The h Bar - 2016-01-24

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Mike Miller

01:13

@Danu: Well, if you can prove mirror symmetry, some people will be very excited. If you know more details I'd love to talk about this a little; feel free to email me.

Anuj Mishra

01:32

Please help in removing the duplicate tag from my question How do we know that light is made up of electric and magnetic fields?
My question is similar to a question previously asked but satisfactory answers were not given to the previously asked similar question and was closed

So please help in removing the duplicate tag so that a satisfactory answer can be achieved to my question

dmckee

01:56

As far as I know there are no single killer experiments. What does exists is a huge pile of experiments which agree with the predictions founded on Maxwell's equations (starting with $c = 1/\sqrt{\epsilon_0 \mu_0}$ where the two constants are measured in experiments not involving light at all).

Maxwell's equations explain ray optics; and diffraction experiments (in optical and microwave and radio bands); and radio; and (with sufficient understanding of the atom); and prisms, rainbows and the colors on oil slicks or soap bubbles; and the limit on the magnification of a optical microscope; and so on. The list just doesn't stop.

If I wanted just one, I might go with the microscope thing because it annoys people to no end.

3 hours later…

Bernard Meurer

05:07

@DanielSank are you currently powered on?

0celo7

he is not

Bernard Meurer

Dammit, I need to setup remote ssh access on him

2 hours later…

GBeau

06:42

@FenderLesPaul USC, Wisconsin Madison, Harvard Chemical Physics

@FenderLesPaul and Columbia Applied Physics

1 hour later…

Danu

08:01

@MikeMiller I don't think I'll be proving anything :(

DanielSank

08:15

@BernardMeurer wat?

2 hours later…

David Z

09:55

@AnujMishra the linked duplicate has been reopened

Danu

10:45

@MikeMiller My thesis will probably be related to the physics-side of the mirror symmetry story.

2 hours later…

John Duffield

12:56

@0celo7 : I root for relativity, but I have to say spacetime is an abstract thing. We live in a world of space and motion. Not in some static block universe. Like Ben Crowell said, objects don't move through spacetime, objects move through space.

Secret

user image

Hi guys I have a question

(attached is the answer key to the question of "C and D, which one rolls faster down the track")

I don't get how's the tangential velocity differs from the C and D cases. Aren't both have the same rotational KE (as they have the same $\omega$) and thus the tangential velocity at all points of the sphere between the two cases would match exactly?

That is, in case C if the tangential velocity v at the equator is $v'$ than shouldn't I expect the same v to be found on the equator of the sphere in case D?

innisfree

13:16

@DavidZ I can see why the hw policy drives so many people crazy

Secret

How does changing the point of contact will make such a different to the tangential velocity and hence how quick the ball reached the bottom?

Now suppose if I had a ball positioned such that its two points of contact is on its equator and an off centred part like so

user image

Then how to calculate what the tangential velocity and the velocity of the centre of mass be (because this problem is asymmetric)?

innisfree

@DavidZ I don't think it was necessary to close or delete the answer, but I guess I'll reluctantly accept to the community's judgement on it, and it might get enough reopen votes

David Z

@innisfree which one are you talking about here?

Secret

For the above asymmetric case, which radii I should use to calculate $\omega$?

innisfree

@DavidZ I hope it isn't unreasonable to (briefly) make a case for reopening

David Z

13:26

You can always make a case for reopening a closed question, but I'm still not sure which one you're talking about

innisfree

13:49

@DavidZ this q physics.stackexchange.com/questions/231428/…

David Z

Ah, OK. I suppose your argument for reopening is the same you posted in the comments? If so, my response is simply that that's not a valid defense against closing a question as homework-like.

innisfree

@DavidZ OK, no big deal. We won't become enemies :)

David Z

Oh, of course not

(why would we?)

busukxuan

14:31

Hey guys I'm not sure if this is the right place
Is it ok that I share a custom dark theme CSS for Physics.SE here?

David Z

14:45

Yeah, sure, no reason that would be a problem

Mike Miller

15:02

@Danu: Well, you can still teach it to me. :)

1 hour later…

Danu

16:04

@MikeMiller In a few months, hopefully :)

Mike Miller

16:37

@Danu: Mm, but do I have the patience for that?

Danu

Well, I won't be busy explaining for a few months, it'll just start after a few months :p

Mike Miller

Ah, that sounds easier to handle.

