The h Bar

user228700

10:21

@S007 Will check it out!

user228700

When given that $\sigma$ bonds have circular symmetry, what does that really mean..? Circular symmetry about what, exactly?

user228700

@JohnRennie :-)

Anonymous

@Kaumudi Well "doubt" is correct in Indian English :) ...btw en.wikipedia.org/wiki/Circular_symmetry

Anonymous

In geometry, circular symmetry is a type of continuous symmetry for a planar object that can be rotated by any arbitrary angle and map onto itself.

Anonymous

You will understand circular symmetry better if you know Newman projections

user228700

10:27

@S007 Wtf? Okay, I'll read about this some more.

Anonymous

You rotate the bond about any angle but if you view from one side the view remains the same

user228700

@S007 Ik :-) 0celo7 refused to buy into the Indian English argument tho.

user228700

@S007 Hm, okay. Thanks!

Anonymous

@Kaumudi It really doesn't matter if someone doesn't accept Indian English. But it is not right to force British English or American English upon someone :) I will continue to use "doubt" as a synonym for question. After all language is man-made!

Anonymous

10:34

This is Newmann's projection

Anonymous

of ethane

Anonymous

And you can see the circular symmetry here

Anonymous

@Kaumudi

user228700

@S007 Hm, yes I can, thanks very much bro! (xD)

Anonymous

@Kaumudi Not again!!!! XD

user228700

10:35

@S007 Indeed :-) However, the purpose of language is to make communication as clear and concise as possible and I'm not sure if loads of people understand that we Indians mean "question" when we say "doubt".

user228700

@S007 What's the big deal, anyway?

Anonymous

It is just a joke :P

Anonymous

Better you don't know :P

user228700

:-P OK.

Anonymous

@Kaumudi They better start understanding what we mean when we say "doubt"...LOL XD....we have a majority here :-P BTW india is the country which has the highest number of english speakers in the world ;-) en.wikipedia.org/wiki/…

user228700

10:39

I've another quick question, BTW. Under VSEPRT, Shape→Don't take lps into account and Geometry →Take them into account. Is there any more to it?

user228700

@S007 :-P How effing ironic.

Anonymous

@Kaumudi I didn't get your question actually..could you say it in other words ?

Anonymous

>>>>> Under VSEPRT, Shape→Don't take lps into account and Geometry →Take them into account

Anonymous

What does this mean ?

user228700

Well, the basic question is, with regard to VSEPR theory, what is the difference b/w the terms shape and geometry?

user228700

10:43

What I said before is the answer I found on the mighty internet. I was wondering if it's correct (?)

Anonymous

Shape means bent or tetrahedral or.....

user228700

So, this would mean that the shape of a water molecule is bent, but the geometry is tetrahedral.

user228700

Dyou see what I mean?

user228700

(Don't scream at me, I found this on many websites :-P)

Anonymous

No. Shape and geometry is the same here. For water molecule you just say that sp3 is the hybridization

user228700

10:45

Some websites say it's the other way around. i.e. Shape→Take lps into account and Geometry →Don't.

user228700

@S007 Not hybridization yr!

Anonymous

Well, I would follow wiki en.wikipedia.org/wiki/VSEPR_theory#AXE_method

Anonymous

They say both are synonymous

Anonymous

shape and geometry

Anonymous

The term used when LP are included is "Electron arrangement"

Anonymous

10:50

Basically lp included "electron-pair geometry" and lp excluded means "molecular geometry"

Anonymous

see intro.chem.okstate.edu/1314F00/Lecture/Chapter10/VSEPR.html

user228700

Yeah, I found that as well ^

Anonymous

In jee if you go through the past papers you will find that geometry=shape in all cases like this

user228700

But look:

user228700

user228700

10:52

^ Solomon and Fryhles.

Anonymous

There is some confusion there. But as

Anonymous

I said

Anonymous

JEE considers geometry=shape

user228700

But yeah, no, the most common opinion seems to be that there is no difference b/w molecular geometry and shape.

user228700

@S007 Yeah, I was just a little thrown off 'cause Solomon & Fryhles, u know.

Anonymous

10:55

I would'nt trust solomon and fryhles for jee actually :-P....everything depends on the paper setter

Anonymous

better if we follow jee pattern

user228700

True that :-P

user228700

Okay, thanks (again :-P)!

