The h Bar

and i'd like to get out being hand wavy to having a concrete understandign of what i'm doing, ideally doing my own math (i.e. checking equations myself, writing simulations myself)

@0ßelö7

Maybe, now this is a tall order, it would be cool too if the stuff i read in pop science websites, I could actually then read in their original research paper form, and verify up to the math

0ßelö7

4:16 AM

@frogeyedpeas ok, read Hall's "Quantum Theory for Mathematicians"

@JohnRennie morning?

time for a benchmark?

John Rennie

It's a bit early in the morning for benchmarks. Give me a chance to drink my coffee first :-)

John Rennie

4:49 AM

@Kaumudi.H morning :-)

user228700

Morning! :-) How's ur coffee?

Anonymous

@Kaumudi.H Hi, no class today?

user228700

@Blue Hey :-) No, I'm on Onam vacations till the 10th.

user228700

And you? What are you doing here?

Anonymous

@Kaumudi.H Ooo...nice. I have class in 30 mins :P

John Rennie

4:51 AM

Coffee is good. Mum buys this posh coffee so I'm drinking that :-)

user228700

@Blue Ah :-P Have a good day! :-)

user228700

@JohnRennie Ah, haha, I see :-)

user228700

You drink instant coffee, don't you?

John Rennie

Normally yes.

But my Mum buys this coffee that is a mixture of instant coffee and ground coffee.

user228700

I've heard that coffee connoisseurs look down quite harshly upon instant coffee drinkers! (And that would include even me for we don't have a coffee machine!)

John Rennie

4:58 AM

So it has the convenience of instant coffee but has some of the flavour of freshly ground coffee.

user228700

Ah, nice. Hence posh, eh? :-)

John Rennie

I'm not a coffee connoisseur :-) It's just something I drink first thing in the morning to get me started.

user228700

Yep, yep, I know :-)

John Rennie

@Kaumudi.H posh = expensive !

user228700

Well, right, but I assume that blending in some of that ground coffee is, by definition, going to be more expensive than instant coffee!

John Rennie

5:01 AM

Yes, and I guess that's fair enough. It must cost the manufacturer more to make.

user400188

By definition? What if you grow your own coffee? =P

user228700

^ Ah, I hadn't considered that!

user400188

By the way, is anyone here a postgraduate or higher? I need some advice on finding time to self study (defining self-study to be study you do for enjoyment, as opposed to study along the lines of your course).

not that I don't enjoy my course. Its just that I like a change of topic every now and then.

John Rennie

I was a postgraduate student several decades ago ...

user400188

Then you would know all about it :)

presumably

Secret

5:09 AM

0

Q: The Bohm interpretation and Schrodinger's cat

I was wondering how the Bohm interpretation of quantum mechanics would affect the Schrodinger's cat experiment. Would it imply that the cat would be in one state and never in superposition, so the cat would always be dead or alive whether or not the observer looks or not?

possibly will fail if you don't have an ensemble of cats

John Rennie

@user400188 I'm not sure what you're asking. Are you doing a PhD now and finding you want to study something else in your spare time?

user400188

No, I'm an undergraduate.

John Rennie

What degree are you doing?

user400188

The problem I am facing, is that there are a lot of interesting topics like General relativity, that are not part of my course. As such to learn them I'm either going to have to wait or do my own study.

A double in Robotics and Physics

John Rennie

My instinctive reaction is that your priority should be getting a good result in your finals, and you should be cautious about anything that risks taking too much time from that.

As I recall (and it was a long time ago) I and my friends used to use what free time we had doing something completely different e.g. sport (and drinking :-)

I didn't start learning about GR until a few years ago, and I found it enormously enjoyable. But it takes a lot of time and effort.

user400188

5:21 AM

Hmmm. I think I'll take that advice to heart. I have found myself taking some time from my own course to study this particular way. Perhaps I should tone it down.

Thanks @JohnRennie

That said, I do have a Colleague that is teaching myself and another about GR at the moment. We have just begun (currently only up to the maths aspect) , but we plan to tackle the physics eventually.

