The h Bar

0celo7

00:13

@dmckee I got an email from my research supervisor "Hi Ryan,

You did really a great job with your talk! Very impressive!"

dunno how to respond

Danu

Just a short thank you seems good, no?

0celo7

@Danu Yes, and I'll send him the slides.

GBeau

@FenderLesPaul although, looking back to 2014, there seems to be quite some variability for when unis send out at this point - by only about a week, though

0celo7

01:28

@dmckee finally getting paiiiiiiiiiid

dmckee

01:40

Very good. In the old days working for free for a while was a kind of dues paying initiation, but federal employment law has been curtailing the space for that.

So, they should pay you.

0celo7

I'm not getting paid just yet

But the prof sent in the paperwork to HR and he CCd me

Emilio Pisanty

Hey @dmckee, quick question

was user566 a moderator?

cf their comment here

60

Q: Experimental test of the non-statisticality theorem?

Context: The paper On the reality of the quantum state (Nature Physics 8, 475–478 (2012) or arXiv:1111.3328) shows under suitable assumptions that the quantum state cannot be interpreted as a probability distribution over hidden variables. In the abstract, the authors claim: "This result holds e...

> Removed some comments (some offensive and some not) which combined to form a debate which was peripheral to this technical and concrete question. There are plenty of places to have debates on this paper and its deep meaning, an issue which attracted some attention. Here, I'd hope, is a place where people can engage with the specifics of the question asked. – user566 Nov 19 '11 at 6:42

dmckee

There was a pro tem moderator during the beta who has since left the site completely. I can't place that particular interaction but it reads right.

Emilio Pisanty

@dmckee Huh. That's in late 2011, though...

When did the site leave beta?

dmckee

Uhhh ...

Emilio Pisanty

01:46

hah, yeah, who knows

Probably nov 2011 is closer to the end of beta than to right now, though

So yeah, if there's that, then that's probably it.

dmckee

Area51 just says "4 years ago".

Physics – physics.stackexchange.com

Launched Q&A site for active researchers, academics and students

Emilio Pisanty

yeah, there's tooltips everywhere except for that

though actually

dmckee

The big Stack Exchange list of moderators says DavidZ and I (and mbq) were first elected in "2011". No help there.

Emilio Pisanty

End of beta in feb 2011, meta.physics.stackexchange.com/questions/463/…

dmckee

But it takes a wile after the official launch to hold the first election. I seem to recall that it took quite some time.

Emilio Pisanty

01:51

Election page says april 2011

physics.stackexchange.com/election/1

dmckee

I found meta.physics.stackexchange.com/questions/684/election-result from that month too. So I don't know what that is about.

Emilio Pisanty

Huh

Actually

I find it more troubling that there's very little record that mbq had a diamond as well.

That feels like something that should be available to make sense of old discussions, X user had a diamond from Y date until Z.

But oh well

thanks for having a look

dmckee

I know that there have been moderators (network wide) who were asked to leave. And I got the feeling that some of those went off with less than perfect grace. Not sure if that is related.

@EmilioPisanty I suppose you could ask about keeping that kind of record on the mother meta if it's more than an offhand itch to know.

0celo7

02:33

@dmckee What does your HP do that my TI CAS can't

dmckee

Fit into my shirt pocket. Run for years on a single set of batteries. Fail to have an "=" key; as with all right-thinking calculators.

2

Listing the things your TI will do that my trusty HP 11-C won't would take a book.

Ah and something the HP does which I have no idea if yours does or does not, but I can run a low-level hardware diagnostic which will check every path in the CPU for correct function and every key on the machine for registering the input..

0celo7

why do I need that?

@dmckee Your students make fun of you for the first. The second...damn. The third? What?

I can get my calculator to solve complicated algebra problems

dmckee

@0celo7 I run it every time I drop it. Because it is easy.

0celo7

solve(f(x)=g(x),x)

dmckee

If my students make fun of my calculator I go get my slide rule (which I know how to work; I used to be quite handy with it).

2

0celo7

02:39

Then they really make fun of you.

@dmckee Hmm, I never drop my calculator.

dmckee

@0celo7 Actually they've always shut up at that point.

