The h Bar

rob

00:00

I probably shouldn't have posted this, but I needed a laugh.

heather

00:35

@DanielSank um...

DanielSank

02:03

@heather Yes?

@EmilioPisanty Well, first you see a bear stuffing its paws into a bee-hive and eating the honey. Maybe the bear drops some of the comb on the ground. Later, after the bear leaves, you walk over and pick up the honey comb. Remember, you're a half-starved caveman. So you sniff it and it smells amazing so you take a taste. Holy crap that's delicious!

user685272

02:21

The count down has begun:

This user has been temporarily suspended by a moderator and cannot chat for 1 day 16 hours.

That's t - 40 hours @0celouvskyopoulo7

Till blast-off

:-D

Houston let's not have a problem this time.

DanielSank

03:56

@heather

John Rennie

05:43

@user685272 If I were a suspended user I'm not sure I would appreciate your commentary on my suspension. I think you should refrain from commenting on the situation.

user685272

05:53

@JohnRennie what is it about my "commentary" that you would not appreciate Sir?

I mean, people count down New Year's Eve, no?

This, in my opinion, is a happy occasion that I've looked forward to.

Those that dislike him don't know how to read him.

John Rennie

Colloid science strikes again!

0

A: Why Refill of Gel Pen contains some transparent type liquid?

I'm not sure this is really physics, but since you asked the clear fluid is called the ink follower. The gel is a suspension of pigment particles in an aqueous solution of a polymer. The gel has to have just the right properties - it has to be thick enough to suspend the pigment particles but th...

user685272

Hi @SevenSidedDie

Your avatar is a sevenfacedie ;-)

SevenSidedDie

@user685272 Allo! Yeah, that's the second time someone's made that observation, but I think it's gone over my head… or skimmed my head. Does “side” only apply formally to 2-dimensional objects, or something like that?

user685272

06:08

Yes.

Faces of a cube = 6

SevenSidedDie

@user685272 You mean ~~sides~~ faces of a cube = 6?

John Rennie

Face and side can be used interchangably.

user685272

True dat^

John Rennie

A heptahedron (plural: heptahedra) is a polyhedron having seven sides, or faces.

SevenSidedDie

*fwew* Then I'm in the clear. ;) Mostly the name comes from the RPG-community use describing dice as N sided.

user685272

06:13

Thank you Sir @JohnRennie

:-)

You wouldn't use face for a square

You would use side.

Or triangle

The faces of a cube have 4 sides.

SevenSidedDie

@user685272 This is sensible (and tickles my appreciation for the intersection of semantics and ontology).

Kartik

Hi everyone! Is anyone following this: staff.science.uu.nl/~gadda001/goodtheorist ?

2

user685272

Thanks for sharing @Kartik

Kartik

@user685272 will you follow this plan?

I am thinking of making a slack team for this

user685272

dunno yet

Is Kartik's post worth pinning? @JohnRennie

John Rennie

06:34

I think 't Hooft's web site is a useful resource, but I'm not sure it's world changing.

user685272

"More than rudimentary intelligence is assumed to be present, because ordinary students can master this material only when assisted by patient teachers."

He would make a GREAT AMA guest!

::wishfully thinking::

SBM

06:55

hello

Avantgarde

@SBM yo

user228700

07:12

Hi, everyone :-)

SBM

good afternoon

user228700

Coldplay has released a new song!

user228700

Coldplay - All I Can Think About Is You (Official Lyric Video)

►

John Rennie

That's nice :-)

user228700

@JohnRennie :-) I'm glad to be in a position where I can play it on repeat for several minutes while coloring without worrying about a pile of revision to be done!

John Rennie

07:20

@Kaumudi.H True :-)

I keep thinking how amazing it must be that the several years worth of continual stress has now ended. Though I suppose you quickly get used to it ...

user228700

> you quickly get used to it

user228700

Yes, yes :-) I am, however, attempting to use Kurt Vonnegut's advice to keep the beauty of these free moments alive.

John Rennie

I remember the first day I was at Cambridge. I had collected my room key from the Porter's Lodge and I was walking to my room, through the College gardens, when it suddenly dawned on my how amazing it all was. It felt like a dream. But after a few weeks it was all just a normal part of life.

user228700

Wow, that does sound quite lovely, but yes, unfortunately, we tend to get used to even the most loveliest of things like that. Bummer, really.

