The h Bar

Uhh

Did you ask Balarka? This is PhD topology picture stuff

@BalarkaSen same question

Tho I dunno if Balarka's good at the GR stuff

What is big sigma

The spacelike hypersurfaces of the original manifold

So you want to cut out two cylinders

And then double the manifold?

3:12 PM

Basically my problem is that if there's a tube going up identified to a tube going down, I don't think there's a foliation of the manifold such that the leaves wouldn't cross

Not double, just glue the cylinders

I can cobble together an argument for the time orientable case by involving time functions

but the non-time orientable case

who knows what its shape is

I don't even know what the topology is for the simple case where the original manifold is Minkowski space

I'm pretty sure it's locally similar to $\mathbb R \times (\Sigma \# T)$

So I'm guessing it's some line bundle

How many line bundles are there, anyway, just two?

Is it always the trivial line bundle and the $\mathbb Z_2$ one?

hmm. Is there an easy way to get a slightly larger version of a latex symbol, e.g. a bit bigger version of $\epsilon$?

Although really I'm not even 100% sure it's even locally like that

I can't picture such a foliation

@Semiclassical o.O

why would you need that

@Semiclassical \large?

I'm trying to decide if I think the default version is too small, especially when I'm writing stuff like $D=\epsilon E$.

3:25 PM

$\epsilon \large{\epsilon} \Large{\epsilon} \LARGE{\epsilon} \huge{\epsilon} \Huge{\epsilon}$

Anonymous

$\Large{\epsilon}$

so for comparison it'd be $D={\large{\epsilon}} E$

@Semiclassical wtf

yeah, that's not right

it's making E bigger as well

the default is not too small

stop trying to change the natural order of things

3:26 PM

pfff

relevant:

dresdencodak.com/2009/09/22/caveman-science-fiction

$D=\epsilon E$ vs. $D={\large \epsilon} E$...hmm

I'd prefer $D=\varepsilon E$ but I'm not sure I should use a different looking epsilon than Griffiths does

Lunate epsilon is lame

yeah, I'm not a fan of the lunate one

I don't know why they call $\varepsilon$ the variant, because technically it's the canonical one

ya

"thus spoke Knuthustra"

huh, \large doesn't work in mathmode in latex the way it works in mathjax

okay fine I'll submit to the natural order of things

3:44 PM

Going to visit my future flat today

take measurements and whatnot

4:19 PM

I guess perhaps

The appropriate idea would be to try to find the time-orientable double cover

Of this spacetime

I suspect the double cover is gonna be some system of two wormholes

The horror

Sir Cumference

Albert Einstein was born today

Fun facts

Anonymous

3 hours ago, by Yashas

@BernardoMeurer One genius decided to start his life on this day (PI day!) while another chose to end his.

Future Historian

Gentlemen? Question: how powerful is a death beam of 42,600 joules per square metre from a Casaba Howitzer?

This is accounting for a distance of 50 km relative to the target.

So...........

Do you mean 42600 watts? The watt is a unit of power i.e. one watt is one joule per second.

42 kilojoules isn't very much.

A: Ch-ch-ch-changes: Left nav, responsive design, & themes

4:36 PM

3

Please reconsider your decision on voting buttons The voting buttons are an ever-present aspect of a bunch of sites' graphical identity, and they pull a disproportionate amount of weight in making the theming feel like a complete skin rather than just some surface-level attributes. My main site ...

^ consider supporting =)

@FutureHistorian 42 kilojoules is about the energy given off by burning one gram of petrol. One gram of petrol burning per square metre doesn't seem apocalyptic to me ...

4:49 PM

@JohnRennie Can I ask you for some express life-saving help?

@JohnRennie I wrote this whole paper last night without sleeping, it's due in an hour, can you read through it and see if you find any blatant errors? send.firefox.com/download/18844f6e80/#Kh2eX131Q78SCzOghQ3syA

Reading now ..

@JohnRennie There's an error on 1.3 Complexity Theory I'm already aware of (lingering one of) and fixing

Also weird pagebreaks being fixed

> A Turing machine is a particular form of state machine, we shall carefully construct it's mathematical structure, in particular we will use a deterministic, one-tape Turing machine, although as is well know all models of deterministic Turing machines are equivalent.

