The h Bar

so if your expression is on the vacuum of the free theory then it basically just says that the decay is not there at tree level

but you can possibly have non-zero contributions from loop diagrams

@FenderLesPaul proof

see like Peskin and Schroeder chapter 4

or Srednicki chapter something

user54412

@0celo7 You ever been to California?

@ChrisWhite yes, my sister lives in LA

user54412

3:03 AM

Also, my temporary roommate for the first two weeks of college had a cat. Worst. Idea. Ever. Cats need more than a dorm room, and imagine spending all your time next to a litter box.

user54412

(I'm saying this as someone who actually likes cats)

cats are also scary

@FenderLesPaul can you be more specific

@FenderLesPaul I do. ...?

Weinberg says this like I should know it

is it not 11 in Canadia?

3:05 AM

woohoo internet is not off.

@0celo7 it's a weekend.

@0celo7 more specific about?

page, section

just tell me?

lemme get my copy of Peskine

unfortunately don't have any pdfs yet on this laptop

wait, it's only at tree level?

is there no general nonperturbative statement to be made?

if what you wrote down is on the vacuum of the free theory then it's just the tree level term

read sections 4.2 to 4.4

of Peskin

3:14 AM

maybe I should get Peskin

Weinberg is way too vague

Weinberg is a horrible way to first learn QFT

ch2 of Vol1 is the only thing worth reading on a first reading

I've also read good bits of Srednicki

but he's kinda vague too

Srednicki is my favorite

his chapters on renormalization are fucking awesome

I don't quite like Peskin's chapters on the later parts of QFT

but Peskin's first few chapters are excellent

if combined with ch2 of Weinberg

because Peskin doesn't explain any of the deeper foundations of QFT whatsoever

Should I be taking note of this?

probably

I keep thinking I know QFT, but then I don't know it, but then I do but really don't

3:19 AM

The QFT 1 I'm taking has lecture notes arxiv.org/pdf/1110.5013v5.pdf

Also recommends peskin.

@FenderLesPaul @0celo7 The lecturer told the class to read those notes, which he based his off of. This is to learn most of the material, is it well written?

don't ask me any more

haha

those lecture notes are legendary dude

they're basically the notes that a shit ton of modern QFT books are based off of

I'll use them then.

@0celo7 Because I'm being annoying or because you don't feel like talking about QFT?

Coleman also has video lectures online

on Harvard's site

of his QFT course

I don't feel like talking about anything

I'm getting bored of physics

3:23 AM

Then do other physics.

@FenderLesPaul Are they well written for learning from? For a first read is it self contained?

it's probably better to read Peskin

they're definitely well written

@FenderLesPaul can you be even more specific in Peskin

@FenderLesPaul Though you told me to stay away from peskin.

but they're rather condensed and don't have detailed calculations

what I don't understand is the connection between the S matrix and the vevs of stuff

3:25 AM

@0celo7 I really think reading all of sections 4.2 to 4.4 is a good idea

I'm just so confused because everyone does QFT differently

@obe if I did then I take it back idk what I was saying

some have path integrals, some have random Js

I might have meant stay away from the later chapters and use Srednicki for those instead

I can't keep any of it straight

3:26 AM

@0celo7 jus read all of 4.2 to 4.4

the connection will become very clear

Peskin is very detailed about it

Ok, I won't interfere in your discussion. later.

I'm sleeping now

granted I use path integrals all the time in my research and never use the canonical formalism that Peskin introduces all this formalism in

but it's definitely easier to learn it the way Peskin does it

speaking of research

I think Dr. Donovan told me to fuck off until I'm a grad student

:(

3:28 AM

they keep treating me like an idiot for some reason

@obe if you have more questions feel free to ask

if not take care buddy

do they think I'm telling them I watched like nova documentaries or something

@0celo7 yeah that's the life of an undergrad unfortunately

I'll stay for a few minutes, thanks.

@0celo7 Show them your PSE answers. Tell them the books you've read.

they won't recognize them because I'm not trying to work in those fields

3:30 AM

@obe definitely use the notes

one professor told me I won't be able to take the higher level physics classes because engineers don't get enough math and the physics professors do not like putting up with them

but don't use them only

@FenderLesPaul Ok.

@0celo7 Though you are a math minor too.

@FenderLesPaul uh he's also going to class and has notes for the class

@obe yes

I'm going to take the graduate EM class

Jackson fun times 101

if you don't want to be bored of physics

definitely don't take a Jackson class lol

that's the epitome of boring

3:33 AM

it's necessary for plasma

damn Jackson

what about relativistic electrodynamics?

always a necessary evil

what about it?

what about it

is it interesting or important?

