The h Bar

vzn

00:35

@user2723984 there is some anti-fluid sentiment expressed in chat over the years incl by oldtimers/ mods etc. on the other hand theres also a statistical chance ~½ in here voted for trump :P o_O

JMac

03:01

@vzn "on the other hand theres also a statistical chance ~½ in here voted for trump :P o_O" That assumes that approximately everyone here is from the US, which isn't the case at all basically. Also, even if they were, it doesn't mean statistically that the demographics of this chat would lean the same as the overall population combined.

Azmuth

03:54

Yo yo yo!

What's up everyone?

123

Yoooo... :-)

Hi @Azmuth

Azmuth

hiii :-)

vzn

04:23

@JMac its a joke, dude. just like our current president. statistics is full of assumptions isnt it. are you a nate silver fan? :) fivethirtyeight.com

Mohamed Obeidallah

05:17

Hey guys, so here apps.dtic.mil/dtic/tr/fulltext/u2/a351472.pdf and globalsecurity.org/wmd/facility/sandia_nm-buildings.htm the pulsed power linear accelerator which is used as a gamma ray simulator HERMES III generated 22 MeV bremsstrahlung rays with peak diode current at 730 kA and total beam energy at 370 kJ. The peak dose is >100 kRads (Si) and the peak dose rate is >5 x 10^12 Rads (Si).

According to John previously, 22 MeV gamma rays even at 1 kilometer, the intensity of 22 MeV gamma rays is about 0.14.

But here osti.gov/servlets/purl/4460107 the atomic bomb that was drop on Hiroshima have lower energy gamma rays which are only at 7 MeV or so

But despite the lower energy of the gamma rays that are produced, the distance travelled is long

I think longer than the distance the 22 MeV gamma rays travelled

Especially at the page 73 in the third link

John Rennie

@MohamedObeidallah That's a 182 page document! What pages do I look on to find the gamma ray range you're referring to?

Mohamed Obeidallah

@JohnRennie sorry it was page 73 in the third link

John Rennie

@MohamedObeidallah This graph?

Mohamed Obeidallah

@JohnRennie page page 4, page 17, and page 64 seems interesting too.

@JohnRennie yes, thats it

John Rennie

If you look at the red square I've drawn, that marks the intensity falling by a factor of ten.

Then if we look at the horizontal scale the distance taken for the intensity to fall by 90% is about 900m

For the 22 MeV gamma rays we worked out the intensity fell by 86% in 1000m

So the two figures seem reasonable. The range increases with energy so the 22MeV gamma rays penetrate farther, but the range doesn't increase a lot going from 7 MeV to 22 MeV.

Secret

05:32

Simulating Dragons Under Cloth Sheets! 🐲

►

Nihar Karve

Two Minute Papers is an absolute beast

Secret

This will actually be useful in simulating collision between two rigid bodies with complicated surfaces

so may have relevance outside of VR gaming as well

Nihar Karve

I'd love to see more science-ML videos

He mostly focusses on computer graphics (I think that's his field)

Secret

yup

Mohamed Obeidallah

@JohnRennie gotcha. Here nuclearsecrecy.com/nukemap the radiation radius of the Hiroshima bomb is 1.2 km with the description "500 rem ionizing radiation dose; likely fatal, in about 1 month; 15% of survivors will eventually die of cancer as a result of exposure." According to the previous link about nuclear radiation, fission bomb generated 7 MeV gamma rays if I'm not mistaken. Is it reasonable that the 7 MeV gamma rays travel to 1.2 km?

The Hiroshima bomb as we know was detonated in air, not on ground

John Rennie

05:40

I don't think gamma rays are the main killer in nuclear bombs. I think the main offenders are neutrons.

And remember that in a nuclear explosion we get a 1/r² decrease in intensity in addition to the decrease due to absorption by air. All we have been calculating so far is the absorption by air.

Mohamed Obeidallah

@JohnRennie the previous link did said there are neutrons produced in nuclear explosions too, and neutrons are more penetrating than gamma rays if I'm not mistaken. I just read the nuclear radiation article only a little bit because it have too many pages. I'll continue reading the article later

123

06:07

Hi All,

How fixed point in rotation behaves (in term of force) when force is applied at some distance from it?