John Rennie

For me doing the degree and PhD were undoubtedly the most amazing years of my life. I'm really quite envious you have that time to look forward to :-)

user228700

07:27

Haha, well, I am greatly looking forward to it!

user228700

BTW, I've been reading that book, The Ocean At The End of The Lane and, huh, where is this going..? :-/

John Rennie

How far in are you?

user228700

Only a few tens of pages...

John Rennie

Keep at it - all will become clear :-)

user228700

OK :-) And now I'm going to pre-order Coldplay's New EP!

user228700

07:30

It doesn't look too promising but, well, it's Coldplay!

user228700

Done! And it was only Rs. 60! :-o

John Rennie

Presumably there's a new album on the way, and the EP is tracks taken from the album.

user228700

Some of them, yes. Thank God that there is another one on the way!

user228700

When the new album comes out, I will buy those songs not on the EP separately.

John Rennie

07:36

I generally just wait for the album. I rarely buy an EP or single if the tracks are going to be on an album.

user228700

@JohnRennie Usually, I don't buy either. I wait for the whole album to be uploaded to YouTube so that I can download it illegally :-P This time though, I am quite anxious for this EP; a majority of their most recent songs have failed to remind me of their core essence and I am hoping that this one will. ::Fingers Crossed::

John Rennie

That happens with bands. My all time favourite band, Hawkwind, released a new album a few weeks back and ... well ... it's OK, sort of.

user228700

Huh :-| It sure does suck when that happens and especially for me because I wasn't even around when the songs I actually love were released!

user228700

I love the band apart from their music as well though because they do things like this:

John Rennie

I was around when Hawkwind were in their prime, but that was a loooooooong time ago :-)

user228700

07:41

user228700

And most of their concerts are like this:

user228700

Coldplay - Fix You (One Love Manchester)

►

user228700

I'm sorry about blabbering :-P

user228700

@JohnRennie Haha, I see! :-)

John Rennie

Super Bowl 50 halftime show

The Super Bowl 50 Halftime Show took place on February 7, 2016, at Levi's Stadium in Santa Clara, California as part of Super Bowl 50. It was headlined by the British rock group Coldplay with special guest performers Beyoncé and Bruno Mars, who previously had headlined the Super Bowl XLVII and Super Bowl XLVIII halftime shows, respectively. == Background == Coldplay, Rihanna, and Katy Perry were considered as potential acts for the Super Bowl XLIX halftime show in 2015. Perry was soon confirmed as the headliner of the halftime show in October 2014. In late November, 2015, reports surfaced stating...

That makes for interesting reading for me, though I'm sure it's not news to you.

user228700

07:43

Yep yep, that's the one.

user228700

@JohnRennie Certainly not :-) I've watched that performance over 3 or 4 times now...

John Rennie

I have to go through a report for the next few minutes - high latency!

user228700

OK :-)

John Rennie

I'm back! That took less time than I thought :-)

But now I'm going to make that most essential of all psychoactive drugs - A COFFEE!!!

Slereah

Real math people use amphetamines

John Rennie

07:58

Aaaaaaaaaaaaah :-)

Slereah

sprinkle some meth on it

user685272

How many teaspoons? :P

John Rennie

I think I posted my recipe for John's Creamy Coffee (TM) a while back. Let me search ...

May 14 at 16:34, by John Rennie

@HsMjstyMstdn it's really easy. Put milk in the mug and heat in the microwave until it's nearly but not quite boiling. Sprinkle on instant coffee and it should spread out and form a film on top of the milk. Pour on boiling water. It will froth up and form that swirly pattern.

Slereah

"Magnetohydrodyanmics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring."

John Rennie

easy solvability of the Navier-Stokes equations :-)

Sounds like a modern version of The Devil's Dictionary

2

user685272

08:17

A contradictionary :P

Mithrandir24601

@JohnRennie I can still remember walking from the train station to the college just being like "I can't believe this is actually happening" :D Our happy notions were quickly shot down by the admissions tutor telling us all things like the next three years are going to be unbelievably hard and "you're all average" and the terrifying "I know the person who could have taken your place". Was one of the most useful things I've ever been told.

