@PM2Ring I think it's fine as long as you start the message with "Welcome to Physics SE!"

@AaronStevens I for one don’t think it’s fine.

Although I will not deny I have uttered similar comments in the privacy of my own mind, the wiser course of action is to refrain from such postings.

@dmckee Can you expand a bit on this? I’m curious to compare notes: I myself recommend SE especially for Mathematica coding as I find this specific site to be an immensely useful reservoir of pieces of code.

*recommend to my students.

no sense in re-inventing the wheel... although I insist they acknowledge the source.

o/

@ZeroTheHero Sorry, it was supposed to be a joke.

@PhysicsMeta Already answered, I win you silly robot

@AaronStevens no worries. I realize also I was a little “gauche” as I should have made my reply to another comment. Still... let’s agree to stay away (at least in a public chat forum) from this kind of comment.

@ZeroTheHero I didn't ask them to elaborate.

My guess would be that it is a place for them to ...

(a) See multiple different discussions on the same concept with a tight enough focus to not get lost in the detail. Which will help them to develop the fluidity they need in move between different representations fot he same problem.

(b) See lots of different examples of technical writing of varying level of quality, and begin to get a sense of what makes for good technical writing.

(bii) Practice their technical writing if they particpate actively.

(c) And of course we have a small but growing pool of really outstanding answers which are valuable all for themselves.

Yes I also see several virtues to browsing the site. One of them is that it helps to remove a certain loneliness to you see people posting questions similar to the ones students ask themselves.

the kind of “ah yes other people have this issue too” kind of comment.

@ACuriousMind I think my idea is so good, that it simply requires to be documented on the PSE meta, in an of course $\approx$ -25 scored post.

@ACuriousMind ACM wrote "I'd argue that moderators should have an understanding of what this community values". True enough but, I'm afraid, essentially vacuous. It's precisely (IMHO) the values of this community that aren't quite clear and are, in fact, 'in play'. Maybe I'm wrong about this. I don't claim to have the standing to speak for the community. But I'm skeptical that a moderator can, in general, have such an understanding.

@AlfredCentauri this relates to a sentence from a meta post by a mod that bothers me. wrote a comment on it, but it got deleted. feel a great mod is not merely a preserver/ dictator of "values of a community" but is also receptive to feedback/ new directions/ different povs on those values. lets see how the new blood may have some different ideas or "gets assimilated by the borg" :P

@dmckee that is massively heartening :-)

@AaronStevens Don't worry, I realised you were joking. And of course I would never post a comment like that on the main site!

How can I zap my skin with electronics using a 9V battery, but no transformer?

I don't know if it's a "zap" or... but I want to trigger the skin neurons to send info to my brain.

@NovaliumCompany Why no transformer? Are other coils allowed?

@AlfredCentauri You're right that "the community" is not a monolithic entity with a singular opinion. I should have said "an understanding of the different factions within this community and what they value". And yes, there are no formal factions, but there are a few archetypes that can serve well to characterize a large fraction - certainly not all - of the userbase

Anyway, you can do it using a voltage multiplier. You'll also need some kind of oscillator, because the voltage multiplier needs AC input. FWIW, a few decades ago I built a negative air ion generator using a voltage multiplier.

from Two-point functions in AdS/dCFT and the boundary conformal bootstrap equations, it's like the structure constants are derived from the operator product expansion, rather than the converse as I considered and posted here yesterday.

Then I still wonder what principle an operator product expansion is based on to make the expansion.

@CaptainBohemian There are two different viewpoints of operator product expansions. In one approach, it is postulated that the operators of the theory possess OPEs and then you can derive other things from that. In another approach, you start from the Virasoro algebra and then generate the OPEs from them. I just answered a question about that here

@ACuriousMind thank you very much. This post looks apparently just like my question. I will read the detail to see if my question can be resolved.

Hey is there a high pdf version of A comprehensive introduction to differential geometry by Micheal Spivak

All of the ones I found are terribly low resolution

@user212860 amazon.com/Comprehensive-Introduction-Differential-Geometry-Vol/…

@user212860 it's low-resolution in the original =P

isn't there a latex version?

