The h Bar

3:01 PM

@morbidCode I'd be a bit worried about advising on medical matters. Is there a pharmacy nearby where you could ask?

Logically..., moisture is basically water droplets and vapour suspended in air. Then of course a wet hand is basically having 100% moisture on it and thus it will be a bad idea to wet the package.

John Rennie

If you took the medicine to the pharmacy they can tell you how to store and handle it.

Secret

however the details may be not that simple, use johnrennie's advice

ACuriousMind

@morbidCode Yeah, I don't think you should trust strangers on the internet what exactly counts as "moisture" for the purpose of your medicine

Robert Frost

@ACuriousMind Time, measured in seconds is benchmarked against the rotation of the Earth. One 86400th of a rotation to be fairly precise. That is the third-party clock.

Bernardo Meurer

3:03 PM

@morbidCode Store the medicine in the ocean

Guaranteed success

Fawad

~~Try touching switching electric switches. If you get shock,it means your hands are still wet~~

AccidentalFourierTransform

the salt will keep it dry

60% of the time, it works every time

ACuriousMind

@RobertFrost That's the definition of a unit of time, but the notion of time is distinct from the definition of one particular unit for it.

In the end, time is what clocks measure, just like length is what rulers measure. You don't need to know the earth's rotation to build a clock.

User Anonymous

Shameless promotion of question:

0

Q: Is the diffraction pattern of a vertical slit horizontal?

I am familiar with the mathematical aspects of single slit diffraction pattern, at the undergraduate level. Consider the following pictorial representation from the book Optics, by Hecht: The fact that I find puzzling here is - even though the slit is shown vertical, the pattern on the screen...

Can someone please tell me why my symmetry argument doesn't work?

Secret

@acuriousmind We knew that the eigenvalues of a density matrix corresponds to the probability of finding some pure state that form the ensemble, but what do the corresponding eigenvectors physically mean?

Also, while it is known that density matrices can be treated like operators in the computation of the purity of the mixed state (thanks Emilo), von neumann entropy and expectation values, do multiplying two different density matrices together in general have a well defined meaning?

Are there situations where we have both density matrices and state vectors thus resulting in something like…

Robert Frost

3:08 PM

@ACuriousMind Very interesting actually. You are describing, as it happens, time without units, which I'm inclined to think is exactly what my question is asking is possible.

0celouvsky

@ACuriousMind I think it is very relevant because you are assuming how the community feels about the meme. It happens to be true and that is accepted.

@AccidentalFourierTransform what conversation?

AccidentalFourierTransform

that thing about the thing... you know, the thing

0celouvsky

No, I don't know.

morbidCode

Thanks. I'll try to find a pharmacy to make sure. The wet hand is just an example. @Secret, you said "moisture is basically water droplets and vapour suspended in air." How mutch water droplets? When I touched the package a few minutes ago, my hand are just a little damp (as damp as being abel to use a laptop keyboard) and I immidiately dried the package afterwards.

ACuriousMind

@Secret If you understand why the eigenvalues are the probabilities of finding a pure state then you should also understand what the eigenvectors are.

John Rennie

3:12 PM

@RobertFrost can you clarify whether you're talking about time as a label for when things happened or the flow of time i.e. the fact time appears to flow?

ACuriousMind

@RobertFrost Units are just a choice, they have no fundamental physical importance. You're free to take a clock, paint random markings on it and use those as your new unit. I'm still not sure what were talking about, honestly.

Secret

@morbidCode I don't think there is a clear cut value on how moisture is defined, except there is trace amount of liquid (usually water for daily life cases)

0celouvsky

@AccidentalFourierTransform are you talking about Mathematica?

Secret

Ok, I guess I intially have a calculation mistake resulting in not getting the eigenvectors to match the projectors that form the density matrix.

Ok, so that means a density matrix is an ensemble of projectors. Now suppose I have density matrix $\rho$ and $\sigma$, then by expanding the matrix multiplication $\rho \sigma$ I should effectively having a mixed state projected to another mixed state. But what physical situations do the need arise to project a mixed state to another mixed state?

Also do we never have density matrices appearing alongside with state vectors, since any state vect…

0celouvsky

3:29 PM

Hello?

