The h Bar

2:34 AM

I'm sure I saw a question from this site pop up in the hot questions list earlier today. It was something about "tickling the dragon's tail". (Since this was the hot questions list, that phrase must have been in the title of the question.) Noticed it as I was clicking away from the page, so I came here to look for it. No joy. Was it merely a figment of my imagination?

Kyle Kanos

15

Q: Why did "tickling the dragons tail" by Louis Slotin not cause an explosion?

I have been reading the excellent Command and Control by Eric Schlosser and discovered more about Louis Slotin's experiment with "tickling the dragons tail" and the infamous Demon Core. What I don't understand; and please excuse my naivety, is when the accident on May 21st, 1946 occurred and Slo...

nuclear-physics

Imagining, you are not

tpg2114

6:03 AM

@KyleKanos Have you ever worked with characteristic decomposition for Navier-Stokes?

David Z

7:45 AM

@VianEsterhuizen flashlights with incandescent bulbs will produce a full spectrum (not exactly the same as the sun, but reasonably similar), but newer flashlights with LEDs will only produce a few bright lines. Which might be an interesting thing to do in its own right.

TRiG

11:13 AM

@KyleKanos Thanks.

user54412

12:00 PM

@Danu No, there are many important causal equations that are first order in time. The problem is with elliptic equations (e.g. Laplace) and parabolic equations (e.g. heat).

user54412

Consider the Euler equations for fluid dynamics: these are often expressed in flux-conservative form, d/dt (conserved quantity) + div(flux) = 0

user54412

they have finite information propagation speeds

user54412

in relativity, all these equations look like 4-divergence(some tensor) = 0, and again everything is local, which is why we use these equations in simulations

user54412

as a prof of mine once said, nature only uses hyperbolic equations, we invent elliptic equations by setting things to 0 in search of steady-state things and the like

user54412

the ADM equations are examples of highly nonlinear but still very much local equations that are first-order in time

user54412

12:09 PM

@ChrisWhite (I should have said explicit simulations -- implicit solvers tend to need information across the entire domain at some time to figure out the entire domain at some later time)

Danu

1:37 PM

@ChrisWhite Hah, I didn't know one can respond to one's own comment

Thanks a bunch for the good explanation, by the way.

Hippalectryon

2:49 PM

Does anyone know the English equivalent of what the French call ARQS ?

Jim

What does the acronym stand for?

Hippalectryon

Approximation des Régimes Quasi Stationnaires

Basically when studying electricity, you neglect the fluctuation of the current (as in a wave) inside the wire

That means that the cables must be short enough, etc

Jim

Try Quasistatic approximation

Hippalectryon

Oh thanks, that sounds exactly like it

Kyle Kanos

@tpg2114 As in using the Jacobian of the hydro PDEs?

Waffle's Crazy Peanut

3:11 PM

@ManishEarth I never really thought of that! It just didn't occur to me. Well, if the tides can get so high while orbiting the BH, so can the rocket, no?

@Jim: You should say something about that, (being a rocket-science enthusiast!) :D

2 days ago, by ManishEarth

@Phonon Well, they leave Earth with a 3 stage rocket, and then they leave a planet with 160% Earth's gravity with just the 3rd stage-module-thing. The planet was orbiting a large BH -- there have been explanations about the slingshotting necessary to leave that grav pull; however it's hard to explain escaping a planet with so less fuel

Jim

@Waffle'sCrazyPeanut Do you really want to hear me rip that to shreds?

Waffle's Crazy Peanut

@Jim Yeah!

Jim

So they are sending people to another planet with a 3 stage rocket? Great! That's the only way you'll get them there

But you have to use all three stages to leave Earth

There will be some fuel from stage 3 left over, but not all of it

Also, the first stage is the most powerful and absolutely necessary to leave Earth's gravity

It's also a solid propellant stage usually, because those offer more thrust even though a lower ISP

Kyle Kanos

@Jim 6 stage rockets then :D

3 to leave earth, 3 to get there?

Jim

@KyleKanos that makes no sense

Kyle Kanos

3:16 PM

I know

That's why I said it

Jim

The only way to leave the other planet is with a more powerful 3 stage rocket

Kyle Kanos

Of course, I'm not really following the original conversation

Waffle's Crazy Peanut

@Jim Yeah, but isn't that enough to get to that (imaginary) orbiting centrifugal station?

