The h Bar

AccidentalFourierTransform

2:03 PM

I've decided that I'm a "shut up and don't calculate" kind of physicist

is "shut up and go watch tv" a good attitude towards QM?

s.harp

Probably not

Slereah

It is best to avoid interpretation wars

111

2:50 PM

Hello everyone. There's this line in my book, it says,
'There is one line through the charge, such that, if the velocity of the charge is along this line, there is no magnetic force. We define the direction of the magnetic field to be along this line.'

I just want to ask why this has been defined this way. Is this arbitrary, or is there some physical interpretation to the definition?

s.harp

The expression for lorenz force is $F=v\times B+E$ if you forget about the constants. Can you see why $F$ is zero if $v$ is parallel to $B$ and $E$ is zero?

111

Yes, of course.

sin of the angle goes to zero, that is why.

s.harp

Now if $v$ is not parallel to $B$, the cross product is not $0$, so you get a force

111

Right.

s.harp

If they want to give a particle a velocity in a magnetic field so that the magnetic force is zero they are left with no choice other than making this velocity zero or making it parallel to $B$

111

2:54 PM

Yes, but why do we call this direction as the direction of B?

s.harp

well two vectors are parallel iff $v_1=\lambda v_2$, so they point in the same direction

or in the opposite direction, but thats just a sign after all

AccidentalFourierTransform

what did the sushi say to the bee?

s.harp

Sushi has no organs with which to talk

and no capacity for agency either

111

Sushi said, thank god you're not a fish!

AccidentalFourierTransform

ʷᵃˢᵃᵇᶦ

111

2:59 PM

And, I still don't get the answer. How does the parallel thing relate to my question? It's giving me a lot of brain fog. -_- :(

s.harp

You want the particle to move in such a way that it experiences no magnetic force right?

This is the goal of your undertaking as I understood it

111

I don't know, it says that there is one line through the charge where it doesn't experience any magnetic force, and that line is defined to be the direction of magnetic field. But why is it defined that way? I'm homeschooled , so I can't even ask any teacher. -_-

AccidentalFourierTransform

why do things have to be defined some way?

its just a definition, move on

s.harp

no

in this case its not a definition

111

1. I didn't ask you.
2. I can't take physics as easy as you take it.

@s.harp It's written the direction is defined that way.

s.harp

3:03 PM

If you move a particle along a trajectory its velocity will always point in the direction of the tangent vector of this trajectory

AccidentalFourierTransform

klol

s.harp

and we have just seen, that the only possible velocity vector for which there is no magnetic force on the particle is one that is parallel (that is points in the same direction) as the magnetic field

AccidentalFourierTransform

@s.harp you are taking $F=v\times B$ as given

s.harp

this means that there is only one line along which the particle can move so that it does not experience any magnetic force

AccidentalFourierTransform

OP's book is looking for a more primitive definition

if you accept that F=v\times B is correct, then the whole discussion is trivial

s.harp

3:05 PM

I see

the goal of the passage is define the action of the magnetic force then?

AccidentalFourierTransform

the book is after a more fundamental definition of B

you may like it or dislike it, but in the end its just a definition

the motivation, if any, is just historical

s.harp

I misunderstood the passage, I thought they were saying they would define the "line of no force" to be the one parallel to the magnetic field

111

Hold on, I'm not getting anything. The magnetic field's direction is defined to be a vector where the charge doesn't experience force, and just for historical reasons. If that's true, I'll stop believing physics is right. I mean, something was defined for no reason?

s.harp

which is not a definition, because the line of no force is defined as the line a so that the tangent vector would not experience force and the other part follows from this definition

AccidentalFourierTransform

@111 it doesnt make sense to say that "physics is right". There is no fundamental truth to be found in the equations

just models that work (or not work)

the reason to define the magnetic field as it was defined is to match up Faraday's experiments

s.harp

3:08 PM

when he says historical reasons he means that it was defined like this 200 years ago for some reason or another that fit into some view of how this stuff should behave

111

Dude, either you're very wrong, or you're very right, and if the latter is true, you're giving me some serious nausea about taking up physics.

Plus, you speak like my teachers, they too say that 'learn things as they are'.

