The h Bar

0celouvsky

17:07

@BalarkaSen Do you know what this cryptic statement means? "It simply refleets the fact that there are "more" rational functions with prescribed poles than there are polynomials with "corresponding" zeros. See also Section
III.4.G, the Riemann-Roch Theorem."

Balarka Sen

What's the context?

0celouvsky

@BalarkaSen Index of a certain PDE

Balarka Sen

Huh. I have no idea.

Jim

@Secret sometimes measuring it artificially changes the topic state entirely

user228700

17:24

@Yashas: I figured that you might not be bothered to read a whole article about this seeing as you already don't like reading so here, this article contains pictures, not words, about people encouraging reading:

user228700

buzzfeed.com/jarrylee/…

Secret

One thing quantum mechanics seemed to tell little about is what interaction will become a measurement
(be it project the wavefunction to eigenstates (Copenhagen), probability of outcomes given input settings of instruments (operationalist), splitting of worlds in the universal wavefunction (Many worlds interpretation), nonlinearities (Objective collapse), argument does not apply to epistemics approaches (quantum bayesian, quantum logic, ) as measurement is just a way to gain information from the system and not really a physical thing, anything else not mentioned I don't have enough background)

Relational quantum mechanics

This article is intended for those already familiar with quantum mechanics and its attendant interpretational difficulties. Readers who are new to the subject may first want to read the introduction to quantum mechanics. Relational quantum mechanics (RQM) is an interpretation of quantum mechanics which treats the state of a quantum system as being observer-dependent, that is, the state is the relation between the observer and the system. This interpretation was first delineated by Carlo Rovelli in a 1994 preprint, and has since been expanded upon by a number of theorists. It is inspired by the...

user228700

And if you do feel like reading, here is an article talking about the importance of reading fiction:

user228700

forbes.com/sites/ehrlichfu/2015/06/14/why-read-fiction/…

Secret

This one is also interesting because it seemed to be what susskind is hinting at in his theoretical minimum book when he talked abotu how the instrument entangles with the observers

John Rennie

17:27

@Kaumudi.H: hi. Yes, I arrived safely at my Mum's house :-)

user228700

@JohnRennie Good to know! :-)

John Rennie

A long drive, then I had to take my Mum shopping! So I'm a wee bit tired right now :-)

Balarka Sen

i know some people who read, but not fiction. they find fact stranger than fiction i suppose

user228700

@JohnR: So FBAWTFT was OK?

John Rennie

I thought some of the plotting was a bit wayward.

user228700

17:30

Exactly which parts of it are you talking about?

John Rennie

Would Newt and Tina really have been executed after what amounted to a kangaroo court?

DukeZhou

@Kaumudi.H Thanks for posting that Forbes article! I read academic literature, but it was creative lit that ultimately led me there.

John Rennie

And the swapping of cases is straight out of any number of slapstick comedies.

In many places the script gave the impression that the film was relying on spectacle rather than story.

user228700

@JohnRennie No, they certainly wouldn't have but Graves was (spoiler alert) really Grindelwald and he knew that Newt was onto him.

user228700

@DukeZhou Uhh, no problem! :-) Ah, I see.

user228700

17:32

@JohnRennie Sadly, yes.

John Rennie

@Kaumudi.H Yes, but the aurors wouldn't just have done what Graves/Grindlewald said.

I guess I'm sounding negative, but I did enjoy reading it.

user228700

Yes, but...hmm, I'm not too sure if Picquery would've been upset about the execution but on second thoughts, she may have; even though she couldn't have cared less about Newt, Tina wasn't a nobody. Even after all the rule-breaking, it did seem like they still had a great amount of mutual respect for each other and right, an execution was just too harsh.

John Rennie

It's just that compared to a work like Goblet of Fire it falls far short.

DukeZhou

@Kaumudi.H Mythology is my original field, and the insights or artists are valuable in my experience (Hesiod, Aeschylus, Euripides, etc.) SciFi is arguably quite useful.

John Rennie

For some reason I got the mental image of Newt as a much older man. This was before I discovered Eddie Redmayne was playing Newt.

