The h Bar - 2016-10-30 (page 4 of 5)

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user116211

3:00 PM

yes, totally true; and they are not bothering much about the policy making for the homework.

user116211

Things are going....

user116211

And things are going....

user116211

As if there are no problems.

@S007 I don't think this is true.

user116211

@Sanya There are many good posts in Math.SE; the thing I get while googling is Math.SE's post. But lately, they are infested too much with homework posts.

3:03 PM

It is true that the type of users that want to ask questions that are not really suited for PSE are chased away much faster and more efficiently than on other sites. But that's not a bad thing.

Anonymous

@Danu I said something in brackets

Geology is not real science

Lololol

3:18 PM

could someone explain intuitively what magnetic flux is? I can't seem to understand it.

An important part of learning is being shown how to do something. This applies whether you're learning quantum field theory or how to fix cars. However the PSE homework policy prohibits questions that amount to *Show me how to solve this problem*.

Actually I agree that this is a good policy for the PSE to have, but it would be nice if there was a site with the aim of teaching students physics by showing them how to do it. That's all the proposed site is - it is as simple as that.

This doesn't strike me as controversial, and I'm surprised it's generated so much debate.

3

@heather The "amount" of magnetic field lines passing through a given surface.

@ACuriousMind, thank you, that's really clear. =)

Anonymous

"Actually I agree that this is a good policy for the PSE to have" ---- Can we just do an experiment for a month by allowing them and see the results ?

Anonymous

@JohnRennie

3:21 PM

@JohnRennie I don't think the controversial part is what the proposal is, or how it's different from physics.SE. The controversial part is whether the proposal is suitable as an SE site.

user116211

@S007 Like the one done in Chem?

Anonymous

@MAFIA36790 yeah!

@S007 You are free to propose that on meta. However, my personal opinion is: Hell, no!

3

user116211

Yes, Chem has a different situation than Phys.

user116211

Thy have less traffic then physics - one reason if I want to mention.

3:23 PM

@S007, I'd have to agree with ACM on this one. Besides, even if I did support it, I doubt enough others would.

@ACuriousMind for a small subset of meanings of the word controversial. There isn't a right or wrong answer. The SE pay for all this and they don't believe it furthers their ends, so they've said no. That's their right. End of story.

Physics gets 88 questions per day; Chemistry gets 27. An experiment here might be worse than an experiment there, given that we'd suddenly be telling everyone, "Hey! You can now ask more well-written homework questions!", then changing it back 30 days later. We get a lot of homework questions; it would be hard to corral them all.

@JohnRennie, I know, I just keep hoping there's some way to persuade Robert.

Based on what Robert says in his reply to your discussion post we don't have a Bose-Einstein condensate's chance in a Big Bang.

@AcuriousMind So any symplectic (in particular any Kaehler) manifold has non-trivial cohomology in every even degree, corresponding to powers of the symplectic/Kaehler form. I'm wondering what the interpretation is in terms of multiplets arising from Calabi-Yau compactifications of superstring theories. Do you know?

user116211

3:31 PM

@HDE226868 yes and moreover, it's the Homework season.

Is this worth asking on the main site?

@Danu I think that depends on what you wrap on the corresponding non-trivial cycles in homology

The second cohomology is part of the metric moduli, but I don't think the higher cohomologies have such a straightforward interpretation

I vaguely recall there being some "universal" multiplets that don't depend on compactification details. I was hoping for something like that, since this seems like a pretty good thing to call "universal" :P

It's possible, but I don't know it. That might be a nice question for the main site

Any suggestions for a tractable title? It's hard to put this briefly :P

"What is the stringy interpretation of the cohomology arising from the Kaehler class?"

3:39 PM

Yeah, something like that

@ACuriousMind I gave it a shot:

0

Q: What is the stringy interpretation of the cohomology arising from the Kaehler class?

