The h Bar

user434058

6:33 AM

Did you check out the ultra dark mode?

user434058

6:47 AM

2

Q: How to show Current a scalar quantity under coordinate transformation?

I know current is a scalar quantity because it doesn't follow the vector law of addition , but I want to prove it with transformation of coordinates. How to do that? From where to start?

user434058

How do you use co-ordinate transformations to prove that something is a vector (or a scalar)?

user434058

Is it even possible?

John Rennie

7:06 AM

If we transform from coordinates $x$ to coordinates $x'$ then we can write a matrix describing the transformation $m_{ij} = dx_i/dx'_j$.

Then vectors transform according to $v_i' = m_{ij}v_j$

So we just have to check that the object we are studying transforms in this way.

NB this is required but not sufficient. Vectors must transform in this way, but there could be other objects that aren't vectors that also transform in this way.

user434058

7:36 AM

@JohnRennie But while checking whether $v_i' =m_{ij} v_j$, we have implicitly assumed that $v$ ia a vector, haven't we? Or else how could we define $v_i'$ and $v_j$?

John Rennie

8:06 AM

@FakeMod The idea is that you know what $v$ and $v'$ are, and you know what the transformation matrix $m_{ij}$ is. Then you check that $v_i' = m_{ij} v_j$ and if this doesn't hold you know $v$ isn't a vector.

Slereah

morning

Slereah

8:50 AM

Is the general formula for parallel propagation the equation $$P_{\gamma, s, t} X(\gamma(\lambda)) = \int_{s}^{t} \nabla_{\dot{\gamma}} X(\gamma(\lambda)) d\lambda$$

or something to that effect

Then I suppose that would be

\begin{eqnarray}
P_{\gamma, s, t} X(\gamma(\lambda)) &=& \int_s^t \left[ \dot{X}^\mu(\gamma(\lambda)) + \omega^\mu_\nu X^\nu \right]d\lambda\\
&=& [X(\gamma(t)) - X(\gamma(s))] + \int_{s}^{t} \omega^\mu_\nu X^\nu d\lambda
\end{eqnarray}

and give the appropriate result for the holonomy

Except I'm not sure how the path ordering gets in from there

(This is starting from $$\nabla_{\dot{\gamma}} X(\gamma(t)) = \left[\frac{d}{ds} P_{\gamma, t, s} X(\gamma(s))\right]_{s = t}$$)

Physics Meta

9:43 AM

0

Q: Is error-analysis not a physics topic?

I have a question about my Physics Stack Exchange post: How to incorporate the uncertainty of the model coefficients in the prediction interval of a multiple linear regression I am dealing with a rather hard question, and hoped to get some help on stack-exchange. Being a computational physi...

discussion

Novalium Company

I know this is not the place to ask this, but do you guys know a type of commonly found material which I can shape (like clay) into a mold-like form, then I can fill it up with HDPE (or some other suitable plastic) and I put that whole thing in the over an get the shape I want? You get what I'm trying to do.

Basically recycling plastic into a shape that I've cut from some material like clay or something like that.

Slereah

11

Q: Some questions about Wilson loops

Let $G$ be the gauge group whose Yang-Mill's theory one is looking at and $A$ be its connection and $C$ be a loop in the space-time and $R$ be a finite-dimensional representation of the gauge group $G$. Then the classical Wilson loop is defined as, $W_R(C)(A) = \mathrm{Tr}_R[\mathrm{Hol}(A,C)]$, ...

quantum-field-theory research-level yang-mills wilson-loop

There's a proper derivation

Novalium Company

oven*

Slereah

10:42 AM

During a Berkeley meeting of the Manhattan Project, Edward Teller brought up the basic idea behind the hydrogen bomb. You would use a nuclear bomb to ignite a self-sustaining fusion reaction in some other substance, which would produce a bigger explosion than the nuke itself. The scientists got to work figuring out what substances could support such reactions, and found that they couldn’t rule out nitrogen-14. The air is 79% nitrogen-14. If a nuclear bomb produced nitrogen-14 fusion, it would ignite the atmosphere and turn the Earth into a miniature sun, killing everyone. They hurriedly con…

Yuvraj Singh...

11:04 AM

@Jmac I was write both Brian and Stephen hawking does use the word parallel universe.

I have a question does there exist something before the big bang! I mean most of physicist agree that, the origin of our time is when the big bang happen, but some physicists says our big bang was a part of millions of big bang

If the concept of multiple universe exist would there been a different time meaning for all of them?

ACuriousMind

@Slereah At least "turn the earth into a miniature sun" would've been a stylish way to go out :P

Yuvraj Singh...

11:22 AM

Sorry for the typing error I was writing "right" and phone auto correct makes it "write".

@JohnRennie hi

PM 2Ring

@NovaliumCompany What's wrong with clay? I guess the plastic may glue itself to the clay, which will make it a little tricky to remove your plastic item.

