The h Bar

Sir Cumference

04:19

I'm no chemist, but "atom" does not strictly imply electrically neutral, right?

With regard to this

Johan Liebert

06:51

worldometers.info/coronavirus

Why not make more strong travel restrictions?

ACuriousMind

07:03

@SirCumference depends on the context, I'd say. Especially chemists might use "atom" as a shorthand for "neutral atom". As long as its clear what's meant I wouldn't call either usage wrong

Johan Liebert

07:39

10

Q: Will there be Community Promotion Ads in 2020, if so when?

Will Community Promotion Ads be returning this year? In previous years, a post was put up on meta (Chem, Math, Phys) sometime between December and early February. Is it just delayed or is this feature no longer going to be supported?

Aumkaar Pranav

@JohanLiebert Maybe because tourism is an essential industry in too many countries. And it's also harder to impose restrictions in high population tourism centres.

Knight admires Chappo

@JohanLiebert Today I got -8 (if you have access then please see my profile). Did some user remove his account or someone really down-voted those random answers?

Knight admires Chappo

08:10

@JohnRennie What does "outspoken" badge means?

John Rennie

@KnightadmiresChappo there's a "Badges" page on the site somewhere that has all the definitions.

Knight admires Chappo

@JohnRennie Where?

John Rennie

I'm just looking ...

Aha, here

Knight admires Chappo

@JohnRennie What does this 'Post 10 messages in chat starred by 10 different users" means ?

John Rennie

The "post 10 messages in chat" is straightforward.

Do you know what "starring a message" means?

Knight admires Chappo

08:17

@JohnRennie Means I recievd 10 satrs

John Rennie

Yes. Ten different people have starred your posts in the chat.

Knight admires Chappo

@JohnRennie It was awarded to me on 1st March but I can't see anyone of starred messages.

John Rennie

I don't think there is any way to see your starred posts.

You can see all your posts here but I don't think it displays stars.

Knight admires Chappo

@JohnRennie So how I got that badge :) ?

John Rennie

Oh, wait, it does show stars.

But you can't filter it to show only starred messages. You have to go through all your messages to see which ones were starred.

Knight admires Chappo

08:27

Is there a star filter?

Johan Liebert

@KnightadmiresChappo indeed it is a case of serial down voting. I have flagged one of those posts for moderators' attention.

Knight admires Chappo

@JohanLiebert Thank you.

John Rennie

@KnightadmiresChappo no, I don't think so.

Knight admires Chappo

@JohnRennie Okay!

Slereah

08:58

Morgen

Loong

huomenta

John Rennie

09:10

Hooray, we got an award!!

3

Novalium Company

@JohnRennie coolio

ICCQBE

12:21

Hello everybody.

Consider that we have a disc (infinitely thin and full).

If that disc's radius is R, then It's area is $\pi R^2$.

Now, I take a $\Delta{x}$ units thick ring piece from that disc.

That disc's radius is r.

So, $R=r+\Delta{r}$

Now, I calculate the area of that disc part.

$A_{\text{taken ring piece}}=\pi(r+\Delta r)^2-\pi r^2=\pi r^2+\pi {\Delta{r}}^2+2\pi r \Delta r-\pi r^2=\pi {Delta{r}}^2+2\pi r \Delta r$

${\Delta r}^2=0$ so area is $A=2\pi{r}{\Delta r}$

Is everything ok?

tpg2114

@JohnRennie you can see which posts of yours get starred per room : chat.stackexchange.com/rooms/info/71/…

But not globally

Slereah

12:40

Please help, I am in a neverending rabbithole

Trying to find theorems on measurement in GR > define radar coordinates locally > get the proper expression of a photon bounce > get the cosine law for Lorentz manifolds > get properties of the worldfunction > work out bitensor derivatives

It never ends

Almost nobody even does bitensors

Slereah

14:40

fortunately, while there's nothing on bitensors, there's no lack of ressources on connections for pullback bundles

geocalc33

15:28

@Slereah I've never heard of a bi-tensor until now. Why do you reckon there's not much work with them?

Slereah

0

Q: Defining the covariant derivative on bitensors

Bitensors (tensors defined on two different points) are an extension of tensors found in some applications of general relativity, where objects such as the world function, parallel transport operator, heat kernel or propagator are bitensors of some kind. It is unfortunately not commonly done in t...

general-relativity differential-geometry

Plz halp

@geocalc33 Who know

The world function is pretty useful, really

but it's rarely even mentionned

geocalc33

(+1)

I don't understand much of it, but I will try to "halp" lol

I'm really good at finding the right papers

I'm going to start with this: "as far as I can find, if we have two vector bundles $π_i:V_i→M, i=1,2,$ (with typical fiber V_i) the bitensor bundle is defined in the following way"

So you're saying that a vector bundle is a map from a set of fibers?

