The h Bar

2:06 AM

hi. if bob lies down underneath a bed and alice sits on the bed at the extreme edge of the bed with her legs hanging out and bob tries to lift her by putting his hands underneath her and pressing up, how much weight will bob bear if 2 of the 4 bed legs are up and the angle reached is 45 dgs? assume alice weighs 100 kg and the bed 20 kg.

Thormund

2:32 AM

with her legs hanging out? is that detail necessary? feels like you could just draw a force diagram (assuming everything is a point object) and call it a day

Maroloccio

The detail about the leg was just to indicate that Alice's is as far towards the short side of the rectangle as she can be and that she is sitting rather than lying down.

Thormund

fair

Maroloccio

How much weight rests on Bob's hands if he positions them right under Alice and tries to lift her almost straight up? (The bed would actually draw a very slight arc, with the radius being the length of the bed, centred in the middle of the back legs, sweeping until Bob extends his arms fully, but I think this is unimportant)

Thormund

https://chat.stackexchange.com/rooms/54160/problem-solving-strategies
This room might be better for your question

Sir Cumference

=0.5

Maroloccio

2:46 AM

How do you work out that it is half of the total of Alice's weight and the bed's weight?

Maroloccio

3:04 AM

This may be somewhat similar: journals.lww.com/nsca-jscr/Fulltext/2011/02000/…

user434058

6:35 AM

@DanielSank It might be possible that you would be viewing an older version of the page (this usually happens when you use the "back" button). Reloading the page fixes it.

Guru Vishnu

7:53 AM

Two days ago...

a perspicaciously curious mind

8:25 AM

@GuruVishnu ouch! Sticks & stones...

John Rennie

@GuruVishnu what's the link. That needs to be flagged as rude.

I see it has been dealt with :-)

Guru Vishnu

9:10 AM

@JohnRennie It got deleted by a moderator before I even flagged it. I was just seconds earlier before that event and the comment vanished with a reload. However, I thoroughly enjoyed it and saved the screen shot :-)

a perspicaciously curious mind

given that he has "Down with elitists!" in his profile, I don't think that's the last we'll see of him

John Rennie

:-)

a perspicaciously curious mind

in your opinion sir @JohnRennie would you say overall is oxford more "elitist" than cambridge?

John Rennie

@aperspicaciouslycuriousmind neither are elitist.

To get into either you have to be good and it doesn't matter who your parents are.

a perspicaciously curious mind

which has had more royal family graduates?

John Rennie

9:20 AM

I don't know.

Prince Edward was at Cambridge when I was there, though I never met him.

a perspicaciously curious mind

was hawking there also when you were there?

John Rennie

Yes. I saw him around Cambridge several times. He was already badly disabled by that time.

a perspicaciously curious mind

on "a brief history of time" he said they gave him a chance to stay at oxford

> I replied that I wanted to do research. If they gave me a first, I would go to Cambridge. If they gave me a second, I would stay in Oxford. They gave me a first.

Shing

9:51 AM

anyone has read "Modern Classical Physics"?

I am a bit confused by its problem-set.

I need some help...

John Rennie

I've never read it. What's the problem?

Shing

kip use his notation for tensor in quite a different way

I was working on the problem, and not sure if i am right

he defines $\textbf{T} (\text{ _ , _ }) = T_{i,j} \hat{e}_i \otimes\hat{e}_j$

@JohnRennie so $\textbf{T} (\hat{e}_i ,\hat{e}_j , \hat{e}_k ) = T_{ijk}$

then I was working on $ \textbf{Q} ( \text { _ , _ , } A) = ?$

John Rennie

I don't think I can help. Sorry :-(

Shing

I tried $ \textbf{Q} ( \text { _ , _ , } A) = Q_{ijk} A_k \hat{e}_i \otimes \hat{e}_j$

but the solution I found on the internet has no the outer product part.

@JohnRennie that's fine!

