The h Bar

Anonymous

1:31 AM

@Gugg Then why don't we also have a tag for the Heisenberg equation ???

Anonymous

Why only the path integralb and the schroderingalr equateeion? . Lubos Motl would be angry .. .

Anonymous

  motls.blogspot.in/2013/08/…

Anonymous

... (Anti-Schrodinger post) motls.blogspot.in/2013/08/…

Crazy Buddy

2:54 AM

@ChrisWhite Ah... So, both phase & group velocities have nothing to do with carrying information?

Okay.. I screwed up. Now, where's the information in the wave? :)

@RhysW We're entangled macroscopically sweetie. We do the same everywhere :D

@ChrisWhite And, who might that be? Would he care to teach me? ;-)

ShuklaSannidhya

4:04 AM

$a = dv \over dt \implies a dt = dv$ ?

Crazy Buddy

So..???

ShuklaSannidhya

Thats a question.

Crazy Buddy

Actually, don't use \overbecause the fractional part would cover the whole expression... ;-)

ShuklaSannidhya

4:18 AM

oh crap.

I meant $a = {dv \over dt} \implies a.dt = dv$

?

@CrazyBuddy Can we transpose $dt$ like that?

ShuklaSannidhya

4:33 AM

@ManishEarth ?

$dv \over dt$ is not a fraction right?

I've seen in many books that they transpose $dt$ to the other side of the equation. How can they do that when it is not a fraction?

Crazy Buddy

@ShuklaSannidhya Yeah.... I've noticed that in some expressions... ;-)

@ShuklaSannidhya Isn't that a fraction? o_O

Even in my mechanics, they're using the same expression... They just multiply on both sides -_-

Well, $dv$ and $dt$ aren't just numbers to be multiplied on both sides... I think that's more like an approximation, may work for small order derivatives like this one :)

@ShuklaSannidhya: Here's a discussion on that ;-)

physicsforums.com/showthread.php?t=328193

Crazy Buddy

6:01 AM

BTW @ManishEarth: You'd be happy to see this (as a Chrome user) :P

0

Q: Windows 8 - Does Firefox have any incompatibility?

I installed Windows 8 in my laptop. And, my browser is Firefox 22 (as usual). FF22 didn't have any issues with XP or Windows 7. But in Windows 8, it doesn't respond when some page is loading. Say, I've opened a link. Now, I click anywhere on the window (it may be menu, another tab, minimize butt...

hwlau

6:13 AM

@CrazyBuddy No one upvote you, so it is your problem?

Crazy Buddy

@hwlau Nope... We had a discussion (a few weeks ago) regarding browsers...

He clung to Chrome, while I stuck with FF....

hwlau

@CrazyBuddy Actually, i don't like this fast update model. It just make random breaking from time to time.

Crazy Buddy

And, welcome to our chat...

@hwlau What???

hwlau

@CrazyBuddy firefox 20 breaks one of my favorite addon, and it is hard to rollback...

Crazy Buddy

ahh...

hwlau

6:17 AM

And I am not new here...

Crazy Buddy

I welcome people who haven't spoken for a few months :P

hwlau

ic :)

Crazy Buddy

@hwlau: Seems like we've spoke before... Your last convo was with me... My brain... forgets everything :D

hwlau

@CrazyBuddy Yup, I remember you, em, your pic. Probably you have talked too much here

Crazy Buddy

@hwlau Yeah... No one will forget it. Too much??? I'm quite less than Manish :)

hwlau

6:24 AM

@CrazyBuddy Almost everytime I go to chat, I see your logo.

somewhere

Crazy Buddy

Hah... Well, I keep Facebook and SE chat online all time (Just another tab - what's gonna do much?) :D

hwlau

Glad to see that you rate SE the same as your facebook!

Crazy Buddy

@hwlau What? No... Umm.. well, both are chat... In one way or another, SE chat & FB chat are same (only exception that FB chat is private) :P

hwlau

Too little people in this chatroom. I just have a look of other site charroom, there are more people talking compared here

Crazy Buddy

@hwlau Yeah... We're always dead. You should've seen the Physics chat session. Only 3 or 4 people hang along (that too, can disappear someday) :D

@hwlau Canadian... You don't sleep? :)

hwlau

6:31 AM

Just wake up...

