The h Bar

@Slereah Is it solely dependent on the mass density of the universe being greater than $\rho_{\text{crit}}$, or can it be influenced by the existence of phantom energy, etc.?

Slereah

I dunno

Would seem weird that it matters much, though

Sir Cumference

Why?

Slereah

I mean the metric only depends on the stress energy tensor, in the end

DHMO

2:01 PM

@SirCumference I suppose matter is trapped energy

Slereah

If you have more positive energy than negative one, it doesn't matter too much

Sir Cumference

@Slereah I guess

Though I still need to understand all this critical density stuff

Oh, also

Does the mass density of the universe have anything to do with the Big Rip scenario?

AccidentalFourierTransform

2:14 PM

@Slereah hi!

you probably know a lot about Penrose diagrams

I need to learn the basics, do you know of any good reference?

something brief and simple please =P

Slereah

Errr Wald?

Carroll?

Basically you just recast the metric in a conformally flat form

AccidentalFourierTransform

I'll probably go with carroll, I need to learn the basics

nothing rigorous, just to know what the symbols mean

@Slereah I'd like to develop some intuition

yeah, I'll read Carroll, thanks!

Slereah

it's used to have a simple representation of the causal structure

since the causal structure is preserved under conformal transformation

And Minkowski space has a very easy to read causal structure

AccidentalFourierTransform

"easy" ha

not for me, I suck at diff. geom.

Slereah

Well it's pretty easy

The light cones are just 45° lines

AccidentalFourierTransform

2:19 PM

yeah, but that means nothing to me

Slereah

So finding out where causal curves go is just a matter of angles

There's no differential geometry involved there

It's all basic special relativity

AccidentalFourierTransform

can I say that I also suck at SR? =P

man I hate physics

Itachí Uchiha

Hi.Can Faraday's Law of Induction be applied to open circuits?

AccidentalFourierTransform

you can do whatever you want

the sky's the limit

Secret

user228700

2:22 PM

x'D

Secret

Cross cap drawn in a way such that the double line is visible as a circle

AccidentalFourierTransform

259

Q: How do I draw a pair of buttocks?

I'm trying to develop a function which 3D plot would have a buttocks like shape. Several days of searching the web and a dozen my of own attempts to solve the issue have brought nothing but two pitiful formulas below. They have some resemblance to the shape I want, though not quite. Could you...

Secret

The middle loop, if projected to 3D, becomes a line. This is the double line of the projective plane

Slereah

@AccidentalFourierTransform then maybe you should focus on that

Before learning about penrose diagrams

Itachí Uchiha

Hi.I have a question.Can Faraday's Law of Induction be applied to open circuits?

AccidentalFourierTransform

2:26 PM

you can do whatever you want

the sky's the limit

@Slereah but I hate learning :-(

FML

Slereah

Maybe sociology will be more your thing then

Itachí Uchiha

@AccidentalFourierTransform,Was that reply to me?

AccidentalFourierTransform

@Slereah lol

@ItachíUchiha perhaps.

Slereah

Actual sociology

Itachí Uchiha

Could someone give me accurate answer?

Slereah

2:36 PM

Faraday's law is part of Maxwell's laws of EM

It applies to all classical settings

Itachí Uchiha

But if the circuit is open and emf is it's ends,how will the current flow?

*there is emf between

Slereah

If there is no current sounds like a pretty simple application

Secret

need to check

Ramanujan

3:07 PM

@Secret btw for whom these are?

Secret

anyone who may concern

aquire

3:43 PM

sorry for troubling you guys is there a way to learn about power semiconductor devices in 4 hours?

AccidentalFourierTransform

no idea, sorry

Emilio Pisanty

Hey @AccidentalFourierTransform, can I ask whereabouts in Barcelona you work?

AccidentalFourierTransform

@EmilioPisanty I'm a student

@ Universitat de Barcelona

Emilio Pisanty

@AccidentalFourierTransform Castelldefels campus by any chance?

AccidentalFourierTransform

@EmilioPisanty nope, but IIRC that is close to my university

let me check the map

aquire

3:49 PM

no problem@AccidentalFourierTransform its okay.

AccidentalFourierTransform

@EmilioPisanty oh no, it is rather far from where I live/study :-S

Emilio Pisanty

@AccidentalFourierTransform yeah, I figured

AccidentalFourierTransform

@EmilioPisanty btw why do you ask?