@user212860 there's two versions - one with old typesetting, and one with ancient typesetting

if I understand the situation correctly

@Qmechanic I take it that tachyon questions are pretty much automatically off-topic? I was just about to ask that here, in reference to physics.stackexchange.com/questions/477173/collision-of-tachyon but then you closed it.

What is the difference between V=sqrt(ar) and V=sqrt(GM)/sqrt(r) Why is the proportionality of r different for the two?

@PM2Ring physics.stackexchange.com/search?q=is%3Aq+closed%3Ano+tachyon has 100 hits

To clarify, in one, V is proportional to the square root of r and the other V is proportional to the inverse square root of r.

Can someone explain?

@PM2Ring Tachyons can be mainstream, but if the question does not contain any more detail about what it considers a "tachyon" beyond the name, it's usually not

See physics.stackexchange.com/q/166095/50583 for a question with some good answers on how a formal and "mainstream" look at tachyons differs from the naive popular conception of "FTL particles".

Thanks, Emilio & ACM.

@PM2Ring : Echoing ACuriousMind, roughly, it depends on whether OP is referring to faster-than-light or not.

@Paul on what context are you trying to compare sqrt(at) and k/sqrt(r)

Ok. A tachyonic field is worth talking about. A tachyon point particle doesn't make much sense, or at least quickly gets tied up in paradoxes.

@Paul What's V? Velocity? Potential? Is a acceleration?

V is velocity

it related to to velocity initial and I want to know why r has different proportions.

@Paul The equation $v = \sqrt{GM/r}$ looks like the equation for the escape velocity from an object of mass $M$. Is that where you got this equation?

Hii @JohnRennie

@user8718165 hi

@JohnRennie In the morning, you told that the position of the pivot matters in the seesaw case. Could you please explain it a bit now(if you are free)

And $v = \sqrt{ar}$ gives the speed of a particle in uniform circular motion, with centripetal acceleration $a$, and radius $r$.

@user8718165 suppose you place the pivot below the centre of the beam like this:

@JohnRennie oh got it thanks a lot

When the beam is perfectly level (upper drawing) it is balanced because the COM is directly above the pivot. But if you tilt the beam at all the COM moves to the side of the beam and that makes the beam tilt even more.

@JohnRennie I'll try to get the other case myself. If I can't I'll ask you tomorrow

So with the pivot below the beam the level position is an unstable one.

@JohnRennie Okay sir thank you:-)

@Paul well it depends on the forces acting on it, no?

Ok

@Paul assuming you mean a to be the gravitational acceleration $a = GM/r^2$ the two equations are the same.

That's because $ar = GM/r$ so $\sqrt{ar} = \sqrt{GM/r}$

"Theories with a non-symmetric metric tensor. This means that the angle between vectors a and b is not the same as the angle between vectors $b$ and $a$. The difference depends on the electromagnetic field. Einstein favoured this possibility in one of his last attempts. Now it has practically fallen into oblivion."

I've suggested a dupe for this black holes collision question physics.stackexchange.com/questions/477183/… but I suspect there may be better targets.

@bolbteppa Many years ago I met one of the proponents of such a theory, Prof. John Moffat from Toronto (en.wikipedia.org/wiki/John_Moffat_(physicist) )

It turns out not far from the action was an astronomer from Villa Nova University... I can't remember his name... but anyways he was doing measurements of close binary systems...

and his data showed a slight deviation from the motion predicted by GR, but agreed with Prof. Moffat's theory (no surprise since non-symmetric stuff has more "parameters" to fit data)

i remember reading a Discover article about variable speed of light when I was in high school or something

was neat at the time, but i wasn't exactly in a position to be skeptical about such things

I think I remember Magueijo being on some popsci programme, possibly Horizon on the BBC, and talking about his theory. I think he and Linde were interviewed together.

am trying to remember the name of this Villa Nova faculty. He has been the last imperial astronomer to Iran before the Shah fell, and if I remember correctly he had been smuggled out of Iran in some kind of dramatic manner. He is no longer listed on the physics or astro site so I presume he might have retired.

