The h Bar - 2017-05-02 (page 3 of 6)

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4:00 PM

@DavidZ I might have to run soon

OK well I can cover it as long as the internet holds up

In QM, is it clear to anyone why in a system in thermal equilibrium why we expect $\frac{\partial \rho}{\partial t} = 0$ where $\rho$ is the density operator?

Either way, let's get started. Welcome to our biweekly chat session, everybody!

Please keep unrelated messages until after the prepared topics

@DavidZ My question came before your announcement :)

though actually I'm not sure what the prepared topics are, so for now our agenda is

1. Intro and welcome and site questions (5 minutes)
2. Recent physics developments (10 minutes)

Ping me if there's anything else to add

First things first, is anyone new to the site, new to chat, or new to chat sessions?

@Moses well we put other stuff on hold for chat sessions

4:03 PM

@DavidZ No problem.

It seems quite quiet today though. Won't be long ;-)

Quiet doesn't bode well for questions either...

Yeah. When we actually have something to talk about, the chat sessions are livelier.

So let's move on to recent physics - I assume we have some lurkers who might want to chime in. What's new and/or interesting in the world of physics?

Cassini's dive is one of the big focus here

@Secret ooooooh, yeah

has anyone seen good colour pictures of the northern hexagon?

from the recent pass

like, this is nice

saturn.jpl.nasa.gov/internal_resources/893

but there's better pictures by Cassini from farther away

4:09 PM

that's not colored, and I don't recall anything colored in recent images

Proposal on detection of naked singularities via precession of objects orbiting around it

like, this is from whenever ago

user image

Simulating the rings of Chariklo, rings were found to consists of smaller particles than expected

@Secret huh, that's interesting

No phase shift means hypercomplex number extension to quantum mechanics are out of the question for now

Stacking polyaromatics layers to make spin liquid states

Non equlibrium phase transition modelling using compelx hamiltonians (and something else Idon't quite understood)

Study on nonlocal correlations in ground state spin hamitonians

Did we talk about the LHCb B-meson decay discrepancy last time?

AccidentalFourierTransform

4:14 PM

hi people

I think we talked about lepton universality, but not that

Yeah I wasn't sure since it was just announced on that day

@DavidZ nope, but 2.5$\sigma$ are not that exciting ;P

@AccidentalFourierTransform hi

@ACuriousMind Yeah, still something to be aware of IMO

Valleytronics, modelling multivalley dispersion in momentum space

4:16 PM

@Secret you're full of ideas today :-P

Sure, I'm just not a fan of hyping these things. The last bump they had that vanished after more data got people far too worked up, imo

@Secret we might could do with a What is "valleytronics"? question on main

That's our 10 minutes... I do want to mention one thing before we revert to open discussion

169

Q: Brief outage planned for Wed, May 3, 2017 at 8pm US/Eastern (00:00 UTC) (like a fire drill for computers)

MicroVersion: Planned service degradation: All Stack Overflow/Stack Exchange sites read-only for 20 minutes on Wed, May 3, 2017 shortly after 8PM US/Eastern (midnight UTC). If you blink, you'll miss it. Short version: There will be a service degradation for up to 20 minutes shortly after 8PM US...

discussion featured announcements

4

Not terribly important for most people, but it's something to be aware of

@DavidZ as I understood it, the outage may last up to 20 min in read-only, but will likely be shorter?

Supervoid cannot explain great cold spot in CMB. Other possible explanations include random flucturations in big bang, topological defects in early universe, to collision with another universe

2

and that's all from me

4:18 PM

@EmilioPisanty I don't know, but it sounds like they are planning on using the full 20 minutes, based on that post

@Secret oh yeah I heard about that one. Interesting.

OK, so back to open discussion, i.e. we can talk about whatever, and the official chat session ends in 40 minutes

@DavidZ Rather, it is sciencedaily that is full of ideas in this fortnight (that's where I quote most things), not many particularly interesting for this fortnight for theoretical physics however, mostly condensed matter stuff

@Secret Well, that's good, we shouldn't get locked in to theoretical stuff

That's true

22 mins ago, by Moses

In QM, is it clear to anyone why in a system in thermal equilibrium why we expect $\frac{\partial \rho}{\partial t} = 0$ where $\rho$ is the density operator?

in case anyone knows offhand

@DavidZ Wouldn't the definition of entropy make it simple?

4:24 PM

@Moses The definition of "equilibrium" is that the state doesn't change over time.

@DavidZ @Moses What do you mean by equilibrium if it isn't $\frac{\partial}{\partial t}=0$?

