« first day (1610 days earlier)      last day (3323 days later) » 

0celo7
0celo7
12:01 AM
@ACuriousMind I thought that a quantum mechanical force is a curved fiber bundle
cf. your post on entanglement
 
ACuriousMind
ACuriousMind
@0celo7 Ehhhh...well, the gauge theories which we call "forces" live on these, but I've never seen a clear notion of "quantum mechanical force".
 
Sean
Sean
Am I just oblivious, or has Bobie just recently been suspended?
 
ACuriousMind
ACuriousMind
@Sean Drawing conclusions, are we? ;) Yes, I think it's quite obvious what happened there, but since we'll not get any confirmation on that, speculation is moot.
 
Sean
Sean
No I'm just wondering if it was recent or not
And it's not altogether obvious to me, unless he was a part of the sting yesterday
 
ACuriousMind
ACuriousMind
Oh, okay, then I haven't said anything.
But a helpful information is that the suspension periods are fixed as something like: One week, one month, or a year.
 
Sean
Sean
12:24 AM
So he was suspended Monday...
I feel like so much drama goes on behind the curtain. Haha
 
Danu
Danu
12:52 AM
I just tried to reread part of my bachelor's thesis and realized I have no idea what I was talking about anymore
Not sure how I feel about it
 
ACuriousMind
ACuriousMind
I'd find that a bit unsettling :P
I've only read it again and realized that I had no idea what I was talking about back then at some points ;)
 
abt
abt
1:16 AM
@ACuriousMind How do German universities compare to their US/UK counterparts? Particularly for foreign students.
 
ACuriousMind
ACuriousMind
@abt Since I've never been to another university thus far, I can't speak with certainty. It seems to me that we require far less (almost no) "general" courses from undergraduates than in the US, i.e. we study one and only one subject - no required history or English courses for someone who doesn't study history or English, for example.
For foreign students, it depends very much on the subject you study, I think.
In our physics programme, you can come and study without knowing a bit of German - all courses are held in English, but this is exceptional, and for most other subjects, you should have a firm grasp of German, I think
 
abt
abt
Though you still have to write the language requirement exam, at least for undergraduates.
 
ACuriousMind
ACuriousMind
Oh, yes, the undergraduate courses are in German, mostly
It's the graduate level where you'll have no problem at all
I also think it varies quite much from university to university - very little is standardized, and the lectures offered will vary quite much from location to location
 
abt
abt
@ACuriousMind Are you currently a postdoc?
 
ACuriousMind
ACuriousMind
@abt No, I'm in the first semester of my M.Sc.
 
abt
abt
1:30 AM
I apologize I didn't look at your age.
I didn't know that M.Sc in theoretical physics were common, usually it's just Ph.D.
 
ACuriousMind
ACuriousMind
Ah, that's a difference between the American and European system, I guess
We cannot acquire PhDs directly after our BScs - the route is: Bachelor->Master->PhD/Doctorate, no exceptions
And the Master and PhD programmes are entirely separate
 
abt
abt
And there is also habilitation in Germany, though I'm not sure if it's the same as a postdoc.
I know some people here with titles such as Dr. habil or Priv-doz.
 
ACuriousMind
ACuriousMind
No, habilitation is acquiring the "right to teach", or rather, the title of Professor. PhD/Dr. may teach, but they may not call themselves Professor, nor may they have a position as chair of a department or similar.
Priv-doz. are usually people from the industry or non-academic research institutes who teach without being actually employed by the university
I'm not sure what a Dr. habil. is, I've not heard that one
 
abt
abt
It is the title for having completed the habilitation, at least here it is.
I have seem prof dr. habil ____, or dr. habil ____.
 
ACuriousMind
ACuriousMind
Ah, the title for habilitation is Professor, so that's a simple Prof., usually
 
dmckee
dmckee
1:40 AM
@ACuriousMind I had a quantum mechanics class canceled by order of the Secret Service once, but I think you win.
3
 
ACuriousMind
ACuriousMind
It varies quite much from subject to subject how much importance is placed on these titles. I couldn't tell exactly you which of my physics lecturers is a Dr. and which a Prof. Dr., but in other subjects like law or economics, the lecturers will be sure to mention and insist on all the titles they possess.
@dmckee Ah, well, nothing too unusual, we find about one or two of these duds here a year (in one city alone). It's just the first time I was personally too close to one.
People dig up the past in these cases quite literally. And explosively.
 
abt
abt
They are not always properly diffused either, I've read news on unexploded ordinance in Germany that have shattered windows and infrastructure in the attempt of safely disposing them.
 
