The h Bar

8:16 PM

@giulio_hep You should be aware that unless you take pretty strong measures, Google knows who you are and what you have searched in the past, so your results may differ from those seen by other people. It's not the first result when I google it (I get some company or another).

giulio_hep

LOL I work in IT ;-) I'm a programmer

ah... Google doesn't know who am I more than you do

@dmckee btw I'm reading your past posts like this comments about " time ordering can only be reversed for space-like separated events" ...

0celo7

0

Q: If the stress energy tensor $T=0$, does that mean $\delta{T}=0$?

If the stress energy tensor $T=0$ in the vacuum, does that mean the variation of $T$ with respect to $g_{\mu\nu}$ equal zero? $$ \delta{T}=0 $$

Lol

Kyle Kanos

@0celo7 ...but that's not the situation if $T\equiv 0$. — Danu 38 secs ago

ACuriousMind

I seem to have hallucinated asking OP whether their tensor vanishes identically

Kyle Kanos

::*grabs popcorn*::

0celo7

8:27 PM

T(x)=0 at a fixed metric in metric space.

Actually, this space has the structure of a fiber bundle.

ACuriousMind

Or perhaps that was on a question of theirs that is now deleted.

Kyle Kanos

@ACuriousMind He did ask something like that. I told him it would be better as an edit to the above-linked question

ACuriousMind

I definitely asked op whether $T=0$ with or without use of the equation of motion.

Danu

@0celo7 $T=0$ is typically interreted as $T(\text{vars})=0$ irrespective of values of vars.

Kyle Kanos

I wish deleted posts were easier to find

Danu

8:30 PM

I already said that htis is how I interpret the question in an earlier comment. I don't see why you keep on nagging on about this?

0celo7

$T$ is both variable in $x$ and $g$.

Danu

This is a simple notational thing that has nothing to do with GR.

I'll be flagging the comments for obsoleteness (and deleting mine) to avoid clutter.

0celo7

What? How did I not construct a counterexample? I found a $T(x)=0$ and constructed a $T'(x)\ne0$

He's asking about $\delta T=T'-T$, no?

Danu

I'm not really interested in pursuing this pointless discussion any further :P

0celo7

???

Danu

8:34 PM

You can interpret the question how you want; I explained how I interpret it (and I'm under the impression this is the standard interpretation of this notation) and that's all there is to it (my comments are correct when this interpretation is accepted). The other stuff is totally trivial and uninteresting, so why discuss about it?

0celo7

The question clearly mentions the metric! Why are you ignoring that?

Danu

@0celo7 Okay, one last attempt.

ACuriousMind

@0celo7 What has this to do with the question mentioning the metric. When someone says $T=0$, it is perfectly valid to assume they don't mean for a special value of $g$.

Danu

@ACuriousMind Exactly. I'll elaborate for you @0celo7

ACuriousMind

It's also perfectly valid to assume they mean that, hence the question is unclear.

Kyle Kanos

8:36 PM

4 hours ago, by Kyle Kanos

@JimsBond That seems rather typical for mathematical physicists (from what I've seen anyways)

^ see what I mean anyone?

Danu

You interpret the question as $\exists g T(g,x)=0\forall x$ while the standard (imo correct) interpretation is $T(g,x)=0\forall x,g$.

0celo7

But then the question is absolutely useless!

Danu

Your "counterexample" doesn't apply when the question is understood as I explain it. It's also kind of funny that you'd think that it would not be obvious to me :P

ACuriousMind

@0celo7 lol, if I've learned one thing in my time here, that's not an argument either way

Danu

@0celo7 My main point was pointing out how bad the notation in the question was---perhaps OP wants something different, but that's his/her problem. Not mine, in particular.

ACuriousMind

8:38 PM

@KyleKanos No, I am incapable of self-awareness.

2

Kyle Kanos

And apparently I never voted to close on it. I voted to close as unclear

0celo7

Hmm

"vacuum"

Kyle Kanos

Only because OP ought to respond to the debate going on there between Danu & Ocelot

0celo7

That might be the key...