FenderLesPaul

17:07

@GBeau i see

still waiting'

0celo7

17:21

@FenderLesPaul has Chicago sent out yet?

FenderLesPaul

17:47

@0celo7 no

they don't typically send anything until end of Jan apparently

@0celo7 I'm setting my sights on lesser ranked schools now though

0celo7

18:01

Ivy League, undergrad research and everything else and you're still worried about getting into a highly ranked school?

dmckee

@0celo7 The best programs typically get ten times as many qualified applicants as they have places for.

"Qualified" meaning someone who has a reasonable chance of getting through the program.

Programs that are very good but not as prestigious get more applications than places.

And then there is problem of downward mobility for those who want to stay in academia.

Most people get post-docs at places less prestigious than their PhD school, and get faculty positions at places less prestigious than their postdocs (if they get them at all).

user54412

all too true

FenderLesPaul

@0celo7 It's more a matter of preparing myself to be happy wherever I get in I think

not so much having completely given up hope on my dream school, at least not yet :)

for example I would LOVE to go to uchicago but I would also be really happy at UMD College Park because of Ted Jacobson

as an aside, Wald showed a while ago that black hole entropy is noether charge for absolute horizons (i.e. the usual teleological horizons)

does anyone know if such a result also exists for quasi-local horizons like marginally trapped surfaces?

user54412

18:24

what would the charge be? some sort of apparent entropy?

dmckee

If you are not planning on an academic career there is less pressing need to get into a top program (though you still want the best one you can get, of course).

0celo7

@FenderLesPaul I know of the first result, but no clue what you mean by "quasi-local" horizon.

David Z

@FenderLesPaul honestly, your advisor's standing in the field matters more than the reputation of the school. If you go to a minor college and work with one of the top world experts in their research area, then you're probably setting yourself up better than if you hypothetically went to a top school like U Chicago and worked with someone who has a merely average research record.

Of course, the reason that latter case is hypothetical is that you don't get a research job at a place like Chicago without being a world leader in your field in the first place.

0celo7

@DavidZ Wald is at UChicago and is probably the most prominent GR researcher in the world.

(He's the guy FLP wants as his advisor.)

(Classical gravity.)

In terms of quantum GR stuff, probably Ashtekar. Not sure what school he's at.

David Z

Ashtekar is at Penn State (but we've been over that)

0celo7

18:32

@academics Is going into industry for a while a total career killer for pure science/math academic positions?

@DavidZ Oh yeah.

David Z

@0celo7 Depends on the position and the field. In theoretical particle physics, yes, absolutely. Others I couldn't say...

I would imagine the same is true in gravity research though

0celo7

@JohnDuffield Unless you define "motion" that's a pretty worthless comment.

@DavidZ Those are fields that don't have industry equivalents.

Hmm, I might have shot myself in the foot with my question then.

David Z

@0celo7 I would advise against calling comments worthless

0celo7

@DavidZ Ok. @JohnDuffield : It's devoid of content.

David Z

somehow that doesn't seem much better

dmckee

18:36

@0celo7 I've known exactly one professors on the tenure track after a stint in industry, and he struggled to the point that he left the school without applying for tenure.

The professors I asked said that he might have gotten tenure, but that he wasn't happy in the work.

0celo7

Is this different for engineering departments?

dmckee

What seems to be more common is industry people working an non-TT instructors in various ways.

@0celo7 I don't have any personal experience, but I think the wall is less rigid.

And it is certainly true that industry engineers do quite a bit of non-TT teaching.

0celo7

The NE department head is a retired Navy Captain (Admiral?).

dmckee

My wife took a couple of classes from industry folks while she was at Old Dominion doing a ME program.

@0celo7 It's pretty hard to make Admiral as an engineer (thought I seem to recall that Nimitz insisted that the nuclear engineers be line officers rather than EDOs).

0celo7

I don't think he's old enough to be an Admiral.

But I've heard at least one person say he was an Admiral.

Checking...

::raises eyebrow::

He's been a prof for 21 years, no way he was ever in the Navy. The hell?

He was in the Navy for at most 4 years by the looks of things.

Ok, for 5 years.

Dunno where people are getting his rank from.

0celo7

19:11

@all $(x,y)$ is an ordered pair, $(x_1,...,x_n)$ is a ordered what?

ACuriousMind

tupel

or tuple, I guess

0celo7

Yes, thanks.

