The h Bar - 2020-10-01

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12:05 AM

@probably_someone you’re back (at least punctually)

probably_someone

@ZeroTheHero I agree, the moderators here are quite good. I have no problem with this site in particular. But as far as I can tell, all SE sites are connected: supporting one indirectly supports all of them. And the policies that allow such atrocious moderation are pretty fundamental to the design of most SE sites. My current feeling is that this is the only option left that will have any significant impact.

Repeating myself: it’s so sad to see a senior contributor go.

4 hours later…

4:12 AM

@BioPhysicist Yes, I did watch #Debate 2020, I feel like I've wasted 25 minutes of my life...

2 hours later…

Superfast Jellyfish

5:43 AM

https://physics.stackexchange.com/questions/582682/motion-of-points-around-a-triangle/582684
The close reason for this question states “This question does not appear to be about physics within the scope defined in the help center.” Is that the case? I personally think it is otherwise. The question is asking specifically about a concept and not asking for the working out of the problem. So I understand why the answers were deleted. But not why the question was closed

5:57 AM

India pushes bold ‘one nation, one subscription’ journal-access plan

Researchers will also recommend an open-access policy that promotes research being shared in online repositories.

> The Indian government is pushing a bold proposal that would make scholarly literature accessible for free to everyone in the country. The government wants to negotiate with the world’s biggest scientific publishers to set up nationwide subscriptions, rather than many agreements with individual institutions that only scholars can use, say researchers consulting for the government.

Do such things happen?

2 hours later…

7:48 AM

@SuperfastJellyfish because it is not about physics.

That one is all maths. There's no physics there.

Superfast Jellyfish

8:29 AM

“So, why according to particle 𝐴, particle 𝐵 is in uniform motion? What happens to their acceleration vectors while frames are changed?”

To me this looks like OP is seeking an interpretation of the underlying math. Which is very much in the scope of physics

@SuperfastJellyfish But the entire specific problem is wholly irrelevant to the question of how acceleration vectors change under change of frame.

We got 4 answers none of which addressed the question about the frame and just provided more or less detailed arguments why you shouldn't think about this problem in terms of frames and then presented a different solution to the problem

Superfast Jellyfish

@ACuriousMind that perhaps is true. But that can be addressed in the answer. In fact it has to be. I think it stems from OP’s incorrect understanding of uniform circular motion. So I understand deletion of the answers. But not the closure of the question

If you really think that the question in its current form is sufficiently focused to be about physics, you can vote to reopen :)

Superfast Jellyfish

8:44 AM

Yeah I just wanted to understand why it was closed in the first place. I wanted to be informed before making any decision :)

But I think a large part of the question could be cut and then it would be much clearer what the problem for OP is. And the problem itself is really not physical - no physical system moves like that

I'd much prefer if the question elided the reference to the problem entirely and just asked how acceleration vectors change under change of frame

Superfast Jellyfish

@ACuriousMind yeah that’s the core of OP’s question. But wanting them to ask that is sort of a catch-22 don’t you think?

Maybe? I think they already understood that they are confused about how acceleration vectors change under frame

Superfast Jellyfish

9:02 AM

Then perhaps the close reason should be “needs focus” that way OP gets the message to trim the fat which anyway will force them to think about the meat of their question

4 hours later…

shelton Benjamin

12:35 PM

Why is internal energy measured from centre of mass frame of reference?

I adore Zee’s GR book

he has a phenomenal way of conveying information in a non pretentious and boring way

anybody know where he got this quote from “I’ll differentiate any function, even the freaking delta function, as many times as I darn well please!”

Non boring*

He could have meant some imaginary character he invented as part of his narrative

12:51 PM

Oh I see

Geroch used Galileo's characters for an article of his

2 hours later…

2:23 PM

@ACuriousMind I suspect there is a user just selecting "Leave open", "reopen", etc. on all of the queues. I have flagged one question they voted to leave open, but I didn't know if that was the appropriate way to bring attention to it

@BioPhysicist That's exactly the right way, it's fine

Sounds good, thanks

I think this user is trying to fight the system haha

"the system" always wins

Any time something is brought up about the system I think of this

SNL Digital Short: Threw It on the Ground - SNL

lol

2:31 PM

"The moral of this story is, you can't fight the system"

*trust... oops

shelton Benjamin

Why is internal energy measured from centre of mass frame of reference

what's a centre of mass frame

centre of momentum?

If so, it's probably so you can ignore the kinetic energy resulting from bulk motion

2:55 PM

@Charlie the center of momentum frame with the origin at the center of mass

@BioPhysicist I was literally just listening to The Lonely Island lol

@SirCumference ok

@JMac Great minds ;P

3:12 PM

minecraft steve is in smash bros

wow, they actually did it

3:49 PM

I bet at the very beginning the makers of minecraft never expected to make it into Smash bros.

Recently, in my YouTube feed I got a video on personality type test (video was from Vox). It talked about how these tests are used in America in assigning post (such as Manager, etc). Can somebody please explain me about this whole stuff. Is it that important a thing? Is it scientific? Has anyone here given it?

4:29 PM

Hi, everybody.

@BioPhysicist Wait, what?

@ACuriousMind Can I somehow give you a prize for that comment?

@DanielSank it's true

i guess that's one way to refuel the hype for smash bros after months of no updates

4:59 PM

user image

Can someone explain why that example isn’t a vector?