Kyle Kanos

I seem to recall a bit more information in the other post by the guy, but it's deleted & I don't know how to find it

Danu

8:41 PM

Let's please just forget about that boring question.

Kyle Kanos

You're a boring question

Danu

@KyleKanos Judging by yo' momma's repeated uttering of my "title" last night, not everybody agrees!

ACuriousMind

Non-boring question: Does anyone know how the chat decides when to make a dotted line?

0celo7

@ACuriousMind Ugh, boring question. Let's move on.

Danu

(@KyleKanos I hope that doesn't actually offend you; if it does, I'll gladly remove it ;) )

Kyle Kanos

8:43 PM

2

A: Visual indication of last view chat message

There is now a dotted line indicating where we think you last read to in chat.

0celo7

^^boring indeed

Kyle Kanos

And to be honest, I don't think I've ever noticed the dotted line

But now that ACM mentioned it...

0celo7

@KyleKanos Old eyes?

Kyle Kanos

I am 31 & need glasses, so maybe

Physicist137

hi?

Danu

8:44 PM

Hullo

Physicist137

Oh. It works.

Kyle Kanos

@Danu Yeah, I'm not terribly offended by it. Ocelot does the same sort of stuff because he's like 15

Oddly enough, my brother once got mad and me and said a your mom joke to me. I looked at him and laughed. He then realized his folly.

Physicist137

From some chat history (few min ago) I see that everyone is bored. And.. I have a soft-question and the soft-question tag suggested I asked here, so... may I?

Kyle Kanos

@Physicist137 Feel free.

0celo7

@KyleKanos Close enough.

dmckee

8:46 PM

In chat? Anytime.

Danu

@KyleKanos Good, good :D

@KyleKanos Hahaha that's great

Physicist137

Oh. Chat works. I am amazed...

0celo7

@KyleKanos Only with you, curiously.

Danu

Especially that he'd make a yo momma joke when he's actually mad!

Kyle Kanos

@0celo7 It's within a factor of 2, good enough for astrophysics

0celo7

8:46 PM

@Physicist137 Amazed?

Physicist137

amazon.com/gp/customer-reviews/R1X4888WNHAER8/…

This guy seems to think cgs units is better and make physics nicer when studying EM. Is that true?

0celo7

@KyleKanos Within a factor of 10^134, good enough for $\Lambda$CDM people...

Danu

> cgs

Kyle Kanos

@Danu Well it was one of those "You're a !@#$*" back and forths

Danu

> for the real experts

dmckee

8:47 PM

@Physicist137 As long as you can tolerate the people talking about game mod-packs, the vagueries of multiprocessing architectures, and bloody differential geometry textbooks.

Danu

loljk

@0celo7 only $10^{122}$ ;)

@dmckee You talkin' to me HUH?!

0celo7

@Danu What's 12 factors of 10 between friends :D

Danu

@Physicist137 CGS ~ UK

Physicist137

@dmckee what?

dmckee

@Danu I don't know? Which group(s) are you in?

Kyle Kanos

8:48 PM

@Physicist137 Well sorta. There's no epsilon-naught or mu-naught

Danu

@dmckee Take a gander :D

@KyleKanos Also PLANCK UNITS DAMNIT

who would ever use non-Planck units at research level

dmckee

@Physicist137 People talk about a lot of different things in here. I was just picking on some of the regulars.

Danu

unless you need to actually produce like... results?

Kyle Kanos

I don't talk about multiprocessing architectures

0celo7

@Danu This should be the new fad in engineering.

Kyle Kanos

8:49 PM

But I know two people who do talk about that sort of thing....

Danu

@dmckee Don't forget my (amazingly fascinating) love life

Kyle Kanos

Awww, I got excluded from the list :(

dmckee

Someone has been talking about GPU versus FPGA versus CPU recently.

Physicist137

@dmckee ahh

0celo7

@dmckee Guilty on charge 1.

Danu

8:50 PM

@KyleKanos You're too old for this, grandpa! Shoo!

