The h Bar

12:25 AM

0

What is a tensor quantity , how is it different from vector quantity or scalar quantity and why is pressure a tensor. It would be very helpful if you could give me an intuitive idea of tensor quantities.

Queue up an answer that includes the phase "that transforms like" closely followed by @DanielSank's scream of horror.

David Z

12:36 AM

This answer provides good book recomendations but it seems that the editor has made big mistakes in providing external URLs to the books. The names in the hyperlinks do not match with the books we are redirected to. If you try all the links, you will se what I mean. Could someone fix this? Maybe the original editor? — TPVasconcelos 4 hours ago

an editing task for someone with spare time

DanielSank

@dmckee Yo.

dmckee

Finals week here. Grading sucks.

0celouvskyopoulo7

@dmckee I'm tempted to write something about free modules/free vector spaces. It will elicit the same scream.

@dmckee do you ever get people who surprised you with bad grades?

dmckee

@0celouvskyopoulo7 Not like "I was expecting an A but you gave me a D", but one letter grade happens a lot.

0celouvskyopoulo7

like people you thought would get an A but end up with a B?

dmckee

12:52 AM

@0celouvskyopoulo7 Yeah. Or thought they were getting a B and got a C, or occasionally ones who thought they would squeak by with a D and got an F.

I do enough midterms and stuff for people to be pretty clear on how they are doing. But flub the final or forget to turn in the last three homework assignments and you can drop pretty suddenly.

0celouvskyopoulo7

Yeah I had a few flubs on finals. Looks like my midterm grades are carrying me to victory though

dmckee

And my school doesn't do + and - distinctions. So an 88.5% average is a B.

0celouvskyopoulo7

I was sitting on the edge of my seat for my thermo grade

@dmckee At my school it's up to the teachers to decide. I have a stupid A- from a 93 in philosophy.

dmckee

Which hurts. I remind them that they got a full blown A for that 91% last semester and that sometimes takes the sting out of it.

I put a 'most demanding case' grading scale in my syllabus, but reserve the right to make it easier.

But I haven't felt the need to do that much recently.

I'm getting better and writing assignments and test to match the class's preparation.

DanielSank

It's interesting, I always did crap on midterms and better on finals.

0celouvskyopoulo7

12:55 AM

@DanielSank Really? I remember stuff easier for midterms

dmckee

@DanielSank Most classes were like that for me. But E&M and thermal were the other way 'round.

My buddies and I would do a complete review of the semester over dead-week, and I think that made a big difference in putting stuff in context.

DanielSank

Yeah, midterms were easier material, but I almost always had the same problem:

At the midterm, I was still trying to figure out wtf all the words meant and how everything fit together.

dmckee

^ Context and big picture needed.

DanielSank

I would be behind the whole semester until the end when I finally understood why all the formulae were how they were.

@dmckee Also professors explaining wtf things mean.

Or, well, usually them not doing that and me eventually figuring it out.

:D

0celouvskyopoulo7

My measure theory final had stuff from the first week of class

I didn't bother to review

DanielSank

12:58 AM

Stat mech was a good example.

There were all these partial derivatives and Legendre transforms and I had no f------ clue what it meant.

There were formulae about heat engine efficiency, and entropy and who knows what else.

0celouvskyopoulo7

I think I once understood what a Legendre transform was

DanielSank

It took me all semester to sort it out.

dmckee

I'm finally beginning to feel comfortable with stat mech this semester...

DanielSank

If you can explain thermodynamics to me, then I'll thank you.

dmckee

@0celouvskyopoulo7 I wrote a little paper on them for my students this term. That;'s part of why I think I finally get it now.

DanielSank

12:59 AM

I know it's all about Legendre transforms, but that's it.

The quantum II midterm was a disaster.

We had learned about Clebsch Gordon or whatever they're called.

0celouvskyopoulo7

@dmckee I did something similar with the eigenvalue problem on a compact manifold. I wrote a really detailed explanation of the whole thing, and by the time I was half done it just clicked and now I get it

DanielSank

Nobody ever just came out sand said "these are the elements of a change of basis matrix".