3:33 AM

for what

that's a very open ended question

@Rigor GOT MY TICKET

Rigor

@0celo7 :D

it's about as important and interesting as it gets

does QED raise GDP

solving boundary value problems and calculating Green's functions for the 100th time on the other hand...

QED = 15 freedom dollars

per capita

@FenderLesPaul I think that's why Donovan has grad students

so I'm going to look into working for Lang's group in undergrad

3:36 AM

Then @0celo7 learn advanced nuclear physics.

his research is kinda meh but he likes me and told me to come check out the research group meeting

which Lang?

@obe I don't have any idea where to start on that

@FenderLesPaul Maik Lang, U Heidel alumnus

oh

is Serge Lang still alive?

I think he irradiates shit for a living and then tries to break it and do random stuff to it

3:39 AM

@0celo7 Look at the syllabi of the courses.

what is love?

@obe those aren't readily available unless you are taking it

maybe

baby don't hurt me

no mo

@FenderLesPaul misspelled aloe vera

mm aloe vera

Rigor

3:40 AM

Everybody dance now

@0celo7 Other universities...?

@0celo7 UofT

Third year course.

I'm pretty comfortable with QM and QFT when I'm not being retarded

I want something not 3rd year

what are you looking for?

this god damn decay shit

damn gotta start writing NSF fellowship application

le sigh

3:48 AM

does Srednicki explain this connection as well

does Weinberg?

I looked for a good while, could not find it

It's a third year course though the book they use is a graduate book I think.

between S-matrix and correlators?

yes

Srednicki does it fine yeah

does Weinberg

3:51 AM

not as well

there are subtle points that Srednicki doesn't explain though

like

Peskin at least makes explicit the uses of the subtleties but doesn't fully explain them either

it's just one of those things you need a competent professor for

like working in Euclidean time to prepare the vacuum state then analytically continuing to Lorentztian time when computing correlators

which Peskin hand-wavingly does by adding an $i\epsilon$ to the Lorentz time for the asymptotic in and out states

but just by seeing that it isn't really clear what he's doing or why he's doing it because iirc he doesn't explain it as washing out excited modes

so as to prepare the vacuum

these things are also just way easier to understand using path integrals

where does Weinberg even explain the relevance of correlators to the S matrix

section 3.5

but the notation is so fucking terrible idk if it's worth trudging through

where

I read that today

I did not see any correlators

3:59 AM

also section 4.4

well 3.5 is the QM version

and then if you jump to 4.4 you get the QFT version

wait no

it's still the QM version

dammit where is it in Srednicki then

ok so he doesn't explicitly do it he just lifts the QM result and throws it into a QFT framework

in chapter 6 section 1

for Srednicki it's the chapter titled "LSZ Reduction"

which I think is chapter 5

but there are no correlators there

Weinberg never has correlators, does he

he doesn't explicitly write them down no

he just turns them into a Dyson series

god damn Srednicki says see Weinerberg for more info on LSZ

4:03 AM

I think this book just exists for chapter 2

is (5.24) in Srednicki exact

or is that some perturbation BS

its exact

its the LSZ formula

dude does Weinberg even mention that

it just says that the S-matrix projects onto on-shell states

I have no idea honestly

his notation is impossible for me to comb through

but Weinberg mentions that the decay is 0 if the correlator vanishes

4:08 AM

I don't think anyone mentions that

so one has to be able to get that from what he's saying somehow

it's just clear from Feynman diagrams

oO

what the fuck

something like $\langle \phi(x_1)\phi(x_2)\phi(x_3)\rangle vanishing just means the tree level contributions vanish because that correlator clearly only gives the usual tree level diagrams in a $\phi^3$ theory

the usual channels

@FenderLesPaul see the comment right under 10.1.9 in weinberg

vzn

4:17 AM

Quantum weirdness proved real in first loophole-free experiment / new scientist

4:41 AM

@0celo7 ah

6:10 AM

@vzn I'm skeptical.

@Danu I'd participate in "answer the unanswered".

Dan's weekly gripes arising from people failing basic literacy on the main site:
1) Stop starting every damned sentence with the word "so".
2) Do not ever, for any reason, type the phrase "my question is" in a question. Just delete that part, capitalize the word directly following, and you'll have a better English sentence.

user54412

@DanielSank That there were any loopholes left to be closed?

