The h Bar

0celouvskyopoulo7

6:02 PM

it's been a long time since he was famous

He has probably improved

perimeterinstitute.ca/people/Jacob-Barnett

he's a PhD student at Perimeter

peterh

3

Q: What will happen if open a portal in the Mariana trench and another portal in the Sahara desert?

First portal is placed at bottommost point of the Mariana trench and second is placed at point where ocean is farthest from any direction. The 2nd portal is inverted so the water jet hits directly to the ground. Each portal is 10km in diameter and stable. Assuming the system to be a one way trans...

I think, something is not okay with the rules.

Slereah

Lee Smolin is at the university apparently

and Unruh

peterh

I have a very clear intuitive feeling, that this question would be interesting and useful also on the PSE.

Despite that it is obviously offtopic on the current rules, unfortunately.

Emilio Pisanty

@dmckee =P

Slereah

and 't Hooft

Sha Vuklia

6:05 PM

guys, why aren't newton's laws as formulated here? (the answer with the 31 upvotes) https://physics.stackexchange.com/questions/70186/are-newtons-laws-of-motion-laws-or-definitions-of-force-and-mass
everything that's written there actually makes sense.

Slereah

and Ashtekar

0celouvskyopoulo7

@ShaVuklia they are

What are you talking about?

Sha Vuklia

they are always taught differently. i've never seen them rephrased like that.

0celouvskyopoulo7

That's pretty standard

Maybe you should ask your teacher

Sha Vuklia

they don't formulate the first law as "there exist inertial reference frames"

0celouvskyopoulo7

6:07 PM

@Slereah GR is a horrible name for the subject

what does it have to do with relativity anyway

@ShaVuklia That's how I learned it...

dmckee

@ShaVuklia For many people those definition are very, very hard if they don't already know the material.

Sha Vuklia

really? well then you're lucky i guess

Slereah

The other "standard" name of GR is geometrodynamic

dmckee

Not that the usual intro versions are easy, but those are harder.

Slereah

Would u prefer that

Sha Vuklia

6:08 PM

lol, so they resort to something even more confusing? @dmckee

Slereah

Geometrodynamics is fairly rarely used

0celouvskyopoulo7

@Slereah If I write a GR book it would be Einstein Gravity from the Mathematical Perspective.

dmckee

Almost everyone I know in physics agrees that the abstract versions are much clearer once you know the material and many upper-division and graduate books introduce some version of them.

0celouvskyopoulo7

Einstein Gravity is the best word for it

Sha Vuklia

sure, that's why I would stick to the "conventional" laws in high school, but in university, they should introduce the abstract versions. like, for real. in the first year.

Slereah

6:09 PM

Einstein-Maxwell should be of course called electrogeometrodynamics

dmckee

@ShaVuklia People have tried those. In the introductory classroom Newton's versions are more accessible. Unless you already have a strong background in advanced math.

@ShaVuklia At least in the US many student don't get physics in in high school so the introductory college course is there first introduction.

ACuriousMind

Newton's laws are not that relevant once you get into abstraction. Lagrangian and Hamiltonian mechanics are where it's at if you want abstraction

dmckee

That's not ideal, but it is the reality of it.

ACuriousMind

That might also explain why their discussion is often so muddled - at the level where you could appreciate a clear and abstract formulation of Newton's laws, you don't have any use for them anymore

2

Sha Vuklia

@dmckee right, i understand. in any case, i feel a great sense of relief now that i have finally read the abstract version.

0celouvskyopoulo7

6:11 PM

@dmckee What does 'advanced' mean here?

Sha Vuklia

would you happen to know a book that introduces classical mechanics in an "abstract" way? I'm looking through Goldstein's book now, but I'm not happy with it. I would like a book that is extremely formal.

0celouvskyopoulo7

@ShaVuklia Arnold.

Extremely?

Abraham-Marsdon.

Rudolph-Schmidt.

Sha Vuklia

okay cool i'll have a look!

i hope it's in my library!

(i'm at our university library)

0celouvskyopoulo7

Two of those are Springer.

The other is AMS, IIRC.

Any library should have them.

dmckee

@ShaVuklia Marsden might be for you. But you'll need a strong math background.

AccidentalFourierTransform

6:16 PM

or Landau?

IIRC Goldstein was pretty good

Slereah

there's that crazy book that does classical mechanics with fiber bundles

I forget the title

It contains the harmonic oscillator done with the cotangent bundle

AccidentalFourierTransform

amazon.com/Geometry-Physics-Introduction-Theodore-Frankel/dp/…

0celouvskyopoulo7

No.