Crazy Buddy

Humph... No one in Physics.SE sleeps :P

hwlau

probably you dont, always see you here

Crazy Buddy

@hwlau Yeah... I won't (include Manish too) But, he has left me for in charge of this room :)

hwlau

@CrazyBuddy This is a good position for you :)

Crazy Buddy

Or the other way, he's afraid of me... He won't come during daytime, because I'll be around disturbing him :D

There are a few people who (came in the past) became tired of coming to this room because I'd be around (David, Chris, Rhys, Manish)

hwlau

6:35 AM

@CrazyBuddy you get 5k rep by simply opening the other stackexchange account?

There needs someone in chat room to keep it alive

Crazy Buddy

@hwlau What??? Hey... I'm not a sock-puppet :D

hwlau

What do you mean

Crazy Buddy

I mean, I don't have another Phys.SE account (Did I misunderstand something?) o_O

hwlau

I mean you get total rep over all SE, by opening other SE and gained 101.

There are so many SE site this day.

Crazy Buddy

@hwlau Yeah.. Of course...

105 I think...

Only 7k is mine... 10k is from other site :P

hwlau

6:41 AM

Really, there are >100 sites here now!

Crazy Buddy

Yup... :)

hwlau

@CrazyBuddy I am just thinking is it possible to guess the background of the person from their Q&A history?

Crazy Buddy

1 min... I'll brb :D

@hwlau Umm... No we can't. Some can write better English, some can write ugly English, their way of presenting things..

hwlau

You English seems better now

Crazy Buddy

For instance, you can see my questions (I asked from Aug - Oct., then I stopped asking) and answers....

@hwlau That's what I'm saying...

11 hours ago, by ManishEarth

@Crazy wrote a blog post about his SE story

And, don't forget to read my post :)

hwlau

6:52 AM

@CrazyBuddy Nice post

I guess most people know stackoverflow before this site

Crazy Buddy

@hwlau Thank you :)

Yeah... Because, this site was launched by area51 :D

SO is a lot... She was the mother of all ;-)

hwlau

ya, at the time I find SO interesting, I also see this site in the earlier beta.

Crazy Buddy

@hwlau Hey... You're following this from the beta itself..!!! o_O

Quite an experienced user :)

hwlau

yes, but I am not as experience as you. I just go here and there.

Glad to see this site is still here

not like the theoretical physics...

Crazy Buddy

@hwlau Well, you read a lot of posts...

Me? Do you look at my age? o_O

BTW @hwlau: It's my topic going on at Root Access chat room :P

hwlau

7:00 AM

@CrazyBuddy Age? Many kid have much better programming experience than me, so it obvious not the prolem of age

What is root access

Crazy Buddy

Root Access

For all you Super Users out there. You have backups, right?

4

SU chat room :)

hwlau

I find that myself

thx

Crazy Buddy

@hwlau Agreed. But, believe me... I dunno much programming stuff... But, I survived at SO at my first entry :)

hwlau

I just mean that younger person can have better knowledge than me, may be in programming or physics

Crazy Buddy

That's the way mostly everyone think: "I don't have much knowledge. He knows a lot than me"

Everyone do have this inferiority complex (me too..)

But, I've chosen to ignore my Ego :P

hwlau

7:06 AM

Since you are a student...

and it is possible that knowledgable than you in all possible are.

Crazy Buddy

Whatever...

So, do you work? what's your field of specialization?

hwlau

You dont think I am a student?

grad...

Crazy Buddy

@hwlau I thought you aren't...

Ohh.. Grad student. Why am I always getting stuck with grads? :P

hwlau

i tink there are many students here :)

Crazy Buddy

No... Only me, Manish, some people come & go sometimes (not too enthusiastic) :D

hwlau

7:13 AM

err, I don't know what they are work on.

