The h Bar

Bernard Meurer

05:00

And also authenticating some signatures. Well. Since I didn't have a registered signature I needed to add mine to the books

I was thinking it'd be something digital so that any documents office could see everyone's signature...

No. It's quite literally a massive book of signatures

It's so ridiculous lol

John Rennie

In recent years there have been lots of stories in the papers about people using student visas for illegal immigration, so all countries have been tightening the rules.

I'd guess that's why they are giving you so much hassle

Bernard Meurer

Getting this visa is being hell

I filed in all the paperwork today

It's like a folder filled with all kinds of documents you can imagine

hopefully it'll be okay

John Rennie

Good luck :-)

Bernard Meurer

Thanks :)

Now I just need a plane ticket

John Rennie

Anyhow when you're settled ping me here and we'll sort out the laptop. Are you on Facebook? I'm a little relucatnt to exchange e-mail address in public, as it were.

Bernard Meurer

05:04

@JohnRennie Yep. I'm there under the same name, gimme a second

John Rennie

That was quick :-)

Bernard Meurer

You were on my history :p

John Rennie

Perfect. I've been checking and lots of UK delivery companies will deliver to Lisbon so everything should be straightforward.

Now I should probably get back to my servers :-((((

Bernard Meurer

Told you you shouldn't have jinxed it...

John Rennie

Nothing's broken, it's just the routine checks I need to do now.

Checking the servers haven't run out of disk space and their backups all ran normally. All very straightforward but a bit boring.

Bernard Meurer

05:09

That's what the servers want you to think

also, can't you automate that with a bash script?

John Rennie

Now you mention it I've been finding potatoes in the server room. Should I be worried?

Bernard Meurer

:)

This is only the beginning

John Rennie

@BernardMeurer It's all highly automated - you can't look after 500 servers if you need to check them all manually

But at the end of the day a human has to look at the report that picks out any unusual status reports.

Bernard Meurer

Well, I think you can, just not do that and be the messiah of PSE

John Rennie

Scaling is one of the things you have to consider in IT. Suppose it takes 5 minutes per server to dial in and check a couple of logs. For 500 servers that's 42 hours a day.

And we're on course to grow to 1,000 servers in the next few years.

shashank

05:13

Can somebody please explain the precession of a gyroscope. I checked out a lot of answers in the threads but couldn't understand them. Thank you.

Bernard Meurer

@shashank Which answers and what did you not understand about them?

@JohnRennie Scalability is key indeed

I like scripting/automation a lot

It's actually my single project I use the most

github.com/bemeurer/devstrap

shashank

Many people have asked this question before but their answers difficult to imagine.

Bernard Meurer

This saves me a humongous amount of time

shashank

One answer mentioned that as the gyroscope goes lower due to the torque applied by gravity, the lower part of the wheel spins faster than the upper part which causes precession. I couldn't understand why the lower part moves faster.

Bernard Meurer

xivero.com/musicscope

Amazing software

John Rennie

05:21

Was it you saying you'd recently got a reissue of the Black Sabbath albums

Bernard Meurer

Yeah! The SACD remaster

sounds stellar

John Rennie

I lost touch with them after Technical Ecstacy, but the albums before then are amongst my favourite albums.

In 1979 I saw Ozzy Osbourne's first ever live set as Blizzard of Oz :-)

DanielSank

@JohnRennie Is that slang for slow machines?

Bernard Meurer

@JohnRennie All of those are SACD remasters

John Rennie

@DanielSank when you awake to find that one of Bernard's potatoes has uplinked to your brain through your nasal cavity then you'll know what I'm referring to :-)

2

Bernard Meurer

05:25

The Black Sabbath ones that is

John Rennie

Volume 4 is my favourite Sabbath album.

Bernard Meurer

@DanielSank You shouldn't have served potatoes at your wedding

John Rennie

Actually it's close to my favourite album of all time.

@BernardMeurer William Gibson got it mostly right but failed to anticipate potates running Linux.

Maybe I'll look for the remasters, but maybe not. I've got all the early albums on CD, though I've ripped them to FLAC and only play them on my media centre these days.