Slereah

A vector bundle is a vector bundle

from the usual theory of vector bundles

geocalc33

15:48

@Slereah Just asking so I can learn lol. So you want to take the cartesian product of two spacetimes and define a projection function? does this projection have a name? Where did you read about this specific projection?

Yuvraj Singh...

16:04

3

Q: Why does warm water and cold water make different sounds?

Why is there a different sound when pouring water for it being warm or cold? Pouring cold water makes a higher pitch sound than warm water. The difference is clearly distinguishable. I was unable to find any reliable explanation. Please help.

thermodynamics experimental-physics acoustics water home-experiment

I was watching this question.

Can we say pitch variation occur due to difference in speed of sound in hot and cold water.

I am saying this because I have read this $$ v = \sqrt{\gamma RT} $$

Slereah

@geocalc33 If you don't know about vector bundles I would advise reading about them before trying to understand bitensors :p

ACuriousMind

I prefer beef tensors.

Slereah

What about the wurst tensor

My favorite tensor name is the November tensor

So named because Einstein discovered it in november

Yuvraj Singh...

@ACuriousMind sorry for posting question link.

ACuriousMind

I feel like I should be surprised about that name but we live in a world where elementary particles are named after nonsense words from a novel

Slereah

16:14

It has a dumb name because as far as I can tell, nobody ever named it

It was just found in Einstein's notes, without any text

So they called it the november tensor

also not technically a tensor

John Rennie

@YuvrajSingh... that's the speed of sound in a gas, which is heavily temperature dependent.

Yuvraj Singh...

@JohnRennie oh yes.

ACuriousMind

@YuvrajSingh... it's not forbidden to post question links, we just ask people don't advertise their own questions shortly after they're posted

John Rennie

The speed of sound in a liquid is $v = \sqrt{K/\rho}$, where $K$ is the bulk modulus, and both the bulk modulus and density are much less temperature dependent that in gases.

Yuvraj Singh...

But $$ v = \sqrt{\P / fract} $$

Yes yes @JohnRennie

So yes, does pitch depends upon speed sound.

John Rennie

16:18

@ICCQBE yes in the limit of $\Delta r \to 0$, which is often the limit we use as we are often integrating the ring elements.

Yuvraj Singh...

@ACuriousMind sorry sir I have no intention to promote answer it just that my question is related to that question so I have to add the link so I did.

@JohnRennie yes sir? Does pitch depend upon velocity?

John Rennie

@YuvrajSingh... Have a look at these figures

The speed of sound in water changes by about 10% between 0 and 100°C

Yuvraj Singh...

@JohnRennie yes.

I can see the temperature difference.

But does that effect pitch?

@JohnRennie

John Rennie

I would guess the dominant effect is the viscosity of the water. I wonder if I can find data for the temperature dependence of viscosity.

Yes, there's a large viscosity change.

Source

Novalium Company

Good evening, humans

Semiclassical

16:30

@Slereah fun fact: the history guy I colloborate on QM stuff? that'd be janssen

Yuvraj Singh...

@JohnRennie hi.

John Rennie

@YuvrajSingh... hi

Semiclassical

i forget if he actually is the source of the phrase "november tensor" in the literature but he's definitely involved in the history

(as in, documenting the history)

Yuvraj Singh...

@JohnRennie can you refer some book source to read it.

John Rennie

@YuvrajSingh... I don't know of any books that would cover this. Sorry :-(

Yuvraj Singh...

16:33

@JohnRennie ( :-)

Slereah

@ACuriousMind I do

What will you do about it

Yuvraj Singh...

16:56

@Slereah what do you mean?

Johan Liebert

@NovaliumCompany who told you that we are humans! Hahaha

Novalium Company

17:16

I'll be training my first Generative Adversarial Network (GAN) tonight. Wish me luck.

(on a dataset afaik no one has done on)

I'll post a youtube video if it works

Johan Liebert

17:32

-1

A: Why do our hands feel warm when we rub them?

Hands feel warm because friction generates heat, which is then sensed by receptors in the skin. There is no need to artifically increase the complexity of this simple problem with notion of gradients, heat flow and one hand being possibly warmer than the other: I assume you are doing all of this ...

Wow!

Novalium Company

17:43

Why why?

