The h Bar

0celouvskyopoulo7

01:31

hello?

0celouvskyopoulo7

01:46

@SirCumference I see you

0celouvskyopoulo7

02:36

oil rig workers make 6 figures!

dmckee

03:22

Anyone want to venture an opinion on the nomenclature of solid-state physics?

In particular, is it reasonable to define a phonon (in the context of a simple harmonic lattice solid model) as "an excitation of a normal mode"

user228700

04:46

@JohnRennie Oh, I slept all too well, really; 10 hours straight! Nobody can accuse me of not having rested well enough. On the upside, I do feel much better this morning!

John Rennie

04:58

@Kaumudi.H good :-) You sounded pretty downbeat yesterday.

user228700

Not being able to move kinda does that to you :-P

John Rennie

@dmckee Yes, I'd recognise that as a good description.

@Kaumudi.H it all sounds rather dramatic. Is it muscle pain or joint pain? Or both?

user228700

@JohnRennie Both.

user228700

I wasn't kidding when I said "It hurts all over".

user228700

Yes, all over.

John Rennie

05:22

It sounds the sort of thing that must be documented on the web somewhere ...

I did have a look yesterday but didn't find anything.

dmckee

@JohnRennie Thanks, that sounded defensible to me, but I'm venturing away from my expertise. The next question of course, is how to I convince the students that it is reasonable to treat one as a particle, but I think I can avoid the issue for now.

user228700

@JohnRennie Neither did I :-/

user228700

I'd guess that my body is just prone to intense pain. Perhaps I will acquire Arthritis in the future; my mum has it. Hmm, is it hereditary?

John Rennie

@dmckee the obvious analogy is with QFT where the particles are field modes. Actually I suppose by comparison with QFT if the phonons are interacting then there aren't any well defined normal modes.

@Kaumudi.H I have (osteo) arthritis, i.e. the type you get just by being old (middle aged :-) and it isn't anything like what you describe. It's a localised pain in the joint and associated with swelling and redness.

dmckee

@JohnRennie Well, I'm doing (a cartoon treatment of) the thermostatistics of lattice solids tomorrow as the last material in my thermal physics course, so we're using non-interacting models.

user228700

05:27

@JohnRennie Oh, that's a bummer :-( Ah, hmm, I have no idea, then :-/

John Rennie

@Kaumudi.H apologies for the latency, I'm dealing with a dead server. However I have just restored it to life. Hooray for me!! :-)

user228700

Nice! :-)

user228700

Man, the rate at which these servers are dying seems to be increasing of late...

Secret

Well well well what do we have here...:
I just recevied one of those infamous predatory conference scams in my uni email.
This is why microsoft outlook sucks

John Rennie

@Kaumudi.H as the company grows there are more and more servers to look after and inevitably more failures. They are still pretty rare though. Less than one major failure per week, and that's with getting on for a thousand servers.

@Secret why is it Outlook's fault you got a scam mail?

user228700

05:40

@JohnRennie Ah, OK :-)

John Rennie

Anyway, it gives me something to do :-)

Secret

@JohnRennie In both UNSW and USyd outlook email systems, they are the ones where I receive the largest voume of misaddressed, scam and phising emails. I never had that problem in gmail

John Rennie

Outlook isn't the e-mail system, it's just the client. The problem is the mail server used by your university isn't filtering out all the scams. Harass your university sysadmins not Microsoft.

Secret

So it is likely the uni have not configure their scam filtering properly or something, especially there are many times where there are emails from another person with the same few letter names as I am get sent to my email

(Edit: Ok I see)

The most notable case from my experience is I receive various conference stuff from International Desalination Association (that one I recall I have check is a legitimate organisation), but a more detailed look at the recepient it is intended for, turns out to be another user with almost the same email address as me

Which is pretty bizzare since I never knew the existence of this organisation nor had anything to do with it throughout my uni life

Because it is so bizarre and I have yet to find out why I am getting all these misplaced emails, I have actually archive all of those email to deal with them later in the future. From them I learnt how whoever this person who use a similar email address as mine (assigned by the uni) is a member of this organisation and has some research involving silicon nanoparticles and is from China

In short, I suspect my old uni have config the email alias of students and staff incorrectly, resulting in given two email address where the alias have the first 3 letters matching, then both recipients get the same email

Joshua Lin

05:57

@Secret Wait are you from Australia somewhere?