In superstring theory, one usually considers compactifications on Calabi-Yau 3-manifolds. These manifolds are in particular compact Kaehler, hence possess a Kaehler class which gives rise to nontrivial cohomology classes in every even degree. To see this, note that the Kaehler class $\omega$ on $...

string-theory differential-geometry supersymmetry topology compactification

@JohnRennie, I really hope that some Bose-Einstein condensate survives the Big Bang =P

@Danu Looks good, although I disagree with the stylistic choice of writing the question as a blockquote ;P

@ACuriousMind I did it afterwards, to compensate for the lack of a question mark.

Perhaps it wouldn't be so obvious to readers where the actual question is.

3:57 PM

lol, while I was debating whether editing the ae to ä was worth it, Jamal did it :P

I chose ae on purpose because it'd be easier to google for people

But I can respect the edit :P

@Danu I think google is smart enough to search for both ä and ae when either is entered

The SE search, however, is not :P

hahaha

4:18 PM

@ACuriousMind If I found a probability to be $|\alpha+\mathrm i\beta|^2$, should I expand it for the final answer?

Answer to what ?

to a QM problem

@0celo7 I don't understand the question.

@Ocelo7 Please quote it.

um

are $|\alpha+i\beta|$ and $|\alpha-i\beta|$ the same thing

no, probably not

4:25 PM

only if $\beta = 0$, right?

Of Course they aren't.

I dunno the english word but in german it is the "komplex konjugierte".

well, actually, with the absolute value...

@PhysicsGuy, complex conjugate?

Yes, I didn't know if that was the english word for it.

though I'm actually starting to wonder, because with the absolute value, doesn't that make it equal? I'm not sure how absolute values work on complex numbers, I guess.

@0celo7 Sure they are

4:29 PM

Wait....

As vectors in the complex plane, one is just the other mirrored on the x-axis, so their length is the same

Yeah, the absolute value would make them equal. That makes sense.

What does it mean for a quantum system to be "two-level" or "three-level"? Does it represent the number of basis states?

Oh yeah, I didn't perceive the bars

@heather Yes, an "n level system" is a quantum system with n basis states/an n-dimensional space of states.

Hmm, that makes sense. okay.

4:32 PM

Classic examples of two and three level systems are the spin states of spin-1/2 and spin-1 particles, respectively

So a qubit is a two level system, because there are two basis states. So then it can be represented by two level or higher systems, right?

So, for instance, spin or photon polarization?

@heather What do you mean by "represented by"?

A qubit is to some extent an abstraction but they can be "built" in the real world by picking a two-level or higher quantum system and have one of the basis states represent the 1 state and the other represent the 0 state. Right?

Ah, yes

I think that's usually called a "realization" of a qubit

oh, yeah, that'd be a good term to use, thanks

4:35 PM

@ACuriousMind not if $\alpha,\beta\in\Bbb C$, I hope

so then, what does it mean for energy levels to not be uniformly spaced?

$E_n=\omega n^2$

@0celo7 Oh, yes, then not (take $\beta = \mathrm{i}\alpha$). I assumed they were the real and imaginary parts.

@ACuriousMind nope

@heather For example the levels of a hydrogen atom are not uniformly spaced - the energy difference between the first and the second level is larger than between the second and the third.

4:38 PM

so I guess, if I had a multi-level quantum system with more levels than 2 representing a qubit, the first two levels would represent |0> and |1> and photons of particular frequencies caused transitions between the 0 and 1 state, wikipedia says the energy levels can't be uniformly spaced so the energy from the photon doesn't cause a transition to a higher state.

so with your explanation

then the difference between the first and second level has to be larger than the difference between 0 and 1

so that the photon doesn't excite the qubit into a state higher than 1?

hmm, this is starting to make sense

@ACuriousMind If I have an operator $T$, then the spectrum of $cT$ ($c\in\Bbb C$) is just $c\sigma(T)$, right?

Same as for matrices?

Yes

4:54 PM

@ACuriousMind isn't the sound of paper rustling amazing?

No

:/

why don't you like it?

