The h Bar

0celo7

00:57

@EmilioPisanty jesus

is that thing orientable?

Cows

Just made dinner (jambolaya) . Brushed my teeth, and am done with qft for a while. Trying to cramp in as much math time before I return to work hehe

Might be watching a few movies at home tonight and chilling.

It turned out to rain today.

Not soo bad

Emilio Pisanty

02:07

@0celo7 sure

It's a genus-three torus

Watch the video

skullpatrol

Numberphile sounds to much like pedophile :P

Secret

02:29

Last might dream: my house fell into a black hole with the size of an oil tanker and we got speghettified and I felt a sharp pain at my waist as I get ripped into two piece and died exactly two proper seconds after falling in

I the wake up the next day (in the dream's perspective) and told my parents about it, before I finally woke up

skullpatrol

a black hole in a black hole

Cows

02:45

I finally understand what it is to be a person or a human. All it took was some youtube videos . . . . good times.

HsMjstyMstdn

03:34

Hello, everybody

What would be the opposite of "phenomenological" ? It would be "theoretical", right ?

@SirCumference Too real, bro... too real....

Tanuj

03:57

I think I just fell in love with physics

Tanuj

04:12

Are they about calculus ?

loocsieulb

06:06

@BalarkaSen you seeing this shit? ^

@BalarkaSen once in a lifetime opportunity to broaden your taste in music!!!!!

Tanuj

Lol that is not music , those are Calculus videos

0celo7

06:30

Gwen Stefani used to be an 11/10 my god

Tanuj

1.1 that is

@JohnRennie Hey ! Good morning :)

John Rennie

06:46

Morning

skullpatrol

Gentlemen prefer blondes :P @0celo7

en.wikipedia.org/wiki/…

But...

en.wikipedia.org/wiki/But_Gentlemen_Marry_Brunettes?wprov=sfti1

:-D

...so red heads get no attention from gentlemen.

@HsMjstyMstdn in philosophy the study of phenomena as distinct from ontology.

the study of being

Tanuj

07:05

@JohnRennie You there ?

@JohnRennie Are you still on the business trip ?

ACuriousMind

→ 46 messages moved to Trash

John Rennie

@ACuriousMind a little harsh, though it shows excellent taste in music :-)

ACuriousMind

Don't spam the room with videos (music or otherwise), please.

John Rennie

@Tanuj I'm currently working away from home so will be in and out of the chat.

ACuriousMind

@JohnRennie They contained much too little screaming, growling, or electronic guitars ;P

Tanuj

07:15

@JohnRennie Alright , have fun . Let me know when you're free so we can discuss some problems

welcome @Sid

John Rennie

@Tanuj I probably won't have time for any extended discussions though quick questions shouldn't be a problem.

Sid

Hi people.

Tanuj

@JohnRennie Definetly , just small doubts , wouldn't be a problem :)

@JohnRennie just ping me when you're free , we'll move to PSS

@ACuriousMind you just left , and then you just arrived

John Rennie

@Tanuj he works for a living these days. Since it's 08:24 in Germany I imagine he has started work! :-)

Tanuj

oh cool

@JohnRennie you know I've answered so many doubts past two days , on chats and Chem SE , it really feels so good :)

skullpatrol

07:28

You may find this interesting @JohnRennie

Sid

@JohnRennie "08:24" - that's oddly specific.

@Tanuj *questions

Tanuj

okay autocorrect :)

skullpatrol

@ACuriousMind do you know why my Wikipedia links didn't one-box?

John Rennie

@Sid I posted at 07:24 UK time and Germany is one hour ahead of us.

@skullpatrol thanks, that looks like a fun read.

I spent a lot of time in the computing dept at Cambridge - not for my work, I was just friends with a lot of the CS staff. So I've met many of the people who were instrumental in the development of computing at Cambridge.

skullpatrol

cool

Tanuj

07:43

@JohnRennie are you available right now ?

John Rennie

Working for the next half hour ...

Tanuj

alright .

ACuriousMind

@JohnRennie In fact I left to take a shower and am now on my way to work

@skullpatrol no idea

Slereah

@ACuriousMind was it in Texas

ACuriousMind

@EmilioPisanty I can and I will!