@ChrisWhite More that they really closed them all.

I should read the paper.

user54412

I never understood this loophole business.

user54412

I mean, I can always claim my wrong theory is correct by saying prancing unicorns in subspace are to blame for its erroneous predictions. But that doesn't really count as a scientific argument.

@ChrisWhite LOLOLOL

user54412

6:16 AM

@DanielSank 3) Punctuation for some,reason people?don 't know what the symbols mean/ and they ,think spaces go before periods and commas .but not after

Are you my long lost twin?

@ChrisWhite Egad make it stop!

user54412

@DanielSank <3

@ChrisWhite Indeed. What you wrote there is more or less what I go around telling everyone each year at the APS meeting.

I think Justin Dressel (theorist with whom we did the LGBI violation) more or less agrees.

I'm convinced these experiments are worth doing though.

And to be honest I think the loopholes are worth closing, even if the "theories" we've come up with to fit the data while exploiting the loopholes are completely insane.

The problem, of course, is that quantum mechanics is insane too, so the criteria by which we have to judge other ideas are a bit strange.

user54412

@DanielSank Yes, I suppose someone who's employed to do quantum experiments would say that :P

@ChrisWhite Nah, that's not the only reason.

Quantum mechanics has forced us to think really carefully about what a theory of nature really means.

As much as I maintain that quantum mechanics isn't really all that weird, the problem is that any theory you take seriously has to at some point bring up the issue of what all the mathematical stuff means to me the human.

This is highly nontrivial, borders on being philosophy, and really hard to talk about.

user54412

6:24 AM

As someone who also thinks QM isn't quite as weird as it's often made out to be, I feel interpreting it wouldn't be so difficult if we had a solid interpretation for probability in physical experiments.

@ChrisWhite Oooh, I don't know about that.

Explain?

No matter what you say here my answer is going to be something like "but what about Bell violations?".

user54412

Eigenvalues and hamiltonians and spin statistics are red herrings in this regard. Tell me what it means to you for physical outcomes to be (possibly correlated) probabilities, and I can probably bootstrap that into an explanation for what QM means to you the human.

@ChrisWhite That's what I meant when I said "any theory".

I can't link any theory of Nature to my own experience without saying some really weird stuff.

user54412

okay I think we're in something of an agreement then

Quantum mechanics makes it much worse though, because there's this notion of collapse that's extremely hard to describe.

I can use the theory to argue that when a system interacts with a large environment, it evolves in a way such that it's well described by a set of probabilities.

But why, when I measure it, do I personally see one of them?

What the hell does that even mean?

Let me try to answer your question though.

You take two bags. In one bag you put two black marbles. In the other you put two white ones.

If I pick a marble from a bag and check its color, I now know that the second marble will be 100% correlated and in this case the same color.

What's weird about that?

user54412

6:34 AM

@DanielSank Maybe. But given that we agree that "describing Nature" is already something we haven't succeeded at, I don't know what it means for something to be even worse. As far as I know, once I'm happy with classical Nature, I'll simultaneously be happy with quantum Nature.

user54412

@DanielSank Not much, I think. Keep going.

@ChrisWhite I think this isn't quite true, but I'm not sure yet.

@ChrisWhite Nothing's wrong.

I have a notion of some marbles and I can construct correlated probabilities to describe the outcomes of my experiments.

Now, what happens if instead of marbles we have entangled spins?

Now we have this really bizarre thing where we observe correlation in the measurements but you can't trace it back to the initial states having actually been the things we measured the whole time.

The probabilities here do not come from lack of information.

That's weird. Isn't it?

user54412

Maybe

^ Good answer.

@ChrisWhite I mean, look:

We can definitely take the position that it's not weird.

We can just say "stop complaining, that's what the data says".

But here's the one tiny place where it goes awry:

In the marbles case I don't usually think too hard about what it means to link the various notions of objects and their properties to my experiences in measuring them.

Basically, I assume that I am unimportant. My actions in measuring the system create a unidirectional flow of information; I learn some properties about the system and that's it.

In quantum this just doesn't work at all.

I can accept that the information available to me is that of a probability distribution, but once I measure it the system changes in a way not included in the equation of motion (collapse isn't unitary).

Quantum mechanics forces us to regard the measurement process as a bidirectional interaction!

user54412

@DanielSank That unidirectional flow of information is awfully reminiscent of a non-fluctuating dissipator, isn't it? (Just a random thought)