The h Bar

Jimdalf the Grey

17:00

I really need to step up my game

ACuriousMind

My average votes per answer in matter are 119 :D

Jimdalf the Grey

lol, sweet

but I at least have more than 9 answers in newtonian gravity

Kyle Kanos

I've got 30/answer in rocket-science and propulsion

Which, I think, are my best

wait, 32 in astrophotography and jupiter

Jimdalf the Grey

58/answer in quantum-interpretations and singularities

0celo7

How does the transpose work when we exponentiate a linear combination of non abelian generators? Like if we write an element of $O(3)$ as $$\exp\left(i\theta^a t^a\right)$$does it follow that it is orthogonal?

the $t^a$ are antisymmetric

Kyle Kanos

17:07

I've also moved up in the Citizenship score ranking. Now I'm 9th with 32/40

ACuriousMind

@0celo7 Yes, exponentiating Hermitian stuff gives unitary stuff.

Jimdalf the Grey

@KyleKanos How is it ranked?

Kyle Kanos

I think it goes total -> access date

I guess one could say I'm tied for 8th, since Ron Maimon & annav are both at 32 as well

0celo7

What happened to Ron Maimon?

He just left?

Kyle Kanos

Yes, because he refused to stop being a jerk to users

Jimdalf the Grey

17:12

Hey, I should be a 28, not a 27. Thing hasn't updated since the 15th

0celo7

Users as in new users or long-time users?

Jimdalf the Grey

Oh well, still top 20

ACuriousMind

@0celo7 Both. He (and some others) value "frankness" over everything else.

Kyle Kanos

Nice: Dr Seuss is publishing again: usatoday.com/story/life/books/2015/02/18/…

0celo7

"censorious moderatorship"

I thought he was dead...

Lol, reading. The first sentence confirms.

user54412

17:16

There are several people in my field who keep publishing papers years after dying

Jimdalf the Grey

@KyleKanos You're up to 3.47 on the votes/answer rating

data.stackexchange.com/physics/query/120310/…

Nothing compared to my 5.62 XD

0celo7

Ron Maimon has an account on Christianity.SE. Interesting...

Kyle Kanos

@JimdalftheGrey Is that an increase? I don't even remember :(

Actually, I wonder if deleting my zero-upvote, unaccepted answers will help that

Jimdalf the Grey

@KyleKanos it would, the algorithm takes total number of upvotes on answers and divides by total number of answers

user54412

@JimdalftheGrey be careful who's around when you boast ::cough:: 5.72 :P

Jimdalf the Grey

17:23

Yeah, but you and Mark Eichenlaub are like the only 2 regularly active users above me

(shakes fist)

oh, and alemi with 12 votes/answer

Kyle Kanos

I kinda want my -2 to be downvoted again to -3 so that when I delete it, I get the peer-pressure badge

user54412

what would be interesting is plotting that data in the answers/total score plane (or any two variables)

Jimdalf the Grey

maybe I can help with that. Let's just see how bad the answer is

user54412

you can draw model curves of "iso-quality"

user54412

or figure out which points are above or below others for any reasonable iso-quality definition

Jimdalf the Grey

17:26

meh, it's not bad enough that I would normally consider downvoting it

and it does cost 1 rep to downvote...

Kyle Kanos

Answers in the x, score in the y

Line of best fit: y = 4.23x - 25.77

user54412

cool

Jimdalf the Grey

-25.77? That's a suspiciously large offset

user54412

what about total/average score

user54412

I'm picturing those 1/x-like tradeoff curves from basic economics

user54412

17:29

maybe no variables actually look like that here

Jimdalf the Grey

@ChrisWhite not as helpful because users with high votes on a few answers muddle things up. High # of answer users usually have mid to high votes/answer

0celo7

@KyleKanos I'm looking at your -2 answer. Does that process happen or not?

Kyle Kanos

That's logarithmic in total score on the x-axis

Jimdalf the Grey

@KyleKanos it's at -3 now. Go for the badge!

Kyle Kanos

@0celo7 Yes, it does happen.