Its a slow pace, only 1 session together a week. But I think that by the start of next semester (holidays are in between) we will have finished.

I think I'll drop my other topics in favor of my course, but keep this one.

John Rennie

There is a balance to be struck. It's always interesting to learn stuff that isn't part of your course as long as it doesn't detract from your course work. One meeting a week on GR doesn't sound excessive :-)

Avantgarde

@user400188 Which year are you in at the moment, if you don't mind telling us here?

+1, agreed with @John. It's best to first focus on your coursework and finish well enough. 1 day per week on exploring new topics seems alright

user400188

Technically 3rd year. But because the science aspect of my course started late, I'm only taking 1 3rd year physics unit at the moment.

Anonymous

@user400188 GR should be taught in a Physics degree, isn't it ?

user400188

Not in ours. I don't think anyone at our particular university is both knowlageable enough and feels like teaching. Its the same for particle physics.

For those two subjects the uni advises us to go to other universities. For instance, there is one close by that is leading in particle physics.

John Rennie

5:40 AM

@Blue GR isn't normally taught at undergrad level in the UK, though the more able students may do some GR work.

Though maths students may study GR at undergrad level. I think that's possible at Cambridge.

DanielSank

6:09 AM

@Mithrandir24601 what is the name of your institution?

Anonymous

6:41 AM

@JohnRennie Oh...that must be a BSc (3 year) course I guess. Over here most of the good universities do not offer a 3 year UG course in physics. They are mostly 5 year BS+MS.

Anonymous

So they include GR

user228700

6:56 AM

@JohnR: Did I tell you that my driving test is tomorrow?

John Rennie

@Kaumudi.H no, you didn't say. I didn't tell anyone when my driving test was in case I failed :-)

Anyhow, good luck! :-)

user228700

Ah, well, there is a fair chance that I really am going to fail; I haven't been in the driver's seat of a car in over a month.

Avantgarde

@user400188 2Okay

John Rennie

@Kaumudi.H I have to confess it took me a couple of goes to pass.

user228700

Oh, wow, I see. That explains why you stopped telling other people about the date, haha.

John Rennie

7:01 AM

In the end I bought a motorcycle and passed the test on that. Then I spent a year riding the motorcycle before I came back to the car. After a year on the bike I found the car test really easy.

I think a large part of the test is just confidence, and you only really get that with experience.

user228700

Ah, right, that makes sense.

user228700

Since my parents have paid an exorbitant amount of money to the driving school, they will make me pass even if I do technically fail the test, I think.

John Rennie

Really? I guess it's good if you get to pass the test, but it doesn't fill me with confidence about driving on Indian roads :-)

user228700

@JohnRennie You would be right to feel that way! Some of them didn't even need to drive at their tests!

John Rennie

Eek :-)

I guess my advice, for the little that's worth, would be to try relax and enjoy the driving. Probably easier said than done though.

user228700

7:07 AM

Hmm, thanks :-) I might ask the driving school if it's possible to practice once before the test.

user228700

Today evening, perhaps.

John Rennie

Actually that's not a bad idea.

user228700

Yeah?

John Rennie

Though they may charge extra of course.

user228700

They will :-/

John Rennie

7:09 AM

Yes, maybe get the instructor to give you a mock test?

Or at least take you round the usual test routes.

user228700

Yes, I will ask if this can be arranged.

John Rennie

Windows 10 all still working fine?

user228700

Right, so I've called and they told me that there will be a trial at the test center at 9 AM.

user228700

@JohnRennie Yes :-) Time to activate it, don't you think?

John Rennie

@Kaumudi.H yes, if you're happy it's all fine then activate it.

user228700

7:16 AM

OK (Y) The code is in the case containing the battery, isn't that what you said?

John Rennie

Yes. Take the battery out and you'll see a blue green label with the key written on it.

The key is five groups of five letters.

user228700

OK. Will do now and update you.

user228700

Yes, done :-)

John Rennie

@Kaumudi.H It activated OK?

user228700

Yep!

John Rennie

7:28 AM

Good. All set then! That was fairly painless all things considered.