Crazy Buddy

You, Chris, David, tpg are grads... Others --> experienced physicists :)

hwlau

I don't really think that there are many physicists here

probably few dozens only

but I know that there are physics professors read this site, but not registter

probably after the dead of TP.SE, they dont participate anymore

Crazy Buddy

@hwlau Why do you say so? IMO, there are many physicists here...

I don't want to define them... But, they do respond in a way that they're as crazy as a physicist :)

hwlau

Because I don't really know who are real physicist

Crazy Buddy

They're like dark matter. You can't find them :P

hwlau

7:22 AM

Thats why I say so

I think most of them are only watching

There are some good self-learner

and few fake physicists (you know)

Crazy Buddy

@hwlau Pick me.. Pick me... Yeah.. that's right..!!! Me :)

@hwlau What's that fake o_O

hwlau

no good knowledge and not physics professor, but pretend so

Crazy Buddy

@hwlau Maybe their way of expressing makes them look like that...

Why should we care about that? If their answers are right, if they got the right idea, we can encourage...

@hwlau: If you allow me to say, I was a guy who rote things (when I was learning high school) till I entered this site...

I did peek into things which I don't know at all..!!!

But, in some way, I was curious to learn things... My merciless roting gone and this generation of ideas came in...

It took a lot of months really (about 8 or something)

I don't know how I changed, but now - I can't even memorize a single expression ^_^

hwlau

@CrazyBuddy No, You can judge that from their answers, because it is either wrong, very ambiguity and say nothing

Crazy Buddy

Ahh... like that... Yeah :)

hwlau

7:30 AM

and they keep doing so. Especially, they pretend to know much and professional, but iti is not the case

Crazy Buddy

@hwlau You can find some in my answers too (that's history)

Yep... I understand.

hwlau

Though some answer are right, but you know that they are not good at that

Crazy Buddy

But, there are people who struggle to understand these things. They get to know someday :)

hwlau

Everyone know that you are students, so there is no problem

but not the people I am talking about

Crazy Buddy

ah...

hwlau

7:33 AM

anyway, most people are like you

It is good thing that you learn how to udnerstand physics without memorizing expression

that is good sign that you can study further

Crazy Buddy

I know... And, I'm always happy that there are people here to help me, when I got stuck somewhere... :D

hwlau

Many of students keep memorizing and loss all interests in university eduction.

Crazy Buddy

@hwlau That has some serious consequences. (I think you may know) My academics is totally engineering (aeronautical)

If people can't memorize here, they can't survive..!!!

So, I struggle...

Just, I've used to ignore those (too) -_-

hwlau

@CrazyBuddy I do not mean that you don't need to memorize formula, but there are more fundamental things that you should know under the formula/

Crazy Buddy

Yeah... I know. For instance, take my example.

I have a paper called Engineering Mechanics...

2 or 3 chapters is based on inertia tensors...

For that, we have to memorize things from
http://en.wikipedia.org/wiki/List_of_moment_of_areas
http://en.wikipedia.org/wiki/List_of_moments_of_inertia

At least, most of them (neglecting 2 or 3)

I know how these things can be derived (have some intuition on these)...

But, for exams - If I don't memorize any expression, I can't answer anything..!! :/

Even if I try to visualize something for not-getting-it-away-from-mind, I forget ;-)

hwlau

7:45 AM

@CrazyBuddy I am not quite sure about aeronautical. Wouldn't they give you the formula sheets for them unless they ask you to derive it?

Crazy Buddy

@hwlau What? Formula sheets? what are they?

hwlau

A sheet listing only common formula that you need...

Crazy Buddy

My question paper has some problems. I have to recall all my formulas and put them in the problem..!!!

@hwlau Never heard about it :)

hwlau

what?

are you kidding?

Crazy Buddy

Ha ha... That's why I suck..!!! :D

I've to memorize formulas :D

hwlau

7:48 AM

Where is your univeristy?

Crazy Buddy

@hwlau India... Chennai :D

hwlau

i was thinking that it is common practice for physics

ic

Crazy Buddy

@hwlau Nope... I don't think Physics even exists in our College...

A phrase you can find in my blog...

