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Mohamed Obeidallah
Mohamed Obeidallah
2:16 AM
Another newbie question. All of the cold cathode and hot cathode gas discharge/vacuum tubes work by static electricity, right? So, somehow we can say if Thales of Miletus didn't discovered static electricity in the first place there will be no such things as gas discharge/vacuum tubes? But I thought in my opinion even without Thales, someone or some people will independently discover static electricity eventually.
 
 
3 hours later…
John Rennie
John Rennie
5:04 AM
@MohamedObeidallah I wouldn't say they operate using static electricity.
Typically electrons are produced by field emission or thermionic emission, then they are accelerated by a voltage applied between the electrodes. I don't see how static electricity is involved.
 
Mohamed Obeidallah
Mohamed Obeidallah
5:44 AM
ok I just did a quick search on Google that field emission theory was first proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim while thermionic emission can be traced back to Edmond Becquerel, Frederick Guthrie and Thomas Edison. So, we can somehow say there will be no field emission and thermionic emission gas discharge/vacuum tubes without those people?

Field emission is emission of electrons induced by electrostatic fields, and since classical physics, it has been known that some materials, such as amber, attract lightweight particles after rubbing. Electrostatic phenomena arise f…
 
John Rennie
John Rennie
No.
Suppose you have some chunk of metal. Electrons inside the metal have a lower energy than electrons outisde the metal. The difference in energy is called the work function.
So to get electrons to leave the metal you have to supply this energy.
If you apply a really, really strong electric field then this can supply enough energy to get the electrons out of the metal, and that is the basis of field emission.
 
Mohamed Obeidallah
Mohamed Obeidallah
@JohnRennie right. Thanks for your response. Who do you think is the first to demonstrate field emission from another metal to another metal? Ralph H. Fowler and Lothar Wolfgang Nordheim just make the theory only but I think they never demonstrated it.
 
John Rennie
John Rennie
I don't know. Sorry :-(
 
Mohamed Obeidallah
Mohamed Obeidallah
@JohnRennie oh nevermind, so it could be anyone who experimented with metals with an understanding of electrostatics somewhere around 1700 to 1800 and onwards. Because many people with engineering background said the inventor of this and that is not really the first person to create that something, as people always been copy each others idea and then attempted to improved it by trial and error. Thoughts?
 
John Rennie
John Rennie
I've never been very interested in the history of science I'm afraid.
 
Slereah
Slereah
6:37 AM
We have a whole stackexchange for that
if you need to
 
 
1 hour later…
Mohamed Obeidallah
Mohamed Obeidallah
7:40 AM
@Slereah thanks! Will bring this topic there later
 
 
4 hours later…
Slereah
Slereah
11:30 AM
"If the function is both a local diffeomorphism, as above, as well as an injective function then we call it an open embedding"
@bolbteppa Isn't a diffeomorphism already injective, since it's invertible?
Or is it because Schreiber uses a weird definition of diffeomorphism
are local diffeomorphisms not diffeomorphisms
 
ACuriousMind
ACuriousMind
12:08 PM
@Slereah Indeed, a local diffeomorphism is not a diffeomorphism, it's just a map such that every point has a neighbourhood on which the map is a diffeomorphism
it's a stronger variant of the "full rank" requirement for immersions and not Urs' invention, see en.wikipedia.org/wiki/Local_diffeomorphism
 
skullpatrol
skullpatrol
12:52 PM
how's the curfew working out for you? @Slereah
 
Slereah
Slereah
1:07 PM
@skullpatrol Not the clubbing type so I basically haven't noticed
I go home before dark
9
Q: A local diffeomorphism of Euclidean space that is not a diffeomorphism

El MoroCould someone give me an example of a local diffeomorphism from $\mathbb{R}^p$ to $\mathbb{R}^p$ (function of class say $C^k$ with an invertible differential map in each point) that is not a diffeomorphism.. in the real line (1 dim case) that would mean a function with a continuous non null deri...

differential-geometry multivariable-calculus
 
 
1 hour later…
bolbteppa
bolbteppa
2:29 PM
"In the case at hand, several proofs of Hurwitz’s theorem are sketched on Wikipedia, with links to more details. Discovering that any one of these proofs is wrong would instantly hurtle anyone to the status of a mathematical superstar."
 