AccidentalFourierTransform

sorry not sorry

physics is fun though

111

1. I didn't ask for sorry.

2. I think somebody will tell me. Because it takes time, but I usually find a reason behind things in physics.

AccidentalFourierTransform

youre taking me too seriously. 99% of the time Im a troll

most of what I say is random bullshit

s.harp

The fundamental reason behind a definition: It works

111

3:10 PM

3. And the reasons are almost never 'historical'.

AccidentalFourierTransform

wow no, the reasons are always historical

s.harp

You probably have a different understanding of what it means when somebody says something is defined like this for historical reasons then

111

I simply can't believe there is no physical meaning of a definition. :\

s.harp

What is physical meaning?

AccidentalFourierTransform

when a framework (e.g., Maxwells equations) are very well understood, it is easy to come up with good and sensible motivations for the definitions

but this only works a posteriori

once the equations/definitions are known to work

s.harp

3:12 PM

You can find it the definitions I'm sure, its more a personal thing than something that must exist/cannot exist.

AccidentalFourierTransform

the first time something is introduced, it is always very dirty and unmotivated

s.harp

Tell that to witten

111

Physical meaning? I'll try an example. I know that a charge can move in a circle due to magnetic forces, because I can VISUALIZE it. But I can't visualize some highly advanced and intellectual humans defining something just because it works for the observations.

AccidentalFourierTransform

@s.harp i tried but he wont answer to my letters

s.harp

That doesn't sound like physical meaning, that sounds like assigning geometric intuition to a problem

AccidentalFourierTransform

3:15 PM

@111 well, the reason that you cannot visualise random/unmotivated definitions is that you are not used to that

ACuriousMind

@AccidentalFourierTransform I'm not sure that's true - the first time, the motivation is usually very clear, but it's not precisely clear what the useful definitions are. When more work has been produced, there's usually a convergence in the various definitions and approaches, but later texts often omit (or even are ignorant of) the original motivation for a concept.

AccidentalFourierTransform

so far, all youve learnt has been very well motivated

111

I think that I'm at fault then, because you all seem experienced physicists, I'm just a highschooler. I guess I'm being fanatic.

Wait, but why doesn't the same definition apply to electric fields? ( I know it's just one field - electromagnetic - but before that notion came into picture, electric fields were studied separately. )

s.harp

A particle feels electric force regardless of its velocity

So a definition in this vein is not very coherent with what people at this stage have previously called electric force

111

So why isn't a magnetic field's direction not defined this way - it can be any direction except the one parallel to the velocity?

s.harp

3:19 PM

Let $v$ be the velocity so that the particle feels no magnetic force

how many directions are there that are not parallel to this velocity?

ACuriousMind

@111 The nature and exact relationship between "truth", "reality", "meaning", "science" and "physics", among other things, has a long tradition of philosophical discussion that is usually not brought up in science courses. It's good to think about it - you just need to be prepared that what you find is not what you would have expected.

111

@ACuriousMind Am I logically wrong then?

When an electric field can spread infinitely, why can't magnetic fields behave that way?

s.harp

what do you mean

ACuriousMind

@111 About what exactly?

@111 I don't know what that means

Both electric and magnetic fields, in principle, pervade the entire universe.

AccidentalFourierTransform

ew gross, they dont pervade me

no way jose

111

3:23 PM

I mean. My texts say the direction is along a line where there is no force. I say they exist everywhere except that line, what is wrong with my logic? Any mathematical flaw?

ACuriousMind

@AccidentalFourierTransform There's no escape, even my fields are inside of you :P

@111 Well, there is no "logic" there. You just chose a different definition of "field". But note that "everywhere except a line" would not be a vector, but a surface to which that line is perpendicular

111

Yes, exactly, we could use an area vector for the direction at a localized point.

ACuriousMind

And, in fact, that is what the magnetic field is! It's not a vector, it's a pseudovector.

111

Alright. Thanks for the info though. :) But see, what is wrong with my definition?

And why is the other one right?

ACuriousMind

At least, that is what it appears naturally as in the modern covariant formulation of electromagnetism. In the "classical" formulation, it's a vector. So neither you nor others are really "wrong" here - you just chose different definitions

AccidentalFourierTransform

3:28 PM

@111 there are no "right" definitions. Nor "wrong" ones. Just useful vs. useless definitions.

111

Okay, so you're saying, if 70% of the physicists in the world accept my definition, ( just for thinking, I know that's not possible) , they'll start printing that. Right?

Slereah

The proper formulation of EM is with quaternions

111

@Slereah I'm just a highschooler pal.

s.harp

@ACuriousMind in the geometric formulation the magnetic field cannot be defined as an object independent of coordinate choice, what do you mean with psuedovector?

