The h Bar - 2019-05-25 (page 1 of 2)

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12:00 AM

:O did you just call spivak "trash"?

yes

if i could rate that book in the negatives i would

that was the most rushed introduction to manifolds i've ever seen

have you tried any other of his textbooks?

nope. that said, supposedly the one i used is his first and often considered his worst, so he should have better ones

but not really sure what I should be looking for

one of them is just kind of named "calculus"

not sure what you want if Spivak is trash to you

^

12:04 AM

have you read that one in particular?

This review sums up my complaints pretty well

no, I think it's a bad book

@RyanUnger so do you have any good recommendations

what exactly are you trying to learn

basically just get an introduction on manifolds and basic diff geo concepts. I've heard they have uses for analytical mechanics so that sounds interesting

or something useful for GR

if you wanna do GR you're gonna have to learn some PDE

I don't like recommending math books to physics students because I'd have to recommend 20

Do you know general ropology?

12:12 AM

Hmm, well we have an undergrad PDE class here but it's only based on 4 specific ones (e.g. the heat equation, etc.). The graduate version is apparently better but requires a two-year analysis background, which I don't have yet.

where are you

We've been over this, I'd prefer not to say

ohhhhh now I remember

@RyanUnger Only the basics, from Rudin's principles of mathematical analysis

I have a good friend coming to do a post doc there next year

He can really help you out

He's big on helping motivated undergrads

12:14 AM

welp we do have a math help room full of grad students, but that's closed now. I'm just looking for something to read over the summer

Ok if you know Rudin then you should read Lee's smooth manifolds.

Rudin does integration in multiple variables right

@RyanUnger Well I've dealt with Spivak's explanation on that, which is hopefully sufficient

have you seen this?

Truth be told we don't use Rudin here, I've just been reading it since I won't be able to take Analysis II until the spring

@skullpatrol that's too many books

you don't need 7 books to learn what a manifold is

12:18 AM

@RyanUnger I'll look into that, thanks

1:01 AM

Do photons bend spacetime or not? - I thought mass was the requirement for bending space-time. And I would've thought rest mass to be mentioned (not that I know what that means). Why isn't it mentioned, and what does bend space-time?

no, energy and momentum do the trick

@Mazura In GR other things do as well, such as energy, momentum and pressure

2 hours later…

3:26 AM

@Mazura photons are the historical name for "spacetime fabric waves". modern experts still cant quite fully grasp that photons are a kind of epiphenomenon of the interaction of light + matter (atoms). atoms quantize the waves so to speak (via energy transitions). we're nearing the point that this analogy will be more widely understood by scientists. it will help to definitively demonstrate the exact same effect in classical physics. who will be the 1st to reveal it? :)

3:46 AM

Hello, this is a quote from the chapter on conservation of energy from the first Feynman lectures book. "It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.", but don't textbooks generally define it as an ability to do work?

Yes, they do define it that way.

sorry, I missed that?

4:03 AM

out of curiosity, can you give an example of your understanding of that statement? @skullpetrol

no :)

he's basically stating a definition.

no, textbook's

i meant

Energy is the capacity to do work.

whose?

The work that the energy does.

4:08 AM

the work's capacity to do work?

to do itself*

Yes. It is a circular definition.

4:25 AM

In 60 Seconds - Is Energy Capacity To Do Work? - 'Is Energy A Real Thing' ?

1 hour later…

5:51 AM

@Mazura the curvature of spacetime is related to an object called the stress-energy tensor. Specifically there is a measure of the curvature called the Einstein tensor and this is proportional to the stress-energy tensor.

The stress-energy tensor treats mass and energy as equivalent and related by Einstein's famous equation $E = mc^2$. So anything with energy bends spacetime in the same way that anything with mass does.

6:43 AM

i gotta feeling this question is gonna attract a lot of attention :-/

6:59 AM

The PARTICLE DATA GROUP announces:

The 2019 partial update of the Review of Particle Physics
is now available on the web at

http://pdg.lbl.gov/

Constantine Black

7:28 AM

@Slereah , @knzhou , any other: Morning:
Regarding my question on whether QCD phase diagrams and transitions, ultrarelativistic ion collisions, are good for one to work in terms of current research and employment, I didn't mean something full of gold. But do you know if there is research or work to be conducted in contrast with- let's say- other fully theoretical fields( eg quantum gravity). That is stuff that are both theoretic and applied on some level, maybe with some computational side also.

