The h Bar

Q: How many times can I down vote a particular user over in a given time frame?

12:21 AM

@Danu ^

dmckee

12:44 AM

@Sanya Photon Scattering from Atoms in an Atom Interferometer: Coherence Lost and Regained From PRL volume 75 number 21. Chapman, et. al.

I've given that reference a few times on the site and anna v ran down a link to a no-paywal PDF of the paper once, so you can find it.

Sanya

@dmckee hmm, I probably didn't get my meaning across - I just really liked the innocent question "what if the observer [...] interfere with the results?" - it's so strangely on point; but I'll still have a look

thanks :)

chapmanlabs.gatech.edu/papers/scattering_ifm_prl95.pdf

Physics Meta

-3

There is a user who down votes 90% of the time and that is his privilege. How many time can I down vote him before I am interrupted?

discussion support status-declined down-votes

ACuriousMind

1:01 AM

Well, now I know where those downvotes are coming from :P

@ACuriousMind haha

you downvote 90% of the time?

ACuriousMind

No, only 81.7% of the time, as @HDE226868 helpfully calculated ;)

HDE 226868

@ACuriousMind Well . . . I typed them into my laptop and "Cortana" calculated it, but yeah, I did back-of-the-envelope estimation before deciding to go all-out pedantic. :-)

The US Govt. job application system is horrible

I don't understand the lazy gov't worker meme

this is more effort than linear algebra...

I'd rather be doing linear algebra than this

dmckee

@0celo7 Every on-line application process is horrible, though some are worse than others.

Of course I'd be willing to believe that the US governmental one is a nightmare, but that may just represent a philosophical bias.

HDE 226868

1:14 AM

@0celo7 What kind of job are you trying to get? Is it related to ORNL?

@HDE226868 That's next. Right now I'm looking for stuff in DC

I want to live at home

@HDE226868 I have someone telling me he can get me a spot at LANL but I'm not too sure about it.

HDE 226868

@0celo7 Wow. Sounds like the paperwork might be worth it.

user218912

@0celo7 what would you do there?

@IceLord Hopefully build a bomb big enough to wipe out Canadia in case you get too rowdy.

@HDE226868 Yeah. We have lots of joint faculty with LANL for some reason.

done

now for the other 6 positions...

user218912

2:06 AM

@0celo7 :o

3:18 AM

@BalarkaSen why are you up so late?

It's like 9 in the morning here

I woke up an hour ago.

Huh

@BalarkaSen OK, well while you're here, I have Aubin's diff geo book.

(from the library ofc)

Ah, how is it?

lots of problems

Sounds good.

3:23 AM

first 43 pages are covered in GP

basic manifolds stuff

I don't think there's anything profound here

lots of problems

like a third of the book is problems, it's crazy

Problems aren't bad.

i know

I'm just commenting

flipping through it right now

Fair.

he also has solutions to a lot of them

@BalarkaSen the section on flows avoids the gore

I assume that's supposed to be a good thing.

3:29 AM

@BalarkaSen Yeah. Unless you need a technical result on flows.

@BalarkaSen This seems to be a solid intro indeed

3:56 AM

Hi guys

4:06 AM

hello

4:26 AM

@Slereah especially if it's printed on soft paper

How do you think all those greek scrolls disappeared from history

2

Um

the BCH formula is an exercise in Sakurai

user228700

4:52 AM

Hello :-) I was reading about enthalpy and wondered if anybody could quickly tell me why we've defined this new state variable when we've already got internal energy..?

I freely admit that I never really understood exactly what all the multiple different types of thermodynamic quantites mean, but enthalpy includes work done by volume changes.

Oh I remember a great drawing of it

Suppose you have some process like water turning to steam. The enthalpy for the process includes both the change in internal energy (change in energy of the water molecules) and the work done as the water expands into steam.

And the Gibbs free energy adds to this the change in entropy.

user228700

4:56 AM

@JohnRennie Hm. Do you know why we measure the energy of chemical bonds in terms of enthalpy rather than internal energy..?

@Slereah Yes, that's a good illustration. @KaumudiHarikumar does that help?

Plus it has wizards

Death Grips - Exmilitary [Full Mixtape]

►

user228700

Um:

@JohnRennie

user228700

4:57 AM

@0celo7 Morning

hi

some terrible music for you

@KaumudiHarikumar refresh the screen and the image will appear.

if you like this, see a doctor

user228700

@Slereah Haha, yes, that helps, thanks.

user228700

4:59 AM

I still don't understand why we use enthalpy when talking about chemical bonds though...

@0celo7 it's not my style of music, but I think it's worth 48 minutes to give it a fair listen. I'm listening to the radio at the moment, but I'll put the Death Grip on later.

user228700

How does the $P$∆$V$ factor in for bonds..?