Anonymous

welcome

Anonymous

:)

koolman

11:02

How can we do this

@Kaumudi

@JohnRennie @S007 help me

user228700

Due to the fact that I haven't done Electrochemistry properly yet, I dunno the products of electrolysis of each of those but the first step would be to identity what gets left in the solution.

koolman

How

user228700

Have u learnt how to identify the products of electrolysis yet?

user228700

If I remember correctly from the last time I did this, there should be a topic in ur book titled "Products of electrolysis".

user228700

I don't see how else we can predict the nature of the solution after electrolysis.

koolman

11:16

For example of HCl is H+ and Cl-

user228700

No, u need to identify what is released at the anode and cathode...

koolman

But there can be possible cases

user228700

Yes, but there are ways to find the most likeliest of them all.

user228700

Like I said, I need to learn this again, but maybe this will help:

user228700

boundless.com/chemistry/textbooks/boundless-chemistry-textbook/…

user228700

11:19

Also check on YouTube: "Prediction of products of electrolysis".

koolman

Ok

user228700

I'm sorry I dunno anything else to help u :-/

Slereah

guys

Oh wait

No GR guys here

nvm

John Rennie

@koolman Hmm, at a first glance none of them would remain neutral ...

koolman

@Kaumudi i don't get it

@JohnRennie yes

John Rennie

11:22

@koolman Potassium and sodium will both react with the water and make it alkaline, so C and D cannot be correct.

Copper and silver are both fine - they'll just form a layer of metal on the electrode, so the question is what happens to the anion.

koolman

@ok

@JohnRennie but the answer is option c

I have the solution but I could not understand it

John Rennie

Well consider what happens to SO_4^2-

it donates two electrons and becomes SO_3 + O

koolman

John Rennie

The O atom combines with other oxygen atoms to form O_2 and this bubbles away. The suplhur trioxide reacts with water to form sulphuric acid. So you end up with sulphuric acid in the water and the pH goes down.

koolman

Why will it donate two electron

John Rennie

11:28

Ah, of course.

Kyle Kanos

@Slereah You keep reminding us :/

koolman

Sorry , please explain me more clearly

Kyle Kanos

I'm still aiming to learn it, just have other things to do first

Slereah

Do I

Anonymous

@koolman (C) is the correct answer

Anonymous

11:29

K2SO4

John Rennie

@koolman With potassium sulphate the potassium reacts at the electrode to form potassium hydroxide and the sulphate reacts at the other electrode to form sulphuric acid. The two react with each other and form potassium sulphate again.

Anonymous

ucc.ie/academic/chem/dolchem/html/dict/electrol.html

koolman

@S007 how

John Rennie

So overall the pH remains unchanged.

Anonymous

see the link

Anonymous

11:29

ph remains same in k2s04

Anonymous

ucc.ie/academic/chem/dolchem/html/dict/electrol.html

Anonymous

At the cathode :
Hydrogen ions are being removed from solution, thereby leaving an excess of hydroxyl ions which makes the solution alkaline, and
At the anode :
Hydroxyl ions are being removed, so leaving an excess of hydrogen ions which makes the solution acidic.

koolman

Why this process will not happen with other , @S007 @JohnRennie

Anonymous

Because of the reduction potentials

Anonymous

of cu and ag

John Rennie

11:32

@koolman With (A) copper sulphate the copper comes out as copper metal so that doesn't change the pH, while at the other electrode the sulphate forms sulphuric acid. So the pH goes down. Does that make sense so far?

Kyle Kanos

Wow...I'd forgotten I've got 100 flags/day

Been so long since I've used any of them

koolman

But in how will we know reduction potential

*in exam

Anonymous

@koolman you need to remember the chart

Anonymous

reduction potential chart is given in ncert books

Anonymous

cu and ag have larger tendency to exist as neutral species

koolman

11:33

@JohnRennie yeah

@S007 ohhk

Anonymous

koolman

@JohnRennie what about NaCl

Anonymous

nacl forms naoh

John Rennie

@koolman OK, with (B) silver nitrate the silver comes out as metal and that doesn't change the pH. The nitrate forms nitric acid so once again the pH goes down.

koolman

@S007 its too big

John Rennie

11:35

@koolman With sodium chloride the sodium reacts at the electrode to form sodium hydroxide, while the chlorine forms Cl_2 and bubble off. So the pH goes up.

koolman

@JohnRennie ye

Anonymous

but you need to remember...no choice...@koolman

koolman

@JohnRennie oh got it

@S007 but logic of @JohnRennie is also nice

Secret

Nuclear physics question: What is the underlying reason that radioactive decay is random instead of deterministic?

