The h Bar

heather

12:02 AM

okay, sorry about that

why does dinner keep interrupting great conversations

@DanielSank yes, I know; I apologize for this.

=/

DanielSank

It's ok :D

Bernardo Meurer

@heather Have you been talking to a tall German girl lately?

heather

uh...no?

I generally don't ask people their ethnicity during conversation

Bernardo Meurer

@heather Why would you ask someone their ethnicity if you can see them?

DanielSank

@BernardoMeurer wat?

heather

12:06 AM

@BernardoMeurer, I don't think I'd be able to distinguish a German and British citizen just by looking, for example

Bernardo Meurer

@heather Accent.

heather

you were talking about sight

ACuriousMind

@heather Okay, so let's think about combination of quantum systems: We start with two quantum systems, spanned by bases $\lvert \psi_i\rangle$ and $\lvert \phi_j\rangle$. We want to construct the Hilbert space of the system that consists of both of those systems. Is something unclear/uncomfortable about this setting?

Bernardo Meurer

And you about holding a conversation :)

heather

@ACuriousMind, those two vectors are the basis vectors, like $|0\rangle$ and $|1\rangle$, right?

Bernardo Meurer

12:08 AM

My point is, ethnicity is a concept that is based heavily on things that are "physical" about the person. height, accent, skin color, clothing,... So in general you don't have to ask for someone's ethnicity when you're talking to them, and the fact that it's so superficial is what makes it for the most part so stupid as a criteria

heather

@BernardoMeurer, in a nutshell: as far as I know, I don't think I've talked to a tall German girl consistently for the past couple days. =)

whatwhatwhat

Hello everyone!!

Bernardo Meurer

@heather Keep it this way, it's already bad enough without you perverting my neighbour's ideas :P

whatwhatwhat

No one is talking in the CS chat room :/

ACuriousMind

@heather Ah, my bad, they're not meant to be single vector. I mean that we have one system with a basis $\lvert \psi_1\rangle,...\lvert \psi_n\rangle$ and another one with a basis $\lvert \phi_1\rangle,...\lvert \phi_m\rangle$. You may think of them as states labeled by numbers from 1 to n and 1 to m, respectively.

Bernardo Meurer

12:11 AM

@ACuriousMind You suck at physics golf :P

heather

@ACuriousMind, okay, so we want to create a system that contains both systems - a basis that contains both basis?

ACuriousMind

In the simplest case, we have two qubits - two systems with bases $\lvert 0\rangle_1,\lvert 1\rangle_1$ and $\lvert 0\rangle_2,\lvert 1\rangle_2$, where the numbers outside the bracket tell us whether the state belongs to qubit 1 or 2.

heather

okay

and then our overall basis is just $|0\rangle, |1\rangle$, right?

ACuriousMind

@heather Yes, we're fed up with describing those systems separately, and we want to write a system that includes them both.

@heather what do you mean "overall basis"?

heather

the basis that contains both basises (what's the plural of basis?)

ACuriousMind

12:13 AM

@heather Well, $\lvert 0 \rangle,\lvert 1\rangle$ is the basis of one of these systems, but it's not the basis for the combined system.

Bernardo Meurer

@heather merriam-webster.com/dictionary/basis

heather

oh, wait, is it $|00\rangle,|11\rangle$?

Bernardo Meurer

"bases"

heather

@BernardoMeurer, oh. duh. thanks

ACuriousMind

the idea is that we combine all possibilities: If the first system is in the state $\lvert 0 \rangle_1$, the second can be "either" (don't take the either too seriously) in $\lvert 0 \rangle_2$ or $\lvert 1 \rangle_2$.

Bernardo Meurer

12:15 AM

TAKE THAT NATIVE SPEAKER

:P

ACuriousMind

If the first system is in $\lvert 1 \rangle_1$, we again have those two possibilities

So the combined system has four basis states: $\lvert 0 \rangle_1$ together with $\lvert 0\rangle_2$, $\lvert 0 \rangle_1$ together with $\lvert 1\rangle_2$, $\lvert 1 \rangle_1$ together with $\lvert 0 \rangle_2$ and $\lvert 1 \rangle_1$ together with $\lvert 1 \rangle_2$.

We're lazy physicists, so we write "together with" as $\otimes$.

heather

tensor product!

ACuriousMind

We're really lazy physicists, so we might adpot the notation $\lvert 01\rangle = \lvert 0\rangle_1 \otimes \lvert 1 \rangle_2$

But the important thing here is to understand what the "together with" means.