I then brought my (Indian) friend to visit near Christmas time. He couldn't believe he was actually there :)

John Rennie

That's a really awful thing for the admissions tutor to say. What a killjoy. The truth that they should have said is that it's going to be unbelievably enjoyable.

user685272

How is fear useful for learning in the long run? @Mithrandir24601

John Rennie

I didn't think university was hard work. Looking back it was. In fact Cambridge specialises in working the arse off students, though I didn't realise this until I compared my workload with friends at other universities. But at the time it didn't seem like hard work - it was just what you did.

Mithrandir24601

@user685272 It wasn't that - it was the awareness that we were lucky to be there and the forewarning that it was going to be unbelievably tough

I'm still in a bit of shock that I've got evenings and weekends (for the most part) off :P I'm actually going home for a holiday in a little over a week

Slereah

08:25

0

Q: Under what circumstances is the functional derivative an actual function

In general, for a functional $F[\phi]$, the functional derivative is $$\frac{\delta F[\phi]}{\delta \phi} [f(x)] = \lim_{\varepsilon \to 0} \frac{F[\phi + \varepsilon f ] - F[\phi]}{\varepsilon}$$ which is itself a distribution. But for quite a wide class of action functionals, the functional ...

halp

Mithrandir24601

@JohnRennie They did that as well - this was a good 20 minutes/half hour of talking and I'm just puiling out a couple of points to demonstrate how he brought us back down the Earth :P I believe the reason he says that is so that people don't crash too much within the first week and quit

(because, yeah, I know someone who did that :/ )

John Rennie

Now you mention it I remember we got warned that the first few weeks would seem intimidating.

Mithrandir24601

It would have been great to hear the whole "congrats on getting here. You've done a really good job" etc. for the whole talk, but once work started, if I didn't hear the rest of it, I would have just been thinking "there's no way I can do this and I'm pathetic" much more

Slereah

@ACuriousMind Is one of the string theory CFT a spooky phantom field

ACuriousMind

I don't know what a "phantom field" is

Slereah

08:39

Since by the signature of the target space metric one of them has a negative kinetic term

Phantom field is a field with negative kinetic term

like some kind of sinister spirit

ACuriousMind

Ah, well, I think you're obviously correct. :P

Slereah

Also I'm guessing that if the target space isn't flat we can consider that to be field interactions for the worldsheet CFT

Possibly weird nonpolynomial ones since the terms would be like $g_{\mu\nu}(X) X^\mu X^\nu$

ACuriousMind

@Slereah Yes, although note that the $g_{\mu\nu}$ itself depends on $X$. The standard lore is to write $g_{\mu\nu} = \eta_{\mu\nu} + \chi_{\mu\nu}(X)$ as a perturbation of flat space, then identifying the extra terms involving $\chi_{\mu\nu}$ as the $X_\mu$ coupling to their own graviton modes.

Slereah

yeah that makes sense

ACuriousMind

This leads to the lore "String theory in curved space is the same as string theory in flat space with a "vacuum state" that contains a coherent graviton state".

Slereah

08:49

Is there much benefit to doing string theory in non-flat space?

Also does it always conserve conformal invariance?

ACuriousMind

@Slereah Well, technically, all of the compactification scenarios are "string theory in non-flat space"

Slereah

I wouldn't call a torus non-flat

ACuriousMind

No one except for toy models compactifies on tori

Slereah

but yeah I bet the Calabi Yau isn't too flat

ACuriousMind

exactly

Slereah

08:51

does any metric preserve the conformal invariance of the action?

Or do you need it conformally flat maybe

or something

ACuriousMind

If you go back to the proof that the Polyakov action has conformal invariance, it does not involve the flatness of the target metric at all - since it's a symmetry on the worldsheet, it doesn't do anything with the target metric to begin with

Slereah

Alright

Mithrandir24601

YES! :D science.sciencemag.org/content/356/6343/1140 Just in time for my essay deadline! :D

ACuriousMind

What happens is that non-flat metrics spoil the global Poincaré invariance, and you need special manifolds to preserve at least a bit of supersymmetry - that's where the Calabi-Yaus come from.

