Hans Zimmer is the best Composer that ever existed on the face of earth

Master Oogway is the best Philosopher of the time who ever existed in Universe

Change my mind!

GooD MoRniNG.. :-)

Anyone care to give me some help on deriving the spherical harmonics?

This video was uploaded on 26th October and gives deja vu kind of feeling when I look at it now:

@psa what do you mean deriving? Their formula, why they arise in the context of the Laplace equation, etc...

more precisely, producing the first nine terms from acting on a complex number $(x+iy)^j$ ($j=0,1,2$) with the angular momentum operator

put some decoration on the GR shelf

I can't tell whether the decoration is the brass figure or "Quantum Fields and Strings: A Course for Mathematicians" :D

ahhahahahha I was thinking the same

does anyone know a good resource to keep studyng concepts from standard model following an approach similar to the one used by Baez and Huerta in arxiv.org/abs/0904.1556 ? I am mostrly interested in see how unification of $SU(2)$ and $U(1)$ is carried out using group theory and representation theory, but even a generic resource discussing SM using group theory would be nice.

looking from the resource recomendations of the site I think group theory from Pierre Raimond could be it, but I am not sure

Do you mean just gauge theory?

I think what I am looking for is particle physics using representation theory. something that goes a bit deeper respect the paper of Baez.

if there is even some gauge theory that's ok, but what i would like to see is an approach using methods of group thoery

@Slereah You have more books than I've noodles in my packet

@Ratman section 1.1 mentions further reading, e.g. Zee's book has stuff on this

What's Witten (and Dirac, well it does have 1 chapter...) doing on the GR shelf :p

Well, there's odd one out too :P

Yes I have seen the further Reading part, in fact i was trying to look at the Anthony sudbery ref

I'll try to see zee as well, thank you

@bolbteppa Hello sir!

Do you've any idea on how many courses are there in a regular B Tech college and how much do students take?

@bolbteppa some are misplaced

@Azmuth no idea sorry

oh okay...

Also Dirac's book has a section on GR

@Slereah I found the ultimate paper on GR spinor stuff

For spinors in GR I mostly use Springer's GR handbook

It has a nice section on GR spinors

@Slereah have u read all of these?

They're textbooks

I have not read them cover to cover

@Slereah have you completed all?

There's only so many times I can take someone defining what a vector field is

all books?

@Slereah what do u do with all this knowledge?

^Don't ask

I develop mind powers

Yo @Slereah

hey @BalarkaSen

That's a good bookrack

The "Dirac - Lectures on Quantum Mechanics" is for me

Don't look at the shelf below

(the little red book at the top)

It's the Humanities shelf

Lol

Mostly archeology

and philosophy

@Slereah i'm asking seriously :P

Do I need a reason to learn science

no, it can be to satisfy your curiousity

i'm just asking if you're working for nasa or something

I am not

NASA doesn't do a lot of GR

Since they do aerospatial engineering mostly

@Slereah I'm NASA, ask me

@Azmuth How's them rockets

@Slereah satellite theory stuff.... :P

Plenty of people work at NASA, I assume

Cooks, drivers, mechanics, puppeteers

great, so there's a place for me

I'm gonna clean them rockets

I'm gonna fix computers

Imagine the guy managing the google doodles

"I work for Google"

@Slereah so you learn science to satisfy your curiosity but you don't apply/use the knowledge and you haven't applied/used the knowledge? Just wondering, not judging.

I mean it's GR

What would I apply it to

gps :D

make ur own gps

oh nvm

that was SR

GPS is 1 GR thing and a million signal theory and radio waves things

calculate how spacetime curves around ur body :P by knowing your size, mass, earth's size and mass..

actually, what is GR used for lol?

See this bit of ancient wisdom :

@Slereah hehehehehe DO you have that poem GR Book?

Funny one!?

Buy that!

I do not

@Slereah soo, what is GR used for?

Hard to find a copy that will ship to France

@Slereah Buy that and read those to your kids, there must be one in Amazon

I don't think that book has been printed since the 60's

@Slereah lemme check it

there are some, but they are used copies

that are 1) expensive 2) not necessarily shipping here

I guess GR will become a big thing in the future after we start colonizing space. But no one knows if someone won't come and disprove GR by then lol

@JingleBells what does 'disprove' GR mean? It makes many, many testable predictions that have been verified

Of course, it is not a complete theory at all length scales

@NiharKarve I don't know, but for many years people thought that the Earth was flat, then that Earth was the center of the universe/solar system and whatnot. Recently, physicists thought they had figured it all out until the blackbody radiation thing which led to QM... I'm not saying that GR is wrong, but in history, we've got enough evidence to know that things are not always what we think they are.

