The h Bar

Sir Cumference

12:59 AM

@Azmuth It's extinct in the wild :P

not completely extinct

Terran Swett

2:44 AM

Somebody posted the above as a question here: physics.stackexchange.com/questions/581423/…

I see two possibilities. The first is that this person has decided to write some avant-garde physics-themed poetry and post it as a question.

The second is that this person does not speak English and used Google Translate to translate from their native language to English, but mistakenly inserted line breaks in the original, and Google Translate translated each line separately.

I think you have to admit that the result is quite beautiful in its way.

Azmuth

3:27 AM

@BioPhysicist Nah,,, How;s it?

@SirCumference Oh okay, then it's good, we can revive them...

@Slereah So you are Kitty Meeks ?

ACuriousMind

7:20 AM

@TerranSwett We do need more physics poets! Even if it's just Google's translation AI... :P

"A slippery horizontal plane. / It moves from silence, / And in five seconds it was moving away 250 ft." sounds extremely sad to me and I can't quite pin down why

Slereah

8:09 AM

@ACuriousMind you could buy that GR poetry book

Also of course, Tonnelat did GR poems

ACuriousMind

I don't speak French so I can't judge that :P

it certainly looks like poetry

Slereah

well it's song parodies

but close enough

Superfast Jellyfish

8:25 AM

If we’re accepting approximate poetry, Morin has quite a few limericks physics.harvard.edu/undergrad/limericks

2

Azmuth

9:48 AM

Hey all kids!

See this!

twitter.com/abhas_rewcie/status/1308702884410863621?s=20

Official Kevin Pietersen liked my post!

Biggest Twitter achievement soo far!

shelton Benjamin

Which computer software i can use to experimentally verify variation of z factor with increase in pressure of real and ideal gas?

Mayank

@Azmuth Serously

Which post?

Azmuth

@Mayank click the link and check it!

He's still my favorite Batsman!

Mayank

Can you share the post link?

Azmuth

twitter.com/abhas_rewcie/status/1308696705416810496?s=20

Mayank

9:52 AM

Oh got it

Azmuth

His Hindi is also sooo good. He shot a Nat Geo Documentary in Hindi....

Mayank

I can't believe it.

Azmuth

Yep. When he was a kid, he came to India just to learn Hindi... He was also selected to do Hindi commentary in IPL. Probably later matches....

bolbteppa

10:13 AM

[Inception "pre-kick" song] Non, Je Ne Regrette Rien - Edith Piaf (French & English lyrics)

►

That's how I read those lyrics

Slereah

10:42 AM

I'm afraid it's not the one french song people know, no

Faded Giant

@sheltonBenjamin What do you mean, "experimentally"?

REFPROP can give you the compressibility factor for various real gases.

bolbteppa

11:08 AM

I know my French music

Georges Brassens - Je me suis fait tout petit

►

skullpatrol

12:23 PM

Eric Mazur's Millikan Award Lecture at the AAPT Meeting in Edmonton

►

JingleBells

2:53 PM

How did Elon Musk figure out people need electric cars?

ACuriousMind

what

Charlie

not sure what that means

ACuriousMind

Elon Musk is not the inventor of the electric car!

Charlie

the demand was for cars with less environmental impact, not specifically electric ones

JingleBells

I mean, why did he decide to put his money into electric cars. As far as I know, this wasn't a successful thing before.

ACuriousMind

2:57 PM

First two paragraphs of the Wikipedia article on electric cars:

> An electric car is an automobile that is propelled by one or more electric motors, using energy stored in rechargeable batteries. The first practical electric cars were produced in the 1880s.[1] Electric cars were popular in the late 19th century and early 20th century, until advances in internal combustion engines, electric starters in particular, and mass production of cheaper petrol (gasoline) and diesel vehicles led to a decline in the use of electric drive vehicles.

From 2008, a renaissance in electric vehicle manufacturing occurred due to advances in batteries, and the desire to re…

Charlie

Did he not begin developing better batteries first? Before cars

ACuriousMind

the lithium ion battery is not a Tesla invention, and the same article points out the first modern electric car in series production was a Mitsubishi

JMac

I think Tesla was started as a car company. I'm not sure if designing the batteries was always part of their plan; but I'm pretty sure the company always intended to make cars

JingleBells

Why did he decide to put his money there?

JMac

BRB installing my mindreader.

Charlie

3:00 PM

lol

Is Tesla not ultimately a source of funding for SpaceX? I was always under the impression that Elon's main project was mars

ACuriousMind

1899 had a very different aesthetic for cars :P

JMac

@Charlie I just looked it up and learned this, apparently SpaceX was made a year before Tesla was founded...