Bernard Meurer

Czech thy facebook

John Rennie

I've never used ALAC, but that isn't any technical judgement but rather that I had never thought to look at it.

I thought FLAC was supported everywhere?

But maybe not on mobile devices so much ...

Bernard Meurer

05:34

Not by Apple stuff b/c they have ALAC lol

John Rennie

Ah :-)

Bernard Meurer

The compression on ALAC is worse though

John Rennie

I don't know how interested you are in Black Sabbath, but I have several books on them (as epub) if you want to borrow them.

Bernard Meurer

DSD/SACD albums are amazing

I've just begun to listen to them more recently

I was angry because of traffic one day and found out they helped me decompress

The downside of SACD

John Rennie

File sizes?

Bernard Meurer

05:40

Yep

And this is with it being compressed to ALAC

John Rennie

Is it ripped at a high bitrate?

Bernard Meurer

the DSD file sizes are horrendous

It's

Well

Direct Stream Digital

Direct-Stream Digital (DSD) is the trademark name used by Sony and Philips for their system of digitally recreating audible signals for the Super Audio CD (SACD). DSD uses pulse-density modulation encoding—a technology to store audio signals on digital storage media that are used for the SACD. The signal is stored as delta-sigma modulated digital audio; a sequence of single-bit values at a sampling rate of 2.8224 MHz (64 times the CD Audio sampling rate of 44.1 kHz, but only at 1⁄32768 of its 16-bit resolution). Noise shaping occurs by use of the 64-times oversampled signal to reduce noise/distortion...

This is what SACD uses, as opposed to PCM

For example, my recording of Dire Straits sultans of swing

John Rennie

@shashank: sorry, we've been talking about music and ignoring your question about gyroscopes.

Bernard Meurer

is 96KHz, 24 bit and 4608 kpbs

John Rennie

@shashank: Are you asking why the gyroscope moves at right angles to the applied torque?

DanielSank

05:43

@BernardMeurer read about how delta/sigma encoding works.

Bernard Meurer

@DanielSank en.wikipedia.org/wiki/Delta-sigma_modulation

This?

DanielSank

@JohnRennie Oh! Oh! Klepner and Kolenkow's book has a fabulous illustration of why torque works that way!

@BernardMeurer Yes. You need to understand that.

Bernard Meurer

@DanielSank On it!

DanielSank

Good.

Imma go learn how to use firebase.

John Rennie

@DanielSank I might just check that out. I must admit I've always just taken it for granted and never thought to find out exactly why it was.

Bernard Meurer

05:45

> In a conventional ADC, an analog signal is integrated, or sampled, with a sampling frequency and subsequently quantized in a multi-level quantizer into a digital signal.

What do they mean by quantized here?

John Rennie

@DanielSank Firebase! Are people still using that?

DanielSank

@JohnRennie Oh wow! Do you have the book?

@JohnRennie Looks like it: firebase.google.com/…

John Rennie

@DanielSank No, but I'm sure that nice Jeff Bezos can help

DanielSank

@JohnRennie Ok ok ok (I'm so excited).

Imagine a rod with a mass on either end.

Now spin the rod about its center.

Bernard Meurer

Okay I'm spinning it

DanielSank

05:47

Orient it so that the axis of ration runs left to right in front of you.

John Rennie

I have a few friends who write code with Delphi, and in the days before SQL Express go established Firebase was quite often used as a lightweight SQL Server. These days it's a niche product and rarely used in industry.

DanielSank

And lets point the angular momentum to the left.

John Rennie

Hang on, I'll grab a pen and paper ...

DanielSank

ok waiting...

Bernard Meurer

Okay you've lost me and I've hit my computer with a metal rod

2

John Rennie

05:48

Ok, go ahead

DanielSank

Ok angular momentum pointing to the left.

When one of the masses is at the top (and the other at the bottom), imagine you whack the top mass toward the left and the bottom mass toward the right.

"Whack" means "impart momentum to".

Before the whack, the top mass was travelling directly away from you.

Now it's going away and a bit left.

The bottom one is moving toward you and a bit right.

@JohnRennie with me?

John Rennie

Yes, I even managed to draw my diagram correctly - the right hand screw rule :-)

DanielSank

@JohnRennie Good job.

