The h Bar

DanielSank

00:01

@DavidZ Unfortunately it seems to be mostly <1000 rep users who make such titles. Not sure they read the the meta. There is helpful text inside the title entry box saying to be specific, but people just ignore it.

@ACuriousMind Indeed! It just gets old. You know.

David Z

Well, all those posts are meant mostly to be things we can point people at when they complain, and say "we told you so" (or: "here's how to do better next time")

DanielSank

@DavidZ Ah.

David Z

or for the few new users who actually do want to know how to ask good questions

DanielSank

Good point.

Actually, isn't this already in the help center?
...checking...

David Z

Probably something like it, but I doubt it's very specific

DanielSank

00:03

@DavidZ Indeed not

That could use work.

David Z

That's exactly what faq posts are for.

DanielSank

* scratches head

@DavidZ This is a tag I use in the meta? What does it do?

David Z

It's a special tag that can only be added by moderators. It's meant for posts explaining some aspect of how the site works, in more detail than the help center.

The best example of usage is on Meta Stack Exchange

they have a whole series of FAQ posts on how the SE software works

DanielSank

This should be linked in the help center.

David Z

It might be, somewhere

DanielSank

00:09

Excerpt:

Stupid:
HELP! Video doesn't work properly on my laptop!

Smart:
X.org 6.8.1 misshapen mouse cursor, Fooware MV1005 vid. chipset

Smarter:
X.org 6.8.1 mouse cursor on Fooware MV1005 vid. chipset - is misshapen

David Z

I'm almost sure it's linked in one of the FAQ posts on meta SE

DanielSank

Stupid:
Electrostatics question?

Smart:
Electric field from a sphere of charge

Smarter:
How do I compute the electric field outside of a sphere of uniformly distributed charge?

I may make that into a meta post.

David Z

Probably not a bad idea

For the sort of thing that is intended to become an FAQ post, tag it with faq-proposed and people can comment on it and suggest changes etc. Then when and if there appears to be a consensus, a moderator will add faq

DanielSank

@DavidZ: Ok. The title of the post would be "How do I construct useful question titles"?

DanielSank

01:27

@TAbraham: if and when you have a minute I'd like to chat about your question regarding plants and quantum mechanics. The problem you're asking about is called the quantum random walk and is highly studied in the literature.

However, the idea that plants are well described by this model is suspect. Some people point out that transport in plants may exceed the speeds predicted by a classical diffusion equation. However, they fail to understand that for short time scales or length scales the diffusion equation grossly underestimates the transport rate in classical systems.

The continuum diffusion equation is an approximation of a process which really involves discrete objects moving around. There is a mean free path (and associated time) for which the particles don't collide. If you consider time scales much larger than this you can use the diffusion equation.

However, if you consider time scales shorter than this the particle motion is ballistic and are not well described by diffusion.

tl,dr: Don't believe everything you read.

DanielSank

01:41

Anyone here know quantum mechanics?

ACuriousMind

@DanielSank Ís that a trick question? :D

DanielSank

No

I have a real question:

Suppose I have a two level system connected to something else which is in a thermal state.
The two level system's Hamiltonian is
$H = -(\hbar \omega / 2)\sigma_z$

The interaction between the two level system and bath is
$H = A \hat{F} \sigma_x$

All I want to know is this:

If I want to compute a transition probability for the two level system, and I don't care what the final state of the bath is, how do I "integrate" over all states of the bath?

I guess I just sum over the probabilities corresponding to each possible bath final state.

But something about that smells funny.

ACuriousMind

Isn't saying "connected to a heat bath" the same as saying "the system is perfectly described by the canonical ensemble"?

DanielSank

Well, no in the sense that the two level system starts in a state which I prescribe.

It's not in equilibrium at first, although the bath is in a thermal state, like I said.

ACuriousMind

I see

DanielSank

01:48

* strokes beard thoughtfully

ACuriousMind

I'm now actually wondering if you should sum the probabilities or the amplitudes of the final bath states

DanielSank

@ACuriousMind: I think I'm just asking how you write down a probability if you don't care what state part of the system is in at the end. This should be, like, really obvious, but I'm not a smart man.