> I don’t give much importance to my academics, because I’ve already begun to hate “Engineering” which I believe is due to our educational system, currently (for more than a decade) affected by a pedagogical virus. While my fellow students who’ve gotten used to this scheme, can rote everything mercilessly, I can’t even digest a single formula (due to my low IQ or something)…

Madras Institute of Technology

The Madras Institute of Technology (MIT) is an engineering institute located in Chromepet, Chennai (Madras), India.It is one of the four autonomous constituent colleges of Anna University It was established in 1949 by C. Rajam as the first self-financing engineering institute in the country, and later it merged with Anna University. The institute was at that time an experiment in technical education, for it introduced to India new areas of specialization: aeronautical engineering automobile engineering, electronics engineering and instrumentation technology. The MIT is the first i...

hwlau

Is the situation general in India or not?

Crazy Buddy

@hwlau I'm pretty sure it exists in our state... I presume it's all over India too...

hwlau

7:52 AM

@CrazyBuddy MIT, is the people really call it like this

Crazy Buddy

@hwlau Yep :P

hwlau

@ManishEarth Welcome :)

Crazy Buddy

First, when I entered SE, I wrote undergrad student at MIT, people were fooled...

Then, I gave a wiki link to my college ;P

hwlau

Nice man!

Crazy Buddy

@hwlau He'd be reading from the start :)

hwlau

7:55 AM

@CrazyBuddy I always have problem when I go in any chartroom in the middle of a chat

Crazy Buddy

@hwlau Why's that so?

hwlau

@CrazyBuddy I just can grasp what they are talking about in a short time.

without read a lot

Crazy Buddy

Ohh...

2 days ago, by David Z

or rather, don't have this attitude that when "professionals" are talking you need to stay out of their way.

ManishEarth

@ShuklaSannidhya It's not, it's the operator d/dt of v. However, for first order total derivatives, you can split it. It's an abuse of notation, but it works and it's not too bad to use it

Crazy Buddy

3 hours ago, by Crazy Buddy

Well, $dv$ and $dt$ aren't just numbers to be multiplied on both sides... I think that's more like an approximation, may work for small order derivatives like this one :)

I'm righthy then? o_O

ManishEarth

7:58 AM

@CrazyBuddy not really

hwlau

You can think it as $\Delta v$ and $\Delta t$

ManishEarth

Yeah

Crazy Buddy

I said, they aren't numbers o_O

Crazy Buddy

8:23 AM

Hmm... CERN is open :)

Stack Exchange

ManishEarth has made a change to the feeds posted into this room

Crazy Buddy

home.web.cern.ch/about/updates/2013/08/…

2

@StackExchange What did he do??? o_O

RhysW

8:42 AM

@CrazyBuddy looks like he added a feed of new questions tagged quantum mechanics from the chemistry site

Crazy Buddy

@RhysW Yeah.. I saw that :)

RhysW

but you asked what happened 0.o im confused :L

Crazy Buddy

@RhysW I was ragging Stack Exchange for a while :D

RhysW

oh xD

you really are crazy :P

Crazy Buddy

@RhysW I know..!!! That's why I'm very hard to forget :P

RhysW

8:50 AM

thats true

Crazy Buddy

updating profile to include my blog :)

Physics Meta

10:23 AM

0

Q: Are we missing an off-topic close reason

We seem to be missing an off-topic close reason. I think. Unless I'm missing something. When I select off-topic, I can choose from: homework with insufficient research engineering fringe rather than mainstream physics belongs on math.SE or meta.physics But there are potentially plenty of off...

feature-request close-reasons

Gugg

10:34 AM

@ChrisWhite Thanks for your suggestion! (And I'll let you know if it somehow disappoints.)

Crazy Buddy

12:44 PM

in Android, 4 hours ago, by Dan Hulme

Admit it: calling it a rant is a ploy to get people to read the post. :-)

^^^ Response to my blog's title :P

ShuklaSannidhya

12:57 PM

Is ${dy \over dx} = {1 \over {dx \over dy}}$ ?

@CrazyBuddy?

