The h Bar

user434058

1:01 PM

2

Q: Is the slotted mass responsible for the centripetal force in vertical circular motion?

Is the slotted mass responsible for the centripetal force in a vertical circular motion in this particular experiment? I want to do the following experiment, where I hand spin a mass (red) that is attached to the same string with the slotted mass at the other end. I spin the mass by grabbing the...

user434058

Will this question be closed as off topic?

user434058

Like, it's "ancestor" also gained two upvotes and an answer but that question is closed.... I think that this question is also going yo have the same fate.

user434058

This doesn't seem explicit HW to me but there's no certainty that it will not get closed, so I am asking about it here. I don't want to end up answering closed questions.

user434058

1:25 PM

1

Q: Can the battery of a laser provide kinetic energy to laser?

Consider the following scenario : A laser is kept on a frictionless plane and switched on. The laser is emitting light in a particular direction and we know light has momentum. Now as the net force on system is zero, conservation of momentum states that the laser must also acquire a momentum in ...

energy-conservation conservation-laws thought-experiment

user434058

Whatever I said there ☝️ in the comments, is that true or is it just some speculative gibberish?

user434058

Because I am not as much sure about it as I am about my other stuff....

PM 2Ring

@FakeMod I hope not, since I encouraged the OP to try & create a conceptual question about this. I admit that it's still not ideal, but I like to encourage people who make a genuine effort to pose their question in an acceptable way. So if you can think of further improvements, please suggest them in comments, I'm sure the OP will be happy to oblige.

user434058

@PM2Ring I was thinking of answering it.....

PM 2Ring

@FakeMod Great!

As long as your answer focuses on the underlying concepts, that should be fine.

@FakeMod You may be overthinking this. A photon has no rest mass, all of the energy of a photon is kinetic

user434058

1:37 PM

@PM2Ring Is the hypothesis of "change in frequency" any good?

Yuvraj Singh...

@PM2Ring hi.

ACuriousMind

@FakeMod I don't really understand your comment about "thinking classically" about the kinetic energy of photons. A photon is not classical, and $\frac12 m v^2$ does not apply to photons.

PM 2Ring

Lasers aren't particularly efficient in turning electricity into light, they tend to generate a fair bit of waste heat, which radiates in all directions.

Hi, @Yuvraj.

ACuriousMind

In any case, as I just commented the question seems to treat lasers as magical boxes that convert electricity into directed light. Once you specify how the laser does that the "contradiction" will go away.

user434058

@ACuriousMind I was just trying to extend the classical stuff. Now I think that was a bad move :P

user434058

1:40 PM

@ACuriousMind No but the paradox is real. Let's say not lasers but any other device which does this direct conversion is used, what then?

ACuriousMind

what direct conversion?

There are no magical boxes that transform electricity into directed light without any steps in between, and in particular none that do so with 100% efficiency.

user434058

@ACuriousMind Right, I was talking about that magical box

ACuriousMind

Sure, you get a paradox if you posit that a) there is a magical box that turns electricity into directed light with 100% efficiency and b) there is magical box that turns electricity into undirected light with 100% efficiency.

But neither of those boxes exist so there is no paradox.

PM 2Ring

@FakeMod If the laser is free to move on a frictionless surface, or if we have a laser floating in space, yes it will move backwards, due to conservation of momentum. And the frequency of the light will be Doppler shifted down (aka ref-shifted), in accordance with conservation of energy.

Yuvraj Singh...

I was reading a physics magazine today, and and magazine had a question in last, how a physicist is different from a engineer when both of them studied the same subject physics as the main?

user434058

1:43 PM

@ACuriousMind Why is the 100% efficient condition necessary? Even a 50% efficient magical box will exhibit this paradox?

Yuvraj Singh...

I have no answer of that.

Can you help?

user434058

@PM2Ring Right! That's what I am talking about, frequency change will account for energy conservation.

ACuriousMind

@FakeMod what is the paradox, exactly?

user434058

@YuvrajSingh... An engineer takes the value of pi to be 3 ;P

ACuriousMind

If the box is not postulated to be 100% efficient, then there's nothing wrong with the energy that's not going into the light to be converted to kinetic energy of the laser

user434058

1:46 PM

@ACuriousMind The paradox is that, in one case some energy is manifested in the form of the KE of the magical box, whereas in the other cas, the box doesn't even move! Where did the energy go?

ACuriousMind

But those are two different boxes!

user434058

@ACuriousMind But I am talking about a box with constant efficiency

user434058

Like if it's 50%in one case then it's 50% in the 2nd case as well.