It's a shame you don't know more about what your programming courses are going to cover or you could download the software for them now.

user228700

@JohnRennie Not really but it is done and over with now!

user228700

@JohnRennie For now, I am meant to learn Python, familiarize myself with Git/Github and solve those exercises.

John Rennie

You could install Python ...

python.org/downloads

user228700

Yes, that I must do.

John Rennie

It's a small download and a quick install.

You can also install GIT for Windows. You don't have to use unix/linux to run GIT.

Let me have a quick look to see what version of GIT I use on Windows.

user228700

7:32 AM

@JohnRennie Ah, but I think I am meant to.

John Rennie

I'm pretty certain I installed this one:

git-scm.com/download/win

NB that link will autostart the download.

user228700

@JohnRennie Ah, too late! :-)

user228700

I'll do it a bit. Thank you :-)

John Rennie

It won't hurt to have GIT available on Windows. It's a small program and installs quickly. And it works in exactly the same way as on unix.

@Kaumudi.H do it now!! :-)

user228700

:-) I'm about to go make lunch, you see.

John Rennie

7:34 AM

Ah, yes, food always takes priority :-)

user228700

:-) Sandwiches again!

John Rennie

Q. why is there always lots to eat on the beach?

user228700

Uhh, no idea. You tell me.

John Rennie

A. because of the sand which is there

(c) Dad Jokes Ltd

3

user228700

@JohnRennie x'D I cracked up because of the copyright statement more than the joke itself!

John Rennie

7:38 AM

I aim to please

(though I usually miss)

user228700

:-)

John Rennie

It's hours till lunch

user228700

:-( More coffee?

John Rennie

Yes, in a moment I shall make another pint of (posh) coffee

user228700

And I'm off to make lunch! I'll be back to ask desperately for help regarding Git etc. :-P

John Rennie

7:41 AM

Actually I have to go out soon to fix a laptop for one of my mother's friends.

I'll be back about 11:00 BST

user228700

@JohnRennie Ah, this again! :-) Have fun!

John Rennie

Enjoy lunch. See you later.

user228700

OK, bye! :-)

Anonymous

@Kaumudi.H Are you sure your uni has Git in syllabus? For using Github you don't really need to know Git :P The coding club might expect you to know it, though :)

Balarka Sen

Ok, this is unironically my favorite rap song of all time.

ııııııııııııııııııııııı

8:09 AM

@ACuriousMind this is theoretically true but in practice has never happened to me in years of using BitTorrent. If you're just a little careful you can be sure you'll be safe.

Mithrandir24601

9:11 AM

@JohnRennie they also do GR in part II physics :)

user228700

@BalarkaSen Unironically? Really?

user228700

Ah, Memelous again, I see.

user228700

@Blue Yep, the coding club does :-)

Mithrandir24601

@DanielSank the Uni is University of Bristol, the school is school of physics, the department is either physics or centre for nanosciece and quantum information (NSQI) and the group is centre for quantum photonics (CQP)

user228700

@Balarka: Have you watched/read/listened to anything created by Bo Burnham?

Balarka Sen

9:18 AM

@Kaumudi.H I am afraid not. I don't follows stand-up comedians much.

user228700

Ah :-/ Well, his work is truly genius.

Balarka Sen

I see

user228700

Also, he doesn't exactly fit the norms of a quintessential stand-up comedian.

user228700

Here, watch this when you've got the time:

user228700

Bo Burnham's "Sad" from "what."

►

user228700

9:24 AM

I watched the whole show and it was after this performance that it really struck me just how excellent he was.

Balarka Sen

I'll check it out at some point of time

user228700

Cool.

Balarka Sen

so how's college

user228700

@BalarkaSen Miserable :-P How's school?

Balarka Sen

still hanging around. :p

also that sucks

user228700

9:31 AM

@BalarkaSen :-) Have you finished your portions? I expect you have, no, by this point?

Balarka Sen

what's a portion?

user228700

Syllabus :-P

Anonymous

I came back home. Not feeling like attending classes today :P Our comp professor has a voice modulation problem. He makes me feel sleepy. lol

user228700

@BalarkaSen Yeah, thanks.

user228700

@Blue Home?