@BernardMeurer cf. above.

@KaumudiHarikumar Chemical Thermodynamics?

@KaumudiHarikumar Chemical reactions may or may not include a change in volume. If there's no change in volume $\Delta H = \Delta U$ and there's no harm done. If there is a change in volume then we do need to use $\Delta H$.

5:02 AM

Hi, everybody.

user228700

@HariPrasad Erm, yes..?

@KaumudiHarikumar How do you get that nice mobile interface?!

Incidentally, for the Delta sign use \Delta

@KaumudiHarikumar shodor.org/unchem/advanced/thermo

Captain Beefheart and the Magic Band - Doc At The Radar Station (Full Album) (1980)

@0celo7: this is one of the albums I love but you will probably hate:

►

user228700

5:03 AM

@DanielSank Hi there! :-) Um, is urs different?

@DanielSank Haha that's called the "Android"

Did someone ask for good music?

@KaumudiHarikumar Yes.

@HariPrasad I have an Android phone in front of me, and the chat room does not look that like.

It looks the same as on a desktop computer.

@DanielSank Go for mobile view

@DanielSank no

Everyone likes Chuck Berry. "My Dingaling" was one of the very first single I bought as a little kid.

user228700

5:04 AM

@JohnRennie OK, this makes sense.

@JohnRennie Death Grips is legitimately terrible.

@HariPrasad My question: "how to get mobile view". Your answer: "go for mobile view".

:|

not like Miley Cyrus terrible

@0celo7 OK maybe I won't bother listening to it :-)

user228700

Yes, @DanielSank. Let me help:

5:05 AM

@JohnRennie That song is hilarious.

@DanielSank Use a mobile

@JohnRennie listen to the second track

@HariPrasad :| :| :|

@JohnRennie what is going on here

@DanielSank just kidding

5:05 AM

at 2:00

@HariPrasad You are a natural born comedian. Where do I buy tickets for your show ;)

user228700

@DanielSank: You should see this on the bottom right corner of the screen. (If you don't, I'm sorry man :P)

@DanielSank just so that you can spend your money on something useless :D

A professional technical assistance team, we have here.

user228700

5:06 AM

^ Real help!

user228700

@DanielSank Did it work?

@DanielSank click on that options button on the top right corner

then check the desktop view

then refresh the page

the againg uncheck the desktop view

then refresh

@JohnRennie is this LSD music?

@0celo7 the track doesn't seem awful ...

@JohnRennie I'm not making a judgement

5:09 AM

@KaumudiHarikumar You are the winner!

I don't even know how to evaluate it

user228700

@HariPrasad What is all this? @DanielSank: Clicking on that option "mobile" at the bottom right corner should do it.

I never noticed that tiny little link down there.

@DanielSank Oh, I could have told you that.

@0celo7 But you didn't.

5:09 AM

True.

So instead, @KaumudiHarikumar gets the price money.

So you have no reason to believe me

It is a whopping $2.17.

that's like....950 calories at McDonald's

user228700

@DanielSank Glad it worked :-) The desktop interface on the mobile is annoying. Can't upload pictures though. For that, you'll need to click on those three bars at the top of the mobile page and then click on "Full site". That's how you revert back.

5:11 AM

speaking of, I haven't been there in a few months

@KaumudiHarikumar I rarely have reason to upload photos in chat.

@KaumudiHarikumar Thats what I said a few seconds ago

user228700

@DanielSank xD No, it's -$1000. If this is how helping works, I gotta pay you for the discussion we had about entropy.

@JohnRennie Tell me something interesting

@KaumudiHarikumar Thanks for saying that. I have been worried ever since that discussion that I didn't explain it well. You've taken a weight from my shoulders by saying that.

@0celo7 Anything?

5:13 AM

@0celo7 What?

@DanielSank Math or physics related

@0celo7 Ok!

I'm stuck on QM homework

Check this out:

want to not do that for a while

5:13 AM

You know the pythagorean theorem, yes?

@DanielSank I've heard of it, yes.

@0celo7 Very good. Here is a way to say it in English:

user228700

@DanielSank It did help, don't go worrying and all :-)

@0celo7 the way to make an industrial slurry stable is to flocculate it i.e. deliberately destabilise it.

@JohnRennie explain

5:14 AM

The length of a line segment is equal to the square root of the sum of the squares of the lengths of the projections of that line segment onto the various coordinate axes.

@0celo7 do you agree?

Suppose you have a slurry of zeolite particles in water (widely used in many industrial applications). This is a stable dispersion because the zeolite particles carry a negative charge and repel each other.