 
1 hour later…
JingleBells
JingleBells
4:00 PM
Why is Bayes' theorem so unintuitive ;(
 
Slereah
Slereah
Humans are notoriously bad with statistics
 
Azmuth
Azmuth
Can anyone point out if Full RISC $\subset$ CISC Systems, then why the hell, RISC $\not \subset$ i386 or any x86 architecture?
 
John Rennie
John Rennie
RISC isn't a subset of CISC
 
JMac
JMac
@Slereah Anecdotes are just so much more relatable.
 
Azmuth
Azmuth
Then what?
 
ACuriousMind
ACuriousMind
4:07 PM
@Slereah we're bad at most things, really
 
Azmuth
Azmuth
@JohnRennie So, RISC $\not \subset$ x386?
 
John Rennie
John Rennie
RISC and CISC are two separate approaches to designing a processor. Neither is a subset of the other.
 
ACuriousMind
ACuriousMind
the amazing thing is that we can learn not to suck that much
 
Slereah
Slereah
Also RISC and CISC aren't actual math objects
 
Azmuth
Azmuth
@JohnRennie So, which one is supports i386 or x86?
 
Slereah
Slereah
4:08 PM
Unlike say, a random access machine or a Turing machine
There's no definition of RISC
 
Azmuth
Azmuth
@Slereah RISC is just like a convention
 
JingleBells
JingleBells
@ACuriousMind including at being bad? hehe
 
John Rennie
John Rennie
@Azmuth x86 is a CISC instruction set
 
Slereah
Slereah
x86 is very much a CISC
 
John Rennie
John Rennie
Examples of a RISC instruction set would be ARM
 
Slereah
Slereah
4:09 PM
A mess, even
 
Azmuth
Azmuth
As far I know, ARM is compatibly with RISC, so, All of ARM must support RISC-V Compatible instruction set?
@JohnRennie Ohhhhh!
 
ACuriousMind
ACuriousMind
@JingleBells Yes, we're so bad at being bad that we invented whole traditions of knowledge to get worse at being bad!
 
skullpatrol
skullpatrol
who's bad?
 
Azmuth
Azmuth
I see, why I messed so bad
@skullpatrol CISC
 
JingleBells
JingleBells
@skullpatrol michael jackson
 
Azmuth
Azmuth
4:10 PM
RISC >>>>
Another question
 
Slereah
Slereah
CISC has its perks
 
John Rennie
John Rennie
@Azmuth RISC-V is also a RISC instruction set, but it is a different instruction set from ARM.
 
JingleBells
JingleBells
@ACuriousMind but if we're bad at being bad, aren't we good :D?
 
Slereah
Slereah
If you actually have to code assembly, CISC is much more pleasant
 
John Rennie
John Rennie
Neither of RISC-V and ARM are subsets of each other.
 
Slereah
Slereah
4:11 PM
RISC is only nice for people making processors
 
Azmuth
Azmuth
If RISC supports MMIX Instruction sets...
@Slereah RISC >>> Because designing algorithms is easy
 
ACuriousMind
ACuriousMind
@JingleBells that interpretation is a tad too optimistic for my tastes ;)
 
Azmuth
Azmuth
Also, Knuth's MMIX is supported in RISC
That's the greatest advantage
 
Slereah
Slereah
x86 has quite nice functions if you want to actually code assembly
Although to be fair, a lot of complex instructions are an illusion
 
Azmuth
Azmuth
@Slereah So, how much time does it takes to translate 100 lines RISC Instruction set module to CISC?
 