Shing

10:14 AM

@JohnRennie oh I get it now. I am correct, because I wrote a tensor. the solution is correct too, because the author wrote "the relationship between components of tensors in a specific basis".

Slereah

10:35 AM

morning

Shing

morning!

user434058

morning!!

user434058

@GuruVishnu BTW, I also flagged them when I saw them :-) Any comment containing "thanks" or it's variant "thank" gets automatically removed by the system. Mods don't need to handle them.

M.N.Raia

10:57 AM

Good morning/evening/night for everyone!

Hope everyone here is quite healty.

Well, yesterday, one professor of mine said (more or less) that due to the fact we have two kind of particle statistics: bose-einstein and Dirac-fermi (which gives rise to the realization of Fermions and Bosons), we construct field theories which can be a scalar, vector, tensor and spinor, theory.

He said, moreover, that the only classical field theory that we know which is a tensor theory is GR, due to the fact of the metric a (0,2)-tensor field. Other exemples then we have: klien-gordon (scalar), electromagnet…

My thing is: can we construct another mathematical object different from scalars, vectors, tensors and spinors?

By the comment of by professor, maybe the answer lies on statistical mechanics, which is a matter of fact to define a desired particle statistics with desired properties.

Thormund

11:32 AM

@EmilioPisanty Hello sorry to bother

I understand that you do research in quantum optics?

Emilio Pisanty

@M.N.Raia I disagree with (your understanding of) your professor's statement. The fact that field theories can be scalar / vector / tensor / spinor does not follow from particle statistics.

it is simply a natural consequence of requiring our theories to be Lorentz (or, more generally, Poincaré) covariant

@M.N.Raia No. There are only two types of theories that satisfy that covariance requirement, spinors and tensors.

scalars and vectors are special cases of tensor objects.

@Thormund roughly speaking, yes.

Thormund

I've had this problem on my mind for a while, but I'm not sure if I'm thinking on the right track, or if its truly quantum optics

I've started from this paper, where you can determine dimensionless field equations for light propogating in the z-axis. doi.org/10.1103/PhysRevA.63.023809

I've been trying to recover the dimensions to the equation, say after a lens transformation, which I had initially thought to have been simple enough, since I could just use conservation of power on an infinitesimal area, and have the wavefront determined by the dimensionless field equations
But then, I realised I had hit a road block more or less, since I realised that from classical optics, the Guassian beams mean that the light doesn't exactly "travel" in a straight line, since there exists a beam width

Emilio Pisanty

@EmilioPisanty the full story is at

Representation theory of the Lorentz group

The Lorentz group is a Lie group of symmetries of the spacetime of special relativity. This group can be realized as a collection of matrices, linear transformations, or unitary operators on some Hilbert space; it has a variety of representations. This group is significant because special relativity together with quantum mechanics are the two physical theories that are most thoroughly established, and the conjunction of these two theories is the study of the infinite-dimensional unitary representations of the Lorentz group. These have both historical importance in mainstream physics, as well...

@Thormund I don't understand the question

Thormund

Yes its abit of a mess on my part too

Let me try get the latex to clarify give me a second

Emilio Pisanty

take your time. I'm heading off to cook lunch, I can't guarantee I'll be back before ~2h30m from now, but I'll look in at some point.

bolbteppa

11:50 AM

@M.N.Raia twistors

Thormund

$$\newcommand{\sd}{\text{d}}$$For a field passing through a lens, say a parabolic lens, the dimensionless field equations right at the lens at $z=-f$ with focal length $f$ will be some function I would like to not disclose here, which we shall just call as $\boldsymbol{F}_F$.