John Rennie

@koolman I doubt you'd be given anything obscure in an exam. Just remember how the metals react with water. For example sodium does react (violently!) with water but silver and copper don't.

Anonymous

11:37

@koolman Yes it is nice. Provided you know which has relatively higher reduction potential

Anonymous

I remember the series as a song

Anonymous

It is actually easy

John Rennie

The other electrode is a bit harder, but sulpher trioxide and nitric oxide react with water to form acids while chlorine doesn't.

koolman

@S007 ok i will do it

John Rennie

@Secret it is a tunneling process, and quantum tunneling is a random process.

koolman

11:38

@JohnRennie ohhk

Secret

I see

John Rennie

In virtually all nuclear reactions the nucleus has to go through a high energy transition state to break down, and that creates an energy barrier.

Classically the nucleus would never disintegrate, but QM allows it to tunnel through the barrier and disintegrate that way.

koolman

@JohnRennie they react with water to form acidic compound

John Rennie

@koolman what react with water to form acids? SO_3 and NO_2 ?

koolman

Ya

John Rennie

11:42

@koolman Indeed they do. Actually chlorine does react with water to form hypochlorous acid, but it's a weak acid so we normally ignore the slight reaction.

koolman

@JohnRennie @S007 therecan be electrolysis of water also

John Rennie

@koolman Electrolysis of water doesn't change the pH.

koolman

But if there is electrolysis of H+ instead of K+

Anonymous

Actually chlorine with water produces both HCl and HOCl which decompose to oxygen and hcl

Anonymous

@koolman what ? electrolysis of H+ ?

koolman

11:45

H+ from water

John Rennie

@koolman Electrolysis of water turns H+ into H_2 at one electrode and OH- to H_2O + O_2 at the other electrode, so overall the water stays neutral and the pH doesn't change.

koolman

On ucc.ie/academic/chem/dolchem/html/dict/electrol.html

See electrolysis of dilute sulfuric acid

SO4- remains unchanged

Anonymous

even H+ remains constant

koolman

I mean to say if one ion from salt and water is only electrolysed

John Rennie

@koolman if SO_4^2- was electrolysed it would produce SO_3 and O_2. But SO_3 immediately reacts with water again to prouce sulphuric acid. So the end result is that oxygen bubbles off and the sulphate is left in solution.

Anonymous

11:51

Moreover the solution being dilute OH- ions would reach the anode faster than SO4(2-)

koolman

I mean for example only K+ and OH- is electrolysed

Anonymous

@koolman Please take your time and frame the questions properly. It is difficult to understand what you are asking.

John Rennie

@koolman Any potassium formed by the electrolysis would immediately react with the water and form potassium hydroxide. The end result is that hydrogen would bubble off.

koolman

5

A: In my homemade electrolysis setup, only the negative end bubbles?

Your setup, using table salt ($\ce{NaCl}$ plus additives) will initially electrolyse according to the following two reactions, separated by cathodic reduction and aniodic oxidation: $$\ce{2 H+ + 2 e- -> H2 ^}\tag{CatRed1}$$ $$\ce{2 Cl- -> Cl2 + 2 e-}\tag{AnOx1}$$ These reactions assume no coun...

The answer given by jan

According to that how we will do this

Anonymous

@koolman Do what ?

koolman

11:59

My question

Anonymous

@koolman Which question? That is for a more conc. solution of NaCl(the one jan answered)

koolman

Ohhk

I have one more doubt

Can we say product of vapour pressure of two liquid is equal to square of total pressure

John Rennie

@koolman the total pressure is the sum of the individual vapour pressures

koolman

Is there no ther relation

John Rennie

No

koolman

12:11

Ok thanks

Kyle Kanos

Hmm, I've got about 5 days left to get another 1k rep. I'm gonna need a couple HNQs I think

John Rennie

@KyleKanos Give me 10% of the points and I'll ask the question of your choice :-)

valerio92

Hello everybody. I wanted to tell you that I've created a new tag, "liquid-state". I think that such a tag was missing, and it is needed for all the questions which address specifically the thermodynamics and statistical mechanics of liquids, as opposed to those question which concern the dynamics and mechanics, for which the tags "fluid-dynamics" and "fluid-statics" should be used. This is the excerpt I wrote:

"The liquid state of matter is characterized by short-ranged correlations, as opposed to the long-ranged correlations of crystals and the absence of correlations in an ideal gas. Us…

koolman

@KyleKanos me too

Kyle Kanos

@JohnRennie Hmm, I could also just ask the question and answer it myself ;)

John Rennie

12:16

@KyleKanos It's worked for me in the past :-)

Though actually I've generally made those questions community wiki.

