The h Bar

Emilio Pisanty

15:00

Unless someone with specific experience tells you otherwise, you're most likely to be better served by putting the focal plane at the middle of the crystal

Balarka Sen

I don't know if any of this points towards anything useful in analyzing our cognitive reasoning or just a fancy theoretical observation

But in general understanding the topology of a given set of data points is a classic problem

(There are applications in machine learning; one of the simplest thing is thumbprint recognition)

Anonymous

I have heard about the applications in machine learning

Anonymous

also data analysis

heather

@EmilioPisanty i just mean taking the original laser beam and collimating - i'm not doing it after the crystals or anything.

or by re-collimating do you mean something else?

vzn

@heather there are a lot of cheap laser pointers out now, can even buy em in toy stores. used a few myself once in experiment. they come with lenses. wonder if any might work with BBO. not sure what wavelengths they are but thats the main question. ps did you see my writeup of undergrad bell experiments? has 2 parts/ supply lists. vzn1.wordpress.com/2017/09/08/…

heather

15:09

@vzn yeah, I found a ~$40 one on digikey that is the appropriate wavelength/power/etc

vzn

@heather right, saw that, looks cool, but without lens, complicating it, therefore suggest at least looking into whether "off the shelf" laser pointers could possibly work with BBO... lasers tend to be made out of a "few" different sources...

heather

sure.

vzn

its up to you if you want to learn/ add optics for fun, its certainly worthwhile, but for me think it could be some complication & maybe simplification is possible...

Prathyush Poduval

@BalarkaSen did you see this?

EPIC SAX GUY IN GTA 5!

►

thoug the sax is misssing

Balarka Sen

yeah

Prathyush Poduval

15:13

reminds me of granday's darude sandstorm remix :P

Balarka Sen

gta5 is such a big meme

heather

well, i can tell you i'm already reading some about optics just to figure out what i'm doing.

Balarka Sen

grandayy is awesome

heather

i can't say i'm studying it very methodically though.

Prathyush Poduval

his soviet anthems....

are really patriotic

vzn

15:15

@heather where did you get the 405nm req from?

Balarka Sen

i think grandayy is a french guy

Prathyush Poduval

since you mentioned it, it does seem like it

the cartoon mascot looks very french

heather

@vzn 405 nm both seems to be fairly commonly used in spdc experiments and it's a common laser diode wavelength, meaning it will be cheaper. i think there were some other things that contributed to the choice, but i can't remember them from off the top of my head.

vzn

this wikipedia article on BBO seems to indicate the birefringence starts at ~409.6nm? is that right? en.m.wikipedia.org/wiki/Barium_borate

> Barium borate has strong negative uniaxial birefringence and can be phase-matched for type I (ooe) second-harmonic generation from 409.6 to 3500 nm.

Balarka Sen

speaking of USSR memes, you know the hard bass meme, surely? @Prathyush

0ßelö7

15:19

why has this chat become meme central

just curious

@BalarkaSen do you meme up the math chat?

Balarka Sen

no but a bunch of the math guys, including meme, have a discord server which is pretty much literally memes

including me lol

Freud on the loose there

0ßelö7

sigh

ok

vzn

@heather interesting, 405nm is "blue ray"/ violet laser pointer became available in ~2010. note a wide variety of wavelengths are available now. en.wikipedia.org/wiki/Laser_pointer

heather

@vzn not sure - i'll have to look at it

arxiv.org/pdf/1001.4182.pdf

Brogrammer

What will happen to an object moving straight up at less than escape speed?

nasil

15:33

it wont escape

Brogrammer

thats what i thought but i thought it would also

deaccelerate

nasil

yes

Brogrammer

the object wont be able to escape and start going in the other direction

nasil

absolutely

Brogrammer

but if moving at more than escape speed

then it'll just escape?

nasil

15:35

yes

does anyone want to play chess

lichess.org/u1Nh3Cds

just click the link

heather

@vzn the paper i posted above seems to be an experiment using 405nm light with BBO crystals for type I spdc

see for example fig 1b, pg 5

ah @vzn i think i figured out the difference.

> Barium borate has strong negative uniaxial birefringence and can be phase-matched for type I (ooe) second-harmonic generation from 409.6 to 3500 nm

(bolding mine)

second-harmonic generation and spdc are two different processes.

so i think i'm good with a 405 nm laser.

vzn

@heather some very nice/ high finesse optimization/ finetuning/ efficiency work but find it hard to follow, all the terminology without much diagrams. they are excited about BiBO.

heather

i have to go for now

vzn

> Nevertheless, while biaxial crystals such as BiBO may appear to be a better source material, their lack of the rotational symmetry present in traditionally used uniaxial crystals can significantly complicate the compensation techniques presented here...

@heather not an expert on this but iiuc the 1st harmonic & 2nd harmonic are simply referring to the 2 outgoing rays, ie the birefringence.

Physics Meta

15:52

2

Q: Problems with tag notes

On the physics tags page, the tags mathematical-physics and mathematics are described as "Just because your question involves math!..." whereas on the actual pages for the tags, the descriptions of both begin, "DO NOT USE THIS TAG just because your question involves math!" I would have to ass...

