The h Bar

Anonymous

01:29

@bolbteppa Yeah, disappointing. It seems to be just mentioned in a footnote.

bolbteppa

The early papers are weird

Anonymous

Timaeus's two answers are pretty enlightening although I still have a few confusions after reading them.

Anonymous

01:54

@JánLalinský I don't have a copy of Streater's book on me, but pages 3, 60, 71, 98, 124, & 131 contain the word maximal according to a google (though he may talk about it in other parts too). If you only ever want position to have a random variable, then your sample space can be the $x$,$y$,$z$ triples, with a measure induced by the pdf $\Psi(x,y,z)$. But if you want to have random variables for say, momentum, then you need a different sample space unless you want to restrict to situations where the wavepackets separate physically and then use position as a surrogate for the other observables. — Timaeus Mar 14 '15 at 20:37

Anonymous

I don't quite understand this (the second sentence). ^ :/

ZeroTheHero

02:22

@DavidZ is there some restriction on the choice of usernames?

I guess if @dmckee is around that could go to him as well.

rob

@ZeroTheHero What kind of restriction are you wondering about?

Anonymous

49

A: Why are some users able to add special characters to their user name, but I can't enter a Malayalam one?

You are free to use any character that is a member of the .NET \w character class. This includes any Unicode letter categories (Ll, Lu, Lt, Lo, Lm), connecting characters (Pc, connecting punctuation, underscores really) and numbers (Nd), with a dash of hyphen, space and apostrophe thrown in. Yo...

Anonymous

There is some length restriction too. Lemme check.

ZeroTheHero

choice of username which might be... well... in appropriate.

@rob like this one: physics.stackexchange.com/users/219232/dumbfuckjuice

Anonymous

@ZeroTheHero Flag it.

ZeroTheHero

02:26

@Blue not quite sure how to flag this...

rob

@ZeroTheHero Pick a post by the user and raise a custom flag.

Anonymous

@ZeroTheHero If you find a username or profile picture inappropriate or offensive, you should flag any of their posts with a custom message for the mods.

ZeroTheHero

@rob ok will do thanks.

rob

I'll take care of that one presently.

Anonymous

Mods are able to reset usernames to default user[number] format. And the profile picture can be reset to the default gravatar. :)

ZeroTheHero

02:28

@rob thanks.

rob

@ZeroTheHero Glad I happened to be around.

ZeroTheHero

@rob well I was checkin' to see if any mods were around and I saw DavidZ and dmckee's avatar so I tried them first.

Sir Cumference

Howdy

Anonymous

Anyway, since I was looking for it:

ZeroTheHero

@rob I meant to raise the issue a little earlier but the chat seemed to be very quiet.

Anonymous

02:34

Max DisplayName    Max Password Max Email Length
Length Used On SO  Length
------------------ ------------ ----------------------------------------------------
36                 As long as   254 (See: stackoverflow.com/a/574698/1945631)
                   possible

ZeroTheHero

@rob case promptly closed... that what matters.

Anonymous

@SirCumference Holla.

Sir Cumference

Sup

Anonymous

The PSE mods check in the chat few times a day. So if you leave a message for them, they're very likely to notice it, maybe not immediately, but probably within 24 hours.

Anonymous

In serious cases, it's best to raise a flag though, if you feel uneasy about anything.

rob

02:39

Yeah, if you find yourself thinking "hmmm, a mod needs to know about this" then starting from "this" and finding a thing that you can raise a flag on is always the most robust way to proceed.

But raising a flag on a profile is not obvious. Several people have asked how to do it on Meta. No worries about your approach here.

ZeroTheHero

@rob it worked quickly enough.

rob

As my mother likes to say, "Well, we had a good outcome."

David Z

I would always suggest raising a flag for something that needs mod attention, even if we're in chat. In addition to making us aware of the problem, it also establishes a record in the system which we can refer to later if it becomes relevant.

3

Anonymous

@SirCumference Holidays now?

ZeroTheHero

@DavidZ Maybe I'm overly cautious with flags: I know what it is to deal with random inputs and how it can become overwhelming.

David Z

02:54

Very true, and that's a valid concern on larger sites like Stack Overflow, but here we're not in danger of being overwhelmed by flags, so I suggest erring on the side of casting more flags.

ZeroTheHero

@DavidZ all right I'll keep that in mind.