@Slereah Yes. :P

Slereah

07:59

heh, looking at a comment on lubos' blog

"I greatly appreciate your blog - I got to it via a monography of Garrett Lisi in this July's "The New Yorker". You were quoted there, and no surprise, it was truncated in such a way that a layman must think you are an extreme bloat-head..."

Where would one get that impression!

skullpatrol

@ACuriousMind still, 9 out of 10 people is an incredible drop out rate.

Even for Texas :P

Home of RL Moore who infamously walked out on his lecture when he saw a coloured student in the class.

Slereah

08:15

"Originally, "g" was for gravity. Einstein and Marcel Grossman, his mathematician friend who introduced him to tensor calculus and collaborated with him on prototypes of general relativity, adopted it after realizing some time between 1909 and 1913 that the metric tensor depended on the distribution of gravitating matter in a region of spacetime.
After the success of Einstein's final version of the theory the notation was widely adopted by physicists and spread into tensor calculus and differential geometry as well."

Tanuj

@JohnRennie are you free now ? :)

skullpatrol

Don't you trust him to ping you when he's free @Tanuj?

2

Tanuj

@skullpatrol He didn't say he would ping me

@skullpatrol and he didn't agree to when I asked him to, he basically said nothing about it.

skullpatrol

True, but just check back and see if he's chatting.

Tanuj

and according to him , he should have finished working 10 minutes ago

skullpatrol

08:25

That would be the polite way to do it :-)

2

Tanuj

sorry guys , I really don't want to get on anyone's nerves but I get too frustrated when I have pending doubts.

Slereah

0

Q: What do the planets in our solar system smell like?

For example, if Jupiter is mostly hydrogen and and helium, but has ammonia clouds, does that mean it smells like a cat's litterbox? Does Mars smell like iron? Asking in order to give kids a more "tangible" understanding of what various planets are made of.

good question

Tanuj

Hmm if I remember correctly NASA did say Jupiter smells like cat's litterbox .

Slereah

I bet they didn't even smell it themselves

Tanuj

hmm..Well atleast the tags are precisely put .

Slereah

08:31

no smell tag though

Tanuj

that would've been too specific

that shows some effort and logic "Asking in order to give kids a more "tangible" understanding of what various planets are made of."

I wonder if its a kid asking this.

Slereah

I'm guessing some of the gas planets smell like farts

with all that methane

Tanuj

uranus and neptune more like frozen farts

Dawood ibn Kareem

08:48

Methane doesn't smell.

Slereah

09:01

bluh

gluing manifolds together requires some convolutions

to smooth things over

mollifiers

that's the name

skullpatrol

Appease the anger?

Slereah

Yes

The $C^0$ parts are very angry

skullpatrol

Did you see the history of CS link I posted for JR?

2 hours ago, by skullpatrol

You may find this interesting @JohnRennie

\o @Mithrandir24601

Mithrandir24601

09:17

@skullpatrol o/

I haven't seen it, but I've heard about this before from 1st year CS :)

skullpatrol

A lot of interesting history.

Mithrandir24601

(The Peter Robinson who wrote Chapter 7 on "Spreading the Word" was my CS Director of Studies - he was brilliant :D)

skullpatrol

cool

With respect to your aside about undergraduate degrees in CS, the first taught course in CS was a post-graduate Diploma at the University of Cambridge in 1953; Cambridge introduced an undergraduate "Part II" in 1970 as the third year of a science degree; expanded it to two years in 1978 (following one year of maths or science); and to "three" years (really 2.5, because half of the first year was still science) in 1989. Even today they don't have a full three year course of just CS, although it's up to 2.75 years. Source: cl.cam.ac.uk/downloads/books/CambridgeComputing_Ahmed.pdf — Peter Taylor 20 hours ago

> note that there was a time, around 1980, I think, at which you could earn a Doctorate from Carnegie Mellon University in CS, but could not earn an undergraduate degree. The reasoning behind that was that "there was insufficient academic content" at the time in CS, in the faculty's judgement. The doctoral students were, of course, developing that content that could later filter down to a wider audience.

