The h Bar - 2021-05-03

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1:57 AM

Must a representation of the Lorzents group SO(3,1) necessarily be 4x4 matrices?

2:55 AM

Anyone have any good sources for a "crash course" on physics? I'm looking for something that can stay relatively rigorous and interactive, like a textbook, but also encompass all of the beginning middle and contemporary physics. Kind of like bourbaki for physics.

I have a history of physics book that's pretty good that I'm gonna keep reading but if anyone has good sources themselves, that'd be very helpful. Alternatively, if anyone in physics can chat about their knowledge I'd be really interested.

3:26 AM

@JohnT. how about this or this

@Jbag1212 it doesn't have to be, it depends on what you're doing, e.g. you can trivially take two separate copies acting in an $8 \times 8$ matrix etc

4:03 AM

@bolbteppa I'll check those out. Wow I totally forgot about landau lifshitz! I think I tried finding the volumes online but some were missing/unavailable. Still great stuff!

2 hours later…

5:41 AM

@Jbag1212 The trivial rep is a 1x1 matrix!

5 hours later…

10:47 AM

Music Theory and Practice turns 10 years old today ... and they get a congratulations banner

user image

::grumble grumble::

I don't recall us getting the same last year

I don't think there was anything, no

1 hour later…

12:25 PM

will we ever stop getting the question "How to invert the metric in the linear approximation"

2 hours later…

1:58 PM

Hi All..

2:42 PM

If pressure on incompressible liquid is increased on depth. $p = \rho gh$.

How according to pascal law pressure is equal at every point is equal/same? It should be different at different depth as hydrostatic pressure.

@ACuriousMind How these two laws justified?

3:16 PM

@123 It's $\Delta P = \rho g \Delta h$ and the change in pressure at a point in the fluid is transmitted to the whole fluid. It's more of a characteristic of fluids than anything.

3:47 PM

@Slereah well, at some point the universe will enter heat death if we're lucky...

4:05 PM

-2

Q: Can one engage in intercourse with themselves using the principle of quantum entanglement? (details follow in desc.)

Can one engage in intercourse with themselves using the principle of quantum entanglement? If so, how fast would this person need to move/travel in order to succeed?

quantum-mechanics quantum-entanglement

ah, junior high school

ever a source of inspiration

Hello @JohnRennie Sir..

@123 hi :-)

@EmilioPisanty I chuckled at the two "non-mainstream" close votes :P

"intercourse" can just mean any form of conversation. It does not have to take place between the sheets.

@ACuriousMind it looks like you voted as non-mainstream as well, FWIW

4:09 PM

The OP could just mean can you have a conversation with yourself.

@JohnRennie it does, but my sophisticated Bayesian model of internet comments puts an overwhelming probability on this not being what OP meant

Though you get strange looks if you have intercourse with yourself in public.

@EmilioPisanty no, I chose a customer close reason, but it seems "majority reason wins" is what happens when the mod vote is just the 5th vote

@JohnRennie is there anyone not from the Victorian age that actually uses it that way?

A true story: during the time my family lived in the Sudan there was a suggestion that a Sudanese official had become overly familiar with a member of the British Embassy staff. The Sudanese foreign office released a statement that, and I quote, "nothing happened beyond the normal diplomatic intercourse".

I wanted to learn solution of Poisson's and Laplacian equations. Which book is best for intuitive explanation?

4:12 PM

@ACuriousMind perhaps you're being too quick at discounting just how many people from the Victorian age are alive and well in Britain today

@EmilioPisanty They all voted for Brexit.

presumably those wouldn't need us to tell them how time machines work, though ;P

@123 probably Griffiths?

@ACuriousMind oh, no, they were born in the 1960s and 80s and 2000s

they are just still from the Victorian age

@EmilioPisanty Griffiths has too many books. Pls mention the name. I have Giriffith's Electrodynamics. Which i am currently studying but he is not explaining the poisson and laplacian.

btw @ACuriousMind your profile picture looks rather disturbing at small image sizes (looks like a person that's been flayed?), and not much better at full size in your profile page -- consider updating it soonish =)

not that I think it's a problem, but ♦

@123 electrodynamics, yes

4:17 PM

@EmilioPisanty Griffith's electrodynamics just using poisson and laplacian. Not explaining these functions how we come up with these.

I will soon endeavor to find a profile picture that doesn't make people faint in horror :P

@123 "how we came up with" what exactly?

Is there any mathematical book which completely explaining every aspect of these functions.

the differential operators?

or the solutions to the differential equations?

(but it's just tattoos, there's no flaying going on with Xzar that I know of)

4:18 PM

Yes Solution of differential equation. Oops. I was too fast in writing comments.