Secret

Sydney to be exact

Joshua Lin

!!!! haha I'm from Eastwood

that was unexpected
came onto h-bar chat and saw UNSW and USyd and was hit by waves of homesickness

Secret

Ah nice

Secret

Anyway the signs that ring the alarm of that email is because 1) 2nd International Conference on biochemistry and 2) I was addressed as Dr. while I only just started my PhD this year

A quick google search found the predatory organiser Conferenceseries.com, and into the junk mail bin that email goes without being read

(afaik: No reputable conferences are that young)

dmckee

06:40

@JohnRennie Well, These days Microsoft pushes their cloud based system pretty hard. My place uses it.

The backend is not terrible (most of the spam is binned, most of the stuff sequestered by clutter is things from people with whom I have a relationship, but that I don't actually care about. I get one or two attack emails in a bad week, none most weeks.

But the client. That makes me pine for pine.

John Rennie

@dmckee the company I work for use the MS cloud mail as well, and I find it pretty good. But Secret's problem was his university's mail servers.

@dmckee I'm pretty certain Microsoft's cloud mail can be used with any POP3/SMTP client. I assume you use one of the later versions of Outlook - I use Outlook 2010 (part of Office 2010) and I find it unnecessarily fussy.

dmckee

@JohnRennie I imagine it can, but I can't install software on my work computer. Grrrrr!

John Rennie

@dmckee To be fair, Outlook/MS cloud mail is far more than just an e-mail app. It is effectively fully fledged Exchange in the cloud, with all the collaboration features that Exchange supports. So it offers dramatically more functionality than e.g. Gmail.

dmckee

So it the Office 365 supplied web-client for me. I hate web clients that hijack my keystrokes.

John Rennie

But it is a bit of a pain if all you want is an e-mail client.

dmckee

06:47

@JohnRennie Yep. I mostly don't use the collaboration features, though my colleagues do schedule meetings with it, so I have to use the calendar.

Which won't sync properly with the one on my android. Another grrrrr!

John Rennie

The web client is a remarkable bit of coding, but it is even clumsier than the later versions of Outlook. But that's probably inevitable when you're trying to shoehorn an Exchange client into an HTTP connection. That it works at all is pretty amazing :-)

dmckee

Or maybe it will now. Someone said they go t it working, and I haven't tried for more than a year.

John Rennie

There is a version of Outlook for Android. I have never used it because my personal mail is Gmail, but I'm told it is pretty good.

Secret

Is office 365 the same as the outlook MS cloud mail thingy you guys mentioned above?

dmckee

@JohnRennie I use it. It runs on a virtual machine of some kind and takes ages to start. But I can check my mail and look at the calendar when I am away from my desk.

Yeah.

That and much much more. The whole productivity package over the web.

Secret

06:53

ok, then it must be my unis don't know how to bin emails properly with email addresses

dmckee

Come to think of it the Outlook app is also one of two things on my phone that don't respect the quiet hours setting. I really, really don't care to hear that I've gotten an email at 2 in the morning. Just quit already.

Secret

I have 4 gmail accounts each for different purposes. (mostly to plug gaming online forum registeration so they will never spam me with their ad on my main email account)

(although technically I only have 3 cause I forgot the password for the last one and the account was deleted due to extensive inactivity ages ago)

John Rennie

07:17

@Secret I've lost track of the Microsoft product naming. I think they use Office 365 for all their cloud office apps including Outlook. The URL for the cloud version of Outlook is outlook.office.com so it's part of the Office 356 web stuff.

