The h Bar

9:24 AM

@Qmechanic no worries, i don't think my comment was helpful anyway

1:13 PM

@ACuriousMind Let $\theta$ be a Grassmann-valued Majorana 2-spinor. How does one prove the "two-dimensional Fierz transformation" $\theta_A\bar\theta_B=-\delta_{AB}\bar\theta\theta$? I've done a bit of Googling and the Fierz identities I found don't look like this.

ACuriousMind

@0celo7 I haven't the slightest idea what a "two-dimensional Fierz transformation" is. What do the indices ${}_A$ denote?

0celo7

@ACuriousMind Worldsheet components.

ACuriousMind

Ok, what's the $\delta$?

0celo7

@ACuriousMind Neither does Google. It's a strange term BBS uses that is also found in GSW.

@ACuriousMind Kronecker

ACuriousMind

And the $\ddot{\theta}\theta$ is supposed to be a scalar product?

0celo7

1:20 PM

That's a \bar, not \ddot.

It's the standard scalar bilinear, yes.

ACuriousMind

Ah, I see

Well, I've got nothing

0celo7

The only thing I have is that the trace is a true statement.

Whoops, forgot a 1/2 on the RHS. (King of Typos strikes again!)

@Qmechanic Have you ever heard of the "two-dimensional Fierz transformation", Eq. (4.24) in Becker, Becker & Schwarz and Eq. (4.1.22) in Green, Schwarz & Witten?

Qmechanic

@0celo7 : Yes. What's the problem?

0celo7

1:39 PM

@Qmechanic I can't justify it beyond "the trace is a true statement".

innisfree

2:04 PM

"Our most important theories, like Newtonian mechanics and quantum mechanics, are not falsifiable, because they do not make predictions by themselves, but provide general frameworks for more specific theories, which do make predictions." - Weinberg, in a recent interview with John Horgan. I've heard famous theorists concede that ST isn't falsifiable with the caveat that neither is QFT, but this is a new one to me.

0celo7

@KyleKanos Ha!

yuggib

2:26 PM

@innisfree It depends on what he is meaning by "a theory"...in my personal taste, I prefer statements (when concerning to logic) that are a bit more precise. Anyways, he is SW, and he can say whatever he wants ;-)

innisfree

2:42 PM

I'm not sure it does, though. Can you give an honest interpretation of "Newtonian mechanics" that hasn't been falsified? or, better, one that can't be falsified?

It strikes me as a frankly bizarre, ridiculous remark, especially coming from one of the leading scientific minds of the last century

yuggib

@innisfree Every scientific statement can be falsifiable, for it to be scientific...at least on philosophical grounds. My interpretation of the quote is that NM and QM may be seen as general mathematical settings (e.g. QM as the category of operator $C^*$ algebras); within these general settings concrete physical theories are developed (that give predictions), and the latter are falsifiable by means of experiments. Anyways this is only my interpretation

and btw applies to every scientific statement that can be, roughly speaking, written in mathematical language

(it is for that reason that, as I said, I prefer less ambiguous statements)

innisfree

even with that generous reading, isn't NM falsifiable/been falsified? You can't build a theory that describes reality in the mathematical setting of NM.

yuggib

3:03 PM

@innisfree Of course NM is falsifiable as a physical theory that makes predictions on the real world; however the theorems of Hamiltonian mechanics, seen as the mathematical theory of symplectomorphisms on cotangent bundles, are true from a mathematics point of view. Anyways, I really don't know if this is what SW intended...

innisfree

hmm, yes I see your point, here's the full interview blogs.scientificamerican.com/cross-check/2015/05/01/…

0celo7

@innisfree What does the "of all places" comment mean? That is where SW has tenure.

@yuggib Would he refer to that as Newtonian mechanics? I've always thought Newtonian refers to $F=ma$, $\tau=I\alpha$, etc. (I know they are formally equivalent.)

yuggib

@0celo7 Well, when I say Newtonian mechanics I think of mechanics in the classical sense, i.e. everything that is not general relativity and quantum mechanics...but that is just my point of view

the things you mention are just the basic "laymen" formulas

0celo7

3:19 PM

@yuggib I think that e.g. V.I. Arnold would disagree, else he would have written Mathematical Methods of Newtonian Mechanics.

yuggib

@0celo7 A russian would never have called it with the name of a westerner :-D Apart from the jokes, this seems really a not-so-interesting point on terminology

vzn

7:20 PM

> [SW:] First of all, this business of falsifiability is a silly criterion imposed on physical science by Karl Popper, who was looking for some way of discrediting Marxism and psychoanalysis.

whoa, really? not an expert on popper...

psychoanalysis forms a major basis of modern psychology. which is a "soft" science one might expect popper to have some affinity for.

as for marxism, as an economic/ political theory, in many ways it seems to be having the last laugh...

likely the "hard" scientists will always have some major aversions to "philosophers of science" & their "2nd hand/ sideline/ armchair" insights sometimes bordering on contempt....

but note wrt falsifiability SW is defending multiverse theory in the interview. lol, whoa

multiverse theory seems to me sometimes one of the ultimate conceivable violations conceivable of occams razor...

anyway quite a lively/ colorful/ juicy interview in places, need to look at it closer/ in detail!