Kyle Kanos

I can use auto-refresh on Opera to refresh it every 15 minutes for a new view and let it hit HNQ through cheating :D

@JohnRennie That's because you were making canonical questions & answers; mine wouldn't be. I think

John Rennie

I bet the SE track IP addresses to avoid such cheating. You need to set up a botnet. Send a trojan e-mail titled Click here to see naked physicists!

Kyle Kanos

I don't think they do. I tested with a question of mine w/ auto-refresh the other day (got it up by 5 or so)

Er...do we really need a liquid state tag?

heather

hello

Ramanujan

Hello!

valerio92

12:27

@KyleKanos Yes, I think so. I think that questions which address the statistical mechanics and thermodynamics of liquids should not be labeled under "fluid-dynamics", as it often happens. But I may be wrong.

Actually, I also created an "amorphous solids" tag. I didn't want to create a "glasses" and a "quasicrystals" tag, but I think we need something like that. But again, I may be wrong! :-)

Kyle Kanos

But having a liquid-state tag implies we should also have one for solids and gases and plasmas

valerio92

@KyleKanos We have gas and ideal gas. We have plasma physics. We have solid-state-physics...

We also have solid-mechanics

inert-gases....

Kyle Kanos

solid-state-physics $\neq$ solids

valerio92

Would you prefer "liquid-state-physics"?

It is the same for me

Kyle Kanos

I'd prefer not having it at all, TBH. This is just my opinion though

valerio92

12:31

Ok, thanks for sharing your opinion :-)

Kyle Kanos

I see your new tag as a meta-tag: only useful when tagged with other tags

valerio92

I think we need it. Liquid state physics is a well established field with its own formalism and I think it should be included in the tags. But everyone is free to disagree, of course.

heather

I agree with @valerio92, it does seem necessary. It would also be weird to have all the other tags without a liquid state physics tag. Also, how is it a meta tag?

Kyle Kanos

@heather Read the second half of my mentioning meta-tag?

heather

@KyleKanos, yes, and I believe it would be useful by itself (though I'd admit it would be better with other tags, but that is true for any tag, I think).

Kyle Kanos

12:36

What type of physics question could be asked with just the tag "liquid-state"

And was the creation of the new tag discussed in chat here previously? Or a Meta post about it? (the latter is a clear no, since it's not in the list here)

heather

I think you could ask a question purely about liquids, like "at what temp does x become a liquid?" or "when x becomes a liquid it has y property - why?"

things along those lines.

valerio92

Questions about the thermodynamics and statistical mechanics of liquids. For example, I could ask a question about the Percus-Yevick approximation. This question, even if it was closed as too broad, is a good example: physics.stackexchange.com/questions/257105/…

Kyle Kanos

The first would certainly be closed and heavily downvoted....

Secret

smbc-comics.com/comic/2015-01-21

So there's a third guy after angel and devil

valerio92

If we weren't supposed to talk tags here I didn't know, sorry.

Kyle Kanos

12:43

No, talking tags here is perfectly fine & expected

valerio92

Ok, good to know. I don't know if it was previously discussed...I am relatively new to the site.

Kyle Kanos

The normal procedure is that one posts either here in chat or on the Meta site if creating tag X would be useful/needed and then, if the response is positive, the tag is created & used

valerio92

Uhm, ok, sorry, it's my first time creating a tag...

I will follow the procedure next time

Kyle Kanos

It's not a problem, your example is probably right & it might be useful--I am normally (very) hesitant about creating new tags

valerio92

Yes, I understand, and I'm very hesitant too, in fact I thought about it a long time ago but just did it today.

Maybe we should open a discussion on Meta about a specific tag for glasses and/or quasicrystals sometimes. I proposed the "amorphous-solids" tag, but it may end up not being used very much, because people who ask questions about glasses will usually try to type "glass", I figure...

ACuriousMind

13:41

@valerio92 I think the tag is a good idea. If you start tagging questions with it, please keep in mind that each edit bumps the question, so try to do it in small batches as to not flood the "active" page solely with questions newly tagged with it.