Slereah
Slereah
4:12 PM
Since it's just translated in microcode down the line
@Azmuth Depends on the code, I suppose
 
ACuriousMind
ACuriousMind
@Slereah I wouldn't call any coding in any assembly "pleasant" :P We invented high-level languages for a reason!
 
Azmuth
Azmuth
@Slereah I'm designing my own compiler and hence a high level language (roughly), so, to make it compactible on i386/x86 variant and RISC Variant, I've to make porting algorithms
 
Slereah
Slereah
@ACuriousMind All relative
 
Azmuth
Azmuth
ARM has the best support of all!
 
skullpatrol
skullpatrol
Object Orientation?
 
ACuriousMind
ACuriousMind
4:14 PM
If you're writing a compiler and not targetting LLVM as a backend you're not going with the times.
 
Slereah
Slereah
@Azmuth I wouldn't advise porting x86 to RISC
 
Azmuth
Azmuth
@ACuriousMind Try doing reverse engineering, you'll realize, how good haven is Assembly
 
John Rennie
John Rennie
Another compiler? Does the world need another compiler?
 
Slereah
Slereah
Unless it's a school project or something
 
Azmuth
Azmuth
@Slereah Not even for compiler design?
 
Slereah
Slereah
4:15 PM
The proper way would be to compile the high level language in both
 
Azmuth
Azmuth
@JohnRennie required in my engineering course, just a mini, small version to be tasted on virtual environment
@Slereah school project
 
Slereah
Slereah
If you translate twice, it's going to be fairly inefficient
 
Azmuth
Azmuth
@skullpatrol structured Oriented
 
skullpatrol
skullpatrol
hmm
 
Slereah
Slereah
If it's a school project ask your teacher maybe
If he tells you whatever just go ahead
 
JingleBells
JingleBells
4:16 PM
I understand the intuition behind Bayes' theorem and I understand how prior data affects a new hypothesis probability but I can't relate that to the equation. Should I try to make sense of the equation or? I don't know to what level it's possible to grasp this thing.
 
Azmuth
Azmuth
@Slereah The most painful thing is converting all of sighed datatypes into unsigned and vice versa, I keep making mistakes everytime
@Slereah Just about a 100 line MMIX Code in RISC-v, and we've to translate it to i386/x86 varient
I've yet to study CISC
Backlogs :(
@Slereah college project
actually...
uni project :P
@skullpatrol Have you completed all of MMIX from Knuth?
 
skullpatrol
skullpatrol
nope
 
Azmuth
Azmuth
So I guess translation must be easy for you :P XD
just kidding
 
Slereah
Slereah
The best of course is to compile in OISC
 
Azmuth
Azmuth
@Slereah nu.... RISC
@Slereah Professor gave a challenge to write a code that prints it's own code @JohnRennie
 
Slereah
Slereah
4:19 PM
 
Azmuth
Azmuth
try try :) XD
 
Slereah
Slereah
@Azmuth That is called a quine
 
Azmuth
Azmuth
@Slereah never heard such creature till now!
 
Slereah
Slereah
Plenty of examples on the internet
 
John Rennie
John Rennie
@Azmuth that's a fairly standard problem.
 
Azmuth
Azmuth
4:19 PM
@Slereah lol, you are very smart!
 
John Rennie
John Rennie
Steady on
 
Azmuth
Azmuth
@JohnRennie c'mon it's just 1 week of class
here's my code:
 
Slereah
Slereah
If you're allowed to use the RAM containing the code itself, fairly easy
 
ACuriousMind
ACuriousMind
@JingleBells It's pretty straightforward: In the Bayesian interpretation, $P(A)$ is your prior belief for $A$ being true, and now you learn that $B$ is true. So you update your belief that $A$ is true to the conditional $P(A|B)$. The factor $P(B|A)/P(B)$ measures how "good" the evidence $B$ provides for $A$ is: If $B$ is completely independent from $A$, then this factor is $1$ and nothing changes - it tells you how much more likely $B$ is to be true if $A$ is true than otherwise.
 