Slereah

Do they just increase the version number at every compilation?

John Rennie

@Slereah yes, like most software there is a multi part version number a.b.c.d and the last part is incremented with each new build. Obviously the first two numbers are only changed with a new consumer release.

Slereah

07:43

That's a release every three weeks for 20 years

John Rennie

07:57

Ah, my facetious comment detector failed to trigger. The primary version number of the latest Office is 16 I think i.e. 16.b.c.d for some values of b, c and d. The 365 in the name is marketing fluff.

Slereah

09:20

Aaaaah I am full of caffeine

Give me a physics to do

caffeine and no breakfast

Phase

09:38

huh

Is the short range repulsive nature of two atoms close together really because of the electrostatic force?

ACuriousMind

@Slereah Here's a physics: Solve inflation.

Phase

I always thought it would be pauli's principle

Slereah

@ACuriousMind Fix prices at the state level

Bam

Next problem

Phase

@Slereah how about a light maths exercise? Prove the non trivial zeroes of the Riemann zeta function all have real parts 1/2

ACuriousMind

@Phase The usual lore is that it's both repulsion of the electrons and Pauli exclusion

Phase

09:41

Which is stronger at the very beginning of it being repulsive?

Because I assume at close range Pauli is stronger

Punching the electrons away etc

John Rennie

11

A: Is the electromagnetic force responsible for contact forces?

Suppose you take two helium atoms and try to push them together. There will be a short distance repulsion and we'd normally describe this as the Pauli exclusion force, so it does indeed sound as if the exclusion principle generates a fundamental force. However suppose one of the He atoms has bot...

Phase

ty

ACuriousMind

@Phase Depends. You need to compute the electrostatic repulsion and the degeneracy pressure for your specific situation and see which is stronger

I'm reluctant to pronounce any of them universally "stronger" than the other - it varies with the actual situation

Phase

Can I make an egregiously unjustified guess?

ACuriousMind

I'm pretty sure you can, I'm just not sure what you think it will achieve :P

Phase

09:44

eh, nevermind actually just realised a hole

John Rennie

@Phase it's a somewhat subtle argument. If we have an interacting system then as we confine this system the energy spacing of its levels will increase. The exclusion principle tells us that electrons must occupy some of the higher levels, and therefore the total energy increases.

So you need both the interaction and the exclusion principle.

In the case of two atoms the interaction is electromagnetic.

Phase

Are the covalent bond energies of compounds related to the energy needed to take the atoms to a configuration where the electrons can overlap?

Balarka Sen

That's a fun fact and fun explanation. I didn't know atoms got repulsed in when they're close.

Slereah

that's why u don't drop through your chair

John Rennie

@BalarkaSen Really? We'd collapse into black holes if that wasn't the case ...

Balarka Sen

09:48

We would?

I know 0 physics so maybe I'd be fine with that :P

John Rennie

@Phase that's rather vague. Are you thinking about the energy change involved in bringing the two atoms in from infinity towards each other?

ACuriousMind

@BalarkaSen Apperently, you're the only person who hasn't wondered why matter doesn't go through other matter, then ;P

@JohnRennie Ehhhhh, that sounds a classic case of attempting to use physics to predict what would happen (turn into black holes) if physics was wrong (atoms had no repulsion).

Phase

@JohnRennie No I mean changing the electron configurations

John Rennie

@ACuriousMind well we know that if you make electrostatic interaction negligable compared to other forces you get neutronium.

Well, we think we know that :-)

@Phase that doesn't make sense.

Balarka Sen

@ACuriousMind My realm is abstract topos theory and dialectical spiritualism. I don't think in terms of matters.

ACuriousMind

09:51

I guess matter doesn't matter to you, eh?

Balarka Sen

Da.

