The h Bar - 2018-01-22 (page 1 of 2)

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12:52 AM

@BalarkaSen winner winner chicken dinner

4 hours later…

Captain Bohemian

4:37 AM

@Slereah are superconformal field theory and conformal field theory on superspace the same?

5:22 AM

::laughs maniacally:: what if mass is negative hehehehe

1 hour later…

user228700

6:46 AM

Hello, everyone :-)

user228700

@JohnR: Are you around?

@0celo7 then u'll have to prove the negative mass theorem like the nerd u are

7:12 AM

@Kaumudi.H yes, sorry for the slow response - dealing with wayward servers.

@Kaumudi.H what's up?

user228700

Ah, right.

user228700

@JohnRennie Hangouts?

7:43 AM

Hi guys, today I encountered a very strange Heisenbug that behave kinda quantumly

So the bug (whose origin is still unknown) result in some files in a series of files in a concatenation to be placed in the wrong place, such as file number 3-5 is being replaced by the entry 1-7 and 6-2 being replaced by 2-4

Some tracing found that the original files of 3-5 actually contains content for 1-7 for unknown reasons (similarly for 6-2)

Now, here's the quantumly bit: When I correct those two faulty entries and rerun the concatenation code again, now file 1-7 disappears even though I never touched its original copy!

so to further this quantum joke, somehow files 1-7 and 3-5 became entangled with the observable of existence in an anticorrelated manner. Still trying to figure out why

(and yes you guessed it: 2-4 disappeared)

Ok, some further troubleshooting I can explain the disappearance of 1-7 and 2-4: They are created by a different geometry script. As a result their corresponding entries is missing the line of atomic numbers, therefore, the molecule software failed to read those molecules in due to being the wrong xyz file format

Still checking the reason for the 1st though

8:14 AM

@0celo7 The whole dlmf site is 3.07GB (not including the 4665 nonexistant files that the site claimed to have)

9:02 AM

user image

even the table of content is awful

9:31 AM

I think the answer misinterpret the question?
https://physics.stackexchange.com/questions/381390/photon-thought-experiment

"an element α ∈ πr+1(φ) can be killed by surgery"

Brutal

10:03 AM

@0celo7 yeaaaaah boi

@BalarkaSen is there a special name for gluing a manifold to itself

I can only find informations for gluing two different manifolds

can you be more specific

you can glue anything to anything

Take a manifold, take two shapes $S_1, S_2$, remove them $M \setminus (S_1 \cup S_2)$, get some diffeomorphism between the boundaries $h : \partial S_1 \to \partial S_2$

There's no name for this that I know.

is there a book that discusses the topic

10:11 AM

You're adding a tube at $\partial S_1 \sqcup \partial S_2$

p. much

Though not necessarily a tube

@Slereah I don't see why a book would discuss this specific construction

0

A: Can the mouths of a worm-hole be torus shaped?

A wormhole with a torus-shaped mouth is a perfectly well-defined spacetime, yes. Via the usual cut-and-paste construction method, you can do the following : Take a copy of $\Bbb R^{n-1}$, remove two non-intersecting tori $T_1, T_2$. This gives you the manifold with boundaries $$M = \Bbb R^{n-1}...

^because this

I seem to recall it is used in the classification of $2$-manifolds

which is just $S^2$ minus a bunch of disks and then identified

Not in any proof that I know of

It's just a convenient construction

yeah, just wanna check if it's a manifold indeed

10:17 AM

Depends on what you are gluing

I guess it's probably not too hard to adapt the 2 manifold proof

What you did is a manifold, for sure

Locally it's gluing a half-ball to another half-ball

"it’s a structure where the neighborhoods of points are locally homeomorphic to open spheres in some ℜn"

Oh no

Fraktur

Anonymous

10:36 AM

@Slereah How to shift to next line in gnuplot (without the current line being executed)? I need to write something like this:

Anonymous

plot "print_1012720" using 1:2 title "Flow 1", \
"print_1058167" using 1:2 title "Flow 2", \
"print_193548" using 1:2 title "Flow 3", \
"print_401125" using 1:2 title "Flow 4", \
"print_401275" using 1:2 title "Flow 5", \
"print_401276" using 1:2 title "Flow 6"

Anonymous

There should be some shortcut key I guess

I do not know

Anonymous

Oh, okay no probs

Anonymous

Oh it seems just typing the next command without the plot variable is enough

Anonymous

10:42 AM

I don't need to do anything extra other than placing a `,\` at the end of each line

Anonymous

Phew

Is it true that a vector under a transformation of the metric does not change? Or how does a vector transform under $g_{\mu\nu} \rightarrow g_{\mu\nu} + \alpha h_{\mu\nu}$?