Kyle Kanos

But now reading Manish's comment, yeah that makes no sense

Jim

@Waffle'sCrazyPeanut I have no idea what station. I never saw the movie

Waffle's Crazy Peanut

3:18 PM

ah, okay...

Then, a few questions...

David Z

-4

A: Sound - what happens with the particles when a wave passes

ALL IS ABOU T THE WAVE S WAVES HAHAHAHAHHAHAHAHAHAHHAHAHAHAH

FYI all: that is exactly the sort of thing the Very Low Quality flag was designed for.

Waffle's Crazy Peanut

@DavidZ I just flagged it for the mod :)

@Jim: They said that the planet orbited a BH, and had 130% gravity compared to Earth's...

But, I can't quite get how the tides got as high as a mountain. And, they came frequently...

David Z

@Waffle'sCrazyPeanut Yeah, I just wanted to say something because like 6 people flagged it for 5 different reasons. I mean, technically, yes, most of those reasons were valid (it isn't an answer), but VLQ is the preferred flag for it, and it seems like a lot of people don't know that.

Jim

How high do neap and spring tides get on Earth? and that's with the Sun's puny gravity

@DavidZ I only saw it in the LQ review queue

Waffle's Crazy Peanut

@DavidZ Oh, alright... :)

Jim

3:22 PM

@Waffle I'm curious what they used for the gravity slingshot

Waffle's Crazy Peanut

@Jim But, that's because we're quite far and it's less massive compared to that BH which should be at least as massive as 10 suns.

David Z

@Jim ah, well I confess I never use that queue (I have my hands full with the mod queue) so I forget what options it gives you. But you should choose whichever one corresponds to "delete" (as I'm sure you did)

Jim

@Waffle Yeah, the tides should be massive. I've heard the BH was as massive as 100 million suns. And I tried to do the math, which I'm pretty sure came out to the planet being within the Roche limit of the BH

So that would be strong tides indeed

Waffle's Crazy Peanut

@Jim It's an advanced flight-tech thing - I can't really comment on its escape

Kyle Kanos

@DavidZ That was already in the LQ queue this morning

Waffle's Crazy Peanut

3:25 PM

@Jim Yeah, then why can't a simple rocket escape the planet? Why should it need 3 stages? It should be easy for the rocket to get off that thing, no?

David Z

@KyleKanos I only just noticed it :-/

Kyle Kanos

If enough of us users flag it for deletion via the VLQ queue, does it get sent to you guys?

Jim

It needs 3 stages because the gravity is huge and 3 stages allows you to eject a lot of mass when those fuel sections and boosters are empty and useless

And the gravity slingshot will only work to get you out of the system if you slingshot around the planet. To do that though, you need to get outside its sphere of influence, which will require massive delta-v; ie 3 stage rocket if manned

David Z

@KyleKanos I'm actually not sure. I can't find anything detailing how the queue works. This is the best I have so far:

11

A: What are the guidelines for reviewing?

Guidelines for reviewing Low Quality Posts Questions appear in the low quality post queue both by algorithm and by flags from users. Basic workflow Check if the post can be improved. If it can, by all means Edit it. A typical example are code-only answers, which can benefit from additional ex...

user54412

So I just got an email excitedly telling me the grad student bar will have earlier hours. Apparently we were unhappy with the prospect of drinking away our sorrows too late at night?

user54412

3:30 PM

Also, it will be open on Thanksgiving. Because holidays for grad students are best celebrated by drinking alone in the basement.

Jim

@ChrisWhite I'm jealous. They closed our grad student bar. I'll never understand why

@Waffle'sCrazyPeanut Point is, I don't see how one rocket could leave Earth, bring people to a more massive planet, and then be used to return them to Earth. It's not feasible. You would need the initial rocket to be absurdly huge. The more sane thing would be to send 2 unmanned and 1 manned rocket. The unmanned would carry the first two stages of the return rocket and the manned rocket carries the passengers and the third stage of the return rocket. That's the most feasible way of doing it

Kyle Kanos

Wait...there's a thing called "grad student bar"

Jim

I've heard Kip Thorne did an analysis and said the physics of that movie were okay. I'm pretty sure he only did the analysis of the BH. Because the rocket science and planetary science seemed bogus and under-considered

@KyleKanos not for me any more

Kyle Kanos

Not that I'm a bar-goer mind you (too expensive)

But that's interesting that such a thing exists

Waffle's Crazy Peanut

@Jim I'll brb in a minute...

