The h Bar - 2018-04-14 (page 3 of 3)

« first day (2718 days earlier) ← previous day next day → last day (2211 days later) »

9:37 PM

@Pieter Hi. I was reading through the comments on this post on Leonid Schneider blog that saw your comment (my guess, based on the name and profile picture) about that guy (A. Tiwari) books published by Wiley.

@EricSilva so in regular QM you can ask about something called a transition amplitude which tells you about the probability to go from one state to another

a useful formulation of this is in terms of a path integral, which is a weighted integral over some measure space of paths

@lılostafa Yes. There is little difference between Wiley, Elsevier and the predatory publishers

it turns out people working in Brownian motion can do this very rigorously, i.e. the measure can be defined and the weight makes sense in the integral

Actually we have been contacted a few days ago by this guy to write a book chapter for them!

in QFT, the measure is harder to construct in general

9:39 PM

ok

So he is still active. Incredible. Does he still mention Linköping University?

the functions you have to integrate are also worse

so there's a dual formalism, called the canonical formalism, where you view your fields as distribution-valued operators on some Hilbert space

problem is, there is no general rule for what the Hilbert space should be

it's not know in many important cases

@Pieter I don't think so. But he's working with Scrivener (and Wiley)

so ignoring this issue, you run into another issue that the PDE these things should satisfy is nonlinear when you have interactions

so then you have to make sense of a product of distributions, which is in general not possible

id have to see this in motion for it to sound like not gibberish

9:41 PM

@Slereah would be a guy to ask about that

What did I dooo

you can give @EricSilva an example of why QFT is wrong

it's all a lie

it is

y tho

@Pieter it's been actually a quite complicated case for me, as the other guy (prof. Turner) seems to be a well-known researcher, and the books are published by Wiley after all. (and therefore asked this question on Academia)

9:42 PM

ELIknow nothing about anything

@lılostafa I do not know what Scrivener is really. But that Wiley is still having him is so weird.

here is the Very Bad Theorem

I'm still not completely sure whether it would hurt me as a grad student to be among the writers of a book published in that series

"Suppose that $\phi_1$ is a free hermitian scalar field of mass $m > 0$, and $\phi_2$ is a local field covariant under the inhomogeneous group $SL(2,C)$. Suppose further that the fields $\phi_1$, $\dot \phi_1$, $\phi_2$, $\dot \phi_2$ satisfy the hypothesis of theorem 4-14. Then $\phi_2$ is a free field of mass $m$."

Everything seems OK except a couple of blog posts by Leonid Schneider. VBRI press (which is a predatory publisher) is not involved

9:45 PM

Basically if you work with the free Hilbert space, as you usually do in QFT, you can only have free theories

Tim Gowers, among others, have begun a campaign against predatory publishers.

@lılostafa Tiwari was at Cranfield University together with Turner. He came to Sweden together with his boss. Turner has been in advertising video for these boat conferences on the Baltic. And Turner lost all respect when he lashed out at Schneider the way he did.

In QFT we usually pretend that this isn't the case, and it kind of works

Except for the part where everything is divergent all over the place!

@lılostafa I doubt that it could hurt you. There are so many people that are his coauthors. They get millions of cash from Swedish research funders.

idk enough for this to sound meaningful to me

9:50 PM

maybe sam is too crackpot for a first exposure

rude

But I would stay clear of the guy. And of the big publishers like Elsevier. I would never work for them for free.

the basic problem in QFT is that we assume that the Hilbert space doesn't matter

Since it is true in QM, via the Stone-von Neumann theorem

But the SvN theorem breaks down in QFT

maybe yo should tell him what SvN says in QM

actually @EricSilva take a QM course

Meaning that it's not guaranteed that we can apply different theories to the same Hilbert space

and expect operators to act the same

9:52 PM

what kind of university student doesn't know quantum mechanics :P

@0celo7 i wanna but scheduling :(

i was supposed to take it this quarter but complex analysis conflicted and math comes first for me

quantum mechanics is the foundation of modern analysis

Of course all Hilbert spaces are isomorphic in quantum theory, but the important part is that we can't expect the field operators to act the same way if they're for different theories