Charlie

Yeah, I think that's also the purpose of StarLink, which I'm very much looking forward to (if it lives up to the hype), same as paypal, just a way to pour money into SpaceX

Sir Cumference

Huh, turns out Python can cross match two 300 line lists in a flash

I was worried doing something that's like O(90000) or whatever would take an eternity and tried to optimize it before even testing it

ACuriousMind

3:15 PM

300 lines isn't that much even with the most poorly optimized algorithm on a modern computer

also, O(90000) is not how big-O notation works :P

that it went quick tells you nothing about the complexity of your algorithm - complexity is about how it scales when the input size gets larger/smaller

Mayank

@ACuriousMind Can you migrate a question to MSE?

ACuriousMind

@Mayank I can, but please just flag the question you think should be migrated instead of asking here

there's an explicit off-topic flag for questions users think should go to math.SE

Superfast Jellyfish

@ACuriousMind to be honest, I quite like this design :P

Ah. So this where my newfound usage of “:P” comes from! For the past week or so I have been (over)using that emoticon. Now I see where I picked that up from

ACuriousMind

:P

Mayank

@ACuriousMind This one “ in need of moderator intervention”?

Charlie

3:28 PM

:P is definitely the most versatile text face

ACuriousMind

@Mayank no, it's a close flag - under "community-specific reasons": "This question belongs on another site in the Stack Exchange network"

Mayank

3:41 PM

Sorry, I am new. I don't know where close flag option is.

ACuriousMind

oh, you probably don't have enough reputation to do that, sorry

Mayank

Can't I simply delete it and post it on MSE?

ACuriousMind

oh, it's your question? Then sure

Mayank

@ACuriousMind Thanks :P

BioPhysicist

Is there any sort of action that is taken against users who repeatedly post solutions to homework questions in the answers / comments?

ACuriousMind

3:50 PM

@BioPhysicist if we notice it (either on our own or because they get flagged for it) they can be sent a mod message reminding them not to do that. I don't know if we ever suspended someone over it off the top of my head, but at least that's not standard procedure

rob

@BioPhysicist Answer: yes, but out of respect we try to keep details of such actions out of the public view.

BioPhysicist

For sure, I don't think it needs to be visible

Alan Abraham

4:05 PM

I am taking AP Research this year, and the main thing we have to do is conduct a research study over a topic of our choice and write a (~4000 word) paper on it. I want to do a physics topic because I have some experience in competition physics and I think I would enjoy doing it.

However, I have no actual experience in physics research and I don't think I can do anything that would require assistance from University-level equipment because of the current pandemic. I need some help brainstorming, so any assistance will be appreciated.

JMac

4:22 PM

@BioPhysicist But if it's not visible, how can you tell if you're helping prop up the mainstream dictatorship?

Sir Cumference

@ACuriousMind yeah i expected i was gonna mess that up lol

been a few years since i used big O

@ACuriousMind i assumed doing a linear search for each member of one list to see if it's in the other list would be something like 300^2 computations

ACuriousMind

@SirCumference sure, it is, I stand by "that's nothing on a modern computer" ;)

@SirCumference I see you're not in the pocket of big O.

Sir Cumference

@ACuriousMind true, but my "modern computer" takes 15 seconds just to finish a basic mathematica computation :P

19 hours ago, by Sir Cumference

I'm prob stupid but does anyone understand why Mathematica is giving three surfaces?

ACuriousMind

symbolic computation is very hard for computers

all they can do is add numbers and compare bits :P

Sir Cumference

yeah, i guess i'm not too experienced in what's fast vs. slow for computers

i was genuinely shocked when bash sorted a file of 130,000 lines for me in an instant. expected my computer to crash

Jack Rod

4:37 PM

Can anybody help in why I am not able to access pse website using my pc and i am able to acess the same website using my phone

ACuriousMind

Algorithms scaling quadratically - like your linear list search - can be the right choice if you know the input set is small enough your machine can handle it. For example, the naive multiplication of matrices is O(n^3), but there are specialized algorithms that are something like O(n^2.6). Unfortunately, the constant and linear factors in front of that n^2.6 that big-O omits are so large this becomes only actually better than the naive computation at truly gigantic n

Jack Rod

All the websites which I use regularly are out of the reach now

I am confuse can anybody help in this please

ACuriousMind

@JackRod "I am not able to access pse website" is not enough information for anyone to help you. How are you trying to access it? What error are you getting? Do other websites work?

Jack Rod

Using Chrome or any other browser

ACuriousMind

@SirCumference we didn't invent quicksort for nothing!

Jack Rod

4:40 PM

It is saying ip address could not be found

Site cannot reached

ACuriousMind

...that would indicate your computer has no internet connection :P

Jack Rod

It has I can access YouTube

And Google

Not specific sites which I use on regular basis

Including twitter

ACuriousMind

try clearing the dns cache, cf. superuser.com/q/203674

Sir Cumference

@ACuriousMind it's rare to find such convenient names as "quicksort"

Jack Rod

it is not responding

Sir Cumference

4:45 PM

@JackRod maybe try a proxy or vpn (assuming they're legal in your country)?