ACuriousMind

One box emits directed light, the other doesn't. They clearly do not have the same internals (since they do different things), so what's the problem here?

If both boxes are 50% efficient then it's perfectly consistent that the directed one converts 45% into heat and 5% into its own motion (percentages probably not kinematically plausible) and the other converts 50% into heat. Where's the paradox?

user434058

@ACuriousMind Well like absolute conversion percentage in light is same.

user434058

1:49 PM

But I think we are digressing here. We should rather assume ideality and attribut frequency change as the answer.

Yuvraj Singh...

@PM2Ring what will be your answer?

user434058

Or rather we should close that question!

user434058

Jk

ACuriousMind

@FakeMod I don't see how that's a paradox

PM 2Ring

@FakeMod This discussion reminds me of a question I answered when I was still fairly new to Physic.SE (but I'd been on SO) for a few years. But instead of a laser, the energy source is radioactive decay. The OP's question asks about an impossible scenario that doesn't conserve momentum, so I had to fix that in my answer, but I did briefly address that "magical" scenario in a postscript. physics.stackexchange.com/q/343250/123208

@YuvrajSingh... The question about the physicist and the engineer?

Yuvraj Singh...

1:52 PM

@PM2Ring yes

user434058

@PM2Ring That's an awesome answer! But sadly, I barely know a tiny bit of special relativity :(

user434058

@ACuriousMind I think I'll have to think about this. Maybe, I will talk about this when I have found some strong convincing ideas of either side of our arguments.

PM 2Ring

@YuvrajSingh... The physicist is concerned about the laws of nature underlying the physical world. The engineer is concerned about the application of those laws in practical ways, especially regarding how to make useful efficient machines, and other structures like bridges, buildings, ships, spacecraft, etc.

Yuvraj Singh...

Can we differentiate them as theoretical and practical physicist.

PM 2Ring

@FakeMod Thanks! It's not hard to learn a little bit of Special Relativity. The basic mathematics is just Pythagoras' Theorem. :) But it can be hard to wrap your head around the concepts, especially treating time as a dimension similar to (but not identical to) the 3 spatial dimensions.

user434058

2:00 PM

@PM2Ring Sure! Learning relativity is on my to-do list :)

PM 2Ring

@JohnRennie Uh-oh. It looks like we have an "unhappy customer" on a dupe we closed. physics.stackexchange.com/q/541113/123208 I've tried to help in the comments, but I don't know what else to say, and the OP doesn't want to edit the question to explain why it's not a dupe. (He's also giving Anna V a hard time in the comments on her answer, but that's a separate issue).

user434058

@PM2Ring Answered!

PM 2Ring

2:17 PM

@FakeMod I'll look at it later. I really should try to get back to sleep... :)

user434058

@PM2Ring No worries :) Get a good sleep.

user434058

BTW How long do the people here sleep? My parents constantly lecture me to sleep less.

ACuriousMind

It's perfectly normal for parents to do that :P

(it's also perfectly normal for children to ignore them)

Yuvraj Singh...

:-P

Slereah

2:42 PM

@ACuriousMind is full of youthful rebellion

ACuriousMind

I guess I am (though the youthful part is slowly dwindling :P)

Slereah

Potter Puppet Pals: Wizard Swears

►

^@ACuriousMind

ACuriousMind

it me

John Rennie

@FakeMod sleep less? Or lie in your bed watching YouTube videos on your phone less? :-)

Slereah

I don't even know how you're supposed to compute the holonomy group in general

Every holonomy is an infinite sum of infinite integrals, and you have to do it at every point for every curve???

Doable for very symmetric spaces, but beyond that

seems a tad harduous!

Although I guess... If a subgroup of a lie group has the same dimension as the lie group itself, is it the same group?

You'd only need to get $N$ such holonomies in the best case

also wait, the holonomy group is the same at every point of the manifold, right?

Although that means that finding a lower bound for the holonomy group isn't too hard, but the exact group

much harder

ACuriousMind

3:00 PM

@Slereah The holonomy group on every point of a connected component is - as a subgroup of the gauge group - conjugate to the group at every other point in that connected component

Slereah

Not too hard to compute for flat space

ACuriousMind

(proof sketch: choose any path from $q$ to $p$ and call the parallel transport along it $g$. then $gPg^{-1} = Q$ for the holonomy groups $P$ at $p$ and $Q$ at $q$ since going back and forth along the path turns a loop based at $p$ into a loop based at $q$)