ACuriousMind
ACuriousMind
Sometimes they have to detonate them because they can't figure out a safe way to defuse them, but it usually goes well
@dmckee But now, of course, the question: Why did the Secret Service cancel your class?
(And don't say you'd have to kill me if you told me ;) )
 
dmckee
dmckee
@ACuriousMind The president (Clinton) was coming to speak on campus and one of the gabled windows in the attic had a view of the platform. So they evacuated the whole building from one hour before until two hours after.
The part we didn't understand is why they detailed six people to guard all the entrances instead of two people to guard the single attic access.
In any case, we all fell out to our favorite watering hole for microbrews.
 
abt
abt
2:08 AM
There are too many things to learn, I'm very lost.
 
dmckee
dmckee
2:25 AM
@abt It's easier to learn one or a few things at once than to try to swallow several disciplines. Of course, the latter course is more enticing because it feels like making "real" progress toward your goals while the former approach will feel like baby steps.
 
 
1 hour later…
0celo7
0celo7
3:38 AM
@ACuriousMind No one said it had to be one-sided ;)
 
 
9 hours later…
ACuriousMind
ACuriousMind
12:33 PM
@0celo7: Let me know what you think about the answer(s) to the question about string non-linearity. The first part of the current answer seems fishy to be - AFAIK, strings don't break, and that's not what non-linear means here.
 
FreeMind
FreeMind
1:23 PM
Hi
I have problem finding out what special terms/jargon mean in Quantum Field Theory. For instance, what is meant by state-dependent?
@ACuriousMind I am totally confused about the jargon used in QM. I think it's because of the combination of it with the probability theory. Some technical terms appear as I go further which makes the text intangible for me.
 
0celo7
0celo7
1:46 PM
@ACuriousMind I am unsatisfied with that answer as well.
@ACuriousMind I might like the second part though. It we have a strong excitation of the string, any additional excitations will have a harder time propagating.
(Assuming that's what he meant.)
 
FreeMind
FreeMind
@0celo7 Please tell me what is meant by state-dependency?
 
0celo7
0celo7
@FreeMind Can you give any context?
 
FreeMind
FreeMind
Variance is state-dependent. (Correct)
 
0celo7
0celo7
Ok, how are you defining variance? (Going Socratic here.)
 
ACuriousMind
ACuriousMind
@FreeMind It means that the variance of the quantity an operator measures is not a property of the operator, but of the state, since variance is $\langle A^2 \rangle - \langle A \rangle^2$, and the expectation values $\langle A \rangle$ are properties of the quantum state, since $\langle A \rangle = \langle \psi \rvert A \lvert \psi \rangle$.
Do we seriously not already have a question "What is mass?"
 
0celo7
0celo7
1:59 PM
@ACuriousMind D'aww
@ACuriousMind I answered it :P
I'm pretty desperate for rep
 
ACuriousMind
ACuriousMind
@0celo7 Yeah, sure, but I can't believe that question is not a duplicate
I can't find one, though
Although this is one of the more basic questions which actually can have interesting/elaborate answers
 
0celo7
0celo7
@ACuriousMind Nah, it's straightforward in classical physics.
 
FreeMind
FreeMind
@ACuriousMind What is mass? :)
 
ACuriousMind
ACuriousMind
@0celo7 Yes, but nothing about the question says "in classical physics". The question is boring if you restrict it to that, if you ask me :P
 
0celo7
0celo7
@FreeMind Combination of energy and Higgs.
@ACuriousMind Tags.
 
ACuriousMind
ACuriousMind
2:03 PM
@0celo7 Yeah. But then it's boring. I'd prefer to have one question *"What is mass?" with answers for all levels than having the possibility of multiple questions all asking about particular areas.
 
FreeMind
FreeMind
@ACuriousMind The problem is the further I go the whole QM goes into darkness instead of shining as classical physics.
 
0celo7
0celo7
@ACuriousMind Well is "energy + Higgs" right?
 
ACuriousMind
ACuriousMind
And I really can't believe that among all of the very basic questions this site has, "What is mass?" is not already among them.
 
0celo7
0celo7
@ACuriousMind Did you look?
 
ACuriousMind
ACuriousMind
@0celo7 Yes, and I didn't find any question like that. Only of the kind "What's the difference between inertial and gravitational mass?" or somesuch
 
0celo7
0celo7
2:05 PM
Damn, Ludacris' new album is on point.
@ACuriousMind none :P
 
ACuriousMind
ACuriousMind
@0celo7 It's not wrong, since these are the "reasons" for masses, but it kinda eludes the question what mass is
 
FreeMind
FreeMind
@ACuriousMind By the way was it sarcastic asking me you might have to ask me what the mass is? >> Do we seriously not already have a question "What is mass?"
 