David Z

Cool, thanks :-)

ZeroTheHero

@DavidZ well to be honest maybe it's an occasion for me to thank you and the other mods for the very patient work of keeping the show on track.

David Z

You're welcome, but really it seems to work pretty smoothly these days without us doing much.

ZeroTheHero

yes for all the complaints about the quality of some of the questions the quality of the reviews has (IMO) increased and the mods act quickly when appropriate.

@DavidZ you trained us well ;)

user400188

03:10

Can someone please explain to me, in a simple way, what a collision kernel is?

Anonymous

@user400188 Chat's not a great place for "explain me broad topic X" type of questions. :P

user400188

Thats a shame. I don't know what I am asking for enough to formulate a good SE question on it.

Anonymous

Try to do some research, mention the exact points where you're getting stuck, etc.

user400188

I wish I knew where I was getting stuck.
So far as I can tell, its simply a term that has come up in kinetic theory several times (in equations and text) which I don't have a physical interpretation for. It appears to be a probability of some kind, but I don't know what it is a probability of.

David Z

@Blue Well, if all a person wants is a brief high-level overview, I think chat is the perfect place.

Anonymous

03:23

@user400188 Here's a PDF (Notes on the Boltzmann Equation).

Anonymous

It seems to be the term on the RHS of the Boltzmann equation.

Sir Cumference

@Blue New Years already passed :P

unless ya mean holiday break, in which case yep

rob

@user400188 Links or references to such occurrences can be helpful in letting other people understand your context better.

user400188

@Blue I read that PDF earlier, but it doesn't give a good overview of what it is. They seem to assume the reader already knows and are only concerned with calculating it.
Thanks anyway.

@rob Thanks for the suggestion, it would have saved @Blue some time if I had followed it too.

Anonymous

@DavidZ Of course, users are free to ask, but based on my previous experience they usually don't receive great responses, unless someone is really willing to spend their time on them. Also, it's always good internet etiquette to "show your research effort", I think.

David Z

03:27

Ah, I see what you mean. I suppose it depends on whether there's someone in the room with experience in the topic being asked about.

Anonymous

@user400188 You can ask a Physics SE question based on that PDF. Give the context of the Boltzmann's equation and ask for the physical significance of $Q$ in the equation. ;)

Anonymous

Although that looks like a PDE-related term. Might be worth a try on Math SE as well.

user400188

This is the link where I first saw the collision kernel mentioned: https://doi.org/10.1063/1.1674619.
@DavidZ I was hoping someone in the room had dealt with them before, which is why I asked.
:48418738 I would always show research effort in a main site SE question, but in the chat, I have been under the impression, that it's fine to show it over the course of a conversation, as opposed to all at once.

rob

I have a dim recollection of reading "scattering kernel" as a shorthand for the energy/angular dependence of a cross-section. There's a neutron transport Monte Carlo code that I used a few times where different materials presented different "scattering kernels" to the beam.

I thought it was a strange use of the word and finished the project without really internalizing a meaning for it.

user400188

Also, just researching the things needed to ask the question with a few helpful pointers (your link back to the pdf) has given me an initial understanding of what it is. A PDE accounting for the forces acting between particles in collisions.

rob

03:35

A little reading just now makes me think that a well-written question on Physics and/or Math would be a good resource.

Anonymous

@user400188 I was just telling you ways to increase the likelihood of getting a useful answer, in chat. :)

user400188

I checked physics SE and there is only one question that mentions a kernel, However the answer that mentions them appears to use the term for something else. Like a kernel of corn or something.
I think I might ask the question anyway (after checking math SE) so that at least one site has a reference people can use on them.

As for increasing the likelihood of a useful answer.. I usually ask the chat first in case there is a really simple one somebody knows, or if they can give hints that let me figure it out myself, them formulate a main site question which can go into detail.
Every so often, someone is really willing to spend their time on them in the chat, and I find that those are the best type of answers you can receive.

ZeroTheHero

@user400188 pity the paper is firewalled.

Anonymous

@user400188 "Every so often, someone is really willing to spend their time on them in the chat, and I find that those are the best type of answers you can receive." --- sure, you should feel free to try your luck.

rob

One dictionary gives a definition of "Mathematics . the set of elements that a given function from one set to a second set maps into the identity of the second set." I can imagine a person coming from an S-matrix background applying this to scattering: it's a map from input vectors to output vectors.