@ZeroTheHero not an expert on this but isnt this (variable speed of light ideas) tightly connected to universe expansion? did everyone see the recent news?

Hubble measurements confirm that the universe is expanding 9% faster than expected, researchers claim / dailymail

@vzn It may be but 30 years ago when I heard about this it was not.

maybe was this guy: en.wikipedia.org/wiki/Edward_Guinan

the John Hopkins press release seems to be this: hub.jhu.edu/2018/03/02/…

@ZeroTheHero btw think new ideas about universe dynamics, incl dark matter/ energy, which seem to be arriving rapidly these days, all tend to be compatible/ align with/ even point to... fluid paradigm :)

more recently, there's this article in scientific american: scientificamerican.com/article/…

any new data usually sends the theorists in that field bananas... Let's see what they come up with!

havent seen this proposed, but isnt a simple idea that the expansion rate varies over time? like real explosions...? presumably this has been proposed somewhere...

my simple solution is to leave cosmology to the cosmologists :P

@Semiclassical lol cosmology = space ie distance :P

You knew our world's source code has gone haywire when scammers is unable to get away with their scams, but actually get bankrupted (with a lawsuit) and thus all that scammed money vanished and benefit no one

But I like that

I wonder how we can make the glitches in this world bigger...

you see... if our world is a simulation, it is a lot easier to produce glitches and shut the world down

@Secret that seems like a undesired result...

lol

@Qmechanic physics.stackexchange.com/questions/477238/… This doesn't look like a dupe of that question to me. Did you mean to use the link BowlofRed posted? physics.stackexchange.com/questions/184377/…

@Secret I disagree. A sim has to behave as if it's a real world. If the inhabitants of a sim can tell that they're living in a sim, then that sim is a failure.

@vzn The Big Bang has little in common with an explosion. Eg, in an explosion stuff gets hotter, not colder. According to Novikov, the BB is more like a process of freezing or crystallization.

@PM2Ring maybe, or think the parallels havent been fully studied. can probably dig up refs that support my case but then presumably few will read them. its clear we still dont know some of the basics. an explosion is a phenomenon from fluid dynamics :)

universe as simulation? try the truman show 1998 (1 yr before the matrix...) imdb.com/title/tt0120382

That wasn't a simulation though. It was a TV show where the star didn't know he was being recorded & broadcasted

@KyleKanos (lol) it has many elements of simulation based on dictionary defn. protagonist lives in a simulated world etc

The simulation sub-genre is much older than that. See en.m.wikipedia.org/wiki/Simulated_reality_in_fiction And I guess it's even older, if you consider the Hindu idea that this world is an illusion.

Plato’s cave is arguably a prototype of that as well

I was reading about sensory substitution and what if we could send a book in the form of little zaps in the cheek for example really fast over and over again? Maybe the brain will be able to make sense of it?

@PM2Ring "maya" (a few millenia old concept)

(In part spurred by one item from the moderator questionnaire) I know it’s possible to be suspended for posting poorly received questions but can one be suspended for posting poorly received answers?

@ZeroTheHero isn't that what happened to JD?

I doubt we’ll get an answer there

I dunno... It's more curiosity than malice on my part...

@KyleKanos I have a pro question and I hope you can answer.

I need to refer to a GitHub page where the user name contains two consecutive ff...

I run into this issue: fileformat.info/info/unicode/char/fb00/index.htm

i.e. if I try to cut and paste from the pdf there's some messup because of the double ff...

is there an easy workaround?

methinks just going to a different font might do the trick...

@ZeroTheHero We usually rely on the automatic question/answer blocks. Manual suspensions for low-quality contributions are very rare (since we believe that generally downvotes are already an appropriate sanction for posting LQ stuff) and only happen in cases where a) the automatic blocks did not kick in but nevertheless b) the user's contributions are so numerous and so consistently low quality (as measured by being downvoted) that they represent a serious drag on the site.

I think I can count on one hand the times we've handed out such a suspension since I became a mod.

@ACuriousMind thanks for the info. I didn't know that.

Something like that?

yeah Thanks I'll look into this.

@KyleKanos yeah that worked perfectly. +1

Excellent!

I usually do the copy+paste+replace method myself