John Rennie

@MohamedObeidallah I been doing some Googling and I don't think the radiation from the initial flash is all that dangerous. If you were close enough to be killed by the radiation you'd be killed by the blast anyway. When we talk about radiation poisoning from nuclear bombs we normally mean being poisoned by the fallout over a longer period.

skullpatrol

The blast heat turns sand into glass.

John Rennie

Humans have been turning sand into glass for thousands of years. A nuclear bomb just does it a bit faster than usual.

Mohamed Obeidallah

@JohnRennie thanks for the info. I'll ask again if there's something I don't understand.

John Rennie

@MohamedObeidallah I had a look at the blast calculator web page and it does show death from radiation out to over a km. I don't know how realistic this is. I guess it depends on how intense the radiation released by the bomb was.

Mohamed Obeidallah

06:18

@JohnRennie does the intensity of the nuclear explosion depends on the energy of the gamma rays or the numbers of gamma rays produced?

John Rennie

I don't know. I would guess both the energy and intensity depend on the size of the bomb.

(I do hope the CIA are not monitoring this chat :-)

2

Mohamed Obeidallah

@JohnRennie if we look at the size of the strongest nuclear bomb ever so far. The size of it is so enormous. There's nothing worrying about this chat, we're just trying to understand how far radiations extend and then the appropriate ways to be safe from them. Distance and shieldings are the most important :D

Sorry CIA lol :P

I may as well started living in a big steel house

John Rennie

We could probably find gamma ray absorption coefficients for lead if you wanted to calculate how much lead you'd need to shield you.

Or likewise for concrete.

Azmuth

@NiharKarve What a day to be alive.

I just got a message from my professor friend <3

Nihar Karve

06:41

@Azmuth who?

Azmuth

@NiharKarve Go and watch any 2 min papers.

Nihar Karve

of course, I've been a huge fan for the last couple of years

Azmuth

And you don't know what What a time to be alive is!?

Nihar Karve

No, I was asking who your professor friend was

Azmuth

@NiharKarve Prof RA Mishra

Nihar Karve

06:50

who's that?

Azmuth

Works in IEEE, checking papers....

Nihar Karve

oh, very nice

forgive my ignorance

skullpatrol

6

Q: Will we have a winter bash 2020 this year?

(or some other end of year fun?) At the end of last year, there was discussion suggesting that "winter bash 2019" (with the hat thing) was the last one (see here to learn more). Now that December is near, I would like to know if the 2019 winter bash (with the hat thing) was indeed the last one? A...

Nihar Karve

07:07

I wish the actual website's question tags were as 'pushy' as these, it's so satisfying

Manas Dogra

07:41

The above picture is from David Tong's notes on EM theory.
Which quantum mechanical derivation of Ohm's law is he possibly refering to?

Is it Kubo's formula for conductivity in linear response theory or something else?

Slereah

As far as I know, the standard QM way to get Ohm's law is the Sommerfeld model

Free electron model

In solid-state physics, the free electron model is a simple model for the behaviour of charge carriers in a metallic solid. It was developed in 1927, principally by Arnold Sommerfeld, who combined the classical Drude model with quantum mechanical Fermi–Dirac statistics and hence it is also known as the Drude–Sommerfeld model. Given its simplicity, it is surprisingly successful in explaining many experimental phenomena, especially the Wiedemann–Franz law which relates electrical conductivity and thermal conductivity; the temperature dependence of the electron heat capacity; the shape of the...

B. Brekke

11:59

In a cooper pair the electrons have momentum $k$ and $-k$, it seems to me that they are moving in opposite directions. How can they give rise to an electrical current?

John Rennie

6

Q: How does current flow in superconductors if Cooper pairs have zero momentum?

I've been reading a lot of condensed matter textbooks, which state both that the net momentum of a Cooper pair in a superconductor is zero, and that Cooper pairs have momentum when they carry current. How can these two statements be consistent? If a Cooper pair has zero momentum, how can current...

quantum-mechanics condensed-matter solid-state-physics superconductivity

RishiNandha Vanchi

14:11

uh I came across an article

im very much doubting its legitimacy because they are quoting none of the facts they say

is this true

phys.org/news/2017-04-physicists-negative-mass.html

this version seems more legit, still not sure but news.wsu.edu/2017/04/10/negative-mass-created-at-wsu

ACuriousMind

@RishiNandhaVanchi What do you mean "they are quoting none of the facts"? They give the article they're referencing at the end, and you can read it at arxiv.org/abs/1612.04055

phys.org is mostly just an aggregator for press releases, not some scientific journal, so if you really want to know what they're talking about you always need to look at the actual article they're talking about

RishiNandha Vanchi

okay got it thanks

skullpatrol

14:30

Why will google not translate: Es bleiben im Raum: Keitel, Jodl, Krebs, und Burgdorf.