Bernardo Meurer

Physics: Mathematics golf

2

ACuriousMind

With just 0 and 1 as basis vectors, you can't see it, but say we have systems with 0,1,2 as basis vectors - how many basis vectors of the combined system do we get if the combine them, @heather? (It's okay if you say you don't know)

heather

12:21 AM

@ACuriousMind, I don't know, sorry

ACuriousMind

@heather It's okay to not know, and you don't need to be sorry :)

The idea is that we have to combine all possible basis vectors with each other

So, we pair the $\lvert 0\rangle_1$ with the $\lvert 0\rangle_2,\lvert 1 \rangle_2,\lvert 2 \rangle_2$ for the second system into $\lvert 0 \rangle_1\otimes \lvert 0\rangle_2,...$ etc., and likewise for the $\lvert 1 \rangle_1,\lvert 2\rangle_1$.

Am I making some sort of sense?

This is one of the parts of quantum mechanics that seem utterly incomprehensible at first and will seem utterly natural later, so it's alright to be confused

It is also, by the way, the key to understanding what an entangled state is, so...don't hesitate to try to take this notion apart. The only single statement of QM I would possibly rank over this is the Stone-von Neumann theorem but it's a bit too soon for that ;)

heather

okay, I think that makes some kind of sense.

it's like all the possible combinations

ACuriousMind

In other news: HATS ARE LIVE!

heather

hats?

HATS!

do you have the link?

ACuriousMind

@heather Exactly! So, for two systems of three basis vectors, how many basis vectors are there for the combined system?

heather

12:32 AM

6?

or wait, no, sorry

9

ACuriousMind

@heather You should see a...snowflake next to the part of the top bar where you usually view reputation increases

@heather Correct!

heather

yay!

(for both hats and getting the answer right.)

ACuriousMind

@heather Now, the point is that we have, by the basic assumptions of quantum mechanics, to allow linear combinations of these basis states. So the nine possible combinations of basis vectors give us a nine-dimensional Hilbert space.

heather

okay, that makes sense.

ACuriousMind

The "tensor product" of two vector spaces is just what mathematicians call the space spanned by all possible combinations of their basis vectors, i..e in our example just the 9D space we constrcuted.

heather

12:36 AM

oh...that makes tons of sense!

::lightbulb::

ACuriousMind

Warning: This notion of tensor product will need to be seriously adapted when you learn about tensors in field theories/special relativity/general relativity, but it is fine for now

heather

okay.

thank you so much!

I have to go now.

ACuriousMind

@heather Okay, see you

heather

New avatar already, man =)

Physics Meta

12:58 AM

0

Q: Winter bash 2016 - hats are live!

Hooray, hats! (Shameless plug towards a hat.) http://winterbash2016.stackexchange.com/ Hat options: I am your father - post/vote on meta Running ragged - earn 150 rep on three non-SO sites in 15 days 011 - collect 11 hats Snapchat - answer in 30 minutes, +3, and accepted Other hats I haven't ...

discussion

5

Junaid Aftab

1:37 AM

Yes, Caroll's book seem like a good choice. I think we may be using it for some parts of the GR course as well, along with either Hartle/Schutz. But I guess Caroll seems to go way too fast with the material; also, we may review SR early on in the course so I can do Caroll once the term starts.

Any other suggestion of a book that'll not only help me get a feel for tensors and the manipulations that come with it but also some worked examples that build on big picture of SR. I guess I should have worked through the problems from Taylor in a timely manner, so I could have just revised it now.

user116211

2:27 AM

It's the Hats!!

user116211

ACM has already donned it ;)

user116211

0

Q: How to get free Dirac Lagrangian in terms of left and right-handed fermions?

How to get Lagrangian of free Dirac field $$\mathcal L = -\psi^{+}\gamma^4\gamma^\mu \partial_\mu \psi - \psi^{+}\gamma^4 m \psi$$ in terms of Left and Right-handed spinors $\psi_L$, $\psi_R$ ?

dirac-equation

user116211

Homework?

user116211

I'm at least tagging it with homework.

DanielSank

2:47 AM

Hi, everybody.

user116211

@DanielSank o/

user116211

@Bernardo is ruling the Star-Board ;)

user116211

And here I get my first two hats \o/

Bernardo Meurer

@MAFIA36790 If only my german neighbour could see it

DanielSank

@MAFIA36790 \o

user116211

2:50 AM

Devil hi?

DanielSank

oops

edited

user116211

;)

DanielSank

How to get hats?

user116211

@DanielSank, prepare for the hats!