0celo7

@dmckee I make fun of you...

that should be enough to deter you

Prof is emailing me from his iPhone

DanielSank

Met @BernardMeurer today. That makes two hbar regulars I've met in real life.

0celo7

Why don't we just text?

DanielSank

Is that a win?

dmckee

02:41

@0celo7 I don't care. If you keep it up long enough I'll bust out the manual and reconstruct my program for finding both roots of a quadratic polynomial (real or complex) on it.

0celo7

@dmckee You should

DanielSank

@EmilioPisanty, thank you very much for the bounty.

user54412

@DanielSank Just remember I was your first :P

DanielSank

@ChrisWhite Always, baby.

I didn't realize @BernardMeurer was just graduating high school :D

user54412

The overachieving kids who join this chat...

dmckee

02:43

@Chris @Daniel The important question here is do either of you gentlemen have a proper calculator lacking in a "=" key?

user54412

@dmckee Calculator? You mean the python installation on my computer, right?

DanielSank

@dmckee Is that some kind of incredibly nerdy way of expressing affection?

dmckee

@ChrisWhite It's actually quite impressive.

DanielSank

@ChrisWhite Eh?

dmckee

@DanielSank Only for the elegance of post-fix notation.

DanielSank

02:45

@dmckee I'm confused. When you say "the important issue here"... is this a reference to previous discussion? I just dropped in so I don't know what's going on.

user54412

I don't own a separate machine anymore -- lost too many of them.

dmckee

@ChrisWhite That's fair.

DanielSank

@ChrisWhite I Google a lot of my arithmetic and unit conversions...

user54412

^ of course you do...

dmckee

@DanielSank Yeah. 0celo7 thinks he knows something about quality calculators. Or perhaps he's just razzing me for being like his dad.

DanielSank

02:45

It's pretty nice, actually. You can type things like:

dmckee

I asked google for EST to UTC conversion today. And it did what I wanted.

DanielSank

> Planck's constant * 5 GHz / Boltzmann's constant in Kelvin

0celo7

beautiful

DanielSank

@dmckee I see. I am somewhat immune to his comments these days.

0celo7

@DanielSank wow

is that code for you're ignoring me

DanielSank

02:47

@0celo7 Wouldn't be much of a code, now would it?

0celo7

you're probably bad at codes

DanielSank

@0celo7 Heh, @BernardMeurer was showing me that he loaded Linux onto one of those.

@0celo7 See, that's why I have you on ignore :)

0celo7

</3

dmckee

I am an unreconstructed and unrepentant RPN evangelist.

0celo7

wtf are you my dad

DanielSank

02:48

@dmckee Good for you, sticking with what works for you.

0celo7

he swears RPN is the best thing ever

it's probably the worst notation

by far

user54412

everyone I know who knows it swears the same

dmckee

I was a math club nerd in high school.

0celo7

me too

DanielSank

@dmckee Are you a slide-rule ninja?

dmckee

02:49

I mostly did number sense, but when our calculator guys didn't come the contest I'd sub in.

0celo7

but damn son you're a bigger nerd than anyone

DanielSank

@dmckee Number sense?

dmckee

All the winners of the calculator contest used RPN machines. Because they are both faster to key and less error prone. That is all.

DanielSank

@dmckee That is all.

0celo7

"calculator contest"

DanielSank

02:50

Move along.

0celo7

can your HP do Fourier coefficients

dmckee

@DanielSank A mental math test. 10 minutes. 80 questions. +5 for right, -1 for skip, -3 for wrong. If you skip two that marks the end of your test.

DanielSank

@dmckee Woah.

Jeez... I spent most of high school doing homework and chasing girls around.

Well, there was the swim team too I guess.

dmckee

@DanielSank Which was to your advantage. I was still to asocial to chase girls.

Took half of university to cure me.

DanielSank

@dmckee Ain't nuthin' wrong with that.

0celo7

02:52

mental math?

homework?

I did none of those things

I can't solve quadratic equations

HDE 226868

@0celo7 Whatever did you do in high school?