Slereah
Slereah
Otherwise it's trickier
 
Azmuth
Azmuth
4:21 PM
@Slereah here's the answer.... 
main(){char*p="main(){char*p=%c%s%c;(void)printf(p,34,p,34,10);}%c";(void)printf(p,34,p,34,10);}
 
ACuriousMind
ACuriousMind
Code Golf is full of various quines and related challenges
 
Azmuth
Azmuth
@Slereah Try :)
@Slereah So you are a physicist and a BTech Engineer too!? Insane!
 
Slereah
Slereah
Am I?
 
Azmuth
Azmuth
@ACuriousMind interesting
 
John Rennie
John Rennie
Most physicists are secretly computer nerds.
 
Azmuth
Azmuth
4:22 PM
@Slereah How do you know all those virtualisation and computer assembly?
 
skullpatrol
skullpatrol
@JohnRennie but he is smart, sir :-)
 
Slereah
Slereah
I kinda hate computers, actually
@Azmuth School, mostly
 
ACuriousMind
ACuriousMind
@JohnRennie I would dispute the "secretly" part ;P
 
John Rennie
John Rennie
:-)
 
Azmuth
Azmuth
@JohnRennie Do you have a computer degree?
 
John Rennie
John Rennie
4:23 PM
0
Q: I need to destroy the Earth. Is this what it might look like?

Crash GordonThis is a small but critical part of my story; I need to kill off most of humanity (leaving a 5- or 6- digit number of survivors) and render the planet uninhabitable long-term, but survivable for a short time (as short as a few hours is OK). So I've been thinking about this scenario: A large com...

physics apocalypse
I have days like that too.
 
Azmuth
Azmuth
@Slereah so they taught you Assembly and compiler design in school?
@skullpatrol yep :)
 
Slereah
Slereah
@JohnRennie qntm.org/destroy
 
skullpatrol
skullpatrol
@ACuriousMind your secret is out
 
Slereah
Slereah
@Azmuth Just assembly
 
Azmuth
Azmuth
@ACuriousMind Well, one might counter argue, how you are a pro Programmer and you didn't tell us anything except Physics part :P
 
Slereah
Slereah
4:24 PM
Of course, if you want a more GR way to destroy the earth, thunderbolt singularities are the way to go
 
John Rennie
John Rennie
> This is not a guide for wusses whose aim is merely to wipe out humanity
 
Azmuth
Azmuth
@Slereah Reverse Engineering, Compiler Design, Virtual Machine, simulation, algorithms?
 
Slereah
Slereah
Nah
 
ACuriousMind
ACuriousMind
@Azmuth Uh...I've actually talked about programming and/or my job in this chat plenty of times
 
Azmuth
Azmuth
@JohnRennie Thanks for the info, I'm interested :P
 
Slereah
Slereah
4:25 PM
I have a software engineering degree, so it wasn't very focused on assembly
 
Azmuth
Azmuth
@ACuriousMind I just heard it 4 days back....
@Slereah I too have a soft eng (in progress) degree
hehehe XD
Now, I mostly spend time with AI and coolio stuff... :)
 
Slereah
Slereah
AI is boring
Boo
 
Azmuth
Azmuth
Machine Learning and AI (Unsupervised, AGI, RL, Deep RL) are boring!???!! okay, i trust u
 
John Rennie
John Rennie
Just don't get me started on laptops.
Amazingly I have finally started selling off my immense pile of old laptops on ebay.
 
Slereah
Slereah
(I work in AI)
 
Azmuth
Azmuth
4:27 PM
@JohnRennie Wooowooowooow!
 
ACuriousMind
ACuriousMind
 
Azmuth
Azmuth
@Slereah Data Science to be accurate
that's not AI! :P
 
Slereah
Slereah
Didn't know you knew my job better than me!
 