@NovaliumCompany That may or may not work. There are two major categories of plastic: thermoplastic and thermosetting. You can melt & reshape the former, but not the latter. See en.wikipedia.org/wiki/Thermoplastic which links to the article on thermosetting. You should be ok with HDPE, since it's thermoplastic.

John Rennie

@YuvrajSingh... hi :-) I have to dash off and get lunch now I'm afraid. I'll be around later this afternoon, or tomorrow morning as usual.

5

Q: Before the Big Bang

I've heard this saying before I don't know about anyone else. It says, "What ever was before the Big Bang is something physics can't explain..! Is this saying true (accurate)?

cosmology time universe space-expansion big-bang

Yuvraj Singh...

11:38 AM

@JohnRennie ok sir!

@PM2Ring a very good evening sir!.

PM 2Ring

@DavidZ I fully agree. I was about to VTC that post as non-mainstream, but Qmechanic beat me to it. But then I decided to google for oxford negative mass, and found that ref for Jamie Farnes.

@YuvrajSingh... Good evening, Yuvraj prabhu. ;)

Yuvraj Singh...

@PM2Ring prabhu!

PM 2Ring

:)

Yuvraj Singh...

@PM2Ring have you notice my question?

PM 2Ring

@YuvrajSingh... Yes. Standard Big Bang theory cannot talk about time before the moment that the Big Bang started. In fact, it can't talk about the exact moment of the Big Bang, and it can't really describe what happens in the very early phases of the Big Bang until we have a good theory of quantum gravity.

See en.wikipedia.org/wiki/Chronology_of_the_universe#Planck_epoch

Yuvraj Singh...

11:51 AM

OK.

Do you believe in concept of multiple universe? @PM2Ring

PM 2Ring

Now there are various speculations that go beyond standard Big Bang theory, and those theories may talk about time before the Big Bang. But even in those theories, the instant of Big Bang is so extreme that it is very difficult or totally impossible for things before the BB to affect things after the BB. So the chain of cause and effect is broken, or at least extremely chaotic, by the BB.

@YuvrajSingh... Maybe. :) I found such theories very attractive when I was younger, but these days I'm not so sure. But I don't think we have enough evidence to make a decision either way.

JMac

@YuvrajSingh... There's a difference between using the word parallel universes, and saying that parallel universes definitely exist and aren't just one possible explanation. That's what I was asking about yesterday, because I would find it a bit weird to see a science communicator like Brian Greene state an interpretation as fact; though it wouldn't be that crazy.

PM 2Ring

Personally, I think there is only one Universe, but that Universe may contain countless sub-universes that cannot influence each other. And I guess it's ok to call those sub-universes parallel universes.

Balarka Sen

@Slereah @ACuriousMind Physics

Slereah

Huzzah!

Yuvraj Singh...

12:06 PM

@PM2Ring thanks sir. Actually I was reading some papers published by Stephen hawking, so where he uses the word parallel and in his interview too he said about parallel universe.

@Jmac OK.

PM 2Ring

@YuvrajSingh... Yes. He's talking about the standard Many Worlds Interpretation, which gives a tree-like structure of "parallel" worlds. So all those worlds are connected into a united branching structure.

ACuriousMind

@BalarkaSen Neat-o. A blog? I guess isolation is getting to you, too, hm? ;)

Balarka Sen

Lmao

I was inspired I suppose

ACuriousMind

At least it's not a podcast :P

Balarka Sen

But no, I am rather happy that the rest of the world is adapting my lifestyle

Self-isolation is what I call daily routine

4

PM 2Ring

12:14 PM

I prefer the picture promoted by David Deutsch, which I described in the answer I linked here yesterday. physics.stackexchange.com/questions/536522/… It's really the same structure as the standard MWI, but I think it's a better way to describe it.

ACuriousMind

@BalarkaSen People are freaking out while I'm just happy that "staying home and playing video games" is finally recognized as doing something for society ;P

Balarka Sen

Exactly

JMac

@ACuriousMind I've been subconsciously training my whole life for this.

PM 2Ring

12:30 PM

@JMac A lot of MWI proponents do seem to have the attitude that MWI is the true interpretation, and people who don't agree just don't have the intellectual courage to embrace its awesomeness. At least, that's the impression I get from pop-sci books & articles that promote MWI.

JMac

@PM2Ring Yeah, I was pretty curious about Brian Greene though because he's a pretty good communicator, and I don't remember him suggesting that MWI was the interpretation in books or anything. From what I remembered he had a more neutral tone about interpretations; but maybe I'm just remembering wrong.

Novalium Company

@PM2Ring Thank you. I've already familiarized myself with the thermoplastics and thermosets and I think I'll be using something that is HDPE (since it's most easily found). I'm thinking of using clay but my only worry is that the clay would melt or blend with the plastic.