DanielSank

How?

user116211

2:51 AM

@DanielSank winterbash2016.stackexchange.com

Bernardo Meurer

@DanielSank How on earth do you accidentally type a Cthulhu like that?

user116211

And there are secret hats too.

Bernardo Meurer

Somehow no one told Chemists that using magic numbers is bad practice

DanielSank

@BernardoMeurer I was doing Russian lessons.

I typed without looking in the chat box and didn't notice I was still in the Russian keyboard.

Bernardo Meurer

I should make a twitter, maybe I'll get some friends and be famous

user116211

2:57 AM

Heard of the Great Yahoo Hack @bernardo?

Bernardo Meurer

Yahoo is a meme

Oh fuck

@DanielSank I have a fever

MY CALCULATOR HAS NO BATTERY

I CANT MONITOR MY TEMPS

user116211

@BernardoMeurer :(

Bernardo Meurer

MY CHEM EXAM IS TOMORROW

user116211

@BernardoMeurer uff ;/

Bernardo Meurer

ABORT MISSION

DanielSank

2:58 AM

@BernardoMeurer Pobre cito.

Bernardo Meurer

@DanielSank Dude why are you being mean to me

I only give you love

DanielSank

^ False

user116211

@BernardoMeurer 0celot would want proof.

Bernardo Meurer

@DanielSank Oh please, that is super true

Dude what do I do

user116211

You are making it an axiom, @bernardo.

DanielSank

3:00 AM

@BernardoMeurer about what?

Bernardo Meurer

@DanielSank A fever the day before the exam of the one subject that absolutely reks me every time

DanielSank

@BernardoMeurer Get sleep. If you're still fukt tomorrow, go to doctor.

What is your uni's policy?

Bernardo Meurer

You either go to the exam or you get rekt

v. good policy

DanielSank

k

Go to exam then, since you have zero choice.

And stop getting sick.

Bernardo Meurer

I ain't going to some doctor because i have the flu! I'll call the Slovenian and ask her to make me tea or something

user116211

3:04 AM

@DanielSank Dabble in Chemistry, irrespective of what career you want to go.

DanielSank

@MAFIA36790 What?

Bernardo Meurer

And pretend like I'm 300x more sick than I am in hopes of getting compassion because I am needy

Actually no, my plan is bogus, she went home yesterday

THERE IS NO HOPE

DanielSank

@BernardoMeurer That sounds like a mistake.

@BernardoMeurer You are so melodramatic.

I was telling a friend about you, and her one comment was that you sound dramatic :P

I found that amusing.

user116211

And now the question boils down to how $1/T$ acts as an integrating factor.

user116211

It's even explained in Fermi AFAIR.

Bernardo Meurer

3:10 AM

@DanielSank Tsc, she clearly does not appreciate my other faults enough

user116211

There are Vader and Yoda hats!!

user116211

But no Indy Fedora ;/

user116211

@DanielSank, I learnt a beautiful fact last week on variational calculus.

user116211

Generally the boundary term we get while minimising the action is neglected for it gets vanished; but actually it plays a "more active role".

user116211

To get a unique solution, we need a proper number of boundary conditions.

user116211

3:23 AM

When the imposed conditions are not sufficient, the boundary term of $\delta S$ comes to rescue.

user116211

It furnishes the right number of boundary conditions we need for a unique solution.

user116211

And this is the beauty of variational problems.

user116211

Not a highly sophisticated fact, but full of insight.

user116211

0

A: Theoretical proof of the constant of speed of light $c$ in vacuum in all frames of references

Of course not. This is physics we are talking about, not mathematics. There is no "theoretical proof" of anything.

DanielSank

@MAFIA36790 How to get?

user116211

3:28 AM

@DanielSank For the former, you have to vote/post in meta; for the latter, you have to use the SE search for 3 consecutive days.

user116211

> Religious statements and assorted nonsense such as string theory are not elements of this class.

user116211

What is going on with this answer? O.o

user116211

Hope that Lubos have not seen that post.

DanielSank

4:17 AM

I got a chat server working in python with no libraries!

It's crappy and I need to fix the protocol up a bit, but it works and uses nothing but sockets and the select system call.

user116211

@DanielSank, you are in the hats Leaderboard.

DanielSank

@MAFIA36790 wat?

how

user116211

@DanielSank winterbash2016.stackexchange.com/leaderboard

user116211

@DanielSank Because you got hats?

DanielSank

I only got one!

Run this, and telnet into it from a few different separate terminal sessions and you can chat with yourself!

user116211

4:30 AM

@DanielSank And now Bernardo entered the Leaderboard.

