The h Bar - 2017-01-19 (page 3 of 3)

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8:02 PM

@BernardoMeurer Newtonian mechanics, strictly speaking, is wrong yet we have many answers relying on it and nobody thinks something should be done about that

Bernardo Meurer

@ACuriousMind Indeed

But what if the old theory was deprecated in favour of a new one entirely?

Again, Newtonian mechanics was deprecated in favor of a new theory and nobody thinks anything should be done about that

@BernardoMeurer Note that, since our current theories match our observations, every "new" theory must necessarily have some sort of limit in which they become the old theory. "Entirely new" theories cannot arise; science is an incremental effort.

AccidentalFourierTransform

in short, no new discovery will ever prove that we were entirely wrong

@ACuriousMind I need help with inflation

user image

$\phi$ is a real scalar field

BTW what happened here:

physics.stackexchange.com/posts/306222/revisions

I dont understand what anna tried to do

8:18 PM

@AccidentalFourierTransform If you look at the other answers, she wanted to add something to her own answer and accidentally added it to another answer

Bernardo Meurer

@DavidZ @ACuriousMind Got it. It was just something that came to mind :)

@AccidentalFourierTransform What exactly do you need help with?

I'm not going to compute a Feynman diagram :P

AccidentalFourierTransform

@ACuriousMind but then she deleted something that was part of the original answer, right?

@ACuriousMind well, first of all how can a scalar couple to photons? =P

@AccidentalFourierTransform lol, good point

Well, a complex scalar could

AccidentalFourierTransform

@ACuriousMind the potential energy of the scalar is given as $V(\phi)=\Lambda\cos\phi/\mu$

so it is real

I wrote an email to my professor asking how could a scalar couple to the photon without breaking gauge invariance

and he told me that the coupling $\phi F_{\mu\nu}F^{\mu\nu}$ is gauge invariant

but then I told him that such a term has vertices with four photons and one scalar, and therefore the process $\gamma\gamma\to\phi\phi$ has no tree-level amplitudes

and then he told me that the vertex has two photons and one scalar, and that the diagram is indeed a tree diagram

8:23 PM

But I'm not sure why they think the vertex should be scalar-photon-photon. By minimal coupling, the coupling should be scalar-scalar-photon-photon from the $D_\mu\phi D^\mu\phi$ kinetic term.

AccidentalFourierTransform

but that only works for complex fields, right?

Yeah, right

@AccidentalFourierTransform Why would it have four photons?

AccidentalFourierTransform

because $\phi F^2\sim \phi A^4$

...no, that's wrong

AccidentalFourierTransform

goddamit

8:25 PM

$F$ contains only one $A$, differentiated.

AccidentalFourierTransform

yeah now I see it, thanks

In non-Abelian theories $F^2$ has three- and four-gluon vertices, but not for photons

AccidentalFourierTransform

also, what is the exact decay process? $\phi\to ?$

or maybe $2\phi\to2\gamma$?

What's wrong with $\phi \to \gamma\gamma$?

AccidentalFourierTransform

hmm

kinematics? =P

idk

that doesn't seem right for some reason

8:28 PM

It's usually forbidden by charge conservation, but since $\phi$ is uncharged because of that weirdo interaction term, it should be admissible

AccidentalFourierTransform

screw renormalisability anyway

@ACuriousMind well thank you :-)

np, what do you call such a particle, anyway?

AccidentalFourierTransform

Walter

I'll call it the faxion - it looks like an axion but it's not, it's fake.

@AccidentalFourierTransform did you listen to the album?

AccidentalFourierTransform

8:32 PM

May 3 '16 at 18:58, by AccidentalFourierTransform

@user507974 then definitely Walter

@obe lol, I downloaded it and put it in my mp3 to take it to the gym

but, lazy as usual, didnt go

@AccidentalFourierTransform ok lmk when you listened to it.

I HAVE TRIED A MILLION TIMES but my ray diagram of a concave mirror when an object is over center of curvature is NEVER CORRECT!!

AccidentalFourierTransform

@obe will do :-)

user image

the object should be right under c

whats the problem here? :(((((((((((((

the diagram is correct

AccidentalFourierTransform

the problem seems to be that you are studying optics

8:36 PM

?

AccidentalFourierTransform

optics is a problem on its own

lol

AccidentalFourierTransform

also, did I ever say that I hate cosmology?

but seriously how do i make it?

AccidentalFourierTransform

did you try to make the lines thicker?

8:38 PM

nope

its all in one go

but the last one is accedentally divided in 2 but it doesnt make any difference anyway..