Physicist137

A harder question then: If I ask it, will it be closed with high probability? :D

0celo7

Who's talking about geometry books?

Kyle Kanos

Maxwell's equations in cgs

Danu

@Physicist137 Yes; it's opinion-based.

0celo7

@Physicist137 YES

Danu

8:50 PM

Guaranteed to be closed

Kyle Kanos

Maxwell's equations in SI

Danu

Especially now that we know ;)

@KyleKanos I think this looks better actually

0celo7

TIL Lorentz force is a Maxwell Equation, but only in SI.

3

Danu

@0celo7 lol :D

Physicist137

I see not too much difference between the two... Except CGS is unusual (to me at least).

dmckee

8:51 PM

@KyleKanos How'd Lorentz sneak in there?

Kyle Kanos

@0celo7 Where've you been?

I JUST GOOGLED THE EQUATIONS AND DIDN"T LOOK AT THEM CAREFULLY, OKAY!

Danu

@KyleKanos Banned from PSE

Physicist137

And if it is primarily based question, then this guy was lieing? After all, if it is based on opinion and thus there is no clear advantage in using CGS units over SI units at EM.

Danu

@Physicist137 "Which unit system is better?" is obviously opinion-based

Since, by definition, unit systems don't matter.

Kyle Kanos

@Danu Well I am an anti-science, anti-academic SE gullomized zombique politician,

0celo7

8:52 PM

@Danu But muh Planck mass

Danu

@0celo7 What about it?

Physicist137

@Danu well... I disagree.. some units systems are cleary a mess. So, on those cases its not opinion based... if it is a mess, its a mess... =).

Kyle Kanos

@Physicist137 Lying? Probably not, just telling his opinion.

Danu

@KyleKanos Welcome back home, dear old friend!

gonenc

@Danu totally agree!

Danu

8:53 PM

@Physicist137 No, I disagree.

The trick is in the usage of "better". What if I define "better" to mean "just like this one crappy unit system"?

Physicist137

@KyleKanos But he said "Physical clarity". So, somehow he said it is simpler and offers more physics insight to learn in cgs.

Danu

@Physicist137 Units never really can offer clarity :P

dmckee

Seriously "which systems of units is better" is essentialy like "six versus a real editor like emacs". Or "which formatting guidelines suck the least compared to 1TBS". Or something.

Kyle Kanos

@Physicist137 There's less "baggage" with respect to permittivity/permeability

Danu

Although total non-dimensionalization can lead to confusion :D

gonenc

8:54 PM

@Physicist137 it basically gets rid of the $\epsilon_0$ and screws the equations imo

Danu

But one can get rid of $c,\epsilon_0,\mu_0$ ALL at the same time

Kyle Kanos

@dmckee IDK about least, but Whitesmith's gotta be dead-last

Danu

Porque no les tres?! :D

Physicist137

oh.. I see latex doesn't work here? $\pi$.

Danu

@Physicist137 It does

Kyle Kanos

8:55 PM

91

A: Should chat have TeX support?

I will leave the original post for historical reference, but as mentioned in the Update below, all four bookmarks are located on this installation page. There are four bookmarks: start ChatJax installs MathJax and starts a loop that renders $\LaTeX$ as needed. This is intended for use in chat, ...

Danu

You need ChatJax

Kyle Kanos

The bookmark to use is in there. Just save it & then load it in the same tab as Chat

Physicist137

ah. I see..

dmckee

@KyleKanos LOL. Yep.

Physicist137

@KyleKanos Oh. I've learned how to reply. =). In any case, cgs seems to have $4\pi$ baggage.

Danu

8:57 PM

@ACuriousMind have you ever seen notation like the following: $\wedge_\text{comp}$?

dmckee

@Physicist137 The thing about unit systems is that you should pick one to stick with while you are learning (use the one you main teacher likes most) and just be aware that there are others choices out there.

