The h Bar - 2016-09-26 (page 2 of 5)

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1:00 PM

@ACuriousMind I am trying to make the polarisation density of some dielectirc (say a polystyrene ring or ball) to align such that it gives a D field with only a curl component and no radial component. If I recall, synchrontron use magnets to steer around the moving charge, thus I am not sure if the polaarisation density (which are basically dipoles) will necessary respond in the intended way to a magnetic field (might need to revise on the calculaton on that)

I think they are

@0celo7 I don't know.

So logically, the obvious step is to subject the polystyrene in a circular electric field, which the only source I am aware how to produce it is via a changing magnetic field

@Secret : sounds like the current in the wire.

user image

Kinda, except we are using faraday law thus changing magnetic flux took the role of I ,and the E is then produced from it

ACM: I might have massively overlooked known methods given that I am terrible at intuition for charge interactions with magnetic fields

Now googling whether electric dipoles will precess in magnetic fields...

1:08 PM

@SwapnilDas mass is indeed a form of energy, and we interconvert mass and energy every day at colliders like the LHC. But energy is conserved and can't just disappear. You could convert mass to the equivalent amount of energy, but I can't see an obvious reason why that would lower the total energy of a system.

-4

Q: do zero possess some magical powers???

ZERO Zero,an aberrant number,isn't it? I mean just look at it.It's a shape,a circle.How can a circle be a number?And yes,if a circle can be a number then why not a square,a rectangle or a triangle? This circle is a stupefaction.It's a repository of mysteries.It's ludicrousness often obfuscate me...

complex-geometry

@JohnRennie Does my above investigation on trying to produce a dielectric with circular arrangement of polarisation density make sense to you? Meanwhile still googling...

@Secret : See section 11.10 of Jackson's Classical Electrodynamics where he says "one should properly speak of the electromagnetic field Fμv rather than E or B separately".

@0celo7 I'm pretty sure it's very easy to write down an explicit homeomorphism---namely multiplication by an element of the group.

@Danu IMO, the most 'magical thing' about zero elements in most algebraic structures is that they are additive identities and/or multiplicative absorbers, which is why we cannot divide by zero. Trailing zeros being used to indicate the orders of magnitude is because of how we define the decimal system

1:22 PM

@Secret No need to convince me... I just thought itwas a hilarious post

@Danu The most hilarious thing is the tag

@yuggib :D

HOW DOES THIS RELATE TO HOLOMORPHIC LINE BUNDLES?! WHY IS THERE ALWAYS A ZERO SECTION?

Sorry, bad habit of always writing neutral sounding things to a clearly hilarious post... I am too used to make sense of absurity

zero too magick 4 me

@Danu because zero was made by the devil

1:26 PM

Nature and Nature's laws lay hid in night:
God said, "Let Newton be!" and all was light.
It did not last: the devil, shouting "Ho.
Let Einstein be" restored the status quo.

user116211

@JohnRennie expected Maxwell too...

Yeah, I mean, come on! Let there be light $\Leftrightarrow$ Maxwell

user228700

@JohnRennie :-) Busy today?

user228700

@Danu (For beginners):

user228700

Problems with Zero - Numberphile

1:38 PM

@KaumudiHarikumar quiet today, otherwise I wouldn't be filling my time by searching out obscure rhymes :-)

I would go for a cycle ride and at least get some exercise, but it's cold, wet and raining in (my bit of) the UK at the moment.

user228700

@JohnRennie I see :-) I asked because I haven't seen u here today that much.

user228700

@JohnRennie I see. It's close to 32°C here :/

@KaumudiHarikumar too hot!

My bit of the UK is around 20-25C in the summer and for me that's perfect. Warm enough to be pleasant but cool enough you can still be physically active.

user228700

@JohnRennie Actually, not too hot for the people living in these parts. We've had close to 45°C in the summer!

user228700

We have only one season :P No spring, no winter, no "rainy". It's summer all year round.

1:45 PM

Incidentally, re the are rotations vectors: you've probably been told that a vector is something that has magnitude and direction, but the definition of a vector is more complicated than this. Generaly we define a vector as an element of a vector space.

