The h Bar - 2019-11-03

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Aaron Stevens

03:35

0

A: In adiabatic expansion does the internal energy of an ideal gas decrease?

Suppose that I have a perfectly-insulated syringe closed at one end and a frictionless piston on the other. The syringe initially contain ideal gas of volume V. So right now the gas is exerting a force on the piston and there is something (maybe even you) supplying an equal but opposite forc...

Is my answer really that bad?

Scientist Smith YT

@JohnRennie That's one great idea! It makes sense that the Current wouldn't be harmful. Glass is a great insulator let alone the 100,000 ohms of the human skin. (I've measured my skin before and got up to 980,000 ohms) so I wouldn't be surprised if the resistance could be greater. The idea that guy had in the video was the capacitance factor. The conductivity of the near vaccume and the insulation of the glass then the skin being a conductor. (Of course in relation to each other)

@JohnRennie Would the capacitor idea be a good idea as well?

@JohnRennie This is in response to your reaponse to @Akash.B . Lots of times we shouldn't rule out things that can be possible. Laws and rules of physics are set by people that have proof that has been proven over and over again. But not every possibility has been put through trials and tested for that. And as society gets smarter and begin to have greater ideas some laws of physics that great people have set can possibly be broken.

@JohnRennie We should always keep our minds open to every possibility that comes our way. That way great minds like yours, mine and everyones else's can make better and more efficient ideas and things. A lot of things that I've learned and taught and even seen have changed what I originally thought. And in some cases I thought some things were impossible. But I kept my mind open. The space time and dimensions ideas blew my mind. I still have lots to learn too.

2 hours later…

More Anonymous

05:26

@Slereah Are you on physicsoverflow? If not I think this is the right stackexchange? physics.stackexchange.com/questions/511647/…

PM 2Ring

06:09

Declan Traill, the Energy Field Theory guy, has clarified his question a bit, and included the diagram and argument from his article; previously he just linked to the article. A couple of us have tried to encourage him to ask a clear focused question, and to not claim that SR is wrong, but he doesn't seem to get it.

However, I feel his question should be reopened so that a clear refutation can be posted. He wrote this question in response to Marco Ocram's suggestion in comments on a now-deleted answer that I linked here a couple of days ago.

More Anonymous

07:08

@AaronStevens I guess who reads it? To a pro perhaps? He seems well versed in this stuff

Aiman Al-Eryani

07:41

hi, I have a question where I'm sure I'm overlooking something silly. We know $\psi_0^*\psi$ is the probability density of a state at time $t = 0$. In time $t$, the wave function evolves by a unitary operator $\psi_t = U_t \psi_0$. The new probability density at time $t$ is then $\psi_0^* U_t^* U \psi_0 = \psi_0^* \psi_0$. This says that the probability density is constant regardless of the state. But I know this can't be for non-stationary states..

2 hours later…

ACuriousMind

09:20

@AimanAl-Eryani You need to be more careful about what objects you write down. Use bras and kets to distinguish states from functions! For any state $\lvert \psi\rangle$, the probability density is $\psi^\ast(x)\psi(x) = \langle \psi \vert x \rangle \langle x \vert \psi\rangle$. You only evolve the $\lvert \psi\rangle$, not the $\lvert x\rangle$, so this is not invariant under unitary evolution.

ACuriousMind

09:34

@EmilioPisanty How do you feel about formulae vs. formulas - showing off or just personal preference?

2

@Slereah Sorry for the late answer - the (anti-)commutation relations define a (super-)algebra, and you're looking for its irreducible representations. Stone-von Neumann shows that the CCR are in the lucky case where they only have one irreducible representation.

Aiman Al-Eryani

09:49

@ACuriousMind ah! of course. That was silly, haha. Thanks a lot!

More Anonymous

@ACuriousMind what is the usual wait time when you flag something for a mod? And how does the mod contact you?

ACuriousMind

@MoreAnonymous Depends on how difficult the flag is to handle. Some things will be dealt with as soon as any mod sees the flag, others will require that we first consult each other or do some investigation. The moderator will not usually contact the flagger unless it's necessary to resolve the flag.

More Anonymous

@ACuriousMind Ah ... How does he contact?

(if he does)

ACuriousMind

@MoreAnonymous Main tools are 'moderator messages' (private messages sent on the main site, mainly intended for warnings or suspension messages) or creating a private chat room only visible to moderators and the user.

More Anonymous

@ACuriousMind Also is this welcome for this physics stackexchange or stackoverflow

https://physics.stackexchange.com/questions/511647/covariant-version-of-the-cosmological-principle

@ACuriousMind Thanks

Slereah

09:53

@ACuriousMind how lucky are we in more general cases

ACuriousMind

@Slereah Unlucky, cf. Haag's theorem

Slereah

Is there a unique rep for the Z2 sort of operators

And if operators commute, is the total hilbert space the product of those hilbert spaces

ACuriousMind

@MoreAnonymous What would it do on SO???

Slereah

@ACuriousMind I mean I know, but I mean more simpler cases

I am wondering about the relativistic point particle case

The spooky ghost fields act on the Hilbert space Z2 it is claimed

More Anonymous

@ACuriousMind Sorry it should have been physics overflow :P Also if we're on the same page on the physics overflow ... Maybe it has a better chance of being answered?

ACuriousMind

09:56

@MoreAnonymous PO is entirely unrelated to physics.SE.

More Anonymous

@ACuriousMind Ah ... But is there a meta where someone asks for when to use which? In the comments I was recommended to go there

@ACuriousMind This means no cross posting violation?