Mithrandir24601

Yeah, I forgot that most people don't know this kind of thing - Peter Robinson did the Diploma (we occasionally joked that he predated CS), although the statement that "Even today they don't have a full three year course of just CS" is quite misleading -

by the same measure, they don't have a full four year course of physics (Strictly speaking, by the same measure, they don't even have a full three year course on e.g. physics), so implying that there isn't a lot of it (which the comment seems to be trying to do) is missing the point that the aim is to give a broad background

skullpatrol

09:36

Hmm, I wonder why places like Germany don't have this philosophy?

Mithrandir24601

(oh, as we're comparing like with like - they don't have a full 3 year bachelors in physics either and strictly, it's not even a full 2 year course on physics for the BA, so by that measure, there's more CS than physics :P)

@skullpatrol All I know is that it was perfect for me, as I didn't know whether I'd rather do CS or physics pre-undergrad. I'm now doing essentially a combination of the two, so it couldn't have turned out much better :)

skullpatrol

Did you do a grade 13 in grade school? @Mithrandir24601

Mithrandir24601

@skullpatrol We don't have 'grade school's in the UK :P

(so I don't have a clue what the equivalent to that is)

skullpatrol

How many years of pre-undergrad did you do?

Mithrandir24601

@skullpatrol 14, I believe

7 in primary school, 3 pre-GCSE, 2 GCSE, 2 A-level

skullpatrol

09:42

Thanks, that answers my question :-)

Mithrandir24601

:)

John Rennie

@Tanuj I'm around for about half an hour ...

user55789

09:59

Hello hello test test $x$ $x$ does this work

Rip, it doesn't

John Rennie

@user55789 you need the macros to enable chatjax

user55789

"The macros"

John Rennie

23

A: Any chance of MathJax in chat?

As a workaround while this request is pending, there exist several client-side workarounds that can be used to enable LaTeX rendering in chat, including: ChatJax, a set of bookmarklets by robjohn to enable dynamic MathJax support in chat. Commonly used in the Mathematics chat room. An altern...

user55789

I just installed Greasemonkey and I've loaded up the user script

ScriptS*

SOUP and ChatJax

No dice

But anyway, onto my question then I guess

I have a little confusion about the explicit evaluation of the free particle action, I can distinguish a few approaches but I cannot justify any of them in way I can justify formally

skullpatrol

Try this

user55789

10:04

Doesn't work either, but I'll look at it later

Thanks though

skullpatrol

np

user55789

Sooo, suppose we're looking at $\int_0^t dt' \dot x^2(t')$

Shing

I was calculating the energy density of capillary waves with Debye method (pretty much Debye model in 2D), and I assumed there is a frequency cutoff for capillary waves as well. However, when I checked my work with solution I was quite surprised that the solution suggests there is no such a cuttoff.

I have been thinking the actual difference between a typical Debye solid and capillary waves, but I couldn't really come up one convincing myself there is no such a cutoff.

user55789

I've seen a variety of people just using partial integration and the on-shell condition $\ddot x = 0$. Sounds fine, but suppose I would use

$ \int_0^t dt' \dot x^2(t') = d{dt}\int_{x_i}^{x_f} dx x$, i.e. I'm treating the time derivative as a differentiation under the integral sign operation...

Actually it's supposed to be d/dt'

Woops

Shing

um... prehaps any capillary waves is observable to naked eyes, and hence the wavelength is larger than the atom intervals?

skullpatrol

10:11

@JohnRennie did they have "A-levels" around in your time as a pre-undergrad?

John Rennie

@skullpatrol Yes, I took A levels.

Shing

We had A-level in Hong Kong. The "pure math" is damn hard

skullpatrol

Two years worth?

John Rennie

@skullpatrol Yes. The last two years at school. From age 16 to 18.

skullpatrol

10:25

Interestingly they are undergoing some reform:

> Controversially, various A Level courses have been abolished from 2017 as part of these reforms, including Archaeology, Creative Writing and Home Economics among others.

Souce: Wikipedia

So much for the "broad background" ideal :P

Emilio Pisanty

12:16

@ACuriousMind that's.... full-on heresy

why would you 'fess up to that?

skullpatrol

No respect for Spivak?

Emilio Pisanty

@skullpatrol I mean, I guess if you were a declared Satanist then you'd have no problem with blasphemy in public

Tanuj

@JohnRennie are you around ?

