The h Bar

Semiclassical

01:03

arxiv.org/abs/1910.10688

Woo

bolbteppa

01:19

Nice

schn

02:22

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

04:45

@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.

Slereah

06:42

@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

Slereah

08:47

Now I am curious

So I am reading a number theory introduction

bolbteppa

09:23

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 :\

Slereah

11:36

"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

@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. :)

bolbteppa

@Slereah it is

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

Slereah

this was actually the Pauli-Fierz theory

Aaron Stevens

13:09

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.

JMac

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

Aaron Stevens

13:29

@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?

Slereah

13:49

@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

bolbteppa

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

Slereah

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

bolbteppa

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)

Slereah

@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

Abhas Kumar Sinha

14:07

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.

Ezze

what about mathoverflow

Abhas Kumar Sinha

@Ezze does it have peer to peer paper review?

Ezze

that supposedly similar aint it

Abhas Kumar Sinha

no

they are only similar when it comes to names

mathoverflow is more like mathematics stackexchange

JMac

@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

Ryan Unger

14:24

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

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

Slereah

Math isn't objective reality

Abhas Kumar Sinha

k

verdelite

Is this the only physics chatroom?

Slereah

there are others

but this is the cool one

Abhas Kumar Sinha

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

Slereah

14:31

not that I'm aware

arxiv ain't that bad rly

it's no vixra

Abhas Kumar Sinha

@Slereah heard for the first time...

Ezze

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

Slereah

I'm afraid this is too complex for us

Abhas Kumar Sinha

@Ezze Poet in physics stackexchange

Slereah

We all are universe

verdelite

14:35

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

Abhas Kumar Sinha

@verdelite what is first filter with affilation?

Ezze

it is to filter with affiliation, first.

Abhas Kumar Sinha

@Ezze what is affilation?

Ezze

workplace

the ones providing u with coffee

verdelite

It is like where you work. what is your institute

Abhas Kumar Sinha

14:37

which institute? school?

Ezze

Well, depends on where you work

Abhas Kumar Sinha

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

Ezze

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

verdelite

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.

Abhas Kumar Sinha

ah okay...

Slereah

14:40

Don't look down on the McDonald people

They make good fries

Abhas Kumar Sinha

@verdelite McDonald... Hehehehe

@Slereah research paper on fries...

Slereah

I'm sure there are some

@JohnRennie do you know any fries physics

Abhas Kumar Sinha

@Slereah what? research paper on fries?

Ezze

has to be, yeah

Ryan Unger

@Slereah no, but math is either correct or not

there is a lot of wrong mainstream physics

Slereah

14:41

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

making them more addictive and cost efficient

Ezze

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).

Ryan Unger

the GR lunch talk next week should be LIGO stuff

Slereah

it does look delicious

Abhas Kumar Sinha

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

Ryan Unger

so hopefully not complete nonsense

Slereah

14:43

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

bolbteppa

@RyanUnger String theory clearly ate your lunch money

Abhas Kumar Sinha

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

Ryan Unger

no one takes string theory seriously

Slereah

Damn now I want some french fries

Ryan Unger

only string theorists and the media

Abhas Kumar Sinha

14:44

@RyanUnger stephen hawking

Slereah

@RyanUnger Tell that to Witten

Abhas Kumar Sinha

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

Ryan Unger

what?

Ezze

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

Slereah

Don't date a string theorist then I suppose

Abhas Kumar Sinha

14:47

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

Slereah

"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

Abhas Kumar Sinha

@Slereah hehehehe...

Ezze

You like the paper dont you

Slereah

Well no, but I do love fries

Ezze

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

Slereah

14:48

.oO{🍟}

$$\Huge{🍟}$$

Abhas Kumar Sinha

klol

verdelite

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

Slereah

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

Abhas Kumar Sinha

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

$$ \ddot \smile $$

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

Ryan Unger

mfw princeton doesnt have access to the journal of food engineering

is this a real journal

Abhas Kumar Sinha

14:49

no

Slereah

I'm sure the McDonald scientists have it

verdelite

@Slereah it is Word, not Words

Ezze

@RyanUnger 3.625 impact factor on it

Abhas Kumar Sinha

we need a journal of spongebob engineering too to make burgers

Slereah

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

All their articles are great

Ezze

14:50

My old university had a class called sensory perceptions

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

Also touch tem.

Abhas Kumar Sinha

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

hehehehe

Slereah

"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!

Abhas Kumar Sinha

Spongebob engineering

JMac

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

Abhas Kumar Sinha

@JMac over analytical engineering

JMac

14:54

@AbhasKumarSinha But what did they "engineer"?