@JingleBells Absolutely, it was just a quibble on your usage of "disproven". Physical theories can't really be proven or disproven from axioms the same way mathematical theorems can

@JingleBells The wrongness of e.g. Newtonian mechanics compared to QM is different than the wrongness of the belief in a flat Earth, and conflating the two is dangerous. GR - like Newtonian mechanics - is well-tested in many, many predictions. If it is wrong, then it is wrong in the sense that it is not the whole truth, but it must arise as a limit of the underlying truth, since we know it makes correct predictions.

The belief in flat earth makes no correct predictions at all - it can indeed be disproven by basic observation (some Greeks already knew how to do that), and nothing about it is even approximately correct.

The whole purpose of the scientific method is that we can be certain our current beliefs cannot be disproven like that - since observation and experiment conform to our predictions, we know they are at least somewhat correct.

@ACuriousMind you should do a Peter Shor and try out ELL

Twitter, SE, TG, WP, Pinterest, Chrome was down in India

what all has happened?

Linux Debian server too

@NiharKarve I'm not sure whether you're saying I should start answering questions there or if I would benefit from learning how to speak English :D

XD

@ACuriousMind I've never asked, what is your profile picture? It seems eerily familiar.

it's Kim Kitsuragi from Disco Elysium

When it comes to learn to speak English, I had the first English language course online yesterday.

how did it go?

the instructor asked each person in class to answer questions throughout the class, rather than the usual lecture, in which only the lecturer talks.

well you can't learn to speak a language if you just listen to it :P

I once had an English class with Dr. Sashi Throor, I only later realized that they spoke english

Shashi Tharoor*

whatever

Has anybody used cohomology in physics outside of a GR context?

the lecturer said the connection is poor when I was asked to make self-presentation and said I need to make self-presentation again some time when the connection is better. It's like the poor connection is a common problem. I have encountered it several times when I interacted with instructor in the online course last semester.

@CaptainBohemian Your English already seems quite good to me!

@NiharKarve yes, comes up a lot in beyond-the-standard-model and string theory contexts, in particular when compactifying

@NiharKarve that English language course is a compulsory course in this physics program. I am not sure if it is only for students from non-native English speaking countries.

@CaptainBohemian ah, ok

often the (co)homology of the compact manifold is more or less directly related to the particle content of the effective theory on the non-compact part

that makes sense intuitively

but, ya know, anything simpler? I was inspired by Baez's piece on cohomology in circuits

the English language course is designed for PhD students and we are required to make presentation about scientific papers.

@NiharKarve you can think about a lot of stuff related to potentials in terms of cohomology

the existence of a potential for some field $F$ with $\mathrm{d}F= 0$ is the Poincaré lemma - contractible spaces have vanishing cohomology,. essentially

I used de Rham cohomology in my last physics assignment

Lol

nice, what for?

To argue that some field is conservative

(co)homology

@ACuriousMind Mass “has a cohomological significance, it parametrizes the extensions of the Galileo group.” (Santiago Garcıa, hep-th/9306040) - from nLab (the other two items are regarding electrodynamics and string theory)

so e.g. you can think about the Aharanov-Bohm effect as the solenoid in the middle being effectively removed from spacetime, making the cohomology non-trivial, so that the 4-potential $A$ is not globally defined (and also so that Stokes' theorem does not apply and you can turn the integral along the path $\int_\gamma A$ into a $\int_S F$ where $\partial S = \gamma$.