An example would be this: thphys.uni-heidelberg.de/~berges/index.htm

physics.stackexchange.com/a/481837/8563 (10k+ link) @Chris it's uncommon to see a moderador in the LQP queue nowadays. Isn't it better to just leave these questions to the queue's standard flow? It carries a bit more weight if there's six community members saying "look, just no", I think.

8:11 AM

12

Q: Massive user removal in one night?

I woke up this morning to find that I suffered massive drop in points across network because of lots of users were removed . What happened to those users? Was there a database cleanup?

discussion support deleted-accounts

Murray Gell-Mann gave structure to the subatomic world (and passed away this Friday).

8

:-(

RIP

F

2 hours later…

10:00 AM

@EmilioPisanty dammit, I told him not to open the box.

10:17 AM

@JohnRennie wait, what? What box?

@Slereah are you handing out grades to news stories?

user image

@EmilioPisanty I feel like it's a Schrödinger's cat reference, but I'm not sure where to go from there

@Slereah pfffffffff

Kids these days

@ACuriousMind huh. My first reaction was Pandora's box, which made even less sense

Or maybe John mailed a letter bomb to Gell-Mann.

It's a mystery

10:51 AM

@ACuriousMind the world may never know

user image

(from mathwithbaddrawings.com/2019/04/24/…)

user image

That one is also brilliant

while we're at it; may I post one of my favorite Gell-Mann interviews?

Murray Gell-Mann - Einstein (33/200)

11:22 AM

@ConstantineBlack Every subfield has work to be conducted, duh...

QCD stuff is another one of those ~500-1000 person subfields. It doesn't seem to have noticeably better or worse job prospects than its neighbors.

The fact that there are computers involved doesn't change things, almost every subfield uses computers.

If you want to decide whether to do it, you should just see whether you're actually interested in it. If you want better job prospects you need to generally move a lot lower in energy than QCD.

@EmilioPisanty : Murray Gell-Mann at NY times.

2

2 hours later…

1:11 PM

@ACuriousMind yes, it was an attempt at a Schrodinger's ~~cat~~ Gell-Man joke. A failed attempt. Oh well.

1:32 PM

@JohnRennie how obvious is it to joke at the bid to get away from unelected European bureaucrats now needs to go through an unelected prime minister?

A high-rep user was deleted. He was active on many sites. It is so sad :-( And not only because the 60 rep I've lost network-wide.

Particularly given bbc.co.uk/news/uk-politics-48395211

you're not alone @peterh

Asking for a friend

The friend is me

hello, does anybody know what is the physical interpretation of negative quantum conditional entropy? Positive conditional entropy means that looking at one of two subsystems you can learn something about the other, zero conditional entropy means that the two are uncorrelated, what does negative mean? That looking at one subsystem afterwards you know less about the first? is there a criterion to know whether a state will have negative quantum conditional entropy?

Novalium Company

1:41 PM

I got drunk for the first time yesterday :P

I will never drink again in my life.

Never is a long time.

Novalium Company

Never.

@peterh 60 rep? You were lucky. I lost 550 rep just on the PSE.

@user2723984 Wikipedia says: "The negative conditional entropy is also known as the coherent information, and gives the additional number of bits above the classical limit that can be transmitted in a quantum dense coding protocol. Positive conditional entropy of a state thus means the state cannot reach even the classical limit, while the negative conditional entropy provides for additional information. "

@JohnRennie It was a user that had cast many votes across the network, see meta.stackexchange.com/q/328642/263383

@EmilioPisanty the leading candidate, Boris Johnson, is utterly unprincipled and interested only in his own status. It would be funny if I wasn't British.

1:46 PM

@ACuriousMind Yes, I read that sentence but it was somewhat enigmatic. I know about dense coding, I was looking for a more hands on guessing game interpretation that yielded some entanglement criterion. Entanglement means that the conditional entropy is nonzero, but what distinguishes a state with positive or negative conditional entropy?

Novalium Company

What is your first experience getting drunk guys?