Missing full stops, and "its" not "it's"

> A Turing machine is a particular form of state machine. We shall carefully construct its mathematical structure, and in particular we will use a deterministic, one-tape Turing machine, although as is well know all models of deterministic Turing machines are equivalent.

> A Turing machine is a particular form of state machine, we shall carefully construct its mathematical structure, in particular we will use a deterministic, one-tape Turing machine. As is well known,however, all models of deterministic Turing machines are equivalent.

@JohnRennie Fixed :)

"A deterministic one-tape TM is speciﬁed by a ﬁnite set " Abbreviations without explicitly defining it? I am not sure if this matters. It's obvious in the given context but... idk if it matters

A: How wrong are the classical Maxwell's equations (as compared to QED)?

4:56 PM

-1

The problem is with one charge in empty space, even it is run with acceleration, it still cannot sent out the wave. It cannot sent out the radiation. For an emitter to send radiation wave out, it need an absorber to receive the wave. This is what the Wheeler-Feynman's absorber theory tell us. I...

what?

@Yashas Another nice one :)

Fixing

p13: Lacan shows us throughout his seminars that, as oppose to Saussurre's classical work should be Lacan shows us throughout his seminars that, as opposed to Saussurre's classical work

Fixing :)

@EmilioPisanty I am tempted to flag that as spam, given how they end with a link to their own paper on the subject

@Semiclassical I'm tempted too, but they do explicitly mark it as their own work

5:00 PM

Yeah.

"and it follow’s from Cobham-Edmonds thesis"

@BernardoMeurer Up to section 1.7 it all looks fine, but at that it seems to swerve into woo woo territory ...

@Semiclassical but then again that journal looks...

@Yashas Fixed, thank you :)

Anonymous

@DanielSank It would be interesting to have an answer to this question from you: quantumcomputing.stackexchange.com/questions/105/…. I have almost zero knowledge about the experimental aspects

5:01 PM

not particularly reputable

@JohnRennie It's completely batshit crazy

Am I missing something?

No? It's just pretty weird overall

@EmilioPisanty it also seems to mostly have only one paper per issue

UPDATED: send.firefox.com/download/a6f629fbc6/#wXj_aGxStAvQa5Ed2s9R5g

@EmilioPisanty If you could take a look at the overall formatting I'd be grateful :)

5:05 PM

"V (G),E(G)" missed a space after the comma?

@EmilioPisanty the author list for that paper is goofy as well

[1]Shuang-ren Zhao, Mutualenergy Group, London, Canada.
[2]Keven Yang, Mutualenergy Group, London, Canada.
[3]Kang Yang, Mutualenergy Group, London, Canada.
[4]Xingang Yang, Mutualenergy Group, London, Canada.
[5]Xintie Yang, Aviation Academy, Northwestern Polytechnical University, Xi’an, China.

@Yashas Yep, fixed :)

At least we know where they got the username from :P

@BernardoMeurer is there a very strong reason why you're doing double spacing?

@EmilioPisanty The prof. mandated it

5:07 PM

also, what's with footnote 16 immediately after a definition number?

She's not particularly muscular though

But very bossy

ditto 18

@EmilioPisanty Ah, yes, I forgot to fix those

That's because the definition comes from that reference

also, §1.4.1 has subheadings that are larger than the section heading?

but it should obviously be at the end of the sentence

5:08 PM

@BernardoMeurer exactly

@EmilioPisanty 1.4.1 has what?

Oh

I see

p. 9 looks very lopsided

@EmilioPisanty I used \paragraph{} for those, what should I use? (Regarding 1.4.1)

it looks like there is imposed whitespace above the figure and that looks wrong

I know that's not what you intended

but it still looks like it

Alright, I'll shift the figure to the left

5:10 PM

just put the figure centered between two paragraphs

no need to get fancy if it's not going to work anyways

footnote 21, shouldn't Ibid. be in italics?