Brogrammer

By what factor would a person’s weight be increased if Earth had 10 times its present mass, but the same volume?

John Rennie

@Brogrammer you should be able to answer that. What is the equation for the gravitational force? Hint: Newton's law of gravity

Brogrammer

F=gm1m2/r^2

i meant big G

John Rennie

Correct!

Brogrammer

why is the volume mentioned tho

anyways

John Rennie

15:57

If the volume stays the same that must mean the radius of the Earth stays the same ...

Brogrammer

o

so is it 10 times as much then

ACuriousMind

@vzn No, they don't

John Rennie

@Brogrammer Yes. It is as simple as that.

Brogrammer

i wasnt sure if that was a trick question

but apparently its that simple

vzn

@ACuriousMind ok afraid of that. confused because wikipedia article mentioned birefringence/ 2nd harmonic generation in same sentence :| ... so (n00b question) in SPDC are both signal/ idler photons same wavelength?

Brogrammer

16:03

In 1996, astronomers discovered an icy object beyond Pluto that was given the
Designation 1996 TL 66. It has a semimajor axis of 84 AU. What is its orbital period according to Kepler’s third law?

kepler's third law ik says

the square of a planet orbital period is directly proportional to the cube of the average distance of the planet from the sun. Based on the rule, we can say the orbital period

well idk what 84 au is

and how i can apply to the formula

but it says semimajor axis

so im assuming thats the radius

vzn

(seems like wikipedia could be more clear on that) en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion#

0ßelö7

@JohnRennie can you recommend good value/money PC speakers?

John Rennie

It depends what you want. On my desk computer I use a TV soundbar and that works really well. On my living room computer I have proper hifi speakers going through a proper hifi amp and that's on another level again.

Is it just for gaming and listening to YouTube videos?

Brogrammer

can someone tell me if my answer is correct

i got around 756 years

for the orbital period

0ßelö7

@JohnRennie yeah

my monitor's built in speakers can't beat the jet engines on my 1080

John Rennie

16:17

How much do you want to spend?

0ßelö7

ideally nothing

can I be one of your charity cases?

jk, idk how much these things cost

John Rennie

Well this is what I use on my PC

Sid

Why don't you simply plug in ear phones to your computer and listen?

John Rennie

@Sid It's nicer using speakers

0ßelö7

@Sid don't want to mess up my hair

Sid

16:21

Maybe, it's just me but I feel earphones are clearer. Speakers tend to become disturbances for others..

0ßelö7

And someone stole my earbuds

@JohnRennie do I have enough room? gyazo.com/6e865b0e1ebab4432fc0ac3f052c3f41

I like to put my keyboard up under the good monitor when writing on paper

So I can't have anything else there

John Rennie

It would be a bit tight for space. I don't think the soundbar I linked would fit.

I can't really advise you as I don't know much about PC sound.

I would just Google PC speaker reviews and buy whatever has got a reasonable review.

0ßelö7

ok

Prathyush Poduval

16:59

@BalarkaSen The one in which russians stomp to the badass song? Why wouldn't i have heard of it:P

Deadpool Musical - Beauty and the Beast "Gaston" Parody

►

^@BalarkaSen what do you think of this?

0ßelö7

17:26

@JohnRennie my pagefile usage was 26GB

I don't know what that means but it can't be anything good

Run like hell

17:59

Hey guys I have a silly question about a passage i don't understand, can someone help me with it? I have this

$(exp \{ -i \omega_{\mu \nu} \Sigma^{\mu, \nu}\} )^{\dagger} = exp \{ +i \omega_{\mu \nu} (\Sigma^{\mu, \nu}) ^{\dagger} \} $

while, since $\omega_{\mu \nu}$ is a real and antisymmetric object I think it should be

$(exp \{ -i \omega_{\mu \nu} \Sigma^{\mu, \nu} \})^{\dagger} = exp \{ +i (\Sigma^{\mu, \nu}) ^{\dagger} (\omega_{\mu \nu})^{\dagger} \} = exp \{ +i (\Sigma^{\mu, \nu}) ^{\dagger} (\omega_{\nu \mu}) \}) = exp \{ -i (\Sigma^{\mu, \nu})^{\dagger} \omega_{\mu \nu} \})$

I posted here cause I thought it wasn't good to open a question in the main site about this computation

0ßelö7

aren't the $\omega_{\mu\nu}$s just numbers?

Run like hell

yeah

0ßelö7

so why would you $\dagger$ them

Run like hell

but the whole thing is a matrix

0ßelö7

$\Sigma^{\mu\nu}$ are matrices

so $(\omega_{\mu\nu}\Sigma^{\mu\nu})^\dagger=\omega_{\mu\nu}(\Sigma^{\mu\nu} )^\dagger $

Run like hell

18:02

ok, for each value of the indexes Sigma is a matrix

0ßelö7

oh come on mathjax

@Runlikehell yeah, so?

just look what I wrote

$\omega_{\mu\nu}\Sigma^{\mu\nu}$ is a linear combination of matrices, so you dagger the matrices

Run like hell

yeah I considered the $\omega$ as matrices, you pointed it out they were just numbers , thanks. God I lost like 20 minutes on that and other 10 minutes to ask the question here in latex...