AccidentalFourierTransform

the proper formulation of EM is with beer

Slereah

3:30 PM

$$\Box 🍺 _\mu= 0$$

111

I've to go to the market now, thanks for bearing with me, all this time. I'll try to find the meaning behind this, if I know, I'll message it here.

ACuriousMind

@s.harp Looking at the field strength $F$, you get temporal components $F_{0i}$ which naturally form a 3d vector and spatial components $F_{ij}$, which naturally form a 3d-bivector. Its 3d Hodge dual is the magnetic field vector, but such Hodge duals behave as pseudovectors, not vectors (under e.g. parity). Equivalently, one might argue that ${\star}F$ instead of $F$ is the true dynamical field strength of electromagnetism

AccidentalFourierTransform

it turns out that ice cream+wine is not a good combination. I have a boo-boo on my stomach

ACuriousMind

@AccidentalFourierTransform the solution is clearly more wine.

AccidentalFourierTransform

you sure? ok, brb

k im back

@ACuriousMind hey sexy ( ͡° ͜ʖ ͡°) how u doing

ACuriousMind

3:36 PM

Oh dear, you're one of those drunks :P

AccidentalFourierTransform

i can be if you want me to ;-)

s.harp

@ACuriousMind I'm not sure I understand that kind of a construction without it being more explicit. I get if you choose a coordinate chart you do a linear transform and take a smaller chart so that you have at a point "time" and "space directions. Now you define a cotangent vector on the hyperplane where the time component is zero to have components $F_{0i}$ and take the hodge dual on this hyper plane where the metric comes from the pullback of the normal metric

Is that right so far?

FrancescoS

Hi guys. Is anyone able to explain me why in Extended SuperSymmetric theories, we have to take the consistency constraint N<= 8 into account?? Both for massless and massive multiplets. What are the inconsistency due to spin>2 massive particles?

ACuriousMind

@s.harp Yes, I'm assuming I'm in Minkowski space. In general curved spaces I don't think you can split the EM tensor into electric and magnetic fields.

s.harp

Well, given a coordinate system you can do it. But it doesn't define anything sensible imo

ACuriousMind

3:40 PM

@FrancescoS Search for Weinberg-Witten theorem

@FrancescoS See also: physics.stackexchange.com/q/14932/50583

FrancescoS

@ACuriousMind I am reading the theorem, which involves MASSLESS particles. Any trace of massive particles, as far as I know. Even for massless particles, the existence of a conserved current is essential?

s.harp

For me "pseudo-vector" always meant an anti-symmetric 2 form in $3d$ euclidean space, you can pretend it has vector components but you get different signs sometimes

FrancescoS

@ACuriousMind the post doesn't treat spin > 2 particles (massive)

AccidentalFourierTransform

there are pseudovectors in any number of dimensions

@FrancescoS you should read the first few chapters of Weinberg's QFT, Vol III

s.harp

@Accidental well, it might be analogous via a top $-1$ form

since ${n\choose n-1}=n$

AccidentalFourierTransform

3:43 PM

yep its the same

ACuriousMind

@FrancescoS Well, when you have more than N=8 SUSY (in 4d), then you get that the gravity multiplet (which is massless) has things more than spin 2 in it.

@s.harp Exactly, and the Hodge dual provides an isomorphism between such 2-forms and vector fields in 3d, but the dual fields transform differently under parity, hence "pseudovector".

FrancescoS

@ACuriousMind well, but does a spin 5/2 massive particles lead to inconsistency ??

s.harp

@ACuriousMind the problem is that the formulation is in $4d$ space, where I don't like to think of $3d$ pseudo vectors as being geometrically meaningful unless you do some dumb stuff like pulling it back onto hyperplane like I sketched

AccidentalFourierTransform

I have a theory of spin i particles

It does qualitatively explains brilliantly some of the consequences of electromagnetism, as far as I have analyzed them.

ACuriousMind

@FrancescoS Yes, if you couple it to anything (because then it will gain "charge" and Weinberg strikes). And if you don't, then you might as well have left it out.

AccidentalFourierTransform

3:47 PM

@FrancescoS yes because particles of any spin greater that 1/2, massless or massive, have to be coupled to conserved currents

Slereah

What kind of meme particle has spin 5/2

AccidentalFourierTransform

or otherwise you have problems

s.harp

@Slereah the fat quark

Slereah

Is there even a name for the 5/2 spin equation

Or do you just slap Wigner's generic equation on it

FrancescoS

@AccidentalFourierTransform Is this statement due to which theorem? Weinberg-Witten?