@ACuriousMind Does "let $V^k$ be the set of ordered $k$-tuples of vectors" make sense to the average physicist

ACuriousMind

Uh, why don't just say it's the $k$-th product of $V$ with itself?

0celo7

Does everyone know what that is?

ACuriousMind

Someone who doesn't will not be more enlightened by "set of ordered $k$-tuples" :P

0celo7

19:17

I sure didn't know what that meant after reading Zee's GR book

is it $k$th or $k$-th

@DanielSank I'm finally going to write this thing. How is: Let $V$ be a real vector space and $V^*$ its dual. Let $V^{ k}$ be the $k$-th Cartesian product with itself and likewise for $V^{*k}$. A multilinear map
$$t:V^{*k}\times V^l\to\mathbb{R}$$
is called a *tensor of type* $(k,l)$. "Multilinear map" means that $t$ is a linear map on each factor of $V$ or $V^*$.

test

dunno what's up with the italics not working

hubot

19:42

Should I read my book about physics from begin though I understand issues from 1 volume but I haven't done all exercises from all chapters?

I was finished at Gauss law of electrostatics.

0celo7

@ACuriousMind I'm assuming "smooth" is known by everyone

@DanielSank After our mess of a discussion I'm really at a loss of how to define the tangent space well

FenderLesPaul

20:53

@ChrisWhite yeah that's what I was thinking, some kind of apparent entropy

@dmckee a career in academia is in fact the goal

@DavidZ right so I also would be happy at Penn State due to Ashtekar

@0celo7 quasi-local horizon as in a definition of the horizon that isn't global like the usual definition

0celo7

what does that mean

give me a definition, not what it's not

FenderLesPaul

well if you consider a hyper surface foliated by a family of say 2-spheres and consider the outgoing and ingoing null normals $l^a,k^a$ to the 2-spheres then one local definition of a horizon is such a hyper surface for which

(a) $\theta_l = 0$, $\theta_k < 0$

this is called a marginally trapped tube

0celo7

$\theta$?

FenderLesPaul

there are other local definitions such as dynamical horizon (which additionally restricts the hyper surface to be space like and assumes $\mathcal{L}_k \theta_l < 0$

$\theta$ being the expansion of the associated null congruence

0celo7

Ah

FenderLesPaul

20:58

e.g. $\theta_l = q^{ab}\nabla_a l_b$ where $q_{ab}$ is the 2-metric on the spheres

0celo7

Yeah I remember that.

Ok so the ingoing null normals are becoming "denser"?

FenderLesPaul

yeah

0celo7

Alright, I buy that.

FenderLesPaul

so it "traps light" in a sense

it's an observer dependent definition of the horizon

because it assumes a specific foliation

but it's local

whereas the absolute horizon is a very global definition

0celo7

Yeah. I don't remember that being in Wald.

FenderLesPaul

21:00

yeah these definitions didn't exist when he wrote the book

I think Sean Hayward was the first person to introduce them in like 1994

I'm interested in them because Raphael Bousso uses them for holographic purposes

so having a notion of entropy on these marginally trapped tubes (which Bousso calls "future holographic screens") would be nice

0celo7

I want to learn the Shoen-Yau proof of the Pos E Thm.

It's pretty horrible though.

FenderLesPaul

Isn't that really nasty?

yeah

0celo7

It spans 4 papers or so.

But it's an exercise in minimal surface theory, which seems to be a combination of geometry and PDE.

So I could see doing my senior thesis in math on it.

So a really long thesis and then Appendix: Witten's Proof. 4 pages :D

FenderLesPaul

Editten

that sounds fun

0celo7

Perhaps.

I'd also consider learning some twistor theory, but I want my thesis to be somewhat relevant to something.

PDE/Calc of Variations has lots of engineering applications, obviously.

FenderLesPaul

21:05

twistor theory is so niche even ancient nordic mythology is jealous

3

0celo7

Exactly.

It allows you to rewrite certain PDEs on Lorentzian manifolds or something

@FenderLesPaul to be fair, ancient nordic mythology gave us Skyrim

FenderLesPaul

that is true

0celo7

twistor theory gave us...well, it probably ruined Penrose

FenderLesPaul

it does have real world applications

I think Penrose notation ruined Penrose

0celo7

so I was just staring at $d_1(x,y)<\delta\Rightarrow d_2(x,y)<\epsilon$ for 30 minutes and then it hit me that means if $d_1(x,y)<\delta$ then $d_2(x,y)<\epsilon$

I think I had a stroke or something

guest

21:12

Do you feel serious pain? @0celo7

0celo7

no

guest

Why did you say you think you had a stroke?