I'm really tired (and it's too late to take a nap) and I want to do something productive. I don't know what to do :P

help meh

Johan Liebert

Can someone comment on the quality of my question and how can I improve it? It is getting a lot of downvotes.

Johan Liebert

17:58

39

Q: Our Commitment to Responding to Meta and Moderators

As Teresa mentioned in our Q1 roadmap blog post, we are working on establishing our commitment to responding on our Meta sites and to our moderators, by setting up a new Meta engagement policy. The goal is to increase staff participation on the various Meta sites, particularly Meta Stack Overflow...

discussion featured stack-exchange community announcements

It looks like things are changing.

@JohanLiebert I just tried but I got an error and the option disappeared. Do you know of a way to force it? — JimmyJames 1 min ago

@ACuriousMind can you move their discussion to chat?

@tpg2114

ACuriousMind

@JohanLiebert Please use flags to request mod actions, not chat - that leaves a much better trail. Besides, more than 20 comments on a post triggers an auto-flag anyway.

Semiclassical

@Slereah yeah, confirmed it. the phrase "november tensor" is introduced in this preprint: mpiwg-berlin.mpg.de/Preprints/P264.PDF

Slereah

heh

Semiclassical

and I know the first author :)

Johan Liebert

@ACuriousMind no he said that some error occurred while doing so. So I told him that I would ask the moderators about this.

@ACuriousMind done (flagged)

ACuriousMind

18:10

@JohanLiebert Well, we can't do anything about strange errors. Bug reports go to meta. In any case I was able to move everything to chat without errors

Johan Liebert

@ACuriousMind thanks for doing that.

Aaron Stevens

@JohanLiebert I really like that post. You can't tell anything is wrong at first, but then it hits you :)

Johan Liebert

@AaronStevens "but then it hits you" can you tell what this means? I'm too bad at English!

Aaron Stevens

@JohanLiebert I just meant that the beginning of the answer is totally fine, and then suddenly they start getting offensive

Johan Liebert

@AaronStevens yeah me too. When I read the first few lines I thought that he might be aiming at something constructive but then... you know.

ACuriousMind

18:18

@JohanLiebert Just for later reference: You downvoted and commented, but that would have absolutely been a case where just flagging as rude/abusive and moving on would've been more appropriate

tpg2114

^ plus downvoting for good measure

Johan Liebert

@ACuriousMind yeah you are right. But I was a bit surprised by the response and hence forgot to do that.

JMac

@JohanLiebert I kinda almost agree with a small piece of what he's getting at; but it's presented in the worst way. I think yes, you did over-analyze the situation; which led to the question. At the same time, it's not like your over-thinking really led to a bad question. You're asking about an aspect of the physics that most people ignore because it just seems "obvious"; but as you point out, the mechanics aren't as straightforward as that answerer is insisting.

It's like because he thought the situation was simple, that you trying to dig deeper was a waste of time. It's funny, because his explanation seems to miss what you are actually questioning.

ACuriousMind

@JohanLiebert No worries, just making sure - if you see something like that, don't hesitate to flag.

We can only act on r/a content if we're made aware of it. There's little harm in flagging one post too much (we'll just dismiss the flag) but arguably potential lasting harm in flagging one too few (broken windows, etc.)

Johan Liebert

@JMac also the main problem is that when I was thinking of that situation I missed the real point the the nerve endings are situated a little deeper into the skin and are not at the surface and hence that lead to the question. But after thinking a bit about I realized the problem and thought to delete it, but then it was quite late.

Poor biology!

ACuriousMind

18:28

@JohanLiebert My main problem when reading your question was that you assume the reader knows that our hands only perceive heat transfer, not temperature as such - but while I may once have learned that it wasn't really present in my mind, and you mention it only off-hand in your first sentence. So I just went "Well, they feel warm because of friction, duh!" and it took me several tries until I got what you were really asking.

Johan Liebert

Discussion on question by Johan Liebe…

Imported from a comment discussion on physics.stackexchange.co...

Aaron Stevens

@JohanLiebert Biology is fun ;)

Johan Liebert

@ACuriousMind yeah my bad.

@AaronStevens it indeed is but not when you don't know the terms properly.

Aaron Stevens

@JohanLiebert oh for sure. I experience that all of the time haha.

"I know you are using basic biology terms...but I have no idea what you are talking about"

Johan Liebert

@ACuriousMind I thought that it might be well know among the science community (I myself learnt it from some YouTube video (Veritasium for example)) so I didn't elaborated it any further.