0celo7
0celo7
@ACuriousMind Of course, that's the QFT reason. GR is like "lol idk".
@ACuriousMind Ok, I'll expand my answer.
Fermion masses are the Yukawa coupling constants, right?
And boson masses are the expectation value of the Higgs field?
 
ACuriousMind
ACuriousMind
@0celo7 Nah, even in QFT, you can also say something like "the first pole of the propagator", talk about the Kallen-Lehmann spectral representation and such things. One needs to explain why the factor of the "mass term" in the Lagrangian really shows up as the mass $p^2 = -m^2$.
At least, that's the interesting part of the question to me.
Mass generation by the Higgs mechanism is a slightly different question, in my view
 
0celo7
0celo7
Then there is other mass due to $E=mc^2$ such as nuclei masses.
@ACuriousMind Ugh, that dumb pole.
 
ACuriousMind
ACuriousMind
2:09 PM
@0celo7 Please don't use the concept of relativistic mass :P
There are good answers by Ben Crowell and others why one shouldn't talk about that
 
0celo7
0celo7
@ACuriousMind I'd love to hear why nuclei are so heavy in your mind then.
I didn't think I was using relativistic mass...
Just the energy of the quark-gluon soup.
 
FreeMind
FreeMind
The interesting question is why do we believe that Reductionism principle works? Is it a kind of faith or due to induction?
 
ACuriousMind
ACuriousMind
@0celo7 They are bound states, nothing to do with $E=mc^2$, to me, but with the fact that the mass spectral function isn't just a Dirac delta comb at the $n$-particle states
@FreeMind Induction is a kind of faith ;)
 
0celo7
0celo7
@ACuriousMind What do you make of "This path can be so curved that the wave ends up travelling through itself such that it displaces its own path into a closed path."
@ACuriousMind Hmm, should I ask a well-posed mass question?
 
FreeMind
FreeMind
@ACuriousMind I have bad memoirs in philosophy class about it with a old prick professor.
 
ACuriousMind
ACuriousMind
2:12 PM
@0celo7 Well, I can't tell you if you should, but I think it is one of the more interesting "What is X?" questions.
 
FreeMind
FreeMind
@0celo7 Have you read the whole GR?
:O
 
0celo7
0celo7
@ACuriousMind In some approaches of GR mass is loosely defined as "something that the gravitational field listens to" and more concretely as asymptotic curvature.
 
ACuriousMind
ACuriousMind
@0celo7 See, that I know nothing about - I only know the classical and the QFT view.
And @FreeMind, I really wasn't being sarcastic, I think "What is mass?" is a question so basic that it should have already been asked, and also a question that should have generated varied, but correct and non-trivial answers.
 
0celo7
0celo7
@ACuriousMind Well my question would be something like: what is mass in classical physics, quantum physics, general relativity and string theory. Furthermore, please compare and contrast these definitions.
Is that too broad?
 
FreeMind
FreeMind
@ACuriousMind I thought you were giggling at my question :D
Anyway, whenever people laugh at my questions, I get more self-confidence, I just think I am on the right path :)
 
ACuriousMind
ACuriousMind
2:17 PM
@0celo7 I'd rather ask more specifically whether the different views of mass become the same in the classical limit, I think. Hm, one should probably read the Wiki articles before hand, I see that at least for classics and GR, this question seems to be adequately answered there
I do miss the proper QFT treatment, though, especially with renormalization.
 
0celo7
0celo7
@ACuriousMind So why is the pole of the propagator at the mass squared? Is it because the pole is at the physical particle, i.e. the mass shell?
 
FreeMind
FreeMind
@ACuriousMind Had you been exposed to Hamiltonian Mechanics before starting QM? I know nothing about canonical transformations and stuff :(
 
ACuriousMind
ACuriousMind
@FreeMind Yes, my second-semester classical mechanics course was Lagrangian and Hamiltonian mechanics.
 
0celo7
0celo7
@FreeMind If you have access to Shankar, he explains all of that with an eye on quantum physics.
 
ACuriousMind
ACuriousMind
@0celo7 I think it's rather because the first pole (suggestively written $m^2$) is by inspection at the point where $p^2 = -m^2$.
 
0celo7
0celo7
2:23 PM
@ACuriousMind What do you mean by that?
 
ACuriousMind
ACuriousMind
@0celo7 Well, QFT is SR-compatible, so it's natural to say that the square of the momentum of a state is its mass, isn't it?
 
FreeMind
FreeMind
@0celo7 I read electronic books, so tiresome. I look at it. I think it has been a month that I have been reading the introductory parts of different books. Burned out.
 
0celo7
0celo7
@ACuriousMind Well yes. Oh I see what you mean.
 
