The h Bar

0celo7

18:02

Does anyone know of a tool to calculate Fourier series?

FenderLesPaul

it's called the brain

user54412

I was just going to suggest paper and pencil

HDE 226868

@ChrisWhite Thanks; I wasn't aware of that difference.

@ChrisWhite My condolences.

FenderLesPaul

@ChrisWhite that happened to a friend of mine but he committed suicide

he was at the top of his class and was headed for great places

0celo7

18:24

@ChrisWhite @FenderLesPaul I already did that, I want to check it now...

Ugh, doing limits...WHY

ACuriousMind

@0celo7 Have you tried typing "Fourier series of [your function here]" into WolframAlpha? :P

0celo7

@ACuriousMind Huh, no

Wonder if that works

It's piecewise, that's the problem

dmckee

Alpha should be your first stop for CAS if you don't run CAS as a matter of course.

0celo7

Well I left my laptop at home

Which has a CAS on it

And I left my CAS calculator there too

dmckee

Hm. I don't have a goto CAS, because I've been waiting for a open source one that's both powerful and relatively easy for a duffer.

I don't like to be dependent on them, so I don't want to spend the time to get good at one.

So that means I turn to Wolfram Alpha a few times a semester. Relatively nice system.

Danu

19:33

Heyo

yuggib

oyeh

Danu

I settled on a supervisor

yuggib

who would that be?

Danu

Brunner

My first choice would've been Mayr, but he's not taking any students :(

...but he told me to go to his collaborator, which is Brunner :)

and he'll serve as an unofficial second supervisor of sorts

0celo7

algebra problem set involves p-adic numbers, no clue what those are

I'm fukt

Danu

19:38

Expansions in powers of different numbers than 10

It's simple in principle.

Just go to any ol' algebra book.

0celo7

I'm sure it's in my book. But the snow day messed up the schedule.

He moved the problem set by a week, which is good. I don't know these words.

I was trying to get a head start on it...that's not going to happen.

yuggib

it's strange what they call mathematical physics in the physics department of LMU...

Danu

@yuggib Why?

It seems there is a good spread, from FA/MQM people to geometry/string theory etc

yuggib

I am talking about the group "mathematical physics and string theory"

Danu

Well that's not the only group doing mathematical physics.

That's the group doing string theory.

@DanielSank So you have the same problem with setting $c=1$?

0celo7

19:43

This exercise...what? (i) Prove that Ramanujan's statement is true. (ii) Prove that Ramanujan's statement is false.

It's the taxi cab thing

Woooooooow

yuggib

@Danu yeah, but I frankly don't see the mathematical physics part...do they publish in journals of mathematical physics?

0celo7

Turns out he said "integer" and not "natural number"

Danu

@0celo7 Is that an "or"? :P

@yuggib Hah, duh!

Sorry but you're sounding awfully biased.

yuggib

why?

Danu

"do they publish in journals of mathematical physics"

I mean... really?

yuggib

19:45

well, that would reflect the fact that they use the mathematical standards of mathematical physics

the difference between theoretical physics and mathematical physics is quite evident in my opinion

Danu

You're probably not very well-versed in high energy mathematical physics, eh? :P

yuggib

it depends

Danu

Mayr, for instance, is a nice example of mathematical physicist in HEP community: inspirehep.net/…

yuggib

I know (a little bit) the connes/van suijlekoom works

Danu

He publishes a lot in dedicated math.phys. journals.

...but also in other journals!

(he doesn't publish a lot in general, regrettably :P)

yuggib

19:48

the only one that is slightly math phys is adv.theor.math.phys.

but only slightly :-P

0celo7

Wtf is math phys anyway

Danu

Or my supervisor: inspirehep.net/…

yuggib

iamp.org/page.php?page=page_start

at least here there are some comm math phys

Danu

And Proc.Symp.Pure Math.

:P

hahaha

yuggib

anyways, another hint that your not so much in the math community is that you use spires as a search engine....so theoretical physics :-D

Danu

19:50

@yuggib It's the only one that keeps good track of citations!

MathSciNet is crap.

yuggib

@Danu that's exactly physicist talking

Danu

Also, it doesn't keep track of high energy physicists.