PM 2Ring

@JMac Yeah, it's been a while since I read anything by Brian Greene, so I can't remember what his attitude is, I was making a generalisation.

Novalium Company

And I'd also need to be able to apply pressure on the mold and if the mold is made out of clay, would that hold?

PM 2Ring

@NovaliumCompany You need a high temperature to "fire" clay, and make it glassy. You are unlikely to get that temperature in a normal oven, you need a kiln for that. And that's way hotter than the temperature you need to melt HDPE. You (probably) don't need to bake your clay mould, but it will need to be dry (eg sun-dried), and it will shrink a little when it dries. Hopefully, that won't be an issue for you.

I'm not sure how to stop the plastic sticking to the mould. Maybe dusting the mould with talcum powder will help. Some talc may stick to the plastic, but maybe a stiff brush will remove that.

@NovaliumCompany Maybe. It depends on the pressure, and the shape of the mould. Clay is fairly brittle. Why do you need the pressure?

Novalium Company

12:48 PM

@PM2Ring So the plastic fits better in the shape

How To Recycle HDPE Plastic The Easy Way

►

I want to do something like this but without building the wood thing. I have a big oven so maybe I can put something heavy over it while it's in the oven.

PM 2Ring

@NovaliumCompany Ok. That shouldn't require too much pressure, but I've never tried this myself. I suspect that your main problem will be preventing air bubbles, especially if your shape is complicated. There may be some useful info here: en.wikipedia.org/wiki/Injection_moulding

Novalium Company

@PM2Ring Thanks, I did my research on injection molding and I'll most likely do that in the future (once I get a few hundreds of thousands of bucks from my biz) but right now I need to make it in the oven :P

I guess I'll try with clay, see what happens.

PM 2Ring

@NovaliumCompany How big is your object? Maybe find someone with a 3D printer to make it for you.

JMac

@NovaliumCompany Does it need to be a plastic for the final product? There might be other options like 2 part epoxies or something. That can literally be poured into a mold as a liquid and then hardens without heat. The biggest hurdle would probably be air bubbles if those are a problem for what you're making; but even that can be minimized, or even eliminated with a vacuum chamber and a way to shake the bubbles out

Novalium Company

@PM2Ring I've considered that, as well as the other available plastic manufacturing processes. My object is not complicated in terms of shape and local 3D printing services are expensive. I also need to be able to produce it quantity (if I have orders) and 3D printing filament as well as the 3D printer itself are way beyond my budget.

@JMac I've considered that, polymer casting. The shapes I need are not complicated at all and that's why I think the oven HDPE plastic melting thingy is the most suitable at first.

Thanks for the help guys!

I know I behave strangely sometimes and I'm sometimes inappropriate so thank you for helping me.

You'll get 20% off the product once it's released ;)

ahh I should stop being so dramatic

just delete my last 4 messages please

Vivek

1:38 PM

hellos

tpg2114

2:05 PM

@AaronStevens I worked out the answer using the Fundamental Shower Theorem of Calculus -- for what I'm trying to do, the spectral space function I want is sinc*(1-2*sum_{negative bands) bandpass_filter). Which means my physical space kernel would look like BoxFilter - 2*sum_{negative bands} Convolution(BoxFilter, InverseFT(bandpass_filter))

JMac

2:48 PM

@tpg2114 "the Fundamental Shower Theorem of Calculus" I assume this is the one where suddenly it all makes sense in the shower?

tpg2114

@JMac Indeed. All math becomes clear when you walk away from paper and computers and break things down mentally.

Semiclassical

"the shortest path between two mathematical points is through the shower"

tpg2114

@Semiclassical Very concise statement of the theorem!

Aaron Stevens

@tpg2114 Ah ok. Glad you got it to work out

Now if only I could figure out how to finish up my doctoral work in the shower :(

JMac

@tpg2114 I think it's pretty universal for problem solving. Sometimes you just need to step away. I was trying to do a numerical methods assignment in matlab and couldn't get the code to work for the life of me. The next morning when I went to get a coffee I somehow managed to solve the problem without even looking at the code (and I'm not exactly a coder).

tpg2114

2:55 PM

@JMac Yup, I frequently just have to go wander around when I'm stuck. It's pretty awesome how a change of scenery and putting yourself in a situation without the "crutch" of papers, books, or computers helps your brain focus in on the real issues

I've been known to dream up the solution to bugs or issues in my code from time to time

Like -- literally while sleeping, in my dream something clicks and it works when I put it in in the morning

JMac

Yeah I think I remembered dreaming about the problem the night before too. I definitely remember dreaming my way through a few heat transfer problems. I had basically spent like a week cramming, it wound up invading my thoughts in a pretty helpful way.