Well now look at the angular momentum vector.

It's pointing left, but a little bit toward you.

John Rennie

@DanielSank Aaaaaaaaaaaah it's been rotated anticlockwise

DanielSank

ZOMG!!!!!!!

Which is exactly how you would define the torque of your whacks!

John Rennie

05:52

While the torque is perpendicularly out of the paper

DanielSank

I mean, the whacks have a torque toward you.

John Rennie

No ..

Hang on ..

DanielSank

::hangs on::

Bernard Meurer

You guys are insane. I'm going to bed before I break something

Night!

John Rennie

It must be too early in the morning, I've forgotten how to define the direction of the torque vector.

It's out of the paper towards me isn't it?

DanielSank

05:54

@JohnRennie yes

Well I can't see your drawing, but as I've described things, the torque is toward you.

John Rennie

Good, I was right the first time. And that's why the response is perpendicular to the torque.

DanielSank

And the angular momentum of the rods and masses has gone from being left to being left plus a little towards you.

John Rennie

@shashank: did you follow that?

Too late, he's gone :-)

DanielSank

@JohnRennie oh well.

It's all in the book.

My dad (PhD in physics) was really wowed by that one.

John Rennie

This one?

amazon.co.uk/Introduction-Mechanics-Daniel-Kleppner/dp/…

DanielSank

05:59

Uhhhh yep!

That's the one!

That book is really great except for the bit on oscillations.

IMHO teaching oscillations without just going over the Fourier transform first is ridiculous.

It's like trying to push an ice cube up a metal slide with a toothpick.

Bernard Meurer

DanielSank

Hey, give credit where it's due!

(I made that)

Bernard Meurer

He did make it

and it was also his idea

Love you Dan :*

DanielSank

Yeeeesssssss. You're learning, my slave.

John Rennie

DanielSank

06:02

^ Well that was fast.

...but not as fast as @BernardMeurer OH SNAP!

Bernard Meurer

Oh my god hahahaha

DanielSank

@JohnRennie How'd you find that?

John Rennie

Top result - not that I condone such behaviour.

DanielSank

@JohnRennie Wow. I am very naive.

So you can go online and read books without paying for them, eh?

John Rennie

I feel a bit ambivalent about the piracy of textbooks. On the one hand people won't write good textbooks if they're just going to be pirated and they don't get paid. On the other hand it means students in the most deprived circumstances have access to high quality books.

DanielSank

06:06

@JohnRennie Well, presumably that's why books are less expensive outside of the US.

The "pay what you can afford" model is interesting, but how to enforce?!

I think a lot of students don't understand that it takes years to write a book.

John Rennie

Well that Klepner book is £45 (about $75 I think) in the UK. How is a student from the third world going to afford that?

DanielSank

I wonder how much of the retail price goes to the author.

@JohnRennie Not a chance in hell.

John Rennie

@DanielSank bugger all

DanielSank

@JohnRennie Is that British for "not much"?

John Rennie

@DanielSank Yes :-)

DanielSank

06:08

@JohnRennie Oh. You know this from real information?

I'd really like to know more.

shashank

Thank you guys. The image helped a lot .

DanielSank

I have 50% written a couple of books.

@shashank \(^^)/

@JohnRennie that's why I come to this room.

Two people got "aha"'s this evening. That's awesome.

John Rennie

Casual conversation with friends suggest that there's little money in writing textbooks, though many status points if it becomes widely read.

But I can't honestly claim I know this for sure.

DanielSank

@JohnRennie Pfffft, status points. Who needs 'em?

John Rennie

Few of us are entirely immune to the approval of our peers :-)

I don't think I've ever met someone who genuinely didn't care what people thought of them, and if such people exist I probably wouldn't want to meet them.

DanielSank

06:12

@JohnRennie indeed.

John Rennie

I would love to write the definitive book on special relativity - using a geometrical approach rather than bloody light clocks - and I would love for that book to become a standard text for first year students.

DanielSank

However, consider this:

I wrote a document about the Fourier transform some years ago. I presented it to an undergraduate journal club. They loved it. When I showed it to some folks here they literally laughed at me.