DanielSank

@HDE226868 Trolled here.

dmckee

MM video games.

0celo7

@HDE226868 shitload of video games, probably

HDE 226868

@0celo7 Why am I not surprised?

@DanielSank Well, besides that.

0celo7

02:55

@DanielSank I don't troll.

Also why are you allowed to call me a troll

@HDE226868 Wow you too?

I was pretty smart in high school, knew some QFT some string theory

I've forgotten all of that.

HDE 226868

@0celo7 Honestly? You do have an astounding ability to come off the wrong way.

0celo7

Ok, that's a new one

user54412

I used to know math. Then that slipped into physics. Now all I know is astronomy. Life is downhill past age 18.

Alfred Centauri

@dmckee I've had my trusty HP-48sx since '90. While a low-level hardware diagnostic is certainly a plus, my HP-48 will also play Eddie Van Halen's "Eruption" as well as act as a remote for my TV.

DanielSank

@ChrisWhite Oh give me a break. You know as much math as ever, it's just not in L1 cache any more.

oops, gotta go each Chinese food in observance of the new year.

Ciao.

dmckee

02:57

My wife has a couple of those. But she never has it when she wants it, so she borrows my 11C.

But I'd like to hear one play "Eruption". Just because a calculator rocking it would be cool.

0celo7

@DanielSank good for him

I can't remember what happened last week

Alfred Centauri

10

A: How can I find the value of $\ln( |x|)$ without using the calculator?

And let's not forget this method (read off of the Ln scale). Image source

dmckee

@AlfredCentauri That's a nice slip stick. Looks like my Dad's.

Emilio Pisanty

@DanielSank My pleasure. Good to have you on the site.

0celo7

How does RPN work on a CAS

user54412

03:01

this chat makes me want to dig out my slide rule

dmckee

I've seen a CAS that took lisp-like input. Like that but with the operation symbol moved to the end.

0celo7

Who are the kids in here, actually?

@3507, @HDE226868 and @BernardMeurer?

@ChrisWhite you too?

Bernard Meurer

@0celo7 let's get Linux on that calc

0celo7

Does the average PhD person have a fucking slide rule?

I'm not nearly nerdy enough to get a PhD

user54412

no, but the average engineer does

Bernard Meurer

03:09

@DanielSank Just got to my hotel again. Bus was nice, cool people

0celo7

@ChrisWhite That's wrong.

user54412

@BernardMeurer put linux on a slide rule, then I'll be impressed

Bernard Meurer

@ChrisWhite Sorry, I'm currently working on getting Linux on my altoids can. While still holding altoids in it

0celo7

I don't even know what Linux is

Bernard Meurer

Pleb

Actually you're only half a pleb, because OSX is UNIX based

0celo7

03:12

using Windows now

Bernard Meurer

Then you're not a pleb, just a masochist

dmckee

@0celo7 Many people who have learned to use them simply want to own one because there are the elegant weapons of a more civilized age.

Or to dispense with the tomfoolery they are just plain neato.

Like planimeters. Mine's a polar.

0celo7

@dmckee Is it bad that I think they're the opposite of that?

My CAS is neato

Bernard Meurer

@ChrisWhite Actually, now that I look at it I think I could get Linux on it?

@0celo7 my CAS runs Doom 3; shush

HDE 226868

@0celo7 What was second semester of senior year like for you?

dmckee

03:19

@0celo7 I don't know about bad, but you and I obviously have a different sense of style.

HDE 226868

For me, the first week of it has been an enormous letdown.

dmckee

I have an acute appreciation of tools that get a lot out of a little. Even though they're much less capable than their modern counterparts.

Sir Cumference

Wait, this might sound like a stupid question, but can pair production produce quarks and antiquarks?

0celo7

@HDE226868 boring

dmckee

@SirCumference Yes. That's just the time reversal of Drell-Yan.

But you don't observe the quarks, you observe jets.

In fact the evolution of the jets to muons production ratio in $e^+ + e^-$ is one way to measure the charges of the heavy quarks

HDE 226868

03:22

@0celo7 Wheeeee.

dmckee

Sir Cumference

So...er...could a quark and antiquark annihilate to produce gluons?

dmckee

Well I think it gets written as two vertexes, but the process can go forward.