However this level of detail is only addressed well into a degree level course.

user228700

@JohnRennie Yes, that is the definition that we've been taught. "Vector space"..?

A vector space (also called a linear space) is a collection of objects called vectors, which may be added together and multiplied ("scaled") by numbers, called scalars in this context. Scalars are often taken to be real numbers, but there are also vector spaces with scalar multiplication by complex numbers, rational numbers, or generally any field. The operations of vector addition and scalar multiplication must satisfy certain requirements, called axioms, listed below. Euclidean vectors are an example of a vector space. They represent physical quantities such as forces: any two forces (of the...

user228700

@JohnRennie OK :/

If this is something that interests you then by all means go for it, but many of us kind of shrug and get on with life without worrying too much about it :-)

user228700

@JohnRennie Riight. I was only interested because I wasn't able to correlate rotational vectors to the definition that we've been given.

user228700

1:49 PM

Also, I was wondering..? When we speak about circular motion, we talk about these "linear" velocities and accelerations too.

Rotations do form a space, but it isn't a vector space. Exactly what it is I have long since forgotten. I think infinitesimal rotations for a vector space, but again the details have faded into the past.

user228700

@JohnRennie Never mind vector space! I don't feel the need to understand it now :P I'm mostly just confused about these "linear" properties of rotational motion...

All velocities and accelerations are linear. Concepts used in circular motion, like angular velocity, are constructed from the (instantaneous) linear motion.

user228700

@JohnRennie OK..?

Suppose you're standing at the side of a road watching a car drive past. Does the car have an angular velocity? Yes, if we take you as the reference point the angular velocity is just $\mathbf r \times \mathbf v$, where $\mathbf r$ is the vector from you to the car.

We can always define an angular velocity. It isn't anything especially fundamental.

Circular motion is special only for it's high symmetry, which means it has various conserved quantities.

user228700

1:56 PM

@JohnRennie Oh!

user228700

But angular velocity's been defined a little differently in my textbook :/

user228700

It's defined mostly just in terms of rotation but that's not correct..?

That doesn't mean your textbook is wrong. There are various ways to define angular velocity and the different definitions are useful in different circumstances.

user228700

@JohnRennie OK, I see.

Ok, it seems dipoles don't necessary travel in circles under a perpendicular uniform magnetic field. Looks like the synchrotron example does not necessary work, but let's give it a try in a small scale setup...

2:00 PM

@KaumudiHarikumar I assume this definition is intended for extended bodies, where it's fairly obvious we can rotate the entire body around some pivot point. But you can in principle break up the body into an infinite number of point masses, and for each point mass $\omega = \mathbf r \times \mathbf v$.

user228700

@JohnRennie Hm, OK...

Have you covered moments of inertia yet?

user228700

@JohnRennie Yes, although I'm not too clear about the concept yet.

You'll find that moments of inertia are calculated by splitting up a body into infintesimal mass elements $dm$ then integrating to get $I=\int r^2dm$.

@Danu My thoughts exactly.

user228700

2:05 PM

@JohnRennie Yeah, yeah, that much I know...

user228700

@0celo7 Hey. I had read your question about probability. You guys aren't taught all this in high school..?

We are

I'm just bad at it

user228700

@0celo7 Oh, I see.

user228700

There are some great lectures on YouTube. Check 'em out maybe...

I think it's a little ridiculous my gen-ed probability class is taking more time than quantum mechanics, topology, and analysis put together

but that's the way the world works

user228700

2:07 PM

@JohnRennie OK, I'm gonna go read about this some more. Thank you :-) Have a nice day! (Whatever is left of it anyway :P)

user218912

@0celo7 same...

I might take that back. This QM homework is pretty hard

user218912

:P

user218912

I'm learning topology right now.

user218912

gonna try these problems.

user218912

2:17 PM

fuck this book doesn't have problems.

Hey, is it normal for reputation to simply fizzle when a bounty has gotten no answers?

I would ask on the meta, but apparently you need 5 rep to post there...

@tuskiomi Yes, that's working as intended

Well, that's not too helpful, now is it?

Stack Exchange taking reputation into the ether, that is.