AccidentalFourierTransform

user image

there you go, you're welcome

THATS THE EXACT SAME THING MY TUITION TEACHER DID WHEN I SHOWED IT TO HER!!

god..

lol

AccidentalFourierTransform

the trick is to not give a damn

are u on 9gag ?

anyway

lemme repost the question

I HAVE TRIED A MILLION TIMES but my ray diagram of a concave mirror when an object is over center of curvature is NEVER CORRECT!!

user image

what r u doing

8:46 PM

making a ray diagram

8:59 PM

]\

9:18 PM

@ACuriousMind 0celo7 asks - do a rotation and a parity transformation always commute?

hello

@obe What do you think?

10:27 PM

@ACuriousMind he says if he knew what to think he wouldn't be asking.

@obe Not talking to him, talking to you

I find the discussion in the comments here rather interesting.

@AccidentalFourierTransform Hmmm. Zooming way in, your upper-right corner reflection is taking place a little behind the mirror.

@heather ohai

@DanielSank Thanks for the thorough answer :)

10:40 PM

howdy danny boyo\

hi heather, how do you do

@Sasha ive never seen you here before

@Sasha Sure!

howdy

@Skyler Don't call me that. Or call me that if you can beat me in a Bo5 Smash Bros set.

@Sasha It was a good question. Did you see the discussion in the comments between @EmilioPisanty and me?

I just got here. I'm new to stackexchange as a user in general... though I've used it for info many a time re:code snippets

@Sasha We're very happy to have you. Your question about the squeezed state cooling was good.

10:44 PM

@DanielSank Yes, I did see it.

It's interesting. I think it's interesting to see how different people think about the same issue.

@DanielSank i might have some time this weekend

To me, the idea that the squeezed state cooling can "in principle go to arbitrarily low temperature" is an irresponsible and even foolish statement in a scientific context.

Good points on both ends. I was nitpicking about their use of language, but you're right that it isn't definite that there's no theoretical limit on squeeze cooling.

The theory almost surely includes assumptions that fail at some point.

This is far, far too often missed by even very good physicists.

10:46 PM

@Sasha The article in your question reminds me of one of my final questions in stat mech.

I agree. I expect grandiose misinterpretations of papers by journalists, but it irked me to see that on NIST.gov

I'm hypersensitive to this because a few years ago there was a big mystery in my fiend about why the quantum state measurement in superconducting qubits failed in a particular way.

Everyone was confused because the theory predicted that the state measurement should not fail. I got involved and eventually figured out what was going on, and it turned out that two of the major assumptions of the theory were just flat out wrong in the limit people we trying to apply that theory!

I run into this a lot in fluid dynamics. which equations work here again???

Having experienced that, I strongly dislike statements about things working arbitrarily well in principle. Those statements are just lazy and kind of dishonest.

@Sasha Right!

We were supposed to figure out the most cooling you could achieve with adiabatic cooling using magnetization on earth

10:48 PM

@Skyler Nice!

That's a pretty simple calculation, yeah?

supposed to lol, did you succeed?

Yea

Excellent :)

sounds like my prof was shouldve quized the journalists

he'd tear them a new one, he's particularly savage old chinese man (im sure daniel knows who im talking about)

Zee?

10:51 PM

yep

AccidentalFourierTransform

si

Speaking of fluid dynamics, if you glue a petri dish to a bass driver hooked up to a cheap two channel amp and strap a ring of LED tape around the edge of the petri dish, fill it with silicon, and get the frequency of the LEDs slightly out of phase with the correct frequency of the bass driver...

you have a ~$60 demonstration of quantum behavior that's visible to the naked eye ;)

Not that it's actually caused by quantum behavior. It just looks eerily like it.

@Sasha (quantization intensifies)

AccidentalFourierTransform

Im looking for a new TV series to watch

any recommendations?

so @DanielSank, advice

I bought this LED driver for a project Im working on, but theres a bit of an issue

user image

how the heck do i solder that

10:55 PM

uhh... hope you have steady hands

@Skyler There are several ways.

You need solder-fu.

AccidentalFourierTransform

@Skyler with a hammer

@Sasha ikr, as if my solder game was so good to begin with

@Skyler That's a surface mount part. Are you connecting it to a pcb? If so, please show image.

@DanielSank na, i didnt realize its surface mount but it does electrically what i need

10:56 PM

@Skyler If your hands are shaky, do a single shot of vodka and wait ten to thirty minutes, if that's legal for you.

