The h Bar

@Slereah - I understand for the religious books, they give it away for free because they would want more proponents/appreciators, but does the same also hold for technical literature?

Slereah

2:07 PM

no

Only the pdg that I know of

The Dark Side

^ That.

Alright thanks.

Slereah

I guess it's financed with TAX PAYER MONEY

The Dark Side

Actually it is even better.

According to your AMA info, you were at the university of Nantes.

Now that prompts the question:

Which country's citizens pay the taxes that fund PDG publishing? :P

Slereah

Not quite sure

I think it's part of CERN funding

So all members of CERN

The Dark Side

If XYZ country doesn't participate in that fundraising, those citizens get it for FREE !!

home.cern/about/member-states

I count 28 members.

user685272

2:15 PM

maybe it's some rich guy who thinks generating interest in physics is a worthy cause?

The Dark Side

Through free distribution of Particle Data Booklets?

user685272

why not?

The Dark Side

That would be more of a deterrent.

People would run away.

From HEP at least.

user685272

not the ones who are genuinely interested

The Dark Side

Yes.

But rich guys would generally tend to promote popular Physics than specialized Physics, so that one could draw more people from a smaller age demographic.

user685272

2:21 PM

who knows what these rich guys think these days

The Dark Side

True.

user685272

Pop Physics attracts a lot of crack pots

Fawad

@TheDarkSide question ban. I asked on chemistry now :D

user685272

you've been question banned?

Fawad

Yes :(

user685272

2:27 PM

for how long?

Fawad

That’s common on physics.se for new learners

Maybe 6 months

heather

it's one of those automated-by-the system bans @Fawad, isn't it?

user685272

:O

heather

It's not for 6 months then.

if you fix the questions that have been downvoted/deleted, and ask for help on Physics Meta to figure out how to fix them, you will be allowed to ask questions much sooner.

user685272

do it^

try to work with the system

people will try to help you

Fawad

2:33 PM

Apr 11 at 6:46, by Secret

@Fawad Me being question banned 4 years ago due to 4 poorly received question (with one of them due to mislead by a crank youtube thing and get hastly downvoted and closed) is the main reason I pop up in this chat. Roughly a month later, the ban wore off, although I rarely had the need to ask questions in main until recently

heather

1. a month $\neq$ 6 months.

Slereah

@heather Proof?

heather

2. it appears from that comment they didn't try to undo the ban by working with the system.

3. the ban varies from person to person, depending on the starting conditions and what is done to fix it.

Balarka Sen

1 is totally equal to 6

ACuriousMind

@Fawad That's not true, the proportion of users affected by the automated question bans is really tiny, and most users learn what kind of questions we don't like before they get an extended automated ban like you. It's not because you are a "new learner", it's because you continued to post questions that showed no effort at all after being told that you should take care to do so.

heather

2:34 PM

@BalarkaSen, @Slereah not helping =P

Balarka Sen

Well, what do you want us to say? 1 = 6 is an established fact of mathematics.

The Dark Side

What?

How?

Balarka Sen

It's also a Millennium prize problem

The Dark Side

21 secs ago, by The Dark Side

What?

ACuriousMind

@BalarkaSen Well, it is true in $\mathbb{Z}/5\mathbb{Z}$... :P

user685272

2:36 PM

1*0 = 6*0

1/0 = 6/0

The Dark Side

:: Facepalm ::

Fawad

@heather my last question was asked on 9th march 2017 (nearly 4 months before..)

Which has 2upvotes.

p 365

2:39 PM

@Fawad have you done anything to try to fix it?

Balarka Sen

@Slereah Dem filthy logicians

user685272

how dare you call the father of analyse "filthy"

Fawad

@heather I flagged my -ve voted question to make anonymous but moderators didn’t did anything.. (I was advised not to make question anonymous)

The Dark Side

@BalarkaSen Modular arithmetic?

Balarka Sen

it's true mod 5 yeah

heather

2:42 PM

@Fawad, sorry, ask @ACuriousMind or another mod for help fixing things, but your "anonymous" fix won't help anything. just because it's anonymous to the outside world doesn't mean it's anonymous to the system.