Secret

13:55

Caption: [Division by zero] This is why you cannot have 0 as the inverse of a nilpotent element in any associative algebra:

1. the multiplicative identity becoming a zero term result in the controlling properties of the zero term to spread to all entries of the cayley table of the multiplicative identity, even if it is only one sided
2. The nilpotent element unpack the 0 that is part of the zero term of the multiplicative identity such that all elements involving the nilpotent element, the identity and zero are expressed in terms of the nilpotent element

(Sorry typo: You cannot have 0 as the multiplicative inverse of a nilpotent element in ANY binary algebra, not just associative ones)

valerio92

@ACuriousMind Yes, absolutely. Thanks!

Secret

Having 9 entries in the * cayley table being locked up in terms of the nilpotent element, and then these elements are being specified by the existence of the multiplicative identity, it means it is doomed to impose a null semigroup onto the + structure

The case where the inverse is not zero for the nilpotent element is currently under investigation

ACuriousMind

1

Q: Studying Quantum mechanics from Schwinger's book

I'm gonna start studying some more advanced than introductory quantum mechanics. I partially studied Classical Electrodynamics from Schwinger's book and I totally loved it, so I was thinking about going on with the same guy. Now After studying quantum mechanics from Griffiths I'd like something m...

On-topic?

I seem to think that asking for "reviews" of specific books is off-topic, but I can't find anything explicit in the meta posts.

Danu

14:20

@ACuriousMind Sounds opinion-based to me.

koolman

How they have written equation of equilibrium

ACuriousMind

They appear to have printed it. (Please ask a more specific question, I'm not sure what you want to know)

koolman

ACuriousMind

@Danu Yeah, that works, I guess.

koolman

This is from ncert

How the equation of equlibrium is written

ACuriousMind

14:26

It's written with symbols. Are you trying to ask how it is derived?

koolman

Yes

@S007

Do you know

ACuriousMind

It appears to me they just used the definition of torque and the analog of Newton's law for rotations, i.e. $\tau = \dot{L}$.

Anonymous

@koolman Can you show me the question ? Basically Torque= (moment of inertia) (angular acceleration) = (magnetic moment) * (magnetic field)

Anonymous

* is cross product

koolman

@S007 thank you , i got it

Anonymous

14:30

@koolman :)

ACuriousMind

14:51

5

Q: Is hunting animals, crafting and attacking/defending from enemies intellectually equivalent to quantum physics?

Our brains were shaped after natural selection. Which means that, as long as we were being affected by it, our brains were changing, evolving. Once we stepped out of nature and stopped being targets of natural selection, our brains (structurally speaking) stopped changing. Civilizations rose, h...

evolution brain natural-selection intelligence human

wat

Physics Meta

15:02

1

Q: Tag(s) for amorphous solids/glasses/quasicrystals

I would like to discuss the creation of one or more tags for amorphous solids, glasses and quasicrystals. There are many question on the site which address specifically glasses and the glass transition, for example this one and this one and this one. Other questions are about quasicrystals, lik...

discussion tags

John Rennie

@ACuriousMind It's a good question though. How come a lump of squidgy grey stuff evolved to help use run away from carnivores turns out to be good at algebraic geometry?

As usual I mention algebraic geometry as the quintessence of mathematics that I've heard of but don't understand. I don't doubt there is even more esoteric maths out there but if so it's so esoteric I haven't even heard of it :-)

ACuriousMind

@JohnRennie My silly-but-not-totally-silly answer: Because mathematics is mostly pattern-matching and the lump doesn't care whether it's matching tiger stripes against a jungle background or formulae against a math background ;)

John Rennie

I'm sure it one of those accidental side effects. If you construct something complicated enough to identify and escape threats then it turns out to have unexpected emergent properties.

Anonymous

ACuriousMind

Also, how do you know we're good at algebraic geometry? For all we know there's an alien race out there who figured out all the mathematics we know on a weekend.

Anonymous

15:06

I solved and found that options (B) and (C) are correct. But any idea what should be the condition for BP to remain taut ? I'm a bit confused as to whether the fourth option is correct... @JohnRennie could you answer ?

John Rennie

@ACuriousMind :-) I can be sure only that I am not good at algebraic geometry.