:(

Kenzo Tenma

@skullpatrol it does translate for me (it says its German): There remain in the room: Keitel, Jodl, Krebs, and Burgdorf.

skullpatrol

Thanks, pal.

JMac

@vzn TBH it doesn't seem like a joke when you're always in here basically only to imply physicists are closed minded from what I see. And yeah, statistics is full of assumptions, so saying something is statistically likely when it doesn't match any of the assumptions is obviously silly. I like to point out when people say wrong things like that... sorry.

Faded Giant

14:51

@JohnRennie I could run the numbers for gamma radiation through Microshield if you need anything.

I also have MCNP here; however, that would be using a sledgehammer to crack a nut.

Charlie

If I could ask a quick question, regarding the C*-algebras used in classical and QM, Wikipedia says a C*-algebra has "an involution satisfying the properties of the adjoint". An involution is a function that is its own inverse, is this implying that the elements of the algebra have as their inverse their Hermitian conjugate? $A^\dagger=A^{-1}$?

The wording is a bit confusing to me

OH no I think I get it, it means $(A^*)^*=A$

Azmuth

15:11

Ama be watching Kung Fu Panda Today

sad day, but whatever...

Nihar Karve

@Charlie yes, that's correct

Anybody like math3ma?

Azmuth

No.

Nihar Karve

Is that an "I don't know her" no or an "I don't like her" no?

Azmuth

First

Nihar Karve

But if you've ever wanted to dip your foot into category theory, math3ma.com is the place

Azmuth

15:14

No

Wiki Exists

Check this cool Math Website

Nihar Karve

Er, why did you just link to the website I was just talking about?

ACuriousMind

15:30

@NiharKarve looks nice (but I already know most of the basic stuff there, I think) - what I always find difficult with math in blog form is how to decide where to start reading

like, start chronologically, and read a lot of intro stuff I might not care about? read the latest post and go backwards? some other approach?

Nihar Karve

16:02

@ACuriousMind what I do is go to the most recent post that interests me, then backpedal through her links to more basic material if I don't understand something

I'm sure you know him already, but John Baez is my go-to for applied category theory

ACuriousMind

Yeah, Baez is reliably interesting and a good writer

Azmuth

Hello friend

Welcome to fsociety!

Nihar Karve

16:21

Unbelievably consistent as well - he's been doing his 'This Weeks Finds in Mathematical Physics' gig for a long, long time now

John Rennie

I say to people "Did you know he's Joan Baez's cousin?" and everyone under 60 answers "Who's Joan Baez?". Sigh.

ACuriousMind

I only know Joan Baez as the cousin of John :P

John Rennie

To be fair that singer songwriter folky stuff has always bored me senseless, so I know Joan Baez only as a member of a set of musicians that I avoid.

Nihar Karve

16:41

She's been inducted in the Rock 'n' Roll Hall of fame apparently

Nihar Karve

16:54

any good refs on photonics, anyone?

ACuriousMind

@NiharKarve physics.stackexchange.com/q/131093/50583?

Nihar Karve

17:20

Great stuff, thanks (I'll check resource recommendations first from next time)

Sir Cumference

17:56

is the Latex on proofwiki broken for anyone else?

Sir Cumference

18:07

well, not rendering

ACuriousMind

if you give a link I can check

123

Hi All

Sir Cumference

18:32

@ACuriousMind E.g. on the front page, I see this

ACuriousMind

looks fine on my end

BioPhysicist

20:22

The more I tutor, the more I am convinced that education systems should favor teaching introductory abstract math / proofs rather than more computation heavy math subjects

ACuriousMind

@BioPhysicist have you heard of/read Lockhart's lament?

BioPhysicist

Oh yes. It is great haha

ACuriousMind

I think you're going in the same direction with that, but I also think that the emphasis on computation in elementary math education stems from a time before omnipresent computers

these curricula weren't designed to teach people math so that they understand what math is about or can do proofs, they were designed to teach them the computation skills needed in a world where you actually have to do computations on your own in everyday life

Ratman

@BioPhysicist do you mean that about high school math ?