Aaron Stevens

I do the opposite. I let my problems keep me up, and they don't get solved :P

tpg2114

We used to joke in grad school that the most important lesson is learning how to balance the stimulants (coffee) and depressants (bourbon) so you could stay awake when needed and fall asleep when needed... We'd also tell people who asked what language we used for our giant code that it was written in bourbon

I'm not advocating self-destruction. In hindsight, we did not make great choices in how we coped with things

Aaron Stevens

@tpg2114 That is my issue. I need to be more self-destructive

tpg2114

Hah -- snuck my edit in!

Aaron Stevens

3:03 PM

@tpg2114 It's too late. I have begun

tpg2114

Working in a partially-finished basement, with a single incandescent lightbulb overhead feels very dystopian. I'm in some post-apocalyptic white-collar job.

JMac

@tpg2114 Is the bulb hanging like mid-room height from a cable and slowly swaying back and forth? If not, you're really missing out on the ambiance

tpg2114

@JMac It's not swaying... the basement is not climate controlled, so there's no mechanism for the air to move

But otherwise, pretty much spot on

I do have those high, small, rectangular windows that look out into... those semi-circle cutouts in the ground that go around basement windows. I can see tiny slivers of sun reflecting off the privacy fence. There's a surprising amount of wildlife that wanders by

I saw a squirrel jump down into one of the window well things, and then get attacked by a snake. So I made my wife go get rid of the snake and am contemplating moving out of the house, possibly burning it down in the process.

JMac

@tpg2114 You need to implement some sort of eerie breeze, or possibly find a way to make it sway without a breeze. Then I'm sure your productivity would skyrocket. The quicker you finish work, the quicker you can get out of your creepy-ass basement.

tpg2114

@JMac I'll need to make sure the breeze makes a rocking chair slowly rock also

That's more of an attic thing though I guess

JMac

3:10 PM

I think it would work in the basement. Especially if you put it in a dark corner so that you can only be mostly sure that there isn't someone on it.

tpg2114

The beauty of a single low watt bulb? Every corner is a dark corner!

JMac

But obviously if you have an attic full of dolls it would be much better up there.

Aaron Stevens

@tpg2114 If the basement is circular you have an infinite amount of really small dark corners?

user434058

3:26 PM

Can anybody confirm the validity of this comment? 👇

user434058

The light in the medium persists of the original light wave and the light wave created by the oscillation of the electrons. The latter can further be splitted into a component, which travels with the vacuum speed of light and a component, which seems to travel with the slower speed of light. Individually they all travel with the vacuum speed of light, but in sum, the components with the vacuum speed of light cancel each other out, as is stated in the Ewald-Oseen extinction theorem, which is quite counter-intuitive. E.g. for visible light this extinction length is $\approx 1\,mm$ — clevor 1 min ago

Semiclassical

trying to decide if this practice quiz problem that the prof provided for my students actually makes sense:

"What is the equivalent capacitance (in μF) between points A and B if all the capacitors have a capacitance 1.5 μF but then a material with dielectric constant 9.5 is inserted into one capacitor, filling the space between its plates? Supply your answer to three significant figures."

The phrase "capacitance between points A and B" doesn't make a lot of sense to me

ACuriousMind

@Semiclassical Makes perfect sense to me - if you attach wires to A and B and view everything in between as a black box, what's the capacitance of that box?

Semiclassical

looked at what the prof meant, and i guess i see the point: if you put a voltage across points A and B, what is the equivalent capacitance of the resulting circuit?

yeah

JMac

It's just like equivalent resistance.

Semiclassical

3:34 PM

i think i'd be happier if it was drawn with nodes at A,B

that doesn't really change it but it makes it more obvious in my brain

user434058

0

Q: Electric field due to a ring/disk at a point straight above a hypothetical point on the axis of ring/disk

Its easy to find the electric field due to a uniformly charged ring or disc at a point on the axis of ring/disc. But I cant figure out a way to find the electric field or even potential at a point at height z above or below the axis of charged ring/disc. Please help me with this. We have to fi...

homework-and-exercises electrostatics electric-fields gauss-law

user434058

Should I answer this?

user434058

This seems like a HW question to me, but I am surprised that it isn't closed as well, so is it fitfor the site?

user434058

I don't want the question to get closed after I answer it ;)

JMac

@FakeMod It's only been 3 hours, questions don't always get closed right away. It has 4 votes to close as homework though... so I would probably avoid that one.

ACuriousMind

3:37 PM

@FakeMod What do you mean "isn't closed at well"

It's 3 hours old and already has 4 close votes.

(now I was the fifth :P)

user434058

Got it! Thanks!