What do you mean by "a change of the metric"

Do you mean a change of coordinates?

Do you mean that you're changing the metric itself or the coordinates

10:46 AM

(obviously a change of coordinates isn't going to change the vector)

if you mean the former, then yes, the vector only depends on the coordinates

well, the vector doesn't change, but its components do

From the metric you give I wonder if you're considering gravitational waves. If so that is a real change and the vector will change.

wot

I think the issue is what do you mean by a vector

If you just mean a vector field without any further specification, then yes, it will not change

If you mean a physical vector field like the EM field, then it may

are invariant interval of spacetime still invariant among accelerating frames?

What do you call an interval

Do you just mean $ds$

And are we in flat space

10:56 AM

sorry for confusion, I mean in flat space, & when I say interval I mean $c^2dt^2-dx^2$

Then no, it is not invariant.

You will get another metric in an accelerating frame.

@Slereah I see. thanks for the help. It is a shame I did not study relativity in depth :/

The big book of special relativity with stuff on all topics is "Special relativity in general frames" by Gourgoulhon, if you're interested

he goes into accelerated frames

@Slereah Thanks, I have just bought the hard cover reprint of Gravitation by MTW, Not sure if it is good place to start learning these stuffs?

@Shing MTW is quite a hard book.

Accelerated motion is dealt with in chapter 6, but there are easier approaches to it in other books.

Obviously the geometry of spacetime is the same whether you're accelerating or not, but using an accelerated frame involves a change of coordinates so you're writing the same metric in different coordinates. For constant acceleration the appropriate coordinates are the Rindler coordinates.

If you search the site for rindler metric I've mentioned them lots of times in answers

11:14 AM

@JohnRennie thanks! is it a good place to learn tensor? I always have hard time calculating tensors.

11:27 AM

I have yet to find a clean way to superimpose all possible frame of references onto a single diagram

11:39 AM

user image

hahahahahahahahahahahaha

:dies:

:-D

X_X

12:07 PM

@Shing MTW is probably not a good starting point

Although good idea to buy the hardcover, get it while it's still cheap :p

12:30 PM

I am super terrible at dealing with things at different places, e.g. working with cross sections and working one frame at a time

But on the other hand and paradoxically, I cannot handle analysis as well as algebra because too many things are happening at the same time

Perhaps, I thrive well in a spacetime where space is highly nonlocal but time is discrete

link.springer.com/article/10.1140/epjp/i2018-11841-3

lol I always like those google searches inspired from these free association events

Hi @ACuriousMind

12:59 PM

why does your math.se profile not exist? @JohnDoe

@user685252 It does exist...

Not when I click on it.

@user685252 Oh okay, not sure, it does when I click on it.

odd...

@JohnDoe Is this you?

71 5

improper-integrals poisson-distribution probability-distributions special-functions

1:05 PM

@JohnRennie Yeah

That link works.

Weird. The link in the chat profile points to https://math.stackexchange.com/users/100431/john-doe

That's what happens when you delete your profile but not the chat account :P

@ACuriousMind If you have a chance please see my post.

Only on mobile, will have a look this evening

1:08 PM

@Slereah I see you’re peddling my book recs

Not like anyone listens to them

@ACuriousMind Thanks much appreciated.

He's a jet setting executive now, always on the move :P

@0celo7 tbh it's pretty much my only SR book

Except for that French one

\o @Blue

Anonymous

Hi

1:16 PM

@0celo7 what do you recommend for an introduction to special relativity?