Bernard Meurer

@ChrisWhite, I'm convinced I can run Linux on a slide rule, get me one

dmckee

@BernardMeurer Needs a memory virtualization unit. That's why it takes a 386.

0celo7

03:30

@HDE226868 what

HDE 226868

@0celo7 Sarcastic excitement.

0celo7

What

HDE 226868

(Sigh) I'm using humor.

dmckee

@HDE226868 What? Without a smiley? Is that allowed?

Bernard Meurer

@dmckee A 386?

HDE 226868

03:33

@dmckee Apparently not! But :-)

Aaaaand I should be going. G'night, all.

dmckee

When linux was first written it was targeted at a intel 80386 because that chip had the proper virtualization hardware.

0celo7

@HDE226868 What

I don't see the humor

HDE 226868

@0celo7 I'm using sarcasm, which is - never mind.

dmckee

And the earlier chips in the same series did not.

HDE 226868

Oh, and I found a paper I could have used for a project . . . which I finished 7 months ago.

Bernard Meurer

03:35

@dmckee That Linux joke was too obscure for me. You should worry.

dmckee

@BernardMeurer Nawh. It's just a sign of my age.

Bernard Meurer

Never had the joy of using a 386

dmckee

I have. And a i80286. And a m68000. And a i8086. And a 6502.

0celo7

@HDE226868 I know what sarcasm is

but I don't see what you're being sarcastic about

dmckee

In fact I used to know the whole 6502 assembly language, including how long most operations took (this was before pipelines so there was a fixed amount of time).

Bernard Meurer

03:41

@dmckee Oh, wow! That's actually quite amazing!

dmckee

@BernardMeurer There were 29 opcodes and four addressing modes. It's not like it was a big language.

I also wrote assembly on the m68000, but I was never even close to knowing everything. I mean, who uses the BCD opcodes? And some of the addressing modes were pretty rarely used.

0celo7

I have no clue what you're talking about.

dmckee

@0celo7 No reason you should unless you are curious about what happens in the CPU above the level of the gates but below the level of structured programming.

Alfred Centauri

@dmckee how's about a Z80 running at a blazing 2MHz? (Heathkit H-89 running CP/M)

0celo7

@dmckee I have no clue what you're talking about.

dmckee

03:48

@AlfredCentauri Earlier than my time. My first machine was a Apple ][+.

But that was a 1.14 MHz machine.

Bernard Meurer

I was born too late to use all the cool stuff, sigh

0celo7

I might as well just stop going to school.

dmckee

@BernardMeurer You can still get many of those cores in a microcontroller form. But frankly things like rasberry pi and aurduino are simply better.

0celo7

I can't come close to your plane of nerdiness.

dmckee

@0celo7 Says the guy who reads abstract algebra for the fun of it. This is just a matter of different niches.

0celo7

03:51

You must be mistaking me for someone else.

I'm pretty sure I hate all math, especially my abstract algebra class.

ACuriousMind

@0celo7 Substitute "abstract algebra" with "Wald and HE". dmckee's point still stands :P

Alfred Centauri

@dmckee If I'm not mistaken, the Apple ][+ and the Heath H-89 were essentially of the same era. However, I first learned assembly on a 6502 Heath MCU trainer liker this:vintage-computer.com/heathkit3400.shtml

dmckee

@AlfredCentauri You could be right. I was thinking that the Z80 was on the market about 18 months before the 6502 so the very earliest hobby machine were built on it.

0celo7

@ACuriousMind Pretty sure I hate those too

Oh god, @adults, how do I taxes?

user54412

@0celo7 you don't, because you don't make money?

0celo7

03:59

@ChrisWhite I have the account info for the grant

Tomorrow I'll make an appointment with department's HR person

So, in a very short bit, I will

Alfred Centauri

@dmckee your memory is likely better than mine. I seem to recall that I got my H-89 around 1982 or so. The Z-80 (and other 8-bit MCUs) had been out for a while by then.

ACuriousMind

@0celo7 I don't believe you.

