The h Bar

4:00 PM

@Slereah good idea... lost, baby thrown out with bathwater™ :( en.wikipedia.org/wiki/Hydraulic_analogy en.wikipedia.org/wiki/Probability_current en.wikipedia.org/wiki/Non-Newtonian_fluid

tatan

Okay... I found this aps.org/publications/apsnews/201606/physicshistory.cfm#4

vzn

also en.wikipedia.org/wiki/Madelung_equations

Slereah

@vzn People love fluid analogies in physics

vzn

@Slereah they also hate/ avoid em... ps more than analogy™ o_O :( :P

Slereah

Although

Here's a simple idea to measure charge

Take two charges

Measure the repulsion of the second charge by the first

Then, change the first charge, until you measure twice the repulsion

Then you have the object with twice the charge of the first

I think that's how they defined the charge, back then, pretty much

John Rennie

4:16 PM

So the new head of NASA is a climate change denier with no scientific background. A bright future ahead then.

Slereah

well I mean

NASA doesn't really have much to do with global warming

though the lack of science is certainly worrying

hey @Qmechanic

Are you wondering if something is too broad

vzn

@Slereah is this related to the fluid analogy or somethiing else? seems like non sequitur wrt prior topic

Slereah

No, that's just an easy way to define the charge

Same as the definition of the mass : the proportionality constant between force and acceleration

Except for charge that constant will actually be $m/q$

vzn

@Slereah ok. no disagreement. on other hand there is a way to define "charge" wrt fluid dynamics. involving attraction/ repulsion. lost in the sands of time. maxwell used it. etc

Slereah

well if you define charge as a fluid, you just integrate the charge density

0celo7

4:26 PM

@JohnRennie Nice

Maybe we can finally move away from peaceful space exploration

Slereah

Time for SPACE WAR

0celo7

The fact that we don’t have space lasers that can vaporize ground targets is ridiculous

Slereah

Space treaty my man

People didn't want it to be too easy to nuke each other

John Rennie

@0celo7 the Russians don’t have space lasers that can vaporize ground targets either

0celo7

What are they gonna do if we put nukes up there first

Slereah

4:30 PM

I do tho

Wait

What if

0celo7

@JohnRennie the Russians are allies, I’m more concerned about the Chinese

Slereah

There's a plan for a gravitational wave detector in space

What if that's just a ruse

to put lasers in orbit

0celo7

A device that can vaporize terrorists and measure gravitational waves would be the pinnacle of American science

John Rennie

@Slereah because LISA absolutely needs terawatt lasers to work :-)

Slereah

@JohnRennie Well, it doesn't not need a terawatt

John Rennie

4:32 PM

We'll measure the optical path length to the Kremlin ...

Slereah

How powerful is the LIGO laser anyway

0celo7

Put it up there with a nuclear reactor

@JohnRennie why would you suggest lasering the friendly Russians

John Rennie

Good idea. We need more radionucleotides in low Earth orbits.

0celo7

It’ll stop global warming

John Rennie

It would certainly gives us something more pressing to worry about.

Perhaps our children would be sufficiently mutated that they could stand the heat.

Slereah

4:35 PM

Praise Mutant

0celo7

You don’t have children so you don’t get a say

Maybe people after 50 who don’t have kids should no longer receive government services. Huh. What a great idea

No more water for you

Slereah

@0celo7 you're not gonna make a lot of friends in the Catholic Church

Sid

@0celo7 Water should be privatized. :P

Slereah

Late stage capitalism

John Rennie

@0celo7 water supply isn't a government service in the UK. It is a privatised industry and we pay a lot of money for it.

Sid

4:40 PM

@JohnRennie New Era of ~~human~~ mutant beings!

eulB

mmm water refreshing

0celo7

@JohnRennie great!

vzn

huffingtonpost.com/entry/…

John Rennie

@0celo7 to be fair the privatised water industry works ok. Water charges are high, but then they would be anyway as the infrastructure needs massive investment.

vzn

wired.com/2017/05/…

Slereah

4:52 PM

Government privatization is p. much a scam

All the infrastructure paid for by the tax payers passing to some company

vzn

nybooks.com/articles/2014/12/18/…

@Slereah huffingtonpost.com/entry/…

0celo7

Maybe we should privatize everything. After a generation, poverty will be eliminated. Genius plan?

