The h Bar

@0celouvsky It was interpreted that way. It all started when I said girls have a weaker right brain but a better left brain. No idea how it got worse after that.

0celouvsky

Ah well. You can make India great again and we'll forgive you

Anonymous

5:19 AM

@Yashas Just leave it. It was more of a joke. :-P

0celouvsky

@JohnRennie I take your silence to mean I won't be getting this book?

John Rennie

We'll see (that's parent speak for no)

0celouvsky

@JohnRennie I'm supposed to get a tax return that will cover a large part

I bought three books today already so I'm kinda bleeding cash

John Rennie

Now I'm intrigued. Which of your parts do you propose to cover with the tax return, and are you sure it's large or is that just the usual exaggeration?

0celouvsky

@JohnRennie It's supposed to be like $137. It's a lot for me :P

Upon second reading, I see what you did there

You're sick

can we please not have room owners sexually harassing citizens?

thanks

John Rennie

5:24 AM

@0celouvsky You've only just realised that? :-)

Slereah

Apparently in the early GR topology days, people didn't use $I$ and $J$

They just defined $\ll$ and $\leq$ for sets

Anonymous

@Yashas Need some help in thermo

Yashas

What's the Q?

Anonymous

Come to the other room

Mostafa

Take a look at the finalists of Wikimedia's picture of the year contest:
https://commons.wikimedia.org/wiki/Commons:Picture_of_the_Year/2016/R2/Gallery

John Rennie

5:34 AM

I like the ravens

Mostafa

Because they're from England, right?

:)

Slereah

My favorite wikipedia pictures are the really bad maps you sometimes see

once in a while, for some obscure historical event, you'll find some really awful MS Paint drawn map

user228700

For the record, girls aren't weaker with regard to acing any of these exams. The reason behind the sex ratio in these colleges is that in most families in India, women are not encouraged to prepare for these exams. Heck, a large chunk of women aren't even encouraged to study beyond 10th standard. Even if they're allowed to do it, they are not supported through the ordeal (lasting for 2-3 years).

Mostafa

Ravens of the Tower of London

A group of six captive ravens live at the Tower of London. Their presence is traditionally believed to protect the Crown and the tower; a superstition holds that "if the Tower of London ravens are lost or fly away, the Crown will fall and Britain with it". Historically, wild ravens were common throughout Britain, even in towns, the tower being within their natural range. When they were exterminated from much of their traditional range, including London, they could only exist at the tower in captivity and with official support. Local legend puts the origin of the captive raven population at the...

a superstition holds that "if the Tower of London ravens are lost or fly away, the Crown will fall and Britain with it"

Yashas

@Kaumudi.H I never meant to be rude towards girls; I just mentioned it (I also stated it that I read it somewhere right after I wrote that girls have weaker right side brains): thoughtcatalog.com/lorenzo-jensen-iii/2015/06/…

Mostafa

5:41 AM

Maybe they're talking about Brexit.^

Slereah

time to shoo those ravens away

John Rennie

@Mostafa I just think it's a nice picture. I've never been to the Tower of London so I've never seen the ravens there. I have seen wild ravens in Wales, where they are still fairly common.

Yashas

@JohnRennie temperature is a function of kinetic energy or internal energy?

John Rennie

Well internal energy is a function of kinetic energy - well sort of.

Yashas

internal energy includes other kinds of energies too, right?

user228700

5:45 AM

And also, @blue: The sexuality of a person doesn't change as one progresses through life. I realise that you meant this as a light-hearted joke but I wanted to make this point crystal clear anyway because at the moment, many problems are caused by this perception that sexuality can be changed, especially in India.

Anonymous

@Kaumudi.H I know I know

John Rennie

@Kaumudi.H: now would be a good time to practice the deep breathing exercises and meditate on the fact that male humans take a remarkably long time to grow up.

Anonymous

That was a joke

Slereah

I've seen medical cases that involve sexuality changing

Of course they involved skull trauma

user228700

@JohnRennie :-P I live in India so I am now an expert at these deep breathing exercises you speak of.

