Did we install mhchem?

@SirCumference not yet, but you can manually load it using \require{mhchem}

for example $$\require{mhchem}\ce{2H2 + O2 -> 2H2O}$$

hello @vzn

@vzn That is because the underlying network models it well... If the underlying network didn't model it well, they would get bad performance... The best model they can get is one where the physics of the problem matches the "network" 1:1.

@Mikhail there is some new use of machine learning in physics, its early days, but this is one of the early uses of it in QM/ schrodinger eqn, looks intriguing, yes it will be used for modelling and could potentially create new models where humans have trouble. theres another recent experiment that was able to decrease decoherence in qubits using ML. etc... an emerging field, exciting possibilities/ potential to be explored...

@heather hi. hey was thinking, dont give up on coding factoring just yet :)

↑ lol freudian slip? vzn1.wordpress.com/2017/02/10/…

Hi, everybody.

Pregunta

Do stars generally have more electrons than protons?

I doubt it.

@DanielSank Well, the thing is that there are far more electrons than protons in a white dwarf. That's why they are supported by electron degeneracy pressure and not proton degeneracy pressure

So I'm wondering if that proton:electron ratio comes from the original star

@SirCumference White dwarves are not charge neutral?

@DanielSank I thought they were. But it turns out that the protons are safely locked away in the nuclei of carbon and oxygen nuclei (which are bosonic), and there are not enough protons on the surface to be degenerate.

As I understand it, the WD is supported by electron degeneracy pressure because there are a lot more free electrons

But the star is net neutral, right?

@DanielSank No clue

@Slereah Pregunta (possibly a stupid one)

Is the comoving distance to the edge of the observable universe increasing at the speed of light?

@SirCumference Gravitationally bound objects are, to excellent approximation, electrically neutral.

@rob wtf then...

I'm hella confused

@SirCumference Why?

White dwarves

Electrons are degenerate, protons and other nuclei aren't.

I've heard that the reason they are supported by electron degeneracy pressure rather than proton degeneracy pressure is because there are far more electrons than protons

Degeneracy pressure depends on mass. At the same number density, electrons become degenerate before protons.

@rob Yes, I know. But I've certainly seen somewhere that white dwarfs have many more electrons than protons, such that there are almost no free protons in a white dwarf.

@SirCumference I think that most white dwarfs are mainly carbon, nitrogen, oxygen by mass.

@SirCumference Since white dwarfs are what's left of the core of a star that has exhausted its fuel, you'd expect them to be hydrogen-poor and helium-rich, at least.

@rob Well in most cases, yes (except for ones from common envelopes, I assume)

@SirCumference Common envelopes are the "let's worry about that problem later" of introductory astrophysics questions.

@rob Er, I'm sorry?

@SirCumference Never mind, sorry.

@SirCumference It's the transition from hydrogen burning to helium burning that drives a star off the main sequence to the red giant phase.

@rob Er, no, that drives it into the horizontal branch

It becomes a red giant once it fuses hydrogen in the outer layers.

Horizontal branch (helium fusion) comes after that

@SirCumference Oh, you're right.

@0celo7 Well Bourbaki is a nice book for rigor but I don't think any of the results are really new

@vzn But forward FDTD style or MOM style simulation of the problem already exists. Instead of learning the model you can just use the model :-)

@SirCumference In any case, the hydrogen survives only in the outer envelope of the star, and gets ejected as the planetary nebula. The core that remains in the white dwarf has been processed by fusion and the hydrogen is missing --- but the electric charge is still there.

@rob I thought you said it was electrically neutral?

@SirCumference Yes. The electrons are still there. The nuclear charge is still there, but as helium or carbon or whatever rather than lone protons. The total is electrically neutral.

@rob Hmm...can any fermionic nuclei form in a white dwarf?

Actually, since only carbon, oxygen, magnesium and neon are really dominate...

@SirCumference "Fermionic nuclei"? What do you mean?

You mean nuclei with odd mass number, $A$?

@rob Composite fermions, in the same way that carbon and oxygen nuclei (for example) are bosonic

It's possible. There's not still fusion in a white dwarf, but whatever was left when the fusion stopped in the parent giant will still be there.

Huh, but aren't carbon, oxygen, magnesium and neon all bosonic?

My understanding is that carbon dwarfs are common because the CNO cycle is a catalyst for proton fusion to helium.

The CNO cycle includes both even- and odd-mass isotopes: C-12, C-13, C-14; N-14, N-15; O-16, O-17.

I'd say carbon-oxygen white dwarfs are more common than oxygen-magnesium-neon white dwarfs because the latter requires higher temperatures (more massive stars) to create those elements

But the cross sections and lifetimes work out so that the even-even nuclei are more abundant. The stable odd-A nuclei are allowed, but they capture the protons more efficiently than the others, and the populations in secular equilibrium are smaller.

@SirCumference Yes, but carbon white dwarfs are also more common than helium white dwarfs, yes?

@rob Wait, what "helium white dwarfs"?

You mean the ones from common envelopes?

@SirCumference Exactly.

Um, I'm still kind of confused

Oh, wait, I see your point

All right, thanks :)

I've enjoyed chatting with you, @SirCumference.

