The h Bar

Sha Vuklia

3:00 PM

Anonymous

@JakeRose Sort of....but I'm trying to explain it more fundamentally

Sha Vuklia

guys, this exercise is about the fact that if you point a laser at a piece of glass, the damage will first occur at the end of the glass (at 2)

Anonymous

It's not really a "correction"

Anonymous

It's very natural

Jake Rose

Mhmm okay

Sha Vuklia

3:01 PM

they explain it by looking at the waves and the corresponding irradiance

where the irradiance is given by $I=\dfrac{v\epsilon}{2}E_0^2$

now it turns out that $I$ is proportional to $n^2 E_0$

ACuriousMind

@ShaVuklia What damage?

Jake Rose

$\int_{r}^{r-dr}-\vec{F}_g\vec{dr}$?

Sha Vuklia

damage due to the energy

I'm not sure exactly how it works

maybe absorption?

they do say that we can neglect absorption

which is exactly my problem

I don't understand why they don't include $tE_i$ and $tt'E_i$

ACuriousMind

I'm not convinced that usual lasers will do any damage to glass, absorption or not

Sha Vuklia

my book refers to it as an "intense laser", and refers to the damage as "laser damage"

oh they also say:

the damage is proportional to the energy the surface absorbs

ACuriousMind

3:05 PM

Wait...how on earth is $E_i + r E_i = 0.8 E_i$ supposed to work? Is $r$ negative?

Sha Vuklia

yes, $r$ is the Fresnel coefficent

where $r=E_r/E_i$

and since $\theta=0$, we have $r_\perp=r_{parallel}$

same for $t$

Anonymous

@JakeRose If you and your house are on the same line and a bus is moving away from your house on that line, then that bus is moving towards you, on that line isn't it? So if the object you're trying to move is being pulled "towards" the body then $d\vec{r}$ is negative, but the displacement of that body w.r.t you would be $-d\vec{r}$. So the work done by you would be $(-\vec{F_g}).(-d\vec{r})$

Jake Rose

Im confused

Anonymous

Note that this tiny work $dW$ is negative

Jake Rose

You said the bus is moving away from me and moving towards me

Sha Vuklia

3:08 PM

this is the slide from my teacher btw

Anonymous

@JakeRose Typo, sorry

ACuriousMind

@ShaVuklia Alrighty. So what's the question/what are $E_1$ and $E_2$ supposed to be?

Sha Vuklia

the amplitude of the electric field

and $E_1$ is apparently the part that isn't absorbed at surface 1

and same for $E_2$

ohhh

hm

I guess it also has to do with constructive interference

there is a phase shift too, I think I have to take that into account too

Anonymous

Now you simply need to sum up over all those tiny displacements $(-d\vec{r})$

Jake Rose

Okay I think I get it

ACuriousMind

3:10 PM

@ShaVuklia Yes, there is a phase shift at the frint since it's from a denser medium into a less dense medium, but not at the back

Anonymous

You'd get something like $\sum_i(-\vec{F}_g).(-\Delta \vec{r})_i$....can you now convert that to an integral?

Sha Vuklia

but then why are they even considering $tE_i$ and $tr'E_i$ if there is destructive interference @ACuriousMind

and I'm guessing they don't take into account the absorbed wave, because it can't interfere with another wave at the surface

so we can neglect that wave

oh actually it's the other way around

ACuriousMind

@ShaVuklia I don't understand the question. They're computing the total resultant E-field on the side from which the beam is incident. For the first interface that's $E_i + r E_i$ and for the second interface that's $t E_i + tr' E_i$.

Mithrandir24601

@Blue I was just out a kung fu (we were doing throws today - so much fun :) ), so I'm just getting this now. If you give me a bit to get showered etc. I can have a look then. Also, because I'd like more traffic on the QC chat, do you want this conversation there? (Feel free to say no)

Sha Vuklia

@ACuriousMind well my question is why not include the transmitted $E$ field too?

ACuriousMind

3:15 PM

Including it where?

Anonymous

@Mithrandir24601 Throws? (Don't know what that is :P). Sure...sure...I'm a bit confused with a few other terms in the paper as well :P Let's shift to the QC chat

Sha Vuklia

including it to get the total $E$ field

$E_1=E_i+tE_i+rE_1$

ACuriousMind

@ShaVuklia At what point do you think the transmitted wave superposes with the incident and/or reflected wave?

Jake Rose

@blue

@Blue

Lets consider the opposite situation

Moving a mass from r to infinity

Sha Vuklia

why should it superpose?