Azmuth
Azmuth
lol, Yep, me too!
XD :)
 
Slereah
Slereah
You're lucky that I'm too bored by it to discuss it further
 
Azmuth
Azmuth
4:29 PM
Y'all soo lucky, you have to have multiple degrees in multiple subjects
 
Slereah
Slereah
Am I
 
Azmuth
Azmuth
I'm just BE -> in CSE
yep
u r :)
 
Slereah
Slereah
I have multiple degrees because I couldn't find a job
 
ACuriousMind
ACuriousMind
@JohnRennie did you run out of shelf space or what's caused this sudden change of heart?
 
Slereah
Slereah
I hated those degrees
 
Azmuth
Azmuth
4:30 PM
@Slereah LMAO, same condition... jobs are very very difficult here
 
Slereah
Slereah
Hell part of why I originally went into theory is because I hated coding
 
John Rennie
John Rennie
@ACuriousMind I literally had no more room for any more laptops :-)
 
Slereah
Slereah
I didn't want to do more bloody simulation
 
Azmuth
Azmuth
I've started searching for unemployment schemes rather than a job
@JohnRennie If I were you, I'd have sold my whole house just to save few laptops for me
@Slereah Oh okay, yes, But, I like CODING!
 
Slereah
Slereah
Boo says I
 
Azmuth
Azmuth
4:32 PM
YOO says I
 
John Rennie
John Rennie
@Azmuth I tried to give them away to Indian students, but the Indian government prohibits imports of laptops even as gifts.
I managed to get one laptop through customs after two weeks of arguing with the customs office.
 
Azmuth
Azmuth
@JohnRennie Yes, all electronic items, you can although give them personally....
 
John Rennie
John Rennie
@Azmuth nope, even as gifts the customs block them.
The only way is to mislabel them as "books" or something that is allowed, but then if the customs spot tem they get seized and you lose the laptop.
 
Azmuth
Azmuth
@JohnRennie Ummm!? You can get your laptop inside, actually.
Why they would block tourists bringing Laptops and Phones?
 
John Rennie
John Rennie
If you visit India you're allowed to bring your own property with you, but even then if the customs suspect you're bringing stuff in to sell they will seize it.
 
Azmuth
Azmuth
4:36 PM
@JohnRennie oh okay... didn't know about it, actually
 
John Rennie
John Rennie
Trust me on this, I've been there :-(
 
Azmuth
Azmuth
haha :) yes, ofc :-)
 
Ankit
Ankit
@JohnRennie is there a reason why electrons with parallel spin experience lesser repulsion ?
I read this fact in my chemistry book and I don't think it's correct .
 
Slereah
Slereah
@Ankit Magnetic term, probably
Since it's the main one where the spin matters
 
Azmuth
Azmuth
@Slereah @skullpatrol @ACuriousMind @JohnRennie write something about me: secret20.com/1913029 :)
 
Ankit
Ankit
4:43 PM
@Slereah does a spinning electron produce electric current ?
@Slereah aslo can you please elaborate on your statements ?
 
John Rennie
John Rennie
@Ankit erm ... um ... is that due to the exchange interaction?
As I recall the maths behind it gets a bit scary.
 
Ankit
Ankit
@JohnRennie what is that ?
 
John Rennie
John Rennie
I bet Wikipedia has a page on it ...
 
Ankit
Ankit
@JohnRennie on what ?
 
John Rennie
John Rennie
It's why if you have a p³ configuration the spins align parallel.
e.g. in nitrogen, 1s² 2s² 2p³ the three p electrons have their spins parallel
Hund's rule? Something like that anyway.
 
Ankit
Ankit
4:50 PM
@JohnRennie I read the above written question under Hund's rule btw.
 
John Rennie
John Rennie
Aha! I cleared out my old laptops just in time. I have to have this!
 
skullpatrol
skullpatrol
niiice
 
Ankit
Ankit
@JohnRennie the mathematics is not clear to me as I am a high schooler . So can you give me the physical reasoning behind the fact taht electrons in two orbital with parallel spin experience lesser repulsion than if they had opposite spin . But why is this ?
Should I ask this question on the main site ?
 