But thanks, at least I understood my mistake

0ßelö7

np

heather

18:18

> Using the chain rule is like peeling an onion: you have to deal with each layer at a time, and if it is too big you start crying.

- Anonymous professor, as per 3Blue1Brown's video on the chain rule

Sid

@heather that is....one of the best things ever said of the chain rule...

Jasper

@heather I don't like this analogy. You just deal with each layer at a time, but it doesn't really make you cry. He's just trying to give an analogy when none is needed, lol.

Leaky Nun

Is energy always conserved?

►

heather

@Jasper not really an analogy; more of a joke quote =) 3blue1brown includes a quote at the beginning of most of his videos.

Avantgarde

18:43

@Balarka oh please no. But I'll watch it because you shared it

John Rennie

0

Q: What if what we observe through telescopes is phase space

Rather than real space as we know it, what if what we observe in space is actually the phase space and not the underlying actual action?

cosmology

Huh?

Abcd

@LeakyNun is it not?

Leaky Nun

@Abcd watch the video lol

Abcd

OK

John Rennie

19:00

22

Q: Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light having a lower frequency which equates to lowering its energy. My question is, where does the e...

general-relativity energy electromagnetic-radiation energy-conservation space-expansion

Leaky Nun

@JohnRennie nice

Abcd

@LeakyNun You study physics too?

Leaky Nun

@Abcd I don't

Abcd

@LeakyNun Which subjects do you study? Only maths?

Leaky Nun

@Abcd right

Abcd

19:10

@LeakyNun But you know programming also if I am not wrong.

Leaky Nun

doesn't mean I study it in uni

Abcd

Ohkay.

vzn

21:01

Synopsis: Quantum Annealers Limited by Temperature / aps physics

↑ o_O possible achilles heel?

0ßelö7

21:40

@ACuriousMind if I had to attribute a name to the sheafy de Rham theorem, whose would it be?

heather

::does a dance:: yes, yes, yes!

it'll still need tons more work, but i've got a terribly written program that can handle multiple qubits in some cases

and i am happy.

dmckee

21:57

@heather ::chuckles:: The 'it's kinda working' dance is even more excited than the 'it finally compiles' dance.

Phase

A program to handle qubits? Can I inquire how / why / anything about it tbh, sounds interesting. Can you compile it without a Quantum Computer?

heather

@Phase yeah, you can compile it without a quantum computer (if you couldn't I'd be screwed ;)

github.com/physicsnerd/Quantum-Computer-Simulator

^code for project

one moment, let me push the code i've got to the repo

Phase

Oh

So its like cnot gates etc?

heather

erm...not cnot gates yet

just single qubit gates

but they work on multiple qubits =P

i know, it's kind of stupid right now.

i just pushed the new code.

@dmckee yeah =)

Phase

I haven't really looked into Quantum Computing much but wouldnt you need to take the tensor product of two operators if you're operating on two states at once?

heather

22:03

if anyone feels like taking a look at my question on SO, i'd appreciate it - stackoverflow.com/questions/46269261/…

@Phase basically, yeah

and i did

Phase

Oh nice, haven't looked through just checking if my very mundane knowledge is even right

heather

you use the kronecker product

so if i wanted to apply the hadamard gate to the first qubit of two, i'd do $I\otimes H$

oh, whoops

no, $H\otimes I$

if i wanted to apply the hadamard gate to the second qubit of two, i'd do $I\otimes H$.

Phase

and if to both, $H \otimes H$

Although tbh idk what a Hadamard gate is RIP

heather

$H =\frac{1}{\sqrt{2}} \begin{bmatrix}1&1\\1&-1\end{bmatrix}$

0ßelö7

@JohnRennie is it wise to run GPU benchmarks to heat up the PC and turn it into a foot warmer?

David Z

22:49

@heather Too much code ;-) When I have something like that where I can't identify where the error is coming from, I'll aggressively trim out parts of the program (starting with those least likely to be related) until I narrow it down to a small chunk that reproduces the error.

heather

@DavidZ heh, yeah. thankfully someone was able to point out it was a persnickety float vs int thing.

weirdest thing was that it came up only when i scaled up, even though it was part of a function that had been there all along...

David Z

Yeah, I guess it might be related to some Numpy optimization or something...

Phase

Is a direct sum just like a geometric inverse of a projection?

0ßelö7

23:40

@BernardoMeurer turning my desktop into a potato:

Jasper

23:53

@0ßelö7 Wow, you have so many computers!

Bernardo Meurer

He managed to install proprietary malware into Linux

Jasper

@BernardoMeurer Is it always a good idea to turn on GPU acceleration in your browser or other apps?

Bernardo Meurer

@Jasper Generally yes

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