ACuriousMind
ACuriousMind
The Källen-Lehmann spectral representation tells you that the first pole occurs at the one-particle states, so it's also natural to identify the first pole as the "mass of the particle". I think, but I'd like for someone to confirm that.
 
0celo7
0celo7
 
ACuriousMind
ACuriousMind
2:26 PM
@0celo7 Ah, no, but I forgot to answer it :D
 
0celo7
0celo7
I don't really understand his picture.
It's an open string, not a closed one.
 
ACuriousMind
ACuriousMind
No, I don't know what he wants to tell us with that, really.
I should also mention that I've engaged this guy in several comments already asking to clarify/correct some of his other answers, and he always reacted rather unfavourably.
So I'd prefer if you post the comment asking for clarification ;)
 
FreeMind
FreeMind
@ACuriousMind Put it as off-topic, haha :D
 
ACuriousMind
ACuriousMind
I'm also rather irritated that your question now has double the views, but not a single additional upvote.
While the other question I bountied gained four votes already. It seems people really don't care for string theory questions when they're not a bunch of incomprehensible formulae :P
 
0celo7
0celo7
Commented
haha
 
FreeMind
FreeMind
2:31 PM
@ACuriousMind You are right about the incomprehensible part. I asked an integral question in MSE, upvote was exploded !
 
0celo7
0celo7
@ACuriousMind Reading about the spectral representation now.
 
FreeMind
FreeMind
@0celo7 Why's that? Although I have the same opinion.
 
0celo7
0celo7
Little kids getting 20 upvotes for a calculus homework and my Sobolev question got 2.
Nothing against you of course.
It's just a homework machine.
 
FreeMind
FreeMind
@0celo7 Haha, funny part -> Little kids :D
 
ACuriousMind
ACuriousMind
@0celo7 All I know about it is from my QFT lecture: thphys.uni-heidelberg.de/~weigand/QFT1-13-14/SkriptQFT1.pdf
 
0celo7
0celo7
2:33 PM
@FreeMind Yeah, I'm 17 :D
 
FreeMind
FreeMind
@0celo7 Seriously? :D
 
0celo7
0celo7
@FreeMind Yah
 
FreeMind
FreeMind
@0celo7 Cool, don't tell anybody I'm 8 :D
 
0celo7
0celo7
@ACuriousMind Ok, I'll give that a look after Srednicki.
 
dmckee
dmckee
@0celo7 The "knot" on the left is topologically equivalent to the simple loop on the right; which is to say it is not a knot at all. Why topological equivalence to a plain loop should be the discriminator for open or closedness I have no idea
 
0celo7
0celo7
2:35 PM
@dmckee I know that, but I don't see what topology has to do with this.
 
ACuriousMind
ACuriousMind
@dmckee The answer links that picture under "wave travelling through itself". It's rather unclear what the user wants to tell us, at least to me
 
FreeMind
FreeMind
@ACuriousMind Since you have passed so many cool courses, please give me a lecture which explains the tensors in the most applicable way with solved examples.
 
0celo7
0celo7
@FreeMind He can't solve any problems without using indices, and that's just not gonna happen :P
 
FreeMind
FreeMind
@0celo7 Mother merry sweety :D
 
ACuriousMind
ACuriousMind
@FreeMind A tensor is a multilinear map. What kind of expanded explanation do you seek? (I got nothing in the way of "calculate this tensor", or something like that, if you seek that)
 
0celo7
0celo7
2:39 PM
@FreeMind You want calculation problems? Calculate the Riemann tensors for all of en.wikipedia.org/wiki/…
Good luck.
 
FreeMind
FreeMind
@ACuriousMind I got headache jumping from one text to another filling the empty spaces of each text. I can't learn without some examples done by myself in addition to the book solutions.
 
0celo7
0celo7
@FreeMind What kind of examples are you looking for?
 
FreeMind
FreeMind
@0celo7 Solved examples, something applied and explained so I know what I am doing when I am tackling a problem without having solution. By just reading bunch of text, I cannot trust what I am doing. Concisely, I don't call myself driver by reading a book without driving a car.
 
ACuriousMind
ACuriousMind
@FreeMind Solved examples for what? Tensors are everywhere, they appear everywhere, but rarely are the main topic.
 
FreeMind
FreeMind
@ACuriousMind I haven't gone further than tensors so I don't know where they are !
 
0celo7
0celo7
2:48 PM
@ACuriousMind In those notes, in Eq. (2.31), I don't see how the second term doesn't contribute a pole.
Or does it, but at the bound state?
Is that what they mean by "first pole"?
 
ACuriousMind
ACuriousMind
The latter
 
0celo7
0celo7
I see.
 
