The h Bar - 2019-07-31

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Emilio Pisanty

00:51

@dmckee well, this got out of hand =/

note that OP is not asking about spin-orbit or electron-spin-to-nuclear-spin coupling (fine and hyperfine structure resp.), but rather the electron-spin-to-electron-spin magnetic interaction, which is weaker than both of the above

unless I missed something huge, this is because (i) spin-spin coupling doesn't have any clear qualitative effects on the spectrum (say, splitting degeneracies the way FS and HFS do), and more importantly (ii) because it's weak, it's overwhelmed by the electrostatic structure, which separates spin and orbit, and when you couple this with the coupled spin-spatial structure of the coupling, it essentially averages out to zero once you start taking matrix elements

there's probably an interesting question to be asked there in terms of why spin-spin interactions are so damned weak

(for starters: the Bohr magneton is larger than the nuclear magneton -- so why is electron-electron spin-spin coupling less important than the HFS electron-spin-to-nuclear-spin coupling?)

almost certainly with an answer along the lines I just outlined, but it'd be nice to get a solid grounding in the literature for it

if it exists, though -- it's very likely that the precision-spectroscopy-theory literature just kinda moved on, say, to the Foldy-Wouthuysen manipulations here, without ever bothering to stop and explain why this was happening

Still, I suspect asking such a question is beyond OP's powers

Emilio Pisanty

01:19

oh, and also: apologies if anybody is put off by the sometimes-slightly-rough tone. But frankly, this site's tolerance of the repeated posting of misinformation by that OP, and the abuse of this site that it represents, is beyond my understanding.

1 hour later…

PM 2Ring

02:46

@EmilioPisanty Nice answer. IMHO, that OP certainly deserves a slap (or several), I just hope that your actions aren't seen as bullying, or arrogant.

FWIW, here is a neglected recent question about angular momentum and Hawking radiation.

1 hour later…

Aaron Stevens

04:18

@EmilioPisanty I think your language in that answer is perfectly acceptable. People who come to this site looking for help in understanding physics should be ready to accept that help. This is not a site for people to be reassured about their faulty understanding.

John Rennie

04:48

Objects falling into a black hole explode because the exceed the local speed of light

2

3 hours later…

Emilio Pisanty

07:45

@PM2Ring @AaronStevens Well, whaddayaknow, straight accept by OP without even a comment.

Maybe this does mark a turning point and we won't see the same misinformation getting re-posted?

I'm not particularly hopeful though

PM 2Ring

07:59

@EmilioPisanty Me neither, but let's be optimistic. ;) At least he's prepared to accept correction when his misinformation is pointed out. So while I agree he's annoying, he's not beyond redemption.

John Rennie

@PM2Ring unlike some other members of the site

PM 2Ring

@JohnRennie Exactly. And I'm not just talking about JD.

@JohnRennie Speaking of whom, how does he reconcile his "clocks stop at the EH" with "stuff explodes into gamma rays at the EH"?

John Rennie

@PM2Ring even for him that's a bold claim.

It just shows a complete failure to understand the basic principles of differential geometry.

How can anybody boldly make claims that contradict all work in GR back to Einstein. It's a cognitive position that just seems incomprehensible.

"Every GR textbook ever published is wrong and I'm right"

PM 2Ring

It's downright delusional.

Loong

@JohnRennie staff.science.uu.nl/~hooft101/gravitating_misconceptions.html

John Rennie

08:09

@Loong yes :-)

Slereah

@JohnRennie You'd have to be either very bad at physics or very good at math to make such a claim

Slereah

08:32

You know

I'm not even sure I know any metric for the pair of pants spacetime

I mean it's not too hard to find a Riemannian one

Conformal metric to a sphere minus three disks

but making it decent sounds hard

I can't really think of a coordinate system that would make it easy to deal with such a manifold

Bipolar coordinates are pretty close but they can't deal with the third boundary easily