Balarka Sen

i was joking anyway

The Dark Side

@BalarkaSen We don't have 5 mods in this room right now. :P

Only 2.

user685272

flag it as "offensive" and we will

mod 2 becomes mod n

The Dark Side

Mod 3 now !!

Mod 4 now !!

user685272

wtf, I was JOKING!!!

PearsonArtPhoto

2:46 PM

Bah. Don't flag things that don't need fixing, and leave us poor mods alone.

The Dark Side

What happened? Did something get flagged?

user685272

@PearsonArtPhoto this is all my fault, sorry

PearsonArtPhoto

LOL. Don't worry about it, just do better in the future.

user685272

will do

thanks

The Dark Side

ROFL

Balarka Sen

2:47 PM

bertrand russell is definitely a filthy logician

nothing offensive bout that

user685272

-_-

The Dark Side

It all started with a $1 = 6$ joke !!

user685272

where is the ignore button

The Dark Side

Hahaha ...

user685272

Balarka

Actions
invite this user…
hide posts
ignore this user (everywhere)

John Rennie

2:49 PM

@Fawad: in electrolytes the mobility of the carriers goes up with temperature because the viscosity of the electrolyte decreases.

Balarka Sen

whatever that boats your goat

i mean, floats your boat

John Rennie

@Fawad: In metals the mobility of the carriers decreases with temperature because they get scattered by the thermal vibrations of the lattice.

Fawad

@JohnRennie oh. I need to learn what mobility is. Thanks.

user685272

@BalarkaSen you almost got me banned

Balarka Sen

your welcome

user685272

2:51 PM

good job

John Rennie

@user685272 That's kind of your fault. If Balarka Sen told you to put your butt in the oven would you do it? :-)

Balarka Sen

lol

tbh I wouldn't put my butt in the oven even if i told that to myself

user685272

@BalarkaSen ~~your~~ you're

John Rennie

... so long as the oven wasn't turned on I suppose.

SBM

uh what?

Balarka Sen

2:54 PM

@user685272 i appreciate you're concern about my typos

3

John Rennie

:-)

user685272

np

Balarka Sen

Your skullpatrol arent you

Didn't recognize at first.

user685272

9 mins ago, by user685272

@PearsonArtPhoto this is all my fault, sorry

@BalarkaSen who?

"your" is a possessive, while you're is a contraction for you are

Balarka Sen

when you read too much joyce nothing matters anymore

user685272

3:00 PM

it will matter when you lose marks

Balarka Sen

neh

James Joyce - The Ballad of Persse O'Reilly [from Finnegans Wake]

►

user685272

A brilliant student, he briefly attended the Christian Brothers-run O'Connell School before excelling at the Jesuit schools Clongowes and Belvedere, despite the chaotic family life imposed by his father's alcoholism and unpredictable finances. He went on to attend University College Dublin

Balarka Sen

he was a genius for sure

Secret

The surprising pattern behind color names around the world

►

2

But this is still not going to help in detecting qualia. E.g. a person with a shifted color spectrum perception may well see 700 nm as a greenish color (what we name for the color at 500nm) and still name it red, for example

Red is more distinctive because... danger?

Or is qualia just like a gauge freedom, that it does not really affect the overall outcome of how the society evolved. Thus it can be possible that most of us see red as a 700 nm like color, but for some might see red (700 nm) as what we see as the color at 500 nm

that is, like most physical qualities, only the relative sense matters and not so much the absolute sense?

user685272

3:20 PM

it doesn't matter if you're black or white, we all have red blood

Abcd

Secret

[Random statement]

> Of all organisms on the planet, ONLY human beings are known to deliberately harm each other for pointless reasons

Abcd

Why is approximate symbol used instead of minus sign?

(Topic: Error Analyisis)

@DawoodibnKareem Great Explanation. TYSM!!

Secret

That does not make sense, absolute error is the absolute difference between the true value and the measured value, thus that should be a minus sign

Abcd

@Secret At many places in this chapter, that sign has been used instead of minus

Secret

3:27 PM

Do they demonstrate an example calculation with that sign used in it?