Novalium Company

@Slereah ur e vizerd herre

ACuriousMind

And for some specific examples computing the holonomy group is pretty easy: Consider e.g. the classic example of the triangle on the sphere (pole - equator - equator 90° latitude further) rotating a vector by 90° when parallel transporting. Modifying that triangle path allow you to get any rotation, so holonomy is at least SO(3)

If you now also know that you can reduce the structure group to SO(n) on orientable manifolds, you're done

Slereah

"It turns out that the connected holonomy group of an $n$-dimensional indecomposable not locally symmetric Riemannian manifold is contained in the following list: $\mathrm{SO}(n)$; $\mathrm{U}(m)$, $\mathrm{SU}(m)$ ($n= 2m$); $\mathrm{Sp}(m)$ , $\mathrm{Sp}(m) \cdot \mathrm{Sp}(1)$ ($n= 4m$); $\mathrm{Spin}(7)$ ($n= 8$); $\mathrm{G2}$ ($n= 7$). "

THat's a lot of conditions

ACuriousMind

3:05 PM

I think it's always this - you don't really "compute" the group, you get a lower bound by some "obvious" paths and an upper bound e,g, by reducing the structure group and then you add conditions to your manifold until the two agree :P

Slereah

Berger’s theorem says that if a manifold XX is

simply connected

neither locally a product nor a symmetric space

then the possible special holonomy groups are the following

Ah there we go

I assume that if it's locally a product, the holonomy is a product of the holonomies?

ACuriousMind

yes

(you can then reduce the structure group to the product of the structure groups of the two factors)

Slereah

Fortunately I can probably assume that anything probed by measurements is simply connected :p

Otherwise either we probed very far or we just found a wormhole

user434058

@JohnRennie I would never trade sleep for watching YouTube videos. Sleep is way better ;)

John Rennie

Teenagers do need a lt of sleep. As a minimum you should be getting eight hours sleep, and up to ten hours is not unusual.

Slereah

3:14 PM

Hm

user434058

@JohnRennie Yup I get somewhere around 8 hrs.

Slereah

apparently, basically any non-flat manifold has the holonomy $\mathrm{SO}(n)$

user434058

user434058

This is nice!

Slereah

Bloody universe

tpg2114

3:22 PM

Interesting...

Slereah

Make sense I suppose

tpg2114

It's not wrong

Slereah

if the universe isn't flat, you can just take any path around whatever source of curvature there is to change the vector's orientation

an arbitrary number of times

and if there's a killing vector I suppose it's just a product of $SO(n)$ with the trivial group

PM 2Ring

2 days ago, by PM 2Ring

11

Q: Why does this very Meta question lack a duplicate target?

The question https://meta.stackexchange.com/q/345815/336163 (asked less than ten minutes ago) was closed as a duplicate less than five minutes ago, but it lacks any semblance of a duplicate target. The header shows: This question already exists: The expected link to the duplicate target is...

bug duplicate-questions meta specific-question community-user

I couldn't get back to sleep... yet.

user434058

3:37 PM

@PM2Ring I had seen that however I thought it was intentional. I took it as a joke :)

PM 2Ring

For more info, see meta.stackexchange.com/q/345289/334566

Loong

Maybe I should ask a homework question on Mathematics

"according to the provisions of the Prussian curricula of 1892"

bolbteppa

G2 manifold

In differential geometry, a G2 manifold is a seven-dimensional Riemannian manifold with holonomy group contained in G2. The group G 2 {\displaystyle G_{2}} is one of the five exceptional simple Lie groups. It can be described as the automorphism group of the octonions, or equivalently, as a proper subgroup of special orthogonal group SO(7) that preserves a spinor in the eight-dimensional spinor representation or lastly as the subgroup of the general linear group GL(7) which preserves the non-degenerate...

"These manifolds are important in string theory. They break the original supersymmetry to 1/8 of the original amount. For example, M-theory compactified on a $G_2$ manifold leads to a realistic four-dimensional (11-7=4) theory with N=1 supersymmetry. "

PM 2Ring

The "best" (worst) homework question I ever saw on the whole network was on Mathematics.SE. It had a useless title, like "Please help! Urgent!!!", and the body of the question was a phone photo of an exam paper, with minimal accompanying text. You could even see the OP's name written at the top of the paper.

Slereah

3:56 PM

there's no lack of phone photo homework questions

but they usually get deleted

what I want is one that says "Answer quick, I'm in the middle of an exam!"

Yuvraj Singh...

@PM2Ring our website is good in terms of that.

Loong

4:15 PM

Slereah

@Loong that's the stuff

I love the hubris

bolbteppa

That Fabri question is a good one

0

Q: Does Fabri-Picasso theorem cause the conservation of charge to fail?