@ACuriousMind Good point.

In either case what are the states to sum over?

ACuriousMind

@DanielSank Given that we never specify what a heat bath is, that's a surprisingly stupefying question.

DanielSank

@ACuriousMind: I'm not entirely sure what you mean.

A heat bath (in this case) is a system which is in a thermal state. I guess.

>.>

<.<

ACuriousMind

We always just wave our hands and say "there's something with constant temperature there" - we never say what, if any, different states with this same temperature it has

DanielSank

01:52

@ACuriousMind: Who's "we"? You got a mouse in your pocket?

:)

ACuriousMind

Heh. Yes, it's me and my giant miniature space hamster that I got from some insane ranger a while ago.

(Don't worry if you don't get the reference :P )

DanielSank

^ I want that.

Seriously though, this should be so obvious.

@ACuriousMind: FWIW this is in relation to my spectral density question.

ACuriousMind

At which point in time are you interested in the "transition rate"? If you wait long enough, the system will have thermalized and we can just use the canonical ensemble, right?

DanielSank

Let's say for "small" times after I let the system go.

I think you get a rate.

ACuriousMind

I must say I have no idea

DanielSank

02:02

@ACuriousMind: Thanks for thinking about it.

@ACuriousMind: Do you want me to ping you if/when I figure it out?

ACuriousMind

@DanielSank Yep, now I am interested whether this is easily solvable, too

DanielSank

@ACuriousMind Ok. Will chat ping you if I figure it out.

ACuriousMind

@DanielSank: Also, regarding your spectral function - I know it defined as $\rho_{ab}(x,y) := \mathrm{i}\langle[\phi_a(x),\phi_b(y)]\rangle$ for scalar fields in a QFT context of scalar fields $\phi_i$. Is your definition different or am I blind?

DanielSank

@ACuriousMind ...processing...

@ACuriousMind: Are $\phi_a(x)$ etc. field creation ops?

What do the subscripts mean?

ACuriousMind

@DanielSank They are the scalar fields themselves, not necessarily free. I'd not call them "creation operators" for that reason.

DanielSank

02:13

@ACuriousMind: Bear with an experimentalist who knows enough to be dangerous.

ACuriousMind

@DanielSank The subscripts number the different fields - we have a scalar QFT of $\phi_1,\dots,\phi_n$ different scalar fields

DanielSank

To me, $\phi(x)$ is the operator which creates an excitation at point $x$.

OH!

Ok, there are multiple fields.

ACuriousMind

You can also just have one scalar field, then the spectral function is just a scalar, and not a matrix, itself

Given only one field $\phi$, it'd still be the expectation value of the commutator of that field, though

DanielSank

Why is your "spectral function" called $\rho$? Also, it doesn't have the units of a spectral function.

Perhaps "spectral density" and "spectral function" are related but different.

The expression you wrote there looks a lot like the thing on the right hand side of the Kubo formula.

Actually it's different. Never mind.

ACuriousMind

@DanielSank Hm. I took a course in non-equilibrium QFT a year ago, and there one essential step was to split the propagator into two parts - its real part, called the statistical function, and its imaginary part, called the spectral function.

It may well be that "density" is a more appropriate term for this thing

DanielSank

02:17

Spectral density surely has dimensions of $(\text{the-thing-I-care-about})^2 \times \text{frequency}$.

I know I've seen that expression before though. I took a condensed matter course which was roughly quantum field theory in non-relativistic systems. That was a while ago, however.

ACuriousMind

Hm...since it is a scalar commutator, it has mass dimension 2. I've never much cared about thinking in not-natural-units. What unit does a spectral density/function have in natural units?

DanielSank

What does $\langle \cdots \rangle$ mean in your expression? Is that a vacuum expectation value?

@ACuriousMind I am resisting every fiber in my body telling me to rant about how much I hate it when people conflate units with dimensions...

@ACuriousMind I have no idea.

ACuriousMind

@DanielSank It occurs in (variants of) something variously called the Kardanoff-Baym or Dyson-Schwinger equations

DanielSank

@ACuriousMind Grah! This is all familiar but not at my fingertips. I need to work things out from first principles or it will never stick. Thanks for the push!