@DanielSank Seems correct. What's the catch?

But if you let the slurry stand the zeolite particles will settle under gravity and form a close packed layer at the bottom of the tank.

@JohnRennie Zeolite? Stable dispersion?

@JohnRennie Believable.

This close packed layer is dilatant and very difficult to redisperse.

5:16 AM

what?

So when you try the pump the slurry out of the tank it basically just sits there giving you a two fingered salute.

is it like slop?

@0celo7 No catch.

can you shovel it out?

But check this out: that statements works for $k-$dimensional objects in $n$-space where $k<n$.

5:18 AM

Objects?

Dilatant systems are generally very hard to do anything with. You end up having to use a pneumatic drill.

Can you be more precise?

If you put a sheet of paper out in front of you in your room, the area of that paper is equal to the square root of the sum of the squares of the 2-dimensional projections of that paper on the three independent planes.

@JohnRennie I need some pictures

is the stuff at the bottom rock-solid?

i.e. projections onto the floor, wall in front of you, and wall to the left of you.

This amazing theorem shows up all over the place.

5:19 AM

@DanielSank Hmm.

It's why you see things like $\sqrt{M^\dagger M}$ as measures in integrals.

I feel like I've heard this from you before, and didn't understand it then

I am unable to comprehend linear algebra

@0celo7 Details here

Calling you Harold from now on.

A: What causes the 'tears' in yoghurt drinks?

Also:

5

Yoghurt is a flocculated suspension of casein micelles. The acid secreted by the bacteria growing in the milk destabilises the casein particles and they aggregate together. Because the volume fraction of the casein particles is high in milk the aggregated particles form a gel. This image shameles...

Harold was my grandad's name

5:21 AM

@0celo7 You don't understand the area of a sheet of paper?

@JohnRennie A fine chap.

@DanielSank I'd have to think about it. Clearly using an integral is overkill.

@0celo7 Integral?

Who said anything about integrals?

@DanielSank It's a manifold w/ boundary, the area is the induced volume integral.

Like I said, linear algebra is incomprehensible to me.

How is the area defined?

@0celo7 The way you expect.

@DanielSank I don't know what to expect.

5:23 AM

What does it matter? You understand the meaning of the theorem, no?

@DanielSank I don't know how it is in the US, but when you go back two generations there's a marked difference in the way people viewed life. My grandad was a great chap and I loved him but many of his views, though typical for his time, would be badly out of place today.

I don't see how to prove it.

And I don't know what "objects" you are talking about.

Regular polygons?

Or products of intervals?

@JohnRennie Naturally.

@0celo7 Start there if you want.

@DanielSank I don't see any reason for it to be true

Why is it true?

@0celo7 Same reason that it works for 1D thingies in $n$-space.

5:27 AM

@DanielSank ...rotate into a plane, draw a picture?

@0celo7 If it helps, go for it.

I don't remember how Munkres proves it.

You can sort of tell that as you rotate the paper around, if it loses area in one projection it has to gain it in the others.

tbh, linear algebra like that is "party trick" math in my book...

along with number theory

@0celo7 wat?

Number theory is scary and amazing.

@DanielSank It's a curiosity

@DanielSank which chapter should I be looking in?

@0celo7 You have Analysis on Manifolds by Munkres o_O?

5:31 AM

Yes

@0celo7 I... we have something in common!

@DanielSank The power of Russian servers is amazing.

@DanielSank Have you forgotten we are both ardent Shankar fans?

A: No magnetic field from a static charge - Is there a simple physical argument to show why?

Someone upvoted this?

1

When I put a magnet on the sides of my dryer drum. Static is reduced. Can you tell me what is happening? I thought the magnets attracted the positive ions which I thought was what static was.

@0celo7 I'm not sure I appreciated that you were such a fan.

Damn good book, though.

@JohnRennie That's a very subtle question.

@DanielSank I've said multiple times it's one of like 5 physics books I consider genuinely good.

And I don't even like physics.

5:33 AM

Haha

Ok, got book... looking for theorem...

Man, Munkres's analysis book is just so good.

One of the best math books I've read.

@0celo7 getting close... page 152... It's near here.

@DanielSank I need to sleep soon...

Ah, it's farther along...

Theorem 21.4

Such a good book.

Zero errors.

Linear algebra, as I thought

@0celo7 Yes.

Amazing result though!

...so you say

5:45 AM

You think not?

Come on, everyone knows the Pythagorean theorem, but few realize that it works for any dimension object.

It's a curiosity. I don't see a real use

Wat?

You use it all the time geometry.

@DanielSank Ok, "any dimension object" needs to be "any parallelepiped"

@0celo7 Locally, any manifold looks like a parallelepiped.