User Anonymous

Hello.

Secret

chat seemed to die a lot tonight

User Anonymous

Shh. Its a secret :P

John Rennie

Frakking, frakking, fraketty, francking frak! I've just spilt a pint of beer over my desk, keyboard, mouse, lap, and so on. Ballcocks! :-(

Luckily it missed my laptop.

My goodness a pint of beer goes a long way when it's spilled on a desk!

Bernardo Meurer

DID SOMEONE SAY BEER?

John Rennie

3:44 PM

SPILLED BEER!!

Bernardo Meurer

HERESY

::grabs torch::

WHO DID IT?

John Rennie

So now I've got no beer - unless I want to lick the desk :-)

Bernardo Meurer

You of all people?

John Rennie

And I've had to spend the last half hour taking everything off the desk and washing down so I'm not left with a sticky desk (insert joke here).

Down Christopher

What did I just walk in on

John Rennie

3:47 PM

Beer, spill, desk, Armageddon!!!

Down Christopher

Huh interesting

John Rennie

"Interesting" was not the word that sprang to my mind :-)

0celouvsky

@JohnRennie please remove that offensive word at the end

Secret

I have been having a lot of quantum questions concerning physical meaning recently due to the quantum chemistry stuff I have read that does not seemed to match what I knew in the physics community

0celouvsky

I mean what even is that

John Rennie

3:48 PM

What? Armageddon?

0celouvsky

"Ballc*cks"

Down Christopher

@JohnRennie How DARE you

@JohnRennie yeah but I don't have beer spilt all over all over my computer, desk and legs

John Rennie

A ballcock is an item of plumbing. Google it.

0celouvsky

And balls are topological objects but we can't talk about them either

John Rennie

It's a valve that automatically controls water level.

Down Christopher

3:49 PM

I like bouncy balls

0celouvsky

A cock is a bird, can I throw that word around?

User Anonymous

@JohnRennie That makes it worse :P

John Rennie

Yes

0celouvsky

Dick is an acceptable name for a person (I have an uncle Richard)

Down Christopher

heh

0celouvsky

3:50 PM

Tits was a mathematician

Down Christopher

Was he big?

User Anonymous

Seriously, where is this conversation heading?

John Rennie

You can even use the word cock horse - though probably best not to swap those two words :-)

Down Christopher

@UserAnonymous I don't know

Everything has gone downhill ever since I joined

User Anonymous

Sigh.

Secret

3:52 PM

For example Equation 1 is what caused me to ask about $\rho\sigma$ and $\rho\lvert\psi \rangle$

0celouvsky

@JohnRennie so if I get banned you will unban me?

Down Christopher

@0celouvsky khanacademy.org/computer-programming/bouncy-ball-function/… my bouncy balls

John Rennie

@0celouvsky Sadly I have no unban powers

Down Christopher

@0celouvsky That is a interesting question

John Rennie

@0celouvsky or ban powers for that matter - well, I can ban you for 60 seconds :-)

0celouvsky

3:53 PM

By your logic I should be able to say that Dick loves Tits and by that I mean by Uncle Rich likes the mathematician J. Tits.

Down Christopher

must not star

0celouvsky

But somehow I don't think that's allowed.

John Rennie

Context is all :-)

0celouvsky

I think it's Jacques

But I'm not sure

@ACuriousMind is the above message a violation?

John Rennie

Jacques Tits

Jacques Tits (French: [tits]; born 12 August 1930 in Uccle) is a Belgium-born French mathematician who works on group theory and incidence geometry, and who introduced Tits buildings, the Tits alternative, and the Tits group. == Career == Tits was born in Uccle to Léon Tits, a professor, and Lousia André. Jacques attended the Athénée of Uccle and the Free University of Brussels. His thesis advisor was Paul Libois, and Tits graduated with his doctorate in 1950 with the dissertation Généralisation des groupes projectifs basés sur la notion de transitivité. His academic career includes professorships...

0celouvsky

3:55 PM

Ayy pretty good

Bernardo Meurer

Lol

What a name

0celouvsky

I guessed that

John Rennie

Tits buildings - snigger

Down Christopher

Ok this is just getting worse

ACuriousMind

@0celouvsky You're phrasing this whole conversation in the wrong terms. This isn't about "forbidden" words in the first place but about the meaning conveyed.