@EmilioPisanty Why isn't E = hf valid

0celouvskyopoulo7

what's up

If in later steps you include the number of photons emitted per second from the laser

I kinda wanted to leave that implicit

$$\frac{dS}{dt} = -\frac{\partial }{\partial t} \text{Tr}(\rho \ln (\rho)) = -\text{Tr}(\dot \rho \ln (\rho) + 1) $$

0celouvskyopoulo7

4:26 PM

@Sanya did you check out Arnold?

@Slereah trace of 1 is large

Hence the entropy is only maximal for $\dot \rho = 0$

@ACuriousMind What about the Shrodinger time evolution of the states?

Do the states not still evolve according to that?

@0celouvskyopoulo7 yeah; I wonder why most of my theoretical physics classes sucked so much

0celouvskyopoulo7

@ACuriousMind perhaps interesting to you?

2 hours ago, by 0celouvskyopoulo7

@BalarkaSen Interesting result of Minakshisundaram-Pleijel: Let $M$ be a compact surface, and $\lambda_j$ be the eigenvalues of the Laplacian. Then $$\sum_{j\ge 0}e^{-\lambda_jt}\sim \frac{\mathrm{vol}(M)}{4\pi t}+\frac{\chi(M)}{6}$$ as $t\downarrow 0$.

Are there any experiment setup that allow one to observe the formation of entanglement in real time when two product states are placed close together to interact?

4:27 PM

@0celouvskyopoulo7 yeah it should actually be $\dot \rho$ too

@Moses Of course they do. So equilibrium is a very special state $\rho$ where $[\rho,H] = 0$.

@mpv @K7PEH @Reversal @LeakyNun Haven't seen you before here. Welcome to the h bar!

@Secret what do you mean by "observe entanglement"?

@ACuriousMind Nuff said

thanks

I'm fairly sure that an argument like that is the reason

4:28 PM

Which is "obvious" for the standard thermal state $\exp(-\beta H)$.

of course you should be careful on what you're doing since those are operators and all

But I think it's something like that

@EmilioPisanty I really don't understand how to jump through the hoops here sometimes

0celouvskyopoulo7

@Sanya classical mechanics is one of the mathematician-approved areas of physics

I am not sure, probably having a setup act as a entanglement witness and over the course of the experiment, one can see the value of this entanglement witness rise and falls as the state become more entangled.

Unlike superposition, I suspect one cannot detect the formation of an entanglement state by inferometry, I am also not sure whether all possible entalgement witness will involve measuring the state

If I include all the information I get disapproval for a full answer, if I don't I get disapproval for an incomplete answer and a statement about missing things

0celouvskyopoulo7

4:30 PM

@Phase I have a solution in mind but the mods wouldn't like it.

@0celouvskyopoulo7 when I have a lot of time, I still want to read Abraham Marsden :D

0celouvskyopoulo7

@Sanya indeed, that's on my long list

I should write a list of books to read...

Well.. You holding back from saying something definitely makes it seem controversial.

@Phase disapproval from full answers is usually linked to obvious homework, which I don't think this is.

0celouvskyopoulo7

I have it in my mind but it's becoming unmanageable

4:31 PM

In any case, my main objection to your answer is that you shouldn't invoke the photon picture at all.

0celouvskyopoulo7

I've made some progress lately

@Phase "Full answers" to non-homework questions are not disapproved of at all.

0celouvskyopoulo7

user image

@EmilioPisanty Why not? He asked why Photons can impart momentum while having zero mass?

Radiation pressure is perfectly explainable by classical EM.

@Phase OP makes no mention of photons.

4:32 PM

@ACuriousMind Just to confirm, are you saying that $\frac{\partial | \psi \rangle}{\partial t} \neq 0$ but $\frac{\partial | \rho \rangle}{\partial t} = 0$ (which is equivalent to saying $[\rho, H] = 0)$ in thermal equilibrium?

are you complaining about the tiny sigma

0celouvskyopoulo7

@Slereah indeed

it is comically small

AccidentalFourierTransform

what is $|\rho\rangle$?

@0celouvskyopoulo7 is the marsden in abraham marsden the same marsden as the one in foundations of elasticity? But anyway yeah, I totally see your point - too many books, not enough time

4:32 PM

He asked how, given that light has no mass

@Moses Yes (although I'm not sure why the $\rho$ is dressed in a ket :P)

0celouvskyopoulo7

@Sanya I don't know who it is

@AccidentalFourierTransform That was a typo. Eagle eyes over here...

:)

@Phase So, you answer that light can nevertheless carry momentum, and invoke the classical picture.

AccidentalFourierTransform

:-P

0celouvskyopoulo7

4:33 PM

user image

Talking about photons in that situation just cements misconceptions.

@EmilioPisanty This doesn't feel necessary, to be honest. I got a downvote for talking about photons instead of waves?