0celouvskyopoulo7

I may or may not continue writing it

DanielSank

If they'd said that, I would've understood it.

0celouvskyopoulo7

@DanielSank Huh

That's how my prof talked about them...

DanielSank

1:01 AM

Then your prof was better than my prof.

0celouvskyopoulo7

You want to put your states in the total $j$ basis and you need to expand in the old one

DanielSank

Why are you explaining it to me?

0celouvskyopoulo7

So it's just the elements of the change of basis matrix

@DanielSank To make sure you know

DanielSank

@0celouvskyopoulo7 Define "total $j$".

This was one of the things that confused me so much.

That's just some phrase, not an operator.

0celouvskyopoulo7

@DanielSank $\mathbf J=\mathbf J_1+\mathbf J_2$

dmckee

1:02 AM

@DanielSank The purpose (well, one of the purposes) is to let you arrange to have the work-like or heat-like (or chemistry-like) energy transfer terms go to zero in your problem.

DanielSank

@0celouvskyopoulo7 What is that?

Why is it bold?

0celouvskyopoulo7

Vector operators

It's the definition of total angular momentum

I can put some tensor products in if you want to be pedantic.

dmckee

The change in plain-ole energy due to heat is zero for adiabatic processes, which is great until you have to deal with a isothermal process. So think about the change in Helmholtz energy, instead and you get back to have the $T$–$S$ term go away. Yeah.

0celouvskyopoulo7

@Slereah There's a Choquet & Choquet-Bruhat paper.

DanielSank

@0celouvskyopoulo7 I have a large yellow book by them, I think.

0celouvskyopoulo7

1:12 AM

Manifolds, Analysis, Physics?

That's by YCB and Cecile Morette-De Witte.

Maybe De Witte-Morette.

god, this paper is using bold and unbold and it's impossible to tell which is which

DanielSank

@0celouvskyopoulo7 What does that mean?

0celouvskyopoulo7

1:41 AM

@DanielSank an operator valued element of $\Bbb R^3$

Fawad

3:26 AM

> My teachers asked me which one is heavier - 1 kg of iron ball or 1 kg of cotton ball. I told him iron ball is heavier . But he didn't agreed and told me both have same weight. So I told him to hit me with 1 kg of cotton ball and I will hit him with 1 kg of iron ball . If both weights equal both will hurt equal . My teacher left the job😁😂 -Facebook post

Avantgarde

5:04 AM

lol

Anyone here?

user228700

Morning! :-)

Avantgarde

Hi Kaumudi :)

How's it going/

user228700

~~obligated to say "OK" but it's going poorly~~ haha. What about you?

Avantgarde

Societal conventions are a pain eh? I'm okay. What's the matter?

user228700

Although, it's going better than before because I've had a beautiful song stuck in my head for a while now.

John Rennie

5:16 AM

If that's another Rick Roll link ... :-)

Avantgarde

@Kaumudi.H Glad it's better now. Coldplay, I see

user228700

@JohnRennie No, no I swear! :-) It's a song by Coldplay but please don't let the name dissuade you from having a listen!

user228700

@Avantgarde Yep <3 Thank you. Are u a student?

Avantgarde

@Kaumudi.H Yes. You as well?

user228700

Yep! Will be joining university in August. What about you?

John Rennie

5:19 AM

August? UK universities start in September.

user228700

@JohnRennie August over here! Sometimes even July.

John Rennie

What date does the university year finish?

user228700

Usually by the end of April/May, I think...

Avantgarde

@Kaumudi.H Oh, all the best with your endeavours! I graduated recently actually. Now onto graduate studies. So you'll be studying physics?

user228700

Thank you! :-) Ah, nice, congratulations! No, I don't think so. Engineering will be it.

Avantgarde

5:21 AM

I had that in mind lol

user228700

...yeah :-|

Avantgarde

@JohnRennie Mine also used to start in Aug. Aug-Dec. Then Jan-May

@Kaumudi.H But you want to pursue physics or nah?