:D

@DanielSank Very vague statement.

5:46 AM

@0celo7 Not really.

@DanielSank Are you claiming this is true for manifolds?

If you parametrize a $k$-manifold in $\mathbb{R}^n$, you need this theorem to do integrals, or something like that.

thinking...

No.

Maybe if you define the integral in some horrible way.

@0celo7 I mean to actually compute them.

"actually compute" -- something I never hope to do

5:49 AM

This theorem tells you how to build the Jacobian determinant in the case that derivative of your change of variables is not a square matrix.

@0celo7 Oh, go to sleep.

Are you thinking about integration over chains?

@0celo7 Maybe. What's a chain?

formal linear combination of simplices

@0celo7 What's a simplicies?

*simplex

5:52 AM

Wow. You actually took that bait.

a simplex is $$\left\{\sum_{i=0}^px_iv_i\mid 0\le t_i\le 1, \sum_{i=0}^p t_i=1 \right\}$$

::Googles "simplex"::

crap

A simplex is crap?

why won't that parse

5:54 AM

\left \{

You have to escape the curly braces.

ah

@0celo7 No idea what that means.

simplices:

k

So triangles.

basically.

they're very useful in algebraic topology

@DanielSank hmm, well

you have some set of vectors $[v_1\cdots v_n]$

and you go out in each direction

your direction is some linear combination $t_1v_1+\cdots+t_nv_n$

and you want each $t_i$ to be $\ge 0$, and the sum of the $t_i$s is exactly 1

5:59 AM

@0celo7 Why is there an $x$?

Typo?

Oh

I'm tired.

Go to sleep.

Probably a good idea

I'll leave so you're not tempted to stay up.

Cheerio

5:59 AM

Ciao

user228700

6:09 AM

Never mind :P

@KaumudiHarikumar the criterion is that the Gibbs free energy decreases i.e. $\Delta G \lt 0$

user228700

@JohnRennie Yes, yes. I was wondering-are there are any physical processes that are non-spontaneous?

user228700

@DanielSank Gosh, you're very kind :P

@KaumudiHarikumar I'm not sure what you're asking. Can you give a (hypothetical) example of the sort of thing you mean.

user228700

@JohnRennie I'm making a distinction b/w non-spontaneous chemical processes (such as, say, the backward reaction of the neutralization reaction b/w an acid and a base) and a non-spontaneous physical process, but I'm unable to come with any physical process that is non-spontaneous.

user228700

6:22 AM

Oh oh crap, hang on, the Carnot Cycle, that's non-spontaneous..?

@KaumudiHarikumar yes, but that's not an isolated system. You need some external mechanism to compress the gas in the Carnot cycle.

If you take an isolated system it can only ever perform a spontaneous process

user228700

@JohnRennie Wait, what you're saying is that the criterion that ∆$G$<0 for spontaneous processes is only in the case of isolated systems..?

If you take some gas and compress it then the $\Delta G$ for the gas alone is positive because (a) its internal energy goes up (b) you do work on it and (c) its entropy goes down.

A spontaneous process is something that happens without any external influence, and all spontaneous processes have $\Delta G \lt 0$.

user228700

So the system has to be isolated; I gotta include my hand that's doing the compressing too, yes?

user228700

@JohnRennie So technically, this gas being compressed is a non-spontaneous process, yeah..?

6:30 AM

Replace your hand by a one hundred ton weight that you place on the piston containing the gas. The total free energy change for the weight falling down and compressing the gas is negative so it happens spontaneously.

user228700

6:47 AM

@JohnRennie So...wait, am I correct in assuming that again, when applying this law, I need to make sure that the system is isolated?

Yes

user228700

@JohnRennie OK. Thank you :-) Do you work during the weekend too BTW?

@KaumudiHarikumar only for a couple of hours ...

user228700

@JohnRennie OK. And also, where do u get ur reading lists from?

user228700

I saw that u have a looong list.

6:50 AM

@KaumudiHarikumar my list is several hundred books long! I keep an eye on various web sites that review new science fiction and fantasy books and I check them for any books that look interesting.

Though my definition of interesting is different from any people's :-)

user228700

@JohnRennie Oh, so u're mostly into sci-fi and fantasy..?

@KaumudiHarikumar for recreational reading I almost exclusively read SFF.

I particularly the New Weird genre of science fiction.

I'm currently reading The Physics of the Dead by Luke Smitherd

user228700

@JohnRennie Oh, I see :-) Didn't know of the existence of such a thing. Then again, there's so much Idk.

@JohnRennie That sounds interesting.

I've been a fanatic science fiction fan since I was a young boy.