@AaronStevens last year studying biology was quite hard for me (even at such a basic level). They kept introducing new terms and I was like :" What the heck is this? "

Wait I am going to modify the question a bit so that there is no further confusion (as far as possible) for the future visitors.

Aaron Stevens

18:38

@JohanLiebert Yeah, exactly haha. It is not like physics where you can build up from essential principles.

The interesting thing about biology is that it is divided more by what system is studied. I was at first so intrigued because a lot of biology talks cover a lot of things that appear to be "basics" because of this.

It would be like a physics talk going over the definition of momentum

In physics everyone pretty much has the same understanding of the basics. For biology each system has its own "basics", so you can't assume one biologist knows the basics that are useful for another biologist

Johan Liebert

@AaronStevens even Feynman had problems with that (the ever expanding vocabulary of biology)!

tpg2114

@AaronStevens That might be over-generalizing a bit. Even within the very narrow world of fluid dynamics, the way we talk about things in combustion is very different than in aerodynamics and so we don't always speak the same language

Johan Liebert

@AaronStevens so how do you people cope with that?

tpg2114

And when I talk to our laser diagnostics or plasma physics people, we're in entirely different galaxies, even though we are in the same research group

Aaron Stevens

@tpg2114 I suppose so. I just meant that when I first started attending biology talks it felt like they would open up with information that seemed like it would be common knowledge for biologists

@JohanLiebert It isn't a bad thing. It is just different

The thing that annoys me most about the pure biology talks I end up attending is that they are all essentially the same exact type of talk: "We modified this gene, and this happened. We modified this mechanism, and this happened. Here is our statistical analysis. Therefore these genes/mechanisms have a role in this process"

Of course what the talk is actually about varies, but that is essentially what everyone does

At least in the biology I have been closely related to. I am sure it is not true of biologists in general

Johan Liebert

18:47

fingers crossed. If I get into IISc Bangalore then I would have to take biology course for three semesters.

Aaron Stevens

Honestly, biology is pretty cool once you get past the biology ;P

Slereah

Biology is mostly ion transport

Aaron Stevens

@Slereah What do you mean?

Slereah

whenever I read about biology, that's what I see :p

bla bla bla calcium ion channel bla bla bla

Aaron Stevens

@Slereah Well calcium is pretty important ;)

I think that would be like saying physics is mostly particles

But not quite

Certainly ion channels are important. But there are many areas of study in biology that do not even think about ion channels

Slereah

18:58

Well they are important!

Johan Liebert

One example : taxonomy

Aaron Stevens

@JohanLiebert Perhaps a species distinction arises due to differences in ion channels though :P

Johan Liebert

@AaronStevens :-)

Aaron Stevens

I doubt that is actually true haha

Johan Liebert

Good Night!

tpg2114

19:17

Boo, my paper was rejected from our biggest/best conference

Aaron Stevens

@tpg2114 Uh oh! Why?

tpg2114

There's only a 30% acceptance rate. I got the reviews back and 1 said Very Good and 2 said Reject, but the 2 said reject because I didn't describe the setup well enough. I got to write a rebuttal to the editor and said that answering their questions would only take a few sentences and provided the missing details.

But it's pretty much impossible to come back from that initial score. I would guess the editor never even re-sent it out or read the rebuttal

The rebuttals only come into play for papers on the border.

Aaron Stevens

Ah, sorry to hear that

tpg2114

On the plus side, I'll just fix up the missing details and send it out to a journal -- the conference actually has a proceedings that counts as archival, so I wrote it as a journal paper to begin with

Or recycle it into another conference so I don't have to write anything new. Heh

Aaron Stevens

In related news. Various people are organizing video conference sessions for people to present stuff they were going to present at the APS meeting

tpg2114

19:24

That reminds me, I need to have something ready for APS DFD

That's a great conference

Aaron Stevens

Where is it?

tpg2114

Chicago

Just before Thanksgiving, that's a crappy time of year to be there

Aaron Stevens

19:39

@tpg2114 Oh well you have some time

Assuming we are all still around by then :P

tpg2114

Yeah, and it's only a 10 minute presentation. Not too bad

Aaron Stevens

Yeah that is what mine is

10 + 2

tpg2114

APS is really interesting and you get to see a ton of cool stuff you wouldn't normally see, but the 10 minute talk is too short to really convey anything technical

At best you can do a "Hey, I do cool stuff like this! Come talk to me later!"