@S007 Suppose the lower string weren't there. You can write an expression for the angle of the upper string as a function of $\omega$. Then if the angle of the upper string is less than 30º to the horizontal that will make the distance to the lower attachment point > $\ell$.

i.e. if the lower string was present it would be taut.

David Z

@KyleKanos I'm coming a little late, but my understanding is that the normal procedure is that if you have enough reputation and you think a tag is necessary, you just make it. You are expected to do reasonable checking to make sure it doesn't duplicate an existing tag, of course. (The rep requirement is so that people who don't know that can't create new tags.)

Making a tag is a relatively minor thing and doesn't generally require a meta post.

That being said... most useful tags exist already. So when someone thinks they have found a new tag that would be useful but doesn't exist, they should probably think to themselves, "hm, why doesn't this tag exist yet?" And that could be a good occasion for a meta post.

But my point is, I wouldn't want someone who creates a tag without prior meta discussion to think they were breaking protocol. (cc @valerio92)

Anonymous

@JohnRennie Great method. I got it now! Thanks a ton :-D

user116211

@ACuriousMind yes, I was shocked with the choice of quantum physics; but of course it's biology and hmm, don't care much.

David Z

15:22

@MAFIA36790 I guess for a lot of non-physicists, quantum physics is the highest intellectual endeavor they can imagine

John Rennie

@DavidZ it is for some physicists as well :-)

user116211

@JohnRennie I thought QFT.

David Z

Sort of the same thing :-P

lol

user228700

@koolman: Dyou know why u've been banned at The Periodic Table?

Anonymous

@Kaumudi LOL XD

Anonymous

15:29

Same Pinch as me!

user228700

Poor guy :-/

Anonymous

I got suspended from Chem SE itself...but it doesn't matter anyway...I use another account :-P

Anonymous

@Kaumudi

user228700

WTF?!

Anonymous

Check my account on Chem SE!

user228700

15:31

xD Well, u brought it upon urself bro.

John Rennie

@Kaumudi Voting irregularities according to koolman's profile. To comment further probably violates policy on discussing thse issues.

user228700

@JohnRennie Ohh. I thought it was because of something else hmm.

ACuriousMind

@S007 Sockpuppetry (using a second account to circumvent system restrictions) is forbidden and will just lengthen your suspension when detected.

John Rennie

@ACuriousMind Is using a second account forbidden? Using a second account to vote up your first account is certainly forbidden, but I'm not sure that just having a second account is a problem.

ACuriousMind

@JohnRennie I said: "using a second account to circumvent system restrictions"

Anonymous

15:36

@ACuriousMind I don't think they can detect. I use a different IP address for that account. Anyways I do not use Chem SE much.

John Rennie

@S007 you're being very candid with someone who is a moderator :-)

ACuriousMind

@JohnRennie Having a second account is fine as long as you don't do anything you couldn't with a single one. Using a second account while one of your accounts is suspended is certainly forbidden.

Anonymous

@JohnRennie XD I realized that just now :-P

John Rennie

Is this a good time to admit I use a second account on the Embarassing Personal Problems Stack Exchange?

user228700

xD

ACuriousMind

15:40

@JohnRennie There's a Stack Exchange for that?

Not that I have any need for it, of course :)

user228700

@S007 Damn, I hope all this honesty won't result in another suspension for u now :-P

John Rennie

@Kaumudi well he (or she or it) is unsuspended in 10 days anyway

Anonymous

@Kaumudi I will create another account with another IP address then :-P...

Kyle Kanos

@DavidZ I guess it has been my experience as I described.

user116211

Hmm, I did have a second account when I signed up in Area 51; but then I requested merging it with my original one; the team has done it then.

Anonymous

15:42

I can generate unlimited IP addresses belonging to any country in the world :-P

user228700

Holy f dude. @S007: Tere ko "it" pronoun se bhi bulane lage. My gut still says u're a he. I respect ur privacy tho so I'll drop this if it's annoying :-P

user228700

Also, excuse my terrible Hindi :-P

Anonymous

@Kaumudi I'll just say "trust your instinct" :-D

user228700

:-P OK bro. (Yeah, I'm not dropping this one xD)

ACuriousMind

@S007 If you continue to announce your intention to dodge moderator action with sockpuppetry I will kick you from this room. The rules are in place for a reason and I will not tolerate outright disrespecting them.