John Rennie

5:48 AM

The spectacular immaturity of testicle bearing humans is a worldwide phenomenon :-)

Anonymous

What I meant was people preparing for competitive exams have to give up on all their sexual desires/love and stuff....they basically turn into "asexual creatures" like monks and nuns :-P It is true for both the sexes I guess :-D Take that as a joke and nothing more @Kaumudi.H

Yashas

@JohnRennie If U = KE + PE then dU = d(KE) + d(PE); if T is a function of U, then T should measure change in potential energy too?

Slereah

@Kaumudi.H Use the American way

(Adderall)

John Rennie

@Yashas true ... I guess temperature is just a function of the kinetic energy then

Yashas

what if KE remains constant and PE increases?

U increases

so T should...?

Fawad

5:54 AM

@Yashas constant?

Yashas

this issue arose out of the following problem: is dU always equal to nCvdT for all substances?

real gases have potential energy in addition to kinetic energy

the same question for dH too

dH is not equal to nCpdT

there is a second term

John Rennie

@Mostafa re Brexit, note these wise words:

Yashas

it can be justified as Cp changes with pressure but that doesn't sound convincing

John Rennie

Slereah

Trying to use JabRef for bibliography but damn thing works poorly

So far I only managed to put HE in the bibliography

A good start but could be better

How does one define a curve on a manifold

Is it just a map from a contiguous subset of $\Bbb R$ to the manifold

or is there a simpler definition

(with more than one point)

I can't think of a simpler definition that would encompass both curves between two points and infinite curves

and curves where the accumulation points don't exist

mathvc_

6:25 AM

Hello everyone! Good morning. I have a question

how

Slereah

pretty broad question

Yashas

I guess he/she hit the enter button too early.

mathvc_

:) im writing!

How category theory could be used in string theory?

is there any literature about it?

Slereah

Category theory is used for quantum field theory

Anonymous

@Slereah :-P good answer

Slereah

6:27 AM

I'm sure it could be used similarly for string theory

Though I'm not sure what functor string theory would be

2D conformal field theories to C* algebras, maybe

mathvc_

How it is used? In parh integral?

Slereah

or something

mathvc_

*th

Slereah

It's used in at least two contexts

Anonymous

@Yashas You got it now?

Slereah

6:28 AM

First, algebraic quantum field theory

Yashas

@blue ?

Anonymous

@Yashas Thermo

Yashas

@blue I am digging through FLP.

Slereah

In AQFT, a QFT is a presheaf from open sets in the spacetime to a C* algebra

Anonymous

Feynman ?

Anonymous

6:29 AM

That's outdated

Yashas

yep

:|

Anonymous

Get a short and sweet book

Yashas

FLP is the best physics book I have ever seen!

Anonymous

:-P

Slereah

and then you have the QFT where...

which one is the functor for cobordisms?

mathvc_

6:29 AM

@Slereah yup. Do u know any useful paper or article about it?

Anonymous

Feynman is more of a story book than a Physics book

Yashas

I have economics xam on 12th :/

Slereah

Ah, the other one is FQFT

Functorial quantum field theory

Yashas

then English and computer science after eco

mathvc_

@blue whatever it is, its funny :)

@Slereah got it!

Slereah

6:31 AM

It's a functor from cobordisms to vector spaces

Anonymous

@mathvc_ Feynman was a funny guy

Anonymous

I've heard

Anonymous

:-D

Slereah

Basically FQFT is the category theory equivalent of path integrals

ncatlab.org/nlab/show/FQFT

ncatlab.org/nlab/show/AQFT

if you need some references

Yashas

@blue that reminds me of the Japan incident

mathvc_

6:32 AM

@Slereah does it make path integral in a more rigorous frame work?

Anonymous

And I've heard that Feynman had a loose character :-P (quora.com/Did-Richard-Muller-ever-meet-Richard-Feynman)

Anonymous

@Yashas

Slereah

there's plenty of rigorous path integrals

Anonymous

Although I love that guy =D

Slereah

The problem is that, as with all rigorous formulations of QFTs, only the boring theories are rigorous

It's always free theories that are rigorous

Anonymous

6:33 AM

@Yashas I've heard of it telegraph.co.uk/culture/10036024/…

mathvc_

@Slereah what u mean by boring theories?