He means Burke.

AccidentalFourierTransform

:(

Slereah

"Real programmers don't use Fortran. Fortran is for wimpy engineers who wear white socks, pipe stress freaks, and crystallography weenies. They get excited over finite state analysis and nuclear reactor simulation. "

Those crystallography weenies

0celouvskyopoulo7

6:22 PM

@Slereah who said that?

Slereah

It's part of the old old "Real programmers" memes

0celouvskyopoulo7

Also how can you not get excited over reactor simulations

Slereah

It's from the early internet days

1983

0celouvskyopoulo7

lol

Slereah

"Real programmers scorn floating point arithmetic. The decimal point was invented for pansy bedwetters who are unable to "think big." "

Loong

6:24 PM

@0celouvskyopoulo7 I have a real reactor a few metres next to my office. ;-)

0celouvskyopoulo7

@Loong Aren't you in Germany?

Loong

@0celouvskyopoulo7 in Finland now

0celouvskyopoulo7

That's cool

We're not allowed to have one on campus by law

But we can use the one at Oak Ridge, roughly speaking

laws are the worst

AccidentalFourierTransform

Fock the police

Slereah

@0celouvskyopoulo7 got any programming classes in your studies?

0celouvskyopoulo7

6:27 PM

yes but not yet

well I had MATLAB

Slereah

What are u gonna learn

0celouvskyopoulo7

fortran

Slereah

lol

AccidentalFourierTransform

loser

Slereah

For some reason I managed to never learn FORTRAN

Even though I learned programming during my physics degree

We only learned Pascal and C

0celouvskyopoulo7

6:28 PM

@Accidental how so?

Sha Vuklia

@dmckee hm yea, I find Laundau and Marsden maybe too much. I guess I will have to stick to Goldstein, after all

AccidentalFourierTransform

fortran is for losers

0celouvskyopoulo7

why?

AccidentalFourierTransform

ergo, 0celo7=loser

Sha Vuklia

except for the issue with force, there's nothing wrong with it for me for now

ACuriousMind

6:28 PM

@0celouvskyopoulo7 KyleKanos would be proud.

Slereah

I think it is slightly wrong to write a 60's programming language in lower case

Sha Vuklia

and they state they're not going to dive into "space, time, simultaneity, mass, and force" at the beginning, so maybe I should just follow them

Slereah

For me it will always be COBOL and FORTRAN

dmckee

@0celouvskyopoulo7 If you have a pressing reason to learn fortran, you should learn one of the newer standards.

Slereah

plz

0celouvskyopoulo7

6:30 PM

@dmckee I'll learn whatever I have to for the class

Slereah

This is how you program in FORTRAN

2

dmckee

There is little reason to learn fortran 77 or digitals minor extension to it from the '80's unless you are going to maintain such code.

0celouvskyopoulo7

@dmckee ok please don't assault me until next semester

dmckee

Basically there are no concepts in that language that you won't find in every other imperative language out there.

0celouvskyopoulo7

I don't know what we're gonna do

Slereah

6:31 PM

@dmckee Is there a reason to learn FORTRAN outside of maintaining code?

dmckee

@Slereah I

I've actually done that. Just once. To say I had. It sucks.

Slereah

heh

dmckee

Debugging and correct especially sucks.

0celouvskyopoulo7

oh god this topology final

Slereah

I hear that it's possible to destroy a punch card reader rather simply by just poking out every hole

The card would usually tear inside the machine and fuck it up

dmckee

6:32 PM

@Slereah Despite the constant bagging on it there is a lot of work still done in fortran and there is a huge toolset of debugged code.

And the newer versions of the langauge support more modern programming styles.

loltospoon

Would time-dependent perturbation theory be a good approximation method for a spatially-varying potential? or is there a better one?

dmckee

@Slereah Generally didn't damage the machine, but it made the operators stop everything and take the reader apart to clear the bits.

Then the boos BOfH deleted you stuff.

AccidentalFourierTransform

@loltospoon if the potential is time-independent there is little reason to use TDPT

not that you cant

but yeah, why would you

Slereah

My favorite old timey language is APL, though

loltospoon

@AccidentalFourierTransform Ok. Any idea of which approximation method would be good then?

AccidentalFourierTransform

6:35 PM

time-independent perturbation theory?

loltospoon

The context is an intro to QM class

Slereah

life←{↑1 ⍵∨.∧3 4=+/,¯1 0 1∘.⊖¯1 0 1∘.⌽⊂⍵}

loltospoon

That was my next guess, yea

Slereah

It's a good language

dmckee

@Slereah Do you actually know it, or just know about it?