The vacuum expectation value of the square of the Noether charge operator $$\langle 0|\hat{Q}^2|0\rangle=\int_{\rm all space} d^3\textbf{x}\langle0|j_0(0),Q|0\rangle$$ diverges in case of a spontaneously broken symmetry i.e., when $Q|0\rangle=|0^\prime\rangle\neq 0$. This argument relies on the a...

quantum-field-theory symmetry symmetry-breaking noethers-theorem poincare-symmetry

bolbteppa

4:34 PM

Where did my comment on there go (did I delete it?)

Aaron Stevens

4:58 PM

3

Q: Given two vectors (with no common point), is a dot product between them possible?

Most of the descriptions I have come across related to dot products between two vectors start off by stating/showing the vectors have a common point* to begin with - makes the notion of an angle between the vectors very easy to deal with in the final formula for dot products. I am showing below...

vectors coordinate-systems geometry

Is this an example of a on-topic "mathematical physics" question? To me it seems like just a math question, but it seems to be gaining popularity

ACuriousMind

That's an example of a completely ill-defined question :P

user434058

@AaronStevens It got HNQed. Didn't expect that.

Aaron Stevens

@ACuriousMind My question or the one I gave a link to?

ACuriousMind

A vector in the standard mathematical sense has no "point of attachment". Whatever OP is showing us is not something that defines two vectors without further elaboration

user434058

1

A: Given two vectors (with no common point), is a dot product between them possible?

There are different ways to answer this question, but a natural one for the purposes of physics is with differential geometry. Some background: (1) Every vector lives in some vector space. A vector space is a collection of vectors which obey particular axioms. Adding two vectors produces another...

bolbteppa

5:03 PM

In math, vectors are always supposed to be at the origin, in physics you're using vectors in space at different points and have to translate them to the origin, so it's really asking about the physics hand-waving use of vectors right

user434058

This answer looks great, but is it?

Emilio Pisanty

why is this still visible three hours after this revision?

how long is the cache for the Featured on Meta sidebar?

a day?

a week?

ACuriousMind

@EmilioPisanty the answer is always caching

bolbteppa

It's answering the question as if it's actually asking about placing vectors, which we always interpret as being placed at the origin of the vector space, at different points of a vector space by literally placing other vector spaces at different points in a vector space

user434058

5:31 PM

Should I give hints to HW questions in comments? To the kind of HW questions which aren't just blatant copy-paste, but the OP shows some (not enough) effort and thus they will probably get closed.

bolbteppa

Why not

ACuriousMind

@FakeMod Comments are for criticizing and improving the post being commented on, or to add related material. A hint is neither of those.

Aaron Stevens

@bolbteppa I think that would be an interesting question, but the OP isn't explicitly asking about it

I could see one arguing that saying the location of the vector "doesn't matter" would mean that the point where you apply a force to a body also "doesn't matter". So that could be an interesting question to explore in terms of relating the math of vectors to the physics of things like torque.

But this specific question isn't asking about physics

they even link to a question of MathSE that seems to be the same question, but they just don't understand the language of it

user434058

@ACuriousMind got it!

bolbteppa

5:47 PM

Right, the notion of an angle doesn't even make sense if they are not at the same point, so taken literally obviously not, but in say mechanics you'll often see vectors which are not at the same point and want to know how the angle changes with time as the vector moves and so you'll have to obviously consider them at the same origin to find the angle even if physically they're not at the same point, which I'm guessing the op means to be asking about

but if we take things literally vectors are always at the same point, the origin of the vector space, so taking things literally the question is invalid, but clearly they think vectors can be at different places and are asking on a physics site so it only really makes sense if you're ultimately moving them to the same origin

user434058

Lately, many new users start answering HW questions out of lack of knowledge of how the system works. I think we should either close questions quicker(requiring lower no. of close votes/flags) or we should put a big banner on the front page saying that we discourage these kinds of questions and any answers to them.

bolbteppa

That would be even more off-putting than getting angry messages about following some silly rules

FakeMod trying to become a RealMod ;)

ACuriousMind

@FakeMod An experiment to lower the close/reopen threshold to 3 is theoretically running, but since Shog (who was in charge of it) is gone it's unclear if/when they'll get around to actually activating it for our site.

Banners are never the solution because users don't read banners

user434058

The problem is that I never wish to downvote a correct and well intended answee by a new user just because it's an answer to a HW question, so I only flag them and occassionally leave a comment instructing them about the site's policies. However, like everyday, today also I have run out of flags :(

user434058

@bolbteppa No no no! Never in my wildest dreams :P The "Fake" makes me sound even more powerful...

user434058

5:59 PM

Jk

user434058

Fake makes me look dumb

user434058

;P

user434058

We should have an impulse tag for classical mechanics questions whichdeal with impulses and collisions....