3

Anonymous

15:45

@ACuriousMind Okay. I will not announce it anymore. Sorry.

koolman

15:58

@Kaumudi because of second account . As u already know

And what do you mean by pinged

@S007 how

Anonymous

@koolman I can't tell it here.

koolman

Then

Parth Maske

16:13

Can I ask a question?

John Rennie

@ParthMaske yes of course

ACuriousMind

@ParthMaske Sure, just ask away, no need to ask permission

Parth Maske

@JohnRennie Thanks. My knowledge about the topic may not be complete but please correct me. When we measure the spin of an electron in a particular direction (say x axis) we either get spin up or spin down. The angular momentum of the universe is conserved, and we say that no of particles = number of anti-particles. This means that a measurement of a particle should instantaneously change the spin of its antiparticle. But I don't understand the meaning of "Instantaneous" ?

John Rennie

> a measurement of a particle should instantaneously change the spin of its antiparticle

No.

Parth Maske

ok? then how is angular momentum conserved ?

John Rennie

16:18

If we have an electron in a superposition of spin states then its angular momentum will have some expectation value in between the spin up and spin down states.

When we do the measurement the superposition collapses and we measure either spin up or spin down, so the angular momentum will in general be different from the expectation value of the superposition.

The difference is transferred to the angular momentum of the measuring apparatus i.e. the total angular momentum of the electron and the measuring apparatus is conserved.

Secret

@JohnRennie I have a computer hardware question: Recently my computer suddenly turn itself off and then reboot. My speaker also give a loud thud whenever that happens. Any idea what could be the reason (cause I have checked my CPU it is not very hot)?

John Rennie

@Secret What operating system? Windows?

Secret

@JohnRennie Windows 10, CPU hardware is HP pavillion and monitor is LG

ACuriousMind

Sigh...I wish arXiv gave its downloads more meaningful names, renaming all the stuff manually is tedious.

Parth Maske

@JohnRennie Are you saying that the change in the angular momentum a particle does not change the angular momentum of its entangled pair

John Rennie

16:24

@ParthMaske Ah, you didn't say you were considering an entangled pair.

Parth Maske

Sorry

ACuriousMind

However, also in an entangled pair it doesn't "change" it. Before measurement neither of the electrons has an angular momentum to speak of.

John Rennie

Typically we consider an entangled pair with a total angular momentum of zero. This needn't be a particle-antiparticle pair, though it can be. In that case measuring the spin of one particle also measures the spin of the other.

ACuriousMind

The crucial thing about an entangled pair is that there is no individual state of an electron before entanglement is broken, so there's nothing to "change".

John Rennie

An entangled pair isn't two particles with some sort of link between them, an entangled pair is a single system.

So it's meaningless to ask about the spin of one of the particles in the entangled pair.

When you make a measurement you are measuring the properties of the whole system. It isn't the case that you measure one of the particles and that measurement somehow ripples through space over to the other particle.

Parth Maske

16:28

@JohnRennie I understand

@JohnRennie @ACuriousMind Thanks. Also, Can we say that every particle has its entangled pair?

ACuriousMind

I'm not sure what you mean by that

John Rennie

@ParthMaske sort of.

ACuriousMind

There are plenty of non-entangled particles

John Rennie

Entanglement happens when any two quantum systems interact with each other. The interaction causes them to become mixed so that they can't be described by two separate wavefunctions.

But in the real world everything interacts with everything else to some extent.

So there is a sense in which everything is entangled with everything else.

But it is not the case that for every electron there has to be one and only one other particle with whicvh that electron is automatically entangled.

Entanglement is a far messier business than that.

I'm sure there was a question on this very subject recently. I'll have a look and see if I can find it ...

Parth Maske

I thought that every particle has its entangled pair, and they were created at the time of big bang. How are entangled pair created?

John Rennie

16:35

> I thought that every particle has its entangled pair

Particles don't have to be created in pairs. It's true that when an electron is created froma photon this process always creates an electron-positron pair. However there are lots of processes where this doesn't happen e.g. muon decay.

Parth Maske

@JohnRennie Ok. I will read about it. I am new to QM, and sometimes I like to wonder against my syllabus. So, any help is like a paradise for me - things just get easier. Thanks

Slereah

17:07

I think I found a paper relating to a question I asked a few months back

Gonna have to answer myself

AGAIN

John Rennie

@MAFIA36790 you realise I'm doing it deliberately just to annoy you :-)

ACuriousMind

For the record, I think that $d$ vs. $\mathrm{d}$ is a purely stylistic choice and one should not edit a post solely for that.