@JohnRennie do it

If you put it on github or something I'll help.

I am not kidding at all. I am much more motivated when working with a partner and even without one I've generated a huge amount of useful technical documentation on all kinds of stuff.

John Rennie

Maybe one day.

DanielSank

@JohnRennie Whacha waiting for?

John Rennie

The trouble is that I have little idea what a first year student will find easy to understand.

The popular books are the ones that managed to hit just the right note for their readers, and there's a certain arrogance in assuming that I can do better.

DanielSank

06:15

@JohnRennie With physicists, assume zero knowledge and infinite intelligence.

@JohnRennie Arrogance?

::slaps sense into John Rennie::

John Rennie

I remember asking a cell biologist why he couldn't engineer a microbe (it was in the content of sewage treatment) and he replied that nature had been doing it for a lot longer than he had and was a lot better at it.

There must have been thousands of books on relativity written over the last hundred years, and the ones that are currently popular got there through a process akin to natural selection.

2

Thinking you can do better than stochastic processes is tempting, but probably misguided.

But I haven't entirely given up hope.

DanielSank

Well I know for a fact I can do better where explaining the Fourier transform is concerned!

I've hated every book I've read on relativity.

John Rennie

Bloody light clocks - if I never see another description of a light clock again it will be too soon.

DanielSank

@JohnRennie yah

Agreed.

Sean Carroll's book and Schutz's books are the closest things I've seen to comprehensible discussions.

But they both fail in certain ways.

Carroll's GR book has one chapter at the beginning which goes over SR.

It's good, but it's too short and has too few examples, etc.

Schutz is really great because he actually tries to tell you what the hell is going on in the math.

However, the writing becomes obscure, and again there aren't enough examples.

John Rennie

I learned what I know of GR from Schutz's book. I think it's an excellent way for an amateur GR head to start, but he sometimes skips steps presumably because he thinks they are obvious - but they weren't obvious to me :-)

peterh

06:24

@ACuriousMind Sorry I've got a lvl1 sleep effect

John Rennie

Anyhow, I have to get back to my servers ...

DanielSank

@JohnRennie That's more or less what i meant by the writing getting obscure.

@JohnRennie okey dokey

John Rennie

07:28

→ 1 message moved to Trash

@Slereah: be nice

John Duffield

@DanielSank : I was referring to what Woit said: "I suspect that there are quite a few physicists, of my generation and later, who don’t necessarily feel “very lucky” to get to live in a period of no significant progress in the past 40 years, with the prospect of none in our lifetimes.".

Slereah

07:44

@JohnRennie is it mean to call his book great

John Rennie

It wouldn't be if you meant it :-)

Slereah

But by deleting the message, aren't you expressing the fact that it is not

John Rennie

08:27

varsity.co.uk/news/10559

Of course it slipped down the rankings a bit when I left :-)

Slereah

Oj Alija, Aljo!

►

Jeff Teoh

11:20

Yo

David Z

5

Q: Handling coherent answers that aren't valid for the site

Over at Physics SE someone posted the following answer to the question "Can one create mass from energy?": Yes. Genesis 2:7 "and Jehovah God went on to form the man out of dust". (God created man from dust.) Genesis 2:19 "Now Jehovah God had been forming from the ground every wild animal...

Bernard Meurer

11:33

@JohnRennie I'll be your model first year student lel

0celo7

12:11

@BernardMeurer why aren't you in Lisbon yet

Shing

12:55

I am thinking if I should use Python to simulate whether dark matter will form a black hole or not as my bachelor dissertation.

Would anyone be kind enough to give me any advices? (I have approximately 6 months, do not know Standard model, know a little bit of Python)

Slereah

That's a pretty tall order

How do you even simulate dark matter?

What model of dark matter are you using

Shing

I have no idea...... I guess it is too tall then?

Slereah

Do you know some general relativity and numerical physics

Including 3+1 general relativity

Shing

nope, I do not know any numerical physics, and very basic of general relativity.