Slereah

Free theories

Not very good to do non-trivial experiments on

mathvc_

Thats indeed boring lol

Slereah

Well, you have some non-free theories that are rigorous

But they're usually 1+1 dimensional

Conformal field theories, sine gordon, thirring, etc

Polynomial fields in 1+1D, also

mathvc_

QED in 1+1 dimension

yup

Slereah

6:35 AM

QED in 1+1 D is trivial, if I recall correctly

Well, not quite trivial, but

There are no photons

mathvc_

A Photon has mass in the theory

Slereah

21

Q: Can light exist in $2+1$ or $1+1$ spacetime dimensions?

Spacetime of special relativity is frequently illustrated with its spatial part reduced to one or two spatial dimension (with light sector or cone, respectively). Taken literally, is it possible for $2+1$ or $1+1$ (flat) spacetime dimensions to accommodate Maxwell's equations and their particular...

mathvc_

if I'm not wrong

Slereah

"No, because the polarization of the electromagnetic field must be perpendicular to the direction of motion of the light, and there aren't enough directions to enforce this condition. So in 1d, a gauge theory becomes nonpropagating, there are no photons, you just get a long range Coulomb force that is constant with distance."

Yashas

I thought the mass of a photon was too small (if not zero) to be measured but using zero as its mass fits in well with the theory. Am I wrong?

mathvc_

6:36 AM

@Slereah Yup thats really true

@Yashas its propagator has a pole at e^2/pi

Yashas

@mathvc_ I am not so far ahead in physics :P still in high school

Slereah

though while there are category theory versions of QFT

Those are only used by math people

Because it's not a very good idea

I'm not sure using category theory really brings up much more insight

mathvc_

Why its not a good idea? I guessed it could be potentially a good idea for unifying and dualities

Slereah

could be, but AQFT can be done without categories at all and it's perfectly fine

mathvc_

Yup but it could make novel insights

I just guess lol

Slereah

6:40 AM

yeah mb

As far as string theory is concerned I'd be fine with any axiomatization at all, really

mathvc_

@Yashas there is no photon actually in 2 dimensions (time+spatial)

Slereah

in the biz we call that a Lorentz surface

mathvc_

@Slereah actually i was interested in gravity-gauge theory

Categorial language may be more helpful there

Slereah

Gravity gauge is a tricky business

Because there's some nuance where diffeomorphisms don't work as a gauge group or somesuch

I don't know the details

it works enough with physicist math but a math people will jump down your throat

mathvc_

U dont think it is a candidate for unified theory?

Slereah

6:45 AM

"In quantum theory, a gauge group is a group of transformations of the algebra of all operators entering the theory, but which leave invariant every hermitian operator representing an observable. On the other hand, a symmetry is an automorphism of the algebra of observables which either commutes with the Hamiltonian, or is part of a dynamical group like the Lorentz boosts."

Yashas

Kids these days

Slereah

I doubt any current theory will work as a unified theory

mathvc_

@Slereah i've seen people in string theory sometimes restrict themselves on only physical states

@Slereah even string?

Slereah

Who knows

I barely know what string theory is, and when I ask what the axiomatization is, people just laugh

Currently it's just an effective theory

mathvc_

I dont know who knows lol

Slereah

6:49 AM

An effective theory that has no predictive power

It's the worst theory

mathvc_

@Slereah yup i agree

Slereah

Loop quantum gravity is probably wrong but at least it's mostly well defined

Really it's probably the only big quantum gravity theory that's somewhat well defined

Slereah

7:57 AM

Is there any complete positive curvature manifold that's not made of spheres

Secret

Conclusion: Biaunal beats are annoying youtube.com/watch?v=MRFC4vAR7yI

NB all this googling begins from:

4 hours ago, by 0celouvsky

google has interdimensional stuff

And so far I found none except spiritual content, which is not what I am looking for

mathvc_

I want to visualize conformal group ($SO(n+2)$) for $\mathbb{R}^n$ with $n>2$. So I started to work with identifying $\mathbb{S}^n$ with $\mathbb{R}^n$. I guessed conformal maps for a sphere is rotations in $\mathbb{R}^(n+1)$, but I conclude the conformal group for $\mathbb{R}^n$ is $SO(n+1)$. Where is my fault?

mathvc_

8:21 AM

any idea?