ACuriousMind

@FakeMod What would it add over collision?

user434058

@ACuriousMind Are all impulses collisions?

user434058

6:09 PM

Doesn't this beg the question as to why the reaction force is different in different scenarios? — Aaron Stevens 3 mins ago

PM 2Ring

I'm not trying to advertise this, I just want to preserve some of this sillyness here for prosperity, in case it gets nuked.

-1

Q: Artificial sun Sculpture

I am designing a dynamic sculpture of the solar system, about 6,500 feet long from the sun to Neptune. At this scale the sun will be 24 inches in diameter, Earth a quarter of an inch. Every hour the sun will rise out of a flaming cauldron. The sun will be heated to 4000° kelvin. Tantalum hafnium ...

sun cooling

user434058

Just saying, but @AaronStevens, "why" questions will slowly tunnel down and we will reach a point where we no longer know the answer.....

PM 2Ring

I can't find a density value for tantalum hafnium carbide, but using an estimate of 13 g/cm³, which is roughly the mean of the densities of the tantalum carbide & hafnium carbide components, a 1 foot radius sphere of that stuff has a mass of over 1500 kg. And using the price given on Wikipedia, that will cost around $14.7 million dollars. But of course that's irrelevant if you're using all the gold in Fort Knox. :D — PM 2Ring 7 mins ago

Aaron Stevens

@FakeMod ?

ACuriousMind

@FakeMod That wasn't my question. You say "we should have", it's on you to explain why you think that tag would be useful

We have collision, and many questions that would conceivably tagged wiht impulse would also have collision, i.e. it's almost a subset

PM 2Ring

6:13 PM

@ACuriousMind Can you bump it to the CMs in one of the mod spaces? Or even in the MSE Tavern chatroom?

user434058

@AaronStevens Like, then we would ask why does the reaction force exist, then why does theelectrostatic force exist and slowly we will reach a question miles away from the original. This is just a thing that I have experienced and so I am sharing it with you. I know you were not going to stretch it this far, but I just felt like sharing this would somehow help you.

Aaron Stevens

@FakeMod oh sorry I wasn't aware you were replying to a comment on another post

@FakeMod Yeah but I am not going that far. What I mean is that the answer seems to be "We know the reaction force must be larger". I fee like it doesn't address the question, it more of just says what we can conclude

ACuriousMind

@PM2Ring We did that a while ago but we can try again now that the dust has settled somewhat

user434058

@ACuriousMind The main use, which I envision, is in the questions which rely on the application of Impulse-Momentum theorem. Or maybe you can even make impulse a synonym of collision. In fact, shouldn't it be the other way around? Like, Impulse is more fundamental than collisions....

ACuriousMind

Well, technically, impulse is just force integrated over time, nothing specific to collisions

It's just not often any use elsewhere

@FakeMod By "use" I don't mean what questions it would apply to. I mean what benefit the existence of the tag would provide over the tags that already exist

We can only apply 5 tags to a question so tags shouldn't be too fine-grained - there is no use in making a tag for every possible physical concept

PM 2Ring

6:19 PM

@ACuriousMind Thanks. The 3 votes thing has made life a lot easier on SO. OTOH, it makes my Python dupe-hammer powers less necessary. :)

user434058

@AaronStevens I completely agree! FWIW, even I and my physics teacher end up spending a lot of time chasing these "why" questions and that's why the rest of the class gets bored :P

user434058

@ACuriousMind That point has satisfactorily convinced me that there isn't a need for the impulse tag. Thanks!

Charlie

Does the sign in the Einstein tensor depend on the metric signature used? I.e. $$G_{\mu\nu}\equiv R_{\mu\nu}\pm Rg_{\mu\nu}.$$

I feel like I heard it said in a lecture somewhere, and it would make sense to me, but now I'm doubting myself

user434058

@Charlie BTW What is your current level of study (if you don't mind mentioning it)?

user434058

Uni, highschool, phd

user434058

6:27 PM

?

Charlie

My undergrad is in chemistry, I'm doing a postgraduate course to convert into physics

user434058

@Charlie Nice! All the best for converting to physics!

bolbteppa

$R_{\mu \nu} - \frac12 g_{\mu \nu} R$ always has a $-$, however the EM tensor on the other side has a sign convention thing going on right

Einstein field equations

In general theory of relativity the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of space-time with the distribution of matter within it.The equations were first published by Einstein in 1915 in the form of a tensor equation which related the local spacetime curvature (expressed by the Einstein tensor) with the local energy and momentum within that spacetime (expressed by the stress–energy tensor).Similar to the way that electromagnetic fields are determined using charges and currents via Maxwell's equations, the EFE are used to determine the spacetime...