AccidentalFourierTransform

3:49 PM

because of the AccidentalFourierTransform theorem I guess =P

I don't know of any theorem, but its easy to understand the reason qualitatively

propagators are always of the form $\sim\frac{p^{2s}}{p^2-m^2}$

if you dont have a conserved current at the vertices, loop integrals diverge

($s$ is the spin of the particle)

s.harp

I'm confused, divergent integrals have never stopped anybody from doing quantum field theory before, why should they now?

David Z

OK, I'm here this time

AccidentalFourierTransform

so yeah, the theory is either free or awfully non-renormalisable

ACuriousMind

@DavidZ Hi!

David Z

@ACuriousMind howdy

John Rennie

3:52 PM

Annoyingly my lunch won't be cooked until 10 minutes into the session. My attention may be limited during the first ten minutes :-)

ACuriousMind

@JohnRennie what will you devour this time?

AccidentalFourierTransform

welcome everybody!

David Z

That's okay, I'm not sure we have anything in particular to talk about

Does anyone have any ideas to put on the agenda?

I guess the best-of nominations will be a good one...

FrancescoS

@AccidentalFourierTransform :) thanks, as @s.harp has already pointed out, why these divergences should be dangerous for the consistency of the theory? Can they be regularised?

ACuriousMind

@FrancescoS Note that if you have a divergence at every vertex, you have an infinite number of divergent diagrams and hence a non-renormalizable theory.

FrancescoS

3:53 PM

@AccidentalFourierTransform ops, ok you are right. The theory is not renormalizable

AccidentalFourierTransform

@FrancescoS to renormalise the divergences you'd have to introduce higher derivative counter-terms

John Rennie

@ACuriousMind vegetable risotto. After the weekend pie fest I'm eating healthily again.

AccidentalFourierTransform

and that is nasty nasty

FrancescoS

@AccidentalFourierTransform So, why are there people working also with N>8 theories? ;)

AccidentalFourierTransform

because you need money to buy food I guess

FrancescoS

3:55 PM

@AccidentalFourierTransform Does it even hold for very important physicists?? I don't think so

AccidentalFourierTransform

I really don't know, maybe there is some sort of fortuitous cancellation in SUSY that I don't know of, that makes higher spin theories well defined

I only know the basics of standard QFT

im a humble student that doesnt really know shit

@FrancescoS I was joking :-P

ACuriousMind

@AccidentalFourierTransform I don't think there is - the fact that all the 10d and 11d SUGRAs without higher spin correspond to string theory/M-theory is seen as a strong hint that strings/M-theory is on to something.

In the sense "all possible consistent SUGRAs arise from string theory"

AccidentalFourierTransform

@ACuriousMind neat

I remember that you said that you didnt really like ST

or that it wasnt exciting or something like that

did that change? you like it now?

FrancescoS

Do composite particles of spin > 2 lead to inconsistency?

Sorry guys :P

AccidentalFourierTransform

@FrancescoS nope

there are composite particles of spin >2

or are there?

FrancescoS

3:59 PM

@AccidentalFourierTransform So, why the components of a supermultiplet have to be elementary particles?

Can we have super partners of spin > 2 which are not actually elementary?

Slereah

Also how do spin 5/2 particles even work

David Z

So I hate to interrupt this little discussion but it is time for our chat session

ACuriousMind

It was only recently (2012) proved that the last two 10d SUGRAs which could not be excluded for other reasons with gauge groups different from those of string theory are, in fact, inconsistent

Slereah

I thought spins like that became very easily non-causal

David Z

Well then, welcome everybody to our biweekly chat session!

yuggib

4:01 PM

@Slereah yep, this is the velo-zwanziger problem

ncatlab.org/nlab/show/Velo-Zwanziger+problem

David Z

Please hold off on unrelated chats until we get through the topics of the day

Terry Bollinger

Which are?

David Z

Though we don't really have much to discuss. It's the usual 5 minutes for welcomes, 10 minutes for recent physics, and then we should touch on the best-of posts on meta

then you can get back to N=8 QFT or whatever ;-)

First things first, is anyone here new to chat sessions, new to chat, or new to the site?

...no?

FrancescoS

me :)

David Z

Hi @FrancescoS, and welcome :-)

(took ya long enough)

:-P

FrancescoS

4:05 PM

But I will try to contribute if I can. ;)

Terry Bollinger

Moving along... :)

David Z

Well we are happy to have anyone who contributes well!

Let's start in on recent physics developments. What's new in the world of physics lately?