DukeZhou

17:35

(so long as people eventually come to the mathematics)

user228700

@JohnRennie How old?

John Rennie

Thirties ish

Balarka Sen

It's complicated to say something like, "history or fiction?" in general. Would I read Lu Xun's A Madman's Diary about a man hallucinating a cannibalistic society around him after reading Zheng Yi's Scarlet Memorials, where you actually see cannibalism happening in Red China, people tearing out people's internal organs and take them with tea?

user228700

@JohnRennie He was 29...

Balarka Sen

I just think I read fiction because surreal makes more meaning than the real for me.

DukeZhou

17:37

@ZeroTheHero I was just reading your answer on quantum states (physics.stackexchange.com/a/326159/152803) and was wondering if this would be an appropriate Stack to ask about the Free Will Theorem?

user228700

@JohnR: What did u think about the Obscurial/Obscurus?

John Rennie

@Kaumudi.H Ah, so I wasn't too far off. In the stills I've seen from the film Eddie Redmayne looks much younger.

@Kaumudi.H I thought it was left rather vague exactly what the Obscurial was and how it interacted with its host.

DukeZhou

@BalarkaSen Cannibalism doesn't seem to have the same stigma in Chinese culture as in the West (Water Margin vs. Atreus and Thyestes, for instance)

John Rennie

I guess there simply isn't time to go into all the details in a film.

user228700

Hmm, I feel that you've approached this as a proper novel rather than a screenplay and if you watch the movie perhaps you would like it a little better because while watching it, it didn't seem like there was any need to explain all that in further detail.

Balarka Sen

17:40

@DukeZhou I agree. History of Chinese society is pretty violent in general.

Or so I feel.

John Rennie

That's always the risk you run reading a screenplay.

user228700

Ohh, but you didn't get to see the mating dance, did you?!

user228700

JKR wrote "Newt performs mating dance" and left it up to Newt and the directors to figure out that bit!

John Rennie

@Kaumudi.H I'm becoming uneasy with the way this conversation is going :-)

user228700

No no, it's actually quite amusing. Here:

user228700

17:42

Fantastic Beasts and Where to Find Them | Mating Dance

►

John Rennie

Wasn't that the hippo/elephant creature, not the obscurial?

user228700

Yep, that's the erumpant.

John Rennie

That's a real test for an actor - attempt to seduce a large herbivore and make it look convincing :-)

user228700

Sigh, but I'm guessing you haven't been convinced to watch the movie, huh?

DukeZhou

@BalarkaSen I suspect that has a lot to do with scarcity (but, as a game theorist, I take an economic view of everything;)

John Rennie

17:44

I have no plans to watch the film ...

user228700

:-( You're missing out but OK.

DukeZhou

Although, sometimes, in the the 4 Classical novels for instance, over the top behavior is often allegorical

John Rennie

@Kaumudi.H well, I know how it ends now :-)

user228700

:-) Have fun at your mum's! I should get ready for bed.

John Rennie

And while we're talking about plotting - the man and woman who start out hating each other end up falling in love. My goodness, there's a new idea in plot development :-)

user228700

17:46

@JohnRennie :-) I bought that because of the actors.

John Rennie

How did you get on reading on your phone?

DukeZhou

@BalarkaSen There's a famous version of the Peach Garden Oath where Xuande reprimands Zhang Fei and Guan for pledging themselves when they still have families to consider (i.e. how can they earnestly put their allegiance to the brotherhood first?) They offer to kill their families to prove their commitment, but that is seen as poor conduct. The solution: Guan will kill Zhang's family and visa versa.)