0celo7

because I think I did

Mike Miller

@FenderLesPaul: It's useful for mathematicians.

0celo7

Seriously?

Just use component notation

Or abstract indices

Mike Miller

21:15

No, twistor theory.

0celo7

oh

@MikeMiller what for?

ACuriousMind

@0celo7 That's commonly a sign to stop staring at equations that are becoming meaningless for now and do something else instead

Mike Miller

It's useful for turning questions about manifolds with some sort of quaternionic structure into questions about complex manifolds. This doesn't appear to be on the Wikipedia page for twistors but there's no way they're unrelated because yhere arent two people mad enough to independently use the word twistor

0celo7

@ACuriousMind I'm interrupting this apocalyptic poem for my lit class with topology

I think I just dozed off for 30 minutes or something, it was strange.

guest

You are strange :P

0celo7

21:21

@MikeMiller I asked one of my profs who does work with curvature flow and he said he hadn't seen twistors used for anything in years

@guest no

guest

Stroke, also known as cerebrovascular accident (CVA), cerebrovascular insult (CVI), or brain attack, is when poor blood flow to the brain results in cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. They result in part of the brain not functioning properly. Signs and symptoms of a stroke may include an inability to move or feel on one side of the body, problems understanding or speaking, feeling like the world is spinning, or loss of vision to one side among others. Signs and symptoms often appear soon after the stroke has occurred...

0celo7

sounds like what I had

guest

no

0celo7

how would you know?

are you a doctor (of medicine)

Mike Miller

@0celo7: That's fine.

guest

21:32

no

hubot

Should I read my book again because I haven't done all exercises from this book?

I seems that I understand this book.

0celo7

doing ALL exercises in a book is a high task

hubot

So should I have done all exercises?

0celo7

That depends entirely on how many there are.

dmckee

As usual, Randall have a nice take on the hot topic of the day.

Superman lies near the bird/plane boundary over a range of distances, which explains the confusion.

0celo7

21:36

If it's like my intro physics book and there's 50-100 per chapter

lol

@dmckee I was reading your thing while typing mine and started copying it

@hubot if it's 50-100 per chapter, definitely not

I think some chapters even had 130 problems

hubot

In my book is average 30 exercises per chapter.

guest

Just do the odds.

0celo7

do the ones you don't know how to do

dmckee

@0celo7 Do the ones you're not sure you know how to do starting with the hard ones.

Generally if you solve the hard ones all the rest are clear.

If you're not making any progress on one of the hard ones, back up to a slightly easier one on the same topic. Iterate.

guest

Do as many as you have time for.

dmckee

21:39

This can save time.

hubot

If this exercises is as tasks in Feynman lectures?

0celo7

just prove all of the theorems in the book without looking at the proofs

dmckee

I've started assigning just a few of the hardest ones, but the assignment also says which easier ones are related to each of the assigned one.

hubot

Is tasks in Feynman lectures by you opinion difficult?

dmckee

That's a lot of work to prepare, but less work to grade because I only have a few to grade.

0celo7

21:41

@dmckee I was amazed by the variability in difficulty for my physics homework last semester

dmckee

Professors have different styles and different expectation for the place of homework in the learning process.

guest

So true ^

ACuriousMind

@guest: You're skillpatrol, aren't you?

If not, then you're at least behaving very similar

0celo7

O.O

how did he figure that out

@ACuriousMind O.O

hubot

Which means that circle in integrate symbol?

0celo7

21:45

Am I the only one amazed by this revelation??

@hubot Line integral along a closed path

ACuriousMind

@0celo7 That wasn't difficult :P

0celo7

@ACuriousMind you say that about everything I struggle with >:(

hubot

And which means circle in double symbol integral? I know that double symbol of integrate means double integral along a surface D.

0celo7

integral along a closed surface

hubot

Is eg. sphere a closed surface?

0celo7

21:49

yes.

@guest no I really do think I had a stroke

for a second I thought the Raiders were a decent team D:

guest

:-/

hubot

What is definition of closed surface and closed path?