@Jim In the movie, they use this centrifugal orbiting station (like the one in 2001: Space Odyssey). The spacecraft from Earth reaches the station, takes it along with it. When they come near this planet orbiting the BH, they shoot one of the spacecrafts to that planet while the station is in orbit around the BH. After a few hours, they escape the planet and return back to the station

Jim

3:45 PM

I assume the station carries literally tons of fuel?

I'm not sure how they get to the station again, but if they do and if the station has a giant engine and lots of fuel, then it could be possible

Waffle's Crazy Peanut

@Jim I guess so, they wouldn't have mentioned about that much (but, that station was really BIG - it could carry 5 other spacecrafts along with its 12 or so modules shaped as a ring

Jim

... what?

5 s/c and 12 modules?

How far in the future is this?

Waffle's Crazy Peanut

@Jim Quite far... Earth is about to get depleted :D

Jim

so where do they get the fuel needed to boost such a large station to a speed that could get people to another planet?

and then slow it down when they get there

bah, they consulted physicists but they never stopped to ask a rocket scientist if this was feasible

Waffle's Crazy Peanut

Jim

3:51 PM

unless those pods are fuel tanks, this baby is going nowhere

Waffle's Crazy Peanut

@Jim: Oops.. I exaggerated about the spacecrafts. Only one could get docked at a time (if I really hadn't forgot what I watched)

@Jim Hah! So, Scifi it is...

Jim

Movie magic

Waffle's Crazy Peanut

@Jim It should also rotate to simulate gravity inside it...

Jim

How many people are in one of those?

Waffle's Crazy Peanut

@Jim Only 3 in the spacecraft. Later, they take control of the station...

Jim

3:55 PM

so, the station is smallish?

about the size of the ISS?

Waffle's Crazy Peanut

@Jim yeah, about that...

Jim

Okay, not as impossible as I thought before. But, there's still no way to leave the planet with only the third stage of a rocket

Waffle's Crazy Peanut

Honestly, I was amazed of the visual effects they've invoked in the wormhole & blackhole. It was nicely done - It was a Science+SciFi mix :)

Jim

I love scifi, but it ruins it when people start saying that the physics and other science is accurate and works out

because it never does

David Z

@ChrisWhite holidays?

Waffle's Crazy Peanut

4:04 PM

@Jim None of my friends were able to accept the physics that appeared in the movie. They can't take it as SciFi, nor they can appreciate real science -_-

You should watch it someday. I'm curious about your review :D

Jim

Yup, that's it. I have now been told to watch that movie by people from literally every area of my life. I may just have to

Kyle Kanos

I'll watch it when it comes to Redbox because who pays $9+ to see a movie?

Waffle's Crazy Peanut

@KyleKanos Yeah, you too

Kyle Kanos

@Waffle'sCrazyPeanut I JUST SAID I'D WATCH IT!

:D

Jim

@KyleKanos the way I look at it is not that I'm paying to see a movie, but I'm paying for the fun social activity with friends that's sure to leave good memories

Waffle's Crazy Peanut

4:08 PM

And, $9? It's only about $2 here!!! O_O

Jim

where are you?

Waffle's Crazy Peanut

India...