Slereah

6:35 PM

@dmckee I know of it!

Didn't learn it so far

I like very succint languages

dmckee

The only person I know who did serious programming in it worked her way through college hacking the stuff, then moved to HR.

Slereah

as u well know

esolangs.org/wiki/Lazy_Bird

dmckee

I met her at a knitting shop near Chicago. Nice, blue-haired old lady. Who hacks APL.

loltospoon

@AccidentalFourierTransform what if I wanted to approximate the energies and wavefunctions of an electron illuminated by visible light?

dmckee

But then that shop was kind of a hangout for women who had been in technical fields.

AccidentalFourierTransform

6:37 PM

the you have to use TDPT

unless $\omega\ll 1$

Slereah

I made a program to actually write more compact code for lazy bird

It was a very bad program

Because it was a brute force program

but it was good enough

you know what, I should make a compiler for it

Let's see if I still have the old python interpreter

0celouvskyopoulo7

@ACuriousMind Hi

What's a filtered $*$-algebra?

loltospoon

@AccidentalFourierTransform what does that condition mean? Is $\omega$ the frequency of the light?

AccidentalFourierTransform

yes

ACuriousMind

@0celouvskyopoulo7 A $^{\ast}$-algebra with a filtration, I'd guess, but I don't know what you'd use that filtration for

Slereah

6:41 PM

pastebin.com/ribgUsfd

0celouvskyopoulo7

@ACuriousMind Is a filtration like a grading?

Slereah

That's the one

Been a while since I worked on it

Let's make a C# version

ACuriousMind

@0celouvskyopoulo7 Yes, just that the grades are not disjoint - each "step" of the filtration contains the one before it

It's the difference between "polynomials of degree $n$" and "polynomials of degrees $n$ or less", roughly

0celouvskyopoulo7

@ACuriousMind Ah. Well the pseudodifferential operators are built like $\Psi=\cup_{d\in\Bbb R}\Psi^d$, and $\Psi^d$ increases with increasing $d$.

Sha Vuklia

Guys, could you say that for a particle, we have defined the “force” of a single interaction as the change of rate of the momentum of the particle (as a result of this interaction). So technically, Newton’s second law says we can add all forces vectorially. So the definition of a force is $\vec F_{\text{single interaction}}\equiv d\vec p/dt$, while Newton’s law states: $\vec F_{resultant}=d\vec p/dt$?

0celouvskyopoulo7

6:46 PM

omg, why are you obsessed with this

Sha Vuklia

because I want to understand it for once and for all, for ffs. i am actually supposed to be doing quantum physics, but i don't even know what a force is

sorry, physics makes me angry

0celouvskyopoulo7

then stop doing it

no one is forcing you

AccidentalFourierTransform

forcing

he

Sha Vuklia

lol

Slereah

It's pretty hard looking back at code you wrote almost ten years ago

Plus I wasn't very good at programming back then

Sha Vuklia

6:48 PM

no i do love it, once the definitions and (experimental) laws are clear. honestly though, that's the least i should be asking:d

Sanya

@ShaVuklia "Guys, could you say that for a particle, we have defined the “force” of a single interaction as the change of rate of the momentum of the particle (as a result of this interaction)." I do not think that's a good definition to be honest

ACuriousMind

@ShaVuklia Yes, you could say that, I think.

Heh. What do you think is bad about it, @Sanya?

Sanya

because the change rate of momentum of the particle as a result of this interaction is not really well-defined if we have more than one force

Sha Vuklia

of course it is!

that's why we have newton's second law:)

and it's not defined, it's experimentally determined

oh wait

i think i know what you mean

but if there are multiple forces,

then we could "isolate" stuff I guess

so we should we talking about "elementary interactions"

Sanya

newton 2 is $\frac{d \vec{p}}{dt} = \vec{F}$

ACuriousMind

6:51 PM

@Sanya Well, we make the usual thought experiment of "turning off all other interactions" :P

Sha Vuklia

yea, I agree with @ACurious

Sanya

@ACuriousMind I do not think that is very intuitive :D

Sha Vuklia

like, we defined force for an interaction with "one force", and everything else is just a composition of interactions then

Slereah

@ACuriousMind Turn off the universe!