0celo7

17:31

What is happening? Does the photon scatter and turn into an electron-positron pair?

Kyle Kanos

Basically.

The fit on that 2nd plot there is y = 0.000441x + 3.36

user54412

It takes the audacity of Hubble to fit a straight line to that

Kyle Kanos

Seems best fit ($R^2$-wise) is power with $y = 1.09x^{0.227}$

NB: I removed the negative scored answerers, so this is anyone with positive # votes and rep > 100

ACuriousMind

@KyleKanos I don't think there is something to fit here :P

Kyle Kanos

@ACuriousMind I agree. But LibreOffice Calc can fit anything

user54412

17:40

@JimdalftheGrey That second plot is what I'm thinking of. If A is above and to the right of B, then A > B unambiguously. But there is ambiguity in the other two quadrants. You could define models of the form (total score)^alpha * (average score)^beta = constant and draw those contours for fixed parameters.

Jimdalf the Grey

@ChrisWhite True, I spoke too soon and should have waited to see the data

user54412

What would be even more interesting (and much more involved) is plotting users' trajectories over time in that space.

Jimdalf the Grey

we'd have to start now

user54412

Perhaps we actually can predict rep years in advance

Kyle Kanos

Thanks Jim :D

Jimdalf the Grey

17:43

Don't thank me

I didn't downvote you. Someone beat me to it

Kyle Kanos

Okay

Thanks whomever

I also lost 26 rep deleting it, but that's not really a big deal

Jimdalf the Grey

you will never recover from this great loss

Kyle Kanos

Even if I died today, I'm sure future visitors will upvote my posts

Jimdalf the Grey

Side note, I should not be implicated in the highly coincidental event that he actually does die today

Also, I'd like the disciplined badge and I have a feeding the bears answer that's sitting at 2 votes. Anyone wanna help me out?

2

A: Frictional Forces

You've forgotten to include the mass of block B in your acceleration. The Force of gravity is only pulling on block B (that is, only block B has a net force due to gravity), so when the system accelerates, the acceleration is applied to the entire system's mass. That is $45N/9.8m/s^2+23N/9.8m/s^...

Thanks! Now we play the waiting game

Kyle Kanos

18:07

Actually, looking at some of my 0-score answers, some of them are decent and I'm saddened that they were looked over :(

Jimdalf the Grey

right?

Kyle Kanos

Like this one:

0

A: Distinguishing Gamma-rays and stars from each other in nebulas

Observations are not made with a single photograph frame, they're made over longer periods of time (for example, a friend takes observations of Herbig stars over 2 minute periods in the J-band with Gemini North). The emissions from stars range from UV to IR (depending on your opinion, it could al...

Jimdalf the Grey

Okay, I have no good examples to show

user54412

I have a meta post about this coming up

user54412

stay tuned for data

Kyle Kanos

18:17

About what?

user54412

about the correlation between quality and votes

user54412

I went through and tried to assign all my answers some sort of quality score, and then I matched those scores up with votes

user54412

I'm just tinkering with the best way to look at the information

Kyle Kanos

Uncorrelated?

user54412

to 0-th order

user54412

18:19

at least it doesn't appear significantly anti-correlated, which is what I feared

Jimdalf the Grey

I don't think there's too much of an anti-correlation. I'm basing this on the fact that the best example I could find from my own questions has 2 votes

2

A: How do I quantize a classical field theory

First off, props for even knowing the proper use of the terminology. I dare say most middle-schoolers probably think classical field theory is about listening to Mozart in a meadow. Now, if I may start at the beginning... You seem to know this already, but I'll restate it for pedagogical reason...

@ChrisWhite making the meta post anyway?

argh! I see so many questions that need to be protected but I'm 2000 rep short of being able to.

Quickly, one of you disable the rep cap and someone else upvote 200 of my answers. This could work

user54412

18:35

@JimdalftheGrey yes, but it may take a day or two to get around to it

Jimdalf the Grey

nah, not worth it then

JamalS

I'm using the multicol package, with two columns. Does anyone know how I can draw a horizontal separator? \hline works, but it's too close to the text above, and I'd like it to not go across the entire column, but be in the center, with 2/3 size roughly.