0celo7

Wtf the book just assumes you know what p-adic numbers are??

yuggib

(anyways, mathscinet is partially crap I agree)

Danu

@yuggib Suit yourself, you don't seem to be interested in hearing anything except your own prejudice ;)

yuggib

19:52

@Danu that's not true

Danu

@yuggib You don't get to determine what you seem to be doing ;)

by the definition of "seem" :P

yuggib

I simply say that what your supervisors do is not commonly considered math phys

Danu

@yuggib I dare say that your "commonly" is restricted to a very small community.

(much smaller than the HEP community...)

yuggib

at least by the community that calls itself international association of mathematical physics

surely it is much smaller than the HEP community

no doubt in that

Danu

So, do you think Witten is a member of it?

I'm curious to know.

yuggib

19:54

of course not...he is a physicist

(as said clearly in his fields medal motivation speech)

Danu

Okay, if you'd like to define mathematical physicist to exclude him, be my guest (but go and stand in the corner!).

yuggib

Mathematical physics is a part of mathematics

Danu

Wrong. That'd be "physical mathematics" ;)

yuggib

and is common practice in it to write papers with the same structure as mathematical papers

Danu

Mathematical physics, in the end, tries to say something about physics.

yuggib

19:57

@Danu It's everywhere called mathematical physics

Danu

Of course you may try to keep in line with mathematical practice, but pure mathematics is not the goal.

yuggib

apart in france, where mathematical physics do not exist

I am just saying it is a problem of names; it is clear that the communities are different

0celo7

@yuggib Is the series "monographs in mathematical physics" improperly named?

yuggib

but if you apply for a position in mathematical physics in italy, believe me you will be considered out of topic

but perfectly on topic for a position in theoretical physics

Danu

Then I think you guys are the ones who have the funny definition ;)

Majorities get to decide on names!

yuggib

20:00

@0celo7 it has titles of people in my community

0celo7

@yuggib Are you familiar with Hawking-Ellis? Is it mathematical physics or just theoretical physics?

yuggib

@Danu well, you can argue that with the italian state university system, or many other ones

on the property of names

or with the international association of mathematical physics

0celo7

@yuggib Is there something that instantly excludes something from being math.phys?

yuggib

@0celo7 well, I don't know

it is just a matter of practice probably

@Danu Would you say that spectral theory of magnetic laplacians on domains with edges would be part of mathematical physics?

It is something I don't do, so I can't be too much biased on that

(I also find it pretty boring)

Chinatsu-creepy-chan

20:22

Hi there!

dmckee

Hi.

HDE 226868

@Chinatsu-creepy-chan Greetings from the ninth planet.

Chinatsu-creepy-chan

Talking to mods iz spooky e_e

:P

Could you guys help me understanding something, please? Simple mechanics.

HDE 226868

Ask and someone may answer.

Chinatsu-creepy-chan

20:38

Suppose you want to move a couch from point 1 to point 2. So you have $\vec{F} = m\vec{a}(1 - M)$ where M is the friction coefficient. But suppose you want to have constant velocity while moving this couch. Then, for example, $\vec{J} = 0$. So I want to calculate the work, which is obviously not null. So $W = \int_{s1}^{s_2} \vec{F}\,ds$. Well, but the force is null because the acceleration is null.

dmckee

What's $\vec{J}$ here?

In any case the net force is null because acceleration is null, but the force applied by the mover (person) is not.

Chinatsu-creepy-chan

So in cases like these I must use $W = \int_{s_1}^{s_2} \vec{F}\,ds = \int_{v_1}^{v_2} m\frac{d\vec{v}}{dt}\,v\,dt = \int_{v_1}^{v_2} m\vec{v}\,d\vec{v}$ ?

@dmckee Impulse. That's the symbol we use in Brazil

dmckee

Sure. That the symbol about half the book use in the US, too, but I didn't expect to see it in this problem.

Chinatsu-creepy-chan

ugh, I won't put vec in all those v's :P

@dmckee It was just an example

dmckee

The problem is asking you to compute the work done by the person, and while the couch is in motion (even constant) the person is still exerting a force because he or she must counteract friction.

Chinatsu-creepy-chan

20:45

So the total energy (both mechanic and kinetic ) is (mv²)/2 ?