Sha Vuklia

lol @Slereah

ACuriousMind

6:52 PM

@Sanya I completely agree but Sha doesn't seem to be looking for intuition ;)

Sanya

@ACuriousMind by the way, I'd be pretty happy with the definition of "a force is the rate of change of momentum of a particle under the influence of only one force"

but it seems kinda circular to me

Sha Vuklia

hahaha yea

AccidentalFourierTransform

@ACuriousMind adiabatic switching off, amirite

Gell-Mann did nothing wrong

0celouvskyopoulo7

@Sanya what?

Sha Vuklia

@Sanya well at one specific point in time, the particle can only accelerate in one way. so we simply define the "force" of that interaction as the rate of change of momentum

AccidentalFourierTransform

6:54 PM

$\dot a_r=0$

ACuriousMind

@AccidentalFourierTransform "Adiabatic", pfffft. Just flick a switch (aka use a Heaviside function to turn it on/off)

0celouvskyopoulo7

That's circular.

How do you know if there's only one force in the defn of force?

AccidentalFourierTransform

RIP continuity

Sha Vuklia

so we don't even have to bother about interactions with "multiple forces"

the force is simply the rate of change of the particle

caused by this interaction

in fact

Sanya

@0celouvskyopoulo7 hmmm?

Sha Vuklia

6:56 PM

we aren't even allowed to bother about "multiple forces", because they weren't even defined at first.

actually i don't know

never mind

0celouvskyopoulo7

@Sanya See my other two messages right after.

Slereah

There is also a clever ruse in mechanics where you have the UNIVERSAL FORCE

That is not measurable at all

Sanya

@0celouvskyopoulo7 "but it seems kinda circular to me" see my message afterwards :D

0celouvskyopoulo7

@Sanya oh :P

Slereah

A universal force affects all objects in the same way, hence you cannot measure any change in position

Sanya

6:58 PM

@ShaVuklia who tells you there is only one?

(interaction=

AccidentalFourierTransform

=bad)

Sha Vuklia

@Sanya well, we have the following thought-experiment: we have a particle and a single interaction. the rate of change of momentum of this particle (as a result of the single interaction) is called the force.

an interaction could be anything, as long as it results in the change of the particles momentum, I guess.

0celouvskyopoulo7

@Sanya I have a project this summer and it's added many books to my reading list :(

books that I have to read

seems a lot less appealing now

being forced to read books

amazon.com/…

@Slereah ^

Newer HE

Slereah

Pankaj Joshi isn't Hawking at all

I really wish I commented that code 10 years ago

0celouvskyopoulo7

what are you trying to do?

Slereah

7:03 PM

Trying to make a new version of that program for this decade

It's a logical combinator interpretor

0celouvskyopoulo7

No Steenrod reference

Insanity

Slereah

he probably assumes that his spacetimes are non-compact

Astrophysics books usually work with non-compact spacetimes

Who needs Steenrod

kvasirr

Is there anyone ?

Sha Vuklia

no

0celouvskyopoulo7

I thought one needs Steenrod for the concompact case

Sha Vuklia

7:05 PM

is there?

kvasirr

world is collapsing

but i will save the world

0celouvskyopoulo7

It's not hard to show that one needs $\chi=0$ in the compact case

that's just Poincare-Hopf

Sha Vuklia

is there even a world? maybe it's just your imagination.

kvasirr

that every truth is not necesseraly true

no is yes

0celouvskyopoulo7

there are better books than steenrod for that

Sha Vuklia

7:06 PM

maybe your imagination is collapsing :P

0celouvskyopoulo7

For instance Guillemin-Pollack + Lee

kvasirr

I want to learn about physics of knots

0celouvskyopoulo7

I know a knot theorist

kvasirr

what kind of forces are implied in it, i will turn it to a game engine

real world knots what I mean with two open ends

0celouvskyopoulo7

Why do you need to know about real world knots with two open ends for a game?

kvasirr

7:09 PM

I will make a game :D

0celouvskyopoulo7

about knots?

kvasirr

yes

0celouvskyopoulo7

Is it about hanging people?