Aaron Stevens

Yeah exactly haha. Although last year when I presented no one came to talk to me later, so oh well

I think they put me in a room that wasn't really what my stuff aligned with

This year I was in a better room, but of course since it was canceled that is no longer the case

The virtual session I am in doesn't really align with my work again

oh well

tpg2114

The acceptance letters for the conference that rejected my paper apparently has some C19 warnings in it -- they may cancel the conference and send the accepted papers to the archival publication without presentation

It's in Australia. I was looking forward to going

Harder to justify spending all that money if I don't have a paper...

I'm submitting another paper to a conference in Greece in Sept., hopefully that one goes better. I need to get on writing that abstract

Aaron Stevens

19:45

Sounds like some fun opportunities. I went to Greece when I was in high school. I don't remember much, but I remember enjoying it haha

tpg2114

I've never been

Aaron Stevens

Well now is your chance!

ACuriousMind

@AaronStevens Funny, I also went on a school trip to Greece back then

tpg2114

It's always nice to travel when work is paying for it!

Although, usually I end up working too much to enjoy the trip. When I was in Aachen last May, they gave us 3 meals a day in the hotel and the presentations were also in the hotel... so I never got outside the entire week, until the train to the airport to fly to the next conference

Aside from the jet lag and the much better food quality compared to meetings in the US, it could have been anywhere local and I wouldn't have known the difference.

Aaron Stevens

@ACuriousMind When I was in high school? :P

ACuriousMind

19:53

Maybe!

Aaron Stevens

Oh I didn't know if you meant when you were in high school

ACuriousMind

Yes, I meant that :P

Aaron Stevens

Ah ok haha.

I was there Summer of 2010 I believe

ACuriousMind

That's...the very year we went there

tpg2114

Uggghhhh. I was studying for qualifying exams that summer.

You kids...

Aaron Stevens

19:54

Ha that's funny

ACuriousMind

So I guess I did go there when you were in high school :P

Aaron Stevens

@ACuriousMind I'll send you all of our pictures and you can tell me if you are in any of them

@tpg2114 I did my QE Spring of 2016. That isn't too far apart

Wait

Spring 2017 haha

ACuriousMind

Haha, I think we went there in September though, so more autumn than summer

Aaron Stevens

Ah yeah. Never mind then

tpg2114

I graduated summer of 2017, after 9 long years...

ACuriousMind

19:56

I remember a lot of looking at ruins and a lot of sneaking Ouzo past teachers :P

Novalium Company

AI

Aaron Stevens

I remember accidentally over-tipping a food delivery boy because I was used to coins not being worth much in US currency

He was very happy

ACuriousMind

heh

Aaron Stevens

@tpg2114 At least you made it out :)

JMac

@AaronStevens How high did the coins go over there?

Oh okay, looks like Euro coins are the same as Canadian, except in Euros instead of dollars

Aaron Stevens

20:06

Yeah I am not sure the exact numbers. I just remember my dad commenting on how high of a percentage tip I gave him

@JMac Looks like there is another "Mac" in town physics.stackexchange.com/users/175857/dmac

JMac

@AaronStevens There was even a community manager named jmac at one point. I don't have the most unique name here.

Aaron Stevens

"D" stands for "Dad"

1

Q: Kinetics of a Particle: Work and Energy Problem

We were given an assignment with this problem and we need to change the value of weight A to $115.945$lb and weight B to $95.945$lb. Upon calculating the total energy, it resulted to a negative value and I cannot find $V_a$ because of it. Can someone explain to me why I can't get $V_a$ and what d...

homework-and-exercises newtonian-mechanics forces free-body-diagram string

@JMac 3/5 teachers oppressively closed this question :P

JMac

20:23

@AaronStevens It's okay. We can discard that data as long as it doesn't align with our feelings.

Aaron Stevens

That sounds too oppressive to me

Student404Mus

20:50

Considering a monotonically nondecreasing potential

$
U(r) \leq 0, \quad\left|U\left(r_{1}\right)\right| \geq\left|U\left(r_{2}\right)\right| \quad \text { for } \quad r_{1}<r_{2}

\begin{array}{l}\text { Assuming that } U(r) \rightarrow 0 \text { for } r \rightarrow \infty \text { and that } \\ r^{2} U(r) \rightarrow 0 \text { for } r \rightarrow 0\end{array}
$

The question is, does the last requirement responsible for the existence of lowest bound states?

PM 2Ring

23:54

@JMac The other jmac was in the MSE Tavern last week, along with Shog9 chat.meta.stackexchange.com/transcript/89?m=8253954#8253954

Discussion on question by Johan Liebe…

Transcript for