Totally not meant to be taken literally

user228700

Not well .__. I plan to download an app to reduce the blue light given off by the phone because I wasn't able to fall asleep for a while after reading my allotted two pages for the night.

user228700

But the reading itself was alright. I think I read far more than just two pages (in the actual book) though; the font was really small :-)

user228700

^ How is this a useful/interesting comment? :-P

John Rennie

17:54

@Kaumudi.H After the end of the world or the end of the exams, whichever happens first, we really need to sort you out with a tablet to read on :-)

DukeZhou

@BalarkaSen At risk of this being a stupid question, does topology extend to connections between geometric objects? (I've read a little about sphere packing, and the problem I'm working on involves the connections of cells and regions in Sudoku-class Latin squares. It's really easy in 2D and 3D, but I'm also trying to understand how it works in 4 dimensions...

user228700

> A tablet to read on

user228700

For a moment, I was really confused because I take "tablet" to mean "pill" :-P

ZeroTheHero

@DukeZhou Can you provide me with a link so I can be sure of what you mean? as in a link to reading material

John Rennie

@Kaumudi.H :-)

DukeZhou

17:57

@ZeroTheHero en.wikipedia.org/wiki/Free_will_theorem

user228700

Don't you have neighbors who'd love a tablet? :-P

John Rennie

Actually no. Much like laptops I have saturated the market.

user228700

:-P Aren't you glad you've found a girl all the way over in India who used to be in possession of...no good electronics, then?

user228700

Ooh ooh, new idea! YOU COULD DONATE THEM TO AN ORPHANAGE!!

Anonymous

@Kaumudi.H I don't find that article giving any evidence of a person who reads fiction being more developed/ living a better life than a person who doesn't read fiction. Not very convincing. Personally I believe that fiction is good mostly for entertainment purpose just like television is. There are very very few good fiction books which actually change your view of the world.

John Rennie

18:00

Hmm, I wonder if there are any orphanages in Chester. I'm not sure if the UK still runs orphanages at least in the Victorian style.

user228700

> There are very very few good fiction books which actually change your view of the world

DukeZhou

@blue have you read Hannu Rajaneimi's Quantum Thief trilogy?

user228700

That would be wrong not only because...well, that hasn't been the case for thousands of people in the world and secondly, as you said before, you haven't read very many novels, have you?

Anonymous

@DukeZhou No. Is it nice ?

Anonymous

@Kaumudi.H I used to read fiction, a LOT upto class 10.

John Rennie

18:02

@DukeZhou I have read the first two books.

user228700

@blue It depends on what you've read, then.

Jim

@blue read some good sci-fi stories. Asimov and the like. I've found them to always be a subtle source of inner humanity for those least seeking to find it

DukeZhou

@blue it's badass imo, and Rajaneimi is interesting in that he holds a degree in mathematical physics

ZeroTheHero

@DukeZhou This is real intense and I would need to read more. I've heard of the result but I am not an expert so I would not provide very knowledgeable points for discussion. I would recommend you post a carefully crafted question.

DukeZhou

@ZeroTheHero You're understanding definitely exceeds mine though. I will put my mind to useful questions

Anonymous

18:03

@Kaumudi.H Indeed it does. However, I still don't have any evidence that a person who reads fiction is more developed/knowledgable/mature than a person who doesn't.

Anonymous

Have you come across any fiction book which has changed your life in a considerable way?

Jim

@blue none exists. It's not a necessary condition. The question is if it will improve you; not everyone

DukeZhou

@blue Euripides The Trojan Women

the impact of that play is not to be underestimated

Anonymous

@DukeZhou I'll read it. So, how did it impact your life? :-)

DukeZhou

nor is the impact of Aeschylus' The Oresteia

ZeroTheHero

18:06

@DukeZhou I wish I could be sure of this. I'm sure there are people here more knowledgeable than I.

user228700

Well, I will give you some evidence by submitting that me and several of my friends have learned many valuable lessons from books but among all, I have learned to be more empathetic. Knowing about the stories of people I've never met from places I've never been has allowed me to "escape the labyrinth of my own consciousness" for brief moments in time and have enabled me to image everybody as complexly as I possibly can.

DukeZhou

Not only my life, but both works impacted Western civilization significantly

Jim

There's a nice quote I like: 'Science fiction is an existential metaphor that allows us to tell stories about the human condition. Isaac Asimov once said, "Individual science fiction stories may seem as trivial as ever to the blinded critics and philosophers of today, but the core of science fiction, its essence, has become crucial to our salvation, if we are to be saved at all."'