Kyle Kanos

@Waffle'sCrazyPeanut Most theaters I've been to charge $9

Jim

Well that explains it

Kyle Kanos

@Jim me = grad student + husband + father of 4 $\equiv$ cheap

Jim

4:11 PM

@KyleKanos No, that is not $\equiv$ cheap. That is $\gg$ cheap

Waffle's Crazy Peanut

@KyleKanos You sure have a lot to worry about... :)

Kyle Kanos

@Jim I do not see how

Jim

another word for worry is care

Kyle Kanos

@Waffle'sCrazyPeanut I don't worry about much, except finding a job

Jim

@KyleKanos sorry, would $\ll$ cheap be better? whichever means it puts you beyond needing to be cheap

Kyle Kanos

4:13 PM

@Jim Ah, that verbalization was what I was missing

Kyle Kanos

Does anyone know why the arXiv doesn't support Latex on its pages?

e.g.: arxiv.org/abs/1406.7037v3

The paper is titled "H$\alpha$ emission-line stars in molecular clouds"

But on the site, it's written "Halpha emission-line stars in molecular clouds"

David Z

arXiv does support LaTeX, but the title you see on the page isn't taken directly from the paper. The submitter has to enter it by hand.

Jim

I always assumed it was done that way to make papers appear more ominous and unreadable. And to generally give you a sense that you're in over your head when browsing

David Z

I dunno, I think they do that well enough with or without MathJax enabled

Kyle Kanos

@DavidZ Hmm. I don't recall seeing any with Latex though. Ever, I mean. Maybe it's just astronomers/astrophysicists who can't do things like that?

Jim

4:22 PM

And a cake is delicious with or without icing. But that doesn't mean we eat just plain cake

David Z

Astro people probably have less need

@Jim AMEN

in other news I miss cake

@KyleKanos hep-ph is full of them, e.g. arxiv.org/abs/1411.6423

Kyle Kanos

@DavidZ Well I take it back then.

Astro people are probably just dumb

David Z

OH here's one with $\ell$ in the title: arxiv.org/abs/1411.6396

Jim

*Jim now subtly reminds us of the distinction between astrophysics and cosmology*

*Well done Jim for making sure you're not included with the group of dumb people*

Kyle Kanos

No, cosmology is part of astronomy & astrophysics

Jim

4:26 PM

You take that back. If it were, I'd understand what those people are talking about

Kyle Kanos

What people? The link I gave?

Jim

Cosmologists don't care about what's in the universe. Just how it got there

and how it affects the universe itself

and "those people" are any astrophysicists

Waffle's Crazy Peanut

LOL...

Phonon

5:39 PM

hi guys, good to see more people hanging around in here as well, usually it's just me and Gauge

Kyle Kanos

Who's Gauge

Phonon

ACuriousMind

Kyle Kanos

....

Okay

Phonon

me and Danu gave him a new nickname, shorter and suited :D

Kyle Kanos

Why Gauge?

Ah

Phonon

5:41 PM

lol

cause he goes crazy for it :D

@KyleKanos how are things with you man? work going well?

Kyle Kanos

Yeah, trying new things now

In particular, Ohmic resistivity

Magnetohydrodynamics

Magnetohydrodynamics (MHD) (magneto fluid dynamics or hydromagnetics) is the study of the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes. The word magnetohydrodynamics (MHD) is derived from magneto- meaning magnetic field, hydro- meaning liquid, and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970. The fundamental concept behind MHD is that magnetic fields can induce currents in a moving conductive fluid, which in turn creates forces...

Phonon

ah

nice nice

Kyle Kanos

A lot of people study magnetic reconnection:

Magnetic reconnection

Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnection occurs on timescales intermediate between slow resistive diffusion of the magnetic field and fast Alfvénic timescales. The qualitative description of the reconnection process is such that magnetic field lines from different magnetic domains (defined by the field line connectivity) are spliced to one another, changing their patterns of connectivity with respect to...

with respect to simulations involving particle acceleration. I'm incorporating it into my model

Phonon

let me read into this, I had a course in micromagnetics once, didn't like it...the math was ok, but the physics is so vague and messy, at least the way we were taught...

Kyle Kanos

At least, I think I am. I have to convince my advisor that it's good to do so

Phonon

5:45 PM

cool! what language you re using?

Kyle Kanos

Fortran, of course ;)

Phonon

I personally use python for data analysis and scripting, and all MC MD's in C++ or C

you and your Fortran D:D

Kyle Kanos

Python's useful for that stuff. Fortunately for me, I never do that stuff so I won't have to learn it

Phonon

^^

@JamalS hey man, what's with that link you shared with the quote?

JamalS

@Phonon It's from Misner, Thorne and Wheeler's text, Gravitation.