Jack Rod

like I opened the site

oh wait

Yashas

How to compute $\left[\hat{U}, x]\right]$? where [] denote commutator

Jack Rod

i just disconnected my lan from computer

Sir Cumference

@Yashas $\hat{U} \hat{x} - \hat{x} \hat{U}$

Jack Rod

now they are running

@ACuriousMind @SirCumference

Yashas

4:48 PM

@SirCumference yeah but I don't know what to do next

Sir Cumference

idk what's your goal

Yashas

do I expand U as a series?

I need to arrive at something like $i\hbar \over m$ or something similar

ACuriousMind

what is $U$? A random operator? A function of $x$? Of $p$? Of both?

Jack Rod

what has happened I am confuse?

Sir Cumference

@ACuriousMind probably time development

Yashas

4:49 PM

@ACuriousMind it's the time evolution operator

ACuriousMind

@Yashas So do you want to do this for a specific $U$? Do you know the Hamiltonian?

Yashas

and $H = \frac{\hat{p}^2}{2m}$ (sorry, I forgot to tell this)

Jack Rod

like if i connect my dongle or lan to the computer the sites are inaccessible

JMac

@JackRod So your wifi works but your ethernet connection is blocking them?

ACuriousMind

@JackRod is this 'lan' your personal network or a public/employer/school whatever? Some will block access to certain sites (though why they'd block SE but not YT eludes me...)

Yashas

4:51 PM

I think it's possible if I expand U but that's a lot of work. Is there a way I can avoid lot of math?

ACuriousMind

@Yashas What are you trying to do then - that commutator is zero without any further computation!

Jack Rod

actually, I have a wifi dongle which I can connect using my

dongle

Sir Cumference

@ACuriousMind i remember my high school blocked the stackexchange imgur domain

i just assume they don't really know what they're doing

ACuriousMind

@SirCumference ah, they probably blocked *.imgur.com because they've heard it contains these "memes" young people like :P

Jack Rod

no, I am connected to the same network using wifi

earlier I was connected using the cable

Yashas

4:53 PM

@ACuriousMind how? It's not obvious.

ACuriousMind

@Yashas What is $U$ in terms of $H$?

(and sorry, it's not zero, forget that)

Yashas

@ACuriousMind $\hat{U} = e^{-\frac{i\hat{H}t}{\hbar}}$

ACuriousMind

@Yashas right, so since $H$ is a function of $p$, $U$ is a function of $p$, too.

do you know how what $[f(p),x]$ is generically?

Yashas

@ACuriousMind No, if that has a name, I will look it up and learn.

ACuriousMind

I don't think it has a name

Sir Cumference

4:55 PM

@Yashas usually with these kinds of things you're gonna have to series expand and use ~~bad~~ math to get the form you want

ACuriousMind

you assume $f$ is a series in $p$ and then do it order by order

Jack Rod

It seems funny but I do not know what is happening the same wifi allow me to use that website and when I connect it using the lan it does not allow me

ACuriousMind

then you stare at the resulting series long enough to realise it's the series of $f'$ :P

JMac

@JackRod I think the top suggestion would be worth checking. forums.tomshardware.com/threads/…

Sir Cumference

i do not miss those problems in qm

the only worse notation than the ones used in QM are the ones I'm seeing in my GR class

i dunno how Einstein summation notation is supposed to make things easier for anyone

ACuriousMind

4:58 PM

@SirCumference What?! Have you tried writing out all the summation signs? It's terrible.

Jack Rod

@@JMac thanks let me try

ACuriousMind

you'll waste half or more of each line just writing a parade of $\Sigma$s

Sir Cumference

@ACuriousMind i'm prob just not used to it, so i'm getting tripped up in trying to understand a lot of formulae

Slereah

just don't use a basis :p

Sir Cumference

doesn't help when the book skips most derivations for some reason

Slereah

4:59 PM

$$\mathrm{Tr}(Z \mapsto (\nabla_X \nabla_Z - \nabla_Z \nabla_X - \nabla_{[X, Z]}) Y) - \frac{1}{2}g(X,Y) \mathrm{Tr}(\mathrm{Tr}(Z \mapsto (\nabla_W \nabla_Z - \nabla_Z \nabla_W - \nabla_{[W, Z]}) W)) = T(X,Y)$$

The EFE without a basis

BioPhysicist

@JMac I just assume that I am always doing that

Sir Cumference

also using $d/dt$ for a convective derivative and using $\partial / \partial t$ for a Eulerian derivative is totally throwing off all the calculus i've used in the last 4 years

in my math classes it's so much simpler, just use $D$ for total derivatives and $D_x$ for partials

Charlie

@Slereah What is the $Z\mapsto[...]$ bit here?

