The h Bar

0celóñe7

12:06 AM

it is algebra

Phase

1:44 AM

@0celóñe7 $a^2 + b^2$ is indeed $c^2$ for a right angled triangle

0celóñe7

1:58 AM

@Phase uh, ok?

0celóñe7

3:47 AM

@JohnRennie greetings

Amazon is sending me emails with SSDs now

I can get an 850 500GB for like 120 bucks

John Rennie

Morning :-)

@0celóñe7 I bet that's not an M.2 disk like your Samsung though.

0celóñe7

no, but probably faster

John Rennie

That's be a standard SATA SSD.

Still fast but not as fast as the M.2 disks.

0celóñe7

my M.2 is slow

John Rennie

I don't think you realise how fast that disk of yours is.

0celóñe7

3:51 AM

when I open my download folder it takes a good 5 seconds to load

it's slow

John Rennie

I've never seen a standard SATA disk anywhere near as fast, even with the lower benchmarks speeds you get.

@0celóñe7 that's not the disk.

It can't be.

There must be something else causing that delay.

0celóñe7

I get the same speed when I run it as secondary after booting on my HDD

John Rennie

I wonder if something is slowing Explorer down ...

0celóñe7

@JohnRennie I have monitored disk activity and it's low

near zero

for some reason the explorer has a hard time grabbing disk usage from the ether

maybe I have a virus hogging my disk and masking it

John Rennie

Grab a copy of swarchive.ratsauce.co.uk/winfile2k8.exe

0celóñe7

4:00 AM

wtf instadownload

John Rennie

That's the old File Manager from Windows NT4.

0celóñe7

you trying to give me a virus?

John Rennie

It's a self extracting executable. Run it and it will unzip itself.

0celóñe7

Windows defender has blocked it

what is this

John Rennie

It doesn't tamper with your system at all, it is just a self extracting zip.

Windows Defender can be paranoid at times. I use that executable on all my PCs and servers. It isn't malware!

0celóñe7

4:02 AM

ok it unzipped

what now

John Rennie

Run C:\win32app\winfile\winfile.exe

0celóñe7

lmao what is this

John Rennie

You should see something like:

0celóñe7

yeah

John Rennie

On the left pane double click Users, then your username then Downloads and see how fast it displays the files.

0celóñe7

4:05 AM

instaintly, but so does inwdows right now

first one in a week I swear

random read is still terrible, sequential read is back up

John Rennie

Ah, OK. So opening Downloads is slow only when you've recently restarted Windows?

0celóñe7

yeah

you think it's defender doing a scan or something?

John Rennie

That will be because Explorer is looking in each file to see if it has an icon and/or other metadata.

0celóñe7

@JohnRennie but the disk usage is like 1%

it's clearly not trying very hard

John Rennie

Next time you restart try using File Manager to look at the Downloads folder before you open it in Explorer. I bet it will still be instant.

0celóñe7

4:07 AM

Hmm, ok

@JohnRennie In other news, my professor scheduled a geometric analysis seminar without consulting me

I'd been asking for a year, he sent me an email with the time, and it conflicts

Scheduling is a damn mess

skullpatrol

So is your team the skins or the titans?

};-)

0celóñe7

neither

skullpatrol

?

0celóñe7

!

skullpatrol

Come to the dark empire

0celóñe7

4:22 AM

@JohnRennie I'm taking a fluid mechanics class. Is that something you like/know about?

John Rennie

If I'm honest I never learned enough about fluid mechanics to really understand it.

From where I'm standing it looks like an ad hoc collection of random attempts to solve basically intractable equations.

Maybe there is an underlying elegance to the subject, but I can't see it from here.

0celóñe7

there are 8 books for the course

ridiculous

oh god, Landau

skullpatrol

Hopefully they're not 8 intractable books.

