The h Bar

tpg2114

00:59

@SpaceOtter That's a trickier question to answer here. Does it give stable orbits in the 3-body system (i.e. when the planet, moon, and rocket are present)? Like, with all 3 bodies there, can you put the rocket in orbit around either of the two bodies?

Sir Cumference

@ACuriousMind here i thought "sir" was a cool name

tpg2114

Alternatively, which numerical method are you using to integrate the equations? Some methods are known to perform poorly in some situations... may not be the case here, but sometimes a "bug" isn't an implementation issue, but a choice of approach issue (subtle difference)

Sir Cumference

well honorific i guess

tpg2114

@SirCumference Personally, if anybody was going to give me an honorific, it better be "Dr.". They could call me Dr. A**hole if they wanted to -- I have definitely earned the first, and most likely have earned the latter ;)

Space Otter

@tpg2114 Yeah been struggling with this code, it behaves as expected in some occasions and very badly in others. Still trying to determine if it is bugs in my code or if it is the Runge Kutta integration becoming unstable due to some parameters reaching extremes

Sir Cumference

01:02

@tpg2114 "Dr. Cumference" someday

does not sound good

tpg2114

@SpaceOtter Yeah... code verification is... not fun

Is it a project for a class or something? Or is it a for-fun thing?

Space Otter

I'm considering simplifying everything and assuming both a stationary earth and moon, which for this case is accurate enough.

Sad that my N-Body isn't behaving well though

Was so impressed with my implimentation and now I'm just beating my head on a wall

tpg2114

Welcome to software development :) I have been working on our simulation code and in the first 3 weeks, I went from our legacy code to adaptive mesh refinement and showed some really cool results with it. But then the next 3 weeks, I've been trying to get it to read and write a stupid checkpoint file

So it's pretty often that you make huge progress quickly, then get bogged down on something silly

And usually, that first 75% of awesome happens really fast. But then the last 25% of little bugs, infrastructure, quality-of-life, etc. take like 90% of the time

Guru Vishnu

06:03

Hi everyone! Is there any simulator to play with electric charges? Especially seeing what would happen if we place a system of three charges (or more) in a plane as time progresses? I checked PhET (which I use normally for learning) but didn't find anything useful for this purpose.

It would be better, if the amount of charge on each particle could be varied and provides some useful quantities like potential energy, kinetic energy, etc.

PM 2Ring

07:26

I have a dilemma. Charles Francis just posted this excellent answer. But it looks so well-polished that I did a little bit of research for possible plagiarism. I discovered that he'd lifted the answer from the book Light after Dark II: The Large and the Small. However, Charles is actually the author of that book.

Some people aren't comfortable with people recycling content like that, but AFAIK there's no site or network policy against it, and the author isn't obliged to disclose that they're recycling, or to mention the title of the prior work. meta.stackexchange.com/q/333708/334566 discusses this topic, but there's no clear outcome: there's one answer that says it's ok, and one answer with the opposite opinion.

I'd like to encourage Charles to be more explicit when he quotes from his books, but OTOH if he does that too much people may accuse him of being spammy. In my 1st interaction with him on the site I told him he needs to be explicit that he's the author when he links to his books, in order to avoid accusations of spamming. So the situation is somewhat delicate...

John Rennie

Moved!

Yuvraj Singh...

@PM2Ring Charles?

skillpatrol

Thank you sir @JohnRennie :-)

Yuvraj Singh...

@skillpatrol yup

PM 2Ring

@YuvrajSingh... Yes. physics.stackexchange.com/users/255997/charles-francis He's only been a member for 49 days, but he generally writes very good answers, and his rep is almost 5000.

Yuvraj Singh...

07:32

@PM2Ring better than Anna v

PM 2Ring

Apr 11 at 7:24, by PM 2Ring

@Loong Anna does have a tendency to answer stuff that ought to be closed... And some of her answers aren't so great. OTOH, I think it's pretty cool that we have her as a regular member on this site. I hope I'm still that sharp when I'm 79 (if I manage to last that long).

Yuvraj Singh...