John Rennie

Ah OK. The year is about the same length but shifted forward a month. In the UK we start in September and finish in June.

Avantgarde

yep

user228700

@JohnR: It will be a busy few weeks toward the end of vacation! I have close to no clothes and the amount of shopping I might need to do Oh God...

user228700

5:24 AM

@Avantgarde I'm not too sure but I've been thinking about it for close to 3 years now and I'm leaning toward nah.

Avantgarde

@Kaumudi.H Ok. I'm assuming that till now, you've only done high school physics. University physics is quite different, so I would say that you can try exploring some subjects while you're there. There would probably be some electives that are offered to all students, irrespective of what their major is (some basic courses like quantum mech, electromagnetic theory, etc)

But yes, anyway, you should do what you feel the most comfortable with

user228700

Right, right. Yes, I certainly plan on doing as much exploring as I possibly can in college. If u've been around for a while, you would've noticed from the transcript that I've been thinking about pursuing cognitive neuroscience after graduation but again, I'm gonna see how things shift and change and decide about that toward the end of my 4 years.

John Rennie

@Kaumudi.H one pair of jeans and a few T shirts. What other clothes do you need? :-)

Avantgarde

Ah, amazing. I have a friend doing research in (somewhat) that. Keep exploring and learning so you automatically gravitate towards your interests and talents.

Neuroscience is pretty interesting. Wish I could get my hands on that too.

user228700

@JohnRennie That has been my philosophy thus far but like I said before, I'm tired of people mistaking me for a guy/not taking me seriously in that regard, if u know what I mean. Sometimes it's a blessing and sometimes it's greatly annoying.

user228700

5:32 AM

So no more T-shirts for me for a while. Going to have to do some hardcore shopping and it's scaring the crap out of me but what has to be done, well, has to be done!

user228700

@Avantgarde Ooh, interesting!

Avantgarde

lol. I hate shopping so much

user228700

Yep, Neuroscience is absolutely fascinating.

user228700

@Avantgarde It's nice to look at pretty clothes but I can only handle it for about 5 minutes before I get really tired of walking around...on and on and on.

Avantgarde

YES^

user228700

5:34 AM

Being an introvert (to a dangerous degree), I get mentally drained as well, by the end of it all, having spent God knows how long in the midst of a large mob.

Avantgarde

If you like music, you can shop with your earphones in all the time.

user228700

I do, yeah, but the trouble is that I'm not going to be shopping alone and it'll be a pain, taking them out every few minutes to consult with my mum.

Avantgarde

haha got it

user228700

Did u major in Physics?

Avantgarde

yes

user228700

5:44 AM

Nice! :-)

Kenshin

6:19 AM

yo dawgs

yes I did major in physics

and math

ematics

@Avantgarde neuroscience is inte5resting so is AI

and philosophy

I'm very close to the ultimate way of livin

@Kaumudi.H so you'll be studying engineering?

cool

is that computer, civil, mech?

how excitin

user228700

7:26 AM

@Kenshin x'D Do u care to share?

user228700

@Kenshin It depends on what rank I get in my exam. I'm waiting for my results.

user228700

(Mostly C.S though)

Avantgarde

7:39 AM

Kenshin was talking to himself there for a while

user228700

x'D

John Rennie

@Kaumudi.H CS?

user228700

Are u questioning that choice or asking for the full form?

John Rennie

I'm asking what the acronym stands for.

CS generally means Computer Science in the UK

user228700

And here as well :-)

John Rennie

7:44 AM

Does that count as engineering?

user228700

Yep, it does. B.Tech.

John Rennie

Ah, that's another difference from the UK. Here CS would be a separate discipline and you wouldn't study it in the Engineering department.

user228700

Ah, interesting...

