The h Bar - 2014-12-02

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1:49 AM

0

Q: Do we have Nobel prize winners on this SE?

Just curious to know do we have Nobel prize winners are on this SE?

8 hours later…

9:42 AM

So, I got up this morning and got to class just to find out it was cancelled. Why am I awake?!

9:53 AM

3

A: What it means to integrate over $n$ variables out of $N$, where $N>n$?

There's really two questions here, one about the definition of distribution functions and one about the derivation of the BBGKY hierarchy. I will address them in turn. Definitions Let's define, for convenience, $\mathbf{r}^n = \mathbf{r}_1,\dots,\mathbf{r}_n$ and $\mathbf{r}^{(N-n)} = \mathbf{...

that's a really good answer

5 hours later…

2:43 PM

Chat session in the morning. Interesting.

Always chat sessions

?

IDK

Just saying

We've got this chat room open always

Nah, I'm just in a time zone that's more than 12 hours off my usual :p

haha right

I got the chat session email and was like "Did it happen? Will it happen? I dunno!"

Ah. Where are you currently?

2:49 PM

West Coast. Portland.

I prefer living in the future on the Eastern Hemisphere.

Ooo, Oregon

What're you doing there?

Mozilla stuff. There's a big coincidental work week

(Since Mozilla teams are distributed across the globe + volunteers like me, there are work weeks a few times a year to get together, discuss in person, and code/whatever. This time all teams have it simultaneously -- so that they can discuss things with other people too)

I see

3:22 PM

cool

I may or may not be able to make it, depending on the internet access

(as usual)

3:56 PM

So anything to discuss?

Chat session: Awkward silence - now also available as a virtual experience on the internet!

Awkward silences are always fun

Currently trying to work through some basic string theory

user54412

astro-ph has... cosmology, cosmology, cosmology, something about a planet, more cosmology...

okay I'm here, for now

user54412

4:05 PM

yesterday I came across some list of the most cited astrophysics papers of all time -- they were all cosmology

@KyleKanos You have a very strange sense of fun ;)

@ChrisWhite Does it ever have more not-cosmology papers than cosmology papers?

@ACuriousMind Indeed I do

user54412

@KyleKanos Don't all cosmologists go drinking and partying in Mexico once a year for a "conference"?

user54412

surely they aren't submitting papers by the end of that :P

what do people do here? is anyone in research?

4:08 PM

Hmm, you never know. There could be a cosmology-research equivalent to the Ballmer peak:

user image

I'm pretty sure the effect is universal, if perhaps the specific parameters vary a bit from field to field or task to task

user54412

@Holtz something like 6 of us in the room right now are grad students, so that would be yes

sweet, what areas are you involved in?

<-- particle phenomenology

user54412

simulating black hole accretion disks

4:13 PM

@ChrisWhite oh somehow I keep thinking you're in cosmology, whoops :-P

sounds awesome

I've just started in AQFT

<-- cosmic ray production in supernova remnants

huh interesting question just posted

or should I say "interesting"

moon sized planets? seems suspicious

user54412

I was thinking of posting a quick calculation, but then I realized that would involve numbers, and I'm just not in a numbers mood this morning

4:16 PM

I think he means that the outer planets were the size of the moon

@DavidZ It's a silly question, but I still wish people wouldn't use drive by downvotes

At least it's not another UFO sighting...

Yeah, i think the question could be phrased slightly better

A good response is to answer explaining why it's unlikely

A reasonable alternative is just to ignore it

and let the autodelete remove it in a few months.

I was recently wondering what kinda of ultimate tensile strength a material would need in order to support a space life

4:18 PM

@JohnRennie when there's no apparent gain to be made by posting a comment, I can understand leaving a downvote without a comment

user54412

@JohnRennie does autodelete delete questions with a score of 0?

actually I personally think it's off topic

@DavidZ Well, planets do look a bit bigger when you're (insert altitude of flight) metres closer to them. They just don't look very much bigger :-)

user54412

@Holtz what do you mean by that?

user54412

it's not too hard to survive vacuum when you're really small

4:21 PM

@JohnRennie An extra 9 km isn't going to help when the planets are >1e8 km away

@DavidZ : Concerning perspective, I'm reminded of skeptics.stackexchange.com/q/23830

@KyleKanos Cor, Jupiter is only a factor of 60 smaller than the Moon as seen from Earth. I would have guessed it was a lot smaller than that.

Well, say you have a really long cable - that goes to sufficiently high enough - with a lift on to ferry people from the surface to a orbiting satellite. I was wondering how possible this would be, since each additional unit of the cable would result in an increase of tension

so you're talking about space elevators?

user54412

@JohnRennie It's always surprising to me that even Neptune is clearly larger than a point source in any reasonable telescope -- the solar system isn't that big

4:29 PM

ah neat. I actually am in that case

4:40 PM

Pretty hot off the press

news.sciencemag.org/physics/2014/12/…

Planck data finally released.

5

@EmilioPisanty Yay!

Oh, yeah, that's what we should be talking about

I meant to write a blog post about that, but I'm too busy now... anyway from what I'm hearing it's not really that interesting

user54412

Not really surprising -- despite the press releases, the fact is Planck is little improvement over WMAP, and WMAP already pinned down cosmological parameters to the percent level

Brandon Enright

@ChrisWhite Visually the CMB map looks much better.

I read somewhere that Plank was near the theoretical limit of the signal-to-noise ratio and we'll never be able to measure the CMB any more accurately. Does that sound even close to right to you?

user54412

@BrandonEnright I think it's fair to say it's no longer an instrumentation limit.

4:53 PM

Yeah, I've heard that.