ACuriousMind

@hubot those are the surfaces (and paths) without boundary

For a path, it simply means that starting point and end point are the same

hubot

And for a surface?

0celo7

@guest IKR

hubot

21:52

What surfaces isn't closed?

0celo7

Hemisphere

ACuriousMind

@hubot Well, click the link under "boundary". I don't know of a "simple" condition like for paths. It means the surface has no "border" - when you have a path on the surface going in any one direction, it can cintinue infinitely long without hitting a border

@hubot A disk, for example

0celo7

It has no open set homeomorphic to an open set of the half space

Hmm, that's no true, is it?

Or is it

Don't know, had a stroke

hubot

Are symmetrical figures closed surface?

ACuriousMind

What's a symmetrical figure?

Danu

21:59

The open disk is not a closed surface.

hubot

Huh, so closed figures is closed surfaces eg. circle, triangle, square?

Danu

You could just look up the definition of a closed subset of $\Bbb R^3$

ACuriousMind

@Danu Nobody said it is, my disks are closed :P

(Funny how the closed disk is not a closed surface)

Danu

(I didn't see the context of this conversation yet)

In any case, a surface without boundary is not yet a closed surface, no?

Your second statement is more precise.

ACuriousMind

@Danu Closed surfaces are compact and without boundary, should've added compact further up there.

Danu

22:04

@ACuriousMind Right :)

Btw did you see the resolution of my thesis-dillema? :)

ACuriousMind

What dilemma?

HDE 226868

@dmckee Heck, it's already been used in an answer on Astronomy.

Danu

@ACuriousMind Deciding on topic/person.

HDE 226868

@FenderLesPaul Hey, Nordic mythology is cool! But yes, niche.

ACuriousMind

@Danu I inferred that you apparently resolved to write on mirror symmetry ;)

Danu

22:06

Nordic mythology = best mythology ^^

@ACuriousMind Yep, or at least something related to it.

Have you ever heard of $tt^*$ geometry (or a $tt^*$ equation, for that matter)

hubot

So why in my course in double integral, where was closed figure wasn't circle in integrate symbol?

ACuriousMind

Never

Danu

Me neither, but that's the first topic my supervisor wants me to learn about ^^

I'm going to be reading part of the book by Hori et al (includin Vafa :D) on mirror symmetry

HDE 226868

@Danu It does have drama. Why does Óðr (Oder) travel away from Freyja? How did Sindri and Brokkr win the bet with Loki, when Mjölnir is clearly faulty? I like Celtic mythology better, though. Nothing like the Mabinogion or Arthur to make you feel good.

Anthony

Wait, are astronauts orbiting the Earth in an inertial reference frame? Shouldn't they be non-inertial because they are spinning around the Earth?

Danu

22:08

@HDE226868 I'm already way out of depth ;)

@Anthony ...but isn't Earth itself a non-inertial frame? ;)

HDE 226868

@Danu I just picked random norse questions from Mythology. Well, not so random, but still.

Danu

(point I'm making is: It depends on what you're looking at)

Anthony

@Danu If you throw a ball in a space ship, it won't go straight, will it?

Danu

Think about a plane.

Anthony

I think I must just be confused.

Inertial just means follows Newton's laws, which for us almost means basically follows Newton's laws.

@Danu Is it just that the acceleration from spinning around the Earth while in space provides minimal fictitious forces?

And again I guess a point to make is that saying we're spinning relative to the Earth isn't necessarily meaningful if we're talking an inertial frame.

0celo7

22:25

6 2

black-holes feynman-diagram general-relativity interactions quantum-field-theory spacetime

Different Sofia I take it?

Danu

If you're going to be comparing two frames that are both accelerating in identical fashion (like two observers on Earth, close to one another), there will not be any non-inertial effects.

@0celo7 ...look at the question :P Obviously!

0celo7

@Danu huh?

Danu

Obviously a different Sofia, I mean.

0celo7

I know what you meant

I don't get the "obviously" part

Danu

Maybe you forgot the type of things the previous Sofia was interested in :)

0celo7

22:28

perhaps!

I did have a stroke you know

Danu

??

0celo7

I kinda mentally died earlier while doing some topology

Anthony

Thanks @Danu

DanielSank

22:48

@0celo7 Yeah, I like that very much.

@0celo7 The italics don't work if you have line breaks. Not sure why.

@0celo7 I think the way you did it with curves can make sense.

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