Balarka Sen

@DukeZhou Well, depends on what you mean by that.

DukeZhou

@Jim that's why Phil Dick is arguably at the top of the heap

user228700

18:08

(Also, I feel like a lot of you are talking about SF which is not the only type of fiction there is)

Jim

@DukeZhou eye in the sky?

Balarka Sen

Geometry usually has a sense of "measure" in it. A metric, eg.

Topology does not.

DukeZhou

Do Androids Dream of Electric Sheep is potentially the most important philosophical narrative of our times

ZeroTheHero

newscientist.com/article/…

DukeZhou

(at least where artificial life and intelligence are concerned)

Jim

18:08

@Kaumudi.H we are scientists and we are talking about fiction. The inevitability seems obvious

DukeZhou

and it's all about the human condition

the nature of identity in relation to reality and subjectivity

Anonymous

@Jim Yes, just like there are some impactful movies. I know only 2 movies which impacted my life in a considerable way. Rest of the movies I watch are mostly for entertainment purpose and not to develop myself. I'd be better off reading physics or maths books.

DukeZhou

and he was writing about Evolutionary Game Theory 5 years before the field was formalized

John Rennie

@DukeZhou I don't think Rajaniemi is a good storyteller. He lets the tech get in the way of the story. The reason I've only read the first two books of the Quantum Thief trilogy is because I wa too bored with them to want to read the last book.

DukeZhou

@JohnRennie Fair enough, but the last book is the best

The All-Defector, and so forth

John Rennie

18:10

I read the first few chapters of the last book, and they didn't seem promising.

Balarka Sen

Fiction changes views and perception, for sure.

Jim

@blue books are often more detailed and lengthy than movies. If there are movies that have impacted your life, isn't it reasonable to believe that there can be books as well that would impact your or others' lives

Anonymous

@Kaumudi.H That's nice. Although I'm not quite sure that you wouldn't have come to the same level of mental maturity had you not read those books. Ah, this seems like a good debate topic :-)

DukeZhou

@JohnRennie What I like about Rajaneimi's trilogy is that the underlying theme can be said to be the unsatisfactory nature of a binary universe

user228700

I am deeply passionate about this subject but I'm afraid I absolutely must get to bed now. However, before I go, @blue: I will share with you-a virtual stranger-a deeply personal experience in the hopes that you will start believing that fiction is important; knowing about tales of suffering and courage displayed by the characters in Harry Potter made me feel hopeful after months of crippling depression.

Jim

18:13

math and physics books (etc) are great for many situations. But fiction adds elements that you can't get from straight knowledge. It also adds life and character

Balarka Sen

@Kaumudi.H That depends on the person reading the books, really.

DukeZhou

@Jim I'd concur by pointing to the influence of narrative philosophy. Half the US is obsessed wit Ayn Rand (being hyperbolic, but you get my point)

John Rennie

@DukeZhou Really? I suspect only a tiny fraction f Americans have even heard of Ayn Rand let alone read any of her books.

Balarka Sen

I do not find that life and death and deeds of characters from fictions influence me. The magic realism, nonsense, chaos and overwhelming search for meaning does.

DukeZhou

There's a theory that we understand reality in a narrative context. Narrative is certainly influential in terms of propagation of ideas.

user228700

18:16

I haven't ever had a proper mentor and books taught me all the things I value now. If you have had a person to teach you these things, fiction might not matter as much, which is how I guess it depends on the person, @Balarka but there is no doubt that books have important messages to impart and imbibing those messages does increase the value of one's life. I feel sorry that you haven't still come across a book that has taught you stuff, @blue.

DukeZhou

@JohnRennie Her philosophy underlies the current strain of libertarianism, and is widely referenced in popular forums

Anonymous

@Kaumudi.H That is great. Point taken. I suppose what I believe is "Any man who reads too much and uses his own brain too little falls into lazy habits of thinking". It depends completely on the person as to how much they can extract/learn from a novel. I haven't though about the HP novels so deeply as you. There is more than what meets the eyes in those books. So a person should not just read books. It is much more important to think about what you have read and apply it in real life.