Hour hol hin (autocorrect lol)

Eric Gargling

Eric Gargamel

1:44 PM

@Slereah what book is that

that is Lee

@Slereah kek

really?

lemme look

huh

you need a PhD to read that chapter plan

just read the whole thing

it's truly one of the better books out there

30 mins ago, by user685252

@0celo7 what do you recommend for an introduction to special relativity?

without autocorrect :P

Anonymous

1:47 PM

@user685252 On his behalf

really?

that's fucking libel

for an introduction?

(it's a joke)

(it's a book by a crazy man)

Say, for a complete beginner novice @0celo7 with only high school algebra

1:56 PM

Someone only knowing high school algebra should not learn relativity

rekt

you might need some calculus, for a start

Jesus the proofs of the classification of surfaces

Anonymous

@user685252 For a basic course just check out Shankar's relativity lectures. It is aimed at medical students, so it will suit your purpose. (You'll need calculus and algebra for that)

so much Morse theory

@Slereah There are combinatorial proofs

why do medical students need relativity

1:58 PM

None of the proofs of classification of surfaces is trivial unfortunately

Anonymous

@Slereah Even Shankar doesn't know. He mentioned that in the lecture :P

Is it for medical physicists???

I don't know

It's the most popular topic in physics

By far.

No love for metrology

user image

@Slereah the best proof is probably whatever is in Hirsch

Likely Something Morsey

2:04 PM

I think they call that climate science

Combinatorics is BS because you have to prove these things are triangulable, which no one wants to do

~~climate~~ atmospheric science

Anonymous

@user685252 Metrologists would not spare you for comparing them to the petty climate guys.

Anonymous

Given that they are already suffer from identity crisis :p

ya, sorry

Anonymous

2:11 PM

@user685252 Uh, btw are you sure you're not confusing meteorology and metrology ?

Oops, you're right.

thnx

2:43 PM

@0celo7 It's not hard.

A consequence of Jordan-Schoenflies

I recently understood the idea of the proof, it's very beautiful

>he recently understood the proof 10 years later

Sounds easy

I can tell you the idea if you want

plz do

Let's think generally. $M$ is a topological manifold and $U$ is a chart in $M$

thinks generally

Proceed.

2:55 PM

Chemistry: The optimisation program somehow create an impossible molecule that crashes any molecule viewer as it is loaded

There is a chart homeomorphism $\varphi : U \to \Bbb R^n$, so you can triangulate a ball $B \subset \Bbb R^n$ of finite radius by $n$-simplices, and pull it back to get a ball $A = \varphi^{-1}(B)$ inside $U$ which is triangulated.

Locally there is NO obstruction to getting a triangulation. Problem is we need to get a triangulation on all of $M$

So, let's say $V$ is another chart in $M$ such that $U \cap V \neq \emptyset$

By making $B$ to be of sufficiently large radius, we can assume $A = \varphi^{-1}(B)$ intersects $V$ too

$V$ has another chart homeomrphism $\psi : V \to \Bbb R^n$. Using that, we get $\psi(A \cap V) \subset \Bbb R^n$ which is a region on $\Bbb R^n$

$A$ is triangulated, so this region is triangulated too (picture: think of it like an unbounded region, a paraboloid with it's interior triangulated by stuff)

The "naive" approach would be to extend this triangulation on this region to all of $V$, by gluing triangles to the faces of the simplices of $\psi(A \cap V)$ that's on the boundary of the region

@BalarkaSen “not hard” followed by “Jordan Schoenflies” is something only a topologist would think to say

If you could do this, the proof approach would be this: Suppose $M$ is compact for now. Let $U_1, \cdots, U_n$ be an open cover by charts. Let $V_k = \bigcup_{i = 1}^k U_i$ be the union of $k$ of these charts.

Inductively extend a triangulation on $V_{k-1}$ to $V_k$

Therefore you have this tower $V_1 \subset V_2 \subset \cdots \subset V_n = M$ exhausting $M$, and you extend triangulation at each stage - end result would be a triangulated $M$

BUT

@BalarkaSen This step is wrong. Homeomorphic image of triangles can be so bad that "other side of a face" would not make sense. Case in point: Alexander's horned sphere. That's a homeomorphic image of a 2-sphere such that other side of the sphere is not simply connected.