ACuriousMind

But I'm not sure my spectral function is related to your spectral function, anyway

DanielSank

02:27

@ACuriousMind I really really think it is.

ACuriousMind

@DanielSank Then I might link you to our lecture notes from that course, 3.4 is where spectral and statistical function appear, and where their evolution equations are derived.

There's no "true" interpretation given, sadly, but I think the spectral part tells you which states are available, and the statistical part how much the system wants to occupy them. (Absolutely no guarantee for this :P )

DanielSank

@ACuriousMind: If you please, without thinking too hard, rate the linked document from 1 to 10.

I'm asking because it's written in a language I don't really know, so I want to know how much time I should spend trying to decode it.

ACuriousMind

@DanielSank Wait, it's written in English, isn't it?!

DanielSank

@ACuriousMind :) I meant that it's written in field theory.

ACuriousMind

@DanielSank And 8, from a biased theorist's viewpoint who also heard the lecture based on it

DanielSank

02:34

These thermal/noise/correlation issues can (and are!) used in simpler formalisms.

ACuriousMind

@DanielSank I believe so, but this is, probably unfortunately, the only time I ever dealt with such things in a quantum (or any, really) setting

DanielSank

and actually I think I just figured something out.

@ACuriousMind Don't get me wrong, I appreciate your help and the resource very much. Just trying to do some time management.

ACuriousMind

Side note: It's really amazing that this kind of QFT can (numerically, but also, somtimes, analytically) predict the thermalization of most systems after "reasonable times" without any additional assumptions.

DanielSank

@ACuriousMind Why wouldn't it?

ACuriousMind

@DanielSank Because so very many (theoretical) statistical mechanics and QFT people treat "out-of-equilibrium mechanics" as something very esoterical, where many approximations and assumptions need to be made in order to get useful results

DanielSank

02:39

@ACuriousMind Indeed. Still, numerically one expects the equations of motion to work, no?

ACuriousMind

@DanielSank True enough. So maybe it's not that amazing that things thermalize, but that we can even look at the process and get pretty simulations. (Quantumly, this is not as easily as it sounds - if you do the wrong resummations of diagrams, your simulation will be absolutely horrible, even if you have enough computational power)

DanielSank

@ACuriousMind Ah, yes, that is pretty cool.

If I had gone into theory nonzero temperature QFT is probably what would have caught my attention.

I have a weird love of the mathematical techniques involved.

You guys are fortunate. As an experimentalist it is difficult to make time to get to the bottom of something theoretically. This is why a lot of experimentalists use what look to theorists like crutches.

I have found, however, that making that time is worth it as long as you're careful not to go down an endless rabbit hole.

ACuriousMind

@DanielSank You seem to walk on them pretty well ;)

DanielSank

It would be nice to have a lot of time for rabbit chasing.

^ I meant that in a nice way.

@ACuriousMind Thanks.

hwlau

@ACuriousMind what does the word thermalization mean in the quantum context

ACuriousMind

02:46

@DanielSank Mostly, it's a series of events where you are just asking "Why didn't someone tell me this from the beginning?!", followed by the realisation that you wouldn't have understood the explanation as "simple" back when you set out to understand $THING. ;)

DanielSank

@ACuriousMind LOL. I know exactly what you mean.

(I also like the bash notation)

Interestingly, the experimental part of experiment is somewhat different.

When something doesn't' work and you finally figure out why you ask "how the hell was I supposed to know this insane way this piece of equipment could fail?"

Then later when you're experienced and you're helping someone younger you just know what's wrong and they think you're a wizard.

Actually, being an experimentalist is kind of like debugging a computer program while God is injecting new bugs in real time.

After a while you get to know His tricks.

ACuriousMind

@hwlau Essentially "that the system is in equilibrium". A necessary condition is that there should be translation invariance of the propagator, and additionally, one should have that it is also periodic with periodicity $T = \mathrm{i}\beta$

(where $\beta$ is inverse temperature)

This singles (mostly) out a single solution to the non-eq evolution equations

(It is, interestingly, the case that there are systems which do not thermalize - it is hoped that there are only a "neglegible" subset of the possible initial conditions)

hwlau

So, it the system open or closed?