To find the dimensionless fields after the lens, we can then propogate those equations by using the following for the paper by vanEnk and Kimble (https://doi.org/10.1103/PhysRevA.63.023809), which is $$ F_+ \left( \rho, \phi, z \right) &= \sum\limits_{s=\pm 1} \int_{0}^{k}\sd k_t \, \frac{1}{4\pi}\frac{sk+k_z}{k} J_0\…

lol rip the equation

Krishna

1:26 PM

I just wish there was a "not-a-question" tag for closed questions. Tagging this (physics.stackexchange.com/q/555660/257049) with nuclear physics is plain weird.

Physics Meta

1:38 PM

-1

Q: Is there any connection between special and general-relativity?

We know that gravity causes relativistic effects according to general relativity which car similar to special-relativity. so is there any connection between general relativity and special relativity? Or any thing in the scientific literature which has attempted to unify the special and general r...

vzn

3:34 PM

@M.N.Raia fluid (unifies them all!) :)

Charlie

I've just read the phrase "if a geodesic is space/time/null-like somewhere, it is space/time/null-like everywhere". What does it mean for a geodesic to have one of those properties "everywhere"? Does this mean if a curve is space/time/light-like in an infinitesimal section of the geodesic then the geodesic is necessarily the same type at all infinitesimal sections along it?

ACuriousMind

@Charlie "everywhere" means "at every point".

Charlie

So if I take any two events on the geodesic and find they are spacelike separated, then immediately it follows that any two events on the entire geodesic are spacelike separated?

ACuriousMind

What is your definition of two events being spacelike separated?

A curve is timelike (or lightlike or spacelike) at a point when its tangent vector is timelike at that point by definition. Saying the curve is everywhere timelike means to say the tangent vector to the curve is timelike at every point of the curve.

Charlie

3:56 PM

I want to say two events are spacelike separated if they can't be causally connected, or that a light beam couldn't travel between them

I see what you're saying, thanks for the help

:)

Afiq Hatta

5:29 PM

Hi all, trying to construct the Cartan-Weyl basis for the fundamental representation of the complexified L(SU(N)) - I think I have it but does anyone know where it's written down so I can check?

Physics Meta

1

Q: Please make MathJax formatting more discoverable to new users

feature-request: please make MathJax formatting more discoverable to new users, by adding it to the post formatting wizard. The Stack Exchange dev team recently released a new version of the Ask page, and we've gone through some of what we can do to make the best of it (though there still remain...

feature-request mathjax new-users ask-page

bolbteppa

5:50 PM

@AfiqHatta exercise 2 maybe?

Afiq Hatta

funnily enough

that's the question that motivated me consulting this chat ahaha

@bolbteppa thanks though

bolbteppa

I think I know where it will be, hold on, I should know this by heart :\

Afiq Hatta

@bolbteppa thanks :) would appreciate it alot

bolbteppa

Ma, Group Theory in Physics, Chapter 4, around (7.77) maybe, search 'Cartan-Weyl' in google books and it should come up?

Afiq Hatta

that's so awesome that you got that down

knzhou i read your notes all the time!!

bolbteppa

6:01 PM

Page 7 of these notes give the $su(2)$ example so if the book reduces to this example it should be the right thing (well $sl(2)$ hmm)

Afiq Hatta

@bolbteppa actually im working from the standard CW basis from SU(2) then trying to generalise up

Emilio Pisanty

@Thormund "lol rip my ability to follow along with what you're asking or care enough to think about it", then, I guess

(sorry to be blunt)

It doesn't look like you gave enough information for your query to be answerable, though.

Thormund

7:13 PM

@EmilioPisanty It's alright thank you for looking through still. It's.... Quite a mess and i guess boiling it down to what I thought was sufficient clearly wasn't so. My bad nonetheless.

a perspicaciously curious mind

why not just write it all out on paper and take a picture of it?

the (upload...) button is handy for that :-)

@Thormund

Thormund

7:43 PM

It's probably at least 5 pages

It's just abit dissappointing that my supervisor doesn't seem to see that I feel it might be a conceptual issue but a code issue

Shing

why tensor is so hard...

bolbteppa

Think of tensors as like vectors and they're fine