@Pieter I think this is the right thing to do (at least morally), but a lot of their chapter writers are respected researchers all around the world. And also I didn't see anything about these books on Schneider's blog. and even contacted Scrivener and they confirmed everything that Tiwari had told us in his email about the book

@0celo7 i might just take our graduate qm course if i can, since it's in the fall instead of spring like undergrad qm

idk tho

9:55 PM

For instance in QM, every theory is such that the operators $\hat x$ and $\hat p$ can be represented by $\hat x \psi(x) = x \psi(x)$ and $\hat p \psi(x) = -i\hbar \partial_x \psi(x)$

But in QFT, this is no longer true

as Schneider has said, he's truly a scam artist :)

For a free theory, you can write $\hat \phi \Psi = \phi \Psi$ and $\hat \pi \Psi = -i \hbar \dfrac{\delta}{\delta \phi} \Psi$, but this will no longer be true for an interacting theory

@lılostafa I understand. Nowadays one needs to be "productive", generate papers. It is not how I was taught by my supervisor (and see what has become of me).

what has become of you?

@lılostafa He is well-adapted to the present ecosystem of science.

9:58 PM

@EricSilva chances are that's a weedout course for first year physics graduate students

they might actively try to kill you there

hence the might

lol

ill ask some of my physics-y friends

the only thing set in stone is that im taking the grad GR course next winter

@skullpatrol Teaching undergraduates at a college with a very small physics department, with mostly teacher students taking my courses. I have some research ideas, but the money is going somewhere else. I am allright, though, should not complain.

ok

10:02 PM

I think I narrowed down the Big QFT Mistake to a few steps btw

Usually at some point in a QFT book, they will say

1) $\langle 0 |\Omega \rangle \neq 0$ because of reasonableness arguments

2) At a time $t = 0$, you have $\phi_{\text{free}}(x, 0) = \psi_{I}(x, 0)$

Those are I think the two lies of QFT

ill save the permalink for this segment of chat for when im not so ignorant

Also if you're doing the path integral, one of the big lie is that $$\int \mathcal D \phi e^{S_{\text{free}}}\sum (1 + ...) = \sum \int \mathcal D \phi e^{S_{\text{free}}} (1 + ...) $$

I mean beyond the fact that the measure is usually not well defined in the first place

Although that one is a different lie from the first, I think!

Different divergence

In words, are you saying the initial free wave function is equal to the initial interaction wave function, and that this is the big lie?

It's one of the thing that they put in QFT textbooks and motivate with plausibility arguments but don't really work yeah

Also that's not the wavefunction

That's a field operator

My reading of Landau is that if you couldn't define the initial and final interacting wave function as being equal to a free wave function, you would literally have no theory, all you can literally do is work with free wave functions and transitions between them, and only in momentum space to boot, considering interactions as they interact is as unrealistic as the paths that no longer exist due to QM, but I'm not sure of this yet

10:17 PM

Yeah if you don't make those assumptions, I'm not sure how you're supposed to build the Hilbert space and the field operators

Hence why it's done

So why is it a big lie

But still, it is a problem in the end

Well it's a lie because mathematically it doesn't work

I don't think I've ever seen anyone construct a Hilbert space for QFT that wasn't the free one, tho

Except one-dimensional QFT, I guess

Since that's just QM

The last time I tried to find out about this stuff I ended up at the bootstrap stuff

Also I don't know how to sync it with Yang-Mills which is always interacting right

I think maybe you could do it for the Sine-Gordon theory

But this thinking did apparently motivate S-Matrix theory a lot

10:20 PM

@0celo7 apparently the grad QM at my school is just the undergrad QM where they dont teach you linear algebra for the first two weeks