ACuriousMind

No idea, I've been digging through papers from ~2000 mainly :P

Slereah

Not much new with CTCs and wormholes, anyway :p

David Z

I haven't been paying particularly close attention myself, but it does seem like there's a bit of a dearth of new science stories lately

I guess there's time crystals

Slereah

4:09 PM

There's always plenty of science stories

They are just for fields we don't follow

David Z

Well, true

Slereah

I'm sure there's exciting news in crystalography or solid state physics somewhere

David Z

"dearth" is relative and context-dependent of course

AccidentalFourierTransform

so... no news is good news?

David Z

Who knows

AccidentalFourierTransform

4:11 PM

who new

Terry Bollinger

@DavidZ my immediate quick reaction to the time crystals idea is just "surely this would fall under temporal quantum uncertainty, and so not be directly observable without destroying the crystal?" Which doesn't say you could not do it...

David Z

@TerryBollinger I guess it depends on the magnitude of the uncertainty. In any case this is something that has reportedly been done by two separate experimental groups.

Though I haven't read the paper or even the full APS Viewpoint

(yet)

Slereah

Also time crystals are old news

David Z

@Slereah Not this paper. It was published 6 days ago.

John Rennie

Has anyone read First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements

David Z

4:15 PM

Anyway, let's move on and take some time to look at the best-of posts on meta.

ACuriousMind

@TerryBollinger Careful, there is no time operator and hence no naive "temporal uncertainty", cf physics.stackexchange.com/q/53802/50583

Terry Bollinger

@DavidZ intriguing. The uncertainty would need to be large enough for the temporal layers of the crystal to "feel" each other's presence, so classical time flatly cannot be allowed for one of these puppies to form. But offhand I can't see any reason why it would be all that different from other forms of long-term temporal uncertainty, including most (all?) quantum computation stragegies..

David Z

9

Q: Best-of PSE 2016: description and categories

As suggested in another meta post, let's do a Physics Stack Exchange "best of 2016". There's a nice precedent for this in the Puzzles and Code Golf site. Objective Reward and draw attention to some of the best content our community has created this year. Have fun :D Procedure Establish cat...

discussion

Terry Bollinger

@ACuriousMind I'll look, I'm sure I'm saying something overly sloppily.

David Z

That is the "master post" describing what's going on, but there are several other posts for individual categories

One has no suggestions yet:

6

Q: Best of PSE 2016 - Best community work

What was, in your opinion, the best answer in 2016 which where two or more authors contributed a significant fraction of the content? Vote either by posting an answer linking to your favorite community work, with a brief description of why it's your favorite, or by upvoting an existing post. Vot...

discussion

Terry Bollinger

4:17 PM

@JohnRennie nope, but "emergent gravity" is an interesting phrase...

ACuriousMind

@DavidZ I think it has no suggestions because that sort of community work doesn't really happen around here.

People usually post their own answers if they have a significant improvement to contribute, not edit it into others.

David Z

@ACuriousMind Yeah, it's not common, but there may be some examples out there.

ACuriousMind

Even most community wiki posts are just by a single author

John Rennie

In my post on Hawking radiation ACM put ina lot of effort.

Also my post of what photons are was largely ACM and Daniel's work though i wrote it alone.

David Z

Maybe those could count.

Qmechanic

4:20 PM

@DavidZ : Is the meta post about community wiki posts, collaborations between different users, or both? The meta post should be possibly be clarified.

yuggib

@ACuriousMind sometimes there is a time operator in QM... see this

David Z

@Qmechanic I don't actually know, or at least I don't consider myself in a position to definitively answer such questions. It's a good question though.

Terry Bollinger

@ACuriousMind that is interesting, but I need to look at the observables distinction more closely than I can in this session.

David Z

There are other categories that aren't so hard to find candidates for, though some of them have only one suggestion so far

6

Q: Best of PSE 2016 - Best answer by a rookie

What was the best answer posted in 2016 by a user whose account was made in 2016? Vote either by posting a link to your favorite answer given by a rookie, with a brief description of why it's your favorite, or by upvoting an existing post. Voting lasts two weeks, i.e. until Jan 29 2017. This is...

discussion

7

Q: Best of PSE 2016 - Best answer directed at the layman

What was, in your opinion, the best layman answer in 2016? A layman answer is an answer directed at those without expert training in physics. Vote either by posting an answer linking to your favorite layman answer, with a brief description of why it's your favorite, or by upvoting an existing p...

discussion

2

Q: Best of PSE 2016 - Less is more

What was the most impressively clear and concise answer in 2016? Note that a clear and concise to a very simple question is not necessarily particularly impressive. Vote either by posting an answer linking to a clear and concise answer and giving a brief description of why that answer should wi...

discussion

Slereah

I say vote for me

I'm great

I don't care which one

David Z

4:23 PM

Just something to keep in mind as you browse the site

Slereah

Tho IIRC I didn't do a lot of answers this year

David Z

That's okay

Somebody did

ACuriousMind

@Slereah 34, if I'm counting correctly.