Altho

Do I need the exact metric to deal with string theory on such a manifold

since it's conformally invariant

not ideal for the boundary conditions, tho

Slereah

09:03

@ACuriousMind can you help me with my pants

3

user2723984

09:14

TIL about the pair of pants spacetime

physicists: theoretical physics isn't removed from reality.

also physicists: what if spacetime was a pair of pants

John Rennie

The spacetime in this context is the string worldsheet

user2723984

yes I guess it isn't as absurd as the name suggests

but the name is hilarious

Slereah

It is sometimes called the trouser spacetime

if that makes it better

user2723984

if I had to guess I would have said that the trouser spacetime is a pair of pants spacetime, but not necessarily the opposite

Slereah

It's all based on the topology, in which case I can tell you that underwears have the same topology

Captain Bohemian

10:03

does how a cover letter is written for a certain position make a difference?

Slereah

I don't want to sound too vague, but I'm guessing it depends

i suspect quite a lot of professors don't give a damn

Captain Bohemian

I have sent cover letters of different versions to different principal investigators, but most of them don't reply me.

the cover letters for applications to principal investigators are harder to get reply than letters asking for position.

Slereah

10:36

13

A: How much dark energy will fit in an average cup of coffee?

Dark energy as expressed by the cosmological constant is, as the name implies, a constant of space. Therefore, in a cup of coffee, we get, for some static observer $t$, and a spacelike hypersurface $\Sigma$ (I'm assuming that in our universe, there exists a neighbourhood that can be foliated in s...

Fixed the uncertainty and significant digits

that guy who downvoted me for forgetting significant digits better vote >:|

bolbteppa

10:58

@CaptainBohemian what kind of position

Aaron Stevens

11:10

@Slereah I saw that comment. Sometimes people get uptight about parts of answers that don't really matter. The OP didn't ask for an answer with the correct amount of digits, and certainly you won't be using that number in any meaningful way anyway.

Slereah

I'm trying to work out the energy in a cup in a Schwarzschild metric, but this proves to be a bit too tricky

I mean I could do it, but that would be too much work

wolframalpha.com/input/?i=integrate+r%5E2+%2F+(sqrt(1+-+a%2Fr))

It involves this integral

bolbteppa

11:26

@Slereah guessing that $u = 1/r$ which gives $ \int \frac{r^2 }{\sqrt{1-\frac{a}{r}}} dr = \int \frac{1/u^2}{\sqrt{1-u a}}(-du/u^2) = \int \frac{1}{u^4 \sqrt{1-au}}du $ can be evaluated using this

Slereah

I mean it's not hard to evaluate

but it is very tedious

bolbteppa

How else would you do it, that solution is pretty ugly haha

Captain Bohemian

@bolbteppa PhD position.

Slereah

@bolbteppa it's just to find out how to define the shape of a coffee cup in the Schwarzschild coordinates

with the appropriate volume

bolbteppa

@CaptainBohemian you're never going to write a good proposal or cover letter on your own

Captain Bohemian

11:35

@bolbteppa do you mean research proposal? but the position offered by principal investigator almost already has research project and doesn't require the applicant to write a proposal.

actually I can only make proposal in the fields I have some knowledge.

bolbteppa

What do you mean they don't respond to cover letters then

Captain Bohemian

most of those positions in conformal field theory and string theory which I can't make proposal.

@bolbteppa I mean my applications to those positions opened by principal investigators mostly don't get reply.

bolbteppa

You mean an e-mail enquiring if it has been filled etc

Captain Bohemian

I mean the applications directly to the emails of principal investigators.

Slereah

Hm

Captain Bohemian

11:40

no, I mean the application.

bolbteppa

So you're not asking about it beforehand

Slereah

If I don't use this formula and use the flat space approximation for the volume, it's just the same formula as before

$E \approx \rho_\Lambda V$

Oh well

I think my answer's good enough as it is

For that dumbass question

Captain Bohemian

@bolbteppa if there is open position announcement, I don't ask for position before applying.