Eric Silva

bad plan

John Rennie

5:08 PM

The problem with privitisation is that many services work best as a monopoly. The post for example. And running a monopoly as a private business is fraught with problems.

Slereah

Also not all government functions can generate profit

John Rennie

Since there is no competition the government has to load up the privatised company with regulations to stop it exploiting the customers.

That rarely works well.

Slereah

though trying to say this to @0celo7 is probably futile since he has no sincere political opinions

John Rennie

This isn't a political opinion. I'm not terribly political. I am an arch pragmatist. Whatever works well is fine by me.

DanielSank

Awwwww yisssss

Hit the rep cap again.

Slereah

5:11 PM

@JohnRennie Everything is a political opinion to some!

DanielSank

@Slereah That's just your political opinion, man.

Eric Silva

@JohnRennie what if we're confronted with the real possibility that none of our options we have in front of us work well

John Rennie

@EricSilva that is pretty much how real life works. You have to choose the least worst option.

Eric Silva

i guess what's least worst depends where youre standing

sounds political to me

vzn

humans are stupid™

DanielSank

5:16 PM

@JohnRennie Except with food, where you can have lots of good options.

amirite?

John Rennie

@DanielSank well for example the UK government has just introduced a tax on soft drinks that contains lots of sugar. That's because kids love sweet drinks and the privatised food companies are happy to supply them. The result is fat kids with rotten teeth.

Slereah

I thought all the brits had rotten teeth

John Rennie

@Slereah wonky teeth, because historically we've not been as fussed as Americans about straight teeth. But I don't think dental health is worse in the UK than elsewhere.

vzn

nytimes.com/2010/07/03/nyregion/03sodatax.html

Sid

@JohnRennie How do they define "lots of sugar"?

John Rennie

5:21 PM

> drinks with a sugar content of more than 5g per 100ml will be taxed 18p per litre and 24p for drinks with 8g or more

Source

Slereah

@JohnRennie what about syrup!

Syrup isn't a drink

John Rennie

You mean like maple syrup or golden syrup?

Slereah

Errr

What do you call the syrups you mix with water to get drinks

John Rennie

Ah, I know what you mean but I'm not sure what they're called either.

eulB

pretty sure it's just called a mix

John Rennie

5:24 PM

I'm not sure how they are taxed, but they aren't a big seller in the UK anyway.

0celo7

@JohnRennie proof?

John Rennie

Proof of what?

0celo7

they're not a big seller

why wouldn't it be

I actually have no idea where those things sell well

Slereah

I buy 'em

It's good stuff

Anonymous

(They are called concentrates by the way)

0celo7

5:26 PM

huh

DanielSank

What on Earth is going on here?

1

Q: Why density matrix?

Why do we need density matrix formalism?

Slereah

@DanielSank why density matrix

John Rennie

@0celo7 Just go to any UK supermarket. You'll find shelves and shelves of bottles ad cans of pop but you'll struggle to find any concentrates.

DanielSank

@JohnRennie gross

@Slereah Read the post though.

John Rennie

@DanielSank true though

Slereah

5:27 PM

@DanielSank odd edit

DanielSank

@JohnRennie Still gross

John Rennie

@DanielSank the sugar tax appears to have worked. Most manufacturers have halved sugar levels to avoid the tax. So hopefully things will be less gross in a few years.

This assumes that the children don't just drink twice as much pop :-)

0celo7

why can't people brush their teeth

oh wait it's Britain

Slereah

@JohnRennie or put some sugar in it

John Rennie

@Slereah the kid will be selling each other bags of sugar in the playground :-)

0celo7

5:31 PM

@JohnRennie free market :)

Sid

@JohnRennie "Sugar is the new cocaine?"

0celo7

sugar and adderall

Anonymous

Sugar dealing

John Rennie

"Oh my God i got burned. This isn't sugar it's cocaine. It doesn't even make the drink sweet!"