John Rennie

I'm around for a few minutes

Tanuj

12:31

ahh okay . I'm again late !

to the PSS shall we ? @JohnRennie

Slereah

12:45

From my calculations I think a wormhole should have fundamental group $\mathbb Z$ in $\geq 4$ dimensions and $\mathbb Z * \mathbb Z$ in $3$

But I am pretty bad at fundamental groups

I guess $\mathbb Z$ would just be closed curves crossing the wormhole $n$ times

And the 2+1 d case has the weird thing where you also have the curves circling the mouth itself

Tanuj

13:39

Guys is anyone of you available ? I have doubt here chat.stackexchange.com/transcript/message/43120640#43120640

001

14:19

@Tanuj you know the inertia formula ?

Tanuj

@001 of course , but do I find $I$ about the point of suspension here ?

001

$ I = frac{1}{2}mr^2 + ml^2 $

Tanuj

do you mean $I=\frac{MR^2}{2} + Ml^2$

001

yes

Tanuj

yes that perfectly fine.

001

14:24

its equal to mlg time theta

you will have a differential eqn

in terms of theta

solve for period

Tanuj

@001 I did that already and it yields an incorrect answer

I think the fact that the string or rope isn't connected to the center but some distance away from it might have something to do with it

001

yes l is R/2 in this case

because the rope is connected R/2 distance away from center of mass

Physics Meta

14:40

0

Q: Answer ban correction

I am banned from writing answers, and all the answers I wrote, the question has been already been deleted by someone. Actually they were homework questions and I answered them, so it is not acceptable that I get banned right? What can I do?

support answers

0

Q: Answering own question: "work" of the question in own answer

After reading the links provided by the mods in a question I recently posted with an answer, I believe that my question conforms in spirit the criteria that the moderators have tried to infuse in our site through their work. While answering own questions is explicitly encouraged in the link righ...

discussion

Anonymous

15:13

Whew, no idea why they unlisted such a great set of lectures:

Anonymous

StatsLearning Lect1/2a 111213 v2

►

Anonymous

@BalarkaSen Got their reply: "Transparent Glass board, we aim the camera at a mirror which reverses the already reversed images (see any writing on our shirts are reversed). We use special Dry erase markers that work on glass!

Andrew"

Balarka Sen

ahhhh

clever

Anonymous

Heh, so our prediction was correct :)

Anonymous

Yup

Anonymous

15:16

Also easier to use than stylus on tablet method

Balarka Sen

more expensive though

Tanuj

@Blue I have a doubt , can you have a look at it ?

loocsieulb

@Tanuj you should run away before you get bombarded with people saying "don't ask to ask just ask" and then have it starred like 5 times.

2

Balarka Sen

lmfao

Tanuj

You see , he still didn't say anything

loocsieulb

15:22

@BalarkaSen did you like the music recommendations?

Balarka Sen

@Tanuj Don't ask to ask; just ask

6

Anonymous

@BalarkaSen I don't really have any idea of how much those cost

Tanuj

he still didn't say anything

Balarka Sen

@loocsieulb I didn't understand the comment

Anonymous

@Tanuj If you have a question just ask it. I am usually not able to answer immediately, since I'm especially busy these days

Tanuj

15:24

@Blue Its in the PSS

loocsieulb

@BalarkaSen check your mentions in your inbox from last night

Balarka Sen

Oh I mean I know you pinged me but I didn't see where the music was

loocsieulb

scroll up from where i pinged you lol

Balarka Sen

Am I being dumb

loocsieulb

a little

Balarka Sen

15:25

OK

loocsieulb

lol

Anonymous

@Tanuj Post the whole question along with your whole confusion here, and wait. I'll answer if and when I can.

Tanuj

@Blue I tried to take the axis of rotation about the center of the disc but that gives me the wrong answer.

Balarka Sen

Oh I didn't see any of it because ACM moved it to trash

what the fuck

loocsieulb

@ACuriousMind way to go

Tanuj

15:28

@Blue Also , does choice of axis of rotation affect my answer ? And could this answer be solved by intution ?

Anonymous

Is it oscillating in and out of the plane of the paper?