Kyle Kanos

$4\pi$ is easier to understand as a constant

Danu

To be understood as $\text{composition}\circ \wedge $

@dmckee Agreed/

dmckee

At some point you need to learn a second one to get a feel for how things change and why people get all ranty about them.

Danu

SI is fine until you get to high energy physics

Kyle Kanos

8:58 PM

@Danu or Astronomy

Danu

@KyleKanos Or astrology

Kyle Kanos

@Danu Okay, this needs to be deleted

2

0celo7

Or Star Wars

ACuriousMind

@Danu No

Danu

I mean, how do I express my emotional frequencies in 1/s? :)

0celo7

8:59 PM

@Danu ...

Danu

@ACuriousMind Okay, so Leeb is just making it up himself ;)

@0celo7 astrology

gonenc

@Physicist137 note that it is not only that but eg the normal Coloumb have units of $ 1 g^{1/2} cm^{3/2}/s$ I hope I got that right...

Danu

I'm a member of one of these large facebook groups where people post dumb stuff about "energy" and "healing" and "angels" and stuff, just for laughs :)

0celo7

@Danu What about it. Didn't Kyle get a PhD in that nonsense?

As if a supernova can predict a job interview...

Kyle Kanos

Danu

9:01 PM

chirp chirp :P

@KyleKanos <3

Kyle Kanos

(actually, I am home, but I'm going to dinner now)

Danu

I should study for cosmololololology

gonenc

@KyleKanos do you actually spend time to find them or do you have an archive of all these memes? :D

0celo7

What. I have "Danny X" and "Daniel X", in the same handwriting, with a different address and phone number.

Petition police time?

Kyle Kanos

@gonenc Google; it takes about 0.5 seconds start-to-finish

Anyways, heading out now

gonenc

9:02 PM

@Danu whatever that is it cannot be any worse than the homoeopathy bs

Physicist137

@gonenc I thought coulomb was a "fundamental" dimension. Or you are talking about cgs? Shouldn't be statcoulomb in this case?

Danu

@gonenc Also part of it. Also, chemtrails everywhere!

@Physicist137 Fundamental dimensions are a myth :P

Except length, of course ;)

gonenc

@Physicist137 yup I was talking about cgs in my above post

Danu

statcoulomb

what does that even mean

Physicist137

@Danu and time perhaps?

ACuriousMind

9:03 PM

@Danu you're wrong, the exception is energy, but we call it mass dimension

Danu

lol quick delete @0celo7

@ACuriousMind you so wrong

ACuriousMind

@0celo7 Call the numbers

gonenc

@Danu what about the chemtrails?

Danu

@gonenc They love posting about them

ACuriousMind

@Danu u wot m8

0celo7

9:04 PM

@ACuriousMind Can't read them!

People need to write better.

Danu

@ACuriousMind i smak u in da gobber cunt

Physicist137

@Danu statcoulomb is the unit of charge in CGS. Right?

gonenc

@Danu oh the conspiracy you mean I got it now :D

Danu

@Physicist137 Never heard of it

0celo7

Is that a zero, 3, 5, 8, or 9?

gonenc

9:05 PM

@Physicist137 yes but check out what statcoulomb is

some funny combination of g cm and s

0celo7

@Danu Charge of electron?

What do you mean by "fund. dimension" anyway?

Physicist137

@gonenc Well.. I don't know CGS. I rather prefer SI :D. But if you are saying so...

gonenc

@Physicist137 you don't have to believe me

Statcoulomb

The statcoulomb (statC) or franklin (Fr) or electrostatic unit of charge (esu) is the physical unit for electrical charge used in the esu-cgs (centimetre-gram-second system of units) and Gaussian units. It is a derived unit given by 1 statC = 1 g1/2 cm3/2 s−1 = 1 erg1/2 cm1/2. The SI system of units uses the coulomb (C) instead. The conversion between C and statC is different in different contexts. The most common contexts are: For electric charge: 1 C ↔ 2997924580 statC ≈ 7009300000000000000♠3.00×109 statC ⇒ 1 statC ↔ ~6990333564000000000♠3.33564×10−10 C. For electric flux (ΦD): 1 C ↔ 4π×2997924580...