3

If there's other stuff that needs editing it's fine to do that along the way, but you shouldn't edit posts just so they conform to your typographic preference.

John Rennie

I'm relaxed about @MAFIA36790 's editorial tendencies. He doesn't do it to old questions so it's not as if he's unnecessarily bumping questions.

I probably should use \mathrm in differentials but I generally don't consider it important enough to make it worth the effort.

ACuriousMind

@JohnRennie Well, you are relaxed but other people might not be. In general it is best to not edit other people's posts unless there is something that needs fixing.

If I saw those edits in the suggested edit queue I would reject them as "no improvement".

dmckee

For writing papers and supplementary notes for my students I use the physics package. \dd{x} gives me an upright differential d. Quickly.

ACuriousMind

17:18

I mean, I also much prefer $\mathrm{d}$ for the differential because $d$ is a variable to me, and I do edit that into posts when I'm already doing other stuff. But just doing that is not worth the edit.

dmckee

Though now that I am starting to write about thermal physics I'm suffering from the like of an inexact differential command in that pack.

Danu

Why don't you define some custom commands?

ACuriousMind

@dmckee What's wrong with the usual $\delta$?

dmckee

I've cloned and modified the author's code and intend to submitt the result back.

@ACuriousMind The authors of the primary texts I'm using like the barred-d notation. And I try to give the students some consistency in this.

@Danu Well, I have. But to my mind the point of a package like that is to collect all the commands into a uniform set.

Danu

@dmckee To me, that's the point of a preamble.

I use many packages in each document.

ACuriousMind

17:22

If I ever had to write a thermo text I'd just call the "inexact differentials" the 1-forms that they are and ditch the strange notation that looks as if it's the differential of something but not really entirely :P

dmckee

@Danu This complain is a bug report, but like all great bug reports to open processes it's nice to have a complete solution to the problem before submitting.

@ACuriousMind I wish. My students would melt under the pressure if I tried to introduce a new mathematical notion at that level.

John Rennie

@ACuriousMind do you remember the discussion you, I and Daniel had about vacuum fluctuations the other day? If so I wonder if you think it's worth me writing that up as an answer to:

13

Q: Are vacuum fluctuations really happening all the time?

In popular physics articles and even some physics classes I've been to, the vacuum of space is described as being constantly full of quantum fluctuations. Supposedly, all sorts of particle-antiparticle pairs at all scales are constantly appearing and disappearing. We end up with a mental image ...

quantum-field-theory vacuum

ACuriousMind

@JohnRennie I don't have any strong feeling about that. The answer that vacuum fluctuations/virtual particles are not what people think they are is on physics.SE already in dozens of places in various forms, and Frederic Brünner's answer there also says "that's not what happens" to the "particles out of vacuum" picture. What would you really add that is not there?

John Rennie

Hmm, yes. I guess what I'd be contributing is explaining what we mean by a particle in QFT. It then follows immediately that a superposition of Fock states fluctuates in the sense of a measurement of $n$ returning a random value.

Then we wave our arms around and say the interacting vacuum is a bit like a superposition of Fock states.

Loong

17:58

@ACuriousMind Nevertheless, this is an international standard and not just a personal choice.

ACuriousMind

18:37

@Loong It is? I've seen so many papers and books that happily use $d$ for the derivative I'm not sure in what sense it's a standard

Loong

@ACuriousMind In particular, it is stipulated in ISO 80000-2:2009 Quantities and units – Part 2: Mathematical signs and symbols to be used in the natural sciences and technology.

However, textbooks have a tendency to ignore such standards.

Many textbooks still use “amu” or “a molar volume of 22.4 l/mol at STP”.

Slereah

Because who gives a damn about ISO standards

Especially for such matters as "the use of italics"

So apparently there is some paper on the chronology protection that lists papers by Banach on solving PDEs on multiply connected surfaces

Good old Banach

but this brings the following question to mind

What's the universal cover of the plane with a handle?

24

A: Homotopy Groups of Connected Sums

The 2nd homotopy group of a connect sum is fairly reasonable to compute. $\pi_i X$ is isomorphic to $\pi_i \tilde X$ provided $i \geq 2$ and $\tilde X$ indicates any covering space of $X$, so we might as well take the universal cover. By the Hurewicz theorem, $\pi_2 \tilde X$ is isomorphic to $...

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