ACuriousMind

3:16 PM

The transmitted wave is in another medium compared to the incident/reflected wave, they simply have no chance to superpose!

@ShaVuklia Well, what is $E_1$?

Sha Vuklia

because apparently they don't even care about interference, because at one interface there is constructive interference, and at the other there is constructive interference

Jake Rose

$\int_{r}^{\infty}-\vec{F}_g \vec{dr}$

ACuriousMind

It's the electric field at some point, right?

Sha Vuklia

yes, at interface 1

ACuriousMind

Well..."at" is too imprecise here

Jake Rose

3:17 PM

Thats the work done by gravity right?

@Blue (just so you see that second message with the equation)

Sha Vuklia

we can't look a bit to the left, because then it doesn't even make sense that the damage occurs within the laser?

I mean, I understand my teacher's slide; there is constructive interference at the back

but the exercise just confuses me

Anonymous

@JakeRose Yup, looks okay

ACuriousMind

@ShaVuklia My point is that at the interface it is unclear what we should consider as "the E-field" - the normal component of the E-field is discontinuous across an ideal interface

Jake Rose

So then the work done by me is the negative of this?

ie. positive

ACuriousMind

So we must do it "a little bit to the right" or "a little bit to the left"

And what your book's computing is both times "a little bit to the left".

Jake Rose

3:20 PM

Oh you take the negative at the end

Got it

ACuriousMind

I agree, however, that the computation for "damage" in the glass should be "a little bit to the right" for interface 1

Anonymous

@JakeRose Right

Sha Vuklia

@ACuriousMind ah alright, well thanks for your feedback

Mithrandir24601

→ 4 messages moved to The Classical Channel

John Rennie

Buuuuuuuuuuuuuuuuuuuuuuurp

Mithrandir24601

3:30 PM

Aw @JohnRennie Why did you leave? :/

John Rennie

Quantum computing isn't really my thing ...

Anonymous

Lol, we got a visitor after so long, that we were thinking of welcoming you with a garland :P

Mithrandir24601

@JohnRennie It could be your thing!

vzn

4:10 PM

> ...Connections between hydrodynamics and AdS/CFT...

user187604

@JohnRennie it's great that you share your food with us....

bolbteppa

@Semiclassical Agreed

vzn

> He was awarded the 2014 New Horizons in Physics Prize by the Fundamental Physics Prize for "his pioneering contributions to the study of string theory and quantum field theory; and in particular his work on the connection between the equations of fluid dynamics and Albert Einstein's equations of general relativity."[8][9][10]

bolbteppa

@vzn what do you think of the little-known fact that general relativity can be formulated in the same spontanously broken symmetry language as the standard model is (which predicted the Higgs)?

vzn

@bolbteppa havent heard of that. do you have a wikpedia ref or otherwise? btw was pondering more your ref on the fundamental constants/ particle masses of the standard model (nice/ thx). would like to learn more details on that, the ref was sketchy.

ACuriousMind

4:21 PM

@bolbteppa Which symmetry is spontaneously broken in GR? It's a gauge theory with local Lorentz symmetry, but I don't see any obvious broken symmetry in its "standard field theoretic" formulation

bolbteppa

@vzn if you're interested, go research it, think about how stunning that simple fact is though

Oh no I've said too much :p

vzn

@bolbteppa does it relate to SUSY?

user187604

@ACuriousMind in which subjects are you guys interested? I will like to ask them here if I have any problem on them?

ACuriousMind

@user187604 There's a list of stuff I'm specifically interested in on my profile, but more generally, if it's quantum I might be able to muster some interest. in any case.

user187604

@ACuriousMind are you interested in mechanics or vector calculus?

ACuriousMind

4:26 PM

Only in so far as they are basic toolsets needed to do actually interesting things :P

bolbteppa

SUSY has it's own symmetry breaking issues, apparently this is a big issue in susy

and even the bosonic string vacuum hints at sbb that hasn't been resolved either apparently

user187604

@ACuriousMind agreed but you all needed to master them to be what you are now.