FreeMind
FreeMind
C y'all
Before going, let me ask a specific question
What are $A^{'ij}$ and $A^{kl}$ in mathworld.wolfram.com/images/equations/Tensor/…
@0celo7 ^
 
0celo7
0celo7
@FreeMind The one with the prime is in a transformed coordinate system.
This is the coordinate transformation formula, it tell you how the components are related in the two coordinate systems.
 
FreeMind
FreeMind
@0celo7 What is $A$ itself? Is it a matrix or vector component? I need an example for it!
@0celo7 Can you give me an example or link exactly explaining this?
 
0celo7
0celo7
2:54 PM
@ACuriousMind It's a tensor component.
Inverse metric tensor.
Energy-momentum tensor
 
ACuriousMind
ACuriousMind
@0celo7 I know that ;)
 
FreeMind
FreeMind
Okay.
 
0celo7
0celo7
Oh
Crap
@ACuriousMind Well just letting you know.
Just checking :P
 
FreeMind
FreeMind
@ACuriousMind Tell me what books have you covered in order since you started physics in university.
 
0celo7
0celo7
@FreeMind He doesn't read books.
 
ACuriousMind
ACuriousMind
3:08 PM
@FreeMind Ehhh...none. The first technical book I've ever read cover to cover was Quantization of gauge systems very recently
 
0celo7
0celo7
@ACuriousMind I don't think you've ever said what music you like.
 
FreeMind
FreeMind
@0celo7 Are you @ACuriousMind ? Or do you read his mind? :D
Who hates rap? :)
 
0celo7
0celo7
@FreeMind I spend a lot of time with him.
@FreeMind Not me.
 
FreeMind
FreeMind
@0celo7 Do you know him in real world?
 
0celo7
0celo7
@FreeMind Nah, different continents.
 
ACuriousMind
ACuriousMind
3:11 PM
@0celo7 Probably because I don't feel strongly about music at all :P I tolerate almost everything that's not German folk songs; I'd think I like metal the most, but that's probably just because most of my friends listen to it.
 
FreeMind
FreeMind
@ACuriousMind Heavy metal after 3 AM, solving QM problems. Lying on the ground watching the roof :)
 
0celo7
0celo7
I don't like how expensive rap albums are.
Kendrick Lamar wants $15 for To Pimp a Butterfly.
 
ACuriousMind
ACuriousMind
@FreeMind Music distracts me, I can't work properly when it's around. And lying on the ground watching the roof usually doesn't happen in the context of solving QM problems to me :D
 
FreeMind
FreeMind
@ACuriousMind You are right, but this happens to me after I have finished the problems not during solving them :)
 
0celo7
0celo7
@ACuriousMind Are we to assume rap isn't your thing?
 
ACuriousMind
ACuriousMind
3:18 PM
@0celo7 Yeah, not my thing. I don't hate it, but I don't like it, either.
 
0celo7
0celo7
@ACuriousMind Huh, now that I actually understand some QFT Weinberg makes sense.
(Reading about the spectral representation.)
 
ACuriousMind
ACuriousMind
@0celo7 Heh, yes, Weinberg is not an introductory text at all. I imagine it makes indeed little sense to someone not already knowing QFT
 
0celo7
0celo7
@ACuriousMind Do you actually have his book?
 
Stan Shunpike
Stan Shunpike
@ACuriousMind that's how I felt about A Zee's text
 
ACuriousMind
ACuriousMind
@0celo7 Yes, the first Weinberg is one of the few books I actually possess. I never read fully through it, though.
 
0celo7
0celo7
3:36 PM
@ACuriousMind Ok, I'm confused by something he says on page 460. He says that the exact propagator cannot vanish at infinity faster than the bare propagator. This is clear to me from (10.7.16). I don't understand why he says that including higher derivative terms in the Lagrangian violates the positivity postulate of QM.
 
ACuriousMind
ACuriousMind
@0celo7 I'm not currently in Heidelberg, so I don't have the book with me, but I think this should answer your question.
 
0celo7
0celo7
@ACuriousMind I know this argument, I'm interesting in Weinberg's specific argument using the spectral representation.
 
ACuriousMind
ACuriousMind
Ah, then I can only help you next week, when I can look into the book.
 
0celo7
0celo7
K
@ACuriousMind Should I try to answer physics.stackexchange.com/questions/173790/… or leave for the VTC crowd?
 
ACuriousMind
ACuriousMind
@0celo7 If you agree it is a duplicate, then answer the original question (if your answer adds something over those already present), not the duplicate. If you don't agree it is a duplicate then answer it.
 
0celo7
0celo7
3:46 PM
@ACuriousMind There's a new comment on the string question.
I don't understand what he's talking about tbh.
 
ACuriousMind
ACuriousMind
@0celo7 His comment doesn't relate to your question at all, as far as I can tell.
 