Emilio Pisanty

0

Q: Proving identity $\DeclareMathOperator{\Tr}{Tr} \Tr\left[\gamma^{\mu}\gamma^{\nu}\right] = 4 \eta^{\mu\nu}$

In the lecture notes accompanying a course I'm following, it is stated that $$\DeclareMathOperator{\Tr}{Tr} \Tr\left[\gamma^{\mu}\gamma^{\nu}\right] = 4 \eta^{\mu\nu} $$ Yet when I try to prove this, I find something different as follows: $$ \begin{eqnarray} \Tr\left[\gamma^{\mu}\gamma^{\nu}\ri...

homework-and-exercises dirac-matrices trace clifford-algebra anticommutator

for your viewing ~~pleasure~~ horror

user434058

Someone please go and add a homework-and-exercises tag to this question. The OP rejected my edit which was doing that.

user434058

0

Q: Point with greatest acceleration - constantly accelerated rolling wheel

This is a problem from Analytical Mechanics, Fowles & Cassiday. I am not sure if the solution in the solution manual is correct, and I am not sure if my solution is correct. A wheel of radius b rolls along the ground with constant forward acceleration $a_0$. Show that, at any given instant, the ...

classical-mechanics acceleration rotation

ACuriousMind

@EmilioPisanty now you're just making things up :P

Emilio Pisanty

3:53 PM

@ACuriousMind no, I thought that nobody was using \let or \def or \DeclareMathOperator, as Martin suggested was a thing, but I decided to query for them just in case

turns out, Martin was right and I was not.

user434058

This user has asked four similar questions and is still wanting answers. What should be done?

Emilio Pisanty

0

Q: Proving identity $\mathrm{Tr}[\gamma^{\mu}\gamma^{\nu}] = 4 \eta^{\mu\nu}$

In the lecture notes accompanying a course I'm following, it is stated that $$\DeclareMathOperator{\Tr}{Tr} \Tr\left[\gamma^{\mu}\gamma^{\nu}\right] = 4 \eta^{\mu\nu} $$ Yet when I try to prove this, I find something different as follows: $$ \begin{eqnarray} \Tr\left[\gamma^{\mu}\gamma^{\nu}\ri...

homework-and-exercises dirac-matrices trace clifford-algebra anticommutator

solved

@FakeMod done.

user434058

@EmilioPisanty Thanks 👍

ACuriousMind

@FakeMod Once again, please use custom moderator flags to call our attention to user behaviour that you think needs some sort of intervention.

Emilio Pisanty

2

Q: Thermal expansion of solid body and calculation of work produced

From the information about thermal expansion on Wikipedia, I found that if I heat a steel rod that's 1-meter in length and 1-cm in diameter steel rod to 100 degrees Celsius, then it will expand by approximately 1 mm (based on linear expansion coefficient).$$ {\require{cancel}} {\def\rod#1#2{\disp...

thermodynamics energy work adiabatic

any thoughts about the use of MathJax on ↑ that one?

user434058

3:56 PM

@ACuriousMind Ohh! I am sorry. I keep on forgetting that. Do I need to flag it now that you know it?

Emilio Pisanty

@FakeMod yes

it gets it onto the system and is then easier to track down the line

ACuriousMind

@FakeMod Yes. Please do not rely on me or any other mod doing anything just because you told us something in chat.

Emilio Pisanty

say, if this user comes back next month and causes trouble, the flag record is there

JMac

@EmilioPisanty I mean, I'm pretty easily impressed, but that's pretty cool. Beats what I could do in Paint.

ACuriousMind

@EmilioPisanty I have two thoughts: 1. "Wow, that's dedication." 2. "Why?"

Emilio Pisanty

3:59 PM

@FakeMod .... and also, helps you work towards those sweet sweet Citizen Patrol / Deputy / Marshall badges

user434058

Shit! I have 0 flags. I can't flag that user.

user434058

Somebody flag that

Emilio Pisanty

@FakeMod it will keep until tomorrow.

ACuriousMind

In other news, I've just dropped a glass water bottle onto my foot. I can injure myself perfectly fine even without leaving the house :P

Emilio Pisanty

speaking of which, I'm coming up on my fourth unawarded Marshall badge

user434058

4:00 PM

@EmilioPisanty Congrats for the other 3!!

tpg2114

@ACuriousMind I've got at least a hundred Gerolsteiner glass bottles upstairs. We use those things for everything, they're bulletproof

Emilio Pisanty

@ACuriousMind @JMac the question was more, should it be allowed to remain?

tpg2114

Well, maybe not literally. Unless it was a small bullet.

Emilio Pisanty

it looks like fragile code to me

ACuriousMind

@tpg2114 Yeah. Unfortunately, my foot is not...

But don't worry, the bottle is fine

Emilio Pisanty

4:02 PM

@FakeMod no, as in, I'm coming up on 2500 helpful flags, but for some unexplained reason the Marshall badge is only awarded once

¬¬

I want my badges

JMac

To be fair, isn't that the case for all the moderation badges?

tpg2114

@ACuriousMind Phew. Glass is a precious resource ;)

ACuriousMind

@EmilioPisanty What does that mean? It's not as if TeX compilers are under active development :P

user434058

0

Q: In the arrangement The tension in string is?

