The h Bar

Tobias Fünke

21:12

morning, qwerty :)

imbAF

When we talk about continuous/ discrete transformations of the configuration of a system, the transformation is called symmetry or symmetry transformation, if a physical quantity is invariant.
And in all the cases considered, we start by checking how the transformation, of any kind, affects the lagrangian and the action
So, my question is, is the term configuration of a system the same as the lagrangian density or action of it ?

ACuriousMind

@imbAF What do you mean "if a physical quantity is invariant"? The definition of a symmetry is just that it leaves the action invariant. Also, who uses the term "configuration of a system"? Do you mean the state of a system?

imbAF

@ACuriousMind en.wikipedia.org/wiki/….

If I have noticed it correctly

ACuriousMind

@imbAF But the configuration space is not the state space: Note that the configuration space consists only of the generalized coordinates, but phase space consists of the generalized coordinates and momenta, and symmetries are transformations on phase space, not configuration space. I don't understand why you would talk about the configuration space in the context of symmetries.

imbAF

A transformation is labeled as a symmetric one, if it leaves the action invariant.
Also you have the definition in the above picture.
But, I would argue that a transformation doesn't change the physical system, rather only its coordinates.

ACuriousMind

21:21

@imbAF that's a very bad definition, where is that from?

imbAF

Which of the two?

ACuriousMind

this "if a property is invariant" from your picture

it's just wrong, the normal definition of a symmetry is if the action is invariant

imbAF

@ACuriousMind you mean the red-lined comment in the picture

that is from the book

ACuriousMind

just some random property being invariant under some transformation does not turn that transformation into a symmetry

imbAF

Ahaaa

well, I am going by from what the book says

ACuriousMind

21:22

likewise the definition of conservation is wrong, the common definition would be that the time-derivative of a conserved quantity is zero

imbAF

and the book is:
https://www.google.com/search?q=qft+for+the+gifted+amateur&sca_esv=5cf0ee39a289ec1c&gl=us&hl=en&pws=0&sxsrf=ADLYWIKg-eV7EZYoOBRFv3VgiCq8c-Z2xA%3A1735852978379&source=hp&ei=sgN3Z6H9FLi59u8P-OXVsAc&iflsig=AL9hbdgAAAAAZ3cRwijSiv1xCWhb18Sk8ew3121aYqSU&oq=qft+f&gs_lp=Egdnd3Mtd2l6IgVxZnQgZioCCAAyChAjGIAEGCcYigUyBBAjGCcyChAjGIAEGCcYigUyBRAAGIAEMgUQABiABDIFEAAYgAQyBRAAGIAEMgUQABiABDIFEAAYgAQyBRAAGIAESPUNUABYsQVwAHgAkAEAmAGkAaABqgWqAQMxLjS4AQPIAQD4AQGYAgWgAtQFwgILEAAYgAQYkQIYigXCAgsQLhiABBjRAxjHAcICBRAuGIAEwgIREC4YgAQYkQIY0QMYxwEYigXCAgoQABiABBhDGIoFwgIHEAAYgAQYCsICCxAuGIAEGMcBGK8Bm…

ACuriousMind

@imbAF my question is what book it is because if this is meant to be a general definition in mechanics it's very bad and you need to find a new book :P

imbAF

Yeah I linked the book

Isn't the link working for you?

"Quantum Field Theory for the Gifted Amateur" is the name

ACuriousMind

well, I already didn't like that book based on its title but now that I see its content I like it even less :P

imbAF

@ACuriousMind I thought of starting with this book before I started with Weingard

ACuriousMind

21:24

I can't tell if Weingard is supposed to be Weinberg or the Weigand notes I keep recommending :P

imbAF

@ACuriousMind SillyGoose also said is bad. I thought it was ok, but perhaps I am wrong. I should drop it?

@ACuriousMind your notes

I thought of reading something simpler before reading that

since I believe that your recommandation

will have condensed information, which I might not be familiar with

Is the book bad?

ACuriousMind

I firmly believe that anything simpler than those notes (or a standard QFT intro book) cannot accurately convey even the basics of QFT in a way that would be useful.

@imbAF I already told you I haven't read it, but this "definition" of symmetry/conservation you posted is simply atrocious

if the rest of the book is of similar quality it is bad, yes :P

imbAF

maaaan... I read 150 pages in like 10 days or less during christmas time

Silly Goose

@TobiasFünke btw i recall a bane of the math notation for state vectors. It can be hard to distinguish indexed coefficients from indexed vectors

imbAF

and now all that goes to waste.....