0celóñe7

some of these books are quite advanced

Secret

I wonder if Balarka like this style of music:

ClubhOuseFunk - (19 EDO - microtonal funk) -- DMT

►

0celóñe7

4:32 AM

@BalarkaSen

skullpatrol

Do you? @Secret

Secret

It's weird, but its a whole new dimension I don't know about

0celóñe7

my power went out

just in my room

skullpatrol

4:51 AM

check the fuse box

0celóñe7

@skullpatrol what kind of advice is that

Of course I turned the power back on

skullpatrol

Advice to prevent it from happening again.

0celóñe7

Check for what?

skullpatrol

How did you turn the power back on?

0celóñe7

Flipped the switch

skullpatrol

5:24 AM

Sounds like you're overloading the circuit

user84215

5:40 AM

After a long negotiation, I have concluded that it is better to start the MSE University project. Currently a bachelor program in pure mathematics is offered (theoretical physics and phd programs would be offered in future). To people who are interested in participating this project as students or tutors, please go to Registration for the MSE University and inform their enrollments.

Anonymous

7:48 AM

@MathematicsAminPhysics I guess this is an informal program (?) and not a officially recognized one

user84215

8:42 AM

@Blue Yes

Dawood ibn Kareem

9:24 AM

@DonThomasReyes It seems to me that the force you want is going to be $\frac{20 \text{kg}\cdot g \cdot \sin 30^\circ - 80\text{N}}{ \cos 30^\circ}$. Think about breaking both gravity and your horizontal force into a component that's parallel to the slope and a component that's perpendicular to the slope, and ignore the component that's perpendicular.

Physics Meta

10:52 AM

0

Q: Moving a question to a different forum or asking for more attention

I'm posting in meta for the first time so please don't mind some culturally outrageous thing that I would end up doing here and inform me about my peculiarities. So I have this question on stack exchange physics, about the fermi level of materials and I have not received any assistance on it as o...

support

Anonymous

11:06 AM

@MathematicsAminPhysics So how many have registered? Who are going to be teach? What will be taught ?

user84215

11:22 AM

@Blue You can see them in Registration for the MSE University. As I said, details would be determined by the committee.

Dawood ibn Kareem

I don't understand. If I take this course, I can say afterwards "I got my bachelor's degree from a chat room", right?

user84215

With a good approximation, yes.

Dawood ibn Kareem

And in the future, you'll be offering PhD's?

user84215

Yes.

Dawood ibn Kareem

So I'd be able to call myself Dr Dawood?

user84215

11:33 AM

After defending your dissertation.

Dawood ibn Kareem

I see. May I comment that I find this whole concept incredibly insulting, to everyone who has a genuine degree from a genuine bricks-and-mortar university?

5

user84215

It is really important that you have a genuine phd degree?

user84215

If you like, I can call you Dr Dawood.

Dawood ibn Kareem

I have an actual degree, not a PhD, from an actual university. I worked very hard for it, for a number of years. It's really important that this is not devalued by a proliferation of people claiming they have degrees, when they really don't.

5

user84215

This project is not suitable for those people whose main concerns are earning degree at all.

Jim

11:39 AM

question: when referring to this chat room from the outside, do we prefer to say 1) "the h Bar", 2) "The H Bar", 3) "the hbar", or 4) "the $\hbar$"?

Dawood ibn Kareem

@Jim Do you mean "say" or "write"?

Jim

@DawoodibnKareem yes

Dawood ibn Kareem

Actually, Rob, I find those insulting too.

skullpatrol

1)

ACuriousMind

@0celóñe7 what's up?

Jim

11:40 AM

everybody says 1)?

djsmiley2k

waaaaaaaaaazzzzzzzaaaaaap

skullpatrol

Chiiiiiiillllin

djsmiley2k

I don't talk about H bar.

ACuriousMind

@Jim 1), since that's what it is actually named. 4) is also fine, I guess, but you have to expect confusion :P

djsmiley2k

First rule of H bar.

Jim

11:42 AM

@djsmiley2k That is why you failed the first rule

djsmiley2k

Wait, first rule is 'Gravity is not a force'

Right.

2nd rule then :D

skullpatrol

4) requires rendering :P

djsmiley2k

I don't have whatever it is, that makes stuff appear properly, here :<

skullpatrol

h

^Cheap hack

Dawood ibn Kareem

ℏ for the same price.

fileformat.info/info/unicode/char/210f/index.htm - it's very quaintly named.