ACuriousMind

5:55 PM

@Phonon Sadly, gauge-theory isn't even among my top 10 tags here

Phonon

@ACuriousMind it just means you two have a very discrete relationship^^

@JamalS the book's been made free?

JamalS

@Phonon: No...

Phonon

@JamalS sigh... thought that was what the quote was implying.....

ACuriousMind

@Phonon You mean discreet, gauge groups are continuous, and not discrete structures ;)

Phonon

@ACuriousMind that was the pun... :D

ACuriousMind

5:59 PM

Oh... :D

JamalS

@Phonon: Of course, the book is kind of free... if you catch my drift :)

Kyle Kanos

Illegally free

Which means you are robbing Misner, Thorne, and Wheeler

JamalS

@KyleKanos: I genuinely feel bad, but at the same time I say to myself, "they'd want you to have it!"

Phonon

:DD

Kyle Kanos

If I wanted the book, I'd probably go get it 2nd hand (i.e., used) because it'd be cheaper.

Phonon

6:01 PM

@ACuriousMind not many people ask much about that stuff anyway, it'll never qualify among your top 10 cause there'd never be enough questions posted for it.

Kyle Kanos

But 2nd-hand sales do not give any proceeds to the authors

JamalS

If I was going to buy the book, then I'd get a new copy

I admit there's nothing like the feeling and smell of a brand new book!

Kyle Kanos

slight context:

2 hours ago, by Kyle Kanos

@Jim me = grad student + husband + father of 4 $\equiv$ cheap

ACuriousMind

@Phonon Yes, I know, it's about one question in three days or so, and I'm not interested in every question that's tagged with that (nor can I answer all)

Phonon

@JamalS so you recommend this book or?

JamalS

6:05 PM

@KyleKanos: Well, I can't chastise you for having kids :)

@Phonon: Yes, and no. It's truly a compendium of general relativity, but there are many other great books, and it depends what you want to study.

@Phonon: If differential geometry is your focus, Danu recommended an excellent book which I'm using for cohomology right now which is Zee's Introduction to Smooth Manifolds.

Phonon

@JamalS alrighty man, really good to know! Any other introductory GR book you yourself would recommend?

JamalS

@Phonon: Some people don't like Weinberg, but I think his Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity is quite good, as well as Wald's General Relativity.

Kyle Kanos

@Phonon I personally like Joel Franklin's Advanced Mechanics and General Relativity

JamalS

@Phonon: MTW's Gravitation is certainly worth the investment; as I said it's very comprehenive, and you will certainly be using it as a reference in the future.

Phonon

@JamalS awesome thanks

@KyleKanos thank you, will definitely look into it

JamalS

6:18 PM

@Phonon: Once you've learnt GR, and at least introductory QFT, I recommend Mannheim's Brane-Localised Gravity which deals with branes and fields in the context of general relativity. It's a very good text, pedagogic, and Mannheim goes into a lot of detail in his calculations. The only downfall of the text may be the explicitness of the calculations, in that he sometimes gets bogged down in small details.

@KyleKanos: How are you getting on with GR?

Kyle Kanos

@JamalS I haven't touched it in a couple days (busy between sick kids & doing my own research)

Phonon

@JamalS this is really useful insight :D,

User Anonymous

2 days. 34 views. 3 upvotes. 2 favorites. NO ANSWERS. :(

Phonon

@UserAnonymous which post you're talking about?

Kyle Kanos

3

Q: Reference on stages of heavy ion collisions in particle physics

Is there any reference (book/review article etc.) where the physics of heavy ion collisions is overviewed? To be absolutely clear about things, I am looking for a introductory review which covers the physics aspects of the progression through the following stages stable nuclei fireball quark-...

particle-physics resource-recommendations heavy-ion quark-gluon-plasma hadronization

That one

Beyond my head

User Anonymous

6:26 PM

Thanks @KyleKanos.

Yep. That one.

JamalS

@KyleKanos: I hope they get better soon; I have a terrible cold too.

Kyle Kanos

Note though that there are some questions that are years old and unanswered

ACuriousMind

@UserAnonymous Psh, two days is nothing. One of my questions went three weeks without answer until I answered it myself

User Anonymous

Thank you for the encouragement @KyleKanos and @ACuriousMind .