Well, the idea is that you pay the bounty for the increased visibility of your question. That an answerer might receive the bounty then is the incentive for people to answer it, but the primary purpose of the bounty is not to transfer reputation from you to the answerer.

As for vanishing into the ether, well, it's generated from thin air to begin with, so I don't see the problem

The problem is that it took away many of my privileges. I cannot even comment now (not that I'm a particularly active on this stack anyway).

I would not have minded this If said question actually got an answer, but now I'm kind of stuck as a Stack infant.

2:26 PM

@ACuriousMind i.e., Rep is not a zero sum game.

Eh, i guess at this point I'm just on a rant, I'll let you guys get back to physics. thanks for the insight.

@tuskiomi Yeah, that's the danger of offering a bounty with low reputation. I'm not 100% convinced that's a good thing, but that's how it works.

@0celo7 Yep

ACM: So I have read some papers about electric dipoles in uniform magnetic fields perpendicular to motion ,and found that they can undergone motion not necessary circular for the centre of mass (with the motion o the charge about the centre of mass being osc in a more complex manner), thus it kinda confirmed my guess about using a
synchrotron method to align the dipoles in a dielectric being deviate from a circle. Therefore it might be not so straightforward. however, I will see what I can come up with using a

small scale circuit with an easily polarisable object as a experiment first before hopping back into the theory

(I am not sure if this summary is short enough to be easy to understand, but I am going to try again on this investigation later)

2:49 PM

@ACuriousMind When I evaluate an ev, does it matter if I do it in the Heisenberg or Schroedinger picture?

i.e. is $\langle \Omega(t)\rangle=\langle\Omega\rangle_t$?

What do you think?

I think that is true

Why?

Because you have $\langle\psi|U^\dagger\Omega U|\psi\rangle$

and you either get $\Omega(t)$ in the middle or $|\psi(t)\rangle$ on the sides

That is correct

3:07 PM

@ACuriousMind dakne

3:38 PM

@ACuriousMind I had a dream last night

I dreamt that "heavy drug use" became a legitimate proof technique

Rappers became the new math gods

There were also clowns but that's nothing new

And I learned sheaf cohomology

I remember years back at a Christmas party I was talking to a nurse. A mental health nurse. She told me that a lot of younger women were in the women's ward because of anorexia. And that a lot of younger men were in the men's ward because of cannabis.

Just saying.

Yeah but if I'm in a metal ward I don't have to argue with you

Metal ward?

Yes. Where you go to get bullets removed from the brain after a failed suicide attempt.

@knzhou The question makes sense. It's a very good question.

@JohnDuffield Men in a mental hospital because of cannabis?

3:54 PM

@DanielSank Pot has been known to trigger latent schizophrenia.

@0celo7 According to ten seconds on internet, that's not known.

@DanielSank : yes. See super-strength skunk linked to psychosis.

There are articles arguing that there's an association that may not involve a causal relation.

@JohnDuffield I don't take scientific evidence from news outlets. Sorry :)

I don't take evidence from scientists because you're all stoners

@DanielSank : then do your own research. You will soon find things like study of patients at south London hospitals finds those who smoked skunk every day had five times the normal risk of psychosis.

3:57 PM

@JohnDuffield Reading just the first few paragraphs of the story you linked reveals that they're not saying there's a causal relation.

@DanielSank : sounds like you're in denial about something.

@JohnDuffield That's another story about the same research paper as the first article you linked.

@JohnDuffield Oh? What's that?

@DanielSank : go and find your own mental health nurse and ask her about the patients.

Lol

@JohnDuffield Why would I do that? Should I do science that way as well? I will go seek out one research astrophysicist and question him/her in order to make up my mind about dark matter! My god, why didn't I think of that before!?

Let's review: so far you've sent me two links to pop news articles both referring to the same research study. Very well. I'd like to read that research study myself before drawing any conclusions. Furthermore, the first article specifically says that the research study cannot be used to claim a causal relation between cannabis and any particular psychosis.

4:01 PM

Plenty of reasons not to do that @DanielSank

Because it's purely anecdotal?

Furthermore, you have taken the liberty to suggest that I am in denial about "something", which suggests that you believe I engage in the use of cannabis. You make this accusation simply because I insist on a modicum of proper scientific analysis on a study about the ill effects of cannabis.