I do not not like it. I have no feeling at all about the sound

do you know what sound it is?

do they play you sounds

I guess you could google

but maybe it's a human thing

I assure you my auditory capabilities are fully sufficient to know what sound it is.

user218912

5:08 PM

@0celo7 so like crumpling it?

Perhaps check out ASMR

user218912

crumpling plastic water bottle asmr is pretty cool

5:43 PM

@ACuriousMind Can the stringy question be turned into a math question, given a math answer, which is turned back into a stringy answer?

6:08 PM

@MikeMiller Perhaps someone can, but I can't: I don't know how to phrase the part about supermultiplets in terms other than actual quantum field theory.

fair enough

then I am of no help :)

@ACuriousMind Maybe. But it might only sound pleasing to humans.

6:25 PM

@ACuriousMind, good to know you have auditory capabilities =)

6:42 PM

@heather That's not necessary

@EmilioPisanty, what?

You can have $E_2-E_1$ smaller than $E_1-E_0$ and still have no photon-induced transitions between $|1⟩$ and $|2⟩$.

Really? Huh. I mean, I trust you more than Wikipedia, but I didn't realize that was the case.

the energy spacings just need to be different, there's no requirement that the top be bigger

the problem with a having a 1-2 spacing smaller than the 0-1 spacing is when the photon can couple into the centre of mass motion

i.e. when you can spend some of the leftover energy into accelerating the system

But why is there no requirement the top can't be bigger? If you have a photon with a frequency high enough to put the qubit past the $|2\rangle$ state, isn't that a problem?

6:46 PM

but then you're not really describing a two-level system, you're doing a two-level system coupled to a continuum

@heather no, it's not a problem

if it's not on resonance, it won't excite transitions

Huh...sounds like I need to read more about this, because I obviously don't understand it well enough =)

But thank you!

@heather no worries

keep in mind, though, that ladder-type configurations are rarely the most useful

Hi all

What is the best book in statistical mechanics?

7:13 PM

I've been recommended to read a book called QED: The strange theory of light and matter. How's this book? I want some reviews, thanks.

@SwapnilDas, I haven't read all of it myself, but I've read some of it and it is awesome. Definitely a good book to read.

@heather Thanks! Does it have math in it?

@SwapnilDas, the part I read didn't, no. I'm guessing that it is somewhat similar to the Feynman lectures in amount of math (because it is also by Feynman).

Hmm. Is it like any other pop sci book?

@SwapnilDas is it for physics of statistics?

7:19 PM

Not at all, from what I read. I'd ask someone else as well, because I didn't read the whole thing =)

@Student404Mus, nope, Quantum Electrodynamics

Do you know a good book in this field Small?

@Student404Mus Hope you got the answer.

I mean in statistical mechanics

@Student404Mus Schroeder.

How far it is good?

7:22 PM

It is very good, according to experts. In what level do you study? (High school or graduate)

Does it contain exercises

Last year undergraduate

Yes, it does.

@Student404Mus In Physics?

Surely

Fundamental physics

Oh God. I'm a tenth grader. I feel small, sorry :(

@SwapnilDas, in highschool?

Anyhow, you're bigger than me =P

7:24 PM

What do u mean @SwapnilDas

@heather Secondary high school, as per India.

@Student404Mus I'm just no one to recommend you books, you are much knowledgeable.

Do you learn statistical mechanics?

@SwapnilDas, oh, okay. I don't know the American analogy for that, but I don't doubt you are farther along than I am - I'm in middle school (as per America).

Ok

@Student404Mus No.

7:25 PM

:)

@Student404Mus, you could check that reference question on physics.SE...one moment and I'll find it

Go ahead

Here we go:

11

Q: Recommendations for Statistical Mechanics book

I learned thermodynamics and the basics of statistical mechanics but I'd like to sit through a good advanced book/books. Mainly I just want it to be thorough and to include all the math. And of course, it's always good to give as much intuition about the material. Some things I'd be happy if it ...

thermodynamics statistical-mechanics mathematical-physics resource-recommendations

Really, if you have any reference request question, you can check the links in this question:

214

Q: Book recommendations

Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this is probably the place to start. All the question...

resource-recommendations faq

Or you can check this nice site, one moment and I'll find it

books.physicsinsider.com

^that one

Yeah that's a good one.

