) $\sim$ talking via electricity

Slereah

the electrophone

Mr. Feynman

I even learned recently that they didn't have words for consciousness so they had to combine Kanji to create new ones. Anyhow, as of today they tend to just take loan words and write them in katakana, mostly

Incidentally, 電話 was one of the words that China later borrowed (but it's written a little bit differently now due to simplified Chinese)

Final fun fact before disappearing: if you instead you combine it with 気 (spirit), you get 電気 which is electricity.

ACuriousMind

14:32

@Mr.Feynman yes, it's the sense of "proper time" (German sometimes Eigenzeit) and "property" - the vectors are "owned by the operator"

Mr. Feynman

And my dumbass hoped polite vectors were a thing in Germany

qwerty

@Mr.Feynman a computer is an "electric brain" in Chinese, which I always thought was unrelated to anything in English, but I was watching some American/British documentary clips/adverts archived from the 1950s(60s?) which referred to computers as "electric brains" (or something very close, I forget) and I was completely shocked that perhaps it may have been a direct translation

Slereah

In French it is an ordinateur, the one who puts things in order

which is originally some ecclesiastical term

^the computer

Basically a bureaucrat sort of position

Mr. Feynman

14:49

@qwerty interesting that for that word, Japanese use コンピューター (konpyūtā) instead

qwerty

@Mr.Feynman classic. not too surprising, in e.g. hong kong you will get lots of transliterations(? is that the right word) alongside translations as well

Slereah

it is the word yes

More Anonymous

17:46

Wouldn't having boundary terms on the einstien field equations affect the geodesic equation?

Slereah

why would it

imbAF

18:33

Hi, one question. We say that the BZ is the wigner seitz cell in resciprocal space. When constructing it, we consider some origin in the reciprocal lattice and, while respecting the Brillouin condition we define the first BZ, which encloses this reciprocal lattice point.
Then I asked my professor if the number of $\vec k$ values coincides with the nr. of lattice point inside the BZ and he said yes. E.g if one were to consider the simple case of the reciprocal lattice of a simple cubic lattice, the BZ would be a cube with dimensions $-\frac{N}{2}-1,\frac{N}{2}$. We considered an example wher…

Mr. Feynman

18:58

@imbAF the number of momenta inside the 1BZ is one thing and points of the reciprocal lattice are another thing

imbAF

19:28

@Mr.Feynman the nr. of momenta is equal to the nr. of lattice points within the BZ

Mr. Feynman

19:56

Which doesn't contradict my sentence

Relativisticcucumber

given the conductivity tensor $\sigma = \sum_n \sigma^{(n)}$ where $\sigma^{(n)} = e^2\int \frac{dk}{4\pi^3}\tau_n(\varepsilon_n(k))v_n(k)v_n(k)(-\frac{\partial f}{\partial \varepsilon})_{\varepsilon = \varepsilon_n(k)}$ i am trying to show that, at $T=0$, the conductivity of a band with cubic symmetry is $\sigma = \frac{e^2}{12\pi^3\hbar}\tau(\varepsilon_F)\int dS \vert v(k) \vert$. i have the $\tau$ figured out and the constant nearly there, [...]

[...] but i keep getting $v_n(k)^2$ inside the integral, not $\vert v(k) \vert$ and for the life of me i cant figure out how to resolve this

ACuriousMind

it seems we have entered the electrodynamics & condensed matter era of the chat

Relativisticcucumber

i have entered the condensed matter era of my life

ACuriousMind

my condolences/congratulations (choose as appropriate)

Relativisticcucumber

acm, i have a q for u. next semester i am taking cmt II. i am thinking of taking qft I, but one of my profs said if im interested in quantum optics/CM experiment that i shouldnt take qft. i said id hoped it would illuminate my understanding of CMT, and they replied that it will not. what do you think about this?