Abcd

@Secret They used minus sign in calculation

Anyway, this was one of the many mistakes in my physics textbook.

I'll ignore it

Such books just confuse students!

Secret

~ only makes sense if they mean there is nonzero absolute error if True value ~ measured value instead of true value = measured value

Abcd

ok'

Secret

So perhaps that's not an equation, but a conceptual statement, either way, the presentation is very unclear

John Rennie

I just couldn't decide whether to have pizza or burgers for lunch ...

user685272

3:53 PM

so you're gonna eat all that?

John Rennie

It has been eaten - all of it! Very nice it was too.

user685272

with beer

John Rennie

Possibly ... :-)

The Dark Side

4:12 PM

@JohnRennie - you were a colloid scientist?

Abcd

@JohnRennie A short question.

Suppose I have a ball in my hand in an elevator capable of accelerating with 7 m/s^2. The elevator starts descending downwards. The moment it starts, I drop the ball. Could you tell me all the acceleration of the things (elevator, ball) in relative motion(read: acceleration) terms?

0celo7

4:52 PM

@JohnRennie you better be using silverware

0celo7

6:07 PM

@JohnRennie 911

"The network adapter is experiencing diver or hardware related problems"

Sid

@0celo7 he he. You still haven't recovered?

You just need to try once..

John Rennie

6:21 PM

@0celo7 what happened? Did the wireless network just stop working?

@TheDarkSide Yes, I worked as an industrial colloid scientist.

The Dark Side

@JohnRennie More 911 help sought from you !!

So, which one of us, me or 0celo7 starts first?

In rough terms, for Mie scattering we would want a scatterer size comparable to, or bigger than the wavelength of light, right?

Whereas for Rayleigh, the scatterer size should be appreciably smaller than the wavelength.

Is that correct, @JohnRennie?

John Rennie

Basically yes. Mie scattering is the intermediate case between very small scatterers and very large scatterers where diffraction effects are important.

I haven't done a lot of work in this area, but as I recall Mie scattering is the complicated one to calculate.

The Dark Side

No problem, nothing theoretical. So, if were to recreate these for a small demonstration, can we create some colloidal suspensions in a solvent, e.g. water?

John Rennie

You can buy colloidal dispersions of various sizes and materials. You can make your own, but getting a narrow particle size distribution takes a fair bit of work.

The Dark Side

Now, if the visible light is 400-700 nm, our rough criteria would be $d<<0.4$ microns for Rayleigh and $d \sim 0.7$ microns or bigger for Mie?

@JohnRennie Yes. I am discovering that part.

John Rennie

6:34 PM

The common types are silica and polystyrene. Typically sillica dispersions are grown by hydrolysing siloxanes while the polystyrene dispresions are grown (unsurprisingly) by polymerising styrene.

Mie scattering occurs when the particle size and light wavelength are the same.

So you want particles in the 400-700nm range.

Much bigger than a micron and the dispersions just look white and boring.

The Dark Side

@JohnRennie Yes. I agree.

John Rennie

We used to get our silica dispersions from a firm called Ludox.

The Dark Side

@JohnRennie Won't that also depend on the path length of the sample?

John Rennie

Google, Google, aha the firm is Grace and Ludox is the brand name of the dispersion

@TheDarkSide short path length = pale white and boring, long path length = bright white and boring.

The Dark Side

@JohnRennie Haha.

John Rennie

6:39 PM

That wasn't meant to be a joke!

The Dark Side

I'm not taking that as one either.

John Rennie

But Mie scattering isn't very spectacular. As I recall you don't get vibrant colours or anything very exciting.

The Dark Side

@JohnRennie All colors are equally scattered, if I'm right. That's why it is "boring".

We would probably want a very narrow size distribution for a very controlled experiment, but any common household stuff that can approximately get the job done?

John Rennie

@TheDarkSide No, there is a wavelength dependance, because obviously the particle size to wavelength ratio is important. But I don't recall it making a huge difference.

What experiment do you have in mind?