so maybe it's the one i should be taking anyway

Because it has a weird thing where you sort of have a superposition principle

I don't really trust QM books which rely on linear algebra tbh

Except Dirac's

Good old wave functions, you can get field operators directly from them

amazon.com/Quantum-Mechanics-Third-Non-Relativistic-Theory/dp/…

Except you don't have wavefunctions in QFT

@EricSilva do you know what book

You have to deal with dirty wavefunctionals

10:22 PM

No picture

@0celo7 no clue

it's either Shankar or Sakurai

if it's not one of those you've got a special snowflake professor

we use shankar for undergrad

Hopefully you will grow out of those books

Overall qft is a gigantic mess if you want to learn it :(

But awesome

Lubos gave a review of Landau on there

QFT is fine if you want to learn it, but like

don't look at the man behind the curtain

10:26 PM

"Landau was a great teacher. The whole series is perfectly organized and the material is divided into pieces of reasonable size which can be swallowed one after another. The reader can appreciate the beauty of every single piece of knowledge. I recommend this series especially to those readers who already have some general knowledge of maths and physics and who are able to study for themselves; it is also a very useful reference for professionals.

The volume on quantum mechanics has taught me a lot about the structure of nuclei, atoms and molecules (in fact also about the chemistry) - and also about the symmetries and the special functions encountered in the field of quantum mechanics. Feynman's lectures on physics are also great (albeit alternative) but the course of Landau and Lifshitz is a standard for anyone looking for the best presentation of physics by 1960s."

qft has a sleazy feel

you never know what's correct

"best presentation" Be still my beating heart

Mithrandir24601

@Slereah I enjoyed learning QFT, but I appear to have forgotten large chunks of it :/

there is certainly a lot of "Trust me on this" moments in QFT

tbf QM is very mysterious too, but the math is by now very well understood and completely contained in Reed and Simon

10:28 PM

@0celo7 Fortunately, the correctness of its physical predictions is not predicated on the rigorousness of its mathematics.

@0celo7 No it's not

Reed and Simon is all free QFT

Free QFT has been put on firm grounds like forever ago

That's not the hard part

@Slereah He's talking about QM, not QFT.

o

a

@ACuriousMind I'm a mathematician, please don't shame me for wanting mathematical rigor

10:31 PM

Wald has a bunch of stuff for nonlinear QFT but I have no idea if it helps

Also I tried reading about renormalization for AQFT but then i realized I don't really know how renormalization works for regular QFT

The horror

that reminds me @0celo7 ive finally cracked open wald

@0celo7 You can demand rigor all you want, I'm just cautioning against conflating rigorousness with physical correctness ;P (Certainly, there are many rigorous theories that are completely unphysical!)

seems like it'll be a good time

I want to learn classical differential geometry

@ACuriousMind Yes, GR is an example of such a theory

@EricSilva good

10:34 PM

I think QFT is probably rigorous if you define it directly from the renormalized S-Matrix

but then it's not a very nice theory to generate more QFT

All those involutes, evolutes, spherical indicatrices

@bolbteppa read my boi do carmo

I feel like he makes diff geom look harder and more abstract than it is

Something like the schaums one is fine for me for now, but of course it's awful too :p

curves and surfaces isnt abstract

@bolbteppa maybe do something easier like linear algbera if you think curves and surfaces are too hard

10:43 PM

do Carmo's baby book is very concrete

my room has so much space now

it's unsettling

My guess is this is the bible of diff geom

Mithrandir24601

@0celo7 I had huge rooms in undergrad. I miss them quite a lot :/

archive.org/stream/lecturesondiffer00forsuoft#page/n13/mode/2up

@bolbteppa apparently “Differential Geometry of Curves and Surfaces” by Kristopher Tapp is very good

10:46 PM

i like montiel and ros's book but it's more mathematically sophisticated

and covers weird shit

Looks like it proves the Jordan curve theorem

JCT is a trivial application of homology

montiel and ros define the integral over a surface as some lebesgue integral over its normal bundle which is kind of weird

lol what

yeah it's weird

10:51 PM

Not trivial to me anyway

the application is trivial, but homology isnt trivial

no it isn't, but anything you're gonna do with geometry will only work in the smooth category, and getting enough approximation theory to carry the result over to the continuous categeory means you might as well do homology

looking at Hatcher, it seems you only need Mayer-Vietoris for JCT

i need to know hatcher better but i cant stand reading it with it's awful font

11:14 PM

@EricSilva according to Balarka the font is great

And adds to the book

where are you speaking to him

tell him he's a big nerd

who has bad taste in fonts

i'll pass it on

@EricSilva I’ve known him for 3 years

I don’t know when he told me that

oh ok

@BalarkaSen Marx debunked Hatcher's font years ago

Sep 28 '16 at 18:36, by Balarka Sen

I like Hatcher's font.