@Skyler you might I'm joking... but I'm really not

AccidentalFourierTransform

I had nothing better to do today

@Sasha judging by the picture you look younger then me, then again ive rocked a beard since i was 14

AccidentalFourierTransform

getting drunk it is

@ACuriousMind I haven't done physics in a while so I dunno :/

@Skyler 25 as of a few days ago

10:58 PM

@Sasha uhh...how old am i again....

@Skyler You need to make a small pcb for that.

ah yes...22

You can try to solder wires but it won't last.

@Skyler He's right. It's to teensy to hitch wires to it securely.

For $30 you can order a bunch of pcb's from Advance Circuits on student special.

gtg

11:00 PM

they'd work temporarily if you were really careful

(wires, I mean)

@Sasha ya, i guess i can at least get that done at home until someother solution pops up

i might be able to get a small batch done on university actually

AccidentalFourierTransform

Germany has legalised weed

we wont be seeing ACM around here anymore

youtube.com/watch?v=Hopd-gKB1Xc btw, this was the setup I referenced

so, @sasha, grad student right?

@AccidentalFourierTransform wait, what?

11:12 PM

@Skyler Nope, undergrad. I've temporarily checked out to get a series of medical issues sorted.

AccidentalFourierTransform

@ACuriousMind dw.com/en/…

but everything is medical if you try hard enough

@Skyler you?

@Sasha undergrad for ~9 more weeks

how long you been checked out for?

Hi guys, can someone advice me what subjects to pick in my Master's degree, I want to work in Quantum Mechanics, what would you advice me, besides grabbing all the QM I can ofc xD ?

@AccidentalFourierTransform I think it might actually be rather difficult to get the required prescription.

11:15 PM

it sounds like youll have to relearn things if its been a couple of years

What are some subject that are important and may not be that obvious*

@Kelthar not sure if in US, but here usually people dont do masters physics

I needed 3 major joints reconstructed because Ehlers-Danlos Syndrome. Also, was diagnosed with epilepsy, Hashimoto's, and a genetic immunodeficiency... so I've been out since 2013.

AccidentalFourierTransform

@ACuriousMind just stab yourself in the eyes, that should do the trick

@ACuriousMind physics.stackexchange.com/questions/306264/…

@Skyler, I'm from Portugal, I'm doing Engineering Physics now

AccidentalFourierTransform

11:16 PM

@ACuriousMind the $\nabla$'s should be $\partial$'s, right?

@Skyler This is my course's program: fenix.tecnico.ulisboa.pt/cursos/meft/…

AccidentalFourierTransform

@ACuriousMind no wait, nevermind

@Sasha wow, been a few tumultuous years for you?

they just all started hitting at once?

@AccidentalFourierTransform I don't that that's a "chronic illness" :P Faking any of the illnesses you can get the prescription for is probably harder and more expensive that just buying from you local dealer :P

@AccidentalFourierTransform hmm? Yes, they should, otherwise that's a pretty recursive formula :D

@Skyler hehe, yeah :D but I've been working off and on in two cool labs, and am about to get co-author published in Cancer Research

11:19 PM

@Sasha noice, taking a little peak at the profil im guessing its related to metabolics of cancer cells

@Skyler yes, but I just did a bunch of mass spec data analysis. I'm not a biologist at allll.

no biophysics in there?

@Skyler I want to work in quantum computing*, lol, wrote QM by mistake...

@Kelthar so experimentalist side

or engineering?

overlaps there

AccidentalFourierTransform

those friggin unpaired parentheses are making me really uncomfortable

11:23 PM

but i guess would you want to be working on the new method or implementing a functinoal computer from someone elses?

also @DanielSank is really rhe person to ask

@Kelthar Just...learn quantum mechanics well and thoroughly first, please. I see far too many questions on this site from people trying to learn something about quantum computing without actually learning QM

@Skyler I did take one class in biophysics, but it's been entirely useless in metabolomics so far. It's OK, though... I'm a decent autodidact.

@AccidentalFourierTransform obligatory xkcd

xkcd references? I'm home.

AccidentalFourierTransform

@ACuriousMind thankyou I'm stressfully cracking my joints because of that

11:25 PM

@Sasha yep, my biophsyics was a really amazing class but probably not useful

@Skyler I find the overlap quite cool actually, I'm not yet sure which of the flavors yet though xD
DanielSank?