@NovaliumCompany Never again...

maybe since classical distributions only have positive CE negative is something that violates Bell's inequality?

Novalium Company

@skullpatrol Wut?

that's a classic video pal

Novalium Company

1:51 PM

@skullpatrol What message does it imply?

Listen closely to the words.

You swore "Never again" the time before.

and the time before that...

Novalium Company

Oh I get it.

Alcohol is addictive, so I stop now or never.

I stop NOW!

@user2723984 I don't think there's a straightforward "guessing game" interpretation of the negative conditional entropy. arxiv.org/abs/quant-ph/0505062 explains in more detail what the Wikipedia quote is trying to say, but it's still a bit technical, of course. Also, non-zero conditional entropy does not necessarily mean an entangled state - classically correlated systems also have non-zero conditional entropy

It is also not only the negativeness in which the quantum conditional entropy exceeds the bounds of the classical one - it also can be larger than the classical upper bound

@ACuriousMind thanks, I'll have a look at the paper

user280247

when an atom is irradiated with X rays or any high frequency radiation of the sort, does it interact or is unperturbed?

2:01 PM

Depends on the atom :P

user280247

(forget about the nucleus) XD

Many of the heavier metals are perfectly fine with absorbing X-rays by punting an electron into a higher orbital.

user280247

Okey, you're right from core->outer shell transitions, but let's forget it

user280247

The point is: is it possible to absorb just part of the incoming photon energy?

Sure, e.g. by Compton scattering on the free electrons in a metal

user280247

2:06 PM

and for an isolated atom?

That sort of scattering can happen for the outer-shell electrons as well

user280247

it's an interesting effect, I should read about it, thanks...

user280247

But still want to insist in this point:

user280247

suppose the spectra of hydrogen. Then it is irradiated it with a non-matching frequency

user280247

Is there any interpretation as to why atoms do not absorb light in this case?

2:09 PM

Because there is no mechanism for them to do so

user280247

Why they can't absorb just part of the radiation?

user280247

Is it a wrong question?

Where would the energy go?

user280247

to an electronic transition

The electron orbitals have discrete energies. The atom cannot absorb photons whose energy does not match the energy difference between two orbitals

user280247

2:11 PM

yes but is it wrong to think the atom as absorbing just a fraction of the photon energy?

user280247

Something like this:

user280247

A + hv 1-> A* + hv2

user280247

v1 is a mismatching frequency, but the atom just take part of that, and release the rest

user280247

still energy is conserved

I repeat, the process in which the photon is not absorbed but only its energy decreases is called Compton scattering

user280247

2:14 PM

But the photon is absorbed

@santimirandarp No, it can only be "absorbed" if there is an electronic transition matching it. That's what "absorbing" means

user280247

I think I cant explain myself, I think on this:

user280247

photon energy= hv= hv1 +hv2

user280247

if the photon can be thought as composed of different frequencies, or something alike, then the atom can absorb part of the energy, the one matching energy level differences

user280247

(I know that possibly the equation is not valid)

2:18 PM

@santimirandarp There's the problem. Photons are indivisible quanta, you can't "split off" energy from them. A photon is not "composed of frequencies", it has a single frequency. That you only get the discrete package that you can absorb or not was the observation (in spectral lines and the photoelectric effect) that sparked quantum theory in the first place!

That's just the way the world turned out to be

user280247

fine. Nice to see you speak about observations and experiments. Thanks...@ACuriousMind

user280247

But if we think of the wave nature of the atom in the same way we think in a chord of a violin or something

user280247

the chord vibrates in every frequency, although only some of them persist on time, isn't it?

@santimirandarp An ideal string can vibrate in a single frequency just fine (it's just the imperfections of reality that get in the way), but in any case such analogies tend to not be very helpful when thinking about quantum objects

user280247

it seems strange, a chord should vibrate...

2:30 PM

Again, it is the photoelectric effect that yields an easy demonstration that photons really come with a single frequency and energy: If the energy corresponding to the frequency of the light incident on the metal is above a certain value, the effect happens (and the maximum kinetic energy of outgoing electrons is exactly the different between the energy of the incident photons and the energy necessary to exit the metal), otherwise it does not.

2:47 PM

I suppose people know they lost rep because they check the green number thing in the user bar?