0celo7
0celo7
Yeah I don't get what pair production has to do with nonlinear dynamics.
 
ACuriousMind
ACuriousMind
If t'Hooft was still active here, we could hope for an answer from him^^
 
0celo7
0celo7
@ACuriousMind So there's something I never really understood in Weinberg. (Pe prepared for a photon propagator question.)
@ACuriousMind I'm considering e-mailing him.
 
ACuriousMind
ACuriousMind
@0celo7 Uh, renormalization is not my strong suit, but ask away
 
0celo7
0celo7
4:00 PM
So he writes $$\Delta'=\Delta +\Delta\Pi \Delta +\Delta \Pi\Delta\Pi\Delta+\cdots$$
Do you know what the symbols mean?
 
ACuriousMind
ACuriousMind
I know $\Pi$ as the photon propagator, but I'm not sure what $\Delta$ is
 
0celo7
0celo7
$\Delta$ is the bare propagator. $\Pi$ is the one-particle-irreducible correction.
 
ACuriousMind
ACuriousMind
It looks like the "formula version" of a sum over all loop orders of a propagator already, though
Ah, yes. I see what diagrams it's meant to be
 
0celo7
0celo7
(This is actually scalar, because I don't understand that either.)
He says "sum the geometric series".
I just don't see how this is a geometric series.
 
ACuriousMind
ACuriousMind
@0celo7 Do pages 68-69 of my (Weigand's) QFT notes explain it better?
Because I'd essentially just repeat what's there, I think :D
 
0celo7
0celo7
4:04 PM
I'll tell you in a bit
Ahhhh
@ACuriousMind Yup.
@ACuriousMind Oh, misread :/
@ACuriousMind Anyway, that was the first of two questions. He says that since $\Pi_{\mu\nu}(q)$ receives contributions only from one-photon-irreducible graphs, it is expected not to have any pole at $q^2=0$.
This is not immediately obvious to me.
 
ACuriousMind
ACuriousMind
@0celo7 Parroting page 147 of the notes: "because this would require a single particle massless intermediate state (whose propagator vanishes at zero momentum), but no such intermediate states occur for the 1 PI diagram relevant for the photon propagator"
 
0celo7
0celo7
Ah, reading that section right now
 
ACuriousMind
ACuriousMind
You simply cannot draw a 1PI diagram with photon legs that is massless
 
0celo7
0celo7
Oh, because there are internal massive fermion states?
 
ACuriousMind
ACuriousMind
Yep. The argument fails when you have massless fermions in the theory, I think.
 
0celo7
0celo7
4:18 PM
I recall handwaving that back when I read this section for the first time.
Good to know I was right.
Ok, you think I should e-mail t'Hooft?
That answer is garbage.
 
ACuriousMind
ACuriousMind
@0celo7 You want to post his answer and cash the bounty yourself :D
 
0celo7
0celo7
...maybe
 
ACuriousMind
ACuriousMind
But I don't see why not. They're his notes, politely asking can't hurt
 
0celo7
0celo7
@ACuriousMind Do I tell him I'm in high school?
I'm not sure if that is relevant.
 
ACuriousMind
ACuriousMind
@0celo7 Do you want a high-school level answer? :P
 
0celo7
0celo7
4:23 PM
@ACuriousMind I guess not.
Hmm, is the subject "A question about your string theory lecture notes"? I've never written a professor before.
 
ACuriousMind
ACuriousMind
I'm not sure if who you are is at all relevant. He would probably be more likely to answer if you could say that you're a physics student, but since you aren't, I don't think it's relevant
 
0celo7
0celo7
@ACuriousMind Ok, this is spooky physics.stackexchange.com/questions/173801/…
Just as we were talking about it.
 
ACuriousMind
ACuriousMind
@0celo7 To echo the people on Academia: "They're people."
(And I think the subject line is okay)
@0celo7 Hah. I've got the feeling it is a duplicate, but if not, you can answer it ;)
 
0celo7
0celo7
@ACuriousMind Guaranteed it's a dupe, and I'm busy with this e-mail now.
Huh, I don't see a duplicate.
 
ACuriousMind
ACuriousMind
Suprisingly, it's at least not an exact duplicate I can find.
I've linked a similar question to OP, though
 
0celo7
0celo7
4:31 PM
@ACuriousMind Argh, your comment is exactly what I would have written.
@ACuriousMind Do you say "Dear" to a prof?
Or like "Yo Gerard"
 
ACuriousMind
ACuriousMind
@0celo7 Is there a different formal address in English?
I'd say "Dear Prof. t'Hooft" and double check the spelling of his name :D
See, it's 't Hooft, good thing I checked
 
0celo7
0celo7
@ACuriousMind haha it's 't Hooft
Frikken Dutch :P
 
ACuriousMind
ACuriousMind
Shh, don't let Danu hear that :D
 
0celo7
0celo7
In America, having an apostrophe in your name is a good way to get a resume thrown out.
 