John Rennie

When you said you were going to study engineering I assumed it would be the usual mechanical or civil engineering.

user228700

Ooh, ALSO, did u know that acronyms are only classified as acronyms if they're pronounceable? Something like CS would be an initialism.

user228700

7:47 AM

@JohnRennie Ah. Well, having a degree in Civil/Mechanical engineering wouldn't be too useful if I wish to pursue a career in cognitive neuroscience.

John Rennie

I suspect there are engineers working in CN - someone has to design the kit they use for studying the brain.

user228700

Right, but I don't think I want to be an engineer working in CN.

John Rennie

Computer Science is good fun. That's basically what I do these days.

Avantgarde

CN?

John Rennie

cognitive neuroscience

user228700

7:51 AM

@JohnRennie I greatly enjoyed learning computer science in high school (it was one of our core subjects).

Avantgarde

oh lol

John Rennie

And if you lose interest in neuroscience there are lots of highly paid jobs in computing :-)

user228700

We had basic C++ in the first year and in the second year, we had stuff like OOP, logic gates, etc.

John Rennie

Gosh, teaching beginners C++ is like teaching them firearm safety by handing them loaded guns :-)

Avantgarde

What is your personal favorite of open source computational engine as an alternative to mathematica?

user228700

7:53 AM

@JohnRennie Huh? What do you mean?

user228700

Ah, we also had data handling (files etc.) and all of that was good fun, yeah, but we had this 300-page chapter on networks and communication with close to 150 terms to learn and oh God, that was the worst.

John Rennie

@Kaumudi.H C++ has lots of facilities for making your app crash in entertaining and innovative ways. Mostly related to abusing pointers.

Most of the more modern languages go to great lengths to try and avoid these sorts of problems.

user228700

> in entertaining and innovative ways

user228700

Hahaha x'D Yes, it's true. I've learned more from all the crashes than I have from my textbook.

John Rennie

@Kaumudi.H Seriously.

Avantgarde

7:56 AM

I never understood files...

John Rennie

You can get obscure bugs e.g. an app will run in debug mode but not when run outside the debugger.

With simple C++ programs it's not usually a big problem, but I've worked on projects like Kodi where there are millions (literally) of lines of code.

user228700

> literally millions

user228700

Gosh!

user228700

The only big project I've ever done was maintaining a proper database and customer interface for a cafe.

John Rennie

5,174,548 lines and counting

user228700

7:59 AM

Wow.

John Rennie

It takes enormous discipline to write apps that big in C++ without memory leaks and crashes due to dereferencing corrupted pointers.

user228700

> dereferencing corrupted pointers

user228700

::realises in shame how much I've forgotten::

John Rennie

To be fair, most introductory courses don't cover all the nerd-speak. You may simply have never come across the term.

Mithrandir24601

@JohnRennie Or even not even compile outside of debug mode...

John Rennie

8:01 AM

@Mithrandir24601 well compile time errors are at least relatively easy to spot and fix

user228700

I certainly don't know what corrupted pointers mean but the basic concept of pointers and dereferencing I've almost forgotten.

user228700

This was our syllabus

John Rennie

A pointer contains a memory address. Dereferencing is looking at the data at the address contained in the pointer i.e. following the pointer to see what it is pointing at. Pointers get corrupted if they get set to zero or a random number, or if the memory they point to has been freed.

user228700

AH YES, THANKS!

John Rennie

Dereferencing an invalid pointer is probably the number one cause of crashes in apps.

user228700

8:05 AM

Sheesh, I've forgotten to such an extent that I couldn't ever recall my syllabus correctly; we had SQL and Boolean Algebra as well.

user228700

@JohnRennie Hmm. A group of our seniors in school had been working on a game and this had been causing crashes. It took them weeks to identify what the problem was and could only do it with the help of my (excellent) teacher.

user228700

...I remember this now that you've elaborated. Our teacher recounted this to us after they'd fixed the bug.