0celo7

@ACuriousMind Believe it.

Proofs, blah

Topology, blah

Topology proofs, blahx2

dmckee

@0celo7 Do you have major investment interest? Lots of pay?

If not you do a 1040EZ, which is managable.

Or you can buy some software to walk you through it.

user54412

@0celo7 Above the reporting minimum? Must be a pretty cushy job for college then

0celo7

04:02

@ChrisWhite What's the reporting minimum

dmckee

Turbo Tax or one of its competitors.

0celo7

I don't know any of these things

dmckee

@0celo7 Read the first couple of pages of the 1040EZ instructions. It covers the bare basics like that right away.

user54412

And then check your state equivalent, which we probably can't help with

0celo7

@ChrisWhite I think it might be added to my parent's thingie because I'm a dependent

dmckee

04:03

@ChrisWhite That's the stinker becuase he may have to do two states.

@0celo7 That's easy then: you send them all the forms you get and let it be their problem.

0celo7

but then I get taxed at their crappy bracket

user54412

yeah, one of which will be "my taxes were taken by the other state kthxbai"

ACuriousMind

@dmckee lol (that's pretty much how I dealt with life so far :D )

dmckee

@ChrisWhite When I had to do Virginia and New Mexico they both claimed the right to be first and would come after you if you tried to duck.

It doesn't make any damn sense, but you can't fight the state revenue any more than you can fight city hall.

VA because I was physically present there. NM because I was employed there. And neither would budge.

user54412

something like that happened to my parents years ago between NY and NJ

0celo7

04:06

I'm a VA resident

but working in TN

user54412

they missed something on one of the forms, and NJ notified them of all sorts of extra money and interest and fines

dmckee

^ That.

Cost me about $400 extra when all was said and done.

user54412

the kicker was NJ was 5 years behind in analyzing the stuff, so my parents had made the same mistake 5 years in a row

user54412

they asked if they could amend the other forms, and were told "we'll fine you when we get to them, with interest based on the date we discover the problem"

0celo7

all my socks survived the laundry this time, nice

3507

04:09

@0celo7 I wonder what happened to those pants you returned.

0celo7

don't know, don't care.

dmckee

@ChrisWhite VA was three and half year behind. That $400 was about $130 for each of three years and a wee little bit for refiling the most recent year.

Bernard Meurer

I hope we have a tax calculating AI when my time comes

user54412

I'm personally hoping for a time when taxes aren't used to fine some people's life choices in order to pay for others', and the tax code is reduced to a single paragraph.

2

Bernard Meurer

Hey guys, how do we know we live in a three dimensional world and not in a very "well drawn" two dimensional one?

dmckee

04:23

@BernardMeurer The aforementioned software is a pretty decent expert system for the task, except that it requires you to correctly identify what things can go into what boxes. but it does save on the drudge work.

0celo7

@ChrisWhite Watch out, you wouldn't have a job if that were reality

user54412

didn't say no taxes just no social engineering and special interests

user54412

also, I've actually held a job in the private sector :p

Bernard Meurer

BREAKING NEWS @ChrisWhite wants no taxes

user54412

time to move to DC and change my license plate I guess

Bernard Meurer

04:31

BREAKING NEWS @0celo7 says @ChrisWhite should be jobless

I think I'd make a good journalist

0celo7

@ChrisWhite if there were no taxes DC would not be the hub it is

because DC is a shithole

DanielSank

06:05

@BernardMeurer Is there a story there?

user54412

07:09

4

Q: Why is stainless steel a poor conductor of electricity?

I recently had a metal plate put in my shoulder and was wondering why stainless steel isn't a good conductor (At least I hope it isn't). Does the alloy just lack free electrons? Why is that?

electricity resistance metals conductors

user54412

Must be one of our most viewed questions. And look at all the deleted junk there. Is there some collective stainless steel paranoia out there?

user54412

07:22

and while I'm scanning the review queue:

user54412

0

Q: Why do bulbs dim when we switch on TV?

Why do the bulbs dim for a moment when we switch on equipments like CRT television or water pump?

electricity

user54412

I understand there are plenty of undeveloped parts of the world, but are there really still CRTs left?

user54412

I haven't heard one in a decade.

