The h Bar

12:35 AM

@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

3:01 AM

@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

3:54 AM

Yo yo yo!

What's up everyone?

123

Yoooo... :-)

Hi @Azmuth

Azmuth

hiii :-)

vzn

4:23 AM

@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

5:17 AM

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

5:32 AM

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

5:40 AM

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

6:07 AM

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

6:18 AM

@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

6:41 AM

@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

6:50 AM

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

7:07 AM

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

Manas Dogra

7:41 AM

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 AM

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

2:11 PM

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

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