John Rennie

@Phase As we bring two atoms together the electron wavefunctions continuously change as the potential due to the positions of the nuclei changes.

Phase

I was just thinking

wouldn't the energy required to lift one to the other from infinity be slightly influenced by the initial state of the electron? Like, if we have two Hydrogens with the same electron state, as opposed to two different ones [differing by spin or orbital]

actually

what even are the conditions needed with the electron configuration of two atoms to covalently bond

is there even one?

Balarka Sen

Aren't there particles which do not satisfy the Pauli exclusion (I think those are called bosons or something?)? Can those "go through" each other, whatever that means?

Phase

It's just linear superposition

like with light

Slereah

09:59

Yes, you can stack arbitrarily many bosons

That's why they can form classical waves

Phase

Light interference from two slits is an example of it

Balarka Sen

Ah, alright

Slereah

ie electromagnetic waves

Phase

otherwise they'd bounce off each other I suppose

and you'd get a gnarly pattern

ACuriousMind

@BalarkaSen Yes - light beams pass through each other without any interaction (to first order, at least)

Slereah

09:59

on the other hand, you can't really have macroscopic fermion waves

since you can stack two of the same momentum, at best

Balarka Sen

@ACuriousMind I see now. That's a nice way to think about stuff.

ACuriousMind

Just imagine if laser pointer beams would bounce off each other...

Phase

@Slereah Whoa hold up there, High school teachers favourite past-time is telling students about the debroigle wavelength of their dog

Balarka Sen

:P

John Rennie

@Phase suppose you have two atoms each with n electrons. Those n electrons sit in atomic orbitals in each atom, two per atomic orbital.

Phase

10:00

@JohnRennie thanks, makes sense

Slereah

@Phase how big is it

John Rennie

@Phase if you now bring the nuclei together you get a new set of electron wavefunctions called molecular orbitals, again each with two electrons.

Balarka Sen

It's really counterintuitive in light (no pun intended) of that that light is made of any sort of particles.

ACuriousMind

@Phase It's a pastime, not a past-time :P

Slereah

@BalarkaSen It's not

It's all a ruse

Phase

10:01

@BalarkaSen to my shallow knowledge, the only way two photons could interact would be either gravitationally, or if one of the photons decayed into an electron and positron, and the photon was scattered or something

ACuriousMind

@BalarkaSen The status of the photon as a "particle" in the ordinary sense is...debatable

Slereah

@Phase you have the non-linear interaction

John Rennie

@Phase populate those molecular orbitals with the 2n electrons and calculate the total energy. If the energy is less than the energies of the isolated atoms a bond will form. If the energy is greater than the energy of the isolated atoms a bond won't form.

Phase

@Slereah ?

Slereah

$$\gamma + \gamma \to e^+ + e^- + e^+ + e^- \to \gamma + \gamma$$

Balarka Sen

10:02

@ACuriousMind I always sort of thought of it like a formalism, to take account of the photoelectric effect and co.

Phase

@JohnRennie ok thanks man

Slereah

@BalarkaSen It's complicated

ACuriousMind

@Phase You can have photons scattering off each other - there's a four-photon interaction mediated by a box diagram with a fermion loop - but its amplitude is so small that you only see it with really high intensity fields

Phase

@Slereah true, I thought about that but I felt like it was already kinda cheating considering one pair production

John Rennie

22

Q: Validity of naively computing the de Broglie wavelength of a macroscopic object

Many introductory quantum mechanics textbooks include simple exercises on computing the de Broglie wavelength of macroscopic objects, often contrasting the results with that of a proton, etc. For instance, this example, taken from a textbook: Calculate the de Broglie wavelength for (a) ...

Slereah

10:03

EM fields are just vectors in the Hilbert space $L^2(TM, dx)$

Phase

@ACuriousMind That's over my head friend

Slereah

or something

I forget

for the free case, anyway

John Rennie

@BalarkaSen particles are surprisingly elusive things when you start looking for them ...