Abcd

$\text{Electric force in medium= } \dfrac{k q_1q_2}{k'r^2}$, @JohnRennie $k'$ is just permittivity of the medium right or is it some other constant?

John Rennie

5:32 PM

Correct

Abcd

Okay

DanielSank

@JohnRennie It's kind of funny to read this as if the children are the masters of their pop intake. Have parents given up?

Abcd

@JohnRennie We were just told the formula, Is there any derivation of it too?

John Rennie

@Abcd that's how you define the permittivity of the medium.

Sid

@Abcd If you want to know the derivation of the formula of Electric Force, then, you can prove it by Gauss Law.

Although, that's kind of cheating

John Rennie

5:34 PM

The force is less in a medium because $\epsilon \gt \epsilon_0$

Abcd

@Sid gauss law is derived from coulomb force

DanielSank

@Sid It's an experimental fact.

2

@Abcd You can do it either way.

Slereah

The electric force can be derived from the Maxwell equation of a static charge

Maxwell equation for $\rho = \delta(r)$

DanielSank

@Slereah What is the Maxwell equation of a static charge?

Abcd

@JohnRennie I dont understand what permittivity is really :(

Slereah

5:35 PM

@DanielSank see above!

DanielSank

@Slereah There's a lotta chat above...

John Rennie

@Abcd it's just a constant. It has some value in vacuum and a higher value ina medium. Just accept it and move on.

Sid

@DanielSank Usually, students who read these stuff are less convinced by "It's an experimental fact. No need to prove it."

Secret

12

A: Does time expand with space? (or contract)

The simple answer is that no, time is not expanding or contracting. The complicated answer is that when we're describing the universe we start with the assumption that time isn't expanding or contracting. That is, we choose our coordinate system to make the time dimension non-changing. You don'...

Slereah

$\rho = \delta(r)$

John Rennie

5:36 PM

@Secret a fine answer

Slereah

In which case the EM field will be $A_\mu = \int \delta(r) \Delta^+$

Secret

It will be interesting if there's an FLRW like expanding universe where time become more dilated as it expands, however, past experience told me I need cross terms in my metric to write something like that

Abcd

@JohnRennie And what's the "definition" you were referring to?

John Rennie

Luckily Duffield wasn't around when I posted that

Slereah

Where $\Delta^+$ is the retarded Green function

Abcd

5:37 PM

4 mins ago, by John Rennie

@Abcd that's how you define the permittivity of the medium.

referring to this^

John Rennie

@Abcd measure the force, q1, q2 and the distance then define $1/4\pi\epsilon$ as the constant of proportionality, k'.

@Secret you don't have to use comoving coordinates. You can choose coordinates where time does dilate.

e.g. conformal time.

Abcd

@JohnRennie so we should have electric force in medium = $\dfrac{1}{4\pi\epsilon}\dfrac{q_1q_2}{r^2}$ instead of $\dfrac{\epsilon}{4\pi\epsilon_o} \dfrac{q_1q_2}{r^2} $

Secret

Ah I see. Btw, Is there a coordinate for the FLRW metric where I have space dependence on $dt^2$ but no dependence on either space or time for the $d\Sigma^2$, sort of like the inverse of the comoving coordinates?

John Rennie

@Abcd Force does equal $\dfrac{1}{4\pi\epsilon}\dfrac{q_1q_2}{r^2}$

Where did you get that second equation from?

Abcd

9 mins ago, by Abcd

$\text{Electric force in medium= } \dfrac{k q_1q_2}{k'r^2}$, @JohnRennie $k'$ is just permittivity of the medium right or is it some other constant?

Secret

5:42 PM

so something like $$ds^2 = -k(\Sigma)dt^2 + d\Sigma^2$$?

DanielSank

$$\text{"Force does equal"} \quad \equiv \quad ''F=''$$

John Rennie

@Abcd OK, I misread your question.

Secret

because I will imagine in that coordinate system it will be cool to see how the extent of time dilation is distributed in the universe as it expands

John Rennie

@Secret $ds^2 = a^2(t)(-t^2 + x^2 + y^2 + z^2)$ where $t$ is now conformal time.