Mostafa

3:57 PM

That's not so bad compared to what I did a few days back: accidentally sat down on my glasses. This is what I'm using until my next one gets ready.
https://i.stack.imgur.com/ETPpI.jpg

0celouvsky

@ACuriousMind Meaning cannot be transmitted via the flag system, so should not be considered.

@JohnRennie "K. Burns, R. Spatzier, On topological Tits buildings and their classification."

John Rennie

@Mostafa bummer :-( At least my desk is now clean.

0celouvsky

I want to read that paper, it's relevant to my interests :)

Down Christopher

@ACuriousMind It is still bad

@0celouvsky Wha!

ACuriousMind

@0celouvsky Oh, I'm not discussing the flag system with you again. I've agreed it's broken for leaving out context several times, what more do you want?

@DownChristopher Sorry, what is bad?

0celouvsky

3:58 PM

@ACuriousMind So why do you insist on not unbanning people for it?

Bernardo Meurer

@ACuriousMind He wants you to do something about it

Down Christopher

@BernardoMeurer No I find it amusing (in a way)

Bernardo Meurer

Instead of "Oh yeah it's kaput ::proceeds to ban people::"

0celouvsky

@JohnRennie that paper contains the Theorem "A compact Riemannian manifold of non positive curvature of rank 􏰎 2 does not have transitive holonomy. In particular, it must be either reducible or locally symmetric."

Bernardo Meurer

@DownChristopher Are you downgoat?

Down Christopher

3:59 PM

@ACuriousMind This chat :P while in context it is not

@BernardoMeurer Nope DownChristopher

It is a PPCG meme and it has become a life choice

ACuriousMind

@0celouvsky I do unban people when the flag was invalid in my view.

Down Christopher

Anyways I gotta go. bye

ACuriousMind

@BernardoMeurer I can't do anything about it - I'm not an SE developer.

John Rennie

@0celouvsky I don't understand that because I don't know what a manifold is :-)

0celouvsky

🙍🏻‍♂️🙎🏻‍♂️

🤦🏻‍♂️

Mostafa

4:01 PM

https://en.wikipedia.org/wiki/Change_of_basis#Change_of_basis_2
Shouldn't it be $t_2=p^{-1}t_1p$ ? (The basis is changed, not the vectors.)

0celouvsky

@JohnRennie I wonder what a building is

I saw a GTM on then in the yellow sale

Bernardo Meurer

@ACuriousMind You can unban people who are banned unjustly from flags. You can talk to SE guys and have more leverage than we do. You can try and do something instead of conforming and avoiding criticism

ACuriousMind

@BernardoMeurer I do the first, I don't have any leverage with SE I know of

Secret

$\Gamma \Theta_k=p_k\Theta_k$ is basically $\rho\lvert\psi\rangle=p_k\lvert\psi\rangle$ in physics terms ... but then if that's the case that's weird cause we never saw density matrices and state vectors to appear in the same time (in fact, state vectors are projectors in the density matric formalism

ACuriousMind

They know chat flagging is broken, there are multiple meta posts about it that I've pointed you all to when we discussed this. It's just not high enough on their priority list (and I'm not sure there's a really good alternative proposed currently)

Yashas

4:06 PM

I got a disputed flag becaz the person reworked his entire post after it was flagged :(

ACuriousMind

@Yashas Well..that's good!

After all, you shouldn't be casting flags to get validated flags, but to improve the site

Yashas

how many disputed flags does it take to get blocked from flagging?

ACuriousMind

@Yashas disputed flags can't block you, only declined ones can

Yashas

:O yay

ACuriousMind

The threshhold for a block by declined flags is 25% declined flags in the last 7 days + more than 10 flags cast

skill patrol

4:12 PM

@Jim once bitten twice shy?

Mostafa

The failing android app of the failing NYTimes has been giving me too many notifications lately...I think I'm going to uninstall it.

Bernardo Meurer

@Mostafa It's proprietary malware anyway

skill patrol

I just shut-off the notifications.