"there exist topological manifolds that admit no smooth structures at all."

which ones

But this CAN be done for 2-manifolds. The reason is Jordan-Schoenflies, which says homeomorphic image of circles in R^2 always have "standard" sides

Nothing can go wrong in 2 dimensions

The proof pushes through and you get a triangulation on $M$

3:15 PM

John Lee defines two examples of manifolds

$\Bbb R^n$ and finite dimensional vector spaces

two very different examples

3

A: Manifold that is NOT smooth

In light of OP's response, here is a construction borrowed from my class notes. However, to show that this manifold does not admit any smooth structure will not be discussed in my answer. The interested reader is welcome to check the other class note taken by Prof. Somanth Basu. Kevaire claimed...

I'm not reading all that shit

Worst manifold

It's not easy to construct such manifolds

Or at least prove it works

Can't we just make a $1$-dimensional one

I am not interested in nonHausdorff nonparacompact dudes

Please don't make me think about it

It's tempting

Let's consider the long line, excent infinite dimensional and non-Hausdorff

its very long

the long Hilbert space

Future Historian

Excuse me.

Does anyone know how to calculate the magnetic field of a planet?

As in: the formula?

3:29 PM

I'm guessing there is no simple formula

It's all magnetohydrodynamic models or whatever

Future Historian

So.........if a planet has 2.3 Earth masses and 1.34 Earth radii, what can that tell me about magnetic field strength?

not much

the magnetic field depends on the internal composition

@BalarkaSen lmao the prof can’t do the problem either

Future Historian

Density of the planet.

5.253 g/cm^3.

So.......Earth-like core, right?

maybe?

if you want a quick and dirty formula I think the field is basically gonna be inversely proportional to the radius

Since the magnetic field is that way

but it depends on many things

Future Historian

3:41 PM

So......given that.......I am guessing about 74.01 microteslas, right?

@Slereah. That is what I found.

@0celo7 rekt in rip

Observer effect (information technology)

In information technology, the observer effect is the impact on the behaviour of a computer process caused by the act of observing the process while it is running. This effect is a manifestation of the uncertainty principle in information technology. The uncertainty principle is attributed to Werner Heisenberg and was originally referring to quantum mechanics. For example: if a process uses a log file to record its progress, the process could slow down. Furthermore, the act of viewing the file while the process is running could cause an I/O error in the process, which could, in turn, cause it to...

O great, first the Lockdown Trio, and now the 8 troublemakers. Looks like h bar really need to be sent into a black hole today

Future Historian

Basically, Earth's magnetic field = 30.76 microteslas * radius^3 = [insert magnetic field strength here].

who are the lockdown trio

Future Historian

Right?

3:44 PM

i want to understand this anime plot

in Mathematics, 13 mins ago, by Maneesh Narayanan

in Room for Maneesh Narayanan and BAYMAX, 23 mins ago, by Maneesh Narayanan

@BAYMAX https://math.stackexchange.com/questions/2616224/what-is-the-guarentee-of-exista‌nce-of-the-function-t-phit-in-the-proof

The identity of the Lockdown Trio is obvious, it is when the h bar becomes the least responsive to users outside the 3

What we have said earlier reveal some hints that the leader is not Balarka, your task is to figure out who is the leader

Hint: Neutrino

Future Historian

Oh wait.

facepalms

Wrong equation.

@FutureHistorian Square only

Future Historian

Oh.

@Secret And who are the eight troublemakers?

3:48 PM

That's a hyperbola red herring

(and I spelt that wrongly...)

Future Historian

@Slereah? As in: radius^2 instead of radius^3?

yes

Future Historian

Oh.

A higgs-bugson[15][16] (named for the Higgs boson particle) is a bug that is predicted to exist based upon other observed conditions (most commonly, vaguely related log entries and anecdotal user reports) but is difficult, if not impossible, to artificially reproduce in a development or test environment. May also be used to reference a bug that is obvious in the code (mathematically proven), but which cannot be seen in execution (yet difficult or impossible to actually find in existence).

Future Historian

Well, what I got was 3.774 Gwb according to WolframAlpha.

How do I convert that to microteslas?

Anonymous

3:59 PM

@FutureHistorian Weber is the unit of magnetic flux while Tesla is the unit of flux density.