Secret

@JohnRennie (That's my previous question btw, but sure)

Abcd

5:44 PM

@JohnRennie Sorry k is 1/(4 pi epsilon naught) and k' is dielectric constant

@JohnRennie Now what is dielectric constant :/

John Rennie

For any medium you can write $\epsilon = \epsilon_r \epsilon_0$ where $\epsilon_r$ is called the relative permittivity.

Relative permittivity

The relative permittivity of a material is its (absolute) permittivity expressed as a ratio relative to the permittivity of vacuum. Permittivity is a material property that affects the Coulomb force between two point charges in the material. Relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum. Likewise, relative permittivity is the ratio of the capacitance of a capacitor using that material as a dielectric, compared with a similar capacitor that has vacuum as its dielectric. Relative permittivity is also commonly known as dielectric...

Secret

@JohnRennie my current question now is checking for the name of a coordinate system which the expansion of the universe as seen in this coordinate system become a spatial distribution of time dilations while the spatial components show no time dependence

John Rennie

Dielectric constant is just another name for the relative permittivity

Abcd

Ohkay, Thanks!

Secret

To elaborate, we knew the following:

Comoving coordinates:

John Rennie

5:47 PM

@Secret I've seen the de Sitter metric written that way ...

Slereah

yeah it's the...

John Rennie

I think they're called static coordinates

Slereah

fuck I forget the name

@JohnRennie samuel-lereah.com/db/spacetimedb/de%20Sitter

no they ain't!

The conformal coordinates are like $$ ds^{2}={\frac{1}{y^{2}}}\left(-dt^{2}+dy^{2}+\sum _{i}dx_{i}^{2}\right)$$

Though that's for AdS

John Rennie

Ah yes, $g_{rr} \ne 1$

Slereah

@JohnRennie fairly obviously not conformal!

to flat space, anyway

John Rennie

5:51 PM

Blame my aging memory :-)

Slereah

If you just mean coordinates of the form $ds^2 = -dt^2 + a(t) d^3x$ there's a few

John Rennie

I think @Secret meant $ds^2 = -a(t) dt^2 + d^3x$

Secret

Well, to clarify:

Slereah

@JohnRennie That's just Minkowski space

you can easily reparametrize it to flat space

Semiclassical

wait

dt^2 and d^3 x?

Slereah

5:53 PM

Just using $dt'/dt = \sqrt{a}$

@Semiclassical I mean a 3-metric :p

but too lazy to write it

Secret

@Slereah @john We knew Comoving is $ds^2 = -dt^2 + a(t)d^3x$
We knew Conformal is $ds^2 = b(y) (-dt^2 + dy^2 + d^nx)$
And I think we are discussing Static is $ds^2 = -c(x)dt^2 + d^3x$?

Semiclassical

oook

Slereah

Generally speaking I don't think it's a good idea to have only one metric component depending on a coordinate

John Rennie

I'm going to call it a night. I'm sleepy after my light meal.

Slereah

Since you can always just do a redefinition of the coordinates of the form $dx'/dx = \sqrt{g_{yy}}$

Actually I think $ds^2 = -c(x)dt^2 + h$ is just an accelerated observer

$c(x) = x$ for a Rindler observer

Secret

6:02 PM

I wonder if static coordinates is the name for the frame of reference where an expanding universe (with spatial coordinates depend on cosmic time) transformed into a non spatially expanding universe but with an uneven spatial distribution of the extent of time dilations

So to be more precise, I am looking for going from:

$$ds^2 = -dt^2 + a(t)d\Sigma^2$$

to

$$ds^2 = - b(\Sigma)dt^2 + d\Sigma^2$$

Slereah

that is just flat space.

Balarka Sen

so is ur face

Secret

but $b$ is not a function of t, it is a function of the set of spatial coordinates $\Sigma$, so dividing by b won't give minkowski space as in JohnRennie's $ds^2=-b(t)dt^2+d^3x$ example?

Slereah

Why not

Secret

uh wait a minute, let me think again...