Gamma

@NiharKarve ah, that's Lie algebra cohomology, which I guess is technically cohomology but I've never seen it as as geometric as "normal" cohomology :P

The $H^2(G)$ I explain in this popular answer as the origin of central extensions is technically a second cohomology group but I've never gotten any great insight from thinking about it like that

That was one of the first answers I ever read on physics SE :)

obviously everything flew over my head

Fun fact, I learnt Lie algebra cohomology from some BRST cohomology notes ACM shared with me

I think its great

ah, the O'Farrill ones?

yeah

Share further

I want to see his handwriting

@BalarkaSen Well, by all means, continue the chain

It's available online: empg.maths.ed.ac.uk/Activities/BRST/Notes.pdf

(it's not notes I took, it's just notes I recommended :P)

@Azmuth and you'd be hard-pressed to find handwritten physics notes online

@NiharKarve Ok

keep mine, If I became legend one day, these notes will help you get some $

@NiharKarve Don't say that

thsoe are the official lecture notes for a bsm course in 2010: thphys.uni-heidelberg.de/~hebecker/BSM/bsm.html

I mean, ironically I have the EPFL lecture notes on GR as an EFT open, most of which are handwritten

the theoretical mechanics lecture series I took with him in 2011 was also handwritten but it seems to have been taken down and replaced by LaTeX in the newer versions of that lecture series :P

@ACuriousMind Your handwriting is good or black ink makes it look good?

On the second link you've shared, all the links are purple for me

very mysterious

@Azmuth it's not my handwriting, it's Hebecker's

okie dookie

my handwriting looks like this (notes from a QFT lecture I attended in 2014)

Excellent deltas :D

that's how I learnt to write it when I took Ancient Greek in school

you took what now

@ACuriousMind I'd suggest to keep proper spacing and intedation, it'd look cooler in your handwriting

I attended a "humanistic school" where you could take Latin and Ancient Greek as foreign languages instead of modern languages

Could've also taken Hebrew but it didn't fit into my schedule :/

also???

I also write delta like that

@ACuriousMind here's my cool handwriting

I suppose the bare minimum of learning Ancient Greek is that you can finally write a xi without butchering it into some forlorn-looking backwards 3

it is quite common even here to study ancient lenguages and literature , I studied Latin in a scientific school

seems adobe removed the links

@NiharKarve part of me dies every time some physicist or mathematician just draws a random squiggle and calls it xi :P

lemme post again

I got to learn French and Spanish

idk, might come in handy if Slereah shares some French stuff at some point

that's the downside of having learned the ancient languages - the only language I speak that's still alive and not my native language is English

I already know french

same here, just english

but I figured there's plenty of opportunities later in life to learn e.g. French if I want to but not quite as many for Ancient Greek

@Azmuth but you must speak at least 3 languages right?

@NiharKarve Hindi, Bhojpuri, Odia, English, French and a bit of Sambalpuri

Well, honestly, I'm expert in none of them

for us it wasn't actually a choiche, in "scientific school" you have English and Latin

@Ratman which country was that in?

italy

I probably didn't appreciate it enough as I would now

@ACuriousMind here it is, my notes

@ACuriousMind I somewhat agree with what you're saying. If GR was to be "disproved", it would most likely mean that it's not the whole truth. But I believe that our brains are very limited and there are many unknown unknowns that are unreachable to our current cognitive capacity. Just because our current brains are convinced that they are measuring reality, it may not necessarily be so.

If you realize that you're a bunch of connected neurons, just like a mouse or an ant, a whole bunch of possibilities opens up as to what happens if you add more neurons and more connections.

@JingleBells I don't think the idea that there is a reality wholly distinct from what we are perceiving is a consistent notion of reality. What does it mean for something to be "real" if not that we can agree on perceiving it?

@JingleBells why do you not apply the same doubt you apply to physics to biology? Maybe we aren't bunches of connected neurons and we'll find out we're something different in the future?

@ACuriousMind I do apply it. I can be wrong.

But then your objection is pointless. Everything can be wrong, all of reality might be a mirage caused by Descartes' demon so why point it out for GR specifically?

@ACuriousMind Because we were talking about GR and not about biology :P

I'm just a bunch of firing neurons, so who am I to be certain that anything is certain. Who am I to be certain that I'm just a bunch of firing neurons?

@JingleBells and if I had said that the sun shone today instead of talking about GR, would you have pointed out that I cannot be certain that the sun shone and I might have hallucinated it?

No, but it's true that the sun MIGHT not be there. I trust my senses as much as I can, but I also trust that my senses are limited. I have an idea of why I said it about GR.

But who knows if the explanation is not just a fabrication of my left brain.

ahh how I missed our talks :D

The point I'm making is, I believe in the limited brain. But as you pointed out - I might be wrong, it's a belief after all.