@dmckee yes

dmckee

It's also typical to simply assume that the little bit of extra needed at the start to get things going is canceled out by the little bit less at the end while letting it stop.

Chinatsu-creepy-chan

@dmckee static friction?

dmckee

The adjective "kinetic" is not normally applied to work, but rather to energy. There is a difference. Work is a property of an interaction, and energy of an object.

Chinatsu-creepy-chan

fixed

dmckee

The mechanic does work to get the couch up to speed and to prevent it from slowing. While it is moving the couch has kinetic energy, but that energy goes away when the couch stops. The work done by the mechanics is not refunded when he lets the couch stop. Instead the energy has gone into frictional heating.

DanielSank

20:47

@Danu Yes. One does not "set $c=1$". Usually one decides that $x$ means "distance divided by speed of light".

@TanMath Plot? Does it work?

dmckee

@Chinatsu-creepy-chan The couch is moving, right? So kinetic friction. OH, was that what you meant by kinetic work? I see. I would write "the work of kinetic friction", becuase the short version is confusing.

Chinatsu-creepy-chan

@dmckee I'm supposing a huge friction force (well, I think that this happens in real life, the couch wouldn't start sliding), so I just consider the couch would stop moving almost instantly after stop push/pulling it

dmckee

You can use the work--energy relationship to figure the energy needed to get the couch up to speed and find the work done against friction to keep it moving with a direct integration $\int_\text{path} F_\text{to resist friction} ds$.

You seem to have all the right ideas, you just need to paste them together with the proper attention to detail.

Chinatsu-creepy-chan

@dmckee is this supposed to be a surface integral? :P

dmckee

No. It's a line integral. I'm just to lazy to bother with detailed notation here because you clearly know how to deal with those.

0celo7

20:56

Laplace equation on a rectangle is messy

Danu

21:08

@DanielSank Not really

I feel like it's more accurate to say that you just express all velocities as fractions of $c$

But I guess that's just semantics

It's the same as refusing to write out $x= 5 (\mathrm{m/s}) t$ (when $v=5$ meters/second), and just writing $x=5t$ instead.

I think one isn't really tied to saying that the dimensions are the same, though I recognize that this is the usual way it's presented.

1

Q: How can one confirm creation of magnetic graphene?

Recently my high school physics teacher contacted me looking for an expert to ask about this situation. I am paraphrasing his correspondence below: I just had a a student accidentally create magnetic graphene. This has been done before, but we did it very simply by exfoiliating with a magnt...

Interesting!

0celo7

21:26

...magnetic graphene in high school?

And a teacher who knows what magnetic graphene is?

DanielSank

@0celo7 Huh, isn't that the simplest possible case?

Danu

I think the disk should be easier.

It has more symmetry.

0celo7

@DanielSank The boundary conditions are horrible.

You have to split it into four functions and solve an almost-homogeneous problem four times.

DanielSank

@Danu Dude... $x=5t$ makes perfect sense if you say that $x$ is position divided by speed of light.

Danu

@DanielSank Like I said, I think it's semantics.

DanielSank

21:28

@0celo7 What? You said "on a rectangle". Boundary conditions are another issue all together.

Danu

I don't like putting it the way you do.

DanielSank

@Danu Important semantics!

Danu

But it's definitely the same equation.

0celo7

@DanielSank Yes, with boundary conditions on the rectangle

Danu

I prefer what I said above.

DanielSank

21:29

@Danu k, for the record I find it confusing.

0celo7

@Danu Yes, the disk is easier.

DanielSank

The velocities thing is fine, but then how do I read $x = 5 t$.

@0celo7 You drunk/high?

Danu

You can read it as $x=5(\mathrm{m/s})t$ if you insist

It's just not important.

Just as writing $c$ is not important :P

DanielSank

@Danu aaaaaaaaaaaahhhhhhhhhhhhhhhhhhhhhhhhhh

0celo7

@DanielSank No, wtf is wrong with you?

DanielSank

21:30

Dammit noooooooooooooo!

@Danu Why would you have implied units in an equation?

Danu

Like I said, I don't think one is actually tied to saying the dimensions "are the same"

DanielSank

Physics equations do not have units!