I couldn't understand the arrangement. Arrangement-

newtonian-mechanics rotational-dynamics

user434058

Flags needed

user434058

4:04 PM

That typo was.....nice

ACuriousMind

(yes, I know about LaTeX 3 or whatever it's called, but (La)TeX as is is essentially frozen afaik)

user434058

@ACuriousMind Isn't it LaTeX2e?

ACuriousMind

That falls under "whatever it's called" :P

user434058

@ACuriousMind :-)

Emilio Pisanty

@ACuriousMind we don't run LaTeX, we run MathJax

which does see a decent amount of development

and which also puts a decent amount of control on the hands of the user

ACuriousMind

4:08 PM

Looking at the code, it doesn't seem to use anything particularly outlandish

Emilio Pisanty

with the Common HTML renderer

ACuriousMind

Hm, alright, you've convinced me ;) So...screenshot the "best" version and add it as an image to the post?

Emilio Pisanty

html-css renderer

preview-html renderer

@ACuriousMind good

yes, I think so too

Aaron Stevens

4:52 PM

@JMac You can get the same badge for each review queue

Like you can get Custodian, Reviewer, and Steward once for each queue

Slereah

5:22 PM

0

Q: Is there any method to know the real age of the universe?

Well, I was wondering about the real age of our universe, I found that it's estimated to be $13.8\times 10^9$ years. Is it an approximation, or laws behind this age?

general-relativity cosmology space

Will I get moderated if I say "count the rings"

Aaron Stevens

@Slereah Only if you can cite your sources that show the universe is a tree :)

1

A: why does not the dielectric field cancel out the capacitor's field?

why does not the dielectric field cancel out the capacitor's field? The polarization of the dielectric in the capacitor does reduce the effective electric field of the capacitor, but doesn't completely cancel it out. The reason is the molecules of the dielectric material are not perfectly po...

Pet peeve: When users post answers that cover what is already covered in other answers :P

To be fair this user was probably making their answer at the same time as I was

JMac

@AaronStevens Yeah, that does bug me too. In this case I'd give benefit of the doubt because his answer is a bit wordier and has a picture, so he was probably writing it as yours was submitted.

It does really irk me when it's like 4 hours later and someone does that though.

Aaron Stevens

@JMac Yeah I usually don't want to go through the trouble putting pictures in my answer :P

But yeah, definitely an issue if it has been a good amount of time since the answer was posted

Slereah

"In quantum mechanics what is called the Dyson formula is what in mathematics is called the iterated integral-expression for parallel transport"

There's the fancy word

Although this is nlab, so that word may only exist there

user434058

Flags needed!

user434058

5:34 PM

0

Q: Relational interpretation vs Many Worlds,which is better?

There’s a simple yet elegant answer to this question. It’s found in the Relational interpretation of Quantum Mechanics by Theoretical Physicist Carlo Rovelli and the recent Wigner’s friend experiment. Simply, no collapse or universal measurement occurs. Here’s an explanation from Relational Quan...

quantum-mechanics quantum-field-theory schroedinger-equation heisenberg-uncertainty-principle quantum-interpretations

Aaron Stevens

1

Q: Can I say $F_s=-kx$ is also equals $F=ma$?

Is $F=ma$ a general formula for all cases in the physical world? or Is it only limited in linear motion?

newtonian-mechanics forces harmonic-oscillator spring

Wow I do not remember voting to close this question

Shows how fast I go through the close queue I guess :P

user434058

5:57 PM

Hi!

user434058

I am bored!

user434058

3 hours ago, by FakeMod

The light in the medium persists of the original light wave and the light wave created by the oscillation of the electrons. The latter can further be splitted into a component, which travels with the vacuum speed of light and a component, which seems to travel with the slower speed of light. Individually they all travel with the vacuum speed of light, but in sum, the components with the vacuum speed of light cancel each other out, as is stated in the Ewald-Oseen extinction theorem, which is quite counter-intuitive. E.g. for visible light this extinction length is $\approx 1\,mm$ — clevor 1 min ago

user434058

Can anybody explain this to me?

user434058

6:13 PM

What should I do when I am out of flags?

Emilio Pisanty

Cat state

In quantum computing, the cat state, named after Schrödinger's cat, is a quantum state that is composed of two diametrically opposed conditions at the same time, such as the possibilities that a cat be alive and dead at the same time. Generalizing Schrödinger's gedanken experiment, any other quantum superposition of two macroscopically distinct states is also referred to as a cat state. A cat state could be of one or more modes or particles, therefore it is not necessarily an entangled state. This is in contrast to the more specific concept of a Greenberger–Horne–Zeilinger state, which by definition...

ah, isn't that Wikipedia page looking great, now that its animation of the Wigner function is...

... not wrong...

the idiot who made the original file had the thing rotating the wrong way 🙄🙄🙄

user434058

When will I lose that "new contributor" banner?