@DanielSank there actually was a winner for "best answer for the laymen"

it acheived 1 upvote

(and 3 downvotes, but downvotes shouldn't count against)

for example, I didn't downvote the posts I didn't want to nominate, I just upvoted the posts I did want to nominate

@DanielSank note the "best baby steps answer" also has a net negative vote yet is selected

so for consistency the "best answer for the laymen" should also have a winner

@Mikhail not an expert on this wrt physics but predict the machine learning models will have some edge over the other models "soon" if not "already". its not the end of human-constructed models, there will be an ongoing interplay between the two. this is starting to happen in mathematics also eg auto thm proving vzn1.wordpress.com/2016/09/16/… vzn1.wordpress.com/2016/01/22/…

At the end of the day you can learn a function like sin(x) or you can represent it as a series of functions like x-x^3/3!+x^5/5!. Due to issues like over-fitting, representing the function in its true form is always preferable. I did some ML for some image recognition stuff.

Was this(meta.physics.stackexchange.com/a/9620/102705) really necessary to ask in AMA ? @Kenshin

Anyway, its your choice...

A small block of mass , attached to one end of a massless spring, is kept on a smooth horizontal surface. The other end of the spring is moved with a constant velocity v in the horizontal direction. Find the maximum total mechanical energy in the spring-block system

Mass is (m)

Hi, could anyone help me in the question?

Any hints could be appreciated.

@SwapnilDas work in the frame moving at velocity $v$. In this frame the end of the spring is stationary and the mass is moving at $-v$, so it's just an SHM problem.

@JohnRennie Thank you. What answer does that yield?

@anonymous just a bit of fun

<The provided answer is $2mv^{2}$>

@SwapnilDas I don't know - I haven't worked through the problem. However that is the easiest method to approach it.

::take a deep breath::

::rewrite the phrase in a less personal manner::

::try to pretend that it is still satisfying that way::

:-) Which question/answer?

I wonder how can one show that the integrated charge density on the plate is indeed $-q$...?

well, the first step is we need the charge density to be bounded, but by what envelope function?

@Kenshin what is the penalty for not voting in Australia?

@skillpatrol a small fine why?

Hey guys sorry to interrupt , I just want an answer..Plz see my qn

Qn no 46) it's somewhat of chemistry but I know you guys can solve it.. it's urgent guys so plz give me some hints to get the answer as I am confused how to do it when temp and concentration is not given

Just curious @Kenshin

How is forcing people to vote "freedom" of choice?

@skillpatrol I don't know anyone who claims that it is

all societies have rules that sacrifice some freedoms for "greater good"

it's only a $20 fine in Australia if you don't vote

but it encourages political participation

@Kenshin AMA events is mainly meant to be useful for future viewers to learn from other's life experiences and avoid common pitfalls . Have a look at the intro (meta.physics.stackexchange.com/questions/7783/…) here. Do you feel your question will be of any help to future viewers ?

Your question is okay for the chat room. However, I'm not sure if it is good for the main site.

@anonymous I have responded to your comment on the question

note comments aren't for extended discussion so I encourage you to reply to the comment here

::makes popcorn::

@Kenshin Oh only $20?

yeah

Is New Zealand the same?

Hello :-)

How are you?

Hello :-D

Fine thanks.

That's fine

Sup girl

sup

means wassap

means what's up

oh

soz

normally if one says they're good I'll say that's good

he said he's fine, so i say that's fine :)

...

..

coool what type

@Kaumudi.H it's ok if you're poor I don't judge you for that

what did you get new dress?

oh

lol that sounds like a funs hirt

been looking at a few houses to maybe buy

yea it will be my first house

ty haven't bought oen yet but beginning the process

@Kaumudi.H Hi.Stopped for lunch?

cheers

The last time you appeared at this time you were ordering your dessert - the disappointing chocolate pudding!

disappointing?

whas the pudding a bad recommendation after all?

fark

shuda gone the mouse

Mouse? :-)

lol

yeah she was deciding between a mouse and a pudding

That was a narrow squeak then

eewww

some of us are eating here

np

Yes. I've always only eaten one big meal, though I do have cereal at breakfast.

well they say caloric restriction prolongs life

(when done in moderation)

It's just the way I'm used to eating. I'm not recommending it as an ideal way to eat, it just suits me.

yeah i do the same

I skip breakfast and lunch usually

@Kaumudi.H coffee isn't food, it's the elixir of life!

I don't drink much coffee tho

yeah!

wow that's heaps

I don't drink water - fish wee in it

yeah water is for bathing in

not for drinking

jks water is gud, but 4L too much

u should aim to have bout 2L

u don't wanna dilute all ur salt

4-5 litres would be a lot in the UK, but with a high temperature you lose a lot of water through perspiration

still tho

do you have diabetes?

@Kaumudi.H drinking too much water is actually dangerous because it messes up your electrolyte balance. It can be fatal in extreme circumstances.

tsk tsk tsk

gotta cut down on that water addiction

You like to live dangerously :-)

what a maverick

In other news, my pushbike is bust!

The chain snapped.

In fact it snapped when I was several miles from home so it was a long walk back.

sorry to hear that John

The bike is so old it needs lots of bits replacing. For example all the gears are worn out.

i recommend installing uber

I don't think it would be economic to replace all the worn bits so I'm going to buy a new one.

wow replacing the whole thign for just a chian?

Yes, I have placed an order for this

@Kenshin Yes, I saw your comment

@anonymous yes the logtic is undeniable

@Kaumudi.H actually I am a bit sad to see it go. I've had the bike for eleven years

@JohnRennie how much does a replacement chain cost?

@Kenshin What ?