But that's a non-trivial definition, like learning etymology by reading classical Greek.

ZeroTheHero

03:42

@rob I know the term but in the context of an integral kernel or a quantization kernel. It's a creature that helps you go from one space to another.

rob

I think that chat is a place where people can work out how to articulate a question in a way that wouldn't succeed on the main site. It's certainly proved interesting in this case.

ZeroTheHero

I think the notion of your dictionary is from representation theory.

integral kernel as in en.wikipedia.org/wiki/Integral_transform

rob

Well, dictionary definitions of technical terms are dangerous territory, and dictionary editors have different levels of carefulness. It's a starting place.

(like Wikipedia is a starting place)

ZeroTheHero

plus physicists are prone to abuse of language.

rob

@ZeroTheHero I believe you mean "driving the evolution of language" :-)

ZeroTheHero

03:47

yeah that too! :)

gentlemen I must bid good evening to you all. As much as students think instructors give them homework it's really the other way around and I must do mine in order to avoid shame tomorrow.

rob

@ZeroTheHero Take care.

Secret

06:58

Color gaunlet blues: When your computer's RGB system failed to faithfully represent the blue color LED colors you actually see

In reality, these blue shades are a lot more RYB blue looking

Slereah

10:17

"The earth is a hollow #dysonsphere with EM-opposed concentric shells making up the body. The moon is small, hollow, self-lit & close (within the Van Allen Belts & within the earth's outer atmosphere). Gravity is really Electromagnetism (Electric Universe), unrelated to Mass. Space-time does not exist. Dark matter does not exist."

Slereah

10:37

vixra.org/pdf/1405.0358v1.pdf

user2723984

10:51

question: if the Dirac algebra $\{\gamma^\mu,\gamma^\nu\}=2g^{\mu\nu}\mathbb{1}$ is an algebra in the sense I'm thinking of, then how come it isn't closed under addition and multiplication?

Slereah

Isn't it

user2723984

at least not obviously to me

not even scalar multiplication

Slereah

The $\gamma$ are only the generators

user2723984

oh

Slereah

Multiplication and addition are still part of the full Clifford algebra

user2723984

10:54

so it's just the algebra spanned by the generator via addition and scalar multiplication?

Slereah

Well not scalar multiplication

user2723984

and matrix multiplication

got it

Slereah

it's a bit more awful

user2723984

oh

why can't things be easy for once

Slereah

The full basis is like $$\{ 1, \gamma^\mu, \sigma^{\mu\nu}, \gamma_5 \}$$

or something like that

I think I'm missing a set

user2723984

10:57

but both $\sigma^{\mu\nu}$ and $\gamma_5$ are just multiplications of $\gamma$ matrices

ah yes, we can kinda turn it into a vector space using those field bilinear things so

Slereah

Yes.

That's why the $\gamma$ are the generators of the algebra

user2723984

thanks

so really it's the same as $M_4 (\mathbb{C})$

Slereah

Well no

It's $C\ell(\mathbb{R}^{3,1})$

user2723984

I have to read more about Clifford algebras, because as it is it looks like via addition and multiplication of gamma matrices we can get whatever 4x4 complex matrix we want, so I don't see the difference

bolbteppa

11:30

If $\{\gamma_1,\dots,\gamma_n \}$ is an orthogonal basis for a vector space $V$ with inner product $\eta_{\mu \nu}$ then the Clifford algebra is a vector space $C(V)$ also possessing a multiplication operation $\cdot$ satisfying $\{ \gamma_{\mu}, \gamma_{\nu} \} = 2 \eta_{\mu \nu}$ for which it's basis elements are $\gamma_{i_1} \cdot \gamma_{i_2} \cdot ... \cdot \gamma_{i_k}$ for $1 \leq k \leq n$ and it has dimension $2^n = (1 + 1)^n = \sum_{k=0}^n {n \choose k}$

bolbteppa

12:16

Of course - a string is reparametrization invariant, thus the modes oscillating along the 2 d.o.f. tangent to the surface should be irrelevant, leaving $D-2$ modes, thus we can set one of the $x^{\mu}$ to be (proportional to) the $\tau$ coordinate. We do this explicitly by looking at our solutions
$$x^{\mu} = x^{\mu}_0 + 2 \alpha' p^{\mu} \tau + \sum_{n=1}^{\infty} \sqrt{\dfrac{2 \alpha'}{n}} (a_n^{\mu} e^{-in\tau} + a_n^{\mu*}e^{in \tau}) \cos(n \sigma)$$
and note, setting $x_0^{\mu} = 0$, we can isolate $\tau$ via $n_{\mu} x^{\mu} = (2 \alpha' n_{\mu} p^{\mu}) \tau$ if $n_{\mu} a_n^{\mu} …