I have my suspiscion that a lot of questions about "what is that phenomenon really about" is that deep down we all think particles are just little billiard balls

2

that just act weirdly

@ACuriousMind I don't know enough about QFT but can't you have photon wave packets with a spread of frequencies just like with massive particle?

@Slereah they're not!?

Yes you may

Actually all physical "photons" are pretty much that

@KyleKanos Probably, or because they obsessively visit the reputation tab on their profile ;P

2:48 PM

since single wavelength photons aren't actually part of the Hilbert space

so the "violin chords" situation described by santimirandarp isn't so outlandish

@KyleKanos It is a hard thing to get rid of

Which is weird because it took humanity thousands of years to even consider that things weren't just a continuous medium

@ACuriousMind I don't do either :|

@user2723984 Yes, but you should think of such packets not as a single "photon" but rather as a superposition of several photons

user280247

I can continue making questions until tomorrow :(

user280247

2:49 PM

so confused

I think the best attitude towards physics is to just consider it as a blackbox

Measurements go in

Future measurements come out

All that is in between, the reality isn't really relevant

Or maybe not, but it's like a wave packet in position space: When you interact with it in a way that requires a definite value of energy, it will appear in one of the definite energy states (or at least very sharply localized around that)

@ACuriousMind mmh I can see what you're saying, but I never thought about electron wave packets as multiple electrons

I mean it is fun to think about but in the end it's not terribly relevant

@user2723984 Yeah, I realized that just now, that was bad wording on my part.

user280247

2:51 PM

@ACuriousMind in ph effect any photon with v>v metal takes the electron out, but this can not happen in orbital transitions?

@Slereah you're crossing the mathematician point

Just checked, my green number thing says 453. Last time I accidentally pressed it was like 2 months ago

@user2723984 Mathematicians don't even have to worry about the measurement part

Also if you think mathematicians don't debate about what is "real"

You are sorely mistaken

user280247

@Slereah but we need models, pictures on our head, to think. Well maybe not everyone, I don't know

@santimirandarp It can - the unbound continuum states are legitimate targets of electronic transitions! But still the entire photon is absorbed, the excess energy now kinetic energy of the free electron

2:52 PM

@ACuriousMind so you're saying that if a photon wave packet interacts with an atom, a measurement occurs and the photon is either absorbed or not absorbed depending on the result

but never partially absorbed and "some frequencies" go away

user280247

@user2723984 :)

That's why high-energy radiation is "ionizing" - this is precisely the process by which the atoms lose electrons

@santimirandarp Sure

But I don't think it's necessarily important to wonder whether those models are real

whatever that means

@Slereah I was joking but I pretty much agree

Well you kno

user280247

2:54 PM

@Slereah Oh yes, I agree, 'reality' is a model too, at least when we speak about it

It's one of those topics that come around a lot

And aftet you've been in physics community for a while, it does get a bit old!

@user2723984 Yes. Of course, where and when the "collapse" occurs depends on your particular interpretation, but the end result is that either you detect the photon afterwards or you don't, corresponding to it being absorbed or not

user280247

@ACuriousMind I'm quite lost but thanks for the patience

user280247

Oh yes I see

It's like what's his name said

You never really understand mathematics

you just get used to it

2:56 PM

Usually that one is attributed to von Neumann

Let's attribute it to me instead

@ACuriousMind of course what really happens is that we go to the Church of the larger Hilbert space, the photon and the atom entangle in a superposition $\frac{1}{\sqrt{2}}(|0\rangle_\mathrm{p}|1\rangle_\mathrm{a}+|1\rangle_\mathrm{p}|0\rangle_\mathrm{a}) $

Stigler's law and all

It is mildly weird I think that so many people are trying to hold onto classical mechanics so hard as the "real" thing when classical mechanics took so long to be accepted as something that made sense

@user2723984 Yup. The "interpretation" part enters when you try to determine when and how exactly this entangled state is disentangled again.

it's not like it's a particularly intuitive thing

moreso than quantum shenanigans certainly

but still

user280247

2:58 PM

@Slereah yes it is

I'm not really sure my monkey brain really believes that things in motion stay in motion

@Slereah We are trained in school for years to see Newtonian mechanics and other parts of classical physics as "intuitive", and to believe we understand it. It doesn't surprise me that many of us try to hold on to that "intuition" that we worked rather hard to get.