Emilio Pisanty

@FakeMod As I understand it, it goes away after a week

... but I swear I've seen an account with your name before. Is this the first account you make?

, you mean

user434058

6:30 PM

@EmilioPisanty This might have been the 4th one or so. I deleted the original one (1722 rep) because it was affecting my studies and then I created one day accounts if I had doubts. This current account is the longest I have stayed here (excepting my first original account).

user434058

You might have seen me here:

user434058

-9

Q: Should contributions to the community be made compulsory?

Just yesterday, I read this and there dmckee♦ posted this comment, and he said: ...relatively small fraction of the eligible users apply their power with any regularity. Now there is already a question regarding this comment by Emilio Pisanty, but that just asks for data. Now, I have writte...

discussion editing flagging review

Emilio Pisanty

@FakeMod Please indicate it on your profile when you do this

deleting accounts because it's interfering with your studies is perfectly legitimate

but it's incredibly disconcerting when this happens

i.e. when there's an account whose age + rep don't match what one recalls and are not assigned to previous interactions and previous participation in the public record

user434058

@EmilioPisanty Ok! I get it. Does it look good now?

user434058

6:49 PM

Should I use $\mathrm d x$ or $dx$?

Charlie

in what context?

i don't think it matters much if you're talking about differentials, sometimes in GR d$x$ is used to represent covector bases

at least as far as I know

user434058

@Charlie generally....(I don't know if that makes sense)

user434058

7:06 PM

@Charlie Bingo! Looks like we were answering the same question :)

user434058

@Charlie And I asked that question on differentials just because I had to use it in the same answer :)

user434058

Is a tag score equal to the total upvotes on the questions containing that tag? If yes then why am I only getting a score of 4 on tags such as thermodynamics and entropy whereas I actually have 6 uovotes on my answers to the questions under that tag?

user434058

I think that it hasn't been updated yet....

ACuriousMind

@FakeMod The tag score is updated like once a day. SE is not real-time in most aspects!

@FakeMod Since the $\mathrm{d}$ is not a variable but an operator, it should be upright.

user434058

@ACuriousMind Got it!

user434058

7:20 PM

@ACuriousMind Hmm...Thanks!

Aaron Stevens

7:33 PM

@FakeMod If I am typing out the MathJax I use $\text d$, but if I see $d$ in another post I won't go in and edit it just for that reason.

Semiclassical

i'm too lazy to do that myself

my knowing -why- it's sensible to do that is not the same as me be willing to do that

ACuriousMind

the human condition in a nutshell

Semiclassical

priorities

Charlie

@fakemod your answer on the guass' law post is missing a $ at the top somehwere

Aaron Stevens

@Charlie I think it is fine to just edit it :)

ACuriousMind

7:42 PM

@AaronStevens Single $ is too little an edit to suggest it

Easy to forget that as a >2k user

Aaron Stevens

@ACuriousMind Ah yes, forgot that

Charlie

^ :(

Aaron Stevens

6

Q: Why do we need Gauss' laws for electricity and magnetism?

The source of an electromagnetic field is a distribution of electric charge, $\rho$, and a current, with current density $\mathbf{J}$. Considering only Faraday's law and Ampere-Maxwell's law: $$ \nabla\times\mathbf{E}=-\frac{\partial\mathbf{B}}{\partial t}\qquad\text{and}\qquad\nabla\times\mathbf...

electromagnetism computational-physics gauss-law maxwell-equations

The first time in a while I have seen a decent question on the HNQ

Loong

@ACuriousMind There was more to do. ;-)

PM 2Ring

@Semiclassical If I'm on a computer, I try to remember to use upright d, but on a phone it gets a bit painful.

Semiclassical

7:53 PM

if i'm writing by hand, i guess i do try to make it look different?

but i just find \mathrm{d}x just too fussy

Aaron Stevens

@Semiclassical \text d works fine too. Fewer characters to type

bolbteppa

No way do I make the effort to do anything but $dx$

Semiclassical

yeah

PM 2Ring

@FakeMod With that question, it's not clear exactly what the OP wants to know. My feeling is that they want to know the QM reason that causes the different frequencies of visible light to have different speeds in the glass, rather than why the different speeds lead to different refraction angles. But they haven't responded to any of the answers or comments. Personally, I would have posted a comment requesting clarification before submitting an answer...

ACuriousMind

@AaronStevens But that's semantically incorrect :P

Aaron Stevens

7:58 PM

@ACuriousMind \textrm ?

ACuriousMind

It's not text!

Aaron Stevens

:)

Semiclassical

it's formatted as text, therefore text

bolbteppa

$\int {\text d}^n x 2^x \ln \Gamma(x) $ vs. $\int \text d^n x 2^x \ln \Gamma(x)$ vs. $\int {\text d}^n x {\text d}^m y 2^x \ln \Gamma(x) \Gamma(y)$ vs. $\int {\text d}^n x d^m y 2^x \ln \Gamma(x) \Gamma(y)$ hmm

So \text only affect the next character in front of it and nothing else?