Anonymous

Unless one is doing that they are simply wasting time!

Jim

A world without fiction isn't one I could live in

@blue ah, that's a good statement there

John Rennie

@DukeZhou well, OK. The libertarian movement has never been of interest to me so I can't comment.

Jim

18:18

but sometimes, a break is nice

user228700

@blue I agree; if you're not going to think about what you've read and engage in productive conversations about the same, then that's only half the experience for me.

DukeZhou

@JohnRennie :) I find it necessary to monitor, since my preoccupation is economic games and automata

Balarka Sen

You seem to be talking about the ethics and life-lessons, which I find are rather uninspiring matters in a fiction. I guess I have non-standard views about things.

DukeZhou

Not against the ideas per se, but "moderation in all things" is a maxim for a reason

Anonymous

@Kaumudi.H Yes. I think I will change the way I framed my sentence. Don't just read fiction. Learn from it. :-) (Unless you are just reading it for entertainment....which is also important sometimes :-P)

Balarka Sen

18:21

@blue Not everything that is read and understood can be applied in real life.

I find that sentence to be a little silly sounding tbh

Anonymous

@BalarkaSen But one should try to. That's my point.

DukeZhou

I might say "be inspired by fiction"

Jim

@BalarkaSen you don't need to think on the obvious ethics/lessons presented. But observing how a person writes, the elements that become appealing, or the general situations of the characters can teach a lot about life, even if the surface messages are useless to you. Reading is as much about how it was said and what wasn't said as it is about the words on the page

DukeZhou

then try to be useful

Any Neal Stephenson fans here? Orbital mechanics was a central theme of SevenEves

Balarka Sen

@blue That trying is totally useless for the elements of fictions I have in mind. Do you have anything other than ethics/life-lessons in mind when you say this?

The philosophy in fictions is to be understood, not applied. What does it even mean to apply existentialism in real life, say? :P

user228700

18:25

You know, I think I've been fighting this battle on the wrong hill; fiction or non-fiction, I believe that reading adds a great amount of value to one's life if one is perceptible and-adding from blue's point-thinks critically about it. I guess I didn't go there because I didn't want for people to point out that Feynman's lectures are technically non-fiction because what I mean by that is just stories; of people and places and things.

Jaime Gallego

@DukeZhou I have read Crypto, Seveneves, and Snow Crash

John Rennie

@DukeZhou I've read lots of Neal Stevenson's books, but I must confess SevenEves looks a bit over long to me.

And the reviews haven't been great.

DukeZhou

@BalarkaSen The reason I like Rajaneimi is his trilogy is about how Game Theory might be applied in a post "singularity" context, where matter and information are interchangeable

In otherwords, in a reality that can be said to be algorithmic

Anonymous

@BalarkaSen See, it is completely okay to read fiction for entertainment only! But if you are reading loads of fiction only for entertainment then probably you are wasting time. It is not possible to apply everything you have read. That's certainly not practical. But you should try to, as much as possible....so that it impacts your life in a positive way.

DukeZhou

Ethical issues are explored, but it also looks at the mechanics

Balarka Sen

18:27

@blue You think fictions which do not present constructive ethics and life-lessons (whatever that means) is entertainment? Then I am amused how wrong-headed an idea that is.

DukeZhou

@JohnRennie I think SevenEves it's his finest work to date, but that's just me

He's not as refined as Gibson, but I love his emphasis on usefulness

user228700

I need to go to bed. @JohnR: I'll re-consider that tablet deal! :-P Goodnight! Have a nice time at your mum's.

Anonymous

@BalarkaSen Maybe we have different definitions of entertainment? :-)

Balarka Sen

Perhaps? What is entertainment to you?

DukeZhou

His allegory about choosing emotion over reason (not listening to scientists in areas where scientists are the only qualified contributors)

John Rennie

18:29

@DukeZhou Anathem is the book I've enjoyed most of his, though Snow Crash was also very good.