@PM2Ring she is most senior physicist

PM 2Ring

@YuvrajSingh... Yes. And she worked as an experimental particle physicist. She certainly deserves our respect. But that doesn't mean we have to upvote everything she does, or to refrain from downvoting when she says stuff that's wrong.

Mind you, she doesn't often say stuff that is downright wrong, but sometimes her answers may not really answer the OP's question, they just talk about related stuff.

skillpatrol

Older scientists tend to do a lot of that...

~~Older~~ Senior

:-)

Yuvraj Singh...

07:48

@skillpatrol I saw your request but that room is my personal

skillpatrol

np, pal

Yuvraj Singh...

@skillpatrol I just put my messages there

Yuvraj Singh...

08:06

@PM2Ring how is Australia now, are you able to control the pandemic?

PM 2Ring

08:30

@YuvrajSingh... Pretty much. From health.gov.au/news/health-alerts/… "Of the 6,667 confirmed cases in Australia, 76 have died and 5,095 have been reported as recovered from COVID-19. More than 474,000 tests have been conducted across Australia."

Our total population is about 26 million.

In comparison, the population of the USA is about 330.5 million, with 890k confirmed covid-19 cases, 82,843 recovered, and 50,372 deaths. :(

Anyway, let's not discuss covid-19 here. There are plenty of other places for that.

PM 2Ring

08:58

After that depressing stuff, here's something uplifting:

Tedeschi Trucks Band - "Anyhow" (Live in Studio)

►

user434058

@PM2Ring Hi.

PM 2Ring

Here's an older one, from around 2010 (not 2016), before Derek & Susan decided to merge their bands together. It's an old song written by Matthew Moore:

"Space Captain” - (Crossroads 2016) [Official Video]

►

@FakeMod Hi.

user434058

@PM2Ring How's your life going? :)

user434058

@PM2Ring I mean... wassup?

user434058

Nvm

user434058

09:11

@Loong FWIW, QMechanic still uses \rm regularly and it was one of his/her answers from where I first came to know about \rm.

PM 2Ring

@FakeMod I tend to spend most of my time at home anyway, so the covid restrictions haven't affected me very much.

Loong

@FakeMod :-(

user434058

@PM2Ring Neither me :) 'cause I am always at my desk studying or wasting my time.

user434058

@Loong Does using an obsolete command in mathjax have any downsides?

Yuvraj Singh...

@Loong I am shocked by your comment for Anna V

Loong

09:25

@YuvrajSingh... ?

user434058

2 hours ago, by PM 2Ring

Apr 11 at 7:24, by PM 2Ring

@Loong Anna does have a tendency to answer stuff that ought to be closed... And some of her answers aren't so great. OTOH, I think it's pretty cool that we have her as a regular member on this site. I hope I'm still that sharp when I'm 79 (if I manage to last that long).

user434058

@YuvrajSingh... Are you talking about this?

Yuvraj Singh...

Thanks

user434058

@YuvrajSingh... But this was said by PM 2Ring...

Loong

And where is my shocking comment?

Yuvraj Singh...

09:27

@Loong sorry it was not you it pm

:-(

Actually @PM2Ring after the comment I have read her answers but I find them content related!

ACuriousMind

Idea for chemistry meta post: "5 comments by Loong that will SHOCK you!"

Yuvraj Singh...

And I feel it is harsh we outspoken if we say her many answer is out of the context!

PM 2Ring

@YuvrajSingh... What's shocking? Anna often answers questions that should be closed as dupes.

Yuvraj Singh...

Like?

user434058

@ACuriousMind "You won't believe what he said in the 4th one"

PM 2Ring

09:36

I'm not saying that I never answer dupes. In fact, I answered a question yesterday that ended up getting closed as a dupe. I usually do a dupe search first, but I forgot to do that yesterday. I guess I got caught up in the enthusiasm of seeing an ok question that I knew how to answer... physics.stackexchange.com/q/546062/123208

user434058

@PM2Ring I feel your pain :))

PM 2Ring

In my defense, the title of the dupe target selected by Qm isn't a very close match to the question I answered. ;)

ACM, any thoughts on chat.stackexchange.com/transcript/message/54179203#54179203

Guru Vishnu

Speaking of dupes, can anyone have a look at this message?