ACuriousMind

@user187604 Sure

That doesn't mean I find vector calculus intrinsically interesting! :P

user187604

Yes also agreed for a physics student mathematics is just a ladder to climb. Nobody likes to climb. But likes to get to the stage he/she wanted. @ACuriousMind

vzn

> General relativity has a Lorentz symmetry, but in FRW cosmological models, the mean 4-velocity field defined by averaging over the velocities of the galaxies (the galaxies act like gas particles at cosmological scales) acts as an order parameter breaking this symmetry. Similar comments can be made about the cosmic microwave background. en.wikipedia.org/wiki/Symmetry_breaking

ACuriousMind

4:31 PM

@user187604 I'm not sure what we're talking about anymore, but I'm not saying I don't find math interesting in itself. Just vector calculus is not among the math I find interesting on its own.

bolbteppa

@vzn I don't know enough about the parameters yet tbh

user187604

@ACuriousMind bye. If I get to your stage I'll ask questions here. Best of luck...

ACuriousMind

@user187604 Oh! If you meant to ask whether I would be willing to answer some questions about vector calculus, then you should've just asked that, not whether I am "interested" in it!

I'm certainly willing to answer questions about stuff I know even if I don't find it very interesting on its own

user187604

@ACuriousMind I'll just have to post the question and not ping anyone. Just wait for answer. Is it okay sir?

ACuriousMind

@user187604 Yes, asking questions in here without pinging anyone is always fine - if someone wants to answer, they will

Also, please don't call me "sir" ;)

vzn

4:37 PM

@bolbteppa btw 18 sounds (almost "suspiciously") low to me. it seems not easy to find a simple analysis/ list of all the fundamental constants of the standard model. aka the expr "buried in the fine print™"

bolbteppa

Standard Model

The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, and not including the gravitational force) in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of the top quark (1995), the tau neutrino (2000), and...

List is there

user187604

@ACuriousMind OK. Sorry for calling that. Bye. Have a nice day

vzn

@bolbteppa ok, 18, but then many fundamental particles masses not listed are presumably not obtained merely adding up masses of their constituent particles... want to understand that better...

John Rennie

6:33 PM

@vzn if you mean the baryon masses they are calculated using lattice QCD. The calculation is reasonably well understood these days, though it's still a heroic undertaking. Last time I looked they were getting the baryon masses right to within around 10%.

Speaking of baryons, this looks an interesting paper:

The pressure distribution inside the proton

Though annoyingly it isn't on the Arxiv. I guess Nature forbids its authors from posting on the Arxiv.

Mithrandir24601

@JohnRennie Yeah, at least for some length of time after the article's published (if they allow it at all, I can't quite remember what I was told before :P )

lılostafa

7:32 PM

@Mithrandir24601 Are you sure? I've even seen papers that are going to be published on Nature but aren't published yet, but are available (and remain available after being published) on arXiv

Semiclassical

"Contributions being prepared for or submitted to a Nature Research journal can be posted on recognized preprint servers (such as arXiv), and on collaborative websites such as wikis or the author's blog." (nature.com/authors/policies/confidentiality.html)

So authors are allowed to post preprints to arXiv. But that doesn't mean they all will avail themselves of that option.

The thing I always find interesting with lattice QCD is the whole issue of the numerical sign problem

Mithrandir24601

@lılostafa Interesting. Maybe it was something that used to be the case?

Semiclassical

7:48 PM

maybe, but that paper is from 2018

so the lack of a preprint in that case probably has nothing to do with Nature's policies

Mithrandir24601

@Semiclassical Yeah, must be the case

philmcole

Hi, I have a pretty basic question I was always wondering about. We distinguish energy into certain forms like potential, kinetic, heat energy etc. Is this distinction artificial in the sense that the universe knows only one form of energy, "energy" and doesn't specify in which form it occurs?

I read the part where Feynman discussed energy using toy blocks. But he only discussed that energy never dissappears but only transforms and not if our specification of potential, kinetic etc. energy is artificial.

Cows

8:46 PM

I now understand why Stephen Wolfram is a genius

Stephen Wolfram

Stephen Wolfram (; born August 29, 1959) is a British-American computer scientist, physicist, and businessman. He is known for his work in computer science, mathematics, and in theoretical physics. In 2012 he was named an inaugural fellow of the American Mathematical Society. As a businessman, he is the founder and CEO of the software company Wolfram Research where he worked as chief designer of Mathematica and the Wolfram Alpha answer engine. His recent work has been on knowledge-based programming, expanding and refining the programming language of Mathematica into what is now called the Wolfram...

I tried doing some mildly non trivial mathematics and quickly understood that the universe is deciding to share some secrets with me

By the same token I can unequivocally declare without need for proof that Stephen Wolfram is a genius

ACuriousMind

@philmcole Depends on what you mean by the universe "knowing" about the forms of energy.

We find these different kinds by knowing that the sum of "all energies" must be conserved and if the sum of what we have is not conserved we go looking for a new form of energy