$\text dx dy$

Semiclassical

there's a few commands that work like that

my favorite being \vec

bolbteppa

8:02 PM

That's not a terrible burden for the few times you really want it to look good but don't want to waste tons of time

Semiclassical

$\vec{a}$ and $\vec a$

Aaron Stevens

@Semiclassical I prefer $\mathbf a$ instead of $\vec a$

bolbteppa

$\mathbf aa$ vs. $\vec aa$

Semiclassical

\vec{a} and \vec a

PM 2Ring

That's pretty normal LaTex, eg $\frac12$

Semiclassical

8:02 PM

there's another instance of that that i use but I forget which one

oh, right

\frac 1 2

$\frac 1 2$

that one i use a bunch

bolbteppa

I've wasted so much time writing those {} brackets e.g. in \vec{a} :(

Aaron Stevens

Or just \frac12 no spaces $\frac12$

bolbteppa

$\frac e 2$

Semiclassical

yeah, that's actually the one

unfortunately, it only works for single characters

bolbteppa

Nice

Semiclassical

8:04 PM

so it does save labor but only in the simplest cases

$\frac{e^x}2$

bolbteppa

$\frac{\Gamma^2(xyz)}22$

This is a whole new world

Semiclassical

but i run into $\frac 1{stuff}$

a lot

little latex discoveries are wondrous

bolbteppa

$\frac {#}2$ $\frac 2{@}$ $\frac 2{#}$

Semiclassical

another one i'm proud of discovering involves the align environment, let me see if I can make it work

\begin{align}
a&=b \\[2cm]
c&=d \\
\end{align}

bolbteppa

Found a way to break the trick already

Semiclassical

8:07 PM

hmm, maybe i'm not remembering it right

oh, it did work

neat

Loong

;-)

Semiclassical

it lets you control row spacing, basically

bolbteppa

Nice, multiple columns are the worst

Semiclassical

which my collaborator who is obsessed with such things was pleased as heck to learn about

Loong

I sometimes use \\[6pt] for that.

Semiclassical

8:12 PM

yeah, the 2cm example was not a realistic one

Loong

For example here: chemistry.stackexchange.com/a/39429/7951

Semiclassical

nice

and a much better way of controlling it

(i forget how my collaborator was doing it previously)

bolbteppa

Holy shi...

0

A: What does a field describe in Quantum Field Theory? What does a linear combination of (vectorial) fields mean?

Quantum field theorists seem to me to be among the most ardent supporters of "shut-up and calculate". When pressed, they will usually say something like "physics is just algorithms to produce experimental results" and suggest that it is naive to suppose that human understanding is possible. I w...

I can't believe I missed the flaw in his definition the first time I skimmed this shocking paper

It's better not to engage this stuff, if someone can delete those last two posts right above this

Nah lets engage

PM 2Ring

8:27 PM

Mar 22 at 16:02, by PM 2Ring

This guy seems to know his stuff. https://physics.stackexchange.com/a/537709/123208 I had a brief look at the start of his paper linked at the end of that answer, Mathematical Implications of Relationism. It looks interesting, but I don't have the skills to evaluate it. True, it's not exactly mainstream, but it doesn't feel like the work of a crackpot.

ACuriousMind

Friendly reminder: Please try to refrain from discussing specific other users in chat where they are not aware and thus cannot respond.

PM 2Ring

Fair enough. Maybe we should invite him to chat here, or in a new room. He might be slightly unorthodox, but he seems happy enough to engage in discussions without getting aggressive.

bolbteppa

9:04 PM

Can the hydrogen atom spectrum ever in any sense be thought of as existing in a finite dimensional Hilbert space?

Charlie

9:30 PM

If I was interested in learning more about smoothness of functions in the context of GR what branch of pure maths would I find useful?

Is that real analysis or functional analysis or something?

bolbteppa

Differential geometry, framed in the language of real and functional analysis

Charlie

Ok thanks

Aaron Stevens

10:37 PM

@bolbteppa I prefer fake analysis

bolbteppa

pseudo-real analysis (ba-dum-tch)

Physics Meta

0

Q: How can you close a thread on the Relational interpretation because of opinion when there's threads on here about Schrodinger's cat?

Listen to these questions on this forum: Q: Schrödinger's cat question Q: The Bohm interpretation and Schrodinger's cat Q: Is Schrodinger's Cat itself an observer? When was there a Schrodinger's cat experiment? These questions are all speculation. My question was based on an actual experiment. Yo...

discussion

Aaron Stevens

11:08 PM

@PhysicsMeta Yeah I am not touching that one

ZeroTheHero

@AaronStevens me neither.

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