Yuvraj Singh...

@PM2Ring that is fair enough, everyone does this!

Guru Vishnu

Yuvraj, Have you authored any books on Physics? re:"everyone does this!"

PM 2Ring

09:43

@GuruVishnu I don't know the topic well enough to make a decision. And you're asking us to decide between answers by Emilo P and David Z... :)

ACuriousMind

@GuruVishnu If one of the questions is very new, close it as the dupe. If all else is equal, close the newer one as a dupe of the older one anyway. If both questions are almost identical, you can also flag for a moderator to merge the two questions but we do that very rarely and it's a mess if they are not really exact duplicates.

Yuvraj Singh...

@GuruVishnu sarcasm?

PM 2Ring

Oops. I meant Emilio.

Yuvraj Singh...

What I meant was everyone do mistakes! And it is

ACuriousMind

@PM2Ring Plagiarism (=unacknowledged direct quotes) is plagiarism regardless of the source. That you hold the copyright over the plagiarised source doesn't change a bit in my eyes, since we're not concerned with the copyright aspect here (cf. physics.meta.stackexchange.com/a/542/50583)

Yuvraj Singh...

09:47

Common in human

Guru Vishnu

@ACuriousMind In this case, I find the answers to suit both questions as A doesn't take direct quote from the question. When is it not advisable to ask for merging? Or why is that process a "mess"? I'm asking this because, Emilio has put a lot of effort in his answer and I think someone seeing the dupe tag may just skip to the original without reading his answer.

user434058

@ACuriousMind Should I ~~waste~~ invest my time in editing (mostly tag edits) questions which are definitely going to get closed/deleted?

ACuriousMind

@GuruVishnu It's a mess because answers often reference the phrasing of the question (e.g. if the question asks a yes-or-no question at the end, many answers will begin with 'Yes.' or 'No.'). Merging essentially just copies all answers and comments from one question to the other, so the questions need to be the same (and not just similar) for this to not make the answers hard to understand

@FakeMod Your call (it's your time), but in my opinion editing questions that you know are going to get closed is only worth it if the edit makes the question no longer closeworthy.

PM 2Ring

@ACuriousMind Ok. I almost left him a comment, saying that he needs to properly cite quoted material, even if he's the author. But when I looked for an official ruling about that on MSE & on Physics meta I couldn't find anything solid (& I didn't see the post you just linked). So I didn't submit my comment. Would you like to say something to Charles about this? I feel it'd be better coming from a mod.

user434058

@ACuriousMind Seems clear to me. Don't edit them.

user434058

09:55

Thanks!

ACuriousMind

@GuruVishnu But "there are two good answers here" is not a reason to not close dupes as dupes. The answerers can always answer the original then (that's one example where it's not bad to copy your post).

@PM2Ring Flagging for mod attention is the correct way to deal with plagiarism anyway, so just raise a mod flag, please

Guru Vishnu

ACM: Fine. Thanks for the clarification. Closing A as a dupe of B.

PM 2Ring

@FakeMod Plenty of closed questions still have useful info. But if they're likely to get deleted, there's no point editing them. (Of course, deleted posts aren't actually deleted, they're just invisible to those of us with <10k rep, and they don't show up in searches).

@ACuriousMind Oh, ok.

Guru Vishnu

@ACuriousMind Just asking, do I need to delete the answer in the dupe and post it in the original or leave the old one as such as create a duplicate answer?

ACuriousMind

Up to you, I'd say. Unless you make an obvious habit out of duplicating answers to get double rep I see no problems either way.

user434058

10:04

@ACuriousMind "duplicating answers to get double rep" Sounds interesting... ::starts copying answers::

PM 2Ring

@FakeMod To make it less obvious, avoid duplicating diagrams and other images from your answers. ;)

user434058

10:21

@PM2Ring This makes me wonder, is there any tool or an A.I. service which can paraphrase your answers without changing the meaning significantly? :D

user434058

One terribly dreadful question which compelled engineers all over the world to commit suicide!!!

user434058

I think I am overdoing the click-baity thing :P

user434058

At least I am less click-baity than CodeGolf's HNQs...