"Throughout his work, Lacan makes use of two grammatically similar, but semantically
**disparate,** terms" I think the comma after disparate shouldn't be there. I'm not sure. I suck at English.

ditto 24 and whereverelse you've used it

@EmilioPisanty I just used the Chicago Style latex pkg

ok

> The Philosopher Slavoj Ziˇzek in his works relating to Ideology

that's some suspicious capitalization

Ah, broken accents

5:12 PM

Oh jeez

second paragraph p. 16 is gynormous

@Yashas Looking at it

@BernardoMeurer that's from my copy paste

@BernardoMeurer do not use "we're" and the like

"Few people seemed to understand what the party was about. Which wasn’t so different from the conference itself, where physicists with PhDs freely admitted that the jargon-laden presentations they’d just attended were incomprehensible."

LOLOL

5:13 PM

does footnote 33 really need a marked-out DOI?

@BernardoMeurer You might want to ask about it in the ELL chatroom. I just found that sentence to sound weird with that comma but it could be that I am used to the wrong way of using commas.

jesus, this is way too technical for a philosophy paper

Anonymous

@0celo7 Modern day philosophy can be pretty technical. :P Subjects like Logic are mostly mathematics

@0celo7 The prof told me to go wild

"While for Saussure the connection between the signiﬁer and the signiﬁer (signiﬁca
tion) is very strict, being an injective-type function described in the Saussurean algorithm,
and unbreakable, Lacan has a very distinct view" This sounds confusing. "and unbreakable" was like surprise!

5:14 PM

Or, rather, she said "Do whatever you feel like"

@0celo7 topic?

@Semiclassical ask bernardo

@EmilioPisanty Fixed the capitalization

Oh, so this is Lacan stuff?

@DanielSank look, they interviewed Mercedes

5:15 PM

@EmilioPisanty What do I do about the large headings?

@BernardoMeurer larger section headers or smaller subheaders

@EmilioPisanty I met her once.

@DanielSank I did my MRes with her

Lacan is definitely outside my realm of interest in philosophy

What's smaller than subsubsection but isn't numbered?

5:16 PM

@EmilioPisanty Oh cool!

@DanielSank imperial.ac.uk/controlled-quantum-dynamics/people/students/… that's us back in the days of youth and innocence =P

the thing I get tired of in philosophy is sophistication for its own sake

You studied with Mark!?

Mark is my Physics Stack Exchange idol!

you may or may not have met Mihai and Naomi as well

@DanielSank yes

Woahhhhhhhhhh

5:17 PM

"Given a set of signiﬁers, a sentence, s, there are a number of diﬀerent " Is a comma supposed to be there after "sentence"? If yes, then the reader may not have a clue what the 's' (some greek character) means (you later define it as the set of signifiers: Graph of paths from the set of signiﬁers s to the signiﬁed S.). This line is very confusing because you have " understanding a sentence s " later.

Naomi is from the same group.

I believe I introduced Mark to PSE?

@EmilioPisanty What happened that year? It's like there was a concentrated burst of awesome.

I'm not sure, it's a bit lost in the fog of time

@EmilioPisanty Thank you.

5:18 PM

@DanielSank you sure about that?

@EmilioPisanty Yes.

My goodness, that's really interesting.

I'll draw your attention to the P section of cohort 1 two years above us

Who's that?

Matthew Pusey?'

@Yashas Fixing :)

@DanielSank the P in PBR?

5:19 PM

Where I come from, this is PBR.

@BalarkaSen summoning you because of sign issues

@BernardoMeurer " understanding a sentence s " " set of signiﬁers s to " what really is 's'?

@Yashas A collection of signifiers

Q: Consequences of the new theorem in QM?

16

It seems there is a new theorem that changes the rules of the game in the interpretational debate on QM: http://www.nature.com/news/quantum-theorem-shakes-foundations-1.9392 Does this only leave Bohm,Everett and GRW as possible candidates?

You might want to go through that entire section again. It looks like a mess.

5:21 PM

" the closeness of it’s path to the path taken " it's should be its?

I can't say I'm not

Wtf

@DanielSank you'll probably also enjoy the work of Kamil Korzekwa and Matteo Lostaglio from the year below me

" Since is trivial, that the problem stated above is a case of ﬁnding the optimal path among a subset of the vertices of a Graph G, we have that the problem is NP-Complete." Hmm? The comma and the way the sentence starts ...