Slereah

I mean basically I think it's always gonna be that formula

It's just that the volume will have a fancier formula

bolbteppa

@CaptainBohemian lol so you're asking people to trust you wont be a 3+ year burden who does nothing and takes the place of a better person based off a paper trail which is compared to maybe 80+ other applications and that's it, why wouldn't you do doing things to minimize the risk factor on their end

Captain Bohemian

11:45

@bolbteppa but if I contact, they would just ask me to apply based on my experience.

they would not discuss anything with me besides directing me to apply.

but after applying, they don't reply me anything.

Slereah

user image

bolbteppa

@CaptainBohemian you know that's not always true

Captain Bohemian

actually I find no matter what topics I express to wish to work on, they would just direct me to the available position in their institute disregarding what the position's topic is.

Slereah

@ACuriousMind There's nothing preventing a worldsheet from being inside another worldsheet

Does that correspond to something specific

Emilio Pisanty

I'm voting to close this question (v2) as off-topic because ref. req. are restricted on Phys.SE. — Qmechanic ♦ 28 mins ago

Captain Bohemian

11:51

that's why I think how cover letter is written seem not to matter.

Emilio Pisanty

@Qmechanic I feel those comments would be much more helpful if they contained actionable information for the OP

Either a link to a policy or a short explanation of what's wrong

As it is it's just turfing out the question and presenting an impenetrable wall to a newcomer user

... not that the question was very good, of course

skillpatrol

...how about a long explanation of what is wrong?

Emilio Pisanty

@skillpatrol define 'long'

skillpatrol

@EmilioPisanty long enough to find out what the OP doesn't understand

bolbteppa

@CaptainBohemian at the end of the day, all it comes down to is convincing someone to take you on, all the rest is fluff (wading through the fluff is by no means easy at all and it's likely one will just give up), but there are other routes than applying to pre-proposed projects where you can't do anything to affect the process

JMac

11:56

@EmilioPisanty Seems like it might be another case of this question seems like a list question

Captain Bohemian

@bolbteppa do you think the positions funded by institute without predetermined project good? Will that kind of position fund you to attend conference if you are admitted?

bolbteppa

@CaptainBohemian there's also an entire continent where a phd application is just a masters application (GRE etc)

skillpatrol

\o @Jim

Jim

yes?

skillpatrol

how's it going, pal?

Jim

12:00

pretty good. Business as usual on this end

skillpatrol

coolio

Jim

you seem surprised to see me at this time of day

but I'm online at this time every week day

bolbteppa

@CaptainBohemian applying to a pre-written project is pretty much the exception as far as I could ever see, like that's the one you should expect to fail, you really should be doing everything you can not just these pre-written ones which box you in before you even start etc

Novalium Company

I'm doing the experiment today. Hope I don't blow myself up. Wish me luck xD

Emilio Pisanty

@JMac a mild case ;-)

skillpatrol

12:02

@NovaliumCompany Good luck, pal

bolbteppa

@Slereah how do you integrate simple functions like $x^2$ or something over a pair of pants :p

Jim

@bolbteppa magnets

Novalium Company

@skillpatrol <3

skillpatrol

<3

bolbteppa

@Jim they took magnets out of pants years ago, or so I've heard (conspiracy?)

Captain Bohemian

12:04

@NovaliumCompany I also think applying for pre-written project has rare probability of success, but a person in SE thinks the opposite to me---they thinks that kind of position has fewer people to apply so they have higher rate of success. But I don't agree with them.

Slereah

@bolbteppa I can't even find any decent metric for a pair of pants

Jim

@bolbteppa what? why am I just hearing about this now? In that case, I don't know how. Do pants still have lasers? That might work

Slereah

Lorentzian one, that is

bolbteppa

Lorentzian pants with an EM field on them = magnets

Slereah

you can just use a variety of flat or spherical or hyperbolic metrics for Riemannian ones

skillpatrol

12:05

@EmilioPisanty I didn't mean to sound rude, pal

just thinking out loud :-)

bolbteppa

@Slereah My sense is this kind of thing comes down to knowing whether you're working with the analogue of a sixth degree polynomial or a fourth degree one even if Ferrari's formula is hard to derive and use, with the fourth degree one we can find a formula for the roots easy enough, for the sixth degree one you have to go through insane hoops to get any kind of formula even for a single root or approximate it etc...