The Dark Side

Something of this tune: measure $I/I_0$ for different colors, for the same suspension, using any simple light sensor/intensity meter.

Mie scattering: Almost same for all.

Rayleigh: Huge change.

($I_0$ being the intensity of the respective color only through the solvent)

John Rennie

6:45 PM

I don't think it will be a very exciting experiment. Rayleigh scattering is generally weak. The sky only looks blue because there is a lot of atmosphere in between you and the Sun.

The Dark Side

I agree with that.

But I'm only exploring the following:

John Rennie

The Ludox colloidal silicas with c. 50nm particle size just look a bit hazy even at very high silica content.

They do look noticably blue in reflected light and yellow in transmitted light due to the Rayleigh scattering, but the colours are pretty weak.

The Dark Side

That is when you pass white light through them?

or on them?

John Rennie

There are some fascinating optical effects you get with dispersions. It's possible to get them to form ordered structures and then you get opalescence.

And if you can get the dispersion right on the point of phase separating you get critical opalescence

@TheDarkSide If you pass white light through the Ludox and view the transmitted light it looks yellow because the blue light is preferentially scattered out of the beam.

Likewise if you view from the side it looks blue because you're only seeing the scattered light.

The Dark Side

That's a very useful link. Hopefully, I will return to that problem some time soon.

John Rennie

6:51 PM

Critical behaviour is a fascinating area.

The Dark Side

@JohnRennie Yes. But suppose we pass only monochromatic light?

And measure the intensities. In and out.

We should see some change?

John Rennie

@TheDarkSide With monochromatic light you wouldn't see anything very exciting. The extinction coefficients would just increase slightly as you decrease the wavelength.

If you're doing the experiment you'd probably be best putting your PMT at the side so you're detecting the scattered light. That will give you the clearest changes as you change the wavelength.

The Dark Side

@JohnRennie Now suppose one selects the path length smartly, won't that translate into an intensity difference?

@JohnRennie Yes. That's true.

John Rennie

@TheDarkSide yes, there will be an intensity difference. But for the small particle sizes even concentrated dispersions don't scatter very much light.

The Dark Side

Now, what I am thinking is the following:

let us move away from the scattering, and talk about absorption.

We have the Beer Lambert law.

That exponential factor means that a large sample length of a small concentration is the same as a small sample length of a large concentration.

Is that fine?

John Rennie

6:56 PM

Yes

The Dark Side

So, predominantly, there are two effects occurring in this problem: absorption and scattering.

Suppose my scatterers are not absorbing.

I don't know how correct is that assumption.

John Rennie

The material that the particles are made from don't absorb in the optical region. All they do is scatter. So what you are calling absorption is just a decrease in the transmitted intensity because some of the light is being scattered sideways.

The Dark Side

No, that's for the scatterers.

We also have a solvent.

Which will absorb in general.

John Rennie

Most solvents don't absorb in the optical region.

The Dark Side

Ohh.

John Rennie

7:00 PM

They will just scatter by Rayleigh scattering.

@TheDarkSide Obviously they don't because they're clear.

There will be some very weak absorption. Water does absorb at the red end of the spectrum, but you need several metres of path length to see it.

The Dark Side

THAT'S EVEN BETTER !!

:: rejoices ::

Electromagnetic absorption by water

The absorption of electromagnetic radiation by water depends on the state of the water. The absorption in the gas phase occurs in three regions of the spectrum. Rotational transitions are responsible for absorption in the microwave and far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur in the vacuum ultraviolet regions. Liquid water has no rotational spectrum but does absorb in the microwave region. == Overview == The water molecule, in the gaseous state, has three types of transition that can...

> Absorption coefficients for 200 nm and 900 nm are almost equal at 6.9 m−1 (attenuation length of 14.5 cm).

> These values correspond to pure absorption without scattering effects.

So, @JohnRennie, if I read it right, that means exhibiting Rayleigh scattering needs no colloidal suspension in water? (In principle?)

John Rennie

The absorption by water in the UV and IR parts of the spectra are because energy is transferred into electronic and rovibrational states of the water molecules.