0celóñe7

11:47 AM

@ACuriousMind Too late now

Secret

What is Polychromatic Music? - An introduction with comparison of modern microtonal instruments.

►

0celóñe7

I ditched measure theoretic probability for Elven lord's modern algebra topics course

Jim

@skullpatrol not really, there's also something special about seeing $\$\backslash hbar\$$

skullpatrol

@djsmiley2k here

Jim

$\bar h$ is just wrong for so many reasons

skullpatrol

11:51 AM

:-)

ACuriousMind

@Jim I have used that symbol a few times, but not to denote Planck's constant :P

Jim

@ACuriousMind I've used $\bar H$, but never $\bar h$. It's like using goto in programming; a lot of languages support it, but any time you use it, you become immediately wrong

ACuriousMind

@Jim In CFT, the conformal weight is occasionally written $(h,\bar{h})$.

And it's lowercase because these two things are just numbers

Uppercase for a number would be just wrong

0celóñe7

@ACuriousMind Uh, energy eigenvalue?

skullpatrol

Why is it so "wrong" to use goto?

Jim

11:57 AM

@ACuriousMind That's too bad. I'm sad to find out that all this time, physicists doing CFT have been wrong

@skullpatrol taboo, look into the history of goto. Essentially it's more trouble than it's worth

0celóñe7

@ACuriousMind here is the book www-users.math.umn.edu/~webb/RepBook

ACuriousMind

@0celóñe7 Yeah, that's wrong.

skullpatrol

BASIC started it.

0celóñe7

@ACuriousMind what? Everyone writes $E\psi=H\psi$

what should it be?

ACuriousMind

@0celóñe7 Still wrong.

Dawood ibn Kareem

11:59 AM

@skullpatrol I seriously doubt that.

ACuriousMind

@0celóñe7 nice

Dawood ibn Kareem

It's clearly named after Kelly.

0celóñe7

@ACuriousMind I figured I could take probability next year but he won't be teaching this course again

skullpatrol

R. Kelly?

0celóñe7

Scott Kelly

@ACuriousMind so is this something you know about?

@ACuriousMind and you didn't say what the energy eigenvalue should be

Jim

12:02 PM

@ACuriousMind avogadro's number

ACuriousMind

@0celóñe7 that book includes a bit more than I know about finite group representations, but yes, it is something I know about

@Jim also wrong

Clearly

Jim

atomic number, Z

0celóñe7

@ACuriousMind I don't think there's any way we get through the book

Dawood ibn Kareem

@skullpatrol gotoresearch.com/kellygoto

0celóñe7

it looks pretty dense

ACuriousMind

12:04 PM

@0celóñe7 My own group theory course covered roughly the first four chapters before we moved on to Lie groups.

0celóñe7

@ACuriousMind wtf, in one semester?

skullpatrol

Thnx @DawoodibnKareem :P

ACuriousMind

@0celóñe7 It was pretty intense (also, a lot of the time-intensive proofs/computations were outsourced to the exercises)

0celóñe7

@ACuriousMind Elven lord loves taking one class for some ridiculous proof

skullpatrol

CS ed.SE would be interested in the taboos of using goto @Jim

heather

12:08 PM

hypothes.is a great web annotation service. works on arXiv papers too.

Jim

@skullpatrol betcha they know all about it

skullpatrol

Yeah, that's a good bet.

Thanks for sharing @heather

0celóñe7

You should clarify whether the "divergence" of the stress-energy tensor is the ordinary or the covariant divergence. — ACuriousMind ♦ 12 mins ago

@ACuriousMind What?

ACuriousMind

@0celóñe7 $\partial_\mu T^{\mu\nu}$ vs. $\nabla_\mu T^{\mu\nu}$.

0celóñe7

The former is not even defined.

It's defined in the minds of physicists.