JamalS

@ACuriousMind: The Wilson line one?

User Anonymous

6:30 PM

@DavidZ - I get your point about the lack of reviews, but how do novices enter the field then?

OK Your field - pA, not AA :P

David Z

Through a PhD program... it's basically an apprenticeship, you know

ACuriousMind

@JamalS Yes, the "redux" one. Probably should've gone to the mathematicians in the first place, it turned out to be all pure group/representation theory

But so I had the pleasure of sitting in the tram one day, staring out of the window, and suddenly the answer hit me :D

Phonon

@JamalS unfortunately I'm reading bout this stuff just as self study as they have nothing to do with my own studies. So I usually just go with the ones (books) that are most readable and self-contained ones,

User Anonymous

@DavidZ And none of the masters feel the need to do some community service by writing things up for guys not in their group? Sad :(

JamalS

@ACuriousMind: Those are the best moments in physics :)

David Z

6:34 PM

@UserAnonymous nope. Because you don't get tenure for writing review articles, you don't get grants for review articles, you don't attract new students to your group or new collaborators or so on. It does nothing to improve the metrics by which physicists measure their success.

User Anonymous

Haha. true :)

David Z

In fact it can work against it. Writing educational materials means you weren't spending that time on research, which other people in the field interpret as you being unproductive.

ACuriousMind

@DavidZ If people think producing good educational material is unproductive, they definitely have the wrong standards

6

User Anonymous

Yes. Something is horribly wrong somewhere.

ACuriousMind

@UserAnonymous That something is called "humans" :P

JamalS

6:38 PM

@ACuriousMind: Agreed, writing a textbook, for example, is certainly not a waste of time.

David Z

That being said, some people do it anyway, despite the academic consequences.

JamalS

@DavidZ: Based on your experience, how do your colleagues (professors) feel about lecturing? Do they consider that unproductive as well?

bobby

@ACuriousMind wow category theory is cool with that homotopy analogy, makes sense of everything! :D

User Anonymous

@ACuriousMind Hehe. True again.

To the anonymous upvoter who just upvoted my question (and I suspect, currently belongs here in this chat room) - thanks, but please say you are in the middle of composing an answer...

:P

David Z

@JamalS Professors aren't really my colleagues, more my superiors, but anyway: I think most of them see it as interesting and (more or less) gratifying, but at the same time they recognize that it's not going to help their career aspirations. Some people just don't want to teach though.

User Anonymous

6:41 PM

Two more upvotes. I wish I could get answers that easily.

bobby

That should be Lee - Introduction to Smooth Manifolds above :) Some great books mentioned! MTW's geometric interpretation of differential forms is poetry!

David Z

@UserAnonymous you know, there is a certain point at which complaining in chat will start to cost you upvotes ;-)

User Anonymous

Oops. Plus, if it is not fetching answers, I better get outta here.

Thank you everyone.

David Z

@UserAnonymous It occurred to me to search "heavy ion collision textbook" on Google. Some good results pop up. I haven't read any of them, so I don't know if they address the specific details you want (and let me reiterate, it is an awfully specific question), but I bet some of them do, to some extent.

amazon.com/Ultrarelativistic-Heavy-Ion-Collisions-Ramona-Vogt/…

I know the author by reputation, she is certainly well qualified to write about this stuff, but that's all I can say...

JamalS

@bobby In what way?

bobby

6:45 PM

What?

User Anonymous

@DavidZ - No, I have seen Vogt (the book I mean). The problem is - it is hardly an authoritative account of everything put together.

JamalS

@bobby: MTW's description of forms as poetry

User Anonymous

So, there are lots of individual pieces with works of many people, but putting all these pieces together is a difficult job for beginners.

JamalS

@UserAnonymous: So you want an MTW of heavy ion collisions? :)

User Anonymous

Haha. Yes. If there is one.

bobby

6:48 PM

Well look at all the pictures in the book :) Here's another description of the material em.groups.et.byu.net/pdfs/publications/formsj.pdf

David Z

@UserAnonymous ah, well if you're looking for an authoritative account, like a comprehensive reference, you're probably not going to find one, not for many years. It's a rapidly evolving field (no pun intended) and everyone involved knows that anything they write will probably be out of date three years later.