Azmuth
Azmuth
@JohnRennie You wrote that? (I guess?)
or @skullpatrol ....?
 
John Rennie
John Rennie
@Azmuth yes
 
Azmuth
Azmuth
4:58 PM
@JohnRennie Yeah!!!! I knew it! :)_
 
John Rennie
John Rennie
@Ankit I'd have to go away and think about it.
 
skullpatrol
skullpatrol
cya
 
Charlie
Charlie
 
Ankit
Ankit
@Charlie what is exchange energy ?
 
Charlie
Charlie
5:13 PM
A quantum mechanical effect between identical particles
I wouldn't worry too much about it
 
Ankit
Ankit
@Charlie was that enough ?
 
Charlie
Charlie
Was what enough?
 
Ankit
Ankit
@Charlie your definition ?
 
Charlie
Charlie
My definition wasn't rigorous no
Don't worry too much about it, it's a bit complicated if you're only just being introduced to Hund's rule
 
Ankit
Ankit
@Charlie Yes I am knew to it but I really don't know why the repulsion is reduced ?
 
Charlie
Charlie
5:16 PM
Like I said don't worry about it for now, just accept that the partial filling minimises the energy
 
Ankit
Ankit
Sorry but it's aginst my rules :(.
Against*
@Charlie Okay let's move ahead.
Can you tell me why every overlapping of two atomic orbitals causes a bonding as well as a anti bonding orbital ?
Like why can't there be two bonding orbital ?
In place of an anti bonding orbital ?
@Charlie are you here ?
Can someone else help me here ?
 
Charlie
Charlie
The anti-bonding orbital is just the higher energy state of the two-electron molecular bond
 
Ankit
Ankit
Why is it higher ?
 
Charlie
Charlie
You will go endlessly down the rabbit hole if you always ask why in chemistry, your best bet is to either get comfortable accepting a lot of things on trust or pick up a quantum mechanic textbook, there is unfortunately precious little middleground
I'm not sure how to answer that
Hello qmechanic
 
Qmechanic
Qmechanic
5:33 PM
@Charlie : Hello.
 
Charlie
Charlie
:)
 
Azmuth
Azmuth
5:45 PM
@JohnRennie Hey, I've checked now, you can import laptops and phones to India... infact, my laptop was purchased from USA. But, there is a separate Visa and process for that.
 
JMac
JMac
@Azmuth There might be a difference between buying a laptop for import and trying to gift one. There's probably some sort of customs or something when you buy it internationally and ship it to India; but if you gift one they might think you actually bought it and are trying to get around any customs fees.
 
Kenzo Tenma
Kenzo Tenma
Gift Tax Act, 1958
Gift Tax Act, 1958 was an Act of the Parliament of India which was enacted to impose tax on the act of giving or receiving gifts under certain circumstances as specified by the Act. == Charges of Gift Tax == As per the Gift Tax Act, the gifts that exchanged hands post 1 April 1958 were subject to taxes as mention in Schedule I. Post 1 April 1987, this was amended to the rate of 30% (Thirty percent) and included gifts that were made in that particular Assessment Year. On 1 October 1998 however, the applicability of Gift Tax ceased to exist.As per the Gift Tax Act 1958, gift (in the form of cash...
 
 
5 hours later…
Stupid question inc
Stupid question inc
10:37 PM
wait wut
The gravitational force exerted by a symmetric sphere of mass M on a particle external to itself is exactly the same as if the sphere were replaced by a particle of mass M located at the centre.
how is this true lol
non uniform density
so by symmetric sphere does it means each layer is uniform or doesn't necessarily need to be uniform?
looks like everyone is offline
 
JMac
JMac
10:57 PM
@Stupidquestioninc I'm quite sure spherically symmetric includes that the layers have to have equal density. So density can vary with radius, but the entire shell at the same radius would need to have the same density for it to be really symmetric.
 

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