ACuriousMind

12:22 PM

@0celóñe7 Exactly, and this is a physics site, which is why we got questions like physics.stackexchange.com/q/346793/50583 and its followup physics.stackexchange.com/q/348085/50583, where one point of confusion was precisely this.

0celóñe7

The second seems to have nothing to do with this

And not sure about the first one either

Jim

@skullpatrol stackoverflow.com/q/2545103

Secret

Partial derivatives on tensors are not necessary tensors themselves, but can we really do contractions like that?

Jim

goto is still a keyword in Java, but will throw a compile-time exception if you use it

Slereah

@0celóñe7 is it not defined on the jet bundle or some shit

Jim

12:32 PM

that's some serious hate if you make the program crash for even using it once in your code

Dawood ibn Kareem

@Jim A compile error is not the same thing as an exception.

Jim

@DawoodibnKareem yeah, my brain thought error but I wrote exception because I'm used to complaining about the latter

ACuriousMind

@Slereah I think it is - it's not a proper tensor field, but it does live somewhere

Slereah

The jet bundle is basically Detroit

Where all the worst things live

Secret

$\partial_\mu T^{\mu\nu}$ this is not mathematically defined?

user84215

12:38 PM

@DawoodibnKareem Excuse me if I have disturbed you.

Dawood ibn Kareem

@MathematicsAminPhysics Well, if it's not a bachelor's degree, you shouldn't call it a bachelor's degree. Similarly, the so-called PhD.

0celóñe7

@ACuriousMind How exactly? Partial derivatives depend on coordinates.

skullpatrol

Thanks @Jim I also found this :-)

0celóñe7

It lives in some function space over an open set defined by the chart

Pretty meaningless

Secret

We can have derivative of matrix functions, so what prevents having a derivative of a tensor function which are multilinear maps?

ACuriousMind

12:42 PM

@0celóñe7 You claim that e.g. the LL pseudotensor is meaningless?

If so, then you have finally abandoned physics for pure math entirely :P

Secret

Are partial derivatives of tensors pseudotensor, I do recall back in the undergrad GR course whenever we tried to transform it into another coordinate, it does not just having a change of sign plus the jacobian matrices that showed it indeed transforms like a tensor?

0celóñe7

@ACuriousMind that thing is defined wrt an explicit asymptotic coordinate system

I'm saying that just saying "ordinary divergence equals zero" is completely meaningless

@Secret what

What's what a covariant derivative does

ACuriousMind

@0celóñe7 So LL's third requirement is meaningless?

0celóñe7

I think all physicists should take a good hard look at the axiomatic definition of connections

@ACuriousMind I just said there's an explicit coordinate system in play.

Secret

@0celóñe7 No, I am not talking about the covariant derivative, "$\partial_\mu T^{\mu\nu}$" seemed to be the ordinary partial derivative of a tensor that is formally resembling a divergence, That cannot even be a pseudotensor?

0celóñe7

12:49 PM

@ACuriousMind But you should know that there are sufficiently bad coordinate systems, like ones where the metric decays as $1/r+1/r^\alpha$, where "invariant" quantities defined by such pseudotensors are in fact not invariant

there are some physicist shenanigans with integration by parts that fail

ACuriousMind

@0celóñe7 Well, no one is claiming the pseudo-tensor is invariant. What they are claiming is that its 4-divergence is invariant (I think)

And I am willing to believe you that there are some mathematical issues with actually defining these objects to a mathematical standard of rigor

0celóñe7

@ACuriousMind Yes, ordinary divergence in a chart, which is specified

I'm saying that writing down $\partial_\mu T^{\mu\nu}=0$ without further commentary is meaningless

Secret

right ok

0celóñe7

Because you can just go to polar coordinates or something and that's no longer true

You see the same issue in $\Bbb R^n$, really. It's a danger of the compact notation we use for GR

ACuriousMind

@0celóñe7 True. Still, I think it would be good to explicitly state that the divergence that's meant in Slereah's answer is a covariant divergence. You know how bad physicists are at this, why make it even harder for them? :P

Secret

12:53 PM

nvm, I misread, it's not the covariant derivative, all is good

0celóñe7

@Secret Actually no, you're right. $\partial_\mu T^{\mu\nu}=0$ has no meaning in special relativity either

You have to specify a Cartesian coordinate system

ACuriousMind

@Secret y'know, since you deleted the message that refers to, someone reading this after the fact is not going to know what you're taking about at all :P

0celóñe7

People like to say that $\partial=\nabla$ when the metric is flat, but that's only true in a flat coordinate system

Secret

@ACuriousMind Opps sorry, that's how it is commonly done in the maths chat, looks like I inherited one of their cultural habits...