Aaron Stevens

@Charlie "Chemist + postgrad physics course --> physicist" reaction?

Charlie

There's definitely some byproducts, confusion over simple topics due to gaping 2 year hole in maths/physics education

Aaron Stevens

6:30 PM

@Charlie Ah yes of course :)

user434058

@AaronStevens I suppose you come under "physics + postgrad bio------>biophysiscist" metabolism, right?

Aaron Stevens

Chemist + postgrad physics course + gaping 2 year hole in maths/physics education --> physicist + confusion over simple topics

Charlie

Having to revisit the basics of linear algebra while trying to study for a GR course is fun

That's pretty much it haha

Emilio Pisanty

118

A: Feedback for The Loop: March 2020

I've linked to this countless times over the past year, but clearly I need to do so again: Effective Apologies Include Six Elements – Association for Psychological Science – APS. The titular "six elements" are: Expression of regret Explanation of what went wrong Acknowledgment of respo...

wow

that is a remarkable comment thread

Charlie

@fakemod thanks, it's going well so far, just a lot to catch up on

Aaron Stevens

6:32 PM

@FakeMod Yeah, you forgot my byproducts of impostor syndrome

Emilio Pisanty

I don't think I've ever seen anything remotely like it on SE

Charlie

@bolbteppa thanks for your response, forgot to reply

PM 2Ring

@EmilioPisanty Shog the CM was a cool dude, but Shog the free agent is even more awesome.

user434058

@AaronStevens Like, every second person studying physics or maths has it....

Aaron Stevens

@FakeMod Yeah I know

ACuriousMind

6:40 PM

@EmilioPisanty Not sure it's that exceptional, looks like quite a few meta threads in which Shog was involved. The only unusual thing is that the other party here is an SE employee and not some upset user :P

PM 2Ring

And that comment from Uchiha Madara is pretty awesome as well.

Aaron Stevens

@FakeMod I just meant I think my impostor syndrome is due to my switch to biophysics. But of course I can't prove this :P

user434058

@PM2Ring I am not able to find that, can you please link it?

user434058

@AaronStevens That's a sign that you aren't overconfident...

user434058

@PM2Ring nvm found it

JMac

6:46 PM

@FakeMod Impostor syndrome is the opposite of overconfidence.

user434058

@JMac Yup! I know it. That's why I said that. Like, seeing the brighter side.....

ACuriousMind

@PM2Ring "looking into it" might happen this month apparently, but since we're not the only site on the list I would expect this to take 6-8 weeks(TM)

user434058

@ACuriousMind Oh! The classic 6-8 weeks....

ACuriousMind

Yes, that's the joke :P

user434058

@ACuriousMind does that TM mean trade mark?

ACuriousMind

6:51 PM

@FakeMod Yes. Things taking 6-8 weeks are an old running gag on SE, cf. meta.stackexchange.com/a/19514/263383

user434058

You should've used ™ for a better effect, but it's ok...

user434058

No you shouldn't....

user434058

It looks very tiny.

user434058

7:03 PM

Why is there a limit on the number of flags that you can raise?

tpg2114

@FakeMod Because it's healthy to go outside and do stuff instead!

user434058

@tpg2114 Are people addicted to flagging?

tpg2114

People are addicted to all kinds of things, so maybe?

I dunno, I couldn't prove it one way or the other

My comment was said with tongue very firmly in cheek.

user434058

But is that the real reason @tpg2114

user434058

I know it isn't....

ACuriousMind

7:06 PM

@FakeMod 1. community moderation doesn't work if a single user flags everything. 2. Gamification - the number of flags/day you can raise goes up with the number of helpful flags. If there was no limit there'd be nothing to gamify. 3. if you could raise unlimited flags, a single bad faith actor could flood all review queues with useless frivolous flags

user434058

@ACuriousMind Out of those 3, only the last point seems convincing to me. But Thanks! That helped.

tpg2114

6

Q: Why is there a limit on number of posts I can flag?

I have noticed that there is a limit for number of posts that I can flag. The limit value seems to decrease without being restored. Why is there such a limit? We earn the ability to flag posts after gaining certain reputation, that means gaining some trust, why add the limit then?

discussion flags limits

One of the answers further down has: blog.codinghorror.com/rate-limiting-and-velocity-checking

user434058

@tpg2114 I see...Thanks!