11:20 PM

Going all out on Java

did wald just call christoffel symbols tensors

wut

Yes

He’s a madman

but y tho

y would he say something so obviously just

not right

i think i was also shocked last time i tried to open this

physicsforums.com/threads/christoffel-symbol-as-tensor.40177

Discussion on the meaning of units vs. dimensions

11:24 PM

This is pretty weird tbh, what does he mean

'tensor field'

@bolbteppa A field of tensors.

@EricSilva for a certain definition of tensor, it might be true

just ignore that line

'A christoffel symbol is a tensor field'

Christoffels are just gauge fields :P ::hides::

yeah it's clear he just operates under a notion of tensor that's wrong so nbd

11:26 PM

'the tensorial nature of the Christoffel symbols are explained as the components of a set of tensors but there is no single tensor whose components are C^mu_ab.'

the transformation law has a second order term in it so that's why it cant be a tensor

'If I understand correctly, the Christoffel symbols are tensors in the respect that they're linear maps from tangent vectors and dual vectors to the reals. Christoffel symbols don't transform like tensors though.'

definition: tensors are things which transform like tensors

2

This is potentially a big deal, if a Christoffel symbol is a tensor by modern standards, but not by old standards :\

@EricSilva you should be very upset by his abstract index notation

Mithrandir24601

11:29 PM

@EricSilva ::gasp:: ::collapses in shock:: :P

$\partial_a$ is not even properly defined

@DanielSank We will all suffer in eternity for lazy people's failure to justify why they don't bother writing annoying constants :P

im only upset at things i cant fix myself

im ok with wald trying to feed me bullshit as long as i trust myself to recognize the bullshit

This is madness tbh

How can he call a Christoffel symbol a tensor, it's like the most drilled-into-ones-head thing in GR-style diff geom that it's not a tensor

\begin{align}
\Gamma_{i' \, j'k'} &= \mathbf{e}_{i'} \cdot \partial_{j'} \mathbf{e}_{k'} \\
&= \frac{\partial x^l}{\partial x'^i} \mathbf{e}_l \cdot \frac{\partial x^m}{\partial x'^j} \partial_m (\frac{\partial x^n}{\partial x'^k} \mathbf{e}_n) \\
&= \frac{\partial x^l}{\partial x'^i} \frac{\partial x^m}{\partial x'^j} \frac{\partial x^n}{\partial x'^k} \mathbf{e}_l \cdot \partial_m \mathbf{e}_n + \frac{\partial x^l}{\partial x'^i} \frac{\partial x^m}{\partial x'^j} \frac{\partial^2 x^n}{ \partial x^m \partial x'^k} \mathbf{e}_l \cdot \mathbf{e}_n \\

@ACuriousMind yep

:-D

11:43 PM

@bolbteppa it's not a big deal, he's just being idiosyncratic about what the word tensor means

@EricSilva FYI Besse does the same thing (or something similarly wrong)

in his einstein mflds book or w.e.?

i need to read that at some point too

not a he

but yes

Besse is like Bourbaki

Lets make everything a tensor while we're at it

so I guess I should have said "do" the same thing

11:46 PM

oh yeah i forgot

i knew that at some point

@EricSilva ayy, Amazon is suggesting GR books to me rn

google is creepy as hell

they know what we're talking about

all it suggests me is GRE shit and Marx shit

i guess it knows

I haven't looked at GR books on amazon in a while

so it has to know we're talking about it

user image

@BalarkaSen this is a meme wrapped in a mystery, inside an enigma

i dont understand this surrealist memery

« first day (2718 days earlier) ← previous day next day → last day (2211 days later) »

Transcript for

Apr '1814

The h Bar

General chat for Physics SE (physics.stackexchange.com). For M...

join this room
about this room

00:00

06:00

12:00

18:00

all times are UTC

site design / logo © 2024 Stack Exchange Inc; legal

mobile