John Rennie

Now, do I dismantle my keyboard and attempt to clean it or do I think ballcocks and throw it away and buy another?

Bernardo Meurer

@JohnRennie Get a new one, get one of the old IBM ones

they are cool

John Rennie

4:17 PM

I like Dell keyboards ...

skill patrol

try to save it first

John Rennie

@skillpatrol hmm, it's a lot of hassle dismantling a Dell keyboard, and considerably more hassle trying to put it back together again.

Bernardo Meurer

@JohnRennie Is it a mechanical or a membrane keyboard?

Mostafa

@skillpatrol notifications are the whole point of having it on my phone so that I don't have to read all the news myself and just get the most important ones.

@BernardoMeurer yeah, a failing proprietary malware.

skill patrol

@Mostafa okay, I didn't know that :-)

Uninstall away.

Mostafa

4:25 PM

@JohnRennie buy one of those fancy ergonomic keyboards...like this:

skill patrol

@JohnRennie this probably won't be the last time you have a spill on your keyboard and the practice will speed you up, no?

Anonymous

@Mostafa Meh, those keyboards are a pain in the ass....they get damaged very easily...my last one broke within 2-3 months...

Anonymous

They are just fancy stuff.

skill patrol

They look fancy.

John Rennie

@BernardoMeurer membrane. I have just taken it apart and beer has got into the membrane, so it's a goner. Oh well. I'm back on the old keyboard that I stopped using because it had got so dirty.

Bernardo Meurer

4:30 PM

Ergodox has an awesome ergonomic keyboard

@JohnRennie Do yourself a favor, get a nice mechanical keyboard :)

John Rennie

I use these:

Anonymous

I'd rather stick to the old fashioned keyboards.

John Rennie

But Dell have stopped making them so I've been having to hunt for them on ebay.

Anonymous

Yep, these are the best ^

Anonymous

And very cheap :-P

John Rennie

4:31 PM

I find these keyboards have a really nice key action.

skill patrol

They always stop making the good stuff.

John Rennie

Probably too expensive.

The keyboards they currently ship with PCs feel a lot cheaper.

Anonymous

The dell latitude laptops had one of the best and most sturdy keyboards.

Anonymous

My old latitude model is running for the past 10 years..with minimal problems

John Rennie

The Latitude Haswell laptops, the E6430, E6330, etc have the really nice keyboards. The newer Latitudes use a chiclet keyboard that isn't anywhere near as nice.

skill patrol

4:34 PM

@blue Wow! 10 years

Anonymous

yes, 10 years! It was my father's laptop actually

John Rennie

The 2007 Latitudes would have been the E6410, E6310, etc series and they are nice laptops.

But the older i3/5/7 processors were a lot slower and more power hungry tha the Haswell chips.

Anonymous

My old laptop was dual core :-P

Anonymous

It still works

John Rennie

My current laptop is dual core.

Anonymous

4:36 PM

Now I am using E4310

John Rennie

As far as I know only that insane power freak @Bernardo uses a quad core laptop.

Anonymous

lol....anti malware freak ;)

Bernardo Meurer

:^)

John Rennie

@blue Nice laptop!

Bernardo Meurer

I compile the kernel a lot

John Rennie

4:37 PM

Also it heats the room nicely on cold days :-)

Bernardo Meurer

having 8 threads/4 cores saves me a lot of time

John Rennie

These days the only really CPU intensive stuff I do is occasional film resampling, and I leave that to run on my server.

Mostafa

@blue This keyboard looks quite old-fashioned.

John Rennie

@Mostafa That's a modified dell keyboard. Brilliant! :-)

Anonymous

@Mostafa hehe

Anonymous

4:41 PM

looks fancy :-P

Yashas

@Mostafa Lately, I have been having some serious problems with transistors.

Why does the base current increase as you increase $V_cc$ in CE configuration?

Anonymous

@Yashas how was the physics test?

Anonymous

100?

Yashas

I can't think of a reason

@blue 99

or less

Anonymous

@Yashas ahhhhhhh

Yashas

4:42 PM

I got a kinematics question wrong -_______-

Mostafa

@JohnRennie I don't know why, but I feel someone that looks like your profile picture should use a keyboard like that....