@Slereah Is your monkey brain Aristotle?

Even Aristotle's mechanics isn't that intuitive

user280247

I think newtonian mechanics are intuitive

I wouldn't go as far as to say that classical mechanics is intuitive just because it is taught to us for years

3:00 PM

Also the obvious thing is to teach QM to children

you can actually, you know, throw a ball

@user2723984 Well sure, but that's also true of aristotelian physics

or a disk on the ice to convince yourself that it only stops because of friction

user280247

at least the law of action reaction is intuitive

Aristotelian physics is roughly true

3:01 PM

I'm not saying Newtonian physics is easy to come up with, but it is easier to accept

@santimirandarp And yet many people are confused about how it applies to a book lying on a table, as evidenced by several popular questions on our site :P

Well it's easy to say that now

You should look at the debates of the era

user280247

Oh I might have done one of those

@ACuriousMind Wouldn't the table get tired of exerting all that reaction force???

@Slereah Apparently not! Smells like there's some way to get free energy hidden there!

3:03 PM

That reminds me of those questions about getting energy from the Casimir effect

"Can we get free energy fr-" "No."

When any energy put into a Casimir plate is basically the same as a spring

You put energy to put the two plates together, that energy is stored in there

then they are pushed apart

Exciting!

user280247

goodbye guys! thanks

@JohnRennie I'm not British and I still don't find it funny. Boris was mayor when I lived in London and it already stretched belief. Him as a leading contestant is mind-boggling.

3:21 PM

@ACuriousMind I think you'd like Kingdom Come Deliverance

it's sloooooooow :P

@JohnRennie Who were deleted, could be found by using the SEDE, to compare the current PSE Users table to the one what we will have after the next SEDE sync. Fortunately, we will have a new SEDE sync in 6 hours (normally it happens weekly). The data of the removed user could be found using the archive.org backups. Although probably it won't be wise to advertise the results.

No, the next SEDE sync will happen in 11 hours. I will soon know it.

@RyanUnger Heh, I've been eyeing it but my poor 660 is the minimum spec, so I don't trust to be able to get it running

I should get a new computer

@ACuriousMind Oh it's a bear to run

It destroys my 1080

It's the same engine as Crysis actually

Hey @RyanUnger

@ACuriousMind One thing you might like but I hate is that you basically can't save

3:27 PM

\o

also the quests are really cool because while they tell you want to do, you can just do something else and it'll work out. like an actual RPG :P

@Slereah hi

Non-linearity and an old-school attitude to saving? Count me in!

it's really stressful. at one point I wasn't able to save for like 45 minutes and had to kill bandits with no training

Please read this link. It says physicsist found shape of electron, any comment or update by you people? medium.com/predict/… — Anubhav Goel 22 mins ago

2

::sigh::

here we go again

4

@ACuriousMind also you have to learn how to read

you're just a peasant...not some savior or prophesized hero

like your guy doesn't know how to read and you can't train in alchemy until you can read

3:32 PM

@ACuriousMind any thoughts on refunding that bounty?

no money back guarantees allowed :P

@EmilioPisanty so far, I think the only cases in which we have ever refunded bounties was when the question was actually off-topic, i.e. when the bounty blocked the ability to close the question. I agree that the reason for this bounty is not a very good one, but, well, users are free to offer terrible bounties, I guess?

@ACuriousMind it's more the fact that any answer addressing the bounty reason would be off-topic for the thread

that's a delicate enough thread as it is

we don't want it to accumulate a bunch of off-topic answers (which might not even be wrong!) that are then hard to clean up

Right, turning one question into another is not a valid use of bounties.

@ACuriousMind exactly

it's (effectively) just hijacking the existing question's score and trying to use it as a spotlight for a separate discussion

not that the bounty-setter is to blame here, they just fell for some awful coverage

but that's the structural role that the bounty is playing in this specific instance

3:41 PM

I have refunded the bounty now (that's after all a reversible decision) before someone answers in the hope of getting it.