There was a really cool mechanism in one of the Assassins' Creed games where you could hang people

kvasirr

no, especially not

0celouvskyopoulo7

You could do it with heavy infantry that were otherwise difficult to dispatch.

kvasirr

7:10 PM

I am sure it only covers hanging

I need the mechanics of tie

in which geometry it requires heavy or weak force to comb it through which angle, in that simplicity manner

not weak force of physics but

parlance

0celouvskyopoulo7

Probably not chat appropriate actually.

kvasirr

I don't know, I can write the code but I couldn't think what this scenario consists of in physics domain that is why I thought it is a physics consultation

Slereah

Ah, I think I finally understood that bit of code I wrote 10 years ago

I wanted to check if the user was using, for instance, s as a variable name

Which would ordinarily show up as ^s

But

it is also possible that this was part of the sequence .^s

Which means "print ^ and then s"

which is a totally different thing

0celouvskyopoulo7

Lol on my thermo exam I had to compute the energy of a rock in BTU

crazy

Slereah

And I had to do a loop because it was entirely possible that .^s was actually part of, say, ..^s, which would mean "print . and then declare s as a variable"

0celouvskyopoulo7

7:19 PM

it's the most GDP unit ever though

Slereah

I think that's why I made that weird loop

So ....^s would be a variable only if the number of periods was even

Slereah

7:33 PM

Also a lot of my code doesn't translate well from Python to C#

kvasirr

python and c# hurts in same sentence

Qmechanic

-3

Q: Why does the finite difference script for solving Poisson equation not work for delta function charge? How to fix it?

I know the charge density at the plates of a capacitor, from which the applied voltage and potential profile is to be calculated. Using the finite difference method in MATLAB script I solved the Poisson's equation that gives the correct result for all kind of continuous charge profiles. I checked...

electrostatics capacitance computational-physics software dirac-delta-distributions

Off-topic? Close as Unclear? Migrate to Computational Science?

Sha Vuklia

7:53 PM

Guys, how can we call the "conversation theorem for linear momentum" really a theorem, when it only involves plugging in zero in the second law of newton?

Emilio Pisanty

@Qmechanic no, just a not-great question

Sha Vuklia

is it because it is such an important fact, that they just state it as a theorem?

Emilio Pisanty

Migration won't help, it's a units issue

it's not unclear, it's a units issue that was dreary but doable to diagnose

ACuriousMind

@ShaVuklia Who said a theorem needs to be difficult to deduce? :P

Sha Vuklia

lol, that's true

but that's the easiest theorem i've ever encountered, then

(just saying)

Emilio Pisanty

7:55 PM

@ShaVuklia so what do you do in situations where you have particles with forces between them that don't obey Newton's third law?

Sha Vuklia

i'm guessing i move then to an inertial reference frame and see what happens there?

gotta go now!

see ya!

Emilio Pisanty

@ShaVuklia for later reading, then

14

Q: Apparent violation of Newton's 3rd law and the conservation of angular momentum for a pair of charged particles interacting magnetically

Consider a system of the two identical point positive charges situated in free space (isolated from influence of any other external fields) as shown in the figure below. Particle 1 is at $(a,a,0)$ and particle 2 is at $(0,a,0)$. Their velocities, at the time under consideration, are as shown in t...

electromagnetism newtonian-mechanics angular-momentum conservation-laws torque

Slereah

h e l p

C# substrings work with a starting index and a length

Python works with a starting index and an end index

Either I make a special function to redo substrings or I have to redo all calculations

kvasirr

you don't have to redo calculations, wait

loltospoon

What approximation method could I use for a quantum system where there is a rapidly applied change in potential?

My guess would be Time-Dependent Perturbation Theory.

kvasirr

8:08 PM

I couldn't understand from chat, what is the problem tidy ?

10 Replies

anybody know much about resonation of various hollow objects in relation to speaker cabinets?

I found this: en.wikipedia.org/wiki/Acoustic_resonance

I'm thinking about building a spherical speaker cabinet, but, I was unsure of how it would resonate

or if that is even important

The wikipedia article has this equation, but, I'm not sure if it applies for what I'm doing

I'm not sure what a "sound hole" entails. I wasn't planning on making a ported cabinet (I'm making an enclosure without a hole)

Which would mean that the resonant frequency would be 0...

But, it could also be referring to the driver (speaker), in which case it would be 8 inches (or .203 meters). I'm not really sure though

Is this worthy of opening a question on Physics SE?

ACuriousMind

@10Replies If you ask about building the cabinet, it would be off-topic as engineering. If you can make it an objective question about sound and resonance, it might be on-topic, but currently I'm not sure what exactly your question is.

10 Replies

I'm asking about the resonance of spheres in general

Not really about how to build this specific cabinet

0celouvskyopoulo7

@ShaVuklia Pythagorean theorem?

10 Replies

I guess a more concise way to put it is "How does one calculate the resonant frequency and or frequency response of a sealed hollow sphere"

AccidentalFourierTransform

9:36 PM

Apr 19 at 18:31, by AccidentalFourierTransform

chat ded

0celouvskyopoulo7