DukeZhou

@JohnRennie Anathem is my 2nd favorite, recently displaced, and Snow Crash has that ineffable quality that elevates great work

Anonymous

@BalarkaSen amusement, pleasure, leisure, relaxation, fun, enjoyment, refreshment, diversion....

DukeZhou

@JohnRennie The train ride over the north pole in Anathem is one of my favorite parts for some reason

@blue I use trash fiction to shut my brain off so I'll stop thinking

Balarka Sen

@blue Sounds like it agrees with my definition. Maybe we have different exposures of fiction? :P

user228700

A clarification: It seems that all this while, I've really only been talking about reading any kind of book but I've been using "fiction" as a header because I didn't want to get into debates about how even textbooks are books! But again, a large number of books out there are fiction so heh.

Anonymous

18:33

@BalarkaSen We can continue this debate some other day. Got to go to bed :-P

Anonymous

It was a good debate though

DukeZhou

(I don't mean that pejoratively, just meaning books and shows that are purely entertainment, as opposed to scientific or philosophical)

Balarka Sen

Sure enough. I think this debate is pointless.

So whatever

DukeZhou

Very interesting folks, here on physics!

Balarka Sen

Everyone who hasn't read Borges, try reading Borges sometimes

DukeZhou

18:35

What Balarka Sen said

Balarka Sen

That's one of the modern fictions which explores things other than merely entertaining science fiction or life-stories (most boring genre of fictions IMO).

user228700

@BalarkaSen Not entirely. I think it's important that @blue change his mind that reading doesn't improve the quality of one's life, which is why I got into all this at 12 AM.

DukeZhou

Someday I'd like to start a discussion on the mathematical analysis of enigmas

@BalarkaSen what genres do you favor, if I can ask?

Anonymous

@Kaumudi.H Let me reframe: Only "reading" doesn't change one's life. "reading+thinking" does :-) Goodnight!

Balarka Sen

@Kaumudi Ok. We'll all die anyway, so

DukeZhou

18:37

what Balarka said

Anonymous

lol

Anonymous

;D

Balarka Sen

@DukeZhou I like artsy stuff. Magic realism, surrealism, symbolism, chaos and dystopia.

DukeZhou

Nice

As a mythographer, I need to keep up with both the high and the low

but mad respect

user228700

@blue That's bullcrap, really. Unless the book in question is stuff like Twilight (and it can be argued that one can think critically even about that one), the experience of reading is not complete without internalising what you've read by thinking critically about it.

DukeZhou

18:40

@Kaumudi.H I only read the first Twilight, just to see, but I thought it was a good take on "just because you're a monster, doesn't mean you have to act like a monster"

user228700

Also, @BalarkaSen: maybe you'll enjoy this: (:-P)

user228700

We're All Gonna Die - A Song

►

Anonymous

@Kaumudi.H I've seen tons people whose "the experience of reading is not complete", like you said. That's why I brought up this point.

user228700

@DukeZhou That's nice :-)

user228700

@blue I see. That has not been the experience for me and most people I know, which is why I was really confused but OK.

Balarka Sen

18:43

i find that most people don't think or read

DukeZhou

what Balarka said

Jaime Gallego

Hey, what are the odds -- five Ayn Rand fans on the same train! Must be going to a convention.

Balarka Sen

not thinking or reading is fine, tho. that wasn't really a derogatory comment :P

DukeZhou

@JaimeGallego excellent cartoon

In the words of Socrates: "I am only not full of sh-t in knowing I am full of sh-t"

Anonymous

@Kaumudi.H Considering the fact that majority of the people whom I see addicted to fiction, simply mugging up formulae for their physics and maths and chemistry and....I've developed a doubt about whether they really stop to think about the novels they are reading....Because I haven't seen that habit being reflected in other aspects of their life. Need to collect more evidence though:-P

Balarka Sen

18:53

That's not a good reason to suspect they don't think but I also don't find a lot of people I know addicted to fictions having great philosophical perceptions of life.

Anonymous

@BalarkaSen My experience is quite similar tbh

DukeZhou

as mine

0celouvsky

@BalarkaSen help

I'm in a planar algebra seminar

Balarka Sen

what's that man

0celouvsky

No clue

It's like a complex of C* algebras on which these diagrams act

Slereah

19:49

@0celouvsky No

Can I downvote a comment

I'm getting dumb comments on my question

DukeZhou

Unfortunately, no.

