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00:24
@SillyGoose there are some operations that you usually do in wavefunction form, that are horrendously complicated in density operator form. That makes it inappropriate for introductory texts except as a thing to show
Hm true i am a finite dimensionally biased individual
merowing!
meow!
00:46
m e o w ~
Hummmm
so for a mixed state
each of the pure states that comprise it will evolve independently? as opposed to a superposition where the eigenstates evolve together
@Allie I dont think this is correct. A superposition will have each energy eigenstate evolve independently too.
01:13
yeah, its probably not correct
in fact im inclined to say its not
let me back up for a second, maybe this question will enlighten me a little more: what stops us from writing the mixed state as a superposition of the pure states?
oh, i found a good answer on SE that answers this question
MEOW!!!
01:30
M E O W ~
ooh
natural spin orbitals
i like this
02:04
i got carried away ~
 
4 hours later…
06:10
@Allie the set of density matrices is convex. Roughly speaking, convex sets have so called extreme elements, which cannot be written as (non-trivial) convex combinations. One can quite easily show that the extreme elements of the set of density matrices are exactly the pure states
oh, sorry, I've misinterpreted the question. just forget it
 
1 hour later…
07:33
@Relativisticcucumber ah yeah it really was! I play with my partner and he carried the game xD we don't do worldle or travle every day, but we usually do whentaken hehe
07:50
hi
08:14
what is the definition of physics according to you? is it just about predictions
if a fortune teller could predict all observations in your life and wire it into ur memory, would that be a substitute for physics??
Carroll says that the most trivial physics theory would simply be the set of all observations
 
2 hours later…
09:46
@TobiasFünke Sie belieben wohl zu scherzen, Herr Feinmann!
You style yourself after Feynmann, @HerrFeinmann, but are you even capable of playing mediocre bongo
@HerrFeinmann hehe
morning
10:40
AI generated nlab pages
2
@Slereah I'm the wish version
similar to this?
@Slereah average accuracy of anecdotes about Einstein
I'm starting to think about the hBar online meeting :P
So, I think that overall Discord would work better than any other option for privacy-related stuff
2
Although stuff like Zoom/Google is fine if you have an account which does not display your personal info
And probably more accessible
 
2 hours later…
12:39
i found something odd
the pdf article and the html don't match, is this common? the pdf had some numeric mistakes in the derivation of formulas, but some are corrected in the html
the article is this one: sciencedirect.com/science/article/pii/S221302091630163X, formula 5 in pdf vs html is different
not planning to join a meeting but there is FOSS that can be used as well, such as standard IRC-channel apps
@misternobody they look the same to me?
don't you have 10.15 vs 273.15 @qwerty?
@Slereah is this math meaningless?
@misternobody ah that's equation (4), not (5)
oh, sorry
there is another value I can't get myself that is in both which is 0.1607
I think the value is 0.189, would it be possible for you to check it?
12:53
the top of the page says > Received 20 February 2016, Revised 13 June 2016, Accepted 14 June 2016, Available online 4 July 2016, Version of Record 20 September 2016.

whereas the pdf just says > Available online 4 July 2016
this seems pretty dodgy
I wonder why the PDF and HTML don't match; the reference is the same
i meant that i expected this to be re-rendered, but i guess this would be expensive
i dont think it's expensive. just seems dodgy
yeah, i informed the author about it earlier, hopefully he complains
13:52
"Folks frequently ask “What are the books that changed your life?” If I tell them, they are usually radically disappointed. I find that curious.