Physics is a good way to lead you to fourth degree ones not the sixth degree ones :p

Emilio Pisanty

12:20

@skillpatrol no snark intended =)

skillpatrol

<3

Aaron Stevens

12:44

Anyone have any god writing tips? I am writing up a fellowship application, but I feel like I am getting in my head too much. So my writing is progressing very slowly and with much uncertainty on my own part.

Emilio Pisanty

@AaronStevens that... sounds like what writing is normally like

or at least, I don't think I know anyone for whom it's different

Aaron Stevens

@EmilioPisanty Yeah I know haha. Sorry, I didn't mean to make it sound like it is a problem unique to myself

Emilio Pisanty

@AaronStevens no, I'm just sorry I have no pro tips to share =/

skillpatrol

I find organizing things in a chart helps, sometimes.

JMac

@AaronStevens You want your god to be powerful, but still relatable so that the reader can connect with them while fearing their divine judgement.

Aaron Stevens

12:47

@JMac OK, you got me :)

skillpatrol

and never create a stone that He cannot lift :P

Novalium Company

@CaptainBohemian What are you talking about?

Captain Bohemian

@NovaliumCompany sorry, that message is for @bolbteppa

Novalium Company

@CaptainBohemian ok :D

Qmechanic

@EmilioPisanty @JMac : Added a short explanation.

Captain Bohemian

12:58

sorry, the above message "I also think applying for pre-written project has rare probability of success, but a person in SE thinks the opposite to me---they thinks that kind of position has fewer people to apply so they have higher rate of success. But I don't agree with them." is to you @bolbteppa

bolbteppa

@CaptainBohemian how could they possibly know that, if anything it being advertised means more would rather than less

Captain Bohemian

@bolbteppa indeed, maybe in that person's field, positions with predetermined projects have fewer applicants, but in theoretical (high energy) physics fields, it's not the case.

actually I don't usually have intrinsic interest in those predetermined projects, but I just try.

in some cases it's because some professors I contact direct me to those positions.

Aaron Stevens

13:38

0

Q: Why doesn't reflection depend upon the energy of the incident light?

Obviously, the color / energy of a light source does not impact its behavior when reflecting off of a mirror. However, the energy of a light source is relevant in iridescence, and when traveling through certain mediums, like prisms. That is, the energy of a photon traveling through a prism deter...

visible-light speed-of-light reflection

OP wants a purely physical explanation without a derivation from an equation.

I feel like one of the cool things about Maxwell's equations here is that the reflection and refraction aspects just fall out of the boundary conditions and wave properties. Oh well.

Slereah

I'm reading some string theory intro and in the middle of it there's just

(COULD PUT THE FADEEV-POPOV STUFF HERE FROM TONG AND PAGES86-90 OF POLCHINSKI AND 3.7 OF KRISTSIS)

that scoundrel

His villainy of stealing from others plain to see!

Aaron Stevens

@Slereah They could have given the proper credit still

Slereah

14:01

cds.cern.ch/record/191979/files/198811125.pdf

this seems cool

This is quite possibly the most horrendous description of a free particle I've ever seen

String theory doesn't seem that bad after seeing how bad point particles get

Ryan Unger

@Slereah what's so bad here

Aaron Stevens

@Slereah Just curious; do you research string theory for fun? Or are you part of some sort of funded project, university, etc.?

vzn

"sometimes slightly rough tone" lol!