ACuriousMind
ACuriousMind
@0celo7 Huh?
Why would someone do that?
 
0celo7
0celo7
4:35 PM
@ACuriousMind Apostrophes are found in ghetto names. I guess a German wouldn't know about that.
@ACuriousMind La'Tanya, La'Quishria, etc.
 
ACuriousMind
ACuriousMind
I have no idea what a ghetto name even is. And I'm not sure I want to know.
 
0celo7
0celo7
Qua'Lifriaqui'Sha'Niquia
La'Taniana'Bo'Vanashrianiqualiquanice
Those are probably fake, but the other two are real.
@ACuriousMind Inner city black culture is quite strange and self-destructive. Crazy names are a part.
 
ACuriousMind
ACuriousMind
@0celo7 lol
 
0celo7
0celo7
Oh god, I'm gonna screw this up...
@ACuriousMind "Dear Professor 't Hooft --

During my endeavor to study physics I had the privilege of reading your pdf introduction to string theory (http://www.staff.science.uu.nl/~hooft101/lectures/stringnotes.pdf). " Too ass-kissey?
This is not the whole thing obviously.
 
ACuriousMind
ACuriousMind
I am not 't Hooft, but my reaction to "endeavor" and "I had the privilege" would be: "...seriously?" :P
 
0celo7
0celo7
4:44 PM
Lol
@ACuriousMind Fine. "While studying string theory I supplemented my textbook with your pdf introduction to string theory (staff.science.uu.nl/~hooft101/lectures/stringnotes.pdf). Unfortunately, I did not understand a comment you made on page 8. "
oh that should be "some comments"
 
ACuriousMind
ACuriousMind
@0celo7 If you then finish off with something like "I'd be very grateful if you could explain why/how...", I'd say it's good.
 
0celo7
0celo7
Yup
 
ACuriousMind
ACuriousMind
 
0celo7
0celo7
@ACuriousMind Too true.
@ACuriousMind Actually, I have emailed professors before.
 
ACuriousMind
ACuriousMind
@0celo7 Liar! You said you hadn't done it
 
0celo7
0celo7
4:54 PM
I emailed one guy about taking a differential equations class, and he said he didn't need the average GPA of the class being lowered any further...
And another was about a scholarship, which I did get.
 
ACuriousMind
ACuriousMind
@0celo7 Ouch. Some people just are jerks. Even if I thought that, I'd not say that unless the guy emailing me had been rude.
(which I assume you weren't :P)
 
0celo7
0celo7
No, but I looked at the grades...one passing.
He posted them online.
Idk why he would make that public haha
 
Gowtham
Gowtham
that was lightning fast in and out
 
0celo7
0celo7
 
ACuriousMind
ACuriousMind
@0celo7 Looks fine to me, except that I don't know what the -- instead of a comma is doing after the address.
 
0celo7
0celo7
4:59 PM
@ACuriousMind My military parents drilled that into me.
Fine, made it a comma.
 
Gowtham
Gowtham
wait , military uses this -- after names ?
 
0celo7
0celo7
woosh
Sent.
@Gowtham Apparently that's how the military addresses e-mails.
 
Gowtham
Gowtham
@0celo7 k
 
ACuriousMind
ACuriousMind
@0celo7 Or your parents have played an elaborate prank :D
 
0celo7
0celo7
@ACuriousMind Less than an hour. I'm better than the grad students :D
 
ACuriousMind
ACuriousMind
5:04 PM
@0celo7: Wrong link
I don't think you wanted to link the mail under "lecture notes", did you?
 
0celo7
0celo7
Lol, nice save.
@ACuriousMind Huh, I'm higher than you in the GR list.
 
ACuriousMind
ACuriousMind
@0celo7 Which "GR list"?
But I see you have answered more GR questions than I have, if that's what you mean
 
0celo7
0celo7
@ACuriousMind Yes.
Also asked a lot more.
O(1/0) more.
 
ACuriousMind
ACuriousMind
5:20 PM
@0celo7 Why on earth would I ask a GR question? :D
 
 
2 hours later…
abt
abt
7:43 PM
To answer it yourself.
@0celo7 Once more, I should not study analysis past the level I have now? I'm concerned about this because most physics students study real/complex analysis in mathematics courses in their undergrad. Therefore would I not be at a disadvantage?
 
0celo7
0celo7
@abt "Most" is incorrect. @ACuriousMind, this chat's resident mathematical badass, hardly knows any rigorous analysis.
@abt You certainly won't need it for cosmology.
 