John Rennie

@Kaumudi.H that's fairly standard for C++ :-)

Most modern languages don't use pointers so they're immune from that sort of crash, though they can still crash in other ways ...

user228700

Ohh, I see...

user228700

I only ever learned C++. Well, and SQL but that doesn't count.

user228700

8:09 AM

I bet we'll have C++ in the first semester or two so I think I'll add "Revise C++" to my to-do list for the summer if it does indeed turn out that I'll be taking up C.S

user228700

...and now it's turning out that I might just not be able to do all of the things on the list :-/

user228700

At this point, my to-do list is comically large and overflowing. I even have "Start learning to play the harmonica" on it x'D

user228700

Ooh, that reminds me: do you play any instrument?

John Rennie

@Kaumudi.H At school I played the tuba

user228700

Oh my, that's a big instrument!

John Rennie

8:15 AM

I haven't played any instrument since, though I used to sing in an amateur operatic society.

user228700

Ah, yes, you've told about the singing before...

Kenshin

CS sounds good

lots of demand for that now and in the future

but i'm sure other engineering branches also have potential

@Kaumudi.H if you got the best score, what branch would you choose?

what branch is the hardest to get place?

user228700

C.S.

Kenshin

interesting

I think in Australia other engineering branches are harder than CS to get a place

Balarka Sen

exhausted

Kenshin

8:24 AM

y

Balarka Sen

walked the way home

Kenshin

y

Balarka Sen

as a step in the proletariat revolution

Kenshin

i don't get it

Balarka Sen

what a bourgeois

Kenshin

8:26 AM

what is proletariat

user228700

Exhausted me as well :'-( Slept for < 3 hours last night.

Kenshin

ru communisit?

y so little sleep Kaumu?

Balarka Sen

Proletariat is the collective terminology for the working class in the postmodernist capitalist society.

user228700

@Kenshin Insomnia :-/

Kenshin

dats no gud

user228700

8:27 AM

Couldn't fall asleep till 3:15, had to wake up at 6:15.

Mithrandir24601

@Kenshin Over here, computer science varies a lot between universities. Some places have it very theoretical with electrical engineering, so that you actually understand what's going on. Others are essentially courses in how to program, yet others focus on things like games development, so how hard the courses are to get into also varies quite a bit

Sid

Insomnia is bad.

Balarka Sen

i slept at 6, woke up at 9

Kenshin

what did u do? just lie in bed awake for hours?

@Mithrandir24601 I see very interesting

user228700

@Kenshin Pretty much, yeah. Tried listening to boring af podcasts but that didn't work so I chatted with some nerdfighter frands for 10 minutes before they kicked me out when they found out that I wasn't sleeping and then yeah, just lay in bed till sleep came at last.

Kenshin

8:29 AM

r u still studying hard?

user228700

Yeah :-/ 3 days to go, man.

Kenshin

oh yeah not long at all

Sid

@Kaumu for?

user228700

bitsat.

Kenshin

until she leaves physics.SE

Sid

8:31 AM

I can bet she can't. Any SE site is incredibly addictive.

Balarka Sen

nah

user228700

^

user228700

Still, I will stick around. In the chat at least for as long as the majority of u guys are here and the main site till, well, I don't know, really; I don't see why I'd leave.

Balarka Sen

i have to decide what i want to do

Kenshin

how come Balarka

what level of life r u at

Secret

8:32 AM

Worldbuilding counterpart to the 3rd Law of Thermodynamics:

Balarka Sen

over 9000

Secret

Clarke's 3rd Law

Any sufficiently analyzed magic is indistinguishable from science

user228700

@BalarkaSen Yeah:

user228700

yesterday, by Balarka Sen

i got teleported from the 60's

Kenshin

so ur relaly old?

user228700

8:33 AM

Right? :-P

Sid

in The Sphinx's Lair, yesterday, by Mithrandir

Xkcd is a worse time sucker than Tvtropes. And SE is worse than both of them combined.

Balarka Sen

i pretend i am old. i don't really connect to the the young generation, morally physically or intellectually

""young"" ""generation""

Kenshin

so you like to date older ladies?

Mithrandir24601

@Kaumudi.H to be fair, I started of in CS, then ended up in Physics...