Yashas

I integrated instead of differentiating -,-

Anonymous

I told you not to give the test :-P

Anonymous

it is useless

Anonymous

John Rennie's profile picture is deceiving

Anonymous

4:43 PM

I thought he would be an angry old man

Anonymous

:-P

John Rennie

Angry? I thought I was fairly chilled (apart from when I've spilled beer everywhere).

Or do you mean the photo looks angry?

Anonymous

Yes, the photo :-P

Yashas

@JohnRennie Your mustache and beard doesn't look real to my eyes o0

John Rennie

Ah. I was quite young when that was taken. It was about 25 years ago.

Yashas

4:45 PM

:|

Anonymous

25!!!

John Rennie

@Yashas It is a genuine beard. A rather straggely one, but a genuine one.

Yashas

Physicists are so involved in physics that they don't get time to shave.

Anonymous

John is a chemist :-P

Yashas

@JohnRennie That beard and mustache looks too god to be true.

skill patrol

4:46 PM

Is this too old fashioned?

Yashas

@skillpatrol I like it.

skill patrol

Me too.

Yashas

Last year, I was begging my school teacher to give me the 386 in the lab

The teacher was so mean.

John Rennie

@Yashas this is the full size version ...

3

Yashas

Were you sleepy when you took that pic?

skill patrol

4:48 PM

Graduation?

Mostafa

@Yashas Based on the equations, becuase $I_C$ increases.

John Rennie

@Yashas I was probably trying to look cool - always a hard task :-)

Yashas

@Mostafa $I_E$ can increase too!

John Rennie

@skillpatrol no, I was in an opera - Die Lustige Witwe by Franz Lehar.

Yashas

@JohnRennie How many GFs did you have in college?

Anonymous

4:49 PM

hehe ^

Yashas

:O @JohnRennie is a singer too!

John Rennie

@Yashas a gentleman never comments

Mostafa

@Yashas Yeah it does.

John Rennie

I was supposed to be a nobleman

Yashas

@Mostafa but how can conclusively say that $I_B$ always increases as $I_C$ increases?

skill patrol

4:51 PM

You sang in the opera?

Yashas

@Mostafa My textbook uses -Vce and -Ib in the graph for input characteristics of a pnp transistor in CE configuration.

The negative sign is causing problems.

The ammeter placed is such that the current enters through the positive terminal but the graph...

Mostafa

Yeah it's somewhat confusing...

Yashas

One last question

In a full recetification device, you connect source->transformer->full-wave-rectifier->filter->regulator

the regular given is a Zener diod

nvm

Anonymous

@Yashas What are your studying transistors for?

Yashas

@blue I am losing one more mark

98 :(

98 last time

98 before that too

Will I ever hit 100 :'(

Anonymous

4:55 PM

Naaaahhhh :-P

Anonymous

Forget it

John Rennie

@skillpatrol only in the chorus. I wasn't good enough to get a starring role. And to be honest they were so short of people they would have taken anyone.

Yashas

-.-

Mostafa

@Yashas The equation $I_C = \alpha I_E$ means that $I_B$ should also change when $I_c$ is changed. ( here )

John Rennie

@skillpatrol good fun though.

Yashas

4:55 PM

@Mostafa o0

@Mostafa Oh the answer was there.

skill patrol

Cool @JohnRennie

Mostafa

Generally when designing real circuits people usually assume $I_B$ and $V_{CE}$ to be constant and find other parameters of the circuit.

Yashas

@blue I lost 2 more marks

@blue this marks will be scaled down to 64

Anonymous

@Yashas you are useless

Anonymous

:-P

Yashas

4:59 PM

@20/20 practical 16/16 assignments

64 theory eamx

Mostafa

@Yashas I don't really remember these (and I unfortunately don't have my notes or my electronics textbooks around now:(

Anonymous

Just chuck it off man

Anonymous

Those exams are stupid

Yashas

The worst question in the test was square law device

6 marks

2 circuit diagrams

2 page answer

Anonymous

square law device?

Yashas

5:02 PM

you didn't have it?!

communication systyems

frequency modulation

amplitude modulation?