Emilio Pisanty

@AccidentalFourierTransform I'm starting a postdoc at ICFO soon

AccidentalFourierTransform

@EmilioPisanty sweet :-D

I'll stop by some day to say hi

Emilio Pisanty

3:54 PM

@AccidentalFourierTransform yeah, I'll be happy to show you around

I can't give any lab tours 'cause I'm politely asked to keep my hands in my pockets when I do go into a lab

but I'll be happy to show you around

talk about physics 'n' whatnot

AccidentalFourierTransform

I take your word :-)

John Rennie

@EmilioPisanty on the grounds that experimental equipment goes wrong the moment a theoretician even goes near it :-)

Emilio Pisanty

@JohnRennie I've been able to successfully stand near experimental equipment without breaking it a number of times, thankfully

admittedly it was mostly not actively taking data

AccidentalFourierTransform

Pauli effect

3

Emilio Pisanty

@AccidentalFourierTransform that's golden

AccidentalFourierTransform

4:05 PM

no, that was Fermi ;-)

John Rennie

@AccidentalFourierTransform so you're excluding Pauli?

AccidentalFourierTransform

ok no more puns for the rest of the day please

DanielSank

4:42 PM

@heather nineteen hats

heather

@DanielSank agh!

Thomas Tom

5:07 PM

Hello, I need emergency help.

(with very simple questions)

Is anybody here?

AccidentalFourierTransform

yes but if it is about newtonian mechanics or electrostatics I cannot help

Thomas Tom

It's actually 100 times simpler.

AccidentalFourierTransform

hmm to me NM or ES are as hard as it gets

Thomas Tom

If you look through binoculars at something (e.g. a bird), then will the image at your fundi (bottom of your eye?) be upside-down or does it depend which side of the binocular you watch look through?

It's a dumb question, but I don't care.

AccidentalFourierTransform

if you dont see it upside down, then it is upside down

your brain will always invert it

anonymous

5:13 PM

@ThomasTom Image on the retina is the inverted image of what you "see". This diagram <http://archive.cnx.org/resources/3544a449315d495866eecc596e6f8558e3d41409/PG11C‌3_028.png> might help

DanielSank

@heather ::maniacal laughter::

AccidentalFourierTransform

do all the hats disappear tomorrow? or do we have one more day of hats?

yeah I just got a new hat!!

and if we have one more day of hats, tomorrow I'll get a new one ^.^

Thomas Tom

Thanks! But I got a two more questions. If you are in for example a transparent aquarium filled with water, looking at some object through the aquarium wall, will the object be bigger or smaller? And the second question is that if you place an object right at the focus of a convex lens, where will the image appear. I think it will be either at the double focus length, between the lens and the object, or also at the focus, but on the other side of the lens?

AccidentalFourierTransform

newtonian mechanics, electrostatics, electrodynamics, optics, thermodynamics, experimental physics, general relativity, condensed matter physics, biophysics, astrophysics, cosmology, ...

and the list goes on and on

I can only help with questions about QFT and maybe QM

anonymous

5:37 PM

@ThomasTom You can calculate lateral magnification using the formula $|\frac{\mu_1v}{\mu_2u}|$

uobabylon.edu.iq/uobColeges/ad_downloads/5_24884_121.pdf

Take $R=\infty$ if the wall of the aquarium is flat.

@ThomasTom If you place an object at the focus of a convex lens then the image appears at infinite distance on the other side of the lens.

@AccidentalFourierTransform That's strange. You know Quantum Mechanics but not Newtonian Mechanics and Electrodyanamics :) Are you joking ?

AccidentalFourierTransform

5:52 PM

@anonymous not really. I think I could solve a basic problem in classical mechanics, but it would take me a lot of time and the method would be highly inefficient

Its been five years since the last time I added two forces, I dont remember any of that

also, its boring =P

Bernardo Meurer

$$F'(x) = f(b(x))b'(x) - f(a(x))a'(x)$$

DanielSank

$$ \lim_{x \rightarrow 0} \frac{1}{x} \int_0^{2 \ln(x+1)} \exp(\cos(t))\,dt$$

Shashaank

Could some one tell me that when a charge q is kept at certain distance from a plane , one side of the plane of gets -q induced charge and the other side gets +q. So the force on an arbitrary charge kept in space in between our charge q and the plane should be only due to our charge q since force due to opposite induced charges cancel . But by the method of images we get a force due to both our q and induced -q. Is it because the plane is grounded and the induced +q flows to the ground.

DanielSank

@BernardoMeurer ged danging it's my internets

Bernardo Meurer

I thought CA was like a modern state with good internet

DanielSank

6:00 PM

I think it's $2e$ because for small $x$ the integrand is $e$, and the upper limit is $2x$

PearlSek

Where the force of electric charges that repell come from ? Can this energy "run out" ?

AccidentalFourierTransform

arrgh

hate it when I spot a grammatical error in a comment of mine after the frigging five minutes period

DanielSank

@PearlSek It doesn't run out.

Force is not the same as energy.

Two things pushing against each other are not using up energy unless those things are also moving.

If your book is on the table, the book and table are pushing against each other, but they are not using up energy.

AccidentalFourierTransform

my book is using up my energy

PearlSek

Isn't energy needed to apply a force ?

AccidentalFourierTransform

6:13 PM

no

Lu Bu

Isn't energy needed to do work.