Slereah

Yeah probably not a good idea then

Shing

13:05

I see, thank you for your advice :)

Qmechanic

0

Q: Non-topological solitons in condensed matter physics

As I know most well-known soliton solutions in condensed matter physics are topological ones: kinks, domain walls etc. In field theory there are several examples on non-topological solitons: Q-balls, oscillons (as long-lived excitations), chiral solitons. Also I know about soliton explanation of...

quantum-field-theory condensed-matter field-theory solitons

Too broad?

ACuriousMind

@Qmechanic It's formally too broad (since it asks for an open-ended list). I'm not wasting a close vote on it though.

Emilio Pisanty

13:31

@Qmechanic It's a bit broad but I'd be inclined to keep it. OP makes a good case (to an outsider) that they are looking for a relatively rare thing, and that a few examples would suffice.

Secret

Today I joined this talk
https://sydneyscience.com.au/event/quantum-physics-and-art/

and

Qmechanic

@EmilioPisanty : My main problem with it is that it is both an actual physics question (the list part) and a res. recom q. We usually try not to mix those. (Here it is implicitly understood that any phys. q is implicitly a request for reference. However an explicit request for res. recom. usually implies the res. recom tag and CW status.)

Emilio Pisanty

@Qmechanic I'm not too bothered by that part tbh

it's certainly not the first and they've usually worked OK as far as I recall

it's often the case that the OP wants examples of X but OP can't know a priori how much of a bite they're trying to take - whether it can be explained in a few lines or whether it's more material and it's an off-to-that-book-you-go type of situation

Secret

Found from the talk that a lot of surrealist art from the 1970 stems from the discovery of a new reality as Heisenberg and others found quantum mechanics

Caption: The Vertigo of Eros, said to be the artist's interpretation on what the atomic world look like

Emilio Pisanty

@Qmechanic re CW I'd say leave it as is. If it does generate only res-recom type answers then it may be appropriate. But here there's definitely merit in selecting good references (i.e. it's not loads of free rep for rattling off Jackson as a good grad-level EM book), so I don't quite see the point even if it does go down that route.

Secret

13:44

Therefore, both artists and scientists are also asking the questions of reality, except one does it by collecting data, and another by expressing an idea and present it in an art work

Aug 6 at 12:18, by Secret

I also found this "attempt proposal of demonstration of spin states" were already realised by the qubit transistor team as they track the trajectory of the spin vector in real time

Emilio Pisanty

Also, @Qmechanic, regarding this one

I'm pretty certain you can show that $\langle\tfrac{1}{2}|Q|\tfrac{1}{2}\rangle = 0$, where $Q$ is the quadrupole operator. However the details have slipped from what I laughingly refer to as my memory. This might give you enough clues to Google the answer. — John Rennie Dec 14 '14 at 10:56

I flagged it recently, it got mauled by the comment handler as in meta.math.stackexchange.com/questions/23698/…

Secret

I also talked to the people after they shared how the artwork is made and the underlying idea, and I found it illustrate nicely what happens in my messy mind where things are all over the place, and ocassionally some stirngs of text will carry a profound emotion on me simply because it links back to very old memories in my life

Emilio Pisanty

It should either be fixed (copy-paste from a clean \rangle and add a space before it), or deleted

John Rennie

14:16

0

Q: What type of image is formed when we use convex lens and concave lens seperately?

Are images inverted? Real? Erect? What type of images are formed?

optics lenses

Erect - snigger

On a less immature level, this seems an awfully lazy question. A few moments of Googling would find the answers.

Qmechanic

@EmilioPisanty : I fiddled a bit. Now the LaTeX seems to display correctly.

0celo7

14:37

Woot, got a free copy of Spivak Vol 1

Emilio Pisanty

@Qmechanic excellent =)

0celo7

Can someone explain exponents of $-1$?

Suppose I have $(-1)^{pq}$ and $(-1)^{(p+q)r}$

all integers

what is the ratio of those

Secret

Would using $-1=e^{-i\pi}$ and then using index rules for complex exponential help simplify them?

0celo7

not sure

I don't remember algebra

ACuriousMind

@0celo7 What is there to explain? $x^a / x^b = x^{a-b}$.

Emilio Pisanty

14:47

@0celo7 what do you mean?

