« first day (3191 days earlier)      last day (458 days later) » 

12:51 AM
@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
 
1:19 AM
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…
2:46 AM
@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…
4:18 AM
@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.
 
 
3 hours later…
7:45 AM
@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
 
7:59 AM
@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.
 
@PM2Ring unlike some other members of the site
 
@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"?
 
@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"
 
It's downright delusional.
 
8:09 AM
@Loong yes :-)
 
@JohnRennie You'd have to be either very bad at physics or very good at math to make such a claim
 
8:32 AM
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
 
9:03 AM
@ACuriousMind can you help me with my pants
3
 
9:14 AM
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
 
The spacetime in this context is the string worldsheet
 
yes I guess it isn't as absurd as the name suggests
but the name is hilarious
 
It is sometimes called the trouser spacetime
if that makes it better
 
if I had to guess I would have said that the trouser spacetime is a pair of pants spacetime, but not necessarily the opposite
 
It's all based on the topology, in which case I can tell you that underwears have the same topology
 
10:03 AM
does how a cover letter is written for a certain position make a difference?
 
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
 
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.
 
10:36 AM
13
A: How much dark energy will fit in an average cup of coffee?

SlereahDark 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 >:|
 
10:58 AM
@CaptainBohemian what kind of position
 
11:10 AM
@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.
 
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
It involves this integral
 
11:26 AM
@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
 
I mean it's not hard to evaluate
but it is very tedious
 
How else would you do it, that solution is pretty ugly haha
 
@bolbteppa PhD position.
 
@bolbteppa it's just to find out how to define the shape of a coffee cup in the Schwarzschild coordinates
with the appropriate volume
 
@CaptainBohemian you're never going to write a good proposal or cover letter on your own
 
11:35 AM
@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.
 
What do you mean they don't respond to cover letters then
 
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.
 
You mean an e-mail enquiring if it has been filled etc
 
I mean the applications directly to the emails of principal investigators.
 
Hm
 
11:40 AM
no, I mean the application.
 
So you're not asking about it beforehand
 
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
 
@bolbteppa if there is open position announcement, I don't ask for position before applying.
 
I mean basically I think it's always gonna be that formula
It's just that the volume will have a fancier formula
 
@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
 
11:45 AM
@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.
 
@CaptainBohemian you know that's not always true
 
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.
 
@ACuriousMind There's nothing preventing a worldsheet from being inside another worldsheet
Does that correspond to something specific
 
I'm voting to close this question (v2) as off-topic because ref. req. are restricted on Phys.SE. — Qmechanic ♦ 28 mins ago
 
11:51 AM
that's why I think how cover letter is written seem not to matter.
 
@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
 
...how about a long explanation of what is wrong?
 
@skillpatrol define 'long'
 
@EmilioPisanty long enough to find out what the OP doesn't understand
 
@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
 
11:56 AM
@EmilioPisanty Seems like it might be another case of this question seems like a list question
 
@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?
 
@CaptainBohemian there's also an entire continent where a phd application is just a masters application (GRE etc)
 
\o @Jim
 
Jim
yes?
 
how's it going, pal?
 
Jim
12:00 PM
pretty good. Business as usual on this end
 
coolio
 
Jim
you seem surprised to see me at this time of day
but I'm online at this time every week day
 
@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
 
I'm doing the experiment today. Hope I don't blow myself up. Wish me luck xD
 
@JMac a mild case ;-)
 
12:02 PM
@NovaliumCompany Good luck, pal
 
@Slereah how do you integrate simple functions like $x^2$ or something over a pair of pants :p
 
Jim
@bolbteppa magnets
 
@skillpatrol <3
 
@Jim they took magnets out of pants years ago, or so I've heard (conspiracy?)
 
12:04 PM
@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.
 
@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
 
Lorentzian one, that is
 
Lorentzian pants with an EM field on them = magnets
 
you can just use a variety of flat or spherical or hyperbolic metrics for Riemannian ones
 
12:05 PM
@EmilioPisanty I didn't mean to sound rude, pal
just thinking out loud :-)
 
@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
 
12:20 PM
@skillpatrol no snark intended =)
 
12:44 PM
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.
 
@AaronStevens that... sounds like what writing is normally like
or at least, I don't think I know anyone for whom it's different
 
@EmilioPisanty Yeah I know haha. Sorry, I didn't mean to make it sound like it is a problem unique to myself
 
@AaronStevens no, I'm just sorry I have no pro tips to share =/
 
I find organizing things in a chart helps, sometimes.
 
@AaronStevens You want your god to be powerful, but still relatable so that the reader can connect with them while fearing their divine judgement.
 
12:47 PM
@JMac OK, you got me :)
 
and never create a stone that He cannot lift :P
 
@CaptainBohemian What are you talking about?
 
@NovaliumCompany sorry, that message is for @bolbteppa
 
@CaptainBohemian ok :D
 
@EmilioPisanty @JMac : Added a short explanation.
 