Jimdalf the Grey

Alright then Chris. Wanna go ahead and protect this question:

9

Q: Why is the nucleus so small and why is the atom 99.999% empty space?

A nucleus consists of protons and neutrons. Both are extremely heavy compared to electrons. Then how come they are contained within an extremely tiny space? And why does the atom consist of 99.999% empty space? I do not understand the mathematics regarding this one bit. Please give me an explana...

atomic-physics

It's on the hot questions list and I just know it's going to attract some crackpot answers

user54412

@JamalS If the line boundaries are defined by columns, then \cline{} works

user54412

\midrule might get the vertical spacing better

user54412

18:47

otherwise, umm, there's always \rule I guess?

user54412

(you might need something like booktabs for some of those commands)

user54412

@JimdalftheGrey I can only protect old questions

user54412

after 24 hours

Jimdalf the Grey

@ChrisWhite Well that's just maddening

Jimdalf the Grey

18:59

Best crackpot answer of the day:

-1

A: Shadows effect gravity?

Many would argue that gravity is merely a shadowing effect of light (of all spectrums) between two objects...just as dark energy is light-emitting galaxies pushing themselves away from each other using radiation pressure...

Jiminion

Sometimes I think magnetism is the shadow of electricity. Not really true, but...

Jimdalf the Grey

@EmilioPisanty Yeah, I didn't watch the link. Are they scientists?

Sabre Tooth

@Danu remember, I am not a member of Physics.SE anymore (not for a while)

I have seen research published on that topic

Emilio Pisanty

@JimdalftheGrey Clicking through would have sent you to one of Feynman's greatest lectures.

Jimdalf the Grey

I see

and he argues that dark energy is galaxies pushing each other apart by radiation pressure?

Emilio Pisanty

19:13

@JimdalftheGrey No. You will learn all the details if you watch it.

Jimdalf the Grey

but I'm at work in a crowded office

videos are disruptive

Qmechanic

0

Q: Futility of developing new fundamental models of physics?

I know that this site is meant for physics professionals to discuss physics as it is accepted and very well established today, so I will keep this as brief and vague as I can. As I learn physics, I habitually put forth effort to reason about the new material in terms of my pet model of fundament...

beyond-the-standard-model

Too broad?

Jimdalf the Grey

@EmilioPisanty Okay, I watched the segment. He's not arguing the point. He introduces and immediately tells us why it's wrong. I stand by what I said that nobody (at least no respectable physicist) would argue what that post claims many would argue

Mark Mitchison

@Qmechanic I voted to close as primarily opinion based. However it is also off-topic since it is not about mainstream physics.

0celo7

How is it most proper to write $SO(4)=SU(2)\otimes SU(2)$? I've seen it with $\otimes$/$\times$ and $=$/$\cong$

Qmechanic

19:22

:20131550: $SO(4) \cong [SU(2)\times SU(2)]/\mathbb{Z}_2$.

Sabre Tooth

seems I am getting more pings from here since I left the site, than when I was a member... weird

0celo7

@Qmechanic why the coset?

Jimdalf the Grey

@SabreTooth It seems we want you back

0celo7

And why do many authors use $\otimes$?

Qmechanic

@0celo7 : Because it (otherwise) is a 2:1 map. Why $\otimes$ is wrong is e.g. explained here.

Kyle Kanos

19:24

@Qmechanic Maybe more opinion-based than too broad, but either would be valid reasoning

0celo7

@Qmechanic does this have to do with the Lorentz group being not simply connected? (Or is that SO(3,1) and unrelated?)

Sabre Tooth

@JimdalftheGrey doubt that, doubt that very much... the latest ping I sort of understand, about atmospheric refraction for @overactor's now bountied post

0celo7

@Qmechanic is $SU(2)\times SU(2)$ the double cover of $SO(4)$? That would explain the $\mathbb{Z}_2$ if I'm not mistaken.

Qmechanic

@0celo7 : Yes.

Jimdalf the Grey

@SabreTooth Well you don't mind if I be optimistic for you anyway?