ACuriousMind

@hwlau It's The Universe™, so I guess it's closed.

hwlau

That's the thing I don't understand, if you are really talking about the whole universe

Using periodicity instead of temperature is confusing to me

Kyle Kanos

03:01

Aw man, the Universe has been TM'd?

hwlau

And if you say that there are system not thermalize, then it is not the whole universe

@KyleKanos TM by god?

Kyle Kanos

Maybe

ACuriousMind

The periodicity technically arises from the invariances thermal equilibrium must have, and using the Schwinger closed time loop formalism for the path integral together with Wick rotation. To get that constraint is really not trivial as far as I can see

hwlau

@ACuriousMind Is the same equilibrium (periodicity) appear in classical system? I mean without any nonlinearity interaction, gravity, etc.

ACuriousMind

@hwlau No, you don't have these propagators and path integrals there, so you cannot transfer this to classical systems easily

Quantumly, you just list the things thermal equilibrium should be invariant under, and then you derive constraints. I'm sure some clever guy has shown that this actually is what we wanted with thermal equilibrium, but it's really not my specialty, so perhaps it's still a big mystery

@KyleKanos Either that or my trademark key is malfunct™ioning

hwlau

03:10

@ACuriousMind So what is the simplest example system that it gets thermalize?

Kyle Kanos

Wait, you have a trademark key? I thought you were a physicist, not a lawyer!?

hwlau

I have seen some serious work on it. But I still don't understand what they want to do with thermal equilibrium in quantum context

ACuriousMind

@hwlau e.g. two high energy colliding wave packets, whose final thermalized state only depends on the total energy density

hwlau

@ACuriousMind Any ref/link? I just want to understand the problem

ACuriousMind

@hwlau look at pages 67/68 of the lecture notes I linked earlier. As I said, I'm not really an expert on this, so I can't do much more than parrot these.

@KyleKanos You mean you don't have one? Poor sod ;)

Kyle Kanos

03:20

^My keyboard

I see no TM button

ACuriousMind

@KyleKanos There is none, I made it up :P

Kyle Kanos

Oh I know

hwlau

It is hard to understand anyway, it needs some background that I don't

ACuriousMind

@KyleKanos Very good, now I'm a bit confused, but don't worry about that

Kyle Kanos

Don't be confused

hwlau

03:24

Also, your notes just remind me that the non-equilibrium statistical theory is also just started.

Phonon

06:57

@ACuriousMind a recent talk given by David Gross on the history and evolution of QFT, (after 2-3 minutes the vid stabilizes). Here's the link

Danu

10:49

I'm getting close to a Reversal badge!!! :D

5 more upvotes... hmmm

where are sock puppets when you need them!

@Phonon Comic sans, lol

Sabre Tooth

link

Danu

@SabreTooth you mean link for my answer?

Sabre Tooth

dang, I already +1'd the answer

Danu

Lol

thanks anyways

Sabre Tooth

as I mentioned sometime earlier... look at the 2nd entry on the star wall... that is now '7'

Danu

10:55

@DanielSank It kiiiinda sucks IMO

@SabreTooth Congrats ^^

Sabre Tooth

lol

Sabre Tooth

11:26

engineering is almost at private beta... maybe my plethora of applied physics questions could be answered there

Arc676

12:12

I'm probably missing something very obvious here (physics isn't my field of expertise) but in this GIF on Wikipedia's page on Relativistic Doppler Effect shows the object as farther away than it really is. Why is this?

Anyone?

user54412

12:33

@Arc676 There seems to be no good documentation as to what is being shown, so I can only guess.

Arc676

The page itself is here

user54412

Yes, the explanation there is lacking too

Arc676

It states that the gray sphere is the object and the colored ellipse is what the observer (blue dot) sees

Only the caption contains info relevant to the GIF

user54412

Here's my guess: suppose the sphere is moving across your field of view, but its point of closest approach is still some distance away from you (my assumption; this is not explicitly stated anywhere)

user54412

Then when the animation begins, the sphere has been moving.

user54412

12:36

At that time, the gray sphere is at a certain location.