Balarka Sen

9:32 AM

oh not really. I have a chunk of physics I haven't done yet

and the inorganic stuff in chemistry

Anonymous

@Kaumudi.H Well, my second home is 5 mins away from uni. :P

user228700

@BalarkaSen Oh, no, not you! I meant your school.

user228700

@Blue You second home? Hostel, do you really mean your hostel?

Balarka Sen

He lives near his university, I suspect

Anonymous

@Kaumudi.H No. It's a flat where I stay with my mom. Sometimes alone

Balarka Sen

9:34 AM

it's all in kolkata after all

Anonymous

I earlier used to live in Guwahati

Balarka Sen

@Kaumudi.H Ah. Well, sort of

user228700

@Blue Oh, wow, that's great! None of the added misery of terrible food!

Anonymous

@Kaumudi.H Yep! :D I can eat home-made food along with canteen food everyday

user228700

@BalarkaSen Right. Have they started pressurizing you yet? :-P

Balarka Sen

9:35 AM

I heard CMI food sucks

Anonymous

@BalarkaSen How about weed?

Balarka Sen

yeah that's easily available

Anonymous

lol

Anonymous

You're lucky then ;)

user228700

@BalarkaSen Huh, my friend told me that it's manageable, actually.

user228700

9:36 AM

And her word can be trusted because literally everybody lives in the same building, IIRC.

Balarka Sen

oh really? i guess i talked to the graduate students so that's 4 years back when they were undergrads

user228700

Yep! Other than the fact that she is the only girl in her class, her experiences over there sounded great, man!

Balarka Sen

thats nice

user228700

@BalarkaSen Oh, speaking of CMI, have you started looking at sample papers etc. yet?

Anonymous

CMI is nice. One problem might be that the course is a 3 years one. I wonder how much they can cover within that time.

Anonymous

9:39 AM

Some people find it difficult to get a direct PhD admit after a 3 year degree course. So they have to do an MS. But well, I don't think Balarka will need that :)

Balarka Sen

@Kaumudi yeah I looked at the last couple year's. mostly working out problems from here and there

relearnt some number theory in the process

user228700

@BalarkaSen Cool :-) If you join, my neighbor might be in your class, depending on whether or not he's able to crack it...in his fourth attempt.

Anonymous

@Kaumudi.H 4th? :O

Anonymous

wauw

Balarka Sen

Engel has a nice book full with clever problems that i occasionally work on; there's this RMO book I have that's also nice

@Kaumudi well depending on whether or not i would be able to crack it, too

user228700

9:41 AM

Right, right :-P

Anonymous

@BalarkaSen Who's saying ;)

user228700

^

Balarka Sen

i know a guy who's in there - he's into differential geometry

user228700

@Blue He completed 2 years of a BSc. Math course in Amrita, Coimbatore before deciding to drop out this year and prepare wholeheartedly for the test next year.

Anonymous

@Kaumudi.H Huh. Honestly I think that's waste of time. He could just focus on getting a good MS or PhD degree in Math

user228700

9:43 AM

I dunno, man, he seems awfully keen on joining CMI, ya know?

Anonymous

Repeating once might be okay. Repeating 3 times is ridiculous.

Balarka Sen

@Avantgarde "Before and after Science" is a nice little minimalist album you should check out. This is my favorite track from there.

user228700

@Blue It's actually quite sad :-/

Mithrandir24601

@Kaumudi.H :(

user228700

@Mithrandir24601 :-) Hi! How's it going?

user228700

9:45 AM

@Balarka: Cutting class today?

Balarka Sen

it's teacher's day

user228700

Ohh, right.

user228700

Doesn't ur school celebrate Tr.'s day?

Balarka Sen

I think so; I don't participate

user228700

Right :-P

Mithrandir24601

9:50 AM

@Kaumudi.H Pretty well actually, thanks :) I just finished my first year of PhD-ing a few days ago, so have a couple of weeks off to sort out my life and go on holiday before it all starts for real :D

user228700

Ah, wow, congratulations! :-) That sounds great!