0celouvskyopoulo7

user image

@Slereah this guy just likes small operators

Ink's not cheap, @0celouvskyopoulo7

Can't print giant sigmas all over the place

Basically, after you reminded me that magnetic fields are one electron operators thus cannot entangled two electrons, I have been wondering what does it physically look like when a state is rotated by some operation similar to CNOT into an entangled state. It is easy to see what's going on on paper as basically the state vector $\lvert s_A,s_B \rangle$ rotates to some new vector $\lvert s'_A,s'_B \rangle$ making it no longer possible to decompose it into product states,
but physically it seemed difficult to understand what is happening since after the state become entangled, you cannot thin…

4:35 PM

@Phase The downvote was because the answer was actively unhelpful - it just got the OP into yet another insoluble quandary, from which he'd be even less able to dig himself out.

The edit makes it less actively unhelpful

Have an undownvote if it matters that much to you

The physical setups that I am aware of that can implement CNOT always involve a bell measurement step, does that mean to establish an entanglement from a product state, I need to do a bell measurement?

I still disagree with calling it "actively unhelpful". It's not the downvote that matters, it's the principle behind it

(hint: it shouldn't. seriously.)

I don't care about rep, but getting downvoted here means that my answer is legitimately thought of as useless, which stings a little and I don't understand how it's useless. It may invoke a photon image, but to me imagining things as particles rather than waves, when both are valid, is simpler

@Phase You've taken an OP that had no idea how to navigate light, and put him in a situation where he has no idea how to navigate light and no idea how to navigate photons

I'd call that actively unhelpful, to be honest

4:37 PM

Why would a layman know how to work his way around Waves any more than Particles though? This is what I don't understand

I gave him the only equations he needed that were particle specific really

0celouvskyopoulo7

why are you Capitalizing?

Everything else is just power and intensity etc

@Phase Well, technically it means that one person thought your answer was not useful.

@Phase Your answer just adds to the conceptual load that the OP needs to cut through for no good reason.

You could equally well just say: Light is a wave and carries momentum with a momentum density $S=uc$ which you get from classical EM [reference], where $u$ is the energy density.

To me that's no more intuitive

at least layman understand the motion of particles from day to day life

4:40 PM

@Phase yeah, well, other people disagree with you, you don't need to fight all of them about it all the time

opinions were expressed, we all move on

@EmilioPisanty I'm just trying to get an answer I can understand, to help me write answers in the future

@EmilioPisanty You already rescinded the downvote, that should be evidence enough that that isn't really what I care about

@Phase I already told you what I think. I don't know what else you want here.

Ok, well thanks for the time you spent discussing it.

I'm not even sure what post you're talking about, actually. Can you post a link (again?) and maybe I can offer another opinion?

@DavidZ physics.stackexchange.com/questions/330463/…

@Phase No need to delete the answer.

One person disagreed with you.

That's all that happened.

4:45 PM

Well, I just wanted to err on the safe side in case it was actually detrimental to OP

It's not as if I deleted my account : p

@Phase Frankly, though, I think the question isn't worth spending much time on.

Uhh... I would consider temporarily undeleting it but it seems I can't view my account right now

OP either (i) has not done anywhere near enough research, or (ii) just needed a pointer to the correct terms to google. Which was hopefully solved in the comments.

@EmilioPisanty maybe, sorry if it seems I get carried away, I promise there's no ill will in my intentions, I just want to get a better gist of what to do on this site

@Phase sure. Asking is the right way to go about it.

4:47 PM

I'm kind of concerned why I can't view my account though

Don't think of it as jumping through hoops though.

@Phase oh, um, well it's totally okay to do that if you want, but I'd personally consider it a bit of an overreaction. Everybody gets downvotes from time to time.

user image

You'll get a feel for answering after a while.

12

Q: User profile pages broken

Going to anyone's profile on MSE, SO or Super User (didn't try others) results in an error page.

bug profile-page

4:48 PM

@Phase you should be able to undelete directly at the answer.

undeleted temporarily, and thanks @ACuriousMind

"Fun" (not really) story: my first answer on the site got downvoted and sat at -1 for a while.

@Phase try again now

Ey it works!

It's fixed now. — patricksweeney 36 secs ago

Bernardo Meurer

4:50 PM

@DavidZ Is manisearth still alive?

I need Rust guidance

@DavidZ Honestly I don't really care about Rep, it's not as if I have anything to prove anyway, the I don't even have a good depth of knowledge having not even completed my bachelors yet. It's just about trying to understand stuff

@BernardoMeurer I assume so. I see occasional activity from him but he's probably busy with other stuff IRL.

@Phase Yeah, I'm not talking about rep either. Just saying every once in a while you get downvoted and most of the time it doesn't really mean your answer is bad.