ACuriousMind

20:10

Well, first of all I wouldn't ever say anyone shouldn't take a course they're interested in :P

Relativisticcucumber

lol ok i think he meant like its not the choice he'd advise. i might be paraphrasing wrong

ACuriousMind

How useful this is for condensed matter specifically I cannot judge because I have essentially no understanding of condensed matter except for viewing it through the biased lens of hep-th QFT

Relativisticcucumber

as someone not interested in hep-th i question the usefulness of qft for my research directions. but on the other hand, i kind of feel like how can i call myself a physics phd student if i havent learned qft.

but i have seen consequences before resulting from my refusal to take advice and my insistence on figuring things out myself.

i am hovering over the rabbit hole but im not sure i wanna jump in

ACuriousMind

well, is this a "I have time for one more course and it can be QFT or something else" or a "If I really stretch myself I can fit in QFT somehow" type of decision :P

Relativisticcucumber

i think its "I have time for one more course and it can be QFT or something else"

ACuriousMind

20:24

so what are the "something else" alternatives you're considering?

user430580

@Mr.Feynman I want to be so famous and recognizable one day that people will use my name like that (I guess having a less generic username would help me a lot). I guess if someone say a very, very bad and recycled joke, one could say "oh god, that sounds user430580ish" :P

Slereah

i refuse to be condensed matter

Relativisticcucumber

@ACuriousMind well i know what electives i want to take in total, and i have time to take them all so its more like "do i want to add qft to the list or devote that time to research" bc the amt of electives im taking meets the minimum requirement eitherway

Slereah

I will forever remain a diffuse substance

More Anonymous

@Slereah cause now you have non stationary endpoints on the action

ACuriousMind

20:29

@Relativisticcucumber then I'd say you can spend the rest of your life doing research but probably not taking courses and later you'd have to learn from a book without a course, so take the course

(I feel compelled to add a "this is not career advice" to this in the same vein as people clearly giving financial advice say "this is not financial advice" to avoid liability)

Mr. Feynman

@ACuriousMind you are clearly speaking as a quantum finance expert now

@user430580 one day you'll be famous like me, young user

Relativisticcucumber

@ACuriousMind that is a v good point

Mr. Feynman

@ACuriousMind this reminds me of why I decided to take a superconductivity course and the professor seemed to be happy because that year there were only theoreticians, so she could make a more "theoretical" course

Then some claims gave me reasons to be believe she thought that varying the action and using EL were two different methods to derive the EoM, so I dropped the course

And when I say "dropped" I mean the lectures. I still have to study that stuff but I hope it will come natural after learning a fair deal of condensed matter at this point

God, why does all my academic self-lore always end like that?

user430580

20:48

BTW could somebody please help me familiarize with this chat room's lore? What does it mean if someone says "MIAO MIAO this~~" "MIAO MIAO that~~"?

Relativisticcucumber

LOL

i think nI uses miao miao to mean "me"

user430580

But why, is there an interesting background lore, a legend maybe?

ACuriousMind

@Mr.Feynman what do you mean "there were only theoreticians"?

Relativisticcucumber

@user430580 i dont think so -- mean maybe they have a personal reason

user430580

ok thanks

Relativisticcucumber

20:50

but iirc they just were like that even prior to joining SE lol

Mr. Feynman

@ACuriousMind that during that year the students taking that class all were in a theoretical curriculum, unlike the previous years

ACuriousMind

I find it a bit strange that the prof would even have that information :P

and even then this is strange - I would expect the "theory level" is a fixed property of the course

Mr. Feynman

@ACuriousMind she asked. Oh, right. Context. If you have a dozen students you can ask :P

@ACuriousMind I think it was more about making some hints at more advanced stuff. The result was one of the most phenomenological courses I've ever seen

Which isn't bad per se, but that course was :P

@Relativisticcucumber did you realize NOW?!

qwerty

21:22

@user430580 nI is a cat, as they have mentioned many times

I have a feeling that if further questions are in order they should be addressed directly to him :p

@Slereah I am dense and doesn't matter

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