@0celo7 But wouldn't talking to a nurse's patients be similarly anecdotal?

@DanielSank : you gather evidence from multiple sources to form a view. What you don't do is dismiss the evidence because you don't like the sound of it.

Hi all

@Alex Hello.

@JohnDuffield Did I dismiss evidence because I don't like the sound of it?

I think I said that the article specifically cautions that the study it describes does not support the theory that cannabis causes schizophrenia.

@DanielSank : your response to what I said sounded overly negative.

4:06 PM

@JohnDuffield Perhaps that's because you have a certain opinion and are surprised or uncomfortable that I don't find the article you linked to have enough evidence to support that opinion?

@DanielSank Sure.

Physics question to interrupt possibly more interesting discussion. Is it true in quantum mechanics, that the eigenfunctions of the Hamiltonian (regardless of the potential) in three dimensions is the same as that of the square of the total angular momentum $L^2$ and any component of angular momentum, say $L_z$?

@Alex I don't see why that would be true.

> The researchers used their figures to estimate that 24% of new psychosis cases in the study population could be attributed to the use of skunk. But it's important to note this figure would not apply to populations where skunk use is less common than in the south London population the study looked at.

@JohnDuffield Read quoted text above.

That's from the first article you linked.

@DanielSank : the bottom line is that the mental health nurse told me cannabis is making a lot of young men crazy, and that there's a right-on politically-correct movement trying to conceal this.

@JohnDuffield One nurse told you that, and so you made up your mind?

That's not very scientific. Nor is your "observation" of my being in denial.

> The figure also assumes that skunk definitely directly causes psychosis, which this study cannot prove by itself.

@JohnDuffield Yet another excerpt from the first article ^

4:09 PM

@DanielSank : I don't make up my mind. I hold a current-best-fit view based upon the available evidence.

@DanielSank Do you agree that the square of total angular momentum and the components of angular moment are commutative and therefore have simultaneous eigenfunctions?

@JohnDuffield Your evidence is two pop-news articles referencing the same experiment, which specifically does not support your opinion, plus one nurse?

@Alex That sounds right :-)

...and I'm not saying your proposition is wrong. I just don't see why it would be true.

...and that could be a reflection of my own ignorance.

@DanielSank : there's more. Do your own research. Remember I don't make this stuff up.

@Alex for a spherically symmetric potential ...

@JohnDuffield But... you make up so much physics... how should I know what you do and don't make up?

4:12 PM

@DanielSank : I don't make up any physics. I just tell you things that you don't know about.

@JohnDuffield Not sure what that means.

Lol

@DanielSank : it means what it says. I'm not some my-theory guy. I'm just some guy who's done his own research. Sometimes it doesn't square with what you've been taught, and you think I'm making it up. I'm not.

By the way, @JohnDuffield, since cannabis acts on the brain, changes one's mental disposition, sets one off on trips, etc. I would not be at all surprised if it does bring out mental psychoses. I'm not in detail there at all. I just don't much appreciate someone dropping one news article, which explicitly warns against drawing too strong conclusions, and then pretending that we understand the issue.

@JohnDuffield I find it odd that you think I reject stuff you say on the grounds that it's at odds with my own opinions.

@JohnRennie Yes I see. So then it is true that the eigenfunctions of the Hamiltonian for a spherically symmetric potential are simultaneously eigenfunctions of the square of total angular momentum operator and the operator components of angular momentum?

4:15 PM

Can someone good at mathematica compute an integral for me? Pretty please?

In fact, I reject stuff you say because it simply doesn't make any sense.

Like this answer of yours.

@DanielSank the (deleted?) cohomology one was better

@JohnDuffield You admitted in the text that you didn't even read the full question before writing your answer!

Why would I pay much attention to such a thing?

Okay

@BernardMeurer okay

4:17 PM

Who'll be my saviour and explain to me my entire Chemistry class

Because he started with the wave function

which I have no clue what it means

@BernardMeurer ::hides::

@BernardMeurer That's stupid.

then he just threw Schrodinger

@BernardMeurer what the hell?