Slereah

13:07

Hm

How to show that the inverse of the Hopf fibration projection implies that for $\pi(a_1, a_2) = \pi(b_1,b_2)$, we have $(a_1, a_2) = (\lambda b_1, \lambda b_2)$ with $|\lambda| = 1$

Easy enough to see if we can express $a_1 = c_1 b_1$ and $a_2 = c_2 b_2$, but is it always the case

What if we have a zero???

I guess I need to show individual cases for zeros

user929304

14:11

@ACuriousMind hi :), I just came across this question physics.stackexchange.com/questions/453073/… and thought it might interest you as well. If I'm not mistaken, the use of annhiliation and creation operators makes no difference in the distinguishability of particles, right?

bolbteppa

15:06

Oh wait

PD not PhD

danielunderwood

What determines the wrong site options? Is it only math and physics meta for this site? I was going to flag this question to go to SO, but that's not an option

Slereah

"The solution has a directional singularity at origin: as one approaches the origin by the z-axis there is no singularity"

Bloody hell

Anonymous

@danielunderwood In case there's no default migration path to SO, use the "in need of moderator intervention" option and explain why you think it should be migrated there. :)

Slereah

We really should add more migration options

Math, chemistry, history of science, philosophy and worldbuilding

Anonymous

Most SE sites have around 2-3 default migration paths in general. But mods can migrate questions to any site on the network.

Slereah

15:20

sure but it would be nice

there's a lot of those

Anonymous

@Slereah Yeah, I guess. It's not too difficult to set up. You'd just need to catch hold of a SE developer or CM. :)

danielunderwood

@Slereah sounds...interesting? Scary?

Anonymous

Might be worth a feature-request on Physics meta.

Slereah

@danielunderwood It's one of those Bad Solutions

The Israel-Khan class of spacetimes

"The space-time described by (4-5), known as the Chazy-Curzon solution [11][12], has a hoop-like structure[35]."

'tis a hoop

danielunderwood

@Blue ahh thanks. That's what I ended up doing

@Slereah I bet the pop-sci reading population would eat up a book on "bad solutions" to GR

Although I don't know how much you could truly do with a pop-sci book on the topic

Slereah

15:29

the horror

danielunderwood

you're into some weird stuff

Slereah

Someone has to

it's one of those rare two body-ish solution to GR

two body solutions in GR get pretty weird

worst part is those solutions are from 1924

So either you read the original paper from before singularities were properly understood or you read a modern paper where they don't bother setting things up

danielunderwood

Who needs to set things up when you have references?

Slereah

Oh yes nothing I love more than having to track down 30 different papers in 5 languages

user929304

@JohnRennie Hi, is it ok if I ask a literature related question?

danielunderwood

15:37

Actually one of my professors got me for that one time. I wrote something with the only description of the equipment being something like "Using the apparatus described in [ref]..."

Slereah

I am exagerrating but only slightly

There's papers in Italian, German, English and French

As we all know a circle is the set of points equidistant to a given point [Euclid]

danielunderwood

I've never had to read papers in other languages, but it sounds tough. I have considered learning a bit of German and trying to read some papers though

Slereah

Fortunately most of the foreign language papers are french which I can read

There's a lot of German though, which I cannot

a lot of early GR is in German

Also I do always wonder if there's like a secret trove of untranslated soviet papers

John Rennie

@user929304 yes of course

Slereah

"It is known that if the Schwarzschild line element of a particle with mass $m$ is transformed to the coordinate system $\tilde{I}$, $l$ is formally the Newtonian potential of a uniform rod with length $2m$."

I have no idea what this means

Calgon take me away

Mostafa

15:48

Any condensed matter guy here?

