The h Bar

1:19 AM

Nice

schn

2:22 AM

In a certain calculus book, when deriving the formula for the centre of mass of a solid as a triple integral, they frequently refer to the torque experienced by a mass element dm as “moments” about the coordinate axis, i.e. dm(x-x0), dm(y-y0), ... what kind of moment is this? Distance times mass?

John Rennie

4:45 AM

@schn If you put your system in a gravitational field $g$ then each $dm$ would generate a force $dm g$, and the moment would be just the usual force times distance. The COM is then the point where the net torque is zero.

6:42 AM

@RyanUnger If you're feeling lazy I'll just take like phone photos of the page

I'm gonna just rewrite it in TeX afterward

8:47 AM

Now I am curious

So I am reading a number theory introduction

9:23 AM

Riemann's 9 page paper claymath.org/sites/default/files/ezeta.pdf

It's like one of the gospel's of nt, need to know :\

11:36 AM

"The massless case is not the massless limit of the massive case"

I hate science

Let's go back to being farmers

PM 2Ring

12:17 PM

@EmilioPisanty Sorry for the delay, I've been offline for a day or so. Here is the Python room, and here are the room rules. Also see Salad Language. :)

@Slereah it is

Don't stray to the farmers over $\lim_{m\to 0} \frac{1}{2} \int d \tau e(e^{-2} \dot{x}^2 - m^2)$

this was actually the Pauli-Fierz theory

Aaron Stevens

1:09 PM

Why has the Close Votes queue been empty lately? It used to be that I would go through the queue, run out of votes/reviews for the day, and there would still be like 30 questions to review.

@AaronStevens I have 81 in my queue, but admittedly I don't do the close vote queue very often

Aaron Stevens

1:29 PM

@JMac I suppose I have just gone through all of those already :) I guess it was going to run out for me eventually

@JMac So what exactly are you trying to do with your fans?

1:49 PM

@ACuriousMind Are point spin 1/2 particles just the SUSY version of Nambu-Goto

AND IF SO

What happens once the torsion comes in

the action seems to be independent of the connection

It's like $$S \approx \int d\tau (-m \sqrt{\dot{x}^2}) + \frac{i}{2} \dot{\theta}^\mu \theta_\mu$$

I would assume that the torsion would influence such things but I'm not sure where it would come in

First issue with that action is, there is no mass term for the fermion, while you're boson inherently has non-zero mass when using that form, and susy requires/shows them to have equal mass

I'm not using the fanciest form but I am more wondering where the connection would come in in a curved version

So from the $S = \int d \tau \frac{1}{2} e(e^{-2} \dot{x}^2 - m^2)$ Lagrange-multiplier action coming from incorporating the $p^2 + m^2 = 0$ constraint we can then set $m=0$ to get the spinning-particle action $S = - \frac{1}{2} \int d \tau (\dot{x}^2 - \theta^{\mu} i \partial_{\tau} \theta_{\mu})$ ('Polyakov' form)

@bolbteppa I'm using natural units so $m = 1$

arxiv.org/pdf/hep-th/9910180.pdf

This does it apparently

But it seems extremely unpleasant

I don't even know how you'd derive that

I mean they do it

But from the Dirac equation

2:07 PM

Like physics overflow- PhysicsOverflow is a physics website that serves as a post-publication open peer review platform for research papers in physics, is there any mathematical alternative to it? please no arxiv

Paper quality in ArXiv is terrible specially when it comes to maths.

what about mathoverflow

@Ezze does it have peer to peer paper review?

that supposedly similar aint it

no

they are only similar when it comes to names

mathoverflow is more like mathematics stackexchange

@AaronStevens Well I was expecting to see a fairly obvious pattern based on the flow direction, blade profile, and direction of gravity. Basically all I learned was that the airflow in my case was not at all what I thought it was. Fans on top that I thought were shooting air upwards were actually sucking air in; but all the dust was still on the bottom of the blades, still not really sure why

But my case also has some weird airflow. It has a tempered glass covering a lot of the fan surface, so the airflow is likely a total mess. It looks cool, but you might be able to see from it's page that the glass clearly restricts some airflow cougargaming.com/products/cases/panzer_g

Basically its really not comparable to a regular axial fan at all, which was unfortunate since I had fans in several different orientations on the case

2:24 PM

@AbhasKumarSinha It's much, much worse for physics.