0celo7

good god your avatar is scary

@ACuriousMind because that doesn't give the right answer...

ACuriousMind

what?

0celo7

oh wait, I'm dumb

carry on

aha, $(-1)^q=(-1)^{q^2}$

what does that say?

Spivak has Satanic markings on it.

Secret

-metry Dif-
Differential geometry?

vzn

15:41

@Secret what team are you talking about?

Secret

Nah, that's just my naive idea on trying to build a demostration of the bloch sphere experimentally

and then someone had done it

Emilio Pisanty

16:04

@0celo7 Is that spivak differential geometry vol 1?

that's odd

my copy looks like ↑ that one

0celo7

@vzn I got a copy of the second edition from a retiring prof

That should be @EmilioPisanty

Do you have the second volume?

Emilio Pisanty

16:24

@0celo7 back home in Mexico

this sort of thing as I recall

though I found vol. 1 distinctly more interesting and useful

vol. 2 is just a trawling dive through the original works of the albatrosses depicted

physics.stackexchange.com/review/suggested-edits/143016 ← really tempted to reject it just to keep "Satanding wave" up

0celo7

@EmilioPisanty volume 2 is the shit

It's also out of print.

Emilio Pisanty

@0celo7 is it actually?

there's a couple of new copies on amazon

amazon.com/gp/offer-listing/0914098713/…

at least the shipping is cheap

ACuriousMind

@EmilioPisanty lol, I guess satanding waves would be more of a case for exorcism.SE, though

Emilio Pisanty

@ACuriousMind lol

Secret

17:09

google.com.au/…

David Z

Thoughts on my comment here welcomed:

2

Q: Orifice Flow Rate: Reference Request

I am writing my first math paper, and I am using a formula for the mass flow rate of a liquid through an orifice. I found this formula mentioned in some online videos (like this one) but I could not find the source of the formula. The formula is $$m' = \frac{(\Delta p)^{\frac{1}{\alpha}}}{R}$$ wh...

fluid-dynamics resource-recommendations

William Bulmer

Ignore this. I am testing my new screen name

Damn

Emilio Pisanty

17:29

@DavidZ Maybe it's just me, but I'm finding it really hard not to look at that title with an entirely toilet-humour viewpoint

David Z

@EmilioPisanty Noted and partially agreed

:-P

Emilio Pisanty

@DavidZ It looks OK to me, but what you propose here sounds reasonable.

It does sound like the OP really does need a solid literature reference to cite, though

The "axioms" will just be the Navier-Stokes equations, or similar, so a "reference to the axioms" is a bit misguided

but if what they want is something to cite, then it may be just fine to keep the tag.

David Z

Eh... I dunno, even if they're just looking for something to cite. Can you meaningfully argue the merits of different sources to cite for an equation? Are there better and worse ones for different people? Will there continue to be new sources with different advantages produced over time?

specific-reference would be more the sort of tag to use when you want a source to cite for something, I think.

Emilio Pisanty

18:03

@DavidZ The point is that while an answer here would be OK, it looks like the OP just wants to be able to write "equation (1) has been shown to be the relevant description of the process [X]"

with [X] being some appropriate reference

which becomes much weaker if it's to some random page on the internet

David Z

If that's the case, it sounds like a specific-reference question

At least definitely not resource-recommendations

Emilio Pisanty

@DavidZ to be honest I'm still not 100% sure what specific-reference actually means

Is it "a question where there really is most likely a single reference that does it"?

David Z

Yeah, pretty much

That's the idea anyway

Emilio Pisanty

because this one doesn't quite do that

it's much more likely to be found in just about any fluid mechanics book if you know where to look

David Z

It may be appropriate to generalize specific-reference to citation-finding or something, to include cases like this

Emilio Pisanty

18:08

@DavidZ Honestly, I don't think it'll be worth the effort

David Z

Then again, no reason the question really needs to have specific-reference or resource-recommendations. I'm about 99% sure it doesn't fit the description of the latter. I wouldn't mind just removing that tag.

FWIW this is the question I had in mind when creating specific-reference

just to give a sense of the kind of thing that tag was supposed to be for

Emilio Pisanty

This one will probably do just fine without either tag

David Z