Bernardo Meurer

@0celouvsky I saw that, I hate U2

@0celouvsky What kind of labor?

DukeZhou

your only defense is to take the high-ground or to smack them down by showing them to be unknowlegable

loltospoon

20:28

Hello all

0celouvsky

@Slereah are you sure that homogenity implies isotropy?

For some reason I thought it was the other way

Slereah

Yeah I checked, I was wrong

0celouvsky

@BernardoMeurer she hasn't said

@Slereah where did you check?

Slereah

Homogeneity and isotropy at one point implies isotropy at all points

0celouvsky

I think I have a proof on PSE

Slereah

20:39

And isotropy at every point implies homogeneity

0celouvsky

Yes, that's actually pretty hard to prove

I don't have that proof

Slereah

I think it's in Stephani

0celouvsky

It's in an old paper by Straumann

(in German)

Slereah

What's a homogeneous spacetime that isn't isotropic

Oh wait

Godel?

0celouvsky

is that homogeneous?

Slereah

20:46

yeah

It's the only homogeneous fluid solution with 5 Killing vectors, even

Balarka Sen

@0celouvsky how do you think about the geodesic field?

0celouvsky

geodesic...field?

Balarka Sen

lemma 2.3 in chapter 3, do Carmo. a vector field on $TM$ whose trajectories are $(\gamma(t), \gamma'(t))$ for a geodesic $\gamma$ on $M$.

0celouvsky

oh, the spray?

Balarka Sen

is that what it's called? then maybe

Slereah

20:53

the ol' geodesic spray

IIRC you can use it to do GR in the optical approximation

Where curvature can be described as "lenses"

Balarka Sen

i am not sure i buy this field at all

Slereah

why not

0celouvsky

@BalarkaSen it's a section of $TTM$, right?

Balarka Sen

seems like it

0celouvsky

so a spray is a second order differential equation

it allows you to encode both initial conditions in the base of the bundle $T(TM)$

for regular flows you really only care about the initial position

but for geodesic flows you want to account for the initial velocity as well

Balarka Sen

20:57

right... i guess i can't see it

0celouvsky

@BalarkaSen you understand that a section of $TM$ is just an ODE, right?

Balarka Sen

Yeah, sure.

0celouvsky

And a second order ODE is just an ODE of a first order one

that's where the idea came from

I don't have a "picture" in mind because I have no clue what the double tangent bundle looks like

but the spray is used in kinetics on manifolds, for instance

it very conveniently lets you encode two initial conditions in one object

Balarka Sen

@0celouvsky I see. That's not a bad perspective.

0celouvsky

@BalarkaSen do Carmo's proofs can be done without $G$, btw.

For instance Petersen has a very different approach to this whole deal

Don't try to read Petersen, my advisor and I spent a week trying to figure out what he was up to haha

But there are different approaches to this

Balarka Sen

21:03

Also I suppose by section of $TM \to M$ being an ODE you really mean that over a chart it looks like $x_i \mapsto (x_i, \sum a_i \partial/\partial x_i)$, so integral curve has to satisfy an ODE. For a section of $TTM \to TM$ all of this happens second order.

0celouvsky

Yep

Balarka Sen

Gotcha.

@0celouvsky Yeah, I am reading Gallot-Hulin-Lafontaine and I think they do this differently.

0celouvsky

@BalarkaSen Btw this result is true on any vector bundle connection. There's always a geodesic spray.

Balarka Sen

Certainly.

0celouvsky

Also, given a spray, one can find a connection such that its integral curves are geodesics of the connection.

Balarka Sen

21:05

Oh, that's nice.

0celouvsky

A spray (without qualifications) is just a fancy term for a section $TM\to TTM$.

Slereah

I'm guessing connections for sprays aren't unique

Since torsion doesn't change geodesics

0celouvsky

They're nonunique up to torsion.