@Student404Mus, does any of that help?

7:29 PM

@heather thanks

@Student404Mus, no problem, any time

Yes it does

Could someone explain intuitively what a bit register is? I don't quite get what it is. Just seems like a bunch of bits in a row to me.

What is your grade @heather?

@Student404Mus, I'm in 8th grade, in America

7:30 PM

Sorry if the question is not rated

No, it's fine =)

8th grade?Secondary school?

8th grade in America is like 12-14, that sort of age. But yeah, secondary school (we call it middle school) and then next year four years of high school, and then college.

I see

How do u find your studies

Hard or easy

Or in between

@Student404Mus What subtopic are u planning to major in?

Just curious.

7:36 PM

Maybe theoretical physics which has another name of atoms and molecules in quantum mechanics something like that

Do you find physics easy?

@Student404Mus, pretty easy, to be honest. Perhaps a little more information would help - next year the actually interesting science classes start. Currently I'm in highschool geometry for math.

@Student404Mus Of my level? Of course.

Elementary maths

This is the nature of studies Al in the world

@Student404Mus, are you asking me something, or...?

@Student404Mus I don't categorize things by easy or difficult. They are beautiful, or they're not. Physics and Math are two most beautiful things that happened to me :)

7:38 PM

From special cases to general ones

Great

This the answer

This is the answer

what is?

or, rather, what is the question?

@SwapnilDas, math and physics are beautiful =)

I'm not asking

Of course. What do you people plan to become? Just curious.

👌 :) I should leave and see you next time

@SwapnilDas, Oh, I dunno. Probably something with physics, math, and computer science. Quantum computing really intrigues me right now, and so does particle physics, for instance. And then pure mathematics. And I enjoy coding in various languages.

@Student404Mus, have a good day (or night, depending on your time zone) =)

7:41 PM

Bye! @Student404Mus Great time with you :)

Even I got to go. Bbye!

@SwapnilDas, have a good day/night =)

@0celo7, I know I messed up the last supremum problem, is there another one I could try again on?

Hey guys i m providing a link to a video,can someone explain what Professor Feynman did in the video during 20-22 minutes.i know calculus a bit.please help youtu.be/kd0xTfdt6qw?list=PLLzGzdSNup63lMYeOpU9Hax6MBsTjdDas

8:28 PM

@heather $\sup\{1-\frac{1}{n}\mid n\in\Bbb N\}$.

This one is tricky!

And we will prove a more general theorem on it later

1 - 1/n such that n is an integer - man, you can be really evil sometimes =)

$n\in\Bbb N\setminus\lbrace 0\rbrace$?

do you guys include 0 in $\Bbb N$?

i didn't think 0 was in $\mathbb{N}$

here it's defined into it:)

@AndrasDeak Never

8:31 PM

:/

@heather natural number

n = 1,2,3,...

@0celo7 get out

@0celo7, right, yeah

@MikeMiller Oh please

we have $\Bbb Z^+$ for $\Bbb N\setminus\lbrace 0 \rbrace$ :P

8:32 PM

@THELONEWOLF. It's a very elegant algebraic proof of Kepler's "equal area in equal time" law.

so I guess you can say that first, 1 - 1/n > 0 so then you have 1/n > -1 or 1 > -n or -1 < n

thank you

but I don't think that's very helpful to what we're trying to do

@THELONEWOLF. The rate of change of the area is the radius crossed with the velocity, $\dot A = \vec r \times \dot{\vec r}$

@THELONEWOLF. Use the chain rule to find $\ddot A$. The two terms are $\dot{\vec r}$ crossed with itself, which vanishes, and a term proportional to the force

@rob You need to tell the \dot what it's supposed to act on with brackets, otherwise it won't render

8:36 PM

@0celo7, you've really stumped me now =)

@ACuriousMind Better?