@ACuriousMind I guess I will start with the notes. I simply thought of preparing myself for it, but apparently I have been losing time needlessly

And the standard QFT intro book, would be i.e Peskin ?

Cuz I have been told that peskin is bad, by others who have read it

ACuriousMind

21:35

I mean, I don't have a favourite among those, the usual choices would be Srednicki, P&S, Itzykon & Zuber or Schwartz (or perhaps Zee but it's also rather casual)

imbAF

Well I guess its better late than never

Tobias Fünke

@SillyGoose nope. the context will do its work hehe

imbAF

@ACuriousMind I asked about the books, because Weigand says that is also important to read one of these books for additional information, in case I would need that. And I thought of asking

Tobias Fünke

the gifted amateur QFT book is not too bad. sure, you should not expect a "rigorous" textbook

but for its purpose I think it is quite good

imbAF

Yeah but, you just saw that its definition for symmetric transformation was not accurate

ACuriousMind

21:42

I mean, Tobias has a point - the book does not present itself as a normal QFT textbook, but more as "for a general audience"

unfortunately, "for a general audience" usually means we're just going to be vague about things :P

Tobias Fünke

@imbAF ...I mean, what did you expect?

yes

imbAF

For definitions to be precise ?

That is the bare minimum

qwerty

yeah I agree with imbAF

like it's not hard to word things carefully if you know what you're talking about

ACuriousMind

I mean I understand the attitude but that's just not how pop-sci works :P

qwerty

burn the fluff

imbAF

21:47

If I wanted pop-sci I could read something more exciting than this

qwerty

lol

imbAF

They should have put a big disclaimer: Half-assed loose definitions

qwerty

do they even realise they're doing that though

handan_toddler

@qwerty but, leave enough residue behind for the struggling learner to follow the thought process

qwerty

confidently incorrect people grind my gears. especially the ones driven by ego or because they're so used to being right on many things they forget humility when speaking outside of their expertise

Tobias Fünke

21:50

I don't understand

with all due respect

qwerty

sorry that was somewhat tangential

Tobias Fünke

its not the purpose of the book to teach you general notions of theoretical physics

that should be clear before reading the book

don't get me wrong: I am also a fan of clear and precise statements

ACuriousMind

@qwerty It's on purpose, again, this is a book somewhere between pop-sci and a textbook, see e.g. the blurb and reviews at the book's homepage

Tobias Fünke

but also one should not always blame the book or lectures etc. One can assume some knowledge of the reader.

imbAF

Actually the book says the following:

Tobias Fünke

21:52

and at the risk of repeating myself: Yes, things can be stated more clearly. But judging a book by single statement or a few intro lines into a chapter... I don't know

handan_toddler

Judge it by the number of typos.

ACuriousMind

they explicitly call about not using "forbidding looking mathematics", which includes for better or worse, in many cases, foregoing precise formal statements for more colloquial language. If you're the kind of person who will do hermeneutics on the phrasing of every single sentence this isn't the book for you

Tobias Fünke

yes, exactly

handan_toddler

LoL "hermeneutics"

qwerty

lol^2

imbAF

21:56

to me the highlighted part, doesn't translate to = "We won't even bother to give proper definitions"

Tobias Fünke

please

ah, nvm

I will stop here

qwerty

@ACuriousMind at the risk of being contrarian I do think that the amateur is more likely to do that than the professional though. a professional can read it and know it's fluff. the gifted student may not

ACuriousMind

@imbAF To be quite honest for this particular case, if the audience is "graduate students", I would expect a graduate student in physics to already understand the notion of symmetry perfectly well.

handan_toddler

"gifted" is a loaded word

ACuriousMind

Like, this is one of the most fundamental notions in physics, if a QFT book is the first time you're thinking about what a symmetry is that's far too late and what have you been doing before

imbAF

21:59

@ACuriousMind Yeah but that would be, included in a group theory course, which wasn't in my program, unfortunately

ACuriousMind

@imbAF No, it would be included in every fundamental course on mechanics

Tobias Fünke

@ACuriousMind that's what I meant some messages above

ACuriousMind

I refuse to believe you have not had a course that dealt with some notion of Noether's theorem before you tried your hand at QFT

imbAF

@ACuriousMind If you are referring to SO, SU etc, hasn't been the case. In classical mechanics we mostly dealt with rotational symmetry mostly

Tobias Fünke

omg

handan_toddler

22:01

:-)

imbAF

@ACuriousMind Of course I have

ACuriousMind

@imbAF then why on earth are you asking me questions here based solely on the hand-waving description of what a symmetry is in this very casual book instead of using your established knowledge of what a symmetry is?