DanielSank

4:54 AM

The chat server is kinda crappy in that it doesn't display the messages in a nice way ('cuz it uses telnet). I'll fix that up soon though, since there's already a solution to that problem in the same repo.

Also, the variable naming isn't great. Pull requests are encouraged.

Pissed off layman

i have enough problems chatting with real people, thanks anyway :-)

David Z

@DanielSank those are better variable names than I typically see

Pissed off layman

do you indulge in self-chat?

sounds like a tv commercial :D

user116211

> Enough with faradays law and maxwell equations all the people go towards them my only

user228700

6:18 AM

@JohnRennie: Dyou know which number to call them at?

user228700

This number: 0800 100 600 is invalid (as given in their website under "Contact Us".)

John Rennie

Ah, that's a UK freephone number

There is a presumably a number for TNT India somewhere on their web site ...

user228700

:-| Okay, I will check...

John Rennie

tnt.com/express/en_in/site/home.html

@Kaumudi.H Call us 1800 208 9999 / 1800 425 9999

user228700

(Y)

user228700

6:26 AM

Wasn't it my e-mail address that u registered with?

John Rennie

I booked the delivery using my own details but gave your e-mail address and your father's telephone as the delivery contact details.

user228700

facepalm. My father just called them, Sir and boy would it have been better to call them a few days ago.

user228700

Apparently, the customs office haven't been able to contact me, which is why it's just been sitting there.

John Rennie

That's odd. I thought you had got a call from the Customs office, or rather your father did.

Anyway, ANYWAY, ANYWAY

user228700

Yes, he did. That was one week ago. I wonder why they haven't been able to contact us.

user228700

6:31 AM

@JohnRennie What!

John Rennie

It's on the way!!!

It's only ON THE WAY TO YOU :-)

user228700

I have not allowed myself to start hoping again. The lady on the other end has noted down my e-mail id and she told my father that the customs office will first e-mail me about it.

John Rennie

There's no reason to lose hope. It's still in the air when it will be delivered - that depends on how efficient the customs office are.

user228700

Now I have to wait for the e-mail. I have a funny feeling about this. Why should the customs office need to contact me? And they already have my father's number, why didn't they call?

John Rennie

But you know it will be delivered.

user228700

6:35 AM

Well, yes, but what on Earth is happening right now? I am finding it a little difficult to believe that there was such a long queue at the customs office. Well, either that, or the customs office hasn't been able to contact me, which is plainly absurd.

John Rennie

Actually, now I think about it, was the call from the Customs office or from TNT?

user228700

Last week? That was TNT.

John Rennie

Aah, so you've never been contacted by the customs office themselves.

So it could be that the contact details were never passed to the customs office, or that the customs office lost them.

Oh my God! Princess Leia and Han Solo had an affair.

My world is shattered!

user228700

@JohnRennie Yes. Gah. Now how much longer will it take? :'-(

John Rennie

It can't take much longer. You'll get it this week.

user228700

6:44 AM

I'm not willing to bet on that. Sigh. The excitement will return only once I have it in my hands, I suppose.

user228700

Ah, nvm.

user228700

I effing hate the customs office for taking this long -_____-

John Rennie

It's annoying that the customs office didn't contact TNT and ask them to contact you.

user228700

Yeah. It's been a week, for God's sake. Did they think they could get away with not bothering?

user228700

I just hope it doesn't a whole week more. If it does, I will get very mad >.<

Secret

7:24 AM

[Scifi] Good luck finding a chemical or physical reaction with such a ridiculously low ground state for the product that once the reaction starts, the heavier stuff becomes exponentially more stable, thus allow all the mass to reach the schwartzchild limit very quickly...

Stark Trek 2009 - Red Matter Explosion Scene

►

which brought another related question: Can there exists thermodynamic process such that the equlibrium of such system has a mass of a black hole...?

koolman

How to find area of triangle in 3D space

user246160

Area of Triangle using Cross Product

►

user246160

7:41 AM

or math.stackexchange.com/a/1483219/388483

user246160

9

A: how to calculate area of 3D triangle?

Heron's formula is easiest as it "requires no arbitrary choice of side as base or vertex as origin, contrary to other formulas for the area of a triangle:" $$A = \sqrt{s(s-a)(s-b)(s-c)}$$ where $s=p/2$ is half of the perimeter $p=a+b+c$ (called the semiperimeter of the triangle). The triangle sid...

Sir Cumference

Howdy

user246160

@SirCumference hello

koolman

Ohk i will see it

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