Radical doubt should be a starting point for an epistemology, it cannot be its end. Yes, the sun might not be there (again, see Descartes' demon), but why does that matter in the contexts of the degree of knowledge (possibly incomplete vs. utterly wrong when compared with observation) I was talking about?

You don't jump out the window just because there might be solid ground directly outside it. That we can radically doubt everything does not mean that we don't trust in knowledge to different degrees.

You can trust in knowledge. Just like I trust in the knowledge that knowledge cannot be trusted.

lol

I don't know what we're debating at this point. :P

I don't understand your point of view. What's the point you're trying to get across?

@JingleBells You seem to believe that "reality" could be completely different from what we perceive. I believe that a notion of reality that has nothing to do with what we perceive is rather useless.

Program on design of assembler using C/C++

question^

Absurd na?

But, it can be done for

CISC x86 based 8086 processor

You dunno how the hell, I'm excited to go to college

Comtupar Engynar

even if there "really" is a world different from what we perceive, there is no point in believing that, since what useful predictions does that make? What would you do with that "knowledge"? How does the "reality" of that world differ from all the other things that are simply untrue?

@ACuriousMind I think it's useless too. I'm just saying that it's possible. I also think that it's useless to doubt everything.

so let's stop :P

Nothing interesting will come out of me doubting the essence of everything

Y'all should listen to Comtupar Engynar and believe that GR isn't good thats why String Theory exists!

dinner time, brb :)

I believe in the scientific method. If observations confirm it, then I trust it. And it's possible that a "different reality from what we perceive" can exist, but as you said, nothing useful or interesting comes out of believing in it.

do we agree here? :P

yeah, alright

@ACuriousMind I guess the confusion arose from the assumption that I believe that GR can be completely disproved, but no, it's just a possibility and a useless one to think about :)

I'm back!

Yoooo people! I know y'all missed me here :)

→ 9 messages moved to ­Trash

@Azmuth what did u eat?

Paneer Butter masala and Naan!

After a long time tho :)

azmuth are you ben 10 one azmuth?

@PrateekMourya Yes

unfortunately ben 10 series now turned to shit

Just In: Joe Biden became 46th President of USA with +14 seats!

Biden wins lol

Biden looks older than Trump, infact trump looks 50(s) or so, BIden looks 90(s)

the internet is exploding right now

Finally...

Hi

Why example of rocket motion can not be explain by newton's law???

Y'all check this out! Norbert Elekes just made an unexpected comeback to Twitter, was vanished for months!

The Twitter exploded, Norbert Elekes is trending!

Poltics in science and science in poltics +

Does any body have the image of sun which clicked on 1916

I am searching it for last 3 hour?

@123 Newton should be sufficient for explaining rocket motion, you just have to realize that you can't use $F = ma = m \ddot{x}$ because rockets depend on changing mass. The more correct form of Newton's second law is $F = \dot{p} = \dot{m} \dot{x} + m \ddot{x} $ (ignoring the vector bits), which is what's needed for rockets (at least for the case I've seen, I imagine real rockets are a bit more complicated).

@danielunderwood until of course we're all intrepid space explorers and we start launching our rockets between a binary black hole system

Hello @knzhou, I just wanted to say that I really enjoyed the lecture notes on your website

What's the Icarus equivalent for a black hole?

Trying to skim the event horizon?

@NiharKarve Thanks! Glad to see people are reading them.

Is anyone on?

Hello @urb

I guess no one wants to have a chat

sup

Nothing much than I have a few questions

I am a layperson and I just am interested

just ask the questions and if anyone wants to answer, he/she will

Do we know what gives speed of light/casualty it's speed limit?

As far as I know light just travels at that speed. That's the way the universe is.

So we don't know why it can't travel faster or slower?

in a vacuum.

I'm not sure. I usually ask questions here than answer, but as far as I know, that's the way the universe is. According to Wikipedia: "There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c". So it's just a constant of the universe.

Okay so we don't know why it's a constant than?

space.com/speed-of-light-properties-explained.html

^ take a look

Thanks

Units are a weird thing. When you think about the speed of light, it is defined in meters/second, but what is a meter, and what a second? A meter is "the length of the path traveled by light in a vacuum in 1299 792 458 of a second." and a second is defined in terms of the radiation frequency at which atoms of the element cesium change from one state to another. So it all breaks down to fundamental measurements of universal properties. (as far as I know, don't take my word too seriously)

Regarding units, I have a question. The SI base units are supposed to be defined independently of anything else, aren't they? So why is a second defined as 9 192 631 770 Hz where a Hz is defined as a cycle per second? It's like, a second is defined by itself. Am I getting this wrong?