Slereah

Don't all manifolds preserve supersymmetry?

I guess you need spin manifolds, at least

ACuriousMind

08:55

You need one with a Killing spinor.

John Rennie

How do you get the times symbol in a circle in MathJax?

user685272

You have to write an essay on that? @Mithrandir24601

ACuriousMind

Turns out, this is basically equivalent to having special holonomy

@JohnRennie You mean \otimes?

Slereah

Or bigotimes

if you like it big

Mithrandir24601

@user685272 I have to write an essay on Quantum Key Distribution/Quantum Communications in space, so yes :)

Slereah

08:56

$$\bigotimes^\otimes_\otimes$$

John Rennie

@ACuriousMind Bing! Thanks :-)

ACuriousMind

It's the tensor product sign, btw

John Rennie

Though all I was doing was tidying up that nonsensical question you just closed, so it's no great contribution to mankind.

I did Google mathjax tensor product but without any immediately obvious hits ...

ACuriousMind

It might well be a question for English Language & Usage, given that it asks about a gerund state ;)

user685272

One small step for man...

Slereah

08:59

Try LATEX maybe :p

John Rennie

I've just seen one in gerund state :-))))

ACuriousMind

@JohnRennie Detexify is very useful.

John Rennie

Gerunds

Though that's typically British humour and I'm not sure how well it translates

Qmechanic

@ACuriousMind : Fuk, I'll do it.

user685272

"Fuk"?

Qmechanic

09:02

Fukaya category $Fuk$.

user685272

FCUK

John Rennie

@user685272 Check the original post

ACuriousMind

@JohnRennie I have no idea what I just read.

user685272

Thanks @JohnRennie

Emilio Pisanty

Have folks seen this? Great paper? Swipe right on the new ‘Tinder for preprints’ app

2

John Rennie

09:04

@ACuriousMind I feared it might be too culturally specific :-) I loved the Molesworth books as a boy but unless you've been exposed to the British public school system they are probably rather mystifying.

@EmilioPisanty Excellent :-)

At last a truly useful app!

Secret

Perhaps we should have a similar idea so that we can find out what type of questions the PSE community interested the most, that will shed more light on the homework policy thingy

ACuriousMind

@EmilioPisanty I rate this "exciting and questionable".

Secret

When it comes to what is interesting, often first impression driven by the intuitive emotions is IMO, more representative [citation needed]

user685272

Good idea @Secret

Emilio Pisanty

@ACuriousMind $\Huge{😂}$

John Rennie

09:08

:: John slumps into a coma at the mention of the homework policy ::

Slereah

If u want something exciting

How about the book written by me and 0celo7

It will br gr8

user685272

::calls 911::

Start working on it on his blog @Slereah

Slereah

I might do a little site for it yeah

user685272

Have you finished Penrose's book yet?

John Rennie

Ooh the postman has just delivered my new SSD. Right, that's the end of work for today!

user685272

09:16

SSD?

@ACuriousMind so you're not in favour of activating the "hbar Tender" bot made by Hippa?

ACuriousMind

@user685272 What?

I don't really care much about that idea either way - if people here want it, it's fine with me. Since it's been almost half a year since we first discussed that and nothing much has happened I'm guessing most share my apathy :P

user685272

true dat

John Rennie

I applied to join the British Apathy Society, but apparently making the effort to apply disqualifies you.

user685272

Baa dum tss

Mithrandir24601

09:33

@JohnRennie oh.

user685272

m.9gag.com/gag/amX1Q74/… @JohnRennie

The British love to put "Royal" in front of everything :P

bolbteppa

gr8 was the name of a gr conference proceedings i was trying to find yesterday

@Qmechanic I've been trying to flesh out your answer here physics.stackexchange.com/a/161810/25851 but the way it was originally proved, any thoughts on this: math.stackexchange.com/q/2324347/82615 e.g. flaws, corrections?