@Yashas No time, but will do for the final version, marked for review

@Yashas Yeah, should be its

@Yashas Fixed already

> To show this, we use the finishing note of Subsection~\ref{sub:tsp}. Since it is trivial, that the problem stated above is a case of finding the optimal path among a subset of the vertices of a Graph $G$ we have that the problem is NP-Complete.

5:26 PM

"i.e. to mistake the self for it’s alter-ego" its?

@0celo7 lol

5:41 PM

Alright, time to deliver!

Thank you guys so much! @JohnRennie @Yashas @EmilioPisanty @0celo7

nw

nbro

Experts needed to improve the following article:

Quantum probability

Quantum probability was developed in the 1980s as a noncommutative analog of the Kolmogorovian theory of stochastic processes. One of its aims is to clarify the mathematical foundations of quantum theory and its statistical interpretation. A significant recent application to physics is the dynamical solution of the quantum measurement problem, by giving constructive models of quantum observation processes which resolve many famous paradoxes of quantum mechanics. Some recent advances are based on quantum filtering and feedback control theory as applications of quantum stochastic calculus. ��2�...

Anonymous

@nbro That's already mentioned at the beginning of the page ;)

nbro

@Blue And? What does that have to do with me sharing the page here to increase its visibility?

Anonymous

@nbro Well, hbarians are lazy. Don't expect them to edit the Wiki. :P

5:53 PM

@Blue I would protest against that characterization, but it would take too much effort.

Anonymous

@ACuriousMind Hey, I was just joking

@Blue If you look closely, I might have made a joke there, too ;)

Anonymous

Oh, lol

flagging

wtf is this slander

Anonymous

I'm writing reports for this silly electron devices lab, and got too bored

Anonymous

5:55 PM

This sucks

Anonymous

@0celo7 Heh :D

nbro

6:06 PM

I suppose you have already shared this new here today. Anyway, here it goes.

nytimes.com/2018/03/14/obituaries/stephen-hawking-dead.html

3

"Stephen Hawking Dies at 76; His Mind Roamed the Cosmos"

These are the kind of news and discussions I would like to see more on these chats, apart clearly from the usual discussion regarding physics problems, etc, etc.

@ACuriousMind Do you have a good reason why the curvature operator should be a derivation of the tensor algebra?

ir probably follows from the usual awful Ricci formulas

index hell, literally

@MadhuchhandaMandal Please be patient and do not post new questions here unless you have good reason to believe they are of particular interest to someone here.

@0celo7 Not off the top of my head, no

Anonymous

6:23 PM

@nbro If you want to see a particular discussion the best way is to initiate it yourself. We do have such discussions here, from time to time

Anonymous

That said, I was bit sad today morning. I spend a major portion of my middle school reading the popular science books by Hawking. Although nowadays I'm not into pop-science anymore, I think that was the beginning of my interest in physics and science as a whole

Is there a way to bookmark questions you want to answer later?

I guess I could use my browser's bookmark system...

@DanielSank favorite

@0celo7 I use favorites for another thing.

I can't believe a mathematical definition of this, and a theorem justifying it, exists:
"This theorem [the theorem of Racah] provides us with the possibility of a precise formulation of the notion multiplet. A multiplet is an irreducible invariant subspace of a group, i.e. a subspace which does not contain a further invariant subspace."

Of course you can't really use this for Poincare, but you kind of can

16

A: Rank of the Poincare group

For semisimple groups (and the Poincaré group is not such), the number of Casimirs (i.e., the number basic generators center of the universal enveloping algebra) is equal to the dimension its Cartan subalgebra (maximal commuting subalgebra) which is the rank of the algebra. This is called Cheva...

Wigner-Inonu strikes again

6:37 PM

why would it not exist

Casimir is a children TV character in France

so it's always funny to see

I literally had no idea what a multiplet was, and couldn't find a definition of this ******* thing in susy

'For such groups the center of the universal enveloping algebra can be characterized by the Harish-Chandra isomorphism , which is less constructive than the Chevalley's theorem.'