Yuvraj Singh...

@PM2Ring have you ever heard Gene Pierson?

PM 2Ring

10:44

@YuvrajSingh... The name doesn't ring a bell. I remember hearing "Reach Out" on the radio back then, but I can't remember if it was Gene's version, or the version by The Four Tops.

Yuvraj Singh...

He was an Australian pop star

In mid 40s

@PM2Ring

PM 2Ring

@FakeMod Pfft. Engineers are happy to use 3.1416 for pi. ;) It baffles me that some people expect more than 15 decimal digits of precision from a 64 bit floating-point number. en.wikipedia.org/wiki/…

user434058

@PM2Ring I would bet that engineers won't even go further than 4 in 3.14159 :D

PM 2Ring

@YuvrajSingh... He was born in the mid 40s. He was a minor pop star in the 60s & early 70s. en.wikipedia.org/wiki/Gene_Pierson I just watched the YouTube clip of him singing Reach Out on the Australian Bandstand TV show. I used to watch that show, so I might have seen him on it.

@FakeMod Sometimes, pi=3 is adequate. :) FWIW, I memorized pi to 100 places when I was in my teens. I still remember it. But I've never actually needed that much precision. :D

About a decade ago, I made an interesting discovery about pi approximations:

in Python on Stack Overflow Chat, Apr 25 '19 at 13:25, by PM 2Ring

@ksalf 22/7 is a well-known approximation for pi, but what's not so well known is that you get a better approximation by subtracting 0.04%. That is, (1 - 0.0004) * 22 / 7 == 3.1416 exactly.

user434058

@PM2Ring 100 digits!!! That's great. Seems like you were nerdy when you were a teen, right? ;)

user434058

10:57

@PM2Ring I believe 355/113 is better.

PM 2Ring

@FakeMod Very. I was one of the first kids at my school to get an electronic calculator, when the first (non Hewlett-Packard) pocket-sized scientific calculators started appearing. Before that, I used to always carry a mini slide rule with me at school.

user434058

@PM2Ring Even I am nerdy, at least that's what other people think :))

PM 2Ring

@FakeMod Yes, that approximation, discovered by a Chinese mathematician, is roughly 3.1415929. But 113 isn't so convenient for mental calculations.

user434058

@PM2Ring Yes, I have never used it anywhere, it's just a fun fact to know...

PM 2Ring

But it is a pretty pattern in base 10 that 1 / pi ~= 113/355

It's a shame that there aren't 355 days in a year. :)

user434058

11:04

@PM2Ring didn't know that...

PM 2Ring

Some ancient Indian mathematicians had a nice pi approximation. I think it used a binary power for the denominator, but my memory is a bit hazy...

Speaking of good binary approximations, 181/128 is a great approximation for sqrt(2), since 181^2 = 32761

user434058

@PM2Ring You are really good at numerical approximations and trivia, but I tend to less like the things which require a significant amount of memorization, like number trivia :)

PM 2Ring

20612 / 6561 is an ok approximation for pi. It might've been discovered by Indians, but as I said my memory on this stuff is rather hazy. I haven't thought about it much in the last 20 years.

@FakeMod Fair enough. Memorization is less useful in the age of smartphones. But back in the day, it could be handy. :) Charles Dodgeson (aka Lewis Carroll) memorized the 3 digit base 10 logarithm table, to make it easier to do quick mental calculations.

user434058

11:23

@PM2Ring Yeah, sure. Moreover, if there was some relation/intuition/concept (which is inherently absent) behind figuring out all these random approximations, I would have been more interested. However, fun is itself a strong reason to explore these fun facts, however I feep bore instead of fun when I try to learn them. Anyways, I need to go offline now, thanks for the nice nuber facts, I hope I remember some of them and productively apply them in the future :)

skillpatrol

62,832/20,000 was later refined to 754/240

PM 2Ring

@FakeMod Well, there is interesting mathematics behind good approximations. The relevant topic is continued fractions

2

skillpatrol

355/113 is a good one to remember

John Rennie

@PM2Ring the Mathologer on YouTube has an excellent video on continued fractions including approximations to pi.