did not know weinberg could be so opinionated/ controversial! seems like a newly revealed side in some ways! the interview is wrt is new book (feb 2015)... any reactions?

amazon.com/Explain-World-Discovery-Modern-Science/dp/0062346652

innisfree

@vzn this is quite good too theguardian.com/science/audio/2015/feb/13/…

an interview with Weinberg about his new book, which I haven't read

i think weinberg underestimates popper's scope and ambition: it's true that he wanted to discredit marxism and freudian psychoanalysis, but he also wanted to tackle major philosophical issues, such as the demarcation between science and pseudo-science and the problem of induction

vzn

7:35 PM

oh! reviewing the book how the hippies saved physics by kaiser. you might enjoy it, would be interested in your opinion. he mentions popper & the problem of "demarcation" in the intro & later, as studied by popper, & also mentions psychoanalysis, marxism, and astrology.

(hah you mentioned demarcation just as was typing that)

kaiser concludes, along with his story & many sociologists, demarcation is a hopeless/ unattainable quest/ mirage...

innisfree

whether marxism is scientific is an interesting one - at marx's small funeral, engels gave an eulology in which he compared marx with darwin. certainly marixism is similar to freudian analysis, in that both provide (meta-)narratives for understanding the world. in that sense, i think they're similar to multiverse ideas.

even if demarcation is impossible, clearly not all knowledge is equally reliable

Icosahedron

8:06 PM

@vzn Not another one of those books...

dmckee

8:40 PM

Would someone who knows QED check this edit: physics.stackexchange.com/review/suggested-edits/81602

If the Wikipedia page is on the subject is right, then the edit should have been rejected and should be rolled back. But I don't have a authoritative reference handy.

vzn

@innisfree agreed demarcation might be related to "reliability". but if popper couldnt figure out demarcation then what chance do we? much psychology is grounded in human neurobiology... was popper against psychology or just psychoanalysis? psychoanalysis is virtually the birth of the field of psychology...

re newtonian mechanics; it seems very strange to me that both physicists & philosophers seem to be talking past each other wrt "falsifiability" of theories... the ["engr-like"-] concept of a theory as an approximation (of reality) seems not to have fully entered the vocabulary/ realization of each...

dmckee

9:00 PM

@vzn It certainly has in the experimental community. We're all about "good enough" and understanding how good that has be.

But that very practical take on correctness is somewhat inimical to the highest level of navel gazing.

vzn

lol science is where it is because of what you just refered to as "navel gazing"

dmckee

9:50 PM

@vzn Well, the successful bits of navel gazing, anyway. Most theory work is not that esoteric. It is not every generation that sees the kind of revolution that comes for successful navel gazing, but they all see theoretical progress.

user54412

@dmckee The units and numbers work out with the edit.

user54412

The wiki equation is for E \propto m_e^2, so a should be qE/m_e \propto m_e^1, right?

dmckee

@ChrisWhite Yes, I see now that the wikipedia article is stating the field, not the acceleration.

So, that's all good. It's the weekend before finals for me and I am feeling wrung out.

Grading, grading, grading ... uhg.

user54412

I don't think I've ever heard of "navel gazing" before

user54412

But of course, there's a wikipedia article:

user54412

10:00 PM

Omphaloskepsis

Omphaloskepsis or navel-gazing is contemplation of one's navel as an aid to meditation. The word comes from Greek omphalos (navel) + skepsis (act of looking, examination). Actual use of the practice as an aid to contemplation of basic principles of the cosmos and human nature is found in the practice of yoga of Hinduism and sometimes in the Eastern Orthodox Church. Some consider the navel to be "a powerful chakra of the body". However, phrases such as "contemplating one's navel" or "navel-gazing" are frequently used, usually in jocular fashion, to refer to self-absorbed pursuits. == See also... ==

JamalS

@ChrisWhite How did you come across that?

user54412

dmckee just dropped the term, and google took care of the rest

Icosahedron

duh

JamalS

Sorry, didn't read the chat logs...

user54412

... and I just realized I'm probably the first person in a while to post nude images to this chat room

JamalS

10:04 PM

It's fine

And they are statues after all...

dmckee

The choice of the term was initially all in jest about the cultural difference twixt theorists and experimenters. But then @vzn took (or feigned?) offense and I seem to be feeling my oats...

user54412

"feeling my oats" - yet another turn of phrase I've never heard before

user54412

@dmckee I've got to visit that part of the country some day, if only to enrich the way I speak

NeuroFuzzy

10:21 PM

@dmckee The very practical take on correctness is the only truth I've came across so far in physics.

@ChrisWhite I... wow. I didn't know that.

vzn

@dmckee probably dislike some abstract/ esoteric philosophy as much as you do, but must admit some real partiality to "scientific philosophers" such as kuhn/ popper, who added significantly to understanding of science as a social endeavor.

> the reverse side has a reverse side —japanese proverb

on other hand also do admit some partiality to the critiques of eg string theory by eg smolin/ woit as verging on a sophisticated form of navel-gazing.

vzn

10:56 PM

btw dmc you dislike the QM movie theme idea? didnt watch any yet, but seems harmless...? did anyone else look? just think of it as physics "outreach" :)

(its supported by various high-profile scientific groups/ societies etc)

dmckee

What QM movie theme?

vzn

@dmckee ?

maybe your students might find something to like there :)

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