Sabre Tooth

19:32

@JimdalftheGrey by all means

0celo7

@Qmechanic Thanks. Is there any special reason why $\otimes$ is used? I'm reading Kaku right now while re-reading Zee and they both use $\otimes$. However, Weinberg, Peskin, and others use $\times$.

user54412

Many physicists mix up tensor and direct products

Qmechanic

@0celo7 : What @ChrisWhite said. Mostly mathematical ignorance.

user54412

Colloquially they think they want a "product," but categorically they really want a sum (i.e. coproduct, $\oplus$)

Danu

19:53

@SabreTooth thought this was righ up your alley though

Sabre Tooth

oh it is

Danu

So maybe give it a shot

Sabre Tooth

I can't post here

Danu

You can make a new account?

Sabre Tooth

why would I do that?

Danu

20:00

...because this is right up your alley.

Sabre Tooth

not really a reason to rejoin a community

0celo7

I don't get it.

dmckee

@0celo7 Both possibilities yield valid interpretations of the plot at the time the curve reaches the horizontal axis.

By "clockwise polar plot" he means that time is measured clockwise from the upward axis.

So if you start with that assumption, and the time that corresponds to +90 degrees (i.e. the rightward axis) you are convinced that the plot is polar (100% certainty).

If you start with the other assumptions and time increase to the right, you come to the other conclusion...

0celo7

20:15

@dmckee Eh...not funny.

dmckee

See xkcd explained, too

explainxkcd.com/wiki/index.php/1230:_Polar/Cartesian

I don't think it is meant to be funny, but more like the kind of visual illusion that you get with the vase that is faces.

A WTF moment followed by clarity followed by further WTFing.

0celo7

So that Wiki's a thing. Thanks.

Sabre Tooth

@Danu I won't rejoin the site, but I will find some of the papers and put them here (when I get time - real life and my current accounts take precedence)

0celo7

20:38

In (12.11) the last one doesn't get canceled but in (12.12) the first one does

They look like they have the same structure. What gives?

Or perhaps the antisymmetric rank four tensor is identically zero in $SU(3)$?

Why is the first one that is crossed off crossed off?

The number of $b$s is less than the number of $a$s...

0celo7

21:15

@Danu you any good at this?

0

Q: Young tableaux of $8\otimes 8$ in $SU(3)$

In Georgi's Lie Algebras in Particle Physics, one finds the following Young tableaux for $8\otimes 8$ in $SU(3)$: I am unsure of all the cancellations. Let us number the canceled tableaus increasing from left to right and top to bottom. There are seven cancellations. I understand cancellations ...

abstract-algebra group-theory representation-theory lie-groups young-tableaux

Danu

22:10

Nope, don't know any group theory

0celo7

@Danu is there any physics discipline besides perhaps observational/computational cosmology that doesn't use group theory to some degree?

Even then, you can use Lie groups to generate solutions to the EFEs

Danu

22:43

No idea

probably also depends on what you define as 'use group theory'

user54412

23:13

@0celo7 All of astrophysics. All of plasma physics. All of theoretical cosmology (all that counts, anyway). All of geophysics. All of atmospheric physics. Vast swaths of condensed matter. All of electromagnetism and thermodynamics and planetary science.

user54412

I promise you 99% of tenured faculty of physics don't know what a group is, and they do just fine without that knowledge

0celo7

@ChrisWhite I would like to point out that electromagnetism is invariant under the Lorentz group.

Or covariant

I forgot geophysics and atmospheric physics is a thing.

user54412

@0celo7 So what? Anything with any structure worth studying has a group lurking there if you want to define it. That doesn't mean you need to (or even should) think about it in terms of groups.

0celo7

But really I had Danu's "mathematical physics" from his bio in mind.

Is geophysics mathematical physics? How is that even defined?

user54412

Geo = earth

0celo7

23:16

Mathematical physics, not geophysics.

I'm not that ignorant.