@ManishEarth ?

Crazy Buddy

@ShuklaSannidhya Of course... The inverse of a derivative. They both are same :)

ShuklaSannidhya

OIC

Crazy Buddy

@ShuklaSannidhya I think you'd have to disturb math users for such things :P

But, but, but... Wait till Manish appears.. He'd say Not really :D

hwlau

1:21 PM

@CrazyBuddy Is there are general plugin that can convert all equation in $ into latex format?

Crazy Buddy

@hwlau Yep... Chat Jax...

We use it often in chat...

hwlau

where can i find that

Crazy Buddy

Well, I used it in Facebook too...

1 sec..

Manish wrote a post on Chem...

3

Q: MathJax in chat (ChatJax offshoot)

This is an offshoot of ChatJax, which enables MathJax along with mhchem on chat. Copy the text below: javascript:(function(){if(window.MathJax===undefined){var%20script=document.createElement("script");script.type="text/javascript";script.src="https://d3eoax9i5htok0.cloudfront.net/mathjax/lat...

discussion mathjax

hwlau

@ShuklaSannidhya Not true in general. Though it is likely true in most physics problem, there are criteria it need to satisfy such as continuity.

Crazy Buddy

Physics is very crude.... Likely true... :P

hwlau

1:25 PM

You will see the divergence for the inverse when dy/dx=0

@CrazyBuddy Because it is likely continuous except some situation

Crazy Buddy

Ohh... I see... ;-)

hwlau

You dont see it in your undergrad courses

eg at phase transition

Crazy Buddy

@hwlau I don't even have Physics in my undergrad courses... That's why I take rest in college and learn these at home :P

hwlau

Nice. You can learn yourself

Crazy Buddy

First, I should learn a few basic things, make them strong, then I should care about more.... :)

Michael Brown

1:51 PM

@CrazyBuddy Tried using that but it was broken. Replacing the "https://d3eoax9i5htok0.cloudfront.net/mathjax/latest/MathJax.js" url with "http://cdn.mathjax.org/mathjax/latest/MathJax.js" seems to fix it. Cool script!

Crazy Buddy

ahh....

@RhysW: You use delicacy right?

(theme on wordpress)

ManishEarth

2:08 PM

@shukla if y(x) is a diffeomorphism, yes. Basically, y(x) must be continuous and have a continuous inverse. Also y'(x) must not be zero in the domain of interest

But dy/dx=1/dx/dy isn't an abuse of notation

hwlau

2:22 PM

@ManishEarth Is the condition you written the necessary and sufficient condition? I forget the details of my analysis class which is many years ago

Crazy Buddy

@hwlau: I was asked a question by a friend...

If I were to add mass to some object (suddenly) bound in an orbit, what would happen...

Alyosha

Hello

Crazy Buddy

@hwlau: I said, the orbiting object is screwed... What do you think?

@Alyosha Hi there... ;-)

Welcome...

Alyosha

Does anyone here know some simple QM?

Crazy Buddy

@Alyosha You'd have to wait for Manish... :D

Or, just ask it away...

ShuklaSannidhya

2:29 PM

@Alyosha QM can't be simple.

Alyosha

When one has to normalise $\psi(x)$, you occasionally encounter the integral $\int_{-\infty}^{\infty} \cos(x) dx$. However, this integral's value is not defined (as it takes different values according to how you go about calculating the area). What is done here?

This is time-independent Schroedinger equation, by the way, although I doubt the answer wouldn't generalise.

I'll give a concrete example.

Let there be a constant potential $V(x)=V<E$. Then

$\frac{\partial^2 \psi}{\partial x^2}=-\frac{2m(E-V)}{\hbar^2}\psi$

$\psi(x)=Ae^{i \frac{\sqrt{2m(E-V)}}{\hbar}x}+Be^{i \frac{\sqrt{2m(E-V)}}{\hbar}x}$

Wait, you have chatjax on Phys.SE, don't you?