Slereah

8:41 AM

When people say that a curve "carries over the time orientation", do they just mean parallel propagation of the vector

Anonymous

9:11 AM

Is internal pressure of a solid/liquid always positive?

Anonymous

@JohnRennie Do you know?

Anonymous

$$P_{in}=\left(\frac{\partial U}{\partial V}\right)_T$$ From the formula it seems it can be negative.

Ben Niehoff

@Slereah projective spaces, Grassmannians, flag manifolds...there are loads of positive-curvature spaces that are not spheres or products of spheres

Balarka Sen

amusing transcript.

Slereah

Projective space is a sphere quotient, though

Ben Niehoff

9:15 AM

@Slereah Also LQG is not remotely well-defined, lol

Slereah

Dunno about grassmannian manifolds

Ben Niehoff

Grassmannians are not sphere quotients

neither are complex projective spaces

Slereah

Alright

Balarka Sen

@BenNiehoff They are.

Slereah

Topology fight!

Ben Niehoff

9:17 AM

explain?

Balarka Sen

$S^{2n+1}$ has a natural $S^1$-action from the $\Bbb C - 0$ action on $\Bbb C^{n+1} - 0$; $\Bbb{CP}^n$'s the quotient.

$S^1 = U(1)$, I mean. Lie group action.

Ben Niehoff

ah, ok, I thought we were talking quotients of spheres by a discrete group

Balarka Sen

Ah, alright.

Then you are right.

Ben Niehoff

but Grassmannians and Stiefel manifolds are not sphere quotients of any kind, I'm pretty sure

Balarka Sen

I think so, but I am not sure.

Ben Niehoff

9:21 AM

more generally, if you take a quotient of compact Lie groups, you can get positive-curvature spaces which are not locally spheres

Balarka Sen

The natural metric on Grassmannian makes it a homogeneous space, I think?

How would one prove it's not a quotient of S^n by some Lie group acting on it?

Ben Niehoff

well, the real Grassmannian is SO(p)/(SO(q) x SO(p-q))...I would think as long as q < p-1 and p > 3, you have something that isn't a quotient of a sphere

I think you can prove it by just looking at what subgroups are available and seeing that there's no way to construct the quotient

Balarka Sen

Well, we're not just acting on S^n by a subgroup isometries, are we? If so that should be a lot easier to prove that it's not Grass(p, q)

I don't know how to do this, I give up. Maybe @0celo7 or @ACuriousMind knows.

Balarka Sen

9:48 AM

@Kaumudi.H Well, the gender ratio on STEM academia in India is also to a small extent caused by women not interacting or showing any interest about it, not just being discouraged by people. Does that mean mental faculty is gender-specific? Absolutely not. It's a resultant of ages of misogynist mentality and discouragement, causing lack of self-confidence among a group of female; what we are seeing today is a responsibility the society has to bear (will it? probably not).

Historically speaking this thing started to break off only about a hundred years ago, when the British colonization took it's form.

skill patrol

10:12 AM

in Mathematics, 4 hours ago, by Jonathan Beardsley

For what it's worth, anyone who's interested in increasing the visibility of female mathematicians might want to consider adding a name to this page, or creating an article for a name already on this page: https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Women_in_Red/Mathematics it could use some crowd-sourced effort

2

Slereah

10:44 AM

Ben Niehoff

11:05 AM

lol

Yashas

That argument is baseless.

Fawad

@Yashas prove it

Yashas

Just because you had the username Ramanujan in your previous account, it does not make you Ramanujan.

Slereah

or does it

Fawad

@Yashas just because you don't know this is my same account,you cannot prove something

Strum

11:14 AM

In physics.stackexchange.com/questions/324022/…

could someone ask what is meant by shape optimization? It is not in the wiki lnk

Btw I am new to the chat, so if such requests are frowned upon please let me know.

Fawad

@Strum welcome!,welcome!

Strum

Thank you :)

Slereah

I just found a theoretical GR paper with 7 authors

What is this madness

The paper is 9 pages long, how much work did they all do

Secret

11:30 AM

Do that paper involve supercomputer calculations, if yes that might be why

Strum

Well if it is a collab. 2 super visors. 2 phd's and their 2 undergraduate writers. Already 6 right there.