Mithrandir24601

@Kaumudi.H it is that :) It's quite exciting :)

user228700

What are you working on, may I ask?

Mithrandir24601

You can and if you give me a couple of minutes I'll even answer

user228700

Please do! :-) [And excuse me if I am unable to ask thoughtful questions later because I don't understand :-P]

Mithrandir24601

10:02 AM

Right, so, in brief, I've just finished a project on what's known as t-designs and my PhD will be on simulating non-Hermitian Hamiltonians on a quantum chip

user228700

Very nice! (As expected, I understood nothing at all! :-P)

Mithrandir24601

@Kaumudi.H so, the PhD project is essentially simulating non-time symmetric processes on a quantum chip

John Rennie

@Mithrandir24601 It's all gone downhill since my day :-)

:: John angrily waves his walking stick in the air ::

user228700

@Mithrandir24601 I...OK! I'm just glad you're having fun, mate :-P

Mithrandir24601

@JohnRennie Nah! GR was so good :) We had Hobson of Riley, Hobson & Bence fame!

@Kaumudi.H Oh, I am :D Thanks

user228700

10:06 AM

@JohnRennie See, if I didn't know you, I'd be inclined to believe that you actually do use a walking stick :-P

John Rennie

@Kaumudi.H I'm not quite that old :-)

Yet ...

user228700

Oh, I know that but somebody else might not is my point :-P

Mithrandir24601

@JohnRennie You don't need to be old to have a walking stick or several (one of my Grandparents used to make walking sticks so I actually had some as a child :P )

(I never actually used them as walking sticks, but I still had some)

Anonymous

I used to use walking sticks to pluck mangoes

John Rennie

I had a shillelagh when I was a little lad but my Dad took it away when I started hitting things with it :-)

Anonymous

10:09 AM

(My grandpa's walking stick :P)

Mithrandir24601

@JohnRennie :/

John Rennie

I was very little at the time ...

It was a well meant but unwise present from one of the family in NI.

Mithrandir24601

Yeah, my Grandparents also had a few of those

Balarka Sen

dude.... what is this game....

Mithrandir24601

*have, to be fair

user228700

10:13 AM

@BalarkaSen You're...you're watching Pewdiepie now?!

Balarka Sen

i catch up with his stuff. he's the number 1 subscribed channel on youtube; source of lots of memes

user228700

Ah.

user228700

@JohnRennie Broke a lot of vases and such things, did you? :-)

user228700

Or were you hitting people? :-P

John Rennie

I don't remember. It was a long time ago :-)

Mithrandir24601

10:15 AM

The only reason I know of a pewdiepie is from this video

John Rennie

For all I know it may not even be a true story!

Balarka Sen

eg this. if you look carefully you'll see a pewdiepie reference in the end

(watch with subtitles btw)

Qmechanic

10:35 AM

Sir I want to ask another question with images on integration but my reputation is less than 10. Kindly help me sir — user134175 2 hours ago

^^^ If you have a moment to spare, you might help OP here with creating his new question.

Balarka Sen

lol

Anonymous

I guess SE doesn't allow image upload with rep less than 10

ACuriousMind

Can users with less than 10 rep post links to the images (uploaded elsewhere), so another user can embed them into the post?

Anonymous

4

Q: "You need at least 10 reputation to post more than 2 links"

I got this error while updating a previously asked question on Stack Overflow: Getting error when importing from sibling package When I originally asked the question (and inserted over 6-7 links in it), I didn't get the above error at that time. My reputation on stackoverflow was 6, both at the...

bug hyperlinks new-users

Anonymous

@ACuriousMind I think so ^

Anonymous

10:41 AM

1 link is fine

ACuriousMind

Well, do we know this users wants to post only one link?