What do you mean by "theoretical cosmology that counts?" What about cosmic string cosmology? I'd imagine that uses groups. Or does that not count?

user54412

Yeah, but cosmic strings are made up, so it doesn't count :P

user54412

There's a whole cottage industry of cosmologists who, instead of solving the problems nature gives them with the tools they can find, look at the tools they have and insist on solving every problem with them

0celo7

Also thermodynamics can be formulated geometrically and most differential geometry texts include discussions of groups to some degree.

user54412

@0celo7 Again, irrelevant. And I assure you tenured faculty, not to mention all others in the field, do not think in this way.

0celo7

Condensed matter physics doesn't use groups for crystal symmetry and such?

(I obviously don't know any CM.)

user54412

23:21

Some condensed matter does. Some doesn't. It's a huge field, and only parts of it are sexy enough to be described to young people these days.

user54412

That's really a perpetual problem that physics has -- the vast majority of the attention it gets is focused on these niche things that are in no way representative of what most physicists actually do.

4

user1667423

Consider a toroidal ring of mass undergoing minor axis rotation. Does this generate a gravitational field going into and coming out of the toroidal ring?

0celo7

@ChrisWhite Certainly any CM physicist knows QM angular momentum. Whether they like it or not, addition of angular momentum is group theory.

Note that @Danu said he doesn't know any group theory. I was questioning how many fields require absolutely none at all.

Also aren't superconductors understood in terms of symmetry breaking?

ACuriousMind

23:49

@0celo7 Yes, but that there is a group that is broken does not mean that you need to know any group theory to describe it

I think there are indeed people who work with "infinitesimal symmetries" every day, but could not tell you what a Lie algebra is.

Mark Mitchison

Especially since the group is U(1) in that case....

So if you think complex numbers are group theory, then yeah it's group theory

0celo7

$\mathbb{C}$ is a group, right?

ACuriousMind

Yeah, like, the most boring group ever

0celo7

$\mathbb{C}-\{0\}$

ACuriousMind

Also, still the most boring group ever

Alfred Centauri

23:52

-2

Q: What are the applications of momentum in physics?

It seems as though kinetic energy is used for everything involving force so I doubt there would be much use for momentum. However I am generally ignorant when it comes to physics so I was wondering if it served any purpose.

newtonian-mechanics kinematics momentum conservation-laws

Mark Mitchison

Sure. U(1) is a group. A very nice group. It's just that "spontaneously breaking a U(1) symmetry" doesn't require group theory to understand. Unless you think that complex multiplication is properly described as group element composition, which technically it is.

0celo7

That was necessary, right?

ACuriousMind

Fields are not very interesting through the lens of group theory

0celo7

Since 0 has no inverse.

ACuriousMind

@0celo7 Under + it has.

0celo7

23:53

True. Is $\mathbb{C}$ a ring or do we need to take out the origin?

ACuriousMind

@AlfredCentauri Given that the user is not new to this site, it almost surely is a prank

@0celo7 $\mathbb{C}$ is a field - it is a group under addition, and $\mathbb{C}-\{0\}$ is also a group under multiplication

0celo7

Oh god I forgot what a ring is. Is a ring a group with two operations?

And a distributive property, right?

ACuriousMind

@0celo7 A ring need not have multiplicative inverses.

0celo7

I see. We require association and distribution, right?

ACuriousMind

@MarkMitchison I think most group theorists would look at you very strange if you described dealing with U(1) as group theory

0celo7

23:57

Do both operations have to be associative in the definition of a ring?

ACuriousMind

@0celo7 Yes. Some people also do not require a ring to have a 1, though.

0celo7

@ACuriousMind the way I understand it, true "group theorists" look at physics group theory in general very strangely though.

Isn't the proper math term "representation theory"?

Danu

@0celo7 the 'trivial applications' don't count as far as I'm concerned

let's put it this way: I know no group theory in the sense a mathematician would use it

or very very basic things only

ACuriousMind

@0celo7 Yeah, what we call group theory is more properly (Lie) representation theory

Danu

a bit about lie groups from differential geometry but that's it

zero representation theory

next semester is my course on it, finally lol

0celo7

23:59

Young Tableaux is too confus for me. Kaku drew some boxes and that was about it.

Transcript for