Future Historian

So, how do I calculate the flux density?

flux per m²

Future Historian

I am trying to calculate the magnetic field strength of Kepler 442b.

@Slereah. Formula being....?

here's the answer : we don't know

Future Historian

O_O

4:01 PM

We have no ground to estimate its magnetic field

Future Historian

Well, that is bad.

Anonymous

@FutureHistorian That won't be a very easy guess.

I mean, it's probably inferior to a neutron star

and superior to a piece of wood

Future Historian

I have the planet's mass at 2.3 Earth masses and 1.34 Earth radii.

but beyond that

Future Historian

4:01 PM

I just want to know how powerful its magnetic field is relative to Earth's.

Anonymous

@FutureHistorian Those two parameters are far less that sufficient to estimate magnetic field

Future Historian

So, what parameters DO I need?

Anonymous

You'll need more data about the interior of the planet

Also we're not even 100% sure why Earth has a magnetic field, really

Future Historian

Hmmmmm. Well, it does have a higher density than Earth, at around 5,253 kg/m^3, so that should give away the planet's composition.

Internally.

4:03 PM

All the other telluric planets have basically no magnetic field

the internal temperature is also important

Future Historian

Comparison: Earth's density is 5,510 kg/m^3.

So.....slightly more dense than our planet.

So, 1.04892442 times more dense than Earth.

Anonymous

Density really tells you nothing much about magnetic field. It could be a piece of rock with solidified core, or it might be rotating too slowly for the core to have any effect.

Future Historian

How fast does OUR core rotate?

Anonymous

If you really want to estimate, learn the necessary physics (namely astrophysics) of planets. I don't know enough to give any accurate or even an estimate for Kepler 422b.

Anonymous

@FutureHistorian Look it up on the net. I don't have it memorized

Future Historian

4:08 PM

Oh.

@JohnRennie holy fook RAM prices

Future Historian

I am already working on that.

That is why I asked, just in case I failed to find anything useful.

guys I have question about academic titles

Anonymous

@2physics "Don't ask about asking, just ask."

If you are not a student and conduct researches as the member of the research group of a professor, but you are not officially hired or paid, can you mention it as Research Assistantship in your CV?

4:12 PM

I would ask the professor in question

they're the person who would be vouching for you

@Blue :D liked that quote

Anonymous

@2physics Well, most people do that, at least at my university. Since you are helping the professor in research that should be okay. But ask them once.

@0celo7 the professor does not have any problem with that

@2physics then what is the issue

@0celo7 the issue is that I wanted to be honest and know the exact term

Anonymous

4:15 PM

If it's not official then just "research intern" would be fine I guess. I've seen my seniors use that term on their CV. Research assistants are usually the ones who are paid (but that may vary from place to place).

I'd say you should really just write there what your prof tells you to write

@0celo7 fun fact: apparently there are in-memory database systems with TBs of RAM

@Blue@0celo7: another question is that should your researches necessarily result in publications? In some circumstances I just have investigated something extensively studied it and also achieved some results but didn't think they are complete considerable for publication..

@ACuriousMind I wanted to get 32GB but it would cost more than a new CPU!

@2physics Well, publications are always nice. Whether any specific thing qualifies for publication depends on your field; again, ask your prof

hi

4:23 PM

@0celo7 what use could you possibly have for so much memory unless you're doing video editing or something similar?

Anonymous

@2physics I would suppose it's helpful but not necessary.

I have 8 GB and I very rarely use all of it

@ACuriousMind I am usually at 10 but i want to do Nintendo emulation

like the big console games?

like wii?

You're usually at 10? What is that, 200 browser tabs? :P

4:27 PM

@ACuriousMind 50

and 20 pdfs

Optimize your work flow instead of buying ram :P

I have 16 so 10 is no issue

But Wii emulation needs 32

Okay, sure, emulating modern consoles needs resources

@0celo7 I only ever buy memory second hand on eBay, so I'm not fussed what the current market rate is ...