I just cleared out of an office, and these are 4 shelves of spines of books that mattered enough to me to bring home. So here they are:"
Seems like a very weird choice to keep
Why would he choose to keep 'A Guide to LaTeX' over something else
bc hes an idiot
14:08
Jan 5 at 1:48, by ACuriousMind
Please phrase criticism or opinions a little more constructively/objectively/less insultingly :P
@DIRAC1930 it's an unusual choice but I'm not sure why it's an object of discussion, it's not uncommon people are attached to seemingly random books for sentimental/personal reasons
Yes you're probably right
I always get him mixed with Weisstein
@qwerty exactly
@qwerty Weisstein is the developer of Wolfram MathWorld right?
@SineoftheTime yeap
14:19
University is a magical time mostly for everything outside of your studies
What are some non physics things u have on ur learning list
it need not be immediate learning list
more like long term (20 years) learning list
Most of the other stuff I like to do is mostly in the arts, or building things
analogue photography maybe. it seems really hard
Wikipedia's illustration of the spectrum of $\mathbb{Z}$
maybe he forgets the syntax of latex, using it every few months or so..
this happens frequently to programmers, and otherwise I guess..
14:32
Why does analogue photography seem hard?
intro to linear algebra seems more worrying in his case, i guess.. (though it's a good author, Strang); pretty wacky bookshelf
good art and technical art is always hard :p but yes it seems difficult to get things like lighting and exposure right
Always meter for the shadows when it comes to film
Unless you are going for a very specific effect with black and white film
@DIRAC1930 oh. i would like to do art too. maybe painting. i don't know how to build gadgets. it seems really hard
@qwerty interesting
i want to learn some neuroscience and philosophy of science maybe
14:47
Sounds interesting
i really want to know every science except for some uninteresting ones
Never gonna make it smh
How will you know science if you don't know rheology or vulcanology or metrology
eww
detective science and psychology should be interesting too
but maybe not metrology
Metrology is super interesting
it reminds me of length measurements
14:52
Also geodesy
Very importany
@DIRAC1930 what are some interesting topics in it
@Slereah it doesn't look too interesting tbh
Just measuring stuff to within a micron or so feels like a superpower
@DIRAC1930 maybe I would be interested in the techniques they use
There are certain physics apparatus that would be fun to build
E.g. a cloud chamber
Achim Leistner is an Australian optician of German origin. During his retirement, he was asked to join the Avogadro project to craft a silicon sphere with high smoothness. Leistner studied optics at Optik Carl Zeiss in Jena, Germany, and in 1953 qualified as a precision optical craftsman. He moved to Australia in 1957, and worked in CSIRO on optical fabrication methods. In addition to precision instruments, Leistner uses his hands to feel for irregularities in the roundness of the sphere. The research team has called his extraordinary sense of touch "atomic feeling". As a result the sphere is the...
14:56
there are even people who grind their own lenses to make telescopes
> In addition to precision instruments, Leistner uses his hands to feel for irregularities in the roundness of the sphere. The research team has called his extraordinary sense of touch "atomic feeling". As a result the sphere is the roundest man-made object ever. If it were scaled to the size of the Earth, it would have a high point of only 2.4 m (7 ft 10 in) above "sea level".
There was a veritasium video on that ages ago I think
@DIRAC1930 oh
He loves the sphere
14:58
i haven't irl experiments in long. i mostly just learn theory
except for basic experiments like dropping objects
it would be cool to do a chemistry experiment and simultaneously know how it works
Then get a chemistry degree…
like the Action lab channel
but chemistry would take too long to learn. they have too many chemicals
Lol
15:00
@RyderRude Lol
It took me 2 years to start doing organic synthesis
similarly for biology. too many beables in their science
i like that physics has fewer beables. just four-five fields and one equation
@Allie oh
@Allie organic compounds are a small part of chemistry, i think
Yeah, no theyre a huge part of chemistry
So much on earth is organic!!!
Including you
15:03
maybe i should learn organic compounds then. then I would be able to understand some chemicals
@Allie yes
I loved learning organic chem it was cool
But i think you need a solid foundation in gen chem if you really want to understand whats going on
Theres a LOT of discussion of orbitals, lewis structures, resonance, electronegativity, ionic/covalent bonds, etc etc
are u able to look at a chemical reaction and really understand its mechanics and even predict what's going to happen before u mix unknown chemicals
Well yes
To a certain extent
really cool
@Allie it would be nice to have a surface level knowledge of chemistry
I know that sodium hydroxide will turn an ester into a carboxylic acid by addition-elimination at the carbonyl :P
Etc etc
15:06
i am aiming to learn so many sciences. maybe i should go for surface level knowledge
Mmm
I would say against that
Dont you really want to dive into something?
It is good to have knowledge in many areas
i want to dive into physics, philosophy, neuroscience and some math maybe
I want to learn more math
My current work is heavily physics based but my chemistry background gives me a good foundation, and i was a biochem major for a short period so I have some understanding of how that all comes together
but not dive into neuroscience. i think i only want a small understanding of it
15:08
But also, jack of all trades master of none something something
@Allie nice
i know some physics and math rn, but it doesn't feel too satisfactory tbh
I feel that I've lost interest in physics
would also like to learn psychology and detective science
@DIRAC1930 oh
@DIRAC1930 i am still interested in the measurement problem and quantum gravity
Lol
let me check if detective science is even a field :P
15:11
Forensic?
Lol
yes. there is. Wiki on Forensic science
Isn't that the degree you can get in the back of a comic book
Lol
When i think forensic psychology i always think of those annoying “body language” youtube channels
And lie detector tests
It is the police, they will just blame the nearest minority
Not the most subtle science
@RyderRude Yes these are interesting but making progress in any of these is essentially impossible
15:15
nobody ever wants to make breakthrough in geodesy
Time to read :3333
@DIRAC1930 yeah... from a researcher perspective, it is hard to make progress.
I heavily prefer reading a physical textbook to a pdf
@Allie I used to be this way but now I prefer a pdf
@Allie i was thinking more like Sherlock techniques
15:17
Pdf hurts my eyes
maybe they teach that somewhere
Isnt sherlock a fictional character
yes. a less exaggerated version of those techniques
Yall r funny lul
The fictional Sherlock expertises in chemistry and logic and paying attention
15:19
@Slereah seems to be resembling Mathgen.
it's a pretty good likeness of nlab articles
I cant pay attention, so yeah sounds fictional
Hi User
Why not aim to be Batman while you're at it
He is also the world's greatest detective
and can do karate
Lol
@Slereah he is making crystal balls to look into future.
@Allie hey
@Slereah yes
15:21
Does anyone like the tv show Sherlock
I like cats….
@Slereah and sleeps with power naps only.
Such lovely creatures
@Allie 🥺😺
@DIRAC1930 No
Shameful waste
15:22
My cat is currently scent marking my phone
I guess that includes all of you
You are his now.
Typical show made by someone who does not like the source material
@Slereah lol
@DIRAC1930 i like it
So many new shows are also ruined by executives who have no respect for art
Ive never watched sherlock, just speaking in general
15:25
Turn every show into character drama swill
@Allie Benedict Cumberbatch's Sherlock?
I heard he does mansplain too much.
Also the first episode is an adaptation of "A Study in Scarlet", but it is missing the most important part (half the book being dedicated to slandering the Mormons)
What is an example of an unprovable statement I. Physics? Like I assume the axioms of qm and according to godel there should exist statements which I cannot prove to be either true or false?
*in physics
@MoreAnonymous There's a bunch of problems known to be equivalent to the Halting problem and therefore unsolvable, you can try to check out which one could conceivably have physical interpretation :p
Mew
15:33
An example would be that some versions of quantum gravity have a sum over all spacetime manifolds, but we know that the classification of 4-manifolds is equivalent to the halting problem
In computability theory, an undecidable problem is a decision problem for which an effective method (algorithm) to derive the correct answer does not exist. More formally, an undecidable problem is a problem whose language is not a recursive set; see the article Decidable language. There are uncountably many undecidable problems, so the list below is necessarily incomplete. Though undecidable languages are not recursive languages, they may be subsets of Turing recognizable languages: i.e., such undecidable languages may be recursively enumerable. Many, if not most, undecidable problems in mathematics...
it's pretty rare that undecidable problems are relevant for physics though
They tend to be problems involving very general objects
^there's one
Hmm .. I see
15:48
So in Bragg diffraction when we talk about the waves of, say, an electron, constructively or destructively interfering,are we talking about the wave function interfering with itself?
like, the superposition of reflections from different planes will either constructively or destructively interfere
16:13
there is a famous problem about the spectral gap, proven in 2015 to be undecidable
hiya tobias
hey allie-gator ;)
im so cold
 