12:58 PM
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
 
@CaptainBohemian how could they possibly know that, if anything it being advertised means more would rather than less
 
@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.
 
1:38 PM
0
Q: Why doesn't reflection depend upon the energy of the incident light?

Feynmanfan85Obviously, 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...

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.
 
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!
 
@Slereah They could have given the proper credit still
 
2:01 PM
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
 
@Slereah what's so bad here
 
@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!
 
@RyanUnger It is a lot of things for a free particle
@AaronStevens for fun
although I welcome all money
 
2:17 PM
@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
 
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
 
@vzn I have not, but I am not really itching to get into string theory haha
 
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
 
2:20 PM
@Slereah So you do your studying at the craft store, then?
 
Hardware store
big strings
hardest part is finding the supersymmetric string at the shop
 
@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)
 
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
 
2:23 PM
QFT also does
 
vzn
@RyanUnger ("most to nearly all") SUSY is certified noshow in LHC
 
@vzn what? They found nearly half of the particles already!
 
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/…
 
I think I'm less good at jokes than I think I am
I meant that only the superpartners are missing
 
vzn
2:27 PM
@user2723984 lol! ok thx for your humor didnt get it at 1st :)
 
@Slereah I want some exotic string for my wormhole. They were all out of stock.
 
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
 
@Slereah but are you a researcher in some other field? Where do you find the time to self study something else for fun?
 
I use an ancient technique called "doing it at the office instead of working"
hush hush
it's a secret
 
@Slereah I do that with Physics SE :)
 
vzn
2:35 PM
secret o_O
 
lol
that explains it
 
I spent all morning working out dark energy in a coffee cup
 
well if you manage to both self study strings and output research, chapeau :)
@Slereah you could publish a paper out of this
 
I don't work in research, alas
@user2723984 probably not
It's fairly trivial
I just did it in excruciating details for comedic effects
 
but what if it was a mug instead of a coffee cup? (With a hollow handle)
 
2:38 PM
The proof is that it's basically the same independantly of the shape
unless we're assuming some very unpleasant cases
 
so no paper for you
 
not for that, anyway
 
@Slereah Just put it on the arXiv
 
vzn
vixra lol
 
I don't have the clout to post on vixra
 
vzn
2:47 PM
its sometimes peer reviewed. after you publish that is. :P
 
I don't think people on vixra have any peers
 
vzn
peerless :o
 
"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?
 
Going to a friend's birthday this week end
camping and whatnot
 
vzn
3:00 PM
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...
 
beach camping
 
vzn
oh nice too, fire? (was on beach 3yrs ago, fond memories)
 
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
 
3:20 PM
@AaronStevens hammer!
 
this kind of weather is indeed suitable for outdoor activities
 
@Qmechanic =)
 
I don't like drinking alcohol. I just want to play in the wilderness crazily because this concrete jungle is so suffocating.
 
4:03 PM
@JohnRennie ?
 
@AaronStevens I've used the dupehammer to close the question you linked as a duplicate
 
@JohnRennie Ah I see haha. Maybe one day I will have my own hammer
 
@AaronStevens you need a gold tag badge in any of the tags on the question.
 
@JohnRennie Yes, I am aware :)
 
4:56 PM
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?
 
@NovaliumCompany Calculate the pH of your solution and compare to the isoelectric point of TiO2 in the literature. You should be fine.
 
5:16 PM
@JohnRennie When are you back from your SciFi weekend?
 
@EmilioPisanty late Sunday evening. Too late, and I'll be too tired, to do anything on Sunday I'm afraid.
 
cool
I need to be in Birmingham for one or two days the week of the 19
I'm trying to juggle my flights
 
I read that as juggle my fruits
Maybe I should take a course in computer hardware so I can understand these papers better...
 
6:01 PM
@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…
7:07 PM
@Loong Cool. John Rennie already checked the pH of charging TiO2, so it should be fine
 
oh fun, the thunder and lightning + heavy rain just arrived to drop the temperature back down a bit
 
 
3 hours later…
9:55 PM
@Slereah why is there a minus under the root in Goto 2.7 :(
 
Good question
det g is negative but so is g_00
oh wait no it isn't
Because Goto uses the Wrong Signature
 
Yeah he says $g_{00} > 0$
He never mentions the signature
 
Well if a timelike vector is > 0
the wrong one
 
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
 
Why not
g_00 is positive
det g < 0
It has to have a minus sign lest it be negative
 
10:02 PM
Why is $\det g < 0$
 
Lorentz metric?
I mean it's string theory
The worldsheet is timelike
 
Yeah I guess a coordinate transformation to 2D Minkowski space is the (or one) way to see it
 
10:20 PM
No that's stupid GR thinking, $g_{00} > 0$ has to imply $g_{11} < 0$, this needs a proper justification
 
whats happening
 
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
 
10:38 PM
$g_{00}$ 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
 
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 $g_{11} < 0$
 
11:09 PM
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
 

« first day (3191 days earlier)      last day (458 days later) »