@ACuriousMind thanks. my professors doesn't care about how it should be called, research assistant, research intern, visiting scholar, assistantship, or whatever. I know publications are nice, lol actually I'm applying to the graduate programs of some US universities and a professor contacted me and asked about what I've done after graduation(I've been graduated in 2015) because he thinks it;s not clear in my CV, he says. I have been working / looking for jobs/

@ACuriousMind and for a period of time I just studied papers and attended some online courses and pursued some researches on my own! also I was in touch with professors and academic environment but I have not that much publications. that's why I'm not sure how I can explain it on my CV..

4:34 PM

@JohnRennie I eagerly await the cyberpunk future where we literally can buy second-hand memories ;)

@2physics well, if there is nothing formal to put into your CV, then that would be something to address in the cover letter

@ACuriousMind alright... and what is your idea about the question of how to call unofficial research assistantship? (suppose professors doesn't care how to call it, but they can confirm that you were a part of their research group)

@ACuriousMind I'll sell my memories of food I have eaten, then buy shares in Pepto-Bismol.

@2physics ugh, I'd probably just call it "research experience"

Anonymous

@JohnRennie Sssh, you should never reveal such great business plans in public. :D I've already stolen it

@Blue btw "asking about asking is asking also" lol

@ACuriousMind yea, Thanks

4:47 PM

@Blue :-)

@JohnRennie I suspect that will be a very tough market competing against all the people eating professional chefs' creations...

Anonymous

Those will cost more

Anonymous

JR can just keep the price at 1/5th of that and reach a larger user-base

@ACuriousMind oh I don't know. People do all sorts of painful and stressing things just to prove they can. Run marathons, climb mountains, learn string theory, etc. Being able to sit though a Rennie meal memory could become a badge of honour.

Captain Bohemian

5:06 PM

is learning string theory a painful and stressing thing?

I think it's an enjoyable thing if there is time to do that.

Over the years many people have made the mistake of assuming that anything I say is to be taken seriously.

Captain Bohemian

I have the disposition of taking what everyone says seriously.

@CaptainBohemian that's an awful disposition, I'm sorry for yo

u

fug

Anonymous

@0celo7 Did you take Captain's sentence seriously?

5:23 PM

lmao good roast

that was surprising coming from you

Captain Bohemian

@2physics I have the same problem as yours.

ola

why does ocelos new picture look like a sock smh

That is discriminatory towards abacus-raccoons

Is that another race celly can get salty at me for being against...

(For reference I have nothing against any race - as I will continue to reiterate, except maybe for abacus-raccoons)

Also how does one write a project proposal form without sounding like a bellend

Why would you want to make an abacus using raccoons? Don't they squeal a bit when you thread them onto the wire?

5:29 PM

@Blue he seems like a person who doesn’t/can’t joke

@JohnRennie Wouldn't that be a raccoon-abacus, not an abacus-raccoon though?

My avatar is the greatest of all time

Raccoons and abacuses (abaci ??) commute

user image

Captain Bohemian

it turns out it's not a cat.

5:42 PM

Ocelo is a furry confirmed.

@JohnRennie it’s Greek so abacopti

@CooperCape that’s never been a secret

8

Q: Plural of "abacus"

A colleague and I were having a discussion as to the proper plural form of abacus. I believe the plural would be abacuses and he feels that the proper form would be abaci. I believe that abacuses is more appropriate as it derives from the root word and abaci is a forked word that is part of the A...

meaning etymology grammar latin roots

1 hour later…

7:04 PM

it's gonna rain

my old nemisis

It's been raining here for the last week or so :P

Since I couldn't find my actual umbrella I had to go goth and take the one with bone butterflies on it we had lying around...

I go through phases understanding this paper

I am at the not understanding phase again

7:20 PM

"From a practical point of view it is impossible to collide more than two particles at a time"

Captain Bohemian

7:35 PM

@Slereah why is so? more than 2 billiard balls can be made to collided. can't microscopic particles be made to do that?

you can but it is unlikely

Schwartz calls the Lorentz invariant phase space the LIPS

7:56 PM

@ACuriousMind yeah, shit weather all round, it seems

we had a bit of cloud cover and some light rain last Thursday

an unpleasant and unwelcome deviation from the usual sunny 15-20°C cold here

8:42 PM

$\text{understanding of paper}(t)=\sin(t)$

period of about 12 hours

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