Balarka Sen

Yeah, meh torsion

0celouvsky

That's rather nontrivial. I think I learned this from Spivak.

@BernardoMeurer I need assistance, text me please.

@ACuriousMind Hilfe hilfe

No sharks but still

ACuriousMind

21:21

@user929304 Hey :) Yup, all is well. Alas, I'm afraid I'm not entirely sure what your question is - I feel you're missing nothing but I can't tell why exactly you think you're missing something

@0celouvsky what troubles you?

0celouvsky

@ACuriousMind I have to compute Berry's phase for a precessing neutron in a magnetic field

What exactly is the interaction there

Slereah

Isn't it just $\vec \sigma \cdot \vec B$

I actually have a whole book about neutrons

I don't remember where I got it

I never even read it really

ACuriousMind

^I concur with Slereah pending further details

Slereah

If you want the QFT version you have to do the whole sigma model shenanigans

0celouvsky

there are no details

they want me to compute the Berry phase??

Slereah

21:29

Who's Berry

0celouvsky

the result is apparently $\frac{1}{2}\Omega$

where $\Omega$ is the solid angle subtended by the neutron

halp?

@ACuriousMind what is $\ket{n,t}$ even supposed to be here?

ACuriousMind

@0celouvsky $\sigma\cdot B(t)$ has two eigenvalues "up" and "down", $n$ denotes which one, $t$ at which time

0celouvsky

how do I even compute this?

any of it

oh wait, there might be something in the book

@ACuriousMind Do you know about planar algebras?

There's something called a "no ghost" theorem and the guy said the name comes from QFT

@BernardoMeurer youtube.com/watch?v=GD_E3t-Louo

ACuriousMind

21:46

@0celouvsky I've never even heard of them

bolbteppa

@BenNiehoff That thing looks incredible, maybe will be able to understand the jist of it during the summer :p

Slereah

I ain't fraid of no ghost

0celouvsky

@ACuriousMind What would be the magnetic field in this problem? $\propto \cos(\omega t)\mathbf i+\sin(\omega)t\mathbf j+\mathbf k$?

dmckee

Anyone else seeing a white bar at the top of the page on the main site?

0celouvsky

no

ACuriousMind

21:49

@dmckee no, but I've seen it on other sites - it's a blocked ad, I think

0celouvsky

lol

mods don't whitelist their own site

wonderful

ACuriousMind

Possibly the "hero image" they recently promised

@0celouvsky yep

dmckee

Hmm.

0celouvsky

@ACuriousMind Well, no

How do I get the $\theta$ in there?

dmckee

The inspector describes it as body.quetion-page.new-topbar

Slereah

21:53

Well

I chugged down a can of redbull

ACuriousMind

@0celouvsky As a factor in front of $k$. Figuring out which one will involve a bit of trigonometry, I guess :P

Slereah

it's time to prove that a system of two warp bubbles violates causality

0celouvsky

@ACuriousMind I don't know trigonometry.

ACuriousMind

@Slereah ...why would you chug a redbull shortly before midnight?

0celouvsky

I could use spherical coordinates, but there are two conventions there, no?

Which one should I use...

ACuriousMind

21:54

(unless you're having vodka with it, then it's okay, I guess :P)

Slereah

@ACuriousMind Oh I woke up two hours ago

Balarka Sen

bourgeois

ACuriousMind

@Slereah Dear lord, your sleep cycle is more messed up than mine

Slereah

yeah

Too bad I have to do things again in two weeks

Jaime Gallego

@BernardoMeurer Holy hell you've got Trans Europe Express

THIS IS LIKE EARLY CHRISTMAS

Slereah

21:57

the only paper with CTCs for warp bubbles is one of those very wordy paper with few equations

So I've gotta buckle down and do things by hand

Bernardo Meurer

@JaimeGallego :)

I have a lot of stuff

Slereah

I have to draw all the little spaceships

Bernardo Meurer

Once I'm out of this island I'll continue upping

So Sunday

@dmckee Are you around?

dmckee

Who wants to know? And why? It might make a difference?

Bernardo Meurer

@dmckee I need C help :P

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