But this doesn't happen across most of the visible part of the spectrum because there are no states that have transition energies in those regions.

The Dark Side

Exactly !!

John Rennie

Water will Rayleigh scatter just like air Rayleigh scatters, but the scattering is weak.

Rayleigh scattering is generally a weak process.

The Dark Side

So, the intensity lost in the forward direction is almost purely a scattering loss?

@JohnRennie What if you increase the path length of the sample?

John Rennie

7:14 PM

When measuring scattering by dispersions in the usual solvents the intensity loss in the forward direction is almost purely a scattering loss, yes.

The Dark Side

@JohnRennie Yay.

John Rennie

If you make the path length arbitrarily big then obviously you can make the intensity loss as great as you want. But there are obvious experimental difficulties.

The Dark Side

True.

John Rennie

I've kind of lost track of where we are going with all this ...

The Dark Side

Our basic problem was Rayleigh and Mie scattering.

We are done with one.

R needs no colloidal suspensions in water (in principle).

But of course, any solute in water which can also add to R, shall amplify the effects.

Am I right?

John Rennie

7:19 PM

solute? That normally means a small molecule.

Like salt or sugar.

The Dark Side

$d < < \lambda$

wink, wink.

John Rennie

Small molecules will have very little effect. After all salt solutions are still clear.

The sorts of dispersions I used to play with had silica particles in the 50-100nm range

The Dark Side

No, but if I'm getting this right, the identity of the small scatterer is irrelevant.

Isn't it?

John Rennie

Provided the scattering centre is small enough the angular distribution of the scattering is independent of exactly what the scattering is being caused by. However the intensity of the scattering can be very dependent on what is doing the scattering.

The Dark Side

So, isn't it small enough with salt and sugar?

John Rennie

7:24 PM

Because different scattering centres interact with light in different ways. A sugar molecule is different to a 50nm colloidal silica particle.

The Dark Side

The yardstick is comparison with $\lambda$, ain't it?

@JohnRennie Ahh. You mean, in shape?

John Rennie

@TheDarkSide no, both are basically spherical.

But the 50nm silica particle creates a far larger refractive index change

If you look at the equation for the scattering intensity then there's a fudge factor $\alpha$ that is the polarisibility of whatever is doing the scattering.

The Dark Side

Yes. You are right.

John Rennie

This will be different for different scattering centres. It doesn't change the angular distribution of the scattering but it does affect the overall scattering intensity.

Anyhow, I have to be at work in 8.5 hours so I need to clock off and start getting ready for bed.

0celo7

@JohnRennie the magical restart fixed it

John Rennie

7:30 PM

@0celo7 Turn it off and turn it on again. It really does fix most problems :-)

The Dark Side

@JohnRennie Just one last question before you leave.

John Rennie

@TheDarkSide Yes?

The Dark Side

If we drop silica 50 nm particles into water, I hope there isn't any chemical reaction involved !! Also, how about fine mud particles?

Instead of well fabricated silica particles?

John Rennie

Fine mud particles are far too large.

The Dark Side

So, they make the cut for Mie at least?

:P

John Rennie

7:32 PM

Mud will be mostly crystals of clays like bentonite or montmorillonite and those are upwards of a micon in size.

They are probably too big for Mie scattering, and they'll have a huge range of sizes.

The Dark Side

Hmmm ... Yes.

Silica gel adsorbs water. But otherwise, Silica + water is OK?

John Rennie

You can't buy dry 50nm silica particles. Ludox comes as a dispersion in water. Typically 20% solids I think. There's no reaction if you dilute this with extra water.

The Dark Side

Ohh. Great!

John Rennie

Silica gel is completely different and won't give you a dispersion when you add it to water.

Making these silica dispersions is quite a complicated business.

I think Ludox is mildly alkaline, but apart from that you could bathe in it if you felt so inclined. It's very safe stuff. There will be a safety data sheet on the Grace web site.

Anyway, I'm off. I'll be around tomorrow if you want to chat further.

The Dark Side

Alright. Thanks a lot @JohnRennie for this detailed discussion.