Although that comes from the overall physics culture where objective reality doesn't seem to matter.

Math isn't objective reality

k

Is this the only physics chatroom?

there are others

but this is the cool one

Is there any way to filter out useless papers in ArXiv which aren't reviewed properly or are useless?

2:31 PM

not that I'm aware

arxiv ain't that bad rly

it's no vixra

@Slereah heard for the first time...

Ah, the final question has been asked

0

Q: what is universe?

Everything that we see is a just a small part. The things that we observe are of our measurable quantities, but actually it is never ending. We say that there is a solar system and a galaxy, but isn’t it all similar to an atom. We have a sun and planets revolving around it, like the electrons rev...

universe

I'm afraid this is too complex for us

@Ezze Poet in physics stackexchange

We all are universe

2:35 PM

@AbhasKumarSinha, first filter with affiliation, of course. it is unfortunate but it works.

@verdelite what is first filter with affilation?

it is to filter with affiliation, first.

@Ezze what is affilation?

workplace

the ones providing u with coffee

It is like where you work. what is your institute

2:37 PM

which institute? school?

Well, depends on where you work

I read in school... I'm afraid my school doesn't publish any journal...

No, no. Filter by affiliation can mean, for example, to only read papers from MIT researchers

yeah. It is like, are you currently working in NIST? in Stanford? If so you may have a paper that is not too bad. If you work in McDonald, maybe not.

ah okay...

2:40 PM

Don't look down on the McDonald people

They make good fries

@verdelite McDonald... Hehehehe

@Slereah research paper on fries...

I'm sure there are some

@JohnRennie do you know any fries physics

@Slereah what? research paper on fries?

has to be, yeah

@Slereah no, but math is either correct or not

there is a lot of wrong mainstream physics

2:41 PM

I'm sure McDonald poured a lot of money into making fries

making them more addictive and cost efficient

What about this one

sciencedirect.com/science/article/abs/pii/S0260877400001898

The viscoelastic behaviour of french fries during deep fat frying after two different types of drying pre-treatment (air drying and osmotic dehydration) was examined under uniaxial compression tests. The stress-strain data of the compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (σmax), the maximum strain (εmax), the elasticity parameter (E) and the viscoelastic exponent (p).

the GR lunch talk next week should be LIGO stuff

it does look delicious

@verdelite where is that filter? I don't find it...

so hopefully not complete nonsense

2:43 PM

Experimental GR is fairly boring and also super hard to make rigorous

@RyanUnger String theory clearly ate your lunch money

@Ezze Where is that filter by affiliation? I dun find it.

no one takes string theory seriously

Damn now I want some french fries

only string theorists and the media

2:44 PM

@RyanUnger stephen hawking

@RyanUnger Tell that to Witten

@RyanUnger the universe is boring because it's no string theory in real.

what?

I mean if the universe is boring then good luck finding an interesting date

Don't date a string theorist then I suppose

2:47 PM

@Slereah no................

"The stress-strain data of the compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (σmax), the maximum strain (εmax), the elasticity parameter (E) and the viscoelastic exponent (p)."

What is the stress energy tensor of a fry

@Slereah hehehehe...

You like the paper dont you

Well no, but I do love fries

I'm definitely having fries tomorrow, uni canteen has french fries fridays

2:48 PM

.oO{🍟}

$$\Huge{🍟}$$

klol

@AbhasKumarSinha The filter is in your brain. Open an article, look at the author affiliation, and make a decision.