0celóñe7

In some fields (good fields like analysis) it is always assumed to be a Cartesian coordinate system, so we can get away with it

But in GR where people like to have coordinate freedom, they also need to shoulder some responsibility and be more precise

@ACuriousMind trying to come up with some flippant Marie-Antoinette response to that

ACuriousMind

12:57 PM

I don't think I've ever seen anyone but Straumann fully commit to the coordinate-free notation.

0celóñe7

@ACuriousMind Wald

Abstract index notation is a beautiful thing

Secret

I think I recall in my undergrad GR course, we wrote something like this for the covariant divergence: $\nabla_{\mu}T^{\mu\nu}$

0celóñe7

He does it incorrectly, but still :P

@Secret Yes, and that has meaning

ACuriousMind

@0celóñe7 Ohhhh, I think many physicsts will defend their notations by claiming it was abstract index notation all along!

0celóñe7

That's the "same" object regardless of coordinate system, so you can be forgiven for not explicitly stating that you're using one.

@ACuriousMind But the problem is that $\partial_a$ has no meaning in the abstract index notation.

You need coordinates to define it, and it's a patently different object than another derivative in a different coordinate system.

ACuriousMind

12:59 PM

I wasn't saying they would be right ;) (I don't really know about abstract index notation :P)

0celóñe7

Wald does write things like $\nabla_a V^b=\partial_a V^b+\Gamma^b{}_{ac}V^c$, which is horrendous

@ACuriousMind In abstract indices you denote the argument of a tensor (seen as a multilinear map) by an index. So if $\omega$ is a 1-form and $v$ a vector, then $\omega(v)=\omega_av^a$, etc.

You can use the flexibility of indices without having to define coordinate systems

That being said, often you want to work with Christoffel symbols, etc., and those are not tensors

And this being a notation, you can't really "prove" anything about it, but it's certainly against the spirit to write something like $\Gamma^a{}_{bc}$.

@ACuriousMind Now Straumann is also dishonest. He doesn't use special symbols for abstract indices. He does everything with indices in one style, then says "interpret the indices as abstract when applicable"

Secret

Christoffel symbols is like one of the most horrible looking thing under a change of coordinates

$\partial_{\mu}\partial_{\nu}-\partial_{\nu}\partial_{\mu}$ is one of the terms you can get if I recall

ACuriousMind

1:16 PM

@Secret That's...not a term that appears in the Christoffels at all.

Anonymous

@MathematicsAminPhysics It'll be better if you don't call those "degrees". You might just say that the MSE University you are designing will offer "courses" (like MOOCs do) by voluntary teachers in chat rooms. You can set course timing as events. Anyone who is interested can register themselves there. However, since it is a virtual chat room we can't do anything if the teachers fail to turn up at the right time.

Secret

10

Q: Christoffel symbol transformation law

It is known that the transformation rule when you change coordinate frames of the Christoffel symbol is: $$ \tilde \Gamma^{\mu}_{\nu\kappa} = {\partial \tilde x^\mu \over \partial x^\alpha} \left [ \Gamma^\alpha_{\beta \gamma}{\partial x^\beta \over \partial \tilde x^\nu}{\partial x^\gamma \over...

differential-geometry tensors

Sorry I misremembered, it appears I only remember there are 2 terms and one of them is a product

Anonymous

It seems ridiculous when you say the MSE university will be offering "degrees" like "Bachelor/Masters/PhD" :P

Anonymous

@MathematicsAminPhysics

ACuriousMind

@Blue There is a dedicated room for feedback where most of what you want to say has probably already been said. Maybe have a look there before you start a new discussion

Anonymous

1:19 PM

@ACuriousMind Oh, thanks. I'll see. I don't think I have much more to say about this though.