Danu

@DanielSank Lolwat?

You high, man :P

0celo7

^^^

Thank you good Danu

DanielSank

If I write $F = m a$ there is absolutely no choice of units anywhere in sight.

Danu

21:31

Lol

Okay, whatever you say :)

DanielSank

@Danu What?

Why would I be required to know that your velocities are to be thought of as $m/s$?

0celo7

You're going to kill @DanielSank one of these days.

DanielSank

Dimensions and units are not the same thing.

Danu

Okay, I think you're massively overthinking this issue. If you want to nitpick about it, count me out :)

DanielSank

@Danu I'm not trying to nitpick. Dude, this issue comes up on main all the time. People really are confused about it.

Calling it nitpicking and leaving the room is a cop out.

Danu

21:33

Okay, then I will leave it to you to enlighten these poor souls.

I do not feel called upon.

DanielSank

@Danu heheh ok

Danu

(I'm not leaving the room)

DanielSank

@Danu I meant that in the figurative sense.

0celo7

So you care to explain why I'm intoxicated

Danu

@DanielSank Now that's confusing because there is a literal sense ;)

DanielSank

21:34

@0celo7 See comments about rectangles.

@Danu My mistake.

0celo7

I did, they don't make sense.

Solving the Laplace equation on a rectangle with boundary conditions on the rectangle is very ugly.

What's so hard to understand about that?

Danu

@DanielSank Last thing I'll say about this: Because it's just easier to have this understood in all your equations, and this is exactly why people leave out $c$ everywhere. As long as everybody knows, there is no issue.

0celo7

*on the edge of the rectangle

Danu

Of course, that leaves a pedagogical... subtlety :)

0celo7

@DanielSank do you just not believe me that it's ugly?

I don't get it.

Danu

21:42

It actually seems fairly simple, unless you have really shitty boundary conditions.

HDE 226868

@Danu That is cool.

Danu

:)

FenderLesPaul

@0celo7 politically incorrect terminology sir

0celo7

@Danu To solve the eigenvalue problem you need homogeneous boundary conditions, which is not the case for Laplace on a rectangle

So if the boundary condition is u=0 on edge of rectangle it is not so bad.

But if you have u(0,y)=g(y), u(L,y)=h(y), etc. it's bad.

@FenderLesPaul no clue what you're referring to

Danu

@0celo7 Nice trick: By linearity you can solve it for nonzero only on one edge first.

0celo7

21:49

Hmm?

Danu

math.psu.edu/yzheng/m597k/m597kLVI7.pdf

0celo7

My book says to split u into 4 parts, each with one nonhomogeneous boundary condition. It's then possible to solve it for each part, and put the whole thing back together

Danu

Right, exactly.

0celo7

Yeah, that's not a nice solution...

Danu

It's pretty nice, IMO

0celo7

21:51

You get four infinite series involving trig and hyperbolic trig functions :/

Danu

If it's (h)trig, it's nice :)

Mike Miller

You can explicitly write down the solution. That is extremely nice.

0celo7

@MikeMiller not if I have to come up with it on a test!

Danu

Hi @Mike

Have you heard of mirror symmetry?

Homological mirror symmetry

Homological mirror symmetry is a mathematical conjecture made by Maxim Kontsevich. It seeks a systematic mathematical explanation for a phenomenon called mirror symmetry first observed by physicists studying string theory. == History == In an address to the 1994 International Congress of Mathematicians in Zürich, Kontsevich (1994) speculated that mirror symmetry for a pair of Calabi–Yau manifolds X and Y could be explained as an equivalence of a triangulated category constructed from the algebraic geometry of X (the derived category of coherent sheaves on X) and another triangulated categ...

DanielSank

@Danu That was what I meant.

@Danu Yeah, and then they go around saying "I set $c=1$" instead of saying what they're actually doing, and all the students are confused.

Why not just say what you mean?

Say "$x$ means $x/c$".

Done.

Danu

22:02

I don't like it :D

DanielSank

k

2

Mike Miller

22:40

@Danu: Yes. I'm in a meeting for the next six hours in a room with neither cell service nor wifi so I may be slow to respond.

Danu

23:41

@MikeMiller fair enough. My thesis will hopefully be related to it :)

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