DanielSank

08:56

@ChrisWhite When playing Smash Brothers Melee one usually uses a CRT to avoid lag issues.

They are coveted in that community.

Danu

09:21

physics.stackexchange.com/review/low-quality-posts/117088

Danu

12:05

@ACuriousMind I corrected a small mistake in your CFT answer; The positive $L$'s all give zero when acting on $\mid h\rangle$, so we need the labels to be $\leq -1$ (you had them all be $>0$)

Bernard Meurer

@DanielSank No, I just make those little headlines that sweep through during TV news and hope no one googles about it

Secret

12:32

https://en.wikipedia.org/wiki/Metric_signature
Why we rarely considered degenerate metric signatures (i.e. why we rarely seen a metric signature where r is nonzero?)

Danu

Because they're completely uninteresting since, in the zero-direction, all points are at zero distance i.e. physically they're the same point i.e. these directions do not exist

(...is the first thing that comes to mind)

Secret

But aren't we have a simailr thing happening for indefinite metrics like (3,1,0)? in that how there's a set of points (the light cone) where any two points are separated by $ds^2=0$. How is that differ from going in the zero direction and finding the metric vanishes there?

user116211

13:09

@Qmechanic: Can you guess the Dark Knight in this puzzle?

Slereah

@Secret : That's a relation between two points, though

For a metric with a degenerate point, every distance between that specific point and any point is 0

Danu

Didn't we have some discussion about reconstructing the metric from the Christoffel symbols at some point?

Danu

13:24

Oh btw @FenderLesPaul @ACuriousMind see below

So it's not as simple as you hoped @ACuriousMind ;)

Slereah

The metric is independant from the connection, though?

I mean you could define the metric by the metricity condition and torsion freeness

I guess

But it is simpler to do it the other way around

No PDEs to solve

Danu

To what extent can one determine the metric from the Levi-Civita connection?

Q: When can a Connection Induce a Riemannian Metric for which it is the Levi-Civita Connection?

As we all know, for a Riemannian manifold $(M,g)$, there exists a unique torsion free connection $\nabla_g$, the Levi-Civita connection, that is compatible witht metric. I was wondering if one can reverse this situation: Given a manifold with $M$ with connection $\nabla$, when does there exist a...

dg.differential-geometry riemannian-geometry

Ah, here we go

9

Q: Does the Levi-Civita connection determine the metric?

Can I reconstruct a Riemannian metric out of its Levi-Civita connection? In other words: Given two Riemannian metrics $g$ and $h$ on a manifold $M$ with the same Levi-Civita connection, can I conclude that $g=h$ up to scalars? If not, what can I say about the relationship between $g$ and $h$? Ho...

riemannian-geometry connections holonomy

Slereah

The relation is a second order PDE

I think it's uniquely determined?

Oh, I guess not

Danu

Well there is always scalar multiplication

Secret

13:42

@Slereah I see, in that case I can see how it fails for the light cone cases because as son we pick one point that is not on the lgiht cone, the distance will not vanish

0celo7

How does one pronounce the "Civita"

Slereah

I think it's like "chevita"

Wait

The example he gives is that g and 2g have the same connection

But those are just diffeomorphisms

Can you determine the metric up to a diffeomorphism is a better question

Danu

@0celo7 Note that the emphasis is on the last a (the correct spelling is really Levi-Cività)

0celo7

My Brazilian geometry prof says it weirdly.

@Slereah Yeah, like that.

@Secret Light cone? Causal structure determines the metric up to a Weyl transformation, IIRC.

Slereah

14:03

That's some fancy words for CONFORMAL TRANSFORMATION

(I will call it conformal 'til I die)

0celo7

14:15

@Slereah Meh, it's standard in string theory.

And "Weyl" is probably the more "physical" name for it.

Slereah

Is there classical string theory

Moses

Hi all

Slereah

Hello Moses

Will u let my people go

0celo7

@Slereah Yes.

Moses

Depends if you can assist with the following question Slereah

0celo7

14:29

@Danu There's a functional analysis book in GTM that says it's accessible to engineers...