A good filter is to close it if it was written in Words

@verdelite author affiliation... That's boring..

$$ \ddot \smile $$

or $$\huge{\ddot \smile}$$

mfw princeton doesnt have access to the journal of food engineering

is this a real journal

2:49 PM

no

I'm sure the McDonald scientists have it

@Slereah it is Word, not Words

@RyanUnger 3.625 impact factor on it

we need a journal of spongebob engineering too to make burgers

"Simulated oral processing, in vitro digestibility and sensory perception of low fat Cheddar cheese containing sodium alginate"

All their articles are great

2:50 PM

My old university had a class called sensory perceptions

They had to look at... stuff... for grades

Also touch tem.

dailymail.co.uk/news/article-7569201/…

hehehehe

"Directional hedonic thresholds for sodium concentration in hamburger"

"Determination of fat content in chicken hamburgers using NIR spectroscopy and the Successive Projections Algorithm for interval selection in PLS regression (iSPA-PLS)"

Oh no I dropped my hamburger into the spectroscopy machine

Our experiment is ruined!

Wait!

Spongebob engineering

@AbhasKumarSinha That definitely seems more like "over analysis" than engineering.

@JMac over analytical engineering

2:54 PM

@AbhasKumarSinha But what did they "engineer"?

I wish for more burgers

@JMac technological advancements in making burgers

🍔

$$\huge 🍔$$

quantum-mechanics hilbert-space mathematical-physics mathematics

0

I am a 4th-year undergraduate student and I have fully read R. Shankar's book on Quantum Mechanics and Griffiths book Quantum Mechanics. I have also done a bit of the Application of QM on multielectron systems,molecules, etc. Not going ahead in the application part, I want to focus on the founda...

is it me, or was a question exactly like this posted yesterday(ish)?

I can't seem to find it

2:56 PM

Pretty much yes

Might have been deleted?

I dunno

potentially it was posted by OP and deleted?

I'm pretty sure it had been closed as a duplicate

physics.stackexchange.com/questions/509923/…

That was the one

good find

how'd you get it?

I still had the tab open

is legend

2:58 PM

pbs.twimg.com/media/…

@AbhasKumarSinha French fry field?

@Slereah exactly...

What's the point of this other than to stop someone getting help with a book recommendation

Math student working on QM... He needs some physics intuition to start with

@Slereah interesting

3:03 PM

Well not necessarily

You can just treat it as linear algebra

@Slereah you use linux debian

anyways, CC @Qmechanic who first closed that thread.

How dare this person ask for a book recommendation where they are forced to add an addeendum specifying that it's not a duplicate as it doesn't cover what they are asking about which apparently the people who closed it just ignored, better 'tell on them' that they posted this question asking for help before where their question was actually ignored and removed, as if they are committing some sin

@bolbteppa sure, anytime anybody's question gets closed or downvoted for any reason, let's just make it OK for them to delete and re-post

@bolbteppa ... If your question is closed and you feel it shouldn't, deleting that question and then re-posting it with the additional information isn't the right approach. You can edit closed questions; and you're supposed to edit them if you feel they were incorrectly closed. You're definitely not supposed to delete it and then ask the same question with the additional clarification.

3:05 PM

I'm sure the added overhead on site moderation won't be a problem

republicworld.com/world-news/rest-of-the-world-news/… ...

who says there isn't any future studying QM

@Slereah there's no connection in the 2D covariant derivative of a spinor, or in the 1D one right, not sure that the expression you gave has one

@bolbteppa yeah they just use the torsion directly

Similarly to the em field

@bolbteppa Seriously, though: this type of snark and sarcasm is deeply unhelpful. You did realize that I was taking the time to explain to the newcomer in question what the rules are and how to go about things? I notice that you weren't doing any of that, so your attempts at grabbing any sort of moral high ground are pretty insulting.

The connection would be $\omega^{\mu \nu}[\gamma_{\mu},\gamma_{\nu}]$ and in 2D the extra $\gamma$'s attached to it make it all go to zero, in 1D it's 0 automatically

3:25 PM

is this real? qr.ae/TWFBTB

@bolbteppa it's the connection of the target space here

ancient-origins.net/myths-legends-asia/…

But unlike the torsion-free case, you can't use the target space metric here

schn

4:03 PM

@JohnRennie Okay, thanks!