Secret

10:04

It might be possible to join the British Apathy Party by applying for it, and then erasing all trace of making the effort to apply :P

Physics Meta

10:19

-2

Q: Cat climbing rope

A cat of mass 1 kg moves up on an massless string which goes along a friction less pulley so as to just lift a block of mass 2 kg.After sometime the cat stops moving with respect to the string .The magnitude of change in cats acceleration is?

discussion support feature-request bug homework

Secret

10:29

2 days ago, by Bernardo Meurer

@TerryBollinger It bothers me the amount of work done in proprietary malware such as Mathematica or Matlab. I think people should always try to use Octave and OS or Free alternatives. However I feel like more and more physicists move to free themselves from the shackles of proprietary software. They write their thesis in LaTeX, put source on GitHub, use GNUPlot or python for their graphs, and so on. It's getting better, physicists are open minded when it comes to software I think

I have been to conferences of both Mathematica and Matlab. IMO, Matlab is a lot more humble than Mathematica. It recognises the importance of cross platform compatibility with popular languages in physics and eingeering such as Python and a lot of recent developments in 2015 focuses on interfacing its code with those applications for various mobile application development.

In contrast, Mathematica idea of compatibility is basically saying that "my code is the best, and thus everyone is going to convert into our code". All demonstrations of software compatibility is focused more on how to c…

Having said that, I can see how Octave is pretty much on par with Matlab (and it even has initerfacing with matlab as part of its core philosophy), thus I am not worried

However, in order for a freeware to surpass mathematica, they have to be able to do at least two things:

1. Abstract algebra symbolic definitions and evaluations
2. Diverse data formats including pictures, 3D shapes models etc.
3. Massive database where it fetch results from that is used in the wolfram alpha engine

meanwhile speaking about open source, there's sage maths

ACuriousMind

10:49

Hm, I have a paper that claims that one can get a Lie algebra structure on the space of 2-forms on a 4-manifold by the pointwise isomorphism $\wedge^2 \mathbb{R}^4 \cong \mathfrak{so}(4)$, but I don't see how that's supposed to define a Lie bracket on the l.h.s. at all.

I mean, sure, if one fixes a basis of the l.h.s. one can define a bracket, but there's no obviously choice for such a basis at all, given that the isomorphism of the space of 2-forms at a point and $\wedge^2\mathbb{R}^4$ is wholly uncanonical.

Balarka Sen

11:24

@ACuriousMind Right, lots of easy dense R's out there.

I suppose you'd find a torus subgroup of SO(3) ("maximal torus"? Is this the right word?) and take a dense irrational line in it.

No, take that back. That just gives a non-closed R, not a dense one.

ACuriousMind

The maximal torus of SO(3) is just a circle, though.

Balarka Sen

Hm, oh

I am bad at this, sorry. SO(3) is rank 1.

ACuriousMind

I'm not sure I'm claiming there is such a dense subgroup, just that the idea gives you a subgroup that's not closed.

That was what this was originally about, right?

Balarka Sen

I guess, yeah.

I suspect if you think about SO(3) as the isometry group of S^2, and then look at the 1-parameter group of rotations which move a specific point around the loxodrome it's passing though, then it would give an R subgroup.

Or something. Best to ignore me completely :)

Balarka Sen

11:44

@Slereah i prefer speedballs

though you probably won't be doing math on those

John Rennie

It took me a moment to work out what a loaded pimeson was. I'm still not clear why it should be loaded.

ACuriousMind

@JohnRennie Pretty sure it's meant to be "charged".

E.g. in German, the word for "charged" is the same as the past participle of "to load", so someone knowing load but not charge would translate it as "loaded" instead of "charged".

The occurence of "formel" (German for formula) in the question text reaffirms my suspicion this is a German

John Rennie

Aha! :-) Yes, the lifetime quoted is for the charged pions so that fits.

Secret

Let me google something...

List of polyglots

A polyglot is a person with a command of many languages. A polyglot may also be called a multilingual person; the label "multilingual" is used for communities as well as individual speakers. Richard Hudson, professor emeritus of linguistics at University College London, coined the term "hyperpolyglot" for a person who can speak twelve or more languages fluently. Other scholars apply the label to speakers of even more languages: twelve, sixteen, or in the most extreme cases, even fifty or more. It is difficult to judge which individuals are polyglots, as there is no uncontroversial definition for...