Moses

In quantum mechanics the time-independent Schrodinger equation says $$\hat{H} \psi_{n} = E_{n}\psi_{n}$$ then in Griffiths book "Introduction to quantum mechanics" it states

$$\langle H \rangle = \int \Psi^* H \Psi dx = \int (\sum c_{m} \psi_{m})^*H(\sum c_{n} \psi_{n})dx = \sum |c_{n}|^{2}E_{n}$$

I don't see how we obtain $\int \Psi^* H \Psi dx = \int (\sum c_{m} \psi_{m})^*H(\sum c_{n} \psi_{n})dx $ since $\Psi(x,t) = \sum c_{n} \psi_{n}e^{-iE_{n}t}$, it is not clear that the exponentials cancel.

Danu

@0celo7 GTM?

0celo7

@Danu Graduate texts in math

Btw, it's not

Maybe the most mathematical of electrical engineers.

Mark Mitchison

@Moses It depends on your definition of $c_n$. You have assumed that $c_n$ are the expansion coefficients of the wave function at $t=0$. Griffiths assumes they are evaluated at time $t \neq 0$. The relation is just $c_n' = c_n e^{-iE_nt}$, where $c_n'$ is the Griffiths definition. Of course in the end you can see that the result is the same.

Danu

I think they always put that kinda stuff in there to sell more copies

0celo7

14:37

@Danu It's in the preface. The author claims to have taught the course using that book and there were engineers in the class.

I don't know why an engineer would take functional analysis.

@Moses I think that's a typo. You might need some variant of "exponentials kill integrals".

Mark Mitchison

@0celo7 It's not a typo.

0celo7

@MarkMitchison How does that integral evaluate to the sum?

Mark Mitchison

@0celo7 Eigenfunctions of a Hermitian operator are orthogonal. The integral gives you $E_n \delta_{mn}$.

Moses

@Mark I don't think that he use $c_n$ defined in that way. He uses $c_n$ with the time dependent exponential to that point, why would he without writing anything suddenly change it. Also how would that show time independence.

Mark Mitchison

Then the Kronecker kills off one sum (and incidentally also the exponential time dependence).

Moses

14:42

If your $c_n$ is s function of time.

Mark Mitchison

Because $|c_n(t)|^2$ is time independent

0celo7

@MarkMitchison Oh, the integral is over $x$. My bad.

Mark Mitchison

But ok, maybe it is a typo. In any case the time at which you choose to evaluate the expansion coefficients is irrelevant. Because their time-dependence is just a phase which disappears when taking the expectation value.

(The expectation value of the energy, mind)

ACuriousMind

@Danu I am not surprised about that typo, I always have trouble remembering which way around it is ;) Thanks.

Moses

@MarkMitchison Okay let me just revise what you are saying.

0celo7

14:48

@Moses Maybe it's easier to see in bra-ket notation where $|\Psi(t)\rangle=\sum_n c_n\mathrm{e}^{-\mathrm{i}E_nt}|\psi_n\rangle$, $\langle\psi_m|\psi_n\rangle=\delta_{mn}$.

Danu

@ACuriousMind I also don't like writing $L_{-n}$ all the time :P

Moses

@Ocelo7 I understand now thanks a lot.

@MarkMitchison You are right it does cancel because of the kronecker

0celo7

@Danu I have a Japanese mechanical pencil where the lead rotates as you write so the tip is always nice.

Danu

Oh, that's neat

Mark Mitchison

@0celo7 Do you have a link for such an item? I want one :)

0celo7

14:53

amazon.com/uni-ball-KuruToga-Mechanical-Starter-1751934/dp/…

Mark Mitchison

sweet

0celo7

I put a little piece of tape on the bottom to test it out, it really works.

Moses

Is the Hamiltonian the only quantity which has an expectation value which is generally independent of time? I know for separable solutions, the expectation values of position and momentum are also independent of time.

0celo7

@Danu Do you like my \mathrms above ;)

Danu

@0celo7 Wait, where?

0celo7

14:58

Above you complaining about -n

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