Loong

@tpg2114 “While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.”

tpg2114

@Loong I really hope you NAA it and leave that comment ;)

Loong

7:13 PM

Hm, I don't have a meta account anymore.

user434058

I flagged it! Don't tell Jeff :P

Loong

I had flagged these arrogant SE people many times before, but it didn't really help.

Charlie

7:52 PM

I have a question, if I submit an edit to a question that is eventually closed, for example as I have done here. If my edit is accepted after the question is closed does that count as the one and only "rebump" of the question after it's been closed?

In other words my edit was submitted before the question was closed, but because I am a <2k pleb my edits require approval, which might mean it gets approved after the thread is closed, and as I've been told before closed questions get a bump if they're edited, under the assumption that they've been "fixed".

PM 2Ring

@Charlie Yes, it does. Many people with close vote privileges understand this, and will refuse pending edits before the final close vote is cast, but not all of then do it.

Charlie

Ok thanks, I will avoid correcting formatting on posts that look unsalvageable content-wise

PM 2Ring

Every edit bumps the question to the top of the Active page. But only the 1st edit of a closed post sends it to the reopen review queue.

Of course, there's a chance that people will vote to reopen if they see a decent question on the front page. But its chances of being reopened are better in a queue full of people who are specifically looking for stuff to cast reopen votes on.

@Charlie Good idea. As the old saying goes, you can't polish a turd. ;)

Charlie

No more turd polishing from me

PM 2Ring

OTOH, people working the pending edit approval queue ought to know better than to approve turd-polishing edits... but some still do it. :)

Stack Exchange is a crazily complicated contraption. But somehow it manages to function ok, most of the time, despite its numerous flaws and weaknesses.

Novalium Company

8:09 PM

@Charlie I like turds cuz they've given me energy

Ahh everyone here has so much rep and I'm sitting here with my 375 :P

Why isn't there rep for getting help

I'd be in top 100

#blessed

#I'mgoingtobed

PM 2Ring

@NovaliumCompany Pity we can't turn all the bad SE posts into fertilizer. Instead, they hang around forever, taking up space on the servers.

ACuriousMind

@NovaliumCompany There is rep for getting help - what do you think question upvotes are?

PM 2Ring

@NovaliumCompany You could spend time making edits, like Charlie does. Sure, it only earns 2 points per approved edit, but it adds up pretty quickly.

Jake Rose

10:39 PM

Im having a dumb moment

oops

Wrong image

Aaron Stevens

Cant post anything on the original post above, so posting this here. It is funny that after @rob announced the locking of this question, the question still remained open just long enough for his comment to get an upvote. — insomniac 19 mins ago

I guess locking a post doesn't lock it's answers

Jake Rose

Looking at this lorentz transform of the minkowski metric

Is it obvious what the inverse of $\Lambda$ should be?

ACuriousMind

10:53 PM

@JakeRose I don't really understand the question - what do you mean by "what the inverse should be"?

insomniac

@ACuriousMind : I'll just repeat what I said the other day

Censoring anything will just make things worse

there was not a hint of anything threatening on my latest comments

ACuriousMind

Tempers are way too high in this discussion, so let's pause it for a few days. If you'd like to keep arguing about this ... don't. Do something else instead. It will still be here later. — rob ♦ 2 days ago

This applies to all users. Stop arguing about this and let it rest.

Jake Rose

As in

I could get it by translating everything into a matrix notation, but it feels rather a round about way of doing it

ACuriousMind

The locking of the comments on the meta post was rather explicitly not an invitation to shift the discussion here, or repeat your comments here.

9 messages deleted

Jake Rose

there we go

That's quite fair

@ACuriousMind oops sorry for the extra work there

Do you get what I mean with the question though?

bolbteppa

11:03 PM

From $\Lambda^{\rho} \, _{\mu} \Lambda^{\sigma} \, _{\nu} \eta_{\rho \sigma} = \eta_{\mu \nu}$ we have $\Lambda_{\sigma} \, _{\mu} \Lambda^{\sigma} \, _{\nu} = \eta_{\mu \nu}$ and so $\Lambda_{\sigma} \, ^{\mu} \Lambda^{\sigma} \, _{\nu} = \delta^{\mu} \, _{\nu}$

tells us that $\Lambda_{\sigma} \, ^{\mu} \Lambda^{\sigma} \, _{\nu} (\Lambda^{-1})^{\nu} \, _{\tau} = \delta^{\mu} \, _{\nu} (\Lambda^{-1})^{\nu} \, _{\tau} $ gives $\Lambda_{\sigma} \, ^{\mu} \delta^{\sigma} \, _{\tau} = (\Lambda^{-1})^{\mu} \, _{\tau} $ i.e. $\Lambda_{\tau} \, ^{\mu} = (\Lambda^{-1})^{\mu} \, _{\tau} $ which is the transpose