Slereah

I'm entirely made of condensed matter

8

Mostafa

I'm looking for resources on the conductivity of single layer (2D) TMDs. Something similar to what we have for Graphene using Kubo formula

"Do not worry about your difficulties in condensed matter physics. I can assure you mine are still greater" (anonymous optics/photonics physicist)

Anonymous

@Mostafa Ask it as a resource-recommendations question maybe? By the way, happy 2019! :)

Anonymous

@Mostafa Physics SE, in general, has a severe lack of condensed matter folks, from what I observed. :P

Mostafa

16:03

@Blue Yeah, but not the just made of condensed matter type :p

@Blue I'll try but I think I've developed a habit of asking in the chat rooms instead of posting on the main sites.

and happy new year :)

John Rennie

Well I've just made a risotto using frozen butternut squash (because I couldn't get fresh) and it has degenerated into a gooey mess on cooking. Epic fail.

user2723984

risotto is an exact science

danielunderwood

Rice has to be one of the hardest things for me to cook. Sometimes it turns into a gooey mess just by itself

Mostafa

@danielunderwood Easiest thing for me (after boiled eggs)

@user2723984 Yeah, JR has published a paper on it

2

4

Screenshot^

John Rennie

16:20

:-)

user929304

@JohnRennie excellent :) I was simply wondering, if there s a book you d recommend for learning the basics of SR and GR for someone with knowledge of CM and QM and v basic SR. For example, I absolutely love your way of discussing/explaining GR questions here on SE, they are very well balanced both intuitively (conceptual lvl) and mathematically (which can easily become v hard in GR). The Feynman book on gravitation I find quite hard to read as it gets advanced very quickly.

Any recommendations, or personal favorites would be highly appreciated (be they books or lecture notes).

John Rennie

@user929304 the problem is that I have a tendency to sacrifice rigor for clarity when posting answers, and no reputable GR textbook is going to do that. So you'll find all the GR books have a more formal feel to them.

Sean Carroll's book on GR seems to be well received, though I don't know it that well personally.

user929304

@JohnRennie that s such a pity... :( I appreciate your candor and suggestion. If you ever come across a text that follows somewhat your style of writing about relativity, please let me know, thanks again.

enumaris

16:40

weez

ACuriousMind

17:07

@user929304 knzhou is already on it, as it appears ;)

user929304

@ACuriousMind yeah :) hard time understanding that answer :PP maybe I have to re-read

AfterShave

19:56

Hey could someone give me a hand with a qft problem

Anonymous

@AfterShave Don't ask about asking, just ask. :P

AfterShave

So I've got the following Lagrangian

Uh nevermind don't need a specific example for this question

Basically if I have an internal line

but its momentum is fixed

do I just drop the propagator for that line?

*fixed and on-shell

user2723984

why should you drop the propagator?

I think you should put the propagator with the fixed momentum

but I'm no qft expert

AfterShave

20:28

well if you don't drop the propagator it's just infinite

or maybe it gets fixed after renormalization?

idk

user2723984

if there are no loops it should be finite

AfterShave

lets say you have two scalars

pi and phi

with a $\pi^2\phi$ interaction

and you try to calculate this correction to phi -> pi pi decay

imgur.com/jBYt55A

how do you treat that internal phi line?

user2723984

ah yes that is probably going to diverge

AfterShave

it's explicitly 1/0

user2723984

because you have an undetermined loop momentum

AfterShave

20:34

no I'm not talking about the loop

I'm talking about the internal line after the loop

how do you treat something like that

user2723984

you just keep the propagator but there is no integral because it goes away in the momentum conservation

why should it diverge?

if p is the momentum of the incoming particle that should be another propagator with momentum p

enumaris

pain

user2723984

$$ \int \frac{ d^4 k}{(2\pi)^4}e^{ip(x-y-z)}\frac{1}{(p^2-m^2+i\varepsilon)^2}\frac{1}{(p-k)^2-m^2+‌i\epsilon}\frac{1}{k^2-m^2+i\epsilon} $$

isn't the diagram something like this?

no wait there's something wrong there should be more momenta

but apart from the exponential factor

AfterShave

$p^2-m^2$ is zero since its on-shell

I guess this has to do with the fact that phi is unstable

if it's not on shell then momentum is not conserved

oh I think i see how it works

phi is unstable so if you sum up all the corrections there's a finite imaginary part on the dressed propagator

user2723984

I mean the diagram diverges either way

but I'm interested too in why that $p^2-m^2$ isn't a problem

let's wait for an answer to your question lol

AfterShave

20:50

Basically if you define the pole mass to be real

the LSZ doesn't kill off the imaginary part of the mass correction

at least that's what I think

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