AccidentalFourierTransform

@ACuriousMind obligatory smbc :D

hes a q.comp guy(to say the least)

@Sasha I also do SMBC and abstruse goose references sometimes, for a little variety ;P

first time ive heard of abstruse goose there ACM

11:27 PM

@ACuriousMind I took this past semester my first subject in QM, 2nd next semester, but my masters starts in September so... I wanted to have an idea

@Skyler wow, that's cool. nice, I'll ask him when he's around

@Skyler Some questions on this site regularly make me think of this one, for example

3

AccidentalFourierTransform

xkcdref.info/statistics

nice ^

@ACuriousMind One of my girlfriends recently asked me a bunch of questions about time. I almost taught her what K-space is before I realized her eyes had glazed over.

AccidentalFourierTransform

maybe I shouldn't ask... but... whats a k-space?

wikipedia is not helping

@AccidentalFourierTransform momentum space, I guess

11:35 PM

Now that I'm remembering, I can't recall how we got on the topic of topology.

AccidentalFourierTransform

K-space (magnetic resonance imaging)

k-space is a formalism widely used in magnetic resonance imaging introduced in 1979 by Likes and in 1983 by Ljunggren and Twieg. In MRI physics, k-space is the 2D or 3D Fourier transform of the MR image measured. Its complex values are sampled during an MR measurement, in a premeditated scheme controlled by a pulse sequence, i.e. an accurately timed sequence of radiofrequency and gradient pulses. In practice, k-space often refers to the temporary image space, usually a matrix, in which data from digitized MR signals are stored during data acquisition. When k-space is full (at the end of the scan...

?

@AccidentalFourierTransform I was referring to compactly generated spaces.

OH, I was explaining what Minkowsky space is

...how did you get to compactly generated spaces from a question about time?

AccidentalFourierTransform

the time is gone, the song is over

@ACuriousMind because she got confused when I said that Minkowsky space has a different metric from the one she's used to working with

and then I had to explain metric spaces, and so on

AccidentalFourierTransform

11:38 PM

ah topology, youre so fun!

@Sasha im surprised she made it to metrics

that sounds like a standard glazing point already

lol, you went full axiomatic on a layman question? Yeah, that tends to produce glazed eyes :D

@Skyler she might be a political science/environmental science major, but she's very intelligent and curious

@AccidentalFourierTransform Topology is fun!

In what other topic is "find a gluing recipe for the three-holed donut" a legitimate exercise?

@ACuriousMind topology is the point at which I start feeling like I need to deconstruct my brain just to wrap it around a concept

11:41 PM

@Sasha still, the prerequisite knowledge for metrics is pretty rare for people

That reminds me of analysis. My introduction to analysis class was taught from the framework of metric spaces, which none of us had ever encountered before (professor was delusional).

During one lecture about open balls vs closed balls, someone had the audacity to ask him, "so, essentially, if I threw an open ball at your face, it wouldn't hit you?"

@Sasha Try mathematical logic if you ever feel topology is getting to comfortable ;)

It's incredible what mathematicians are forcing math to do these days...

XD

Analysis from the perspective of metric spaces is fondly remembered as the math class where our lecture board looked more like a Jackson Pollock crossed with a Picasso.

@ACuriousMind I got lost in that world long ago, when I had to prove the existence of numbers as an initial homework assignment. Maths professors are jerks sometimes.

AccidentalFourierTransform

)

11:50 PM

() twitch

@AccidentalFourierTransform {[({{{[[(([

AccidentalFourierTransform

dude.

@ACuriousMind Yeah that happens a lot.

@AccidentalFourierTransform :)

It's amazing that some people actually manage to learn enough about what a quantum state and unitary transformation is that they do understand quantum algorithms, even though they've never seen a Hamiltonian.

This is a problem though, because they have no idea how to implement the unitaries they want for their algorithm.

AccidentalFourierTransform

11:52 PM

@ACuriousMind oh no, now ascii smileys make me uneasy too

WHAT HAVE YOU DONE TO ME??

This is a problem in my field. Proposing beautiful quantum algorithms is fine but it's a lot more impactful to propose one that the experimentalists can actually run.

Well, perhaps I shouldn't say that.

I hope readers know what I mean here.

@AccidentalFourierTransform ::laughs maniacally::

AccidentalFourierTransform

:-($\phantom{)}$

@DanielSank I'm not sure I do.

AccidentalFourierTransform

a reference to string theory?

11:55 PM

@DanielSank ! Someone told me you have something to do with quantum computing field! I want to work in that as well... Could you advice me on some important yet not so obvious subjects to learn for this?

@DanielSank The programming is always far ahead of the hardware, it seems.

"impactful" is a wonderfully vague word with whose interpretation the whole sentence stands and falls

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