Has your class taken quantum mechanics?

lol

4:18 PM

And everyone was like LOL WAT

what is wrong with Europe?

@0celo7 Things, and stuff.

And then he did something about 1-dim Schrodinger and stationary waves and an electron trapped in a 1-D something

and my that time by brain had suicided

Oh jeeze.

:(

so what the fuck just happened, what's the wave function and what does Schrodinger do?

Literally no one understood a thing

4:19 PM

Well, @BernardMeurer as far as we can tell from experiments etc. thinking of matter as little balls doesn't work.

It's better to think of matter as waves.

your freshman chemistry class is more advanced than my (standard) graduate QM class

welp

Now that's all I'm going to say about it right now, because it sounds like there's going to be a revolution in that course.

@DanielSank Okay, does that have anything to do with the photoelectric effect?

I think the students should talk to the professor.

If you sit and hide and nod your heads while not understanding anything, you're all wasting a lot of time.

You should fix this. You're paying for those lectures.

It's not charity.

The professor is a 1.3 meter oompa-loompa

4:21 PM

Aren't there like thirty of you?

Who's teaching QM to a class that doesn't know calc 1

Talk to the professor.

thirty?

Ok how many?

About 70

4:21 PM

@0celo7 What's the integral?

and the whole year is 220 for my major

@HDE226868 A Gaussian, but turns out I didn't have to do it anyway.

@BernardMeurer Well there you go.

Grab a few and go talk to the prof.

@DanielSank the professor doesn't use a projector, he uses one of those things from the 50's

Do this sooner rather than later.

4:22 PM

@0celo7 Ah. Nasty little guys.

@BernardMeurer Talk to the prof.

A retroproyector

Okay, okay

If that doesn't fix things, go to the department head.

You're throwing your money into a fire otherwise.

But then I'll be marked as the asshole student

No you will not.

4:23 PM

@HDE226868 an $x^2e^{-x^2}$ type Gaussian.

You will be considered helpful by the rest of the students.

I know there's a formula for it (I know how to derive it too), but there's a billion constants

Hm .-.

@0celo7 Oh come now, you know how to do that! :)

Meh

4:23 PM

@0celo7 Change variables to make them go away.

but the problem wanted the answer in terms of $\langle x^2(0)\rangle$, I don't actually have to compute it.

I'll go study the wave function by myself then

@BernardMeurer why?

Also, by the way, keep in mind that most quantum textbooks teach you some stuff that's extremely misleading.

@DanielSank Because I have a test coming in a couple of weeks and I need to know this

@DanielSank i know

4:24 PM

@Alex Yes

@BernardMeurer You need to understand particle in a square well?

my fear is BM will learn QM completely ass-backwards and be screwed when he takes the actual class

Mine too.

gotta go

@DanielSank I need to understand the wave function, Schrodingers equation, where does it come from and what does it have to do with a stationary wave for a 1-dim particle

where it comes from?

that's a hard question

4:26 PM

@0celo7 It came from Schrodinger.

:P

@DanielSank I know the Feynman quote too

@0celo7 Eh?

but the way to motivate it is via Hamiltonian mechanics

@0celo7 ehhhhhhhhh sort of.

Well he started 'deducing' Schrodinger and it yielded this massive sine-filled expression that no one understood

4:26 PM

@DanielSank Feynman: The SE came out of the mind of Schroedinger, there is no way to derive it

True in the strict sense, but totally misleading

One can understand how Erwin came up with it

@BernardMeurer What?

@0celo7 How do you motivate $[x,p] = i \hbar$ without just saying "particles are waves" up front?

The standard way is to demand that the quantum hamiltonian generates a unitary time-translation operator.

Also what's planck's constant

and de-broglies equation?

@DanielSank $p$ generates translations as in classical mechanics.

he just put those on the board

and didn't say a thing

4:28 PM

@BernardMeurer google

@BernardMeurer google

I'm so frustrated man

@BernardMeurer Planck's constant is a constant with dimensions of energy times time.

You require that the translations are unitary, abelian, homomorphism from additive group of $\Bbb R^3$ to unitary operators

I literally want to break shit right now

That basically fixes $[x,p]=\mathrm i\hbar$.