Anonymous

i never read that chapter

Anonymous

lol

Yashas

singnal filtering

Anonymous

too boring

Yashas

how much did u score in ur test?

Anonymous

5:03 PM

which test?

Yashas

CBSE phy

Anonymous

i am not in cbse

Anonymous

i am in icse

Anonymous

i scored 98 or 99 out of 100 iirc in my mock tests

Yashas

that is a nice thing

300+ in all mock tests

and end up 230 in the actual test

Anonymous

5:07 PM

in the exam hall nervousness takes a toll

Anonymous

I expected 250+

Anonymous

Now I am getting about 240

0celouvsky

@Danu I am probably going to be doing my thesis on cobordisms of constant curvature manifolds as studied by Gromov and Lawson. Do you have a good introduction/reference to spin manifolds?

skill patrol

He's busy beginning his own.

0celouvsky

Removed?

skill patrol

5:17 PM

Yeah, it's kinda personal

Hi @Loong how's the coffee? :P

ACuriousMind

@skillpatrol and...how would that be relevant to answering that question? Saying "I don't know one" or "Sure, go read X or Y" doesn't really take a great deal of time.

Loong

@skillpatrol plenty but thin

Danu

@0celouvsky Why not try the "canonical" reference?

@skillpatrol don't worry about my privacy :P Also, I'm not writing about Spin manifolds.

0celouvsky

@Danu Is it a good book?

Danu

I've heard so, yes.

0celouvsky

5:29 PM

Canonical references need not be good.

skill patrol

But they must be canonical:P

Cows

@Danu I haz nudges now?

higg & coulomb branch . . . . I want to know . . .

*higgs

baby referenve

*reference

0celouvsky

I don't think Danu is the kind of guy to appreciate a request like that.

Danu

I've heard it is good.

Cows

ok let me try to make the request better

0celouvsky

5:32 PM

@Danu Apologies, on mobile and that message lagged.

Cows

here goes

Danu

I see

0celouvsky

It was supposed to come earlier.

Cows

I have read that higgs and coulomb branches are to do with vector and scalar multiplets. I also know that we are considering the family of vaccua as moduli space . . . and yada yada via wikipedia. I want to text that is simple enough to show me some operational definitions and computational aspects of what this means.

Emilio Pisanty

@BernardoMeurer around?

Danu

5:34 PM

In any case, I'd be interested if you find a good reference to learn about cobordism. I'd love to know about Thom's original paper...

Emilio Pisanty

or anybody who knows how to deal with svn?

of all things, I know

Danu

nopes

Cows

ok let me try again

What is meant by higgs branch and coulomb branch, and how do they come up in qft not string theory

say for a supersymmetric field theory with many vaucua

. . .

a simple one for example

that I can go and look at

0celouvsky

@Danu Will do. My advisor made it sound like the cool part about it is the analysis though, not the topology. My first job is to be come familiar with Gilkey's book on the index theorem.

Also Hörmander Vol III.

Cows

@0celouvsky can you help me ask the question better

0celouvsky

5:38 PM

No.

Cows

crap lol

Emilio Pisanty

People learning catastrophe theory here?

Cows

can anyone help me ask what I am trying to ask?

Emilio Pisanty

if y'all find a good reference, ping me

Cows

I want a simple example that illustrates the coulomb branch and higgs branch ideas,

Danu

5:40 PM

@0celouvsky Poor you.

I will learn about the index theorem from Roe's book this semester.

It's a very basic book (to the extent that the subject permits it), I think.

Cows

I have read the wikipedia article for it en.wikipedia.org/wiki/Moduli_(physics) . Everything else I find is a ressearch paper

I don't want to read a research paper, because I don't know the fundamentals

0celouvsky

@Danu Does it omit the pseudodifferential mess?

Cows

@Danu have you encountered this , and can you point me toan example?

Danu

Idk. It also uses the heat equation proof though.

Gilkey looks nice.

0celouvsky

Not the reaction I expected from someone who doesn't like analysis ;)

Cows

5:43 PM

@EmilioPisanty can you point me to an example?

Emilio Pisanty

@Cows of how I don't know enough catastrophe theory but would like to learn more?

0celouvsky

@Cows You're being annoying..