$\psi(x)=(A+B) \cos (kx)+i(A-B) \sin(kx), k=\frac{\sqrt{2m(E-V)}}{\hbar}$

$|\psi(x)|^2=A^2+B^2+2AB (\cos^2(kx)-\sin^2(kx))$

$|\psi(x)|^2=A^2+B^2+2AB \cos(2kx)$

ManishEarth

@hwlau dunno, so did I :P

Alyosha

So if you want to normalise $|\psi(x)|^2$, you have to integrate cosine from minus infinity to infinity, which is undefined.

ManishEarth

@aly That's a propagating particle it seems

No normalization

Alyosha

Oh, so normalisation is only for a bounded particle?

ManishEarth

2:38 PM

A free electron also gives a sine wave. That just means that it's propagating as per de Broglie

I think so

Alyosha

So the amplitude can take any value?

ManishEarth

Well, technically a particle in a box isn't bounded

(finite box)

Basically, if the particle is free in an unbounded area, then there's no normalization

Alyosha

Why?

ManishEarth

I think the A has no meaning here. Not sure

RhysW

@ManishEarth some particles arent free? you keep them as slaves? :O

ManishEarth

2:40 PM

Most aren't free

> The wavefunction is not normalizable for a plane wave, but is for a wavepacket.

(Wikipedia page on free particle)

(a plane wave is the 3d equivalent of a sine wave. exp(ik.r)

Alyosha

I think I see why now. If space is homogeneous, then the probability of a free particle being at a point is independent of the point chosen. Thus if you integrate the probability over all space and expect to yield a finite value, $|\psi(x)|^2$ must be $0$, as for all other functions the integral is infinite.

ManishEarth

Yep

ShuklaSannidhya

I don't understand - this derivation

hwlau

$\cos(x)$ is not normalizable wavefunction, so it is not in Hilbert space, but you can use it to construct a normalizable wavefunction.

ManishEarth

So

So yeah, A has no meaning here :p

@shukla fix the link plz? On a talet, can't easily fix it myself

Michael Brown

2:44 PM

Strictly speaking the space of legal states in quantum mechanics (the "Hilbert space") only includes wavefunctions which are normalisable. So the plane waves aren't actually valid quantum states, but it is so convenient to use them that many people pretend they are. This can be justified mathematically by going to a "rigged Hilbert space", but really in the end you always have to make wavepackets.

Alyosha

Does that mean that there is a probability of $0$ of finding a free particle at any specified point in space, or does the whole quantum-mechanical probability thing not apply for free particles?

ManishEarth

@michael but from a basic point of view, they are solutions of the Schrödinger equation.

@aly The momentum is completely known, so the uncertainity in position is...?

ShuklaSannidhya

@ManishEarth Fixed?

Michael Brown

@ManishEarth That alone doesn't make them valid quantum states. :) The Hilbert space is usually defined as $L^2 (R)$.

hwlau

@ManishEarth They are solutions of Time Independent SE

In reality, you have to wait for infinitely long time to see a electron everywher with equal probability

Michael Brown

2:48 PM

@Alyosha Free particles are definitely ok in quantum mechanics. Just to be fully rigorous you need to use plane waves. :)

ManishEarth

@michael but those aren't normalizable either, right?

@michael hmm, makes sense. Didn't know that there were other criteria.

Michael Brown

@ManishEarth Yes they are - that's the space of square integrable functions

ManishEarth

Yep

@shukla. Hmm, dunno. Maybe because it took a partial derivative?

Alyosha

@MichaelBrown So is what is strictly spoken (i.e. the statement that only functions in Hilbert space are valid wavefunctions) incorrect? Sorry for being pedantic but as a novice I can't differentiate rigorous statements from 'oh well, that sort-of works'.

Michael Brown

@ManishEarth Normalizability is usually required. Like I said you can relax it by going to a rigged Hilbert space, but the only fundamental reason for doing so is convenience. Unless you are dealing with literally infinite spaces :)

@Alyosha That quantum states belong to a Hilbert space is an axiom of quantum mechanics. Unfortunately quantum mechanics is usually introduced historically so you may not see the axioms for a while...

ManishEarth

2:54 PM

Yeah. I always thought that normalizability is something that comes out of the equations when applied to realistic situations.