Balarka Sen

The whole MSE university idea is a big utopian nonsense. It has already been settled in various lengthy conversations in chatrooms and piles of downvotes on the MSE meta

I don't see why the OP is pursuing this far fetched trash anymore, but if he so wishes.

0celóñe7

@BalarkaSen I'm taking a course on finite group rep theory

what's a group

help

Secret

Not to mention, I have not even have the chance to launch the ICBM yet! (metaphor)

ACuriousMind

1:36 PM

Please confine discussion of the "MSE university" to its dedicated chatroom. Thanks everyone.

→ 8 messages moved to Trash

0celóñe7

@ACuriousMind What the hell?

user84215

I think it is a dirty work that one wants to criticize an idea in this way.

0celóñe7

Yeah, I am surpsied @ACuriousMind

Has someone told you to censor this?

ACuriousMind

@0celóñe7 What?

You said yourself that some of what I just moved was not "Nice".

And since there is already a room full of discussion about this topic, I see no purpose in having it also spill into here.

skullpatrol

Case closed.

Mostafaııııııııııııııııııııııı

1:44 PM

@ACuriousMind Really? What tool do you use to do simple quick stuff with pictures (like cropping) then?

ACuriousMind

@Mostafaııııııııııııııııııııııı GIMP

Mostafaııııııııııııııııııııııı

Does it have a GUI?

or scripting only?

ahhh I missed the AMA with @Mithrandir24601 :(

Anonymous

@Mostafaııııııııııııııııııııııı Me too. I was sleeping then. :P

Anonymous

It was around 2am-3am over here

Mostafaııııııııııııııııııııııı

Seems you guys have forgotten that bookmarking a conversation is feature that exists @ACuriousMind

Going to bookmark it now

ACuriousMind

1:50 PM

@Mostafaııııııııııııııııııııııı It has a GUI

Giskard42

@Mostafaııııııııııııııııııııııı Gimp's got a gui

skullpatrol

You still there? @0celóñe7

Mostafaııııııııııııııııııııııı

going to bookmark from the beginning of the AMA to this:

yesterday, by AccidentalFourierTransform

later alligator

0celóñe7

@skullpatrol sure

skullpatrol

cool

:-)

ACuriousMind

1:57 PM

@Mostafaııııııııııııııııııııııı Heh, tradition must be upheld :P

Mostafaııııııııııııııııııııııı

OK, for the sake of tradition:

I have bookmarked the AMA with Mithrandir

2

skullpatrol

Thanks @Mostafaııııııııııııııııııııııı

Mostafaııııııııııııııııııııııı

@skullpatrol no problem ;)

consider a free particle in 1D. Is its Hilbert space separable?

ACuriousMind

@Mostafaııııııııııııııııııııııı Yes

skullpatrol

What do all those lines on your username mean? @Mostafaııııııııııııııııııııııı

Mostafaııııııııııııııııııııııı

2:12 PM

@ACuriousMind but separable means it admits a countable orthonormal basis. considering that to each and every real number is assigned an eigenvector of this Hilbert space, how is this countable then?

ACuriousMind

@Mostafaııııııııııııııııııııııı Ah, that's because the $\lvert x\rangle$ are not actually vectors in the Hilbert space!

skullpatrol

Other than being unique :-)

Mostafaııııııııııııııııııııııı

@skullpatrol nothing. Just a totally random thing

@ACuriousMind What??

Can you explain more?

@ACuriousMind What is the Hilbert space of that system then?

ACuriousMind

@Mostafaııııııııııııııııııııııı See e.g. physics.stackexchange.com/q/249851/50583, physics.stackexchange.com/a/90261/50583. The Hilbert space is $L^2$, but the $\lvert x_0\rangle$ would correspond to $\delta(x-x_0)$, which isn't even a function, let alone square-integrable. A countable Hilbert basis of $L^2$ is given e.g. by the stationary states of the harmonic oscillator.