@rob Yep :)

still broken on my screen

welp

I only see \dot\vec r broken, which should be

He didn't fix the second one

8:40 PM

upgraded my phone, can't log in to SE

guess I won't be phone chatting any more

@0celo7 use a different browser?

@AndrasDeak We have $\Bbb N_0$ for 0,1,2,..

Hah! It worked

Interesting, I really never have seen that:) (But I'm not a mathematician, and I don't often read maths in English)

@ACuriousMind Oops

interesting

I can't charge my phone and listen to music at the same time

8:48 PM

so...building a dilution cryostat at home...possible? =P

@heather Sure it's possible. Depends on your budget, though.

@0celo7 new iphone?;)

@AndrasDeak yes

haha

@heather write down the elements in the set

8:49 PM

@rob, the lower the better =) what would be an approximate estimate, you think?

at least you can charge it with your new macbook...oh wait

@AndrasDeak Not upgrading my laptop for a few years

I'm not a mac fan, but judging by mac fan acquaintances, the new macbook pro is less than trivial as a choice now

What does that mean?

@0celo7, okay, let's see. If you plug in 1 for n, you get 0, which is not in $\mathbb{N}$, if you plug in 2 you get 0.5, if you plug in 4 you get 0.75, if you plug in...wait, is it 1? The answer, I mean? Because it will approach 1, but never quite reach it. So it must be 1.

8:52 PM

That is correct

This is a special case of the monotone sequence theorem

Yes! Victory! =)

@0celo7 Well, they overhauled the fn keys with some touchy crap, which is disliked by a lot of people. But what's worse, it features usb-c plugs, so you can only plug your iphone into your macbook pro using an adapter. And it also doesn't have magsafe, which is one of the things I always envied from macs

@heather $1000, give or take a factor of ten

Monotone sequence theorem

How the hell do you charge it then?

8:52 PM

@rob, oh, geesh. Well, um. Okay, then.

usb-c/usb-a adapter

I like magsafe

I'd like it too. I'd love it:)

well, no more magsafe:P

the general stance is "wtf apple?", as I see reactions

And that doesn't even count the rest of the stuff you'd need...darn.

Apple will make a shitload of money regardless

8:53 PM

@heather Cheaper is certainly possible, but creativity is required.

@0celo7 yup

@rob, okay, I'm starting to think I need to read more about how they are constructed.

you don't even push the home button

@heather I was figuring starting from nothing, so you'd need to buy/borrow/steal a vacuum pump

you just put your finger on it and it magics

user218912

8:55 PM

I want an iphone 7 :(

@rob, actually, I think I might be able to get my hands on one of those at no cost - long story.

trying to put on all my stuff now

it's not cooperating

How much would that reduce the price?

@heather vacuum vessel?

@rob, I don't know where I could get my hands on one of those. Hmm. I wonder if ebay would have those...

8:57 PM

"buy/borrow/steal" -> "no cost - long story" *looks suspiciously*

@AndrasDeak, I'm just going to clarify - there is no stealing involved here.

But this vacuum vessel, hmm.

:D

Oh, not stealing that either, I'm just wondering where I'm going to get that.

=D

@rob, well, I'll have to look into that. I know I can get ahold of a vacuum pump, though.

@heather Well, come up with a design that you like. The upper end of the price range I gave you is "everything new from the catalog."

@rob, well, thank you for your help!

2 hours later…

10:54 PM

@heather don't get a phone

they're infuriating

*iphone

no, it's google being stupid

Google? How so?

ah

they sent me a password reset thingie

I don't remember my personal gmail password

what should I make it?

and by "stupid" you surely mean "deviously smart and unscrupulous"

@0celo7 you don't remember...... dude:P

10:56 PM

negative

@0celo7 hunter2

@AndrasDeak I log in once with that password and never do it again

I assume you also don't have 2-step verification, then?

@rob nice

my blog password is in the chat above btw

anyone who finds it can take my blog

I have like 1 sub and stopped using it months ago

nice! Chrome works on my new phone now :)

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