...which it does. A truly free particle isn't really observed iirc.

Michael Brown

The reason you can get away with using plane waves is because quantum mechanics is linear. So you can describe any normalizable state by a superposition of plane waves and work out everything one plane wave at a time, so to speak.

ManishEarth

Yep

I've done that while learning solid state physics on the side

Alyosha

@MichaelBrown You mean you can take the Fourier sum?

Michael Brown

Yes

Alyosha

But it contradicts an axiom, so surely either the axiom is wrong or plane waves don't exist in the real world?

Michael Brown

2:58 PM

They don't exist in the real world, i.e. in the end observable quantities are derived from wavepackets.

ManishEarth

Yeah

The FT is normalizable, the elements themselves aren't

Alyosha

Ah, thanks, I think I get it now. A finite sum of plane waves can be used to approximate a wavepacket, although the sum of plane waves doesn't exist?

ManishEarth

Not necessarily finite

Michael Brown

Another trick that is commonly used is to put your system in a big imaginary box. Now space is finite and the plane waves are normalizable. If the size of the box is much bigger than your system then the edge effects don't matter to any observables. You then take the limit of volume -> infinity at the end of the day.

ManishEarth

I'm doing something similar in my music analysis

Michael Brown

3:02 PM

Really? How does that work in music analysis?

Alyosha

@MichaelBrown I assume this is basically the same as in my crude example of specifying exactly how the integral $\int_{-\infty}^{\infty} \cos(x) dx$ is calculated (e.g. $\lim_{n \rightarrow \infty}2n\int_{0}^{2 \pi} \cos(x) dx$), with the way you expand your box equivalent to the form of the limit of the integral?

ManishEarth

Well, I'm just trying to get a mathematical way of determining how nice two notes sound together. I know the music theory behind it (overlapping harmonics), but I want a way to plug in two waveforms and get a quantitative estimate of how they sound. Probably a futile goal, but a fun thing to play with in y spare time.

So one parameter I was looking at was the energy overlap.

Michael Brown

@Alyosha yeah that's pretty much it

ManishEarth

In that, the infinite-sine-integral rears its head, though it's hidden in the waveform.

So I'm taking limits. Stuff clears up :)

Alyosha

@MichaelBrown Thanks for the help, my QM sources are irritatingly non-rigorous.

Michael Brown

3:06 PM

@ManishEarth Neat! Love to see if anything comes of it.

@Alyosha no worries

ManishEarth

@aly nudge nudge read Dirac snigger snigger

@michael :) I posted some graphs earlier. The energy integral has some issues (which I sorta expected but I looked into it anyway) but I'm going to see if I can mathematically measure beats from arbitrary superpositions.

Alyosha

@ManishEarth $\text{snigger= too advanced}?$

ManishEarth

Not .... the best book to learn from

It's a good read (not that I've read through), but if you want to start learning QM, a very bad idea IMO

It sorta assumes you know many things

Alyosha

I've got access to Feynman III, is it a good starter?

ManishEarth

I've had the singular bad fortune to naïvely try (and fail) to learn GR on my own through MTW, and try to learn QM through Dirac. Yeah, that went well :P

I think so.

Sakurai isn't bad, but I forgot where it starts. Also there are two Sakurais. Be careful about which one you want.

(the Advanced one is about the Dirac/Klein Gordon eqns and whatnot)

Michael Brown

3:15 PM

For what it's worth I would recommend reading as many books you can. :) Feynman is pretty good for physical examples and intuition. Dirac is good too, though he takes a very abstract algebraic approach which might be hard to digest (depends on your taste). My uni used Messiah & Griffiths which are fairly low level but explain things with more examples. Landau&Lifshitz is good but heavy... even heavier than Dirac. Never had the pleasure of reading Sakurai, though I hear good things about it...

ManishEarth

Feynman is usually a good book for anything, though. As long as you don't mind his style. :)

Oh right. Griffiths. Forgot about him.