You started this discussion with this:

50 mins ago, by imbAF

When we talk about continuous/ discrete transformations of the configuration of a system, the transformation is called symmetry or symmetry transformation, if a physical quantity is invariant.
And in all the cases considered, we start by checking how the transformation, of any kind, affects the lagrangian and the action
So, my question is, is the term configuration of a system the same as the lagrangian density or action of it ?

imbAF

Because the definition given in the book, is different, to say the least, from what we were briefly told, in my classical mechanics class. As a result I became confused

ACuriousMind

but if you knew what a symmetry was (which, again, as someone trying to learn QFT you already should), you would know that's a bad/vague notion of what a symmetry is

@imbAF then why would you not lead with "why is this different to what I thought it was"?

imbAF

@ACuriousMind I wouldn't think the book is wrong.

@ACuriousMind But I did give, the definition that I learned. And then I posted what was written in the book.

Tobias Fünke

22:05

sorry

the book

does not give a "definition"

it tries to illustrate the concept

one can argue whether or not it succeeds. but again: It is not the aim of the book to teach you basic notions of symmetry

imbAF

Does that mean, that it should give a convoluted or badly expressed explanation of it?

ACuriousMind

@TobiasFünke This is exactly the hermeneutics problem: The books says a phrase "X is called a symmetry if Y" and this has the shape of a definition. In a rigorous mathematical text, a definition would be phrased in exactly the same manner (just Y would be a proper definition). If you're aware that the book is not a rigorous text, you won't try to interpret it that way, but if you're not, you will.

Tobias Fünke

@ACuriousMind but as you pointed out: the book never claims it is a usual textbook with precise definitions. and based on the title I don't think anyone could expect it to be

ACuriousMind

The funny thing is that if you do this the other way around - always present proper definitions - a different bunch of people will accuse you of being a boring pedant :P

Tobias Fünke

and yes, since it does not give a proper definition $Y$ I would not read it as such, but as an illustration or so. and again: one can argue if it was good or bad or whatever. but for this particular case, I don't see a problem

yes, sure

qwerty

22:10

there's always a middle ground between looseness and pedantry but in general no one agrees where it is lol

Tobias Fünke

@imbAF would you find it better if the book just skipped the paragraph and would rely on your previous knowledge?

ACuriousMind

@TobiasFünke Well to be quite honest based on the title I'm not particularly inclined to be charitable to the book anyway because "gifted amateur" is such an obnoxious phrase loaded with baggage :P

handan_toddler

^

Tobias Fünke

:d

ACuriousMind

personally I find this even worse than the "for dummies" kind of book title

imbAF

22:13

You can say that about every instance, so the book ends up not existing. I don't understand how it's wrong to expect a book, regardless of audiance, not to give the definition correct. I will just give it a rest

Tobias Fünke

read a proper textbook then.

handan_toddler

^^

imbAF

Yeah....I kind of know that by now

qwerty

@handan_toddler yeah I should have put it in quote marks.

handan_toddler

:-)

Silly Goose

22:23

the “in a nutshell series” are proper textbooks; im not sure why they publish under this title though

qwerty

the quirky-title qft book on my shelf is "student-friendly quantum field theory" but i have no idea if it's good or not

handan_toddler

A "nutshell" is usually tough to break open without a nut cracker.

ACuriousMind

@qwerty Maybe there should be a counter-offering: "Anti-student QFT". This book is not for anyone. If you read it, you will suffer. Suck it up. :P

imbAF

Then all the books about QFT, would need the "Anti-student" part

handan_toddler

ie gate keeping

qwerty

22:31

hehe

@Slereah oh thanks for this btw. sorry i didn't see this til now

Silly Goose

Postmodern QFT: written just for you, the reader

handan_toddler

the gifted reader :P

in an impenetrable nutshell

Herr Feinmann

@ACuriousMind in that case it shifts my idea of consistency. You're awfully consistent when you're drunk!

@qwerty Yes, I'm 5

qwerty

22:46

@HerrFeinmann he does a perfect random walk

Herr Feinmann

Which reminds me that I often walk with a weird irregular zigzag pattern and periodically collide with those walking with me. It's a random walk on direction parallel to the ground and perpendicular to the instantaneous motion

Silly Goose

Me with my gaussian walk: ~~~

Herr Feinmann

Oh, thinking of it, I'm not unlike an electron drifting and experiencing collisions

handan_toddler

handan walk

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