Slereah

6:08 PM

It's just solving $$(\frac{dt'}{dt})^2 = b$$

just look at the Rindler coordinates

Secret

right, got $-\left(\frac{dt'}{dt}\right)^2dt^2 + d\Sigma^2 \implies -dt'^2 + d\Sigma^2$ which is indeed minkowski

Slereah

well if $d\Sigma$ is flat, yes

but it will be static anyway

Secret

I thought there exists metric where the spatial coordinates are not changing with time, but the time dilation has variations along spatial coordinates, but I guess any static spacetime with some nonzero stress energy tensor should do

so it's kinda a not-a-real-question, oops

Anyway, I think I got my head clear, thanks

dmckee

@JohnRennie I was watching two morning doves engaged in a battle this morning, too.

Slereah

so much for a symbol of peace

dmckee

6:20 PM

They really put down the pedal for that. Normally they lollygag about unless they are trying to cover a lot of distance.

But these guys were seriously dog fighting.

The loser tried to escape by flying into the bushes, but the winner bobbed and weaved hard to keep the pressure on.

It was quite a display of flying.

vzn

6:41 PM

"dove violence"? "some" humans fight worse than some animals... & all seem to have the capacity o_O

Gombe Chimpanzee War

The Gombe Chimpanzee War (also known as the "Four-Year War" of Gombe), lasting from 1974 to 1978, was a violent conflict between two communities of chimpanzees in Gombe Stream National Park, in Tanzania. The belligerent groups were the Kasakela and the Kahama, which occupied territories in the northern and southern areas of the park, respectively. The two had previously been a single, unified community, but by 1974 researcher Jane Goodall, who was observing the community, first noticed the chimps dividing themselves into northern and southern sub-groups. Later computer-aided analysis of Goodall...

2

Emilio Pisanty

@vzn that's extremely interesting

thanks for that link

my go-to example is giraffes

Homosexual behavior in animals

Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include same-sex sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Research indicates that various forms of this are found in every major geographic region and every major animal group. The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from social species. Scientists perceive homosexual behavior in animals to different degrees. T...

vzn

@EmilioPisanty sure! :) (actually was weak on details myself & was somewhat surprised to now read it.) ps it involves a scientific paradigm shift! cf en.wikipedia.org/wiki/Gombe_Chimpanzee_War#Legacy

Emilio Pisanty

but it's nice to have an example from the other end of the range of supposedly-only-human behaviour

@vzn well, that gets kind of buried under a larger cultural paradigm shift in my view, but yes, it is definitely true.

vzn

7:00 PM

along similar lines, eerie human-animal parallels esp show up in the study of alpha male behavior, which in turn reminds me of the "be nice" policy o_O theguardian.com/us-news/2017/sep/14/…

Semiclassical

7:20 PM

The trouble with being both a math and a physics person at times is that I get obsessed with little bits of logic

To wit: Griffiths discusses the force/torque between magnetic dipoles and leaves the potential energy $U=-\vec{m}\cdot \vec{B}$ as an exercise.

But his derivation of that has them compute the work in two steps: 1) move the dipole, keeping its orientation fixed, then 2) rotate the dipole, keeping its location fixed

Which does indeed give $U=-\vec{m}\cdot \vec{B}$, but seems to be explicitly path-dependent.

(path as both the translational and rotational motion)

I think this isn't actually an issue, but it's taken for granted in this problem

Emilio Pisanty

7:46 PM

@Semiclassical so... you're reading Griffiths, and you're worried about rigour?

Semiclassical

lol

hey, i never said it was a good obsession

Emilio Pisanty

¯\ _(ツ)_/¯

-2

Q: According to Physics, why it is dangerous to travel on the roof of bus?

Last day, I saw an accident of a man traveling on the roof of bus. This man was much confident and brave and think that he is safe while travelling on the roof of bus but actually he was unsafe.He fell down from roof of bus and died. Why it is dangerous to travel on the roof of bus?

newtonian-mechanics kinematics

^ what's with the deletion votes?