Cosmic variance

The term cosmic variance is the statistical uncertainty inherent in observations of the universe at extreme distances. It has three different but closely related meanings: It is sometimes used, incorrectly, to mean sample variance - the difference between different finite samples of the same parent population. Such differences follow a Poissonian distribution, and in this case the term sample variance should be used instead. It is sometimes used, mainly by cosmologists, to mean the uncertainty due to the fact that that we can only observe one realization of all the possible observable universes...

user54412

cosmic variance affects the low-l modes most -- you only get one universe to sample

user54412

high-l stuff is better, so if your theory distinguishes high-l effects, that's good

user54412

but on the other hand we've pushed into the regime where "astrophysical foregrounds" dominate over cosmological signals

user54412

basically, all the interesting physics in the universe -- galaxies, stars, dust -- that pure cosmologists like to pretend doesn't exist in their baryon-free model universes

I guess all the people complaining about putting spherical cows on frictionless surfaces should point the finger at cosmologists for emptying the universe

4:59 PM

Yo guys, I've got a really basic (stupid) qft question

reading Tong's notes on QFT (page 107) (or P&S, or a number of other sources) one usually sees the quantization of the Dirac field

we impose the usual (anti)commutation relations on the Dirac field, and try to find what this means in terms of the creation/annihilation operators

but... $[\Psi, \Psi^\dagger]$... how is this even a valid equation? $\Psi$ has four components, so the first term will be a number but the second a $4\times 4$ matrix...?

I must be failing to see something obvious...

That'd be a really good question for the site, actually

I guess I'll post it... but could you give me a quick answer? :P

@Danu The answer is 42.

:D

well I don't actually know offhand :-P which is why I'm saying it'd be a good question for the site

No, but really... this is ridiculous!

I'm going nuts about this

user54412

5:07 PM

From someone who knows nothing about qft... what's wrong with just plugging (5.4) into the commutator, using the normal "product" between whatever terms arise?

@Danu I think that you should think of both as matrices - like $[\psi,\psi^\dagger] = \psi \psi^\dagger - (\psi^\dagger)^T \psi^T$

@ACuriousMind Really?! Also, how can we possibly 'pull out' the $u, u^\dagger$ out of the commutators

Or rather, componentwise: $[\psi,\psi^\dagger]_{ij} = \psi_i \psi^\dagger_j - \psi^\dagger_j \psi_i$

Don't ask me about details of these ugly spinor computations with the $u$ and $v$, I hate those things

@ACuriousMind Componentwise? I'm not sure... but maybe.

That's the only way out I see for now

I'll take a look at the PDF and see if I can figure it out

unfortunately I'm separated from all my QFT textbooks otherwise I'd flip open P&S, I think I have an idea roughly where in the book they'd talk about this

5:11 PM

it does seem like TOng is surpressing the identity matrix on his identity for $\sum_s u\bar u$, which would point to a componentwise computation

@DavidZ I've already found it; they don't discuss it.

They just say "it is easy to see..." and pull the $u,v$ out of the commutators without comment (exactly like Tong)

I mean, sure, maybe they don't discuss it explicitly, but they include the information needed to figure it out (if you know what to make of it)

Tong used P&S to write his notes :)

oh hey, I didn't even notice the chat session "ended"

One of these days we'll talk about actual site issues, I swear

@DavidZ It's been a while since it's happened, methinks

user54412

That's a good sign, right?

user54412

5:15 PM

Or should I troll meta with some incendiary proposal to give us something to discuss?

@ChrisWhite Yes. Do something about the "apparatus identification" tag you mentioned the other day

@ChrisWhite I dunno, I guess it's good that there are no catastrophes to deal with, but it's healthy to have some discussion of how the site works from time to time

I think the homework policy is on deck, I just keep being too busy to prepare for that discussion

@ChrisWhite: I don't think questions like that would be very useful for future visitors, since it would be very difficult to search for what you're looking for.

We'd probably have a bunch of posts with images, and 'what's this?'

@JamalS I agree... apparatus identification is not really a question of physics either... imo?

@Danu: No, not really a physics question, and if the person doesn't know what it is, how can he know if it's used in physics, and not say, more appropriate in chemistry?

5:28 PM

@JamalS There are a few already, e.g.

0

Q: What is this simple mechanism called?

I often build with Lego and use this mechanism: It converts the rotational movement to linear by making the stick between the two blocks go back and forth. What is this mechanism called?

newtonian-mechanics soft-question terminology

@KyleKanos: Ah, thank you for reminding me. I saw it, but forgot to vote to close :)

user54412

I wasn't serious about the proposal one way or another, but I sense potential controversy

A related Meta post on the issue:

6

Q: Are physics-related notation and terminology questions on topic?

This is prompted by What is the origin of the naming convention for position functions?, which asks why s is used as the letter for position. We've had a few other questions like this; feel free to edit in more examples. Are these questions on topic? They're not really about physical concepts, b...

discussion scope

@KyleKanos And a recent question, where it has become obvious that there is not really consensus about closing or leaving alone such questions.

@ACuriousMind Uhh, that's the same question I linked to two comments up

5:31 PM

Argh

Nvm me then

Question posted: physics.stackexchange.com/questions/150045/…

Please, if any of you has a solution, write an answer!

@Danu I suppose my 42 won't get upvoted?

@KyleKanos We're not on MSE here ;)

Oh yeah

6 hours later…

11:43 PM

0

Q: Code syntax highlighting

Take this code sample from meta.SE:  var a = 3; while( !(a < 0)){ alert("JavaScript code <b>goes</b> here."); a--; } I've written it below (the first line is recognized, processed and eaten by the SE syntax parser): var a = 3; while( !(a <

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Dec '142

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