(When you see this tomorrow)

dmckee

7:41 PM

@Secret Eh? Even seen video of ape or monkey societies?

They have all the same social $#!? that you experienced in high school, only with more body hair.

The Dark Side

Hello @dmckee

Secret

Really? I always thought those are more basic fight for dominance type fights or social class things

Dawood ibn Kareem

@Abcd For as long as the lift continues to accelerate downwards at $7 m/s^2$, the acceleration of the ball relative to the lift will be $g - 7 m/s^2$. Is that what you're asking?

So? What did you want to know?

Abcd

@DawoodibnKareem Yes.

@DawoodibnKareem Why g-7?

Dawood ibn Kareem

7:57 PM

What else would it be?

You could try differentiating (twice) the difference between the amount the ball moves and the amount the lift moves.

$(\int\int g) - (\int\int 7) =\int\int(g-7)$

Jaime Gallego

Use \iint for double integrals

Dawood ibn Kareem

Thanks, I'd forgotten that one. Can't be bothered editing it though - my meaning is clear.

Abcd

@DawoodibnKareem ok, ty

Emilio Pisanty

8:23 PM

guys, we've upstaged Wikipedia

4

5

0celo7

@JaimeGallego or just don't use double integrals

There's literally no reason

Las Vegas Raiders

@EmilioPisanty this is a monumental day

Secret

8:56 PM

So...
Yesterday I was a volunteer for a astronomy local fair visited by many kids. We used tornardoes in water bottles to depict black holes, saying that rotating black holes are basically whirlpools in spacetime and deep wells that light cannot escape. did I comitted a GR crime for being too inaccurate?

If yes, what's a better way to educate 4-7 yr old kids about black holes as simple but as accurate as possible?

Las Vegas Raiders

imho that is too young

Secret

well one of the boys did ask how does tornardo water bottles related to astronomy concepts, and the answer I gave is the above, so I am guessing they might be smart enough to understand basic level physics

ACuriousMind

@EmilioPisanty The only blemish there is that it's very prominently tagged homework-and-exercises...

Las Vegas Raiders

How is that a "blemish?"

ACuriousMind

@Secret I personally don't think 4-7 year old kids need "educating" about black holes at all. It's fine to just tell them a nice story, that's why it's called a lie-to-children

@LasVegasRaiders Because a) that's the single most controversial tag we have and b) it might yield the impression that "homework and exercises" is something physics.SE is for to casual googlers. I'm not sure I'd have clicked on a result that ostensibly was about homework/exercises back when I found this site.

dmckee

9:12 PM

@Secret Monkey B gets beat up by monkey A for some minor transgression. After a short sulk B goes looking for a more junior monkey and on finding C beats him out of the blue.

Hard to find something more along the line of 'just because I can' than that.

Secret

classic bullying prorogation dynamics down the hierarchy, ok I think I agree that it is either beaten or being beaten world (to paraphase undertale)

you do it because your opponent is weaker and you have the ability to do so

In other words, pretty much what you said. In that case, I suspect "pointless bullying" may exist in other animals as well, though I am not sure. It will be quite intriguing if it is only restricted to mammals

Actually an interesting question to ask: Are there scenarios where B beats C without being beaten beforehand? that might rule out the simple reason of retalitation by projecting to something weaker as an explanation

Emilio Pisanty

@ACuriousMind yeah, I know

I'm increasingly less convinced that we need the tag at all

57

A: Can experts identify the model using this part (FOD prevention purposes)?

The device is an old “B.I.C” It can't be said for sure, because there is no marking visible. Still images like this can be found on the externet, Here a very similar one: What is the meaning of “B.I.C” and what is the purpose is another story, this is never explained, and it seems impossibl...

↑ man, is that an instant classic

Sir Cumference

10:22 PM

@EmilioPisanty I didn't think SE allowed jocular answers

Jaime Gallego

@0celo7 Why is that so? My multivariable calc knowledge is nil, but I can see the point of double integrals.

0celo7

10:40 PM

@JaimeGallego just don't write two integrals

You never see that in the analysis literature

Write dA if you have to emphasize

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