0celóñe7

@ACuriousMind Seems pretty square integrable to me. Clearly $\delta^2=\delta$ by evaluating at points, so $\int\delta^2=\int\delta=1$.

@ACuriousMind Hmm, are the dirac deltas a basis of the space of tempered distributions in some sense?

Mostafaııııııııııııııııııııııı

2:25 PM

From that answer: a countable set can span a vector space of uncountable dimension. wow!

Черенки

what are some of the holes in the standard model?

Mostafaııııııııııııııııııııııı

@ACuriousMind but Shankar never talked about these ::feeling betrayed::

0celóñe7

@Mostafaııııııııııııııııııııııı That's wrong.

Mostafaııııııııııııııııııııııı

@0celóñe7 that quote? (from ACM's answer)

Anonymous

@Mostafaııııııııııııııııııııııı I guess it should be a "countably infinite set" rather than any "countable set" ? (Pardon me if I'm saying something silly)

0celóñe7

2:28 PM

@ACuriousMind Uh, did you really say that $L^2(\Bbb R)$ has uncountable dimension?

Mostafaııııııııııııııııııııııı

@Blue yes he means a countably infinite set

Anonymous

@Mostafaııııııııııııııııııııııı Yeah. Thanks

Mostafaııııııııııııııııııııııı

@0celóñe7 what is wrong?

Balarka Sen

It's just that span means something different in infinite dimensional vector spaces.

0celóñe7

Yeah, ACM is mixing terminologies.

He is using dimension in the Hamel sense but span in the Hilbert sense.

Balarka Sen

2:30 PM

I think ACM is aware of these. He's just saying it's uncountable dimensional if we take span in the Hamel sense, but countable dimensional if we take span in the Hilbert sense.

John Rennie

I have a possibly silly question about Feynman diagrams if anyone can help ...

Balarka Sen

"a countable set can span a vector space of uncountable dimension" is just a slogan for that. He's writing an answer for physics.SE, not math.SE, after all.

Anonymous

Wow. This is the first time I'm seeing JR asking a doubt. :P :)

0celóñe7

Yes, ACM is aware of these, but I think that most doubts in physics are because of poorly explained math

John Rennie

If you take a diagram representing Compton scattering then you have a photon and electron coming in and a photon and an electron going out. If you rotate the diagram 90º, i.e. swap the time and spaces axes, then you have another diagram with an electron and poitron coming in and two photons going out.

0celóñe7

2:33 PM

annihilation

John Rennie

So the second diagram looks like pair annihilation to two photons.

Is it a general rule that if you rotate a Feynman diagram 90º then you'll get another diagram that represents something physical?

Presumably this applies to diagrams with any number of vertices since we're only considering what comes in from infinity and what goes out to infinity.

0celóñe7

what the heck is a free module

Черенки

"In theoretical physics, Feynman diagrams are pictorial representations of the mathematical expressions describing the behavior of subatomic particles."

nevermind

Anonymous

demonstrations.wolfram.com/…

Mostafaııııııııııııııııııııııı

@0celóñe7 But why doesn't Shankar mention this??

WHY

Anonymous

2:40 PM

"You can rotate an vertex into eight different orientations, each describing a completely different physical process. Included are electron-positron creations and annihilations, which contain the essence of Einstein's mass-energy relation ." Interesting. Got to read about this

ACuriousMind

@0celóñe7 I expressly said "as a vector space over $\mathbb{C}$", in which case the claim is true.

0celóñe7

@Mostafaııııııııııııııııııııııı because the math is too hard

Mostafaııııııııııııııııııııııı

I'm done with Shankar's QM

4

ACuriousMind

@Mostafaııııııııııııııııııııııı It's not Shankar's fault. Almost no physicist discusses these issue unless they come up - otherwise everyone just pretends one is doing linear algebra. Not saying that's the optimal state of things, but that's how it is

Balarka Sen

@Mostafa loool

0celóñe7

2:41 PM

@BernardoMeurer look ^

John Rennie

@Blue hmm, thanks. That implies that rotating a Feynman diagram does always produce something sensible, though it doesn't actually say so explicitly.