"With images" seems to suggest it would be more than one

Anonymous

Lol...you can tell him/her to link a page which contains the link to multiple images. A link to a google doc page or something like that

Anonymous

justpaste.it

ACuriousMind

That seems...awfully convoluted

Anonymous

:P

ACuriousMind

10:45 AM

I just don't want to give this user instructions like "just post the links" when the system might not allow them to

Anonymous

@ACuriousMind I agree with that

Anonymous

11:02 AM

2

Q: Representation Of Linear Velocity as Cross Product

Why is linear velocity represented as cross product of angular velocity of the particle and its position vector? Why not vice versa? (Consider rigid body rotation)

velocity conventions rotational-kinematics rigid-body-dynamics angular-velocity

Anonymous

1

Q: How do we decide the direction of angular velocity vector?

I know that the angular velocity vector $\vec{\omega}$ points towards the axis of the circular motion and there is a formula $$\vec{v}=\vec{\omega} \times \vec{r}$$ I want to know the origin of this formula which governs the direction of $\vec{\omega}$. And why only this direction, why not the...

vectors coordinate-systems definition rotational-kinematics angular-velocity

Anonymous

Look like duplicates

user228700

11:33 AM

@JohnR: Do you remember that my sister is sending me some books from the U.S?

user228700

Ah, you're gone!

Balarka Sen

@ACuriousMind Do you know much about Jacobi fields?

I am confused about something and the usual geometry crowd doesn't seem to be around

ACuriousMind

@BalarkaSen nope

Balarka Sen

Aw. Thanks though.

0ßelö7

Seriously? You ask him?

Might as well ask a hole in the ground :P

Balarka Sen

11:38 AM

Ah, I didn't see you in the user list.

0ßelö7

I just got here

What is the doubt

Balarka Sen

"doubt" - cringe reacts only

So it's not obvious to me why the exponential map (assume it's defined everywhere aka the manifold is complete) is a local diffeomorphism if the conjugate locus is empty.

Should I think about the sphere picture? Conjugate locus is where a bunch of geodesics meet

So that's a singularity of $\text{exp}$

Oh, I guess I kind of see it. $q$ is in the conjugate locus of $p$ if there is a geodesic $\gamma$ from $p$ to $q$ along which there is a Jacobi field $J$ with $J(p) = J(q) = 0$

0ßelö7

Conjugate locus? Do you mean cut locus?

Balarka Sen

But that should mean the 'spread' of $\gamma$ is trivial near $p$; nearby geodesics starting at $p$ all fall back at $q$

@0ßelö7 The conjugate locus of a point $p$ on the manifold is the set of points $q$ which are conjugate to it, ie, there is a geodesic running from $p$ to $q$ which admits Jacobi field that vanishes at the ends

I think what I said is right; the conjugate points to $p$ are exactly the critical points of $\exp_p$ now that I think about it

0ßelö7

The cut locus contains the conjugate locus but I don't think you're differentiable anywhere on the cut locus. Hard to be a local diffeomorphism in that case...

Balarka Sen

11:49 AM

what is the cut locus

0ßelö7

@BalarkaSen Yes, that's right.

You can think of it's like caustics in a flow.

When the geodesics crash into each other (roughly what conjugate points mean) you get a singularity of the exponential map. It could also be a point of nondifferentiabity.

Like on the sphere, for example.

Balarka Sen

right, true. I didn't think about it this way

Emilio Pisanty

that was fast

0ßelö7

@BalarkaSen there's a relevant MO post on this

Emilio Pisanty

CC @ACuriousMind

OP was even faster than you

0ßelö7

11:54 AM

5

A: Distance function to a submanifold

I'm posting this sketch just for the sake of completeness. This question was already marked as answered by Anton. But I thought of this, more geometrical, argument after the discussion that answered the question in the first time. Notice that the fact that the $\Sigma$ are assumed to be submanifo...

See "assertion"

Balarka Sen

That's a cute proof

0ßelö7

@BalarkaSen It is possible for two points to be conjugate without there being two geodesics that go through the second point. Don't know an example off the top of my head

Balarka Sen

I was wondering what saddle points of the exponential map looks like

I'll think about this. Gotta run for now. Thanks a bunch

0ßelö7

bye

@ACuriousMind " We prove that, up to dimension 7, the main singularities of a complex
potential $q$ having some a priori regularity are contained in the Born
approximation $q_B$ constructed from backscattering data."

lol?

Transcript for