4:27 PM

@dmckee quick question

(asking for a friend)

if an application on an online system for a lecturer position asks you for your current salary, this should be left blank, right?

this is for a position in the UK, my friend is a postdoc at a European research institute

It just sounds like a ridiculous way to hamstring one's negotiating position on salary, should an offer be forthcoming, for no real gain that I or my friend can see

Just write one million dollars

@Slereah they do actually want the job, and lying on a job application is said to be a bad idea ;-)

ACuriousMind

4:46 PM

@Slereah Not quite, there's an additional WZW term from Green-Schwarz's so-called $\kappa$-symmetry.

@EmilioPisanty The advice I read and heard from resume consultants and the like is always to avoid giving current or desired salary data as long as possible.

@dmckee thanks =)

That said, I ran into a lot of systems that tried very hard to force you to answer those questions.

it doesn't seem to be trying that hard

there are required fields on that form and the current-salary field isn't one

If it will accept string input something like "commencerate with my skills and cost-of-living" is proabably harmless.

4:56 PM

Hey look it's @JohnDuffield

Stop spamming my site @JohnDuffield

4

i've got enough spam as it is

@dmckee that's a good idea. I'll keep it in the quiver.

lol someone just asked why we can't get free energy from magnets, and the link they added was a youtube video making a fake free energy device that actually explains it's fake youtube.com/watch?v=BSdSDfOWbNs&lc=

I'm impressed that they managed to find a free energy device video on youtube that actually clears up that it is fake (though obviously that seems to be a real education channel)

5:45 PM

@Slereah He did the same to Motl until Motl told him off

Maybe I can add a Duffield detection algorithm on my site

apparently he never bothered motl afterward

If swearing was enough you'd think he'd leave me alone by now

You're just not interested in real solutions like the rest of the establishment

I'm not even sure what he was on about in the messages

Nor do I care

5:48 PM

Duffield could be a good ally but he hates math too

Well if you want someone who hates physics I hate it too

6 hours ago, by Slereah

I hate science

I have become the sole string town crier

String is old hat

Now it's all about membranes

i've got the choice between taking dynamical systems or statistics alongside graduate stat mech

not sure which would be more insightful

Take math classes

Like Galois theory

5:57 PM

both of them are technically math

the statistics class assumes a measure theory background

@RyanUnger i'm also gonna be taking galois theory

so i can only choose between one of the two i mentioned up there

What stat mech book

Is that when you get gunned down in a duel

@bolbteppa can't see until it's in the Barnes and Noble database, which won't happen till spring

(I wouldn't take either)

welp both are pretty involved in stat mech

5:59 PM

That's what the stat mech course is for

i mean obviously statistics is but dynamical systems is what a lot of the theory is based on

@bolbteppa yeah but for the sake of a stronger understanding

there's some cool theory in there

There's always a wrong way to do physics courses full of axiomatics that wont give any insight :p

honestly i'll probably take DS since it subtly shows up in every topic involving energy

but then again statistics is useful in tons of stuff

argh

"Dynamical systems theory and chaos theory deal with the long-term qualitative behavior of dynamical systems. Here, the focus is not on finding precise solutions to the equations defining the dynamical system (which is often hopeless), but rather to answer questions like "Will the system settle down to a steady state in the long term, and if so, what are the possible steady states?", or "Does the long-term behavior of the system depend on its initial condition?"

An important goal is to describe the fixed points, or steady states of a given dynamical system; these are values of the variable…

@bolbteppa i mean either way, concepts from dynamical systems show up in a lot of places. a huge reason for using energy in physics is because it makes use of DS tools

6:06 PM

You know

one thing I haven't looked into too deeply is

Tipler's proof that a FRLW universe has no recurrence time

Given some conditions

aw man topology is offered too

but conflicts with galois theory

What kind of topology

probably point set

still, the basics are good to know

It can be useful

but there's also an astronomy class

sigh

6:13 PM

The solution is fairly obvious

TIME TRAVEL

6:27 PM

I'd try to do astronomy but 1) big city 2) cloudy weather

Not the best place for it

@dmckee another question

should one list the names and thesis titles of mentored students?