0celóñe7

@Mostafaııııııııııııııııııııııı there are much better uses for physics books

such as keeping fans away from carpeting

John Rennie

Though note that putting books on string theory near a computer will cause any SSD disks inside it to mysteriously run slow.

ACuriousMind

@JohnRennie Maybe some of the data is escaping into extra dimensions.

0celóñe7

HOLY SHIT YOU'RE A GENIUS

Anonymous

2:47 PM

@JohnRennie I haven't read this well, but it seems to discuss the rotation of Feynman diagrams.

Anonymous

You could check out the book. But well, I don't think I know enough to help you with this. My knowledge is limited to pop-sci Feynman diagrams. I hope to take up a serious particle physics course someday in the near future.

ACuriousMind

You're not really "rotating" the diagrams because a Feynman diagram doesn't really have an orientation in space - it's just a type of graph

0celóñe7

@ACuriousMind pedant

Черенки

OK so i want to learn about quantum mechanics and GR. I know single variable calculus and classical physics. I know newtonian mechanics and a modest amount of electromagnetism and optics. Can anyone recommend me path to achieve my goal stated above. thx

ACuriousMind

What this is likely referring to is the different "channels" one gets from diagrams that look as if they are rotated, but which should be thought of as different assignment of "in/out" to the external lines

Anonymous

2:49 PM

@Черенки I'm in the same situation as you. Start with the "for dummies" books and start learning Linear Algebra side-by-side (at least).

Anonymous

I'm doing the same

Anonymous

QM is more about math and less about physics

Anonymous

Same goes for GR

ACuriousMind

@Blue That's news to me :P

0celóñe7

@Черенки read in this order: Abbott, Munkres, Jost (Postmodern), DiBenedetto, Conway (One variable, volume 1), Lee (smooth), do Carmo (Riemannian), Evans (PDE)

Черенки

2:50 PM

shouldn't i have a solid background in classical physics

John Rennie

@ACuriousMind well, OK, you can take the rotation to mean a reassignment of the in and out channels. Anyhow the book @Blue linked says that yes rotating Feynman diagrams (in the sense I mean) doe always result in a diagram that makes physical sense.

Anonymous

@ACuriousMind Sorry. I shouldn't have said....more "about" math

ACuriousMind

I do not think QM is any more "about math" than classical physics is.

Anonymous

It includes a hell lot of math

ACuriousMind

At least not when you understand "classical physics" to consist of Lagrangian and Hamiltonian mechanics, as I do.

Anonymous

2:51 PM

That's what I should have said ^ :P

Черенки

@0celóñe7 what is postmodern, smooth, riemannian and PDE

0celóñe7

@ACuriousMind r/iamverysmart

Balarka Sen

(lol)

ACuriousMind

wat

0celóñe7

@ACuriousMind the intended meme lord understood

@ACuriousMind do you know how to use google

You would get a kick out of it

ACuriousMind

2:54 PM

@0celóñe7 No.

You have infected me with the inability to use it :P

Balarka Sen

lmgtfy.com/?q=r%2Fiamverysmart

Черенки

@0celóñe7 can you properly state your recommendations without abbreviating

0celóñe7

Maybe

Balarka Sen

i wonder what happens if i lmgtfy lmgtfy

shit is bound to get real

0celóñe7

@ACuriousMind there's something wrong if you think Functional Analysis is the same level of math as analytical mechanics

ACuriousMind

2:57 PM

@0celóñe7 There is something wrong if you think QM as done by physicists actually requires functional analysis ;P

Anonymous

@Черенки 0celo is renowned for giving the worst book recommendation to beginners. Don't listen to him. :P

0celóñe7

Actually those are the books i used as a beginner

Черенки

@0celóñe7 so who is renowned for giving the best book reccomendations

@0celóñe7 so what are the names of the books that you used

0celóñe7

I am

I'm taking a PDE course that has 6 books, and I own all of them already. Coincidence?

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