Danu
Danu
@0celo7 It's just exactly like English: Het hoofd = the head. Used to be het hooft (older spelling). Lazy people -> 't hooft
In a name you obviously capitalize it
 
0celo7
0celo7
@abt Note that I am talking about real analysis here. You will need some complex analysis of course.
I think @Danu was looking into complex analysis texts recently.
 
Danu
Danu
One slightly interesting issue is the question of capitalization of the article
Usually when one does not write the first name, the article must be capitalized
...but in the case of 't coming from het this does not happy for some reason
 
innisfree
innisfree
If you start a sentence with 't Hooft, do you capitalize the "t"? 'T Hooft is a tricky name, for example
 
Danu
Danu
7:57 PM
No, never capitalize it
 
innisfree
innisfree
ah our comments crossed! I see
hmm do the Belgians always capitalize? e.g. Jean-Claude Van Damme?!
but e.g. Robin van Persie isn't with a capital.
 
abt
abt
@Danu is ahflors good enough?
@0celo7 I'm trying to find some good lecture note resources.www0.maths.ox.ac.uk/courses/material
Have you tried those? There is also a similar page at Cambridge though with less courses.
 
0celo7
0celo7
@abt Nope.
 
abt
abt
Can you perhaps link some to me?
 
0celo7
0celo7
@abt I think you're ready for Shankar. See if it makes sense, and if it doesn't we can work from there.
If you really want to study rigorous analysis, I can't help you.
 
abt
abt
8:12 PM
I don't want to really, I had it planned though.
 
0celo7
0celo7
@abt Unless you want to study rigorous QM or QFT, you won't need it.
 
abt
abt
Well I do, I suppose.
Since obviously I can start learning rigorous analysis right now, so I thought I may as well get it over with.
 
0celo7
0celo7
@abt You can't study rigorous QM, QFT, advanced GR, string theory and cosmology.
 
abt
abt
Why not? I mean I understand what you are trying to say but I have a low of time to do it.
 
0celo7
0celo7
It's not like you can down a book in a week and move on. Some of these grad students have been studying one thing for years!
 
abt
abt
8:16 PM
Fair enough.
Do you have anything for statistical mechanics?
 
0celo7
0celo7
@abt It's best to focus on one thing.
Although, everyone should know a little stat mech.
Check the link to Tong's notes.
He has two stat mech courses.
 
abt
abt
Yeah I have those, and MIT uploaded 2 grad statistical mechanics courses with lectures recently.
 
0celo7
0celo7
You probably don't need that much. Ask one of the cosmologists what they used if you want to focus on cosmology.
 
abt
abt
@Jimnosperm By the way, how much should I know before I can begin research?
 
0celo7
0celo7
Ask a researcher that.
 
Danu
Danu
8:22 PM
@abt I've heard it's decent.
It really depends on what you want to do
 
alarge
alarge
@abt Depends. Read articles, come up with a new problem, solve it, and publish. Coming up with the problem is usually the difficult part which is why normally you join a group: more senior members can give you a problem (or a partial one) to work with.
3
 
innisfree
innisfree
Yes, I think that's true. As a student, you might work on tough problems, but the problems are in principle solvable, usually with your notes or textbook. A difficulty in research is finding a tractable problem that might bear fruit.
 
0celo7
0celo7
8:46 PM
@ACuriousMind I don't get the distinction between a product bundle $E\times E'$ and its Whitney sum bundle $E\oplus E'$. Aren't both just bundles with typical fiber $F\oplus F'$?
 
 
2 hours later…
Astrum
Astrum
10:18 PM
@tpg2114 thanks for all the information. so if you don't mind me asking... what kind of bomb creates a shock wave of 40 GPa O.o

The questions still remains about how long these shock waves really go on for, the Rankine–Hugoniot conditions are all independent of time. I'm guessing this is in the realm of simulation and no exact solvable DEs exist?
 
tpg2114
tpg2114
10:53 PM
@Astrum It's a matter of how close you are to the material detonating. Things like RDX and HMX (which are components of explosives like C4) have detonation pressures in the 30-50 GPa range within the solid. So the initial blast wave has that pressure, but it does diminish as the wave propagates outwards
And the RH conditions are independent of time because they are in a shock-centered reference frame, so the coordinate system is moving at the shock speed. You could, if you were so inclined, try to do the same analysis with an inertial frame
But if you want to get more complicated than 1D inviscid non-reacting, you're getting into the realm of numerical solutions.
Although if that's something you are interested in playing with, it's not that complicated to write 1D or 2D solvers for shock problems
Supersonic flow is much easier to deal with numerically than subsonic
 

« first day (1610 days earlier)      last day (3323 days later) »