Slereah

also two of them have AOL emails and there is a reference to a geocities website

ACuriousMind

@Slereah Each "it can be shown that" is probably due to a different one of them :P

Slereah

well it does seem to have a lot of tedious calculations

Also they use $H$ for the Hiscock function

Seems a bit much

But I guess if you write long equations, it is best to condense it a bit

Also they're using $\mathcal S$ as a function

I'm not sure I can approve

Bernardo Meurer

11:51 AM

@ACuriousMind Yo

halp

Do you know how to use LaTeX \newcommand stuff?

ACuriousMind

Somewhat. What's the issue?

Bernardo Meurer

I have to write a lot of binary and hex stuff

so I made these two

\newcommand{\hex}[1]{\texttt{0x#1}}
\newcommand{\bin}[2]{\texttt{0b#2}}

To make my life easier

And now my doc won't compile

gist: 04eda820eb53983839f719391360e5f3, 2017-04-09 11:55:07Z

report.log

Here's the log

What am I doing wrong?

ACuriousMind

@BernardoMeurer Enclose the #1,#2 in their own curly brackets, I don't think LateX detects the # in the middle of strings

That is, write \texttt{0x{#1}}

Also, why have you set the argument to 2 on the second one?

Should be \newcommand{\bin}[1]{\texttt{0b{#1}}}

Bernardo Meurer

@ACuriousMind Trying to build it now, one sec

@ACuriousMind Got it :)

Secret

12:07 PM

@ACuriousMind about that question we discussed earlier, a helpful user have left behind a comment clarifying that they are mathematically well defined. Therefore the remaining question is the physical aspect of those operations, which similar to what that user said, may be a bit involved. I will see whether I can find anymore leads about that before I ask the experiment guys

Secret

uh... let me think...

Yashas

2

A: Is the Charge on proton is $3.2\times10^{-19}$ greater than that of charge on electron?

No, The magnitude of charge on both the particles is same but opposite sign. To understand this you can take an example: As you know that the charge on a neutral atom is $0$ and the number of electrons and protons are equal, now if you say that the charge on a proton is more than the charge on ...

how does $a$ being numerically greater than $b$ imply $a+b$ cannot be zero?

Slereah

12:27 PM

fugg

Emilio Pisanty

quantamagazine.org/…

Slereah

I forget what paper has the proof of acausal metrics for systems of several warp bubbles

I need to sort the papers I have

Emilio Pisanty

Interesting case study of when the diffusion of important new results fails, arXiv and all

Sir Cumference

Ugh, I hate being sick

Slereah

"Do not mess with time: Probing faster than light travel and chronology protection with superluminal warp drives"

good title

skull petrol

12:33 PM

What's so good about it?

Slereah

quite sinister

user228700

@SirCumference :-( Cold?

Sir Cumference

@Kaumudi.H Yeah :/

user228700

:-/ I hope u feel better soon. BTW, did u start reading the book?

Sir Cumference

Thanks :)

And I am gonna start soon, I gotta get everything else under control

user228700

12:36 PM

Wokay...

skull petrol

Most colds are pretty hard to get under control.

At first.

Anonymous

Colds are good (atleast in my case). I can get sickness leave from school. I used to be so bored at school.

Anonymous

Thank god school is over finally.

Shing

I am a bit confused, how do we know the existence of some particles by S-martix for a bound state?

I was watching this: ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/…

at the end the prof said: "If you took particles and a square well and roll the particles across the square well potential and measure it as a function of energy, the scattering amplitude, the transmission amplitude, and in particular S21, an element of the S matrix.....

"... and you plotted it as a function of energy, and then you approximated that by a function of energy that satisfies the basic properties of unitarity, what you would find is that when you then extend that function in mathematica to minus a particular value of the energy, the denominator diverges at that energy. You know that there will be a bound state. And so from scattering...."

"...you've determined the existence of a bound state. This is how we find an awful lot of the particles that we actually deduce must exist in the real world. We'll pick up next time."