I am, however, not sure if there exists a universal solution to "lost in translation"

Emilio Pisanty

12:04

@JohnRennie oh

my initial association was to Pimms plus Jameson

John Rennie

Pimms and whisky!!

That's ... words fail me :-)

Secret

Free association (psychology)

Free association is a technique used in psychoanalysis (and also in psychodynamic theory) which was originally devised by Sigmund Freud out of the hypnotic method of his mentor and colleague, Josef Breuer. Freud described it as such: "The importance of free association is that the patients spoke for themselves, rather than repeating the ideas of the analyst; they work through their own material, rather than parroting another's suggestions". == Origins == Freud developed the technique as an alternative to hypnosis, because he perceived the latter as subjected to more fallibility, and becau...

Isn't free association a wonderful and highly unpredictable thing...? :P

Balarka Sen

12:21

jung people are easily fruedened

Secret

well, we all have that to some extent

user228700

12:50

@JohnRennie Dang it, that looks so good.

Secret

1

Q: (No) Interaction dependence on derivatives of the field

The following question came to me reading a paper about different quantization methods in a QFT. You want to get field operators that satisfy (in a scalar case) the KGE and therefore to form a Lagangian density of the form: $$ \mathcal{L}(x)=\mathcal{L}_1(x) + \mathcal{L}_2(x) + \mathcal{L}_{...

quantum-field-theory second-quantization

Magnetic fields are the first thing that sprung to mind. After all you need to be in some moving frame to even feel it

John Rennie

13:37

@Kaumudi.H I must have been suffering coffee withdrawal symptoms because I enjoyed that coffee immensely :-)

user228700

Wow, I see!

user228700

What the heck is the correct way to down energy drinks? In one go or little by little? :-/

John Rennie

@Kaumudi.H Does it make any difference? I doubt your digestive system will care either way. I'd go for whatever your prefer.

user228700

Hmm.

user228700

I don't know why I buy these things. They taste like crap but look fancy shmancy and just the teeny bit... adulty, for lack of a better term.

John Rennie

13:48

I should note at this point that I'm deeply sceptical about the concept of an energy drink. Any drink containing sugar is an energy drink. Drink coffee with sugar rather than spending lots of money on energy drinks.

@Kaumudi.H anything with zero calories is not an energy drink by definition :-)

user228700

No sugar added in this product is what it says on the back.

John Rennie

The calorie is a unit of energy ...

user228700

Right, right :-)

John Rennie

So something with zero calories ...

Bernardo Meurer

@Secret I have no interest in Freeware. I have interest in Free software, free as in Freedom. Being Gratis is not enough. I don't care how much the software costs, I care that it respects my freedom.

user228700

13:50

@JohnRennie I GET IT :-P

ACuriousMind

@Kaumudi.H Uh, "energy drink" for me denotes a beverage with much caffeine and an absolutely unnatural taste, not sugar-free coke :P

user228700

It does have much caffeine.

John Rennie

It probably contains a mild stimulant to justify its claim. Like I say - drink coffee :-)

Bernardo Meurer

@ACuriousMind You get used to the taste of energy drinks :P

@JohnRennie Programming final today :P

ACuriousMind

@Kaumudi.H I know - all coke does - but coke specifically is not an "energy drink" to me

John Rennie

13:51

@BernardoMeurer A written exam? Good luck!!!

Balarka Sen

@ACuriousMind coke is great stuff

ACuriousMind

Not that sort of coke, you drughead :P

Balarka Sen

<3

Bernardo Meurer

@JohnRennie Yep, written

My biggest challenge will be my handwriting, lol

ACuriousMind

@BernardoMeurer I don't think I want to get used to it

Bernardo Meurer

13:52

I have trouble with capitalization

@ACuriousMind ::shrug emoji::

user228700

So OK, according to the back of this can, I just had 32.5 mg of Na with a little caffeine.

user228700

And literally nothing else. It says 0g for everything else.

user228700

@BernardoMeurer Good luck! :-)

Bernardo Meurer

@Kaumudi.H Thanks!

user228700

Hang on, do people still use quotation marks for emphasis?

Balarka Sen

13:55

"i use it for sarcasm"

user228700

Neat edit :-P

user228700

Never use quotation marks for emphasis!

Transcript for