1 hour later…
17:52
@Allie yes
I can't believe that SC books manage to disagree even about something as simple as a DoS
or get the same results with high $\omega_{\text{hand}}$ having
what do you mean precisely? :d
18:09
@naturallyInconsistent AWESOME!
yayyyyyyyyyy
finally i said something correct
yay
and good night
miao miao iz overdue sneeppuuu
oh okay god night
meow meow
There is a factor 2 (about which I don't even want to investigate, it's related to the particle hole spectrum), that some feature and some do not. Tinkham derives the DoS in a way you see all over the literature but I find it not motivated enough.
I would use the definition of DoS. Carsten Timm does, and gets a factor 2 which he motivates, although the connection with the other approach is not clear, but mathematically it comes from a rather questionable choice of the domain of integration ($\epsilon_k-\mu\geq-mu$). Some other people get something slightly different. Other people just copy each other word by word AAAAAAH
And this happens over every little thing :P
meowing
One thing I find frustrating compared to HEP is that, even if in HEP too I find different (not so obviously compatible) ways to handle things, at least basic results and approximations are the same
I think this level of frustration is only comparable to when I read Hawking's papers about the thermal emission of BH!
18:15
good night, allie
@HerrFeinmann you sure it is no unit or spin stuff?
aww you too? :(
ah no :d sorry, I misread
good night, nI :)
im gonna be reading :3
guys how do you not strain your neck when working on problems in a notebook
I feel your pain, Herr Feinmann :/
haha idk
should i like get something that will slant the noteboook towards me?
like a podium does
18:27
In the case of Carsten Timm, he explicitly says it's different from Tinkham's. Both procedures appear rather questionable to me :P
Probably it's like the magnetic fields, all about conventions, but I swear, it's so frustrating AAAAAH
@Allie I have two screens so I can make sure to strain it more symmetrically
hehe
@Allie How do you like P&Y so far? :)
i like it so far, only on the second chapter rn
but i got an introduction to density matrices and reduced density matrices so thats nice
ill return to it later tonight, i try to spread out what im studying cuz i figure theres only so much your brain can process in one day
currently doing electromagnetism, then ill return to solid state for a bit, then ill work on my research and then ill read p&y :)
 