User Anonymous

At least I am not able to build a consistent complete picture.

@DavidZ Yes. But the Physics part of the collision -> fireball -> hadronization upon cooling should stay invariant, no?

It is only that the details get enhanced every few yrs or so.

JamalS

@bobby: I see what you mean; Burke's text on diff geo does something similar.

David Z

@UserAnonymous No, there's lots of room for major changes in our understanding of these processes.

bobby

Yeah Burke is very nice

David Z

6:53 PM

For instance the "ridge" in pA collisions was only discovered 2 years ago, and that has significant implications for the models that can be used to explain long-range correlations in AA as well.

User Anonymous

@DavidZ - What I mean is, like you have some model for hadronization which may be like a spherical cow and has to be progressively made more realistic. Sure, we can do that with time. But why do they hadronize at all, and a zeroth order description would still hold?

David Z

Beyond the fact that quarks and gluons have to hadronize to produce color singlets (i.e. just basic QCD), I don't think much is known about that process. Sure, people can quantitatively characterize the probabilities, but the underlying physical mechanism is still rather mysterious.

ACuriousMind

7:10 PM

@UserAnonymous What do you mean by a model for hadronization? We know that color-charged things are confined (well, we hope they are, it's actually quite difficult to show confinement for the full QCD of which we believe it describes the real world), and so the color-charged particles must form hadrons (or glueballs...) in the confined phase

User Anonymous

7:22 PM

@ACuriousMind - That is true, but I would call that the driving motivation or something like that. With that known, people have made models in hope of describing how this manifests in practice. Like, what would the relative populations of various hadron species come out to be if you start from a given set of initial conditions etc. And how do the fireball parameters like temperature etc. make any meaningful difference.

Or as @DavidZ puts it, people can quantitatively characterize the probablities.

ACuriousMind

8:38 PM

Is it me, or is the $\LaTeX$ rendering in the review queue horribly buggy? It makes reviewing TeX-heavy edits a pain

It's not me, but I cannot find an actual feature request that asks for this to be fixed...

user54412

Hey I complained about that a long time ago

ACuriousMind

@ChrisWhite Hm. Oh well, then it seems we have to live with this

Though the fix "Make the "rendered output" diff engine treat MathJax blocks as atomic units." actually sounds really easy to implement

hwlau

9:13 PM

40

Q: How do I draw a pair of buttocks?

I'm trying to develop a function which 3D plot would have a buttocks like shape. Several days of searching the web and a dozen my of own attempts to solve the issue have brought nothing but two pitiful formulas below. They have some resemblance to the shape I want, though not quie. Could you h...

plotting

ACuriousMind

...

@hwlau While hilarious, that's not a good question, is it? :D

Probably there are only half-assed solutions available yet. — Yves Klett 11 hours ago

As expected, the comments are golden

hwlau

@ACuriousMind But the answers are amazing :)

tpg2114

@KyleKanos Yeah -- if F is the vector of conservative fluxes, I need the \partial F / \partial U matrix, where U is the vector of primitive variables. I've been trying to get the eigenvalues {u,u,u,u+c,u-c} and the eigenvectors but I can't seem to figure out what I did wrong

Kyle Kanos

Hmm, I guess I've just seen the Eulerian ones in papers

tpg2114

I've got the eigenvalues finally, but the eigenvectors aren't right. I was hoping to find the \partial F/\partial U written out somewhere so I could at least rule out mistakes there

Kyle Kanos

9:25 PM

e.g., this one (@ChrisWhite might like this reference)

tpg2114

That's fine, I only need the inviscid flux ones at the moment

Kyle Kanos

inviscid = Eulerian, yeah?