G. Bergeron

8:03 PM

@Semiclassical I'm surprised your beef is with EM, in QM (not even getting to QFT) there are far more dubious mathematical treatment in the usual textbooks.

GENESECT

why doesn't longitudinal waves be polarised

G. Bergeron

@GENESECT There is only one mode to be had per k vector value.

GENESECT

okay

G. Bergeron

@Semiclassical Also, I'm looking at that part of Griffiths now and I can't find that...

Semiclassical

well, I'm not teaching QM right now :P

more to the point, I'm supposed to write up solutions to problems so I'm rather attuned to it

@G.Bergeron problem 6.21 in the 4th edition

G. Bergeron

8:15 PM

@GENESECT because a longitudinal waves are defined as having the displacement of the ''medium'' in the same direction as the propagation direction. As such, there is only one possibility whereas in the transverse case, there space of possible displacement is 2 dimensionnal

ah too late to edit...

@Semiclassical found it

@Semiclassical Look at equation 6.3. This is for the total force, but the same argument can be made infinitesimally to account for the torque.

Or you could always invoke linearity and split B into a uniform and a non-uniform components

Semiclassical

fair enough

G. Bergeron

@Semiclassical And I noticed my copy is already all yellowed-out. I sometimes ask myself why I bought those early undergrad books.

Semiclassical

the way griffiths does it in the solution is compute the work required to move the magnetic dipole out to infinity. if B=0 at infinity, you're done; if not, then you take the reference point for the potential energy to be when the dipole is at infinity and is perpendicular to B

G. Bergeron

This one might be ok, but the Griffiths about QM, I never opened it since that first QM class.

Semiclassical

I had Griffiths for undergrad E&M but not for undergrad QM

G. Bergeron

8:27 PM

For QM 2, we used Cohen-Tannoudji

A much better book that should be used for both classes IMO.

Semiclassical

we used this book for undergrad QM: amazon.com/Quantum-Mechanics-Amit-Goswami/dp/1577663217

which as a mathematical text was mostly fine

but uh, check out the list of 'outstanding features' in the book description

(1)-(4) are fine. (5), however...

G. Bergeron

And for QM 3, we had no book, but it was almost a graduate classes with topics ranging from representation theory in QM, how Galilean transformations leads to Shrodinger equation while Lorentz leads to Dirac, path integrals, quantum information theory, my favorite class of all undergrad!

@Semiclassical LOL

Semiclassical

yeeeah

i was not a fan of those parts of the book

which isn't the part we encountered in the class itself, so it wasn't a big deal from that POV

G. Bergeron

@Semiclassical Well, a man's gotta sell his salad!

Hmm I realized this does not sound the same as in french...

Semiclassical

i guess

a man's gotta earn his keep?

G. Bergeron

8:32 PM

@Semiclassical No, no, it's more along the line of convincing people...

Semiclassical

hmm

G. Bergeron

salad seems not to work either...

Semiclassical

I'm just really not a fan of anything which smacks of "consciousness intervenes on the wavefunction"

and Goswani was very much in favor of that

G. Bergeron

@Semiclassical I'm more than ''not a fan'', I think its rather crazy. In my books, an observer is nothing more than a Turing machine, with respect to physics.

Semiclassical

I was being rather mild :)

G. Bergeron

8:38 PM

Otherwise it's unicorns pooing rainbows!

0celo7

8:53 PM

I almost passed out in the colloquium

How can I be so tired one moment but when I get home I’m not tired at all

vzn

in Computer Science, Dec 11 '14 at 19:13, by vzn

Semiclassical

9:29 PM

@G.Bergeron Mostly I think 'woo consciousness' gives very much the wrong impression. Even if you think that the right way of understanding quantum mechanics is epistemic rather than ontic, it does not follow that stuff like telepathy exists

0celo7

9:48 PM

@Slereah This new Rodianski-Speck paper is very interesting

it's a constructive version of Hawking's incompleteness theorem via a curvature blow-up argument

They show the Kretschmann blows up

interesting, the main theorem requires dimension \le 38

HAH I read it wrong

\ge 38

what a crappy theorem lol

Slereah

10:30 PM

That's not even good for bosonic string theory