Slereah

@RyanUnger It is a lot of things for a free particle

@AaronStevens for fun

although I welcome all money

Aaron Stevens

14:17

@Slereah What is your take on where string theory is now? Do you think it is still a promising theory of describing the universe?

vzn

@AaronStevens have you looked at woit or smolin... or hossenfelder

Slereah

I'm not really at the stage where I could use it to describe the standard model

I'm sure it's fine

At least as fine as QFT is

Aaron Stevens

@vzn I have not, but I am not really itching to get into string theory haha

Slereah

It's just pieces of strings

how hard could it be

vzn

@AaronStevens they all (experts) have great popsci accounts, readable, think worth hearing contrarian povs

Aaron Stevens

14:20

@Slereah So you do your studying at the craft store, then?

Slereah

Hardware store

big strings

hardest part is finding the supersymmetric string at the shop

Aaron Stevens

@Slereah Some might tell you that string was never in stock to begin with

vzn

> fortune and misfortune are like a ~~rope~~ string twisted (old japanese aphorism)

Ryan Unger

String theory isn't really in any state until they find supersymmetry

vzn

strings let one use topology theory in particle physics so its like a (wet) dream for the math physicists... :P o_O

Slereah

14:23

QFT also does

vzn

@RyanUnger ("most to nearly all") SUSY is certified noshow in LHC

user2723984

@vzn what? They found nearly half of the particles already!

Slereah

Heyoooo

vzn

@user2723984 huh? just look at the headlines even recent ones Supersymmetry Dealt Another Blow By LHC iflscience.com/physics/supersymmetry-dealt-another-blow-lhc Why Supersymmetry May Be The Greatest Failed Prediction In Particle Physics History forbes.com/sites/startswithabang/2019/02/12/…

user2723984

I think I'm less good at jokes than I think I am

I meant that only the superpartners are missing

vzn

14:27

@user2723984 lol! ok thx for your humor didnt get it at 1st :)

John Rennie

@Slereah I want some exotic string for my wormhole. They were all out of stock.

Slereah

I don't even know if string theory has any theorems regarding the energy conditions

Oh wait

I actually do remember something

In the gravity-dilaton field limit of string theory, you can have stable wormholes

although obviously the dilaton field is very small, so unlikely to prop up much

I mean since gravity itself stems from string theory, any energy condition theorem is gonna be harsh

user2723984

@Slereah but are you a researcher in some other field? Where do you find the time to self study something else for fun?

Slereah

I use an ancient technique called "doing it at the office instead of working"

hush hush

it's a secret

Aaron Stevens

@Slereah I do that with Physics SE :)

vzn

14:35

secret o_O

user2723984

lol

that explains it

Slereah

I spent all morning working out dark energy in a coffee cup

user2723984

well if you manage to both self study strings and output research, chapeau :)

@Slereah you could publish a paper out of this

Slereah

I don't work in research, alas

@user2723984 probably not

It's fairly trivial

I just did it in excruciating details for comedic effects

user2723984

but what if it was a mug instead of a coffee cup? (With a hollow handle)

Slereah

14:38

The proof is that it's basically the same independantly of the shape

unless we're assuming some very unpleasant cases

user2723984

so no paper for you

Slereah

not for that, anyway

Aaron Stevens

@Slereah Just put it on the arXiv

vzn

vixra lol

Slereah

I don't have the clout to post on vixra

vzn

14:47

its sometimes peer reviewed. after you publish that is. :P

Slereah

I don't think people on vixra have any peers

vzn

peerless :o

Slereah

aip.scitation.org/doi/10.1063/1.524603

milesmathis.com/pi2.html

"I show that in all kinematic situations, π is 4"

vzn

so @Slereah having any fun this summer? what do you do for fun besides phyiscs?

Slereah

Going to a friend's birthday this week end

camping and whatnot

vzn

15:00

oh camping is fun luv it do you get to have a fire? near a river? etc saw 2 delta acquarid meteors a few nites ago, took 30m...

Slereah

beach camping

vzn

oh nice too, fire? (was on beach 3yrs ago, fond memories)

Slereah

dunno

Not in charge of planning

There will be alcohol certainly

no lack of fuel

vzn

got a dose of that too fri nite :) o_O

John Rennie

15:20

@AaronStevens hammer!