Slereah

Yeah but they were all short answers

David Z

Still counts

Anyway, that's about all I've got as far as topics today. I'd say it's time to go back to open discussion

Slereah

4:30 PM

7

A: How to measure Torsion and Non-metricity?

Torsion affects the transport of vectors along a path. More physically speaking, it affects the propagation of spinor fields (EM fields are not affected since exterior derivatives are independent from the connection). Since the torsion tensor is directly equal to the spin tensor, this means that ...

probably my best answer of the year

David Z

@Slereah you could nominate your own answer, I'm sure - it's up to the votes in the end

Slereah

Eh I don't care that much

well

Personally

I think my best answer is this one :

0

A: How to deal with boundary conditions for path integrals?

Hello handsome poster (why thank you kind stranger). The answer to your question it turns out is in Rovelli's "Quantum Gravity", at least insofar as the free scalar field is concerned. This is done in the following way. As you may recall (from Feynman and Hibbs), through various arguments about d...

But I only did it because I really like the poster

AccidentalFourierTransform

@Slereah "of the year", Oct 13 '15

David Z

@FrancescoS having fun yet? ;-)

FrancescoS

@DavidZ ahah not so much :P

David Z

4:34 PM

hehe, yeah... this is fairly slow for a chat session

AccidentalFourierTransform

well i'm having the time of my life

Slereah

Well there isn't much to discuss

AccidentalFourierTransform

you people dont know how to enjoy the little things in life

David Z

@AccidentalFourierTransform Is that an allusion to string theory? :-P

AccidentalFourierTransform

it was an allusion to my ex boyfriend

Terry Bollinger

4:37 PM

@ACuriousMind I recall once going through some of the SR aspects of issues such a length measurement and ending up concluding that xyz length is no more "real" than classical t length, at least if you are realistic about the need to insert an information-integrating observer into the situation. This leaves me initially dubious about whether this conclusion is as solid as the author thinks, or is instead more a reflection of how the equations have been set up.

Nice new icon Danu.

David Z

@AccidentalFourierTransform ah, well that's your personal business

AccidentalFourierTransform

lol

Terry Bollinger

@JohnRennie I'll look up that emergent article. Gettin' too off-physics for me, later all...

ACuriousMind

@TerryBollinger I'm not sure what you mean; in any case, the point about the $\Delta t$ being different from the things appearing in other uncertainty relations is purely non-relativistic. Relativistically, you get other troubles, such as not being able to define proper relativistic position operators in the first place

AccidentalFourierTransform

he left dude

you scared him away

David Z

4:42 PM

@TerryBollinger funny that you leave right before we get back to discussing physics

AccidentalFourierTransform

so... we call it a day?

David Z

Yeah, for the chat session at least.

There was a lively discussion going on when I got here, so people clearly have things to talk about.

ACuriousMind

@AccidentalFourierTransform Yeah, my introductory course was boring and not at all exciting. I've come around to see some of its merits when digging deeper.

AccidentalFourierTransform

@ACuriousMind such as? what nice properties does it have?

I mean, I know the layman explanations and so on

but in technical(ish) terms, why is it so cool/nice?

David Z

BTW see everyone in two weeks for our next chat session!

ACuriousMind

4:56 PM

@AccidentalFourierTransform One thing is definitely what I already said, "string universality": All 10d and 11d SUGRAs are low-energy approximations of string theory, and almost all SUGRAs in the lower dimensions can be obtained by compactifying 11d SUGRA. Contrary to the QFT framework, where you have a giant load of theories which need not share any common features, the stringy (or, well, the SUGRA) framework forces remarkably strong restrictions on the theories it produces

The compactification process itself is intricately connected to all sorts of geometry, and I find it interesting to see how mathematically interesting features and physically interesting features (such as preserving supersymmetry <-> having special holonomy) go hand in hand

There's a geometric intuition to things that ordinary QFT severely lacks. For instance, spontaneous symmetry breaking/restoration directly corresponds to the compactification manifold resolving/developing singularities, and the type of singularity controls the gauge group.

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