Slereah

I wish for more burgers

Abhas Kumar Sinha

@JMac technological advancements in making burgers

Slereah

🍔

Abhas Kumar Sinha

$$\huge 🍔$$

Emilio Pisanty

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...

quantum-mechanics hilbert-space mathematical-physics mathematics

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

I can't seem to find it

Slereah

14:56

Pretty much yes

Might have been deleted?

I dunno

Emilio Pisanty

potentially it was posted by OP and deleted?

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

Slereah

physics.stackexchange.com/questions/509923/…

That was the one

Emilio Pisanty

good find

how'd you get it?

Slereah

I still had the tab open

Ezze

is legend

Abhas Kumar Sinha

14:58

pbs.twimg.com/media/…

Slereah

@AbhasKumarSinha French fry field?

Abhas Kumar Sinha

@Slereah exactly...

bolbteppa

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

verdelite

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

Emilio Pisanty

@Slereah interesting

Slereah

15:03

Well not necessarily

You can just treat it as linear algebra

Abhas Kumar Sinha

@Slereah you use linux debian

Emilio Pisanty

anyways, CC @Qmechanic who first closed that thread.

bolbteppa

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

Emilio Pisanty

@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

JMac

@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.

Emilio Pisanty

15:05

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

Abhas Kumar Sinha

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

who says there isn't any future studying QM

bolbteppa

@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

Slereah

@bolbteppa yeah they just use the torsion directly

Similarly to the em field

Emilio Pisanty

@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.

bolbteppa

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

Abhas Kumar Sinha

15:25

is this real? qr.ae/TWFBTB

Slereah

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

Abhas Kumar Sinha

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

Slereah

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

schn

16:03

@JohnRennie Okay, thanks!

Emilio Pisanty

16:27

@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

Slereah

Just write one million dollars

Emilio Pisanty

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

ACuriousMind

16:46

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

dmckee

@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.

Emilio Pisanty

@dmckee thanks =)

dmckee

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

Emilio Pisanty

it doesn't seem to be trying that hard

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

dmckee

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

Slereah

16:56

Hey look it's @JohnDuffield

Stop spamming my site @JohnDuffield

4

i've got enough spam as it is

Emilio Pisanty

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

JMac

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)

Sir Cumference

17:45

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

Slereah

Maybe I can add a Duffield detection algorithm on my site

Sir Cumference

apparently he never bothered motl afterward

Slereah

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

bolbteppa

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

Slereah

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

Nor do I care

Ryan Unger

17:48

Duffield could be a good ally but he hates math too

Slereah

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

6 hours ago, by Slereah

I hate science

bolbteppa

I have become the sole string town crier

Slereah

String is old hat

Now it's all about membranes

Sir Cumference

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

not sure which would be more insightful

Ryan Unger

Take math classes

Like Galois theory

Sir Cumference

17:57

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

bolbteppa

What stat mech book

Slereah

Is that when you get gunned down in a duel

Sir Cumference

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

bolbteppa

(I wouldn't take either)

Sir Cumference

welp both are pretty involved in stat mech

bolbteppa

17:59

That's what the stat mech course is for

Sir Cumference

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

bolbteppa

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

Sir Cumference

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

bolbteppa

"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…

Sir Cumference

@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

Slereah

18:06

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

Sir Cumference

aw man topology is offered too

but conflicts with galois theory

Slereah

What kind of topology

Sir Cumference

probably point set

still, the basics are good to know

Slereah

It can be useful

Sir Cumference

but there's also an astronomy class

sigh

Slereah

18:13

The solution is fairly obvious

TIME TRAVEL

Slereah

18:27

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

Not the best place for it

Emilio Pisanty

@dmckee another question

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

Sir Cumference

@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

dmckee

18:55

@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

19:05

Does acceleration affect stopping distance?

JMac

@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?

JMac

@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.

dmckee

@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.

JMac

@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

19:21

@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

19:43

@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

dmckee

20:02

"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

21:11

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

21:27

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

bolbteppa

23:17

1

Q: Spin J $A_J(s,t) = - \frac{g^2(-s)^J}{t-M^2}$ Amplitude?

In GSW equation (1.1.2) they define the scattering amplitude for a spin $J$ particle at high energies as $$A_J(s,t) = - \frac{g^2(-s)^J}{t-M^2}$$ mentioning it is an asymptotic approximation to a formula involving Legendre polynomials, and in this stackexchange post it is also defined, and in Wil...

quantum-field-theory

Bounty time

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