:)

0celo7

Benjamin Franklin?

Danu

@gonenc Now what is erg? ;)

gonenc

9:07 PM

Erg

The erg is a unit of energy and work equal to 10−7 joules. It originated in the centimetre–gram–second (CGS) system of units. It has the symbol erg. The erg is not an SI unit. Its name is derived from ergon (’έργον) a Greek word meaning work or task. An erg is the amount of work done by a force of one dyne exerted for a distance of one centimeter. In the CGS base units, it is equal to one gram centimeter-squared per second-squared (g·cm2/s2). It is thus equal to 10−7 joules or 100 nanojoules (nJ) in SI units. An erg is approximately the amount of work done (or energy consumed) by one common house...

Hey I'm no fan of cgs either. In fact I despise it! :D

Setting $\mu_0=\epsilon_0=c=1$ so much better imo

Physicist137

@gonenc Niiice. =). I don't like it too (but its because I don't know it :D).

0celo7

lol, 1 erg = fly doing a push-up

@ACuriousMind Ah, the lower case $a$ is different.

Physicist137

question: Is there anyone here who actually likes CGS?

btw, how do I get this chat ajax?

0celo7

Closer inspection shows the $D$s are drawn in opposite directions.

gonenc

15 mins ago, by Kyle Kanos

91

A: Should chat have TeX support?

I will leave the original post for historical reference, but as mentioned in the Update below, all four bookmarks are located on this installation page. There are four bookmarks: start ChatJax installs MathJax and starts a loop that renders $\LaTeX$ as needed. This is intended for use in chat, ...

@Physicist137 I assumed you meant mathjax for chat

dmckee

9:12 PM

@Physicist137 People from a math background tend to like it because then Guass' Law and the divergence theorem take on the same form.

Physicist137

@gonenc In that case I assume your assumption is correct. :D.

gonenc

@dmckee don't they just set every physical constant = 1?

it makes much more sense to do so

0celo7

You can't set all of them =1, can you?

dmckee

You can set a surprisingly large number of them to 1.

Danu

@0celo7 You can completely non-dimensionalize physics, yes

gonenc

9:15 PM

@0celo7 Even if you cannot you can always change units from chapter to chapter in a maths book.

Danu

It's typically nice to leave a single dimension for physical intuition/clarity

The standard choice is to leave $G$ in (if anything)

gonenc

@Danu why $G$?

0celo7

@gonenc Note Wald and Hawking & Ellis set it to 1 as well.

Danu

@gonenc Because there's no obvious pair that one would like to have identical dimensions that $G$ prevents from doing so.

dmckee

Seriously, $c$ gets set to 1 to make relativity convenient and $\hbar$ to make QM simple.

Danu

9:17 PM

@dmckee And $k_B=1$ because entropy should be dimensionless!

dmckee

If you're doing Genergal relativity you also set $G$ to one, but a lot of us are just not that smart.

@Danu By all means. It just doesn't come up much in my business.

0celo7

@dmckee Weren't you teaching thermo a while back?

Did you impart that knowledge on them?

Danu

@dmckee No, I disagree

dmckee

Aside: I have a strange tick where I always write "simply" when I mean "simple" and vice versa. Frustrating.

Danu

It seems to me that the community is rather split on setting it to 1 or not

dmckee

9:20 PM

@0celo7 I was using Zemansky & Dittman and they stick to SI, except where some of the answer to exercises have clearly not been converted from they earlier, cgs editions of the text.

I do assign term papers and one student wrote on the correspondence of the Boltzmann and Shannon entropies, though.

gonenc

" Gravity does not move objects in spacetime (hence it's not a force), instead gravity warps spacetime and you will warp with it provided nothing gets in your way, when something gets in your way (the ground at your feet), then you feel it." I think this statement is really misleading what do yall think?

0celo7

@dmckee No black hole thermodynamics?

dmckee

@gonenc It's correct as far as it goes. It's just that no one who doesn't already understand the subject will get it.