ACuriousMind

@Phase Then you may substitute what I said with "Through quantum magic, photons can interact with each other, but with very low probability" ;P

Slereah

It's a ray in a Hilbert space with some vector operator $\hat A^\mu : \mathcal H \to TM$

Balarka Sen

10:05

yeah it seems like that

I don't know shit about anything though, so I'm good. The bubble of ignorance is a very safe, warm place

Phase

I can't hear you over my safe space

Slereah

Also what we call "photons" in QFT is a lie

Photons are just modes of the field, they really don't correspond to particles

John Rennie

The radio is playing Hocus Pocus by Focus. Ah, nostalgia is a wonderful thing :-)

Slereah

sounds spooky

user228700

> Hocus Pocus by Focus

John Rennie

10:09

@Slereah isn't that true of any particle?

user228700

Nice :-P

Slereah

@JohnRennie Yes

Most of QFT is a lie

John Rennie

@Kaumudi.H if you like yodelling :-)

Slereah

What we call a particle experimentally is probably more akin to a soliton in QFT

Balarka Sen

So I guess an interesting question I could ask is, what happens in photoelectric effect at a particle-level? Why does electron get bounced off by photons?

user228700

10:11

@JohnRennie I've only ever heard yodelling on The Sound of Music and that wasn't half bad :-P

Swapnil Das

Electrostatic force of nucleus is overcome perhaps @BalarkaSen

Balarka Sen

We're looking at two different kinds of particles so we can't explain by Pauli exclusion as we would if we're bouncing off fermions by fermions ...

Slereah

@BalarkaSen It's Complicated

I don't think there's a non-perturbative definition

Balarka Sen

@SwapnilDas Eh? Electrostatic force between what?

John Rennie

@Kaumudi.H I bought all the Focus albums, and Hocus Pocus is the only decent track on all three!

Balarka Sen

10:12

@Slereah Huh.

ACuriousMind

@BalarkaSen It's boring: The electron absorbs a photon (or "unit of excitation of the electromagnetic field"), and the only place where its energy can go is into kinetic energy of the electron, so it shoots out of the material if the energy of the photon was higher than the potential that kept the electron inside the material.

Swapnil Das

@BalarkaSen Nucleus and valence e-

user228700

@JohnRennie Geez, that's a shame!

John Rennie

Focus - Hocus Pocus

►

user228700

@JohnRennie I'll mark it to listen to it a little bit later :-)

Slereah

10:13

Perturbatively, what we have is that we start asymptotically with the electron field and the EM field with a single particle state in the Fock space

ACuriousMind

In my understanding, it's not really meaningful to ask for more detail on what "absorbing" or "photon" means here - if pressed, one might write down some Hamiltonian and use perturbation theory to model all this, but it doesn't explain anything more, it just gives quantitative predictions to fit the qualitative description

Slereah

And we end asymptotically with the same thing, but the single particle states have different momentum

And then we have some function that relates the two

Called the S matrix

Balarka Sen

@ACuriousMind @Slereah Ah alright. Looks like serious stuff.

Phase

@ACuriousMind is a photon a physical entity⸮

Slereah

Or, if you want to talk about "real" particles, you start off with a coherent state asymptotically

ACuriousMind

10:15

@Phase You need to choose another irony symbol, I only see a unicode box there :P

Phase

Awh..

Slereah

$$\text{?`}$$

Hm

It's supposed to be upside down

Balarka Sen

"?" would be a fine irony symbol.

Slereah

$$\mbox{?`}$$

Phase

@Slereah i think it's actually only meant to be left-right reversed

Slereah

10:17

I think you might need a library

Phase

I thought it was reversed too until like yesterday

Irony punctuation

Irony punctuation is any proposed form of notation used to denote irony or sarcasm in text. Written English lacks a standard way to mark irony, and several forms of punctuation have been proposed. Among the oldest and most frequently attested is the percontation point proposed by English printer Henry Denham in the 1580s, and the irony mark, used by Marcellin Jobard and French poet Alcanter de Brahm during the 19th century. Both marks take the form of a reversed question mark, "⸮". Irony punctuation is primarily used to indicate that a sentence should be understood at a second level. A bracketed...