@Slereah same situation, but let's be real all you'd see in best conditions is a few planets and bright dots

the cool part is the nature of what's out there

6:55 PM

@EmilioPisanty Don't know. The fact of mentoring experience shouold certanly be mentioned as that will interest the hiring committee. Are particulars are particularly impressive? If not maybe let the committee bring it up in the (presumptive) interview stage.

barlop

7:05 PM

Does acceleration affect stopping distance?

@barlop The two are directly related. Acceleration is a change in velocity, and your velocity must change if you are trying to stop while in motion.

barlop

@JMac so then are you saying it makes no difference if you accelerate from 0kph to 30kph really fast, then try to stop. Vs if you have been at 30kph for some time, and try to stop?

or do you not know?

@barlop As long as it's the same vehicle with the same braking force applied, and you're at the exact same speed in each case with no additional forces acting on it, yes.

@barlop This makes no difference in a navie model. Tire heating effects could make a small difference in practical situations, but that is not how @JMac interpreted the question.

When you said Does acceleration affect stopping distance?" he assumed you meant the stopping acceleration.

@dmckee I made sure to add a lot of qualifications before I said "yes". I was starting to write a long list of reasons why it might not occur, but then that seemed like it might have been confusing the subject even more.

barlop

7:21 PM

@dmckee what is stopping acceleration?

a more fundamental qusetion.. is stopping, a force, or a deceleration?

also it's interesting that if I go at 30kph then if I accelerated up to 30kph, the force might be X newtons, where X is a lot greater than 0. Whereas if I reach 30kph and have been doing so for some time, then my force = 0. If the stopping distance is the same then that'd mean that an object with no force is as easy/difficult to stop, as an object with a lot of force 'cos they have the same stopping distance. That seems strange to me

Aaron Stevens

7:43 PM

@barlop Just because a strong force acted on an object in the past does not mean it is acting on it now. One of the properties of Newton's laws is that the history of the object does not matter. If I have an object going 30kph then it doesn't matter how it got to that 30 kph. I just have an object going at 30 kph

Now, if while you are trying to stop your object there are other forces at play, then your "stopping force" might be more or less effective. But this is due to the presence of the other forces while you are trying to stop the object. Not because of the forces that were present before stopping

8:02 PM

"stopping acceleration" is the accleration experienced while stopping.

Here's a hint. Don't use the word "deceleration" for now. It is hard enough to think clearly about kinemetics without introducing that word.

Assuming you've chosen a coordinate system in which the forward direction is positive and the car is currently moving forward, then stopping is an accelartion with a negative value.

grovesNL

9:11 PM

re: physics.stackexchange.com/a/98257/245650 There's an interesting discussion in the comments about Fahrenheit <-> Celsius conversions not being isomorphic to the abstract temperature unit. I was wondering if somebody might be able to clarify what this means when Fahrenheit or Celsius are used in rates? For example, intuitively degF/s being converted into degC/s seems to act as a rescaling (with the offset omitted), and I was wondering how this relates to the framework discussed

ACuriousMind

@grovesNL The thing they don't say explicitly there is that there are actually two different quantities in play here: "Temperature" and "Difference in temperature".

The "non-isomorphy" appears when you try to treat everything that has units of temperature as a temperature, but the simpler viewpoint is that you need to use the conversions with offset for temperatures and the conversions without offset for differences in temperatures.

grovesNL

9:27 PM

Thank you! I think this makes sense to me. So is it true that we can still assign a temperature units to both of these "temperature" and "difference in temperature" quantities, but isomorphy depends on additional context (how the quantities are being used)?

ACuriousMind

Yep