Landau&Lifshitz is dense. And it makes me feel dense too :P

Michael Brown

oh to be clear by Messiah & Griffiths I mean two books, one by Messiah and another by Griffiths :)

ManishEarth

I'm not entirely sure if Dirac is meant for people first learning basic-ish QM

Michael Brown

@ManishEarth He taught it that way though didn't he? The legend goes that at the end of a lecture a student raised something and said "I don't understand this." Dirac sat there silent for a minute until the student said "Well?" And Dirac said "That wasn't a question."...

That's sort of what the book is like. :)

ManishEarth

LOL

Alyosha

3:23 PM

I won't be able to read much variety over the next few months, though, as I'm applying for university and I need to have read thoroughly through a few books rather than learn different bits from different books.

ManishEarth

Grad/undergrad?

(what are you applying for, nit where you are currently)

Alyosha

under

ManishEarth

Cool :)

Great to see high schoolers (or whatever you are) trying to learn qm. I tried to do it at the same stage of life.

Michael Brown

@Alyosha you're asking pretty good questions for someone applying to undergrad. They only start teaching QM at that sort of level at 2nd year undergrad around here.

ManishEarth

We got it at the first year. But it was quite basic.

I got bored, but fortunately the class was receptive and the prof taught some quantum stats just for kicks. That was fun :)

Michael Brown

3:30 PM

We got Bohr model stuff, de Broglie etc in first year. No real QM until second year. :)

Nice :)

ManishEarth

Oh no, we got particle in a box and barrier tunneling and other crazy things. But if you've done them before, all the fun goes away :p

Crazy Buddy

@MichaelBrown I don't have QM at all...

ManishEarth

@crazy switch to a science branch then!

Crazy Buddy

@ManishEarth Switch?

Are you kidding? o_O

ManishEarth

Run away and apply to iiser or iisc or cmi or something!

Maaaybe.

Crazy Buddy

3:33 PM

Ahh... I figured that you're :D

BTW @ManishEarth...

in Android, 20 mins ago, by geffchang

@CrazyBuddy Just read your blog post. It was interesting, but that was definitely not a rant. :)

Michael Brown

@crazy Follow your passion I say! [mumble]caveats about earning a paycheck etc...[/mumble]

5

Crazy Buddy

@MichaelBrown You know what? I don't get your quotes most of the time :P

When they get 3 or 4 votes, I figure out that it was something funny :D

Michael Brown

:D

Crazy Buddy

@ManishEarth: I think I've figured out what's wrong when first blog posts get popular...

Alyosha

I think in Britain they do some Schroedinger in the 1st year

RhysW

3:37 PM

@Alyosha first year of what?

Alyosha

University

Crazy Buddy

Viewers come with lot of expectations, and they show their original face later... (when my blog post sucks)

RhysW

@CrazyBuddy oh good you figured it out yourself i didnt want to have to be the one to tell you :P (Kidding)

Crazy Buddy

@RhysW Don't mock me :P

RhysW

@CrazyBuddy i can do what i want! :P

ManishEarth

3:39 PM

Gtg, have fun guys :)

Crazy Buddy

@ManishEarth Hah..!!! I'm right... Manish disappears with the appearance of me...

RhysW

@ManishEarth see, manish gives me permission to do what i want :P

ShuklaSannidhya

3:51 PM

Hm... So lots of off-topic discussion is going on...

Alyosha

@ShuklaSannidhya Well, fundamentally all human interaction is governed by physics, so all discussion is on-topic in a physics forum.

Perhaps

ShuklaSannidhya

To bring you guys back on-topic I've pulled out a (funny) question right out of my NCERT textbook

> A drunkard walking on a narrow lane takes 5 steps forward and 3 steps backward, followed again by 5 steps forward and 3 steps backward, and so on. Each step is 1m long and requires 1s. Plot the x-t graph of his motion. Determine graphically and otherwise how long the drunkard takes to fall in a pit 13m away from the start.

@CrazyBuddy ^ :D

RhysW

@ShuklaSannidhya infinity, the drunkard will take 2 steps, stop to urinate on a door and pass out on the floor in his own vomit, never reaching the pit

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