@123 if you're looking for a book which offers a complete theory of how to solve the Poisson and Laplace differential equations, and which does so in an intuitive way, I would say you're extremely unlikely to find one

those two things are either/or, I would say

@EmilioPisanty :P Why

Isn't there any good book at that level.

there are good books

but the complete theory gets very technical

@EmilioPisanty Share few name of good books. I will find answer from two to three books.

one way to understand it is this: there simply isn't a closed-form general solution

and there isn't any single method of solution that will work in all circumstances

there is just a large toolbox with many different ways to approach the solution

each tool works well in particular cases

4:23 PM

@EmilioPisanty It means physicist always believe on mathematicians. What they told us for solution of equation and we use them. :/

that's a strange thing to say

@123 I'm sorry if this feels like a betrayal, but here goes: not all differential equations can be solved by humans

there are formal theorems that guarantee that the solution exists

but that doesn't mean that you can write it down

and even less that you can write it down in a form that's useful

@EmilioPisanty It means we have no choice except believe on those theorems.

you don't need to "believe" in theorems, you can prove them!

@123 you don't need to "believe" those theorems

they have proofs

the thing is, there are many more mathematical functions that 'exist' than what we can humanly write down

4:27 PM

@ACuriousMind Oookay... How to understand those theorem intuitively. Otherwise i don't understand the insight of these equations and their solutions.

Pls share few good books name.

imo, "intuition" is just a word for "I have seen this so often I don't need to think about it anymore" like 90% of the time

2

31

Q: Recommended books for advanced undergraduate electrodynamics

What books are recommended for an advanced undergraduate course in electrodynamics?

electromagnetism resource-recommendations classical-electrodynamics

there's already a lot of on-site content for this

kids first learning numbers struggle with addition of small numbers no matter how "intuitively" you explain it to them, and then we grow up and say "addition is so easy, it's just intuitive like putting objects together"

have you looked through it? what did you feel was missing from those books?

@ACuriousMind I remembered your sentence you shared few months back. If we don't understand nature we are ready for quantum mechanics and higher scientific studies. :P

I have Griffith's Electrodynamics, Electricity and Magnetism- Prucell, Classic Electrodynamics John David.

4:39 PM

@123 largely, it's unclear what type of book you're looking for

@EmilioPisanty In term of mathematics solution not for physics. I need math books.

do you want a rigorous theory? If so, you probably need to look at a maths book. But those won't really give any intuition.

You should look at Jackson

it's significantly more technical than Griffiths

@EmilioPisanty Yes for laplacian and poisson we can use geometry intuition in 3-Dimension.

@123 not really

sometimes you can

but the class of cases where intuition truly works is really quite limited

@EmilioPisanty I have Classical Electrodynamics- Jackson. but i did not read it because i found griffith's easy and intuitive. But on poisson and laplacian i stuck.

I will read jackson from now.

4:46 PM

@EmilioPisanty anyways. Jackson. Chapter 1 from section 1.7 details the general properties of the Laplace and Poisson equation. Then Chapters 2 and 3 go one by one through the main mathematical methods to approach the solution.

@123 I should think so, yes.

@EmilioPisanty Yes i have opened that book. Sec1.7 he has poisson and laplacian. I will definitely read it.

@EmilioPisanty I have different question. To study of lagrangian mechanics is there any need to learn functional analysis math course/book first.

@123 functional analysis? no, not at all

why do you think so?

are you sure you mean functional analysis and not the calculus of variations? They're very different things.

probably a case of mistaken nomenclature - the relevant field of math here would be variational calculus - functional analysis is not the "analysis of functionals" in the sense a Lagrangian is a functional

@EmilioPisanty Yes Calculus of variation. My bad

if you meant calculus of variations.... eh. kind of, but not really. You can get by with the introductory material that's provided by the standard analytical-mechanics textbooks.

Goldstein is the go-to standard, but I used Lanczos and Gantmacher when I took the course and I found them extremely useful. With a side of Arnol'd for the ultra-mathematical touch.

4:54 PM

@EmilioPisanty For beginners are these good books. I have goldstien.

I have arnold also

1 hour later…

5:54 PM

0

Q: hooke's law and elastic poetntial energy

A mass $m$ is attached to a vertical spring of elastic constant $K$ and length $L$. The spring is supposed to be of negligible mass. Due to the attached mass $m$ the spring reaches a new equilibrium configuration at a length $L+x$. According to Hooke's law the spring elongation is given by $...

energy-conservation potential potential-energy

It's baaaack

1 hour later…

7:01 PM

Would anybody like to chat with me about representations of the Lorentz group?

7:16 PM

I would, but I don't know anything about that topic :)

Me neither :)

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