4:29 PM

@BernardMeurer Chill, bro.

@0celo7 Indeed.

That's a very "I know the mathematical structure of the theory I want" point of view.

@DanielSank "particles are waves" is how Shankar does it

Not a bad thing. Just saying.

@DanielSank Agreed. But I'm a mathematician :)

Energy times time and distance times momentum both have the same dimensions.

Energy times time is called "action".

That's all I know about Planck's constant.

Also it shows up here:

Plank's constant is there for dimensional analysis @BernardMeurer

It originally came from $E=h\nu$.

4:31 PM

$$ i \hbar \partial_t \left \lvert \Psi \right \rangle = H \left \lvert \Psi \right \rangle$$

that's how you define it

It's the proportionality constant that gives the energy of a photon in terms of its frequency @BernardMeurer

wat

It's totally empirical

^ Yes

Good point.

Ah

4:32 PM

@BernardMeurer Are you telling me they're doing straight up QM before telling you about orbitals, etc?

or quantization of photon energies?

Orbitals?

eh, tautology there

Quantization of what?

quantization of light energy in terms of photons

@BernardMeurer if you put an electric arc through a gas of hydrogen, you see that the light comes out with a few specific wavelengths (frequencies).

This suggests that the atoms can only emit specific wavelengths of light.

4:34 PM

Okay, makes sense

Then there's the photo-electric effect.

Among other things, that more or less proved that the energy of a photon is related to it's wavelength (frequency).

As @0celo7 said, $E = h \nu$ where $\nu$ is frequency.

And what's $h$?

Planck?

Correct.

that's the 8th letter of the english alphabet @bernard

glad to be of assistance.

@Obliv I'ma gonna bitch slap you

user218912

4:36 PM

@BernardMeurer you're taking QM?

@BernardMeurer Chill, bro.

@BernardMeurer I can just feel the flags rolling in ;-)

@JohnRennie Nah.

@BernardMeurer Probably not helpful but Straumann's QFT book (German) has a really good explanation of all this.

@JohnRennie What? That wasn't even rude it was just comical O.o

4:37 PM

He derives it from Maxwell's equations.

@DanielSank we've already determined that the flagging process bears little relation to anything recongnisable as rational ...

@JohnRennie If I had said "I'm going to slap you, bitch" then perhaps :p

@BernardMeurer An important distinction.

@IceLord I'm taking freshman Chemistry in Europe. Apparently it's not that different

Is the prof new?

Does he have a bad rep?

4:38 PM

I don't think so, no

let me check if he's new

I don't know his rep

I don't know anyone in here at all

My advisor has a bad rep and, accordingly, we did Banach spaces on day 1 of intro analysis.

ciao, everybody

Bye

Cia

@ACuriousMind I NEED YOU

@BernardMeurer I'm perfectly capable of helping you, you know.

4:41 PM

@0celo7 Not for the help, but to ask about the European curriculum

What are you doing right now @0celo7?

Printing probability homework

Then I'll finish QM homework, read topology textbook

get on skype

You're teaching me

I'm in the library :o

Tsc

dang it

@0celo7 Erdös and his amphetamines would approve

@BernardMeurer What ails you?

4:52 PM

@ACuriousMind He has to solve the SE for a hydrogen atom in his freshman chem class

Aha...well, Chemistry lectures can be weird, although I typically wouldn't expect that in a class that doesn't have something like "quantum mechanics for non-physicists" as a prerequisite.

QM for non-physicists is a thing?

Europe is a strange country.

It's typically a compressed lecture that introduces the basics of QM together with the mathematical tools needed, which physicists would be assumed to have learned in prior lectures, but other STEM students are not.

@0celo7 It's not a country :P

@ACuriousMind Stop trolling.

@0celo7 Europe is a kingdom not a country.

4:56 PM

@0celo7 What?

@Obliv Oh, ruled by Queen Elizabath.

I'm stupid.

Sorry @ACuriousMind

Methinks it's not me who's trolling.

I wouldn't rule that out, but I see your point.

whose* not who's @acuriousmind

@Obliv Pretty sure who's correct there.

4:58 PM

okay I'll stop trolling too ^^ have to practice some mathemagics now

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