Secret

Given any worldbuilding setting where both magic and science are present, if the magic cannot violate Clarke's 3rd Law, then strictly speaking that setting has no magic

Balarka Sen

8:35 AM

i don't quite know how to reply to that ...

Secret

whether violation of Clarke's 3rd Law is a mathematical impossibility like the 3rd Law of Thermodynamics in the absence of infinite quantities is an open question, however

Kenshin

it's ok no judgement

how old r u tho?

Secret

However, it is unknown if Clarke's 3rd Law is actually well defined enough to be put into a mathematical setting so we can prove/disprove the existence of counterexamples

Kenshin

I need to know what pidgeon hole to put you in

Secret

One trouble is the highly context sensitive nature of language

user228700

8:37 AM

@Kenshin He's in 12th now so that would make him one year younger than me, 17.

Kenshin

ty Kaumu :)

Balarka Sen

biologically i'm a teenager

Kenshin

yeah I thogut u were older

I thought u were prolly 18

user228700

@BalarkaSen Right, but not intellectually, which is presumably why you hardly ever properly respond to "How old are you?" :-P

Balarka Sen

mhm

Kenshin

8:40 AM

yeah intellectually i'm like 15

Sid

@Kenshin that depends on what you consider the average intelligence of a 15 year old.

Secret

I am 22 but I behave like a 12 year old

Kenshin

lol

intellect peaks around mid 20's so basically my intellectwon't peak until mid 30s

Sid

I have a suggestion.

That feed is annoying.

Secret

Clarke's 3rd Law: Any sufficiently advanced technology is indistinguishable from magic

Let $T$ be the set of all technologies. Then Clarke's 3rd Law states
$$\forall A \in \mathscr{S}, \forall K: \mathscr{S}\to \mathscr{S}, \lim_{\{A: \max (dom(K))\}}\mathscr{S}=T$$

$\mathscr{S}$ is the collection of all consistent possibilities and $A$ is one of the possibilities. The maps $K$ indicate the subcollection of possibilities one given possibility can access to

Therefore, a sufficiently advanced technology is expected to have at least one map that has the largest image set

(which means I made a typo, $dom$ should be $img$ )

Clarke's 3rd law basically said that for all such sets that maximise their image set under the maps $K$, the image is equivalent to the set of technologies

Now the issue is, how to define $K$ explicitly

After $K$ is defined, then it will be possible to compute the proof of this problem

Put it in another way, magic is any set $A \in \mathscr{S}$ that does not get mapped to $T$ the set of technologies by all possible mophisms $K$. Clarke's 3rd Law means that $A$ is empty

io9.gizmodo.com/…

The stuff mentioned in the above link is meaningless if Clarke's 3rd Law is mathematically proven

In essence, proving something is a zero sum game, you either win, or the others win. The only case where it is not zero sum is that it is equivalent to the halting problem and hence undecidable

The reason why I construct the zero term algebra investigation some months ago is I want to show whether when they say impossible is really absolute. Impossible is a very strong word, if it is used, to me it often means a mathematical impossibility no matter how witty you are

The idea is that either they are shown to be wrong and they will shut up, or I am shown to be wrong and I will shut up

The goal is the same: When facing any ambiguity, the desire to shut them up tends to one

All unexplained things will eventually be explained, whether by science, by art or (if suitable) by religion

CWV: This world does not tolerate ambiguirities

Secret

9:20 AM

I don't care who is wrong or right, but given any heated topic that can be solved and the answer is either true or false, one party has to shut up after the statement is shown to be true (or false)

Of course:

Most real world problems are a continuum, and there is no true or false, which makes analysing them more difficult

But bleh, rambles are rambles. Better get back to work

Slereah

10:08 AM

"This article is dedicated to Rafael Sorkin, whose friendship and tutoring from third grade onward is responsible for one of us (JF) having spent his adult life in physics and whose work has inspired both of us."

John Rennie

10:50 AM

RIP the SciFi Stack Exchange chat room

Not that we ever discuss contentious things here of course :-)

Secret

What? Mos Eisley is gone?