This is needed to justify that Poincare in qft only has two casimirs, this...

I'm so bad at all that group stuff

"The Poincaré group is a Wigner-İnönü contraction of the de-Sitter group SO(4,1)"

I guess you just send the radius to infinity :p

There are so many things I skip over, and then re-do everything filling in bits, then doing it again to get more bits, it's so difficult

I don't know enough terminology of groups really

all the centers and enveloppes and characteristics

what's a good book on group theory

@0celo7 yo

There is none

6:45 PM

unfortunate

what is the least bad book on group theory

7:02 PM

Ramond's Group Theory is like a base starting point I guess, but you can't trust any of these books, need to be ready to read loads of them :(

Really is the pest of physics

Well the same is true of all fields rly

I don't think there's any GR book that contains all of GR

Except MTW maybe

is there a good focus on lie groups?

I need me a lot of Lie groups

Is there here someone who wants to listen to my noob doubt?

~(no?)~

Anonymous

@Curio Don't ask about asking, just ask!

Anonymous

Ah, that was easy to copy-paste now that it's in the room description itself :P

7:18 PM

@Slereah Brian Hall

Knapp Lie Groups Beyond an Intro has loads on Lie groups, enveloping algebras, centers, characteristics

Well let's compare prices on all these

We know that W=-ΔU, right?

Hall and Knapp are reasonably price

I could probably get both

@Curio No, unless you are being process-specific

$Q= U + W$

That's only for an adiabatic process.

7:21 PM

What do you mean with those letters? I meant work is the opposite of the variation of potential energy

Oh wait

The cheap Hall is actually for rent

Ah!!

Knapp it is, for now

Sorry, I was referring to the first law of thermodynamics

Q- Heat transfer, U = internal energy, W = work done.

@Slereah you need Hall.

7:22 PM

@Curio Yeah, that's correct

Or helgason if you want geometrique

I will acquire Hall via

other means

However consider a spring connected to ceiling

I can send you a legal copy if necessary

And now attach a mass to the spring

7:22 PM

I also have a physical copy but I need that one

It's fine, shouldn't be hard to find

There's an old book on lie groups, lie algebras, with loads on homogeneous spaces which would be good for relativity, I think it's Helgason

is it "Differential geometry, Lie groups, and symmetric spaces"

The spring becomes longer of course. However the work of the spring and the work of the gravity force are different.

@Slereah AMS Book?

7:25 PM

Why?

there's a few different versions of it I think

Academic Press

Oh, the AMS one is "Groups and geometric analysis : integral geometry, invariant differential operators, and spherical functions"

@Curio I don't get your question

No, there’s a GSM version with typos corrected

I have it

It's more on Lie algebras and symmetric spaces than lie groups

7:28 PM

@Abcd where don't you get it?

@Curio What are you confused about?

The forces are the same (in fact all is static now). However the works are different. How is it possible? I said that W=-ΔU, so there is another work. But I don't see any movements

I know that the work of the spring has that 1/2, but then there is another work

@Curio We talk about work that has been done till you have attained the final equilibrium state.

@Curio Yes, there surely is.

Work done by gravitational force.

But I've already considered it

@Curio I don't know why you think the "work of the spring" and the "work of the gravity force" are different. The mass will stop at precisely the height at which the energy gained by the spring is equal to the gravitational potential energy lost by the mass.

7:34 PM

Fp = mg Fs = kx (leave the - now) Fp gravitational force Fs = spring force

@EmilioPisanty How did part 2 of your seminar go?

Fp = Fs Wp = Fpx Ws=Fsx/2

Wp != Ws

@Curio It doesn't have to be equal...

And why

You need to consider additionally the kinetic energy of the mass

7:39 PM

Aaaaah

fuck

@ACuriousMind I wrote you a message but it reloaded and deleted

@ACuriousMind abbreviated version: is it correct to assume that $N$ integrable $\implies$ complex structure is very hard when $M$ is not a priori known to be real analytic?

I think I understand the proof in the analytic case

Thanks

8:18 PM

@Semiclassical fairly well? I was probably over ambitious in the pacing and went too fast

yeah, that's always tricky