I always forget the terms

tpg2114

Yeah

Well, assuming you mean Eulerian as Euler-equations

Kyle Kanos

Yes

That's what I meant

tpg2114

And not as "fixed reference frame" as opposed to Lagrangian formulations

But I assumed that's what you meant

Kyle Kanos

9:27 PM

Quite right

The information you want is in the Appendix of the arXiv link

Though, not the derivation

Now that I think about it

tpg2114

That's okay, if I have the left and right eigenvectors I can at least see if my problem is Mathematica being a PITA

Which it has been all along, so I wouldn't be surprised if that's the case now too

Kyle Kanos

Mathematica is a hunk of junk, so it could be that

tpg2114

Thanks -- I'll dig into this and see if I can find what I need

Kyle Kanos

Of course, it could be a PEBCAK problem too

tpg2114

It is... I'm so annoyed with it

Kyle Kanos

9:32 PM

:D

tpg2114

Could be also -- but when FullSimplify[] seems to think that u + 1/sqrt(1/c^2) is fully simplified even though I told it c>0, there's still something wrong :)

Kyle Kanos

Mathematica's outside my knowledge base. I try avoiding it (wxMaxima suffices quite nicely)

Anyways, I've gotta head out

See ya!

tpg2114

Thanks, see ya

Phonon

@ACuriousMind interesting discussion you had here, although kinda annoying that he kept returning to the same points and ignoring what you were saying bascically.

hwlau

@tpg2114 It does simplify

Where do you have the problem?

Phonon

9:39 PM

@ACuriousMind I wonder, what has made your view of QM so unbiased toward the wave picture? influence of a certain book/teacher?

ACuriousMind

@Phonon Probably the way I learned it: I was introduced to QM purely in the Hilbert space formulation, and wave functions were just $\psi(x) = \langle x \vert \psi \rangle$. From this, it is immediately clear that the wave picture is not fundamentally a good intuition to develop - it depends on the eigenspaces of $\hat x$ being one-dimensional for this to be well-defined, and as soon as you have things like spin, the idea of wavefunction just makes no sense anymore.

Furthermore, there's the same reason I also dislike thinking of quantum fields as "oscillators at every point" - there's nothing oscillating there

At least, nothing observable. I've generally grown to dislike all attempts to develop "intuition" when it comes to all things quantum

Phonon

@ACuriousMind aha, I kinda agree with you, it is generally so easy to be misled by coined intuitions, specially in QM...indeed the dirac notation e.g. would help resolving some of these interpretation issues that most have, but I think at the end of the day, one has to really free him/herself from all classical intuitions first, and only then try to grasp the new ideas underlying QM. And in view of this, I really envy your persistence in trying to convey the proper picture of things!

tpg2114

@hwlau FullSimplify gave me that as the simplest answer, rather than u+c like it should have

I'm not sure why I struggle to get Mathematica to do simple math operations but then people do things like this:

45

Q: How do I draw a pair of buttocks?

I'm trying to develop a function which 3D plot would have a buttocks like shape. Several days of searching the web and a dozen my of own attempts to solve the issue have brought nothing but two pitiful formulas below. They have some resemblance to the shape I want, though not quie. Could you h...

plotting

ACuriousMind

9:54 PM

@tpg2114 Did you notice that @hwlau has just posted this ~40 mins ago? It seems buttocks are really interesting to you people :D

tpg2114

@ACuriousMind Whoops, I did not notice that

Literally just before I said anything in here. Go figure...

I've been working too hard I guess

Phonon

10:37 PM

@ACuriousMind univ ok these days? not too overwhelmed with homeworks and projects?

ACuriousMind

@Phonon All is well, no overwhelming in sight except for that which I inflict upon myself by excessive procrastination ;)

@Phonon: How was your exam? 'Twas this week, right?

Phonon

@ACuriousMind thanks for asking, passed both, went pretty smoothly actually ;D

ACuriousMind

Nice!

Phonon

@ACuriousMind yeah I do that too ;D

@ACuriousMind now I get some time to finally read a bit whatever I want, till the final exams in january.

@ACuriousMind sometimes you just want some time to yourself to be able to read whatever you ve always meant to.... :( but then univ starts hitting you back with more boring and time consuming tasks....

hwlau

10:53 PM

@tpg2114 It worth to post twice :)

user54412

11:22 PM

@tpg2114 My understanding is expression simplification is undecidable -- no algorithm can ever work perfectly -- thanks to Richardson's theorem

user54412

(Note that I don't still get angry at mathematica for failing to do what I consider obvious)

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