PM 2Ring

And for approximating square roots, see Pell's equation, a number theory topic with a long & colourful history, including some excellent work by Indian mathematicians.

John Rennie

11:30

Infinite fractions and the most irrational number

►

PM 2Ring

I think it's pretty fascinating that there are regular continued fractions for various simple powers of e

@JohnRennie That the continued fraction for phi has the slowest convergence has a bad side & a good side. On the one hand, it means it takes longer to calculate really good approximations to phi. OTOH, there are plenty of fractions with small denominators that are reasonable approximations to phi.

Of course, if you want a good value of phi, you don't bother going through the convergents one by one, (i.e., by looking at successive pairs of Fibonacci numbers). You use a quadratic algorithm that can generate $F_{2n+1}, F_{2n}$ from $F_{n+1}, F_n$. Or equivalently, calculate $\sqrt 5$ using Newton's method. But even then, it still converges slower than solving other quadratics via Newton's method.

Mad Spaces

11:55

Hello. Could someone assist me with this homework i have. It a movement of an object trajectory given as follows $ \vec{r(t)} = \left( \begin{array}{c} R* \cos(\omega t)\\\ R \sin(\omega t) \\\ \frac{-1}{2}*gt^2 \\\ \end{array}\right)"$ where as g as earths acceleration. there is some questions asked and i just need help with general understanding. I know that in the X and Y planes there is a circular motion and in the Z plane the object is falling to earth and gaining velocity with

constant acceleration. I know that it should make a helix path going down. However i have some questions that i dont understand.

Does the circular motion here and the radius of the circulation as well as the angular velocity of the circulation change with time? we know that the object increases velocity in Z direction, does this mean that the object will have smaller or higher rotationsfrequencies around his cricle?

What happens to the loss of height in Z direction (we know when it takes helixpath, it goes throught less Z vertical distance as if it would in a normal Fall case) what will change when the speed in Z changes? does the loss increase? decrease? stay stable? how to argument?

my assumption is that the helix will get Sprung (like a spring that gets extend, such that for assume V is so big, that the helix becomes a line! ) Does this mean that the radius change? or it does not?

John Rennie

@MadSpaces I'd be inclined to convert this to cylindrical coordinates $(r, \phi, z)$ as that makes the motion nice and simple.

Mad Spaces

I shall do that right away and come back at you Mister Rennie.

Oh but it is already in cylindrical coordinates.

John Rennie

No, it's in Cartesian coordintes:
$x = R\cos\omega t$
$y = R\sin\omega t$
$z = -\tfrac12 g t^2$

In cylindrical coordinates it would be:

$r = R$
$\phi = \omega t$
$z = -\tfrac12 g t^2$

Mad Spaces

Oh. Well i am going to check on Wikipedia to refreshen my memory about these coordinates

Hmm. This might sound stupid. But i am seeing them as equal.
I have here as a note that to change from cartesian to cylindrical
coordinates one has to do the following transformation
$x=\rho * \cos(\alpha)$
$y=\rho * \sin(\alpha)$
$z=z$

But isnt that literally what we already have since $\rho= R$

Nvm i found a video explaining this

B. Brekke

12:43

Doing Diagrammatic Monte Carlo on a fermionic system I expect to have a sign problem. However, I do not understand what this sign problem is. Initially I thought sampling an alternating series was problematic. Then I heard it is not, but that the normalization is problematic. I don't see it, the normalization looks perfectly fine to me, so does anyone know how this sign problem arises?

PM 2Ring

13:51

Surely there's a good dupe target for this:

0

Q: Why do we believe space and time are intertwined? What is the idea behind it? And how do we arrive at this conclusion?

What led us to believe that space and time are actually interconnected? How did the idea actually originate? why do we believe that this concept is true?

ACuriousMind

any of the questions about the evidence for relativity would do, imo

PM 2Ring

I've spent at least 10 minutes searching, but I can't find anything suitable. Sure, there's stuff about Minkowski spacetime & Lorentz transformations, but I don't think the OP would be satisfied with a screenful of mathematics.