According to Wikipedia: "Caesium atomic clocks are the most accurate time and frequency standards, and serve as the primary standard for the definition of the second." So a second is defined by this measuring device? Like, the amount of time this device takes to do something or make something happen is 1 second, something like this?

Does this mean that every unit in existence is defined from this device? (cuz a meter is the length light travels in x seconds...etc...)

@MathCubes See physics.stackexchange.com/q/230703/50583

@JingleBells You shouldn't say "Hz" in that definition because a Hertz is literally just the unit of "per second". You can count the oscillations, and when you've counted 9 192 631 770 oscillations, we say that's the time span we call a second

it's not dependent on itself, it just relies on counting (and the assumption/knowledge that the oscillation is regular)

@SirCumference tired again? ;)

@ACuriousMind My keyboard's command key has sometimes just not worked lately. Tried to do command+l to get to the search bar and go to facebook, but realized it didn't go through

ah, fair enough :P

THe question is why is it the speed it is

Not why it's exist but why it's it that speed

?

@MathCubes if you're looking for a satisfying answer explained by some theory, afaik we don't have one yet

Okay

they all just take it as a postulate that the speed of light is finite and equal to $c$, because that's what observations show

if it was any other speed, how could we tell :P

Are there hypothesis on it?

it is not meaningful to ask about the numerical value of dimensionful constants

see physics.stackexchange.com/q/78684/50583

I guess it ultimately comes down to a question of why there's a finite speed limit in the universe

Because if there's a finite speed limit, it's gonna take some number

it's fair to say we don't know "why" the fine structure constant has the value it has, but it's not fair to ask why the speed of light has the "value" it has, because ultimately all units are defined in terms of "constants"

Maybe some esoteric theory gives a good fundamental explanation of it, but not anything standard I know of

Well yeah

It probably has to take some value

I guess physics just isn't like math where everything has a fundamental explanation. We kinda just accept that these constants have certain values and work with them

Or maybe I'm totally wrong

I was just was wondering why it's that speed. I know we define the units but the speed I thought must been define by something in the universe.

Probably the same reason the gravitational constant and planck's constant have their value, rather than being something greater or smaller

Which isn't something anyone really knows

Okay gotcha

the last Q&A I linked explains in depth why the "value" of dimensionful constants is not as meaningful as one might naively think

I read that

we can't meaningfully talk about changes in these constants any more than we can meaningfully talk about what their "precise" value means

the real constants you can ask this question about are the dimensionless constants like the fine structure constant

well, you could talk about changes in these constants relative to other constants of the same units

like ask why the speed of light and the planck length differ by the exact factor they do

@SirCumference ...and such a ratio would be a dimensionles constant, no?

...yes

good point lol

Anyways thought on this video

I don't watch videos about physics :P

i do have to wonder if someday there'll be a model that theoretically derives the value of all these constants

like one that could tell us $c$ even if we had never measured it in history

Interesting

@SirCumference if we could discover a model that "forces" all the constants to be a particular value, we'd just shift the question to "why" that model actually models reality

it seems we can never be satisfied

I feel that a lot of the unease people have with free parameters and universal constants is just a symptom of a deeper dissatisfaction that we cannot - ever, in principle - discover by mere science the way the world was "meant" to be

Many gravitate initially towards science because it seems to explain "why" things are the way they are, and then disappointment sets in because at the end it's just a description of how the world is

i mean it does satisfy some of that curiosity as to why things are the way they are

just not all of it

some of the unease about the double slit experiment is satisfied when you know about the wavelike distribution of particles

personally I think of it more as dissolving the "why" rather than answering it, but I'm not gonna claim everyone has to see it like that

wait shoot, which experiment was it that motivated us to model particles with wavefunctions

was it the double slit?

partly? the foundational experiments for QM mentioned are usually the photoelectric effect and the double slit

the photoelectric effect suggest there's something "like a particle" in waves (=light), the double slit with electrons or other particles suggests there's something "like a wave" in particles

So the speed of C doesn't come form it interacting with a field that we know of?

I'm not sure what that would mean, but no, you can't derive the value of the speed of light from QFT

it's an ingredient in (relativistic) QFT, not a result

ok