Slereah

@Charlie To define a function

JMac

@BioPhysicist I find it a lot harder to redeem my points with the Secret Guild of Anti-Intellectualism and Mainstream Dictatorship when I don't have clear evidence. Maybe they're just more lenient with you.

Charlie

5:02 PM

oh wait the $Z$ the vector field in $\nabla_Z$?

Superfast Jellyfish

@Yashas it’s instructional to expand in terms of power series and compute the commutator.

Charlie

I've never seen the riemann tensor written like that

Superfast Jellyfish

@ACuriousMind I find writing $\Sigma$s oddly satisfying.

Yashas

Actually it was quite trivial to expand. It took just 30s to get the answer. I spent 15 minutes googling on how to avoid the expannsion. facepalm

Sir Cumference

@Yashas i spent 4 hours trying to optimize a computation i later realized my computer could do in 5 seconds. i feel ya :P

JingleBells

6:18 PM

In research, do people come up with new discoveries and ideas from the mathematics, or they first get the general idea and then write the maths down?

Sir Cumference

@JingleBells a combination of both

existing ideas in math can inspire new ideas

JingleBells

hmm, got it. I'm a bit confused about how it happens. Do you just sit and look at some formula that describes something and you suddenly decide to move the symbols around or?

How do researchers get ideas? Do they come up with the general idea first, and then write the mathematics for it, or do they somehow look at the mathematics and move symbols around?

Charlie

6:50 PM

The process of "getting ideas" varies hugely I imagine.

Sometimes a problem is solved just by brute force, staring at it until the solution happens to cross your mind. Sometimes an idea that someone else has can inspire a solution. Historically a lot of people have claimed to see solutions in dreams :P

Yashas

Is finding $\left[x(t), x(0)\right]$ same as $[U^*xU, x]$?

Charlie

what context is this in @Yashas?

Yashas

@Charlie This is a subproblem in a free particle problem I am solving. The star denotes conjugate transpose. $U$ is the time evolution operator and [] denotes commutator.

Charlie

ok

Should that read $[U^*x(0)U,x(0)]$?

Yashas

@Charlie yeah

Charlie

6:55 PM

and fwiw the symbol $^\dagger$ usually denotes the conjugate transpose in physics

If you're working in the Heisenberg picture and $x$ is the position operator? I suppose so

Yashas

@Charlie yes

Charlie

Then unless someone wants to correct me, I would imagine the answer is yes

Out of curiosity, is perturbation theory applied to operators other than the Hamiltonian ever useful?

Yashas

I reached here $\frac{t\hat{p}\hat{x}}{m} + \frac{t}{m}\hat{U}\hat{p}\hat{U}^\dagger\hat{x} + \frac{t}{m}\hat{U}\hat{x}\hat{p}\hat{U}^\dagger$ but I need to get $i\hbar t \over m$. Is further simplification possible here? I did cross-check once and everything seemed to be correct.

Charlie

Could you put that in double dollars so it's a bit bigger?

Yashas

$$\frac{t\hat{p}\hat{x}}{m} + \frac{t}{m}\hat{U}\hat{p}\hat{U}^\dagger\hat{x} + \frac{t}{m}\hat{U}\hat{x}\hat{p}\hat{U}^\dagger$$

Charlie

7:08 PM

You're definitely going to have to use the commutator $[\hat x,\hat p]=\mathrm i\hbar \Bbb{1}$

I don't see how to progress from where you are though

Maybe you have made a mistake somewhere, or I am missing something

Superfast Jellyfish

7:27 PM

@Yashas Do you know BCH identity?

Yashas

@SuperfastJellyfish No but I will learn.

Superfast Jellyfish

Try working out problem 2 from ocw.mit.edu/courses/physics/8-05-quantum-physics-ii-fall-2013/…

Yashas

7:49 PM

@SuperfastJellyfish Thank You. I solved it and it helped simplify my original problem really close to the expected answer. There are two extra terms which are very close and should be cancelling but this might be a typo somewhere (dimensionally incorrect too). I will retry tomorrow. Thanks again.

hmm, do operators have dimensions? Am I supposed to be worried about $\hat{x}\hat{p} - \hat{x}$?

I think they should because $\langle\psi|\hat{O}|\psi\rangle$ should have different dimensions depending on the observable. But then $i\hbar \frac{\partial}{\partial x}$ doesn't have any dimensions. I need to check if position wavefunctions have momentum dimensions by default (I think they don't; at least not in particle in a box if I remember correctly).

oops, the $\hbar$ has dimensions. I should sleep and think about this tomorrow I guess. It's 1:30 AM now and I have spent 8 hours straight solving problems :P

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