Bernardo Meurer

@DavidZ Dangit

@Phase: note that I've suggested the question is a duplicate and linked to my answer to the duplicate, and my answer is effectively the same as yours. So I don't think there is anything wrong with your answer.

Terry Bollinger

[minor question: has ChatJax died completely now? Lotta LaTeX on my screen...]

4:52 PM

@TerryBollinger me too

Terry Bollinger

@EmilioPisanty kind of a bummer...

6 hours ago, by ACuriousMind

You likely have an older bookmark, just like me. You need to replace the cdn.mathjax... in script.src by h‌ttps://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AM‌S_HTML

@TerryBollinger yeah, it's odd

^The old Mathjax CDN server died, you need to update your bookmarks

@ACuriousMind I'm running the new bookmarks

Terry Bollinger

4:53 PM

@ACuriousMind thanks!

bizarrely, some TeX shows but other bits don't

that testing message worked

@EmilioPisanty Do you have 2.7.1 and not 2.7.0 in it?

@DavidZ Ok thanks, I'll try to take that on board
@JohnRennie tbh I just winged the post so I'm just glad I didn't make some kind of error in any assumptions

21 mins ago, by Moses

@ACuriousMind Just to confirm, are you saying that $\frac{\partial | \psi \rangle}{\partial t} \neq 0$ but $\frac{\partial | \rho \rangle}{\partial t} = 0$ (which is equivalent to saying $[\rho, H] = 0)$ in thermal equilibrium?

this doesn't

And yeah, it seems the single-dollar TeX breaks but the double-dollar TeX doesn't with the old bookmark

AccidentalFourierTransform

4:54 PM

im using the old bookmark and everything works fine

@ACuriousMind yes

@JohnRennie doesn't work

AccidentalFourierTransform

yes it does! lol

Bernardo Meurer

Broken here as well

@EmilioPisanty Hmmm. Did you try turning it off and on again? :)

so you're saying $ \int_0^{1,2,3}\,\text{testing}(x)dx $ and $$ \int_0^{1,2,3}\,\text{testing}(x)dx $$ behave differently?

well, that's curious

4:55 PM

@EmilioPisanty yeah, for me too - the second one renders but not the first

It works here, but there were lots of different versions of the ChatJax bookmarks and I'm obviously using a version that still works.

Bernardo Meurer

@JohnRennie Just paste your bookmark code here

@ACuriousMind to the extent that you can turn chatjax off, yes

user image

But I don't use automatic rendering. I have a bookmark that renders only on demand.

bizarre

AccidentalFourierTransform

4:56 PM

@BernardoMeurer too long

Bernardo Meurer

Put it on pastebin or smth then

javascript:(function(){if(window.MathJax===undefined){var%20script%20=%20document.createElement("script");script.type%20=%20"text/javascript";script.src%20=%20"https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS_HTML";var%20config%20=%20%27MathJax.Hub.Config({%27%20+%20%27extensions:%20["tex2jax.js"],%27%20+%20%27tex2jax:%20{%20inlineMath:%20[["$","$"],["\\\\\\\\\\\\(","\\\\\\\\\\\\)"]],%20displayMath:%20[["$$","$$"],["\\\\[","\\\\]"]],%20processEscapes:%20true%20},%27%20+%20%27jax:%20["input/TeX","output/HTML-CSS"]%27%20+%20%27});%27%20+%20%27MathJax.Hub.Startup.…

javascript:(function(){if(window.MathJax===undefined){var script = document.createElement("script");script.type = "text/javascript";script.src = "https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML";var config = %27MathJax.Hub.Config({%27 + %27extensions: ["tex2jax.js"],%27 + %27tex2jax: { inlineMath: [["$","$"],["\\\\\\\\\\\\(","\\\\\\\\\\\\)"]], displayMath: [["$$","$$"],["\\\\[","\\\\]"]], processEscapes: true },%27 + %27jax: ["input/TeX","output/HTML-CSS"]%27 + %27});%27 + %27MathJax.Hub.Startup.onload();%27;if (window.opera) {script.innerHTML = config} else {script.te…

@JohnRennie that's unlikely to work anymore

Well, it does.

4:57 PM

cdn.mathjax.org was shut down

And yet it moves

mathjax.org/cdn-shutting-down

Bernardo Meurer

@ACuriousMind Thanks, got it working

@JohnRennie for now

Bernardo Meurer

They kept it just for John

4:59 PM

Hi guys, this is maybe a silly question, but is force, $\vec F$, defined as $\begin{aligned}\frac{d\vec p}{d t}\end{aligned}$? Is $\vec F$ somehow just like a placeholder for $\begin{aligned}\frac{d\vec p}{d t}\end{aligned}$? I haven't done physics for a long time, and I'm kind of confused for the moment.

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