PearlSek

So there's something i don't quite understand

AccidentalFourierTransform

and so me put in work work work work work work

PearlSek

A particle that repells another makes the other particle have more velocity

so it has more kinetic energy ?

Swapnil Das

Hi

@AccidentalFourierTransform So are things in QM and QFT so different that one can even forget Newtons Physics?

Lu Bu

6:24 PM

What i don't understand is that, is conservation of energy just something that is observed or is it derived from mathematics?

Wikipedia says something like this: "The conservation of energy is a common feature in many physical theories. From a mathematical point of view it is understood as a consequence of Noether's theorem, developed by Emmy Noether in 1915 and first published in 1918."

AccidentalFourierTransform

hi @SwapnilDas

Bernardo Meurer

@AccidentalFourierTransform Halp

AccidentalFourierTransform

well, the tools that people use in QM are indeed very different than those of classical mechanics

e.g., there are no forces

@BernardoMeurer tell me

Lu Bu

@AccidentalFourierTransform What makes something accelerate then?

Swapnil Das

Ohk.

Bernardo Meurer

6:27 PM

@AccidentalFourierTransform The integral Daniel posted above

how to solve

or rather

the limit

AccidentalFourierTransform

@LuBu there are potentials, but not forces

@BernardoMeurer ok give me a minute

are you supposed to use some tool in particular?

l'hopital, Taylor, Leibnitz?

Bernardo Meurer

@AccidentalFourierTransform No Taylor or Leibnitz

AccidentalFourierTransform

so L'hopital is ok?

$$
\lim_{x \rightarrow 0} \frac{1}{x} \int_0^{2 \ln(x+1)}\mathrm dt\ \exp(\cos(t))=\lim_{x\to 0}\frac{\mathrm d}{\mathrm dx}\int_0^{2 \ln(x+1)}\mathrm dt\ \exp(\cos(t))
$$

right?

Swapnil Das

@AccidentalFourierTransform A last question if you don't mind. Is Newtonian Mechanics derivable from QM?

Bernardo Meurer

Wat, why is the exponent out of the integral?

AccidentalFourierTransform

6:31 PM

@SwapnilDas yes but it is very complicated and the picture is not completely clear as far as I know. There is not a complete consensus

@BernardoMeurer lol, physicists notation. It is inside, we just like to write $\mathrm dt$ to the far left for some reason

anonymous

@AccidentalFourierTransform :) I see. So you studied it long back ! 5 years..wow!

Bernardo Meurer

@AccidentalFourierTransform Bah, use analysis notation plox :P

And where did the 1/x go?

AccidentalFourierTransform

@BernardoMeurer $$\lim_{x \rightarrow 0} \frac{1}{x} \int_0^{2 \ln(x+1)} \exp(\cos(t))\ \mathrm dt=\lim_{x\to 0}\frac{\mathrm d}{\mathrm dx}\int_0^{2 \ln(x+1)}\ \exp(\cos(t))\ \mathrm dt$$

@BernardoMeurer recall that $$\lim_{x\to0}\frac{f(x)}{g(x)}=\lim_{x\to 0}\frac{f'(x)}{g'(x)}$$

Bernardo Meurer

Ah, derp

Yes

AccidentalFourierTransform

(provided the assumptions of L'Hopital theorem are satisfied, of course)

@BernardoMeurer so far so good, right?

Bernardo Meurer

6:34 PM

Yessir

AccidentalFourierTransform

do you know how to evaluate $$\frac{\mathrm d}{\mathrm dx}\int_{a(x)}^{b(x)}F(t)\mathrm dt$$

?

Bernardo Meurer

now just apply the fundamental thm to integral

yeah

Exactly

AccidentalFourierTransform

Leibniz integral rule

In calculus, Leibniz's rule' for differentiation under the integral sign, named after Gottfried Leibniz, states that for an integral of the form ∫ y 0 y 1 f ( x , y ) d y {\displaystyle \int _{y_{0}}^{y_{1}}f(x,y)\,\mathrm {d} y} ...

just in case

Bernardo Meurer

Cannot use that

AccidentalFourierTransform

damn

Bernardo Meurer

6:36 PM

Ah

Wait

AccidentalFourierTransform

its just the fundamental theorem in disguise

Bernardo Meurer

Yes I can

AccidentalFourierTransform

so we have

$$
\lim_{x\to 0}\frac{\mathrm d}{\mathrm dx}\int_0^{2 \ln(x+1)}\ \exp(\cos(t))\ \mathrm dt=\lim_{x\to 0}\exp(\cos(2 \ln(x+1)))\left[\frac{2}{x+1}\right]
$$

right?

Bernardo Meurer

$2e$

AccidentalFourierTransform

yep

in this case, there is a simpler method

I think

one that doesnt require you to remember Leibnitz formula

actually, nevermind

with what you know, that's probably what you're expected to do

Bernardo Meurer

6:42 PM