Arc676

"point of closes"?

user54412

But the light from the gray sphere's current location cannot reach you instantaneously.

user54412

But you are nonetheless seeing the sphere -- it never disappears from sight.

user54412

That is you must be seeing photons emitted from an earlier time, when the sphere was further to the right.

user54412

(Maybe the sphere is passing through your location; I suppose the same argument would work; but again there's no real telling what the animation shows)

user54412

12:39

So the relative displacement between gray and colored is just an effect of a nonzero light travel time. It's a relativistic effect I suppose, but it's something of a distraction when it comes to what the diagram should be emphasizing (color and shape)

Arc676

I see. Thanks, I didn't think of that. Silly me.. :)

user54412

Put another way, the gray sphere is the "freshman physics object" as measured by a set of clocks and rulers distributed throughout space -- the God's eye view, so to speak

user54412

the colored sphere is the "astronomer's object" where we only ever see photons that come to Earth

Phonon

@Danu you may be interested in this so good this guy.....

user54412

Relativity is often taught entirely in the former sense, but astronomers are in the habit of thinking "on the past light cone" rather than "on my surface of simultaneity"

User Anonymous

14:47

Does anyone know, in exactly which paper did 't Hooft "propose" anomaly matching condition?

Or, should I post this on the main site, as a question?

Perhaps makes sense to ping @ACuriousMind.

ACuriousMind

@UserAnonymous I believe this is exactly the sort of question that falls under specific-reference, though I've misinterpreted that tag before

User Anonymous

See, I scrambled across his list of publications, but I am unable to make out.

Or is this too stupid a question for the main site? @ACuriousMind

ACuriousMind

As I said, I think this is exactly what we allow/want under specific-reference. It's not really a physics question as such, but I think it is on-topic.

User Anonymous

OK Thanks @ACuriousMind . I wish 't Hooft was an active user here, I could've straightaway asked him :(

ACuriousMind

@UserAnonymous He was, some time ago, but not anymore

User Anonymous

14:52

So sad :(

@ACuriousMind - I wonder why would a person so original have a profile pic of this kind:

G. 't Hooft, Utrecht, Netherlands

2.5k 12 23

ACuriousMind

@UserAnonymous I haven't the slightest idea, but perhaps he just likes that monkey? :D

User Anonymous

:D

Physics Meta

15:13

0

Q: Is there a way to check voted comments?

I wonder if there is a way to check more or less quickly if your comments where voted. I specifically had the problem that I wanted to check how far away I was from the "pundit" medal, that requires 10 comments with score of 5 or more. The only way to check that I found is to go through the comme...

support comments badges

ACuriousMind

16:27

"Fun" fact: I vote down more every day than any of you

Sofia

16:45

@KyleKanos : can you help? I downvoted an answer that seemed to me wrong, physics.stackexchange.com/questions/156351/…. But the fellow who answered explained and convinced me. I just insisted that he introduce in the answer the argument by which he convinced me. But the fellow disappeared. Can you help? Not only I want to undo my downvote, I would like to upvote. But I am not permitted after downvoting. What to do?

ACuriousMind

@Sofia The answer has been edited a few minutes ago, so you should be able to simply click on the upvote to change your vote

Gowtham

he just edited it @Sofia .try again

Sofia

@Gowtham : problem solved.

@Gowtham : how do you know there in the room, every "breath" ? It's not a pleasure to be always under the eyes of the "big brother". People need some privacy.

@Gowtham : whatever people do, you immediately know?

@ACuriousMind : ahaaaaa ! You vote down a lot? Aaaaaa!

ACuriousMind

17:05

@Sofia "A lot" apparently means about 5 downvotes per day on this site. Most of them are on questions, I guess, I only vote down answers when they're blatantly wrong or misleading.

Gowtham

@Sofia What O.o it just updated in the main page and i clicked to see the new answer and noticed it was modified XD

@Sofia just trying to help >< no i am neither God nor have a cerebro at home

Sofia

@ACuriousMind : you are from Germany. But your English is, so it seems to me, at the level of mother-tongue. Were you for a long time in England, or U.S.A. or something like that?