ACuriousMind

elementary math education is under pressure to be a commodity like everything else, and the object of value to the market is not abstract understanding of math, it's people who can do office jobs and add some numbers correctly

BioPhysicist

20:34

@Ratman possibly, yes

ACM, yeah, that makes sense

Ratman

I have always been surprised by how high school math has nothing to do with what you see in university

it is like a complete new subject and kind of reasoning

Sir Cumference

@ACuriousMind Lockhart's lament was a valiant effort but went totally ignored by educators

ACuriousMind

you're not wrong

BioPhysicist

On the flip side, I am tutoring someone right now who hasn't heard of 360 degrees and/or 2 pi radians being associated with one rotation around a circle

oops haha

Sir Cumference

@ACuriousMind Not that I agree with 100% of what he says (e.g. I don't think the history of math should be mixed into math classes), but I definitely get where he comes from

One of the worst examples of faulty math educations I've seen are these stupid column proofs taught in US public high school geometry classes

> All proofs are separated into two columns. There are statements on the left-side and reasons on the right-side.

Where do these people come up with this stuff?

BioPhysicist

20:41

Yeah I had that in my freshman highschool geometry

Literally makes you scared of proofs

ACuriousMind

it's an attempt to make logical reasoning into something formulaic one can pretend to teach as a recipe to follow and pretend to grade objectively

BioPhysicist

Plus you have to spell out every single step that you wouldn't normally have to in a typical proof

Ratman

it is merely based on logic

Sir Cumference

@BioPhysicist it's way more convoluted than what mathematicians actually do

just write out an explanation like a normal person

BioPhysicist

Haha. "Like a normal person"

I don't know... some mathematicians are not normal

Sir Cumference

20:42

@ACuriousMind I get that, but it completely fails to prepare students for real math :P

and it's got to be as much of a chore to read as it is to write

ACuriousMind

@SirCumference I'm not defending it! I'm just saying it doesn't even aim to be the thing you and me would want it to be

Sir Cumference

ah, that's fair

ACuriousMind

It doesn't try to awaken curiosity, or to show beauty, or even just to transmit knowledge effectively, so I feel criticizing it for that is kind of missing the point. What you would need to change in the system is not the curriculum itself, it's the goals of math education. It is no accident Lockhart - very accurately - keeps comparing math to art

but art education doesn't intend to teach people a formulaically testable skill, so it is freer to be actually interesting

BioPhysicist

At the very least going through abstract math changes the way you think, more so than learning how to compute

Sir Cumference

@ACuriousMind art education has its own set of problems :P

"work on this project, now work on this project"

there's no point to art when it's done as an assignment instead of being motivated

ACuriousMind

20:47

in order to improve math education you would need to change people's ideas of its goals away from "prepare people for doing accounting at work" to "teach people about the logical reasoning and the art of math"

BioPhysicist

Yeah, it's sad when most people think math is just doing computations

Sir Cumference

I think part of the issue is a miscommunication on what math really is. Teachers and high school students see it as a means to solve an equation, whereas mathematicians see it as a way of formalizing ideas and concepts

ACuriousMind

@SirCumference a lot of struggling artists selling commissioned work for a living might disagree :P

Sir Cumference

When you think of it as the latter, it becomes more interesting and the possibilities are far greater/broader

I remember asking my high school geometry teacher why it's called the "Pythagorean theorem". She said "because no one has proved it yet"

The fact that neither high school teachers nor students learn about basic math terminology is concerning

ACuriousMind

that's not universal, though :P

Sir Cumference

20:56

@ACuriousMind That's true, some schools are much better than others

But at least the exams that public schools prepare you for don't ask anything about math terminology

Which is why it's not always taught

ACuriousMind

well to be fair there isn't much to teach :P "For no reason, mathematicians call true statements by many different names depending on their perceived significance to the writer"

Sir Cumference

well i'm not saying "fact" and "proposition" etc. need to be introduced, but "theorem" is a very basic term for math

Balarka Sen

@JohnRennie I was idly watching this and realized this probably gives the cleanest proof that $\Bbb R^2$ with the $p$-norm and the $q$-norm are not norm-isomorphic for $2 \leq p, q \leq \infty$

Sir Cumference

nowadays I hear "theorem" almost as much as "equation"