Captain Bohemian

this kind of weather is indeed suitable for outdoor activities

Emilio Pisanty

@Qmechanic =)

Captain Bohemian

I don't like drinking alcohol. I just want to play in the wilderness crazily because this concrete jungle is so suffocating.

Aaron Stevens

16:03

@JohnRennie ?

John Rennie

@AaronStevens I've used the dupehammer to close the question you linked as a duplicate

Aaron Stevens

@JohnRennie Ah I see haha. Maybe one day I will have my own hammer

John Rennie

@AaronStevens you need a gold tag badge in any of the tags on the question.

Aaron Stevens

@JohnRennie Yes, I am aware :)

Novalium Company

16:56

The experiment will be done tommorow :P

But in the final solution I'm left with very very little NaOH. Will it be enough to charge the TiO2 particles?

Loong

@NovaliumCompany Calculate the pH of your solution and compare to the isoelectric point of TiO2 in the literature. You should be fine.

Emilio Pisanty

17:16

@JohnRennie When are you back from your SciFi weekend?

John Rennie

@EmilioPisanty late Sunday evening. Too late, and I'll be too tired, to do anything on Sunday I'm afraid.

Emilio Pisanty

cool

I need to be in Birmingham for one or two days the week of the 19

I'm trying to juggle my flights

enumaris

I read that as juggle my fruits

Maybe I should take a course in computer hardware so I can understand these papers better...

Emilio Pisanty

18:01

@JohnRennie OK, flights are sorted, brum visit still to be sorted out

how does breakfast / brunch / lunch on Monday sound? I should probably be in brum by Monday evening though.

if you want to move this off-band my icfo contact is easy to find

1 hour later…

Novalium Company

19:07

@Loong Cool. John Rennie already checked the pH of charging TiO2, so it should be fine

JMac

oh fun, the thunder and lightning + heavy rain just arrived to drop the temperature back down a bit

3 hours later…

bolbteppa

21:55

@Slereah why is there a minus under the root in Goto 2.7 :(

Slereah

Good question

det g is negative but so is g_00

oh wait no it isn't

Because Goto uses the Wrong Signature

bolbteppa

Yeah he says $g_{00} > 0$

He never mentions the signature

Slereah

Well if a timelike vector is > 0

the wrong one

bolbteppa

There are a few ways to see it, but just based on what he writes alone I can't see why it has a minus

Slereah

Why not

g_00 is positive

det g < 0

It has to have a minus sign lest it be negative

bolbteppa

22:02

Why is $\det g < 0$

Slereah

Lorentz metric?

I mean it's string theory

The worldsheet is timelike

bolbteppa

Yeah I guess a coordinate transformation to 2D Minkowski space is the (or one) way to see it

bolbteppa

22:20

No that's stupid GR thinking, $g_{00} > 0$ has to imply $g_{11} < 0$, this needs a proper justification

Ryan Unger

whats happening

bolbteppa

One way to justify the minus sign under the square root properly is going from 5.7 to 5.10 here but Goto seems to see this as so obvious it's not worth mentioning

There is a GR way to show $g_{00} > 0$ should hold, does $g_{ii} < 0$ always hold in GR and is there a way to prove it

Slereah

22:38

$g_{00}$ should not hold

$g_{00}$ can be any sign

he just picked coordinates where this is true

I think he's just making a bunch of common assumptions about the worldsheet

I mean there is certainly a coordinate patch where this is true

bolbteppa

Right yeah $g_{00}>0$ in a coordinate system

He's using the $(+,-,-,-)$ metric due to $g_{00} > 0$ right

So I think the argument around 2.4 to 2.5 shows that $d x_{\perp}^{\mu}$ is perpendicular to the time-like direction and so should be space-like and so we should have $g_{11} < 0$

bolbteppa

23:09

I think that's it, and another proof is given by the link above going from 5.7 to 5.10 without any of this $\perp$ stuff

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