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?
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...
@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.
@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
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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)
@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
@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
@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 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)
@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
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
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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.
@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
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
@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 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 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
@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
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
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...
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
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...
@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.
@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)
@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: 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.
@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: 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.
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-...
@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
@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,
@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.
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.
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...
@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.
That should be Lee - Introduction to Smooth Manifolds above :) Some great books mentioned! MTW's geometric interpretation of differential forms is poetry!
@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.
@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.
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.
@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?
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.
@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
@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.
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...
@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
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
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 :)
@ACuriousMind interesting discussion you had here, although kinda annoying that he kept returning to the same points and ignoring what you were saying bascically.
@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
@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!
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...
@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....