Jake Rose

Why is the ordering important in the indices?

bolbteppa

They are matrices which are not symmetric, for example a planar rotation is a Lorentz transformation which is not symmetric

ACuriousMind

@JakeRose Not sure. I don't think "matrix notation" is a roundabout way here - without all the terrible indices, the first equation just says $\eta = \Lambda^T \eta \Lambda$ and you get the second simply by multiplying with $(\Lambda^T)^{-1}$ and $\eta$

Jake Rose

thanks guys

insomniac

Room for insomniac and ACuriousMind

Thanks guys

Jake Rose

11:13 PM

@insomniac you should really put this into a private room

/ask a mod to make a private room to discuss this stuff

insomniac

@JakeRose : I'll repeat. Not talking to you. Don't get me banned again and again for answering the same question.

For the record though, there is a thin line between moderation and censorship

My issue was with the latest instance of censorshi

Of comments that do make sense (at least enough to be relevant) AND did not contain a hint of aggression

My temper is not really flared up right now

In fact, I waited till I could be sure of this fact, and THEN I posted

on my meta post

Jake Rose

@insomniac truly, I don't care. I'm just saying, if you want to air grievances please do it in a private room with a moderator.

insomniac

ACM (or, the "system", as I learn he has been christened around here) edited it out

Anyway, to finish what I was saying

Jake Rose

@insomniac you're being rather rude to @ACuriousMind. And again, this isn't the correct place to be airing these grievances.

insomniac

if the "system" moderated something out that made clear sense, did not contain a hint of aggression, then it IS a topic for discussion

Jake Rose

11:18 PM

And I can guarantee that all of your rant is about to be deleted.

insomniac

You know

tpg2114

Let's just walk away from this line of discussion please

Jake Rose

Because a)nobody cares b)this isn't the place for it

@tpg2114 agreed

tpg2114

@JakeRose Lead by example ;)

insomniac

last I checked, ACM can answer for himself. No need for you to defend him. Almost feels like an insult to ACM

but anyway

Jake Rose

11:19 PM

@tpg2114 fair fair

insomniac

as I said. eggs to buy. ttyl (for now)

Jake Rose

@tpg2114 could you remove all my messages if you get a chance pls

tpg2114

@JakeRose Nothing rises to the level that needs that. But, it's best to just diffuse things by letting it be. We can always move messages to other rooms if they are a problem here, etc..

Jake Rose

@tpg2114 Got it

tpg2114

Thanks!

insomniac

11:26 PM

Its just... you guys keep positing such things, that I am drawn back to this pit.... @JakeRose : "...could you remove all my messages..." Why? Is there something you said here you aren't proud of?

tpg2114

Please let's just let it slide

Or drop, rather than slide

insomniac

If you are sure you are in the right, why the need to censor? If you are going to turn back and think that something you said is idiotic, better stay shut in the first place, no?

tpg2114

Stores won't stay open forever and unless you have a chicken, the eggs probably won't come to you :)

insomniac

@tpg2114 Yes . I AM leaving

thanks. I do realize that I am hurting many of you. But I feel I have no option. This whole situation is very much in conflict with many principles I have. But sorry for anything hurtful I have said. And @ACM : since you seem to be bearing the brunt of my agitation, doubly sorry to you, but I hope you understand.

bolbteppa

@insomniac it's not worth getting upset over even if you are in the right

tpg2114

11:42 PM

In other news, I went to ride my bike yesterday for the first time in... uhhh... 9 months. And I tried again today, but it didn't go so well.

My Selle Italia SLR saddle was awesome when I was 60 pounds lighter and really in shape 5 years ago. But now, I'm convinced SLR is an Italian codeword for some torture device

bolbteppa

I don't know why that would upset anyone

tpg2114

@bolbteppa Please don't start going this way

bolbteppa

Fair enough

insomniac

Also, that last comment @bolbteppa was not offensive in the least to me

I think I detail my objections pretty clearly in my meta post

don't caricaturize someone's argument. Engage with it

And not everything upsets me. Bolbteppa merely stated (very politely) that they have the right to downvote my questions and answers. I have no problem with that

but yeah, following @tpg2114 's advice, I really should leave if I want to eat eggs tonight (reduced store hours where I live). Ciao

Room for insomniac and ACuriousMind

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