@ACuriousMind Ok. I guess that works, although that doesn't exactly justify why we decided to accept Minkowski spacetime as a physical reality... whatever that means. ;)

ACuriousMind

@PM2Ring Huh? "Spacetime is (at a point/in neglegible gravitational fields) Minkowski" is the very basis of relativity. You cannot accept relativity without also accepting that statement.

If your notion of "physical reality" means this is not enough, I'd suggest to re-examine your notion of reality :P

PM 2Ring

@ACuriousMind It's good enough for me! I just didn't want to bamboozle the OP with a page full of Lorentz transformation matrices. But anyway, I decided to nominate:

5

Q: Why time is considered a dimension?

Why is time considered to be a dimension? And the other 7 (except the 3 dimensions of space, and the dimension of time) dimensions that string theory suggests, why can't they be realized?

special-relativity spacetime time spacetime-dimensions

Feel free to change it or add better targets.

Slereah

14:44

The technology of Satanism is based on the Chaos Force, the mathematics of non linear equations and dark energy matter quantum superimposition in space-like form for temporal power and time-like to disempower, possess light energy matter life form for temporal power and time-like to disempower, possess light energy matter wageslaves.

Can you confirm

@ACuriousMind what if spacetime is superminkowski space at a point

skullpatrol

I suggest you stay away from that room of wizards.

PM 2Ring

15:06

@Slereah Is that from your neural network?

Slereah

I'm afraid that's just garden variety crazy people

PM 2Ring

Oh dear.

skullpatrol

the exotic variety runs dangerously close to cranks

Mad Spaces

Is there a way to express the normalforce if the mass is constantly gaining at a given rate?

dumb as a rock

what do I tell someone if he asks what is central field approximation ? in simple manner

I mean the need for it

in simpler words and without the calculations

Yuvraj Singh...

15:58

@PM2Ring can you explain me in understanding the gravitational lensing?

PM 2Ring

16:23

@YuvrajSingh... There's a Wikipedia page about it, but I suppose it is a bit confusing. I suggest looking at the simple case first, known as Einstein's Ring. Here's a good diagram from that article:

Yuvraj Singh...

@PM2Ring yes sir that's why I reached you!

PM 2Ring

So in that diagram, the light rays are the heavy lines with arrows. The 2 rays on the left side would diverge & miss the Earth. But the heavy mass (the black dot in the lens plane) attracts the light enough so that the rays both converge to the observer's eye, on the surface of the Earth. This means the observer receives more light than he would if the heavy mass were absent.

It also means that he sees that light coming from different places, and not along the central horizontal axis.

Because the light rays are bent, it appears that the light is coming from those 2 open circles on the source plane, rather than from the dark dot on that plane, which is the true source.

That diagram just shows us a 2D slice of what's happening. For the full 3D picture we need to spin that diagram around the horizontal axis. Then those 2 open circles on the source plane will trace out a circle. And that circle is Einstein's Ring!

geocalc33

16:39

physics.stackexchange.com/questions/544391/… reopen?

ZeroTheHero

17:17

@geocalc33 why? Just take any SL textbook and it’s in there...

geocalc33

18:30

@ZeroTheHero what's SL?

I'm assuming you mean SR

and no in fact it's not in any special relativity textbooks

ZeroTheHero

20:38

@geocalc33 Well... for one thing a Lorentz transformation by definition preserves the metric and one just has to show that $\Lambda^{-1}g\Lambda=g$, which is kinda trivial, but your question is an usual way (and probably complicated) way of showing this. Kinda like showing that all points on circle are left invariant by a rotation... Not the best way to show that rotations preserve standard metric.

Physics Meta

20:51

0

Q: Is this just an example of an abuse of moderator rights?

I have a question about my Physics Stack Exchange post: Is it really true that space is continuous? This question was closed by Qmechanic♦ on the grounds that "This question already has answers here": and this is the linked question. Apart from the question I posted having very similar ti...

discussion

geocalc33

21:01

@zero I agree that it's not the best way, and I agree that it's complicated

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