Cows

what is catastrophe theory?

uv catastrophe?

anyways I am not sure what catastrophe theory is

Emilio Pisanty

@Cows no, wait, I thought you were replying to my last message

@Cows if that is what you meant, no idea

Danu

@0celouvsky Reminder: If you are annoyed by someone you can just use the ignore button

Cows

5:47 PM

@EmilioPisanty the only catastrophe I know is en.wikipedia.org/wiki/Ultraviolet_catastrophe but I am not sure that is what you are talking about

0celouvsky

@Danu on mobile, can't

And I still have to see other people talking to him

Danu

@EmilioPisanty Regarding catastrophe theory: I see Arnol'd wrote a book.

0celouvsky

And I'm not the one being annoying in any case

Danu

Are you looking for a mathematical book or not?

Cows

@0celouvsky wait . . . I am annoying. Did not know. Sorry about that

@Danu sorry for annoying. You are free now

Anyways if anyone wants to help me let me know

Emilio Pisanty

5:49 PM

Catastrophe theory

In mathematics, catastrophe theory is a branch of bifurcation theory in the study of dynamical systems; it is also a particular special case of more general singularity theory in geometry. Bifurcation theory studies and classifies phenomena characterized by sudden shifts in behavior arising from small changes in circumstances, analysing how the qualitative nature of equation solutions depends on the parameters that appear in the equation. This may lead to sudden and dramatic changes, for example the unpredictable timing and magnitude of a landslide. Catastrophe theory originated with the work of...

@Danu I just asked because people were mentioning Thom

but that might be something completely unrelated

Danu

I see

Emilio Pisanty

maybe there are plenty of fields grounded on "some obscure Thom paper"

Danu

Thom used to be a great topologist

He did seriously foundational works in the late 50s/early 60s

He introduced/initiated the study of the cobordism rings

I think they're the first example of turning things like "the space of smooth manifolds up to equivalence" into an algebraic structure, in this case a ring.

That's great, because you can talk about generators of the ring and if those turn out to be nice and easy to handle you can prove things about all manifolds of some type by just proving them for the generators of some cobordism ring

bolbteppa

Anybody ever derived the particle supergravity action? $S = \int d t \frac{1}{2}(e^{-1}\dot{x}^{\mu} \dot{x}^{\nu} - i \chi^{\mu} \dot{\chi}^{\nu} - i k e^{-1} \psi \dot{x}^{\mu} \dot{\chi}^{\nu}) \eta_{\mu \nu}$?

Danu

(it turns out many properties that people are interested in are invariant under cobordism)

@bolbteppa Yes.

Stupid exercise

0celouvsky

5:53 PM

How does one derive an action?

Danu

use the fundamental theorem of calculus

lel

stupid joke

It's more like "simplified it into usable form"

bolbteppa

If you set up $S = \int dt ( \dot{x}^{\mu} p_{\mu} - \frac{e}{2}p^{\mu}p_{\mu} - \frac{i}{2} \chi_{\mu} \dot{\chi}^{\mu} - \frac{ik}{2} \psi \chi^{\mu} p_{\mu})$ and eliminate the momentum via it's eom to get $p_{\mu} = e^{-1}(\dot{x}_{\mu} - \frac{ik}{2}\psi \chi_{\mu})$ then plug it in to that action, you're suposed to get that action, $S = \int d t \frac{1}{2}(e^{-1}\dot{x}^{\mu} \dot{x}^{\nu} - i \chi^{\mu} \dot{\chi}^{\nu} - i k e^{-1} \psi \dot{x}^{\mu} \dot{\chi}^{\nu}) \eta_{\mu \nu}$

But I get a ******* annoying extra term which wont go away
$S = \int d t \frac{1}{2}(e^{-1}\dot{x}^{\mu} \dot{x}^{\nu} - i \chi^{\mu} \dot{\chi}^{\nu} - i k e^{-1} \psi \dot{x}^{\mu} \dot{\chi}^{\nu} + \frac{e^{-1}k^2}{8}\dot{x}^{\mu}\psi \chi^{\nu} \psi - \frac{e^{-1}k^2}{4}\dot{x}^{\mu}\psi \chi^{\nu} \psi) \eta_{\mu \nu}$

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