0celouvsky

@JohnRennie apparently tax refunds take several weeks

And I don't get paid for two weeks

Life is hard

Shing

12:53 PM

@0celouvsky I understand how that feels, I need to work to save money for graduate school application, GRE, etc. sigh

ACuriousMind

@Shing What you describe is more a resonance than a true bound state, but the two are related. Searching for that term (resonances) should yield you more detailed explanations. I seem to recall there's a chapter on resonances in Srednicki, for example.

bolbteppa

@ACuriousMind conjugacy classes in a clifford algebra? Make sense to you why they are $[I],[-I],[\gamma_{m_i}],[\gamma_{m_j}\gamma_{m_j}],\dots$?

Shing

@ACuriousMind thanks for the help! I will search for it.

ACuriousMind

@bolbteppa Yes, that does make sense to me :P

Slereah

@0celouvsky Geroch keeps talking about carrying a time orientation along a curve

Is it just parallel propagation of the time orientation vector

0celouvsky

1:04 PM

yeah

that's the construction used in the time orientable cover proof

bolbteppa

@ACuriousMind think I got it actually, thanks

@Slereah lqg well-defined? What about motls.blogspot.ie/2004/10/… ?

0celouvsky

motl is a meme

bolbteppa

@Secret tried binaural beats, seem to work but maybe a placebo...

Secret

I never said about them work or not, I jsut say I am kinda annoyed by it (especially the more warped arranged ones)

bolbteppa

1:19 PM

Feels like unless you use the annoying one, they wont work :p

Yashas

1:37 PM

a photon has no length, right?

length contraction for a photon does not make sense?

Slereah

what would be the "length" of a photon

Yashas

exactly

so length contract does not make sense?

Slereah

It has a wavelength

Which does contract under Lorentz transformation

ie redshifting and blueshifting

Yashas

so this is not really correct?

"Length contraction for a massless particle makes no sense. We can't even define the proper length for the known massless particles such as a photon; let alone calculating the length contraction."

wavelength isn't really the length of the particle, right?

Slereah

Well I mean you can have a massless extended object

There's no special objection to that

Yashas

1:40 PM

o0

even if there was a massless extended object, it wouldn't make sense to talk about length contraction, right?

Slereah

why not

Yashas

you get 0 in the denominator :/

Slereah

Yes, a massless object doesn't have a rest frame

Yashas

0

Q: What is your time dilation/length contraction when you move at $c$?

I was playing around with the equation: T0 = T1 ÷ √[1 - (v2 ÷ c2)] Assume 10 seconds is passing. An object is moving at the speed of light. What is the time dilation? T0 = 10 ÷ √[1 - (c2 ÷ c2)] = 10 ÷ √[1 - 1] = 10 ÷ √0 = 10 ÷ 0 = Undefined / ∞ or -∞ Does this mean it is im...

special-relativity time faster-than-light time-dilation

Slereah

But it can have a length in a frame that isn't its rest frame

Yashas

1:41 PM

I wrote an answer then deleted it :P

I shouldn't have tried to answer while I myself had doubts :D

Slereah

"A time orientable spacetime must certainly have every closed curve time preserving"

Does it

Yashas

:O it went above my head

Slereah

This is not as self evident as you'd think, mister Geroch

Yashas

?

Slereah

I am commenting

Hm, let's see

If we have the time orientation bector $\tau$, then take $\tau$ at $p$, then we parallel transport it $\nabla_{\gamma'} \tau_p$

Gotta show that $\tau_p(q)$ and $\tau_q$ have the same orientation

$U^\mu \nabla_\mu g(\tau_p, \tau) = U^\mu [(\nabla_\mu g_{ab}) \tau_p^a \tau^b + g_{ab} (\nabla_\mu \tau_p^a) \tau^b + g_{ab} \tau_p^a (\nabla_\mu\tau^b)] $

$ = U^\mu [g_{ab} (\nabla_\mu \tau_p^a) \tau^b + g_{ab} \tau_p^a (\nabla_\mu\tau^b)] = U^\mu [g_{ab} \tau_p^a (\nabla_\mu\tau^b)] $

Hm, does that help

That's $\nabla_{\gamma'} g(\tau_p, \tau) = g(\tau_p, \nabla_{\gamma'} \tau)$

Don't think that helps finding the sign

Transcript for