2 hours later…
20:39
@ACuriousMind I see the distinction. Thx for clarifying it
20:52
Regarding Bhabha scattering. In the case where $\sqrt{s}>>m_e$ one can find an expression for the differential cross-section:
$\frac{d\sigma}{d\Omega}\propto 1\sin^4(\frac \theta 2)$ where $q^2_\gamma=-s\sin^2(\frac \theta 2)$, where $q_\gamma$ is the momentum of the virtual photon. While mathematically it is evident that the smaller the angle,the smaller the sinus function and as a result the larger the differential cross section, from a observational standpoint how does that makes sense?
For example when rutherford performed his experiment, he noticed that some particles would deflect from their trajectory a bit, some not at all and some would just return back 180
and he argued that those who went staight on, they didn't collide with anything
while those that deflected or went back, they collided with something.
So back to the Bhabha scattering, how, small scattering angles imply large differetial cross section, hence large cross section, which is a measurement of the probability of some event taking place when scattering occurs
Did anyone read " lifelong kindergarten "? By mitchel resnick, the head of mit media lab
It's a beautiful (imho) book about learning
21:15
nope, but thanks for the suggestion
21:32
Welcome :-) another related one is mindstorms by papert
Mitchel group created scratch, based off turtle by papert s group
21:43
What does it mean that when the de broglie wavelength is large, we are in the classical limit ?
21:56
@imbAF you probably mean small
No large
@imbAF Large De Broglie wavelenght is very quantum
Large would mean small energy. I mean larger object do not exhibit quantum behavior because the have large de broglie wavelength
22:12
No idea what that means but m.youtube.com/watch?v=cYyFPFU6s_A may be useful
Calculates electron vs ball, former having larger DB wavelength
@imbAF you are mistaken, it's the opposite
The quantum effects become relevant when the wavelength is comparable to the "dimensions" of the object
I thought so and I checked the notes, and unless the lecturer is wrong, that is what it is said
May I see?
@HerrFeinmann Then, when the wavelength becomes larger than the dimensions of the electron
that should be the case
@HerrFeinmann Sure, I can write down what was written
Yes, so we figure out if there is a misunderstanding
22:16
Above I made a description of how Bhabha scattering and it's cross section etc was presented to as
After writing down the formula for the differential cross section we make the following remarks
In any case, the wave properties (quantum properties) become relevant with large wavelength and as the wavelength shrinks, you go to the classical limit. After all, think about a classical object $\lambda=h/p$, take any macroscopic sensible momentum
$h$ is so small that this is immeasurably small
you are right about the way you argue
But i will type down what we were told
For another way to see this: you may consider the thermal wavelength. The context is different, but you may know that: low temperature-->quantum, high temperature---> classical (well, roughly). $\lambda_{thermal}~T^{-3/2}$, so for high temperatures it goes to zero (and as I said, high temperature corresponds to classical)
I will wait
large contirbutions for $\theta \propto 0 \rightarrow q^2_\gamma $ is small $\rightarrow$ de broglie wavelength $\lambda_\gamma$ large $\rightarrow$ classical limit $\rightarrow$ electrodynamics is correct $\rightarrow$ determine luminosity ffrom Bhabha scattering for $\theta \approx 0$
1. How does (intuinively) it make sense that $\theta \aprox 0$ implies large differential cross section.
2. Understand the train of thoughts from "de Broglie $\lambda$ large... $\rightarrow$ luminosity and bhabha scattering
@HerrFeinmann I am familiar
1. What do you mean? Don't you have $\sin(\theta/2)$ terms in the denominator?
I'm very confused by this classical limit thing here
22:27
Mathematically as I describe above
is pretty clear
The issue is from an observational standpoint
Wouldn't you agree that the larger the area of effect or interaction
the larger the (differential) cross section
Also because, you consider DB wavelength for matter particles, not photons
I mean, I usually just call it wavelength
Can we begin with 1
If you have an incoming particle that goes into a black box and comes out propagating in the same direction as it came in
wouldn't you argue that no scattering/interacion/collision took place ?
Had there been such an event, there would have been a deviation of the particle from its initial path

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