Balarka Sen

Never in my life did I ever expect to see Harry Belafonte with David Bowie with A$AP Ferg

Anonymous

The language of the question is not very clear

Balarka Sen

all at once

Tanuj

@Blue No , it is oscillating right-left , JR confirmed this

Anonymous

15:38

@Tanuj There doesn't seem to be any fixed axis of oscillation within the body in that case. What I think is that if you tilt it by $\theta$ towards left, the tension in left string becomes $0$ while the tension in the right string increases, which pulls it back.

Anonymous

But due to inertia it keeps moving a bit upwards till tension on right string becomes zero. So, the method to approach it would be to find the restoring torque when tilted by $\theta$ and then use the general $\tau = I\alpha$ method to find $\omega$. Also, no, about which axis you find torque should not matter, because $\alpha$ for any point of rigid body is same. If you change $\tau$, even $I$ would alter, keeping $\alpha$ constant.

Anonymous

That said, I need to leave now.

Anonymous

By "change $\tau$" I meant considering torque about a different axis such that $\vec{r}$ differs and hence $\vec{r}\times \vec{F}$

Tanuj

@Blue I get what you're trying to say , but that doesn't yield the correct result

Anonymous

Pretty sure that's the method. Anyway, I can't help anymore now. Maybe ask on Physics Forums

001

15:58

@Tanuj when you put the equation of motion you add the inertia of the two cables and the disk and it still doesn´t give result ?

Tanuj

nope

nbro

16:09

How can quantum mechanics be a reliable theory if is based on concepts such as "the measurement alters the state of the system", where to confirm the theory you actually need to measure and experiment?

Slereah

@nbro Perform two measurements

Then you know that the first measurement puts the state in an eigenstate of that measurement

nbro

And?

Slereah

then you know the original state and can perform all the experiments you need

nbro

Why exactly do you know the original state? I mean, the measurement makes the superposition state collapse. Why would the observed state after measurement be or not the original state? What do you mean by "original"?

Abcd

@Tanuj it's a physical pendulum right?

Abcd

16:29

@Tanuj What is the right answer??

Tanuj

@Abcd what did you get ?

Qmechanic

0

Q: Origin of physics laws

I am a math student and I have been self studying physics lately. I am going to take a course on electromagnetism this year, so I searched for books that are mathematically rigorous on that topic. And I found some good ones, but none of them explain where did Coloumb's Law came from, it's taken a...

newtonian-mechanics electromagnetism experimental-physics history experimental-technique

Too broad? Off-topic?

2

nbro

What if quantum mechanics is just based on paradoxes or circular reasonings?

Tanuj

I wouldn't say that , no

nbro

It's the most plausible explanation

Anonymous

16:37

@nbro Why do you think so?

nbro

I know little about QM, but it looks to me that there may be mistakes that people have not found yet

Anonymous

Considering the staggering number of smartest-of-their-breed physicists and mathematicians who have worked on it, it is very very improbable that it is wrong. Also no experiment has violated it till date. So, perhaps you need to back up your claim with more solid facts rather than vague statements. I give you that QM is an "incomplete theory". There might be more deeper theories of which QM is a limit, just Newtonian physics is the classical limit of QM.

bolbteppa

@nbro you can take QM to start from the Heisenberg uncertainty principle, derived from experiment, which says there is no idea of the path of a particle, and use the fact that paths exist in some weird quasi-classical limit, and the fact experiments allow you to measure positions but not predict where they will be in the next instant, to set up QM without any guff, so no, not circular

Emilio Pisanty

16:49

@Qmechanic way too broad.

Slereah

Barbashov seems like a pretty good book

Takes its time to explain the relativistic string

bolbteppa

Yeah those early sections really clarified things, still need to write parts up

It looks very off-putting but it's actually a gem

Slereah

I'm not 100% sure how he gets the point particle action from the string action

Tanuj

@JohnRennie Hey ! :)

nbro

3

Q: How do you isolate a single photon?

How do scientists/researchers isolate a single photon (for single photon sources)? How do they know they have isolated it? Is it really totally "isolated"? What is the photon isolated in? Sorry if this is a basic or general question, just really interested to know what this means, considering t...

quantum-mechanics photons vacuum

Answer: "...by lowering the intensity of a light source to the point of zero emitted by the source and then slowly increasing it."

lol

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