@ACuriousMind currently harassing refugees for the local lord and robbing graves. not very heroic :P

Though, come to think of it, @EmilioPisanty, you're just hoping for a new question about that so you can get another HNQ answer in railing against uncritical reproduction of press releases by media, aren't you? ;)

@RyanUnger An RPG in which you can't rob graves isn't a true RPG :P

@ACuriousMind ah but it's illegal in this one

@ACuriousMind I'm not

I have to figure out how to get the hangman to look the other way

3:43 PM

I do want to rail against uncritical reproduction of press releases

but I'd like that kept of HNQ, thank you very much =)

I think my best bet is sneak in the back and distract the dogs with some meat

@EmilioPisanty I'm willing to be that if there's a question about that paper, it will go HNQ

@ACuriousMind ugh

Not that I'm any more excited about that prospect than you are, mind you :P

have you seen the altmetric news listing?

3:46 PM

what article are you talking about

nope

user image

altmetric.com/details/60838462/news

well does the paper claim that

@RyanUnger how so?

"We are able to not only map the shape and orientation of the electron, but also control the wave function according to the configuration of the applied electric fields,”

@ACuriousMind wait can you even access papers any more

3:50 PM

@RyanUnger where exactly is this in the paper?

@RyanUnger Legally, only the ones on arXiv :P

@EmilioPisanty I'm quoting a quote from the Vice article

@RyanUnger that's not a paper

@EmilioPisanty eurekalert.org/pub_releases/2019-05/uob-tgo052319.php

it's from a press release by one of the authors

@RyanUnger also not a paper

@RyanUnger also not a paper.

3:51 PM

oh ffs

I don't know how physics works

there's a difference between papers and not papers

4

idk haven't you seen references like [private communication]

Peer review is the evaluation of work by one or more people with similar competences as the producers of the work (peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer review methods are used to maintain quality standards, improve performance, and provide credibility. In academia, scholarly peer review is often used to determine an academic paper's suitability for publication. Peer review can be categorized by the type of activity and by the field or profession in which the activity occurs, e.g., medical peer review. == Pro...

the positive mass theorem was that way for 35 years!

it's useful, y'all should try it.

3:51 PM

and it was cited hundreds of times

the paper is gibberish to me so I can't make a determination

@EmilioPisanty I don't get it. There's 3 equations in the whole paper. Is it a numerical study?

Or experimental

the PRL? experimental

oh there's a phys rev B paper

@RyanUnger that's what I was going to say - if you want numerics, the PRB is the place to go

if you want an accessible explainer that isn't awful, see the Physics piece, physics.aps.org/articles/v12/56

"secondorder degenerate perturbation theory"

giving me flash backs to QM2

@EmilioPisanty It seems to me like that article is saying that knowing the parameter of the quantum dot is equivalent to knowing the wave function of the electron

@RyanUnger you can deduce the eigenfunctions of the potential if you measure it

but that's not the same as measuring the wavefunction directly

4:01 PM

sure you can always do that

@RyanUnger and the shape of the wavefunction is a completely different object to the shape of the electron

I'm clearly not understanding the claim. What is VICE saying was measured

@RyanUnger yes, you can do that, but there's nothing new about it. We've been measuring potentials and deducing the shapes of wavefunctions for the past 80 years.

Or should I ignore all of this

@RyanUnger that would be my recommendation

4:11 PM

::sigh::

40 mins ago, by Emilio Pisanty

here we go again

:-)

Here I go again on my own

@RyanUnger Hawking had a reference to a private communication or upcoming paper about wormholes having to be formed in pairs to respect parity

I don't think he ever published it

And now it never will be

Who do you respect more @Slereah Hawking or Gell-Mann?

I've never read a Gell-Mann paper

Although of course I have previously mentioned here the weird stuff I've heard about Gell-Mann and the military

Which is very mildly corroborated by some things I've read but absolutely no details

I am very curious to know where it originates from

Though I suppose it would be in poor taste to call him a war criminal the day of his death

4:28 PM

Yes.

4:38 PM

user image

4:50 PM

@RyanUnger There probably should be some kind of consensus understanding of what kind of citation activity on a publicly available while-paper is to be considered equivalent to "peer review".

But I'm not sure what that would look like.

This is especially important in the age of on-going Elsevier boycotts.

@SirCumference I believe that is called an "effective theory".

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