Balarka Sen

namely the unit circle has length going up from $\pi$ to $4$

monotonically

For $1 \leq p, q \leq \infty$ this probably says they are norm-isomorphic iff $1/p + 1/q = 1$ but for that maybe you can argue somehow

Dunno

ACuriousMind

21:02

physicists have long known that $\pi=3$

but $\pi=4$ is a new one

Sir Cumference

@ACuriousMind $=\sqrt{g}$

@BalarkaSen Now that I think about it, I'm curious how a circle would look in other $p$-norms, if we use the definition of points being equidistant from the center

Balarka Sen

check that video out it talks about it

Sir Cumference

oh

should've watched it then

Balarka Sen

PBS whatever

seems like a garbage channel but hey

Sir Cumference

they've got some surprisingly good content sometimes

BioPhysicist

21:14

Perhaps it would even help for students to see one area of mathematics that doesn't involve calculations

Like Graph Theory

Some people get introduced to set theory I suppose

But that is usually in college in the math class for people who don't need math / didn't do well in math in highschool

So by then they probably don't even care

Funny enough, my wife recently took one of those classes and said she absolutely loved doing truth tables haha

user 6232128

what turns most people off are the "word problems"

BioPhysicist

Which is odd because word problems are more fun!

haha

ACuriousMind

the problem is not that word problems that unqiuely terrible it's that they're the most fun that kind of math gets

user 6232128

sure, if you know the translation of words into symbols

ACuriousMind

and at the end they're usually taught by the same focus on a formulaic approach - identify the relevant numbers, plug them into a formula, put the result in an equally formulaic result sentence

they're often not trying to get you to actually figure out anything

BioPhysicist

21:24

Word problems disrupt the formulaic approach

ACuriousMind

some of the most interesting exercises in my physics undergrad courses were strange word problems about archers standing on rotating disks or whatnot, but they weren't interesting because they involved words but because they tried to get us to apply what we learned in a very different context.

BioPhysicist

Yep, this is exactly right

I love the skiier on the snowball problem :P

ACuriousMind

@BioPhysicist yeah, but usually it's like "this is a word problem in the section about quadratic equations so I just need to figure out what quadratic equation the book wants me to solve here" and not "hm, I need to figure out how to actually model this"

so it's still formulaic even if it pretends not to be

BioPhysicist

Are you familiar with the skier on snowball? haha

user 6232128

No, please share

ACuriousMind

21:29

@BioPhysicist I think it's a word dressing for the "particle slipping off a sphere" situation?

BioPhysicist

Yeah haha

My undergrad instructor loved it because there are no numbers yet you can find the angle where the skier will leave the snowball

So he thought it was fun when students would be like "but I don't know this value"

Ratman

ahahaha that's nice

Sir Cumference

@ACuriousMind Oh yours actually applied physics to real life?

Here's mine

BioPhysicist

like mass of the skier or radius of the ball

Sir Cumference

Ratman

21:32

ahahahahahah, I think I have already seen, which text is it from?

Sir Cumference

it was one of my homework problems a few years ago, i assumed it was original given how weird it was

ACuriousMind

@SirCumference well...I'm not sure how "real life" things like "An archer standing on a rotating disk is trying to hit a target on another rotating disk. Where do they have to aim to hit the target as a function of the rotational velocities of the disks?" are, but I thought it was fun (I remember that particular exercise so well because it was an exam question that no one got right but once we saw the solution there was a lot of head->desk action :P)

Ratman

it remembers me something but i can't tell ahaha

JMac

One thing I liked about engineering is that the word problems actually got pretty realistic once we got out of the intro classes.

Ratman

@ACuriousMind, this could be real life, medioeval times were quite crazy

user 6232128

21:35

@SirCumference this is sad.

ACuriousMind

@Ratman sure, they were fighting at carnivals all the time I guess :P

@JMac one might argue that's kinda the point of engineering!

Ratman

you should also consider drunkness in such situations

ACuriousMind

@SirCumference methinks Alice is not the only one on 'shrooms there

JMac

@ACuriousMind Yeah I'd say so. It would be hard to argue the tests weren't appropriate to the subject. That was definitely one of the draws of engineering over studying like physics for me.

user 6232128

In the final analysis, Zenos paradox is just a word problem :P

Ratman

21:42

I got to go back to percolation network, good night to everyone

user 6232128

cya

as opposed to Russell's paradoxes, which are a problem with words.

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