ACuriousMind

@Sofia No, the longest time I had to converse in English continuously were about four days. But I've read quite a lot of English books and watched quite a lot of movies, I guess I'm just good at picking it up

Kyle Kanos

Crud

I answered a question that totally does not belong here

Hate when that happens

Nick

18:16

Greetings h bar.

ACuriousMind

Interesting edit there ;)

Nick

@ACuriousMind Hi, how are you?

Editing is so much fun.

DanielSank

@Danu: r/askscience got more popular and that seems to have degraded the quality. The other problem is a lack of good canonical Q&A which means that the same questions come up frequently. Still, I like that because it's not billed as a "for experts only" forum, people with little background can ask questions about the world around them and learn something.

Nick

@ACuriousMind You seem to know a lot about doing experiments. Are there any guides to which I must adhere before doing even a basic experiment?

Whenever you do any experiment, you have to include the experimental errors - imprecision of the instruments, error in reading off scales, etc. These will lead to errors on every measurement. "Connecting the dots" is never what we do. You have data points with errors, and you do least-square fits to formulae to determine which relation they obey. (Linear regression if you suspect a linear relation) Doing experiments properly is an art/science in itself. — ACuriousMind Nov 23 '14 at 23:12

I have the above quote stuck on my bedroom wall.

ACuriousMind

@Nick I'm one of the purest theorist you'll find, I know nothing about conducting experiments except that which I wrote there :D

Nick

18:25

@ACuriousMind Ofcourse you'll be knowing of some resources on how to conduct a basic experiment and minimize error. There has to be some experimenter's bible lying around somewhere. If only I knew the title.

ACuriousMind

@Nick I had lab courses where we "learned" that

JamalS

@ACuriousMind Don't remind me; I'm dreading those! :(

Nick

@ACuriousMind I skipped all my lab courses to pursue my love of poetry. lol. I learnt nothing except to wear a labcoat.

ACuriousMind

@Nick I never wore a lab coat, those are for chemists :P

Nick

@ACuriousMind ... that explains much.

ACuriousMind

18:29

(Laser) Safety googles, yes, but no coat

Nick

A curious question, when I have to radically update an unanswered question on the main site, should I ask a new question linking to the old one or just change the old question?

ACuriousMind

@Nick If the basic question stays the same, I'd edit it. If it is only about the same topic, but your actual question has changed, I'd ask a new one

Nick

0

Q: Effect of varying distance of light source from a Light Dependant Resistor

I'm doing the experiment described here and I have a few questions regarding it. I've taken a mini-LDR hooked to a multimeter set to $\Omega $ and a $25\,\mathrm W$ Light Bulb to plot the following graph: At every $1 \text{ cm}$, I've taken readings by exposing the LDR to the light for an ave...

electromagnetic-radiation visible-light experimental-physics home-experiment

@ACuriousMind: I got the time to redo the above experiment. Should I update it?

I redid it around 30-40 times, it linearizes perfectly.

But I have this weird resistance error of $\Delta R = -120\Omega$

ACuriousMind

I think you should update it

Nick

@ACuriousMind The only question I have is that resistance. Do you have any clue where that could come from. What does negative resistance even mean.

ACuriousMind

18:41

::shrug:: It means something is fishy ;)

Nick

@ACuriousMind I'm sorry. I'm just an uncoordinated peebrain. The only way I can get that graph pass through the origin is if I lie about the initial reading.

ACuriousMind

@Nick Ah, it is the y-intercept? That could indicate that the relation isn't perfectly linear near zero, or that there is some kind of offset that you did not include

Nick

My aim is to prove $R \propto x$ or $R = kx$ but the line I'm getting is $R = 60x - 120$

@ACuriousMind I have been searching since november on what that offset could be. I am no closer to the answer.

DanielSank

18:57

Hey dudes: is there a resource which takes a mathematically mature physicist from zero to path integrals and actually explains what the heck is going on?

@Nick: I know things about circuits, btw.

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