Shing

damn I forget about calculus... why $$\int \tan x\cdot dx = -\ln\cos x$$

instead of

$$\ln ( - \cos x )$$

if it is about absolute value

Slereah

Well, derivate both?

Shing

both gives me $$ \tan x $$

if I haven't make a mistake...

Phase

You have tho

d/dx -cos(x) is sin(x)

Slereah

10:21

$$-\ln(\cos(x))' = - [\cos'(x) \frac{1}{\cos(x)}] = \frac{\sin(x)}{\cos(x)}$$

Phase

so you'd get -1/cos(x) * sin(x)

Balarka Sen

The latter derivates to -tan(x)

Phase

which would give you minus tan(x)

Shing

oh oh

I see

thanks!

Balarka Sen

In any case you need to be careful about constant of integration.

Antiderivative (or indefinite integral) of a function is not really a function, but a set of functions.

Shing

10:23

okay, I got it, thanks for the remainder.

Phase

No problem, modulus division is my favourite

John Rennie

@BalarkaSen derivate - is that a verb? Shouldn't it be differentiate?

Phase

[BACKWARDS QUESTION MARK]

Balarka Sen

@JohnRennie I can verb anything if I want to.

John Rennie

@BalarkaSen only Americans are allowed to do that

... and Shakespeare

Shing

10:27

only Americans lol

Phase

@JohnRennie meanwhile physicists can adjective anything. Even an adjective. [cough] relativistically [cough]

Shing

you remind me of a joke about how many languages one can speak.

Balarka Sen

"get sammiched"

I also hope it was noticed that I just verbed "verb".

John Rennie

@BalarkaSen we were politely ignoring that :-)

@Phase relativistically is an adverb

Phase

shit true

I'll just stick to relativistic then

Balarka Sen

10:31

lol

grammar jokes need to be pulled off carefully man

Slereah

You know a meat restaurant is serious when their menu is printed on leather

Phase

I have a really good meme saved for something like this though

Slereah

All things must be made of beef

Phase

Doesn't quite apply here but it applies nearly everywhere in FB comments etc

Balarka Sen

good strategy eh

Shing

10:34

lol

Mano Plizzi

hey, anyone has some time to look at my newb question? physics.stackexchange.com/questions/330733/…

Slereah

11:20

Since the Bohm quantum theory involves non-local effects

Are there violations of the DEC from it?

or the FEC, more generally

Does the pilot wave violate the FEC, in some manner

That sounds like a question that would make Motl very mad

I should ask it

Slereah

11:35

"In general this doesn't work because $\nabla_\mu T^{\mu\nu}(\phi_B) \neq 0$ (in non-relativistic de Broglie-Bohm theory energy isn't conserved)"

whaaat

pwallden.gr/mp1006/Struyve.pdf

help

ACuriousMind

Once you delve into Bohmian mechanics, all help is too late for you :P

Slereah

I just wanted to bother Motl

He's doing a foliation of the spacetime to describre the Bohmian mechanics

this is the worst paper

He also never does what he sets to do

He never finds out how to describe the SET for Bohm

Is Bohmian mechanics even at all probabilistic for free particles?

The pilot wave equation doesn't depend on the particles themselves unless there's a potential

Kenshin

Yo

0celouvskyopoulo7

11:51

@ACuriousMind I looked back on the transcript and you were really suffering in that Sobolev class

Did you do anything else there?

Albert Einstein

Hello@Yashas

Slereah

"Bell instead preferred to call the particle positions beables"

0celouvskyopoulo7

Wot

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