@DanielSank I only accept gold medals

@JakeRose given the preceding paragraph, have you checked what happens to it when you rotate the original $p$?

I think so

you just apply the rotation matrix

no, that's not how it's supposed to go

I can’t see how you can’t apply the rotation matrix to it

The original $p$ is definitely a vector. So you have that $p^1\mapsto \cos(\theta)p^1 + \sin(\theta)p^2$ and similarly for $p_2$ under a rotation

let's call the components of the new non-vector $q^1 = ap^1$ and $q^2 = bp^2$

if you plug in how the $p^1$ and $p^2$ transform under rotation, you don't get that $q^1 \mapsto \cos(\theta) q^1 + \sin(\theta) q^2$, but something different, so it's not a vector

Superfast Jellyfish

5:07 PM

@BioPhysicist that part with the kid’s birthday cake being demolished never fails to get me! xD

But P1 and P2 are both just scalar values right

no

if they were scalars they would not be components of a vector!

(but instead not transform under rotation at all)

What

im so confused

if the components aren’t scalars then what are they?

"scalar" does not mean "single number", it means "invariant under transformations"

the components of a vector are numbers but they are not scalars

i see

so how can you have a pair of numbers that aren’t vectors

5:13 PM

acm has antennae that light up when someone uses the word scalar wrong :P

because it’s some place in the $R^2$ plane right?

should be the math r but I’m not sure of it’s latex

well, like in that example - $(q^1, q^2)$ is a pair of non-scalar numbers that's not a vector because it doesn't transform right under rotations

@JakeRose the text you quoted didn't define "vector" as "place in the $\mathbb{R}^2$ place", it defined it as transforming in a certain way under rotations.

mhm

So in this example it would be something other than numbers

I'm not sure what you mean by that

@Charlie just be glad you don't have to see the frothing at the mouth :P

because if it was just two numbers, e.g. $(5,9) $ there’s no way that doesn’t still transform like a vector when multiplied by a and b

sorry about this

5:16 PM

@JakeRose there's nothing about numbers that makes them transform in one way or another

the transformation behaviour is additional information

mhm

When you have a vector $(p^1, p^2)$ and its length $p$, all three of $p,p^1, p^2$ are numbers

but two of them - $p^1$ and $p^2$ transform in some way under rotations, and $p$ does not (rotating does not change length)

And if we define that transformation it messes up the other one

in such a way that it doesn’t work

Well, since we said that $(p^1,p^2)$ is a vector, it transforms such that $(p^1,p^2) \mapsto R(\theta)(p^1,p^2)$

but this means that $(ap^1, bp^2)$ does not

In other words
\begin{align}
\begin{bmatrix} a p_1' \\ b p_2' \end{bmatrix} &= \begin{bmatrix} a (p_1 \cos \theta - p_2 \sin \theta) \\ b (p_1 \sin \theta + p_2 \cos \theta) \end{bmatrix} \neq \begin{bmatrix} \cos \theta & - \sin \theta \\ \sin \theta & \cos \theta \end{bmatrix} \begin{bmatrix} a p_1 \\ b p_2 \end{bmatrix} = \begin{bmatrix} a p_1 \cos \theta - b p_2 \sin \theta \\ a p_1 \sin \theta + b p_2 \cos \theta\end{bmatrix}
\end{align}
unless $a = b$

5:39 PM

Lovely thank you

I think the string field thing goes like this: If $\Phi(X) = \lim_{N \to \infty} \Phi(X_{\mu}(\sigma_1),...,X_{\mu}(\sigma_N))$ is a string field then
ignoring the limit and setting $N = 2$ in $(1,1)$ dimensions this apparently means, for example
$$\Phi(X) = \Phi(X_0(\sigma_1),X_1(\sigma_1);X_1(\sigma_2),X_2(\sigma_2)) = X_0(\sigma_1)^{44} + \frac{2}{3} X_0(\sigma_2)^3 + X_0(\sigma_1) X_0(\sigma_2) + X_1(\sigma_2)^3.$$
and if
\begin{align}
\mathcal{L}(\Phi,\partial \Phi) &= \frac{1}{2} (\frac{\partial \Phi}{\partial X_0(\sigma_1)} ) (\frac{\partial \Phi}{\partial X^0(\sigma_1)} ) + \frac{1}…

@ACuriousMind I do not haz.

BTW Untitled Goose Game got a two-player update.

Ah, UGG2: Electric Goosealoo

6:06 PM

In lightcone coordinates the $X^0$ goes into $\approx X^+(\tau) + X^-(\tau,\sigma)$ and $X^+$ at least does not depend on $\sigma$ so in this case you get a unique canonical momenta associated to $\Phi$

6:27 PM

2

A: Can a free String Field Theory be described in terms of a "wavefunctional" of classical string field configurations?

String field theory is vastly much more sophisticated than that. Let me mention a few points to illustrate why the way you propose to visualize string fields theory is not a good one. Take the example of closed bosonic string field theory. If you look at the highly complicated action you can lea...

6:48 PM

Deep dream is cool

I finally got an intuition of how it works yay

7:07 PM

Does any one know of any "useless" reformulations of quantum mechanics?

The Wigner one you like :p

So mean its not useless

:P

5 hours later…

11:44 PM

@danielunderwood eyy long time no see

danielunderwood

Howdy!

how's life?

danielunderwood

Pretty solid aside from the whole 2020 thing

Crazy times

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