Danu

@gonenc Kinda unnecessarily mystic sounding

dmckee

@0celo7 Not this semester. I have high hopes for a later one.

Danu

9:22 PM

Curved spaces are really quite mundane.

0celo7

@dmckee Not the brightest lot this time?

Danu

There's a nice public piece of art close to my uni demonstrating the interesting things one gets when projecting onto a sphere

(I'm not sure it was intended or not)

0celo7

Have we ever measured the curvature of our universe directly / is that even possible?

dmckee

I'm at a directional state U. The intake has a wide variation between "not so able", and "too much of a slacker to get into the school that would have done them the most good".

Danu

@0celo7 Sure

Spatial curvature is determined by the energy density

dmckee

9:24 PM

We do our best, and the papers really seem to help because they all come asking for assistance and I get some one-on-one time.

0celo7

@Danu Without appealing to GR.

Danu

in FLRW universes

0celo7

@Danu Directly.

Danu

@0celo7 That sounds like a ridiculous request

@0celo7 Define directly

dmckee

We also get some who have gotten their issues squared away and are now ready to see what they can become. Those are the most fun.

0celo7

9:25 PM

@Danu No GR.

Danu

How do you measure the curvature of a sphere "directly"?

spoiler alert: GR-type math

0celo7

@Danu Parallel transport fails.

Danu

@0celo7 lmao

"no GR" amirite :P

0celo7

Math is allowed, G=T is not.

Danu

@0celo7 Then in principle we test spatial curvature by means of parallel transport lol

it's just that there are much better ways to do it

gonenc

9:26 PM

@Danu one should really try that!

Danu

@gonenc Not really :P We know the universe is spatially nearly-flat

0celo7

So it is possible to test for curvature of spacetime without appealing to Einstein's equations?

Danu

It'll be a null-result

@0celo7 It's possible to do differential geometry, of course.

The interpretation however...

gonenc

@Danu it'd be fun to watch a bunch of guys in astronaut suits flying in free space with arrows in their hands

0celo7

How do we get a closed loop to parallel transport?

Danu

9:28 PM

@0celo7 You don't need to close your loop

And in principle, we just make it large enough :P

gonenc

@Danu don't we really?

0celo7

@Danu How else do you compare to your start point?

Oh, we get two dudes to do it?

Danu

@0celo7 You can just do the classic two initially parallel thing

Yeah

0celo7

See if it depends on the path.

@gonenc Got a $100 billion or so?

We could totally do this.

ACuriousMind

@0celo7 Seems you'll have to sit your neighbours' cat a while for that

0celo7

9:31 PM

@ACuriousMind Cats.

There's two of them beasties.

gonenc

@0celo7 ok then we should start a indiegogo-funding-help type-thing :D

0celo7

@gonenc The PSE space adventure.

We already have a rocket designer, @JimsBond.

gonenc

@0celo7 in 1000 years when nasa has nothing to do they'll plagiarise our thought and do the experiment you'll see :)

0celo7

I'm sure @ACuriousMind has some German rocket scientist blood in him.

gonenc

9:47 PM

Lubos has a wikipedia page :O

Luboš Motl

Luboš Motl (born December 5, 1973) is a Czech theoretical physicist by training who was an assistant professor at Harvard University from 2004 to 2007. His scientific publications are focused on string theory. == Life and career == Motl was born in Plzeň, present-day Czech Republic. He received his master degree from the Charles University in Prague, and his Doctor of Philosophy degree from Rutgers University and has been a Harvard Junior Fellow (2001–2004) and assistant professor (2004–2007) at Harvard University. In 2007, he left Harvard and returned to the Czech Republic. Despite being...

0celo7

10:09 PM

Who wrote it?

gonenc

10:38 PM

@0celo7 Rolf h nelson apparently

0celo7

@gonenc What other kinds of stuff has he written?

0celo7

11:45 PM

@ACuriousMind @dmckee Here is the offending picture in Shankar.

Kyle Kanos