ACuriousMind

@MoreAnonymous The canonical meta post for the difference between the two sites is physics.meta.stackexchange.com/q/6196/50583

@MoreAnonymous Whether you post on PO or not is completely irrelevant to anything you do on physics.SE. It is just important that you realize these are not "sister SE" sites like math.SE and MO or cs.SE and tcs.SE. PO is not an SE site at all.

More Anonymous

@ACuriousMind Cool nice hack to know :P

ACuriousMind

I'm not sure what you mean by "hack".

More Anonymous

@ACuriousMind Oh in the same sense of life-hack or bio-hack it's kind of a slang in this sense.

Don't worry I'll be more careful with this kind of english in this chatroom

ACuriousMind

10:09

I know what "hack" means!

I don't understand what's "hacky" about the existence of PO.

More Anonymous

@ACuriousMind Oh for someone like me it's like acceptable "misuse" and of "a rebirth of the untimely passed away Theoretical Physics SE" . It's like wait I get to double the chances of getting a question answered and no price to be paid - This is an amazing hack!

ACuriousMind

@MoreAnonymous Maybe also read Emilio's answer there. You should also know that Dilaton is one of the founders of PO and therefore may not present an impartial view.

More Anonymous

@ACuriousMind Seems to have a different value system and goal as opposed to PSE

2 hours later…

Emilio Pisanty

12:10

@ACuriousMind like any standard-but-slightly-fancy bit of language, there is a certain amount of showing off in 'formulae', but it's pretty minor

I mean

it's not like you're saying 'stadia'

ACuriousMind

@Slereah Wait, what? $\mathbb{Z}_2$ is not a Hilbert space. Do you mean $\mathbb{C}^2$?

Emilio Pisanty

but it's a bit worse that 'algae' or 'nuclei'

@ACuriousMind you sure?

$\mathbb Z_2$ is a vector field over $\mathbb Z_2$

ACuriousMind

@EmilioPisanty Hilbert spaces are by definition over $\mathbb{R}$ or $\mathbb{C}$.

Emilio Pisanty

12:48

@ACuriousMind I use other definitions =P

seriously, do you lose all that much by allowing finite characteristic?

I can see how the topological incompleteness of $\mathbb Q$ will be an issue in proving the topological aspects of Hilbert-space theory

but what results are true over $\mathbb R$ that break over $\mathbb Z_2$?

ACuriousMind

@EmilioPisanty Hilbert spaces are supposed to be complete normed spaces. You can't have complete spaces if the underlying field is not complete.

yuvraj singh

I do not this question is reasonable or not. But my firms ask that if we drop a photon, about 1km from Earth, under the earth gravitational field, what change will be in the frequency.. He said mass m =$hc/c^2$.i do not know wetter this question is correct or not.

Emilio Pisanty

@ACuriousMind is $\mathbb Z_2$ incomplete?

ACuriousMind

Ehhh...strike that

Emilio Pisanty

;-)

ACuriousMind

12:53

What I tried to say but horribly butchered is that completeness over finite fields is not interesting

Emilio Pisanty

that's true

but so what?

If you want to say that $\mathbb Z_2$ is not an interesting Hilbert space then I'm inclined to agree

but why can't it be a Hilbert space?

ACuriousMind

Ah, wait!

The definition of 'norm' already doesn't make sense for fields that are not subfields of the complex numbers

Because the norm is supposed to have $\lvert \lvert \alpha v\rvert\rvert = \lvert \alpha\rvert \cdot \lvert\lvert v\rvert\rvert$

Emilio Pisanty

@ACuriousMind ok. so?

ACuriousMind

@EmilioPisanty $\alpha$ is a scalar - so what is $\lvert \alpha\rvert$ here? The norm itself needs to be $\mathbb{R}_{\geq 0}$-valued.

Emilio Pisanty

@ACuriousMind does it?

ACuriousMind

12:59

@EmilioPisanty Sure, you want to be able to do inequalities with it.

Emilio Pisanty

eh

that's a bit of a cheap way out

but sure

ACuriousMind

I suppose you can define a "norm" that's valued in the underlying field instead, but what you then get is very unlike usual normed vector space theory

Consider that norms on complex vector spaces aren't complex-valued, either

Emilio Pisanty

@ACuriousMind no, the real problem is that the triangle inequality breaks

ACuriousMind

Because you don't have an order to do inequalities with to begin with if your field is finite!

Emilio Pisanty

when things are able to roll back into zero

OK, I'll take it

$\mathbb Z_2$ is out.

bring in the $p$-adic numbers =P

0

Q: What happens to the momenta of photons when light enters a medium

Light refracts as it enters a medium as the velocity decreases because the permittivity and permeability changes. So is there a change of momentum of a photon entering a refractive medium? And if there is a change of momentum, how is the instantaneous change of it justified? Also one more questi...

electromagnetism optics visible-light electromagnetic-radiation photons

leading to

Abraham–Minkowski controversy

The Abraham–Minkowski controversy is a physics debate concerning electromagnetic momentum within dielectric media. Traditionally, it is argued that in the presence of matter the electromagnetic stress-energy tensor by itself is not conserved (divergenceless). Only the total stress-energy tensor carries unambiguous physical significance, and how one apportions it between an "electromagnetic" part and a "matter" part depends on context and convenience. In other words, the electromagnetic part and the matter part in the total momentum can be arbitrarily distributed as long as the total momentum...

oh, boy, that page has some issues

ACuriousMind

13:05

@EmilioPisanty There is actually "analysis" with p-adic numbers, but the p-adic norm is still real-valued :P

Emilio Pisanty

> For example, when there exist dielectric materials in space,
>
> - Is the principle of relativity still valid?

> Even in free space, still there are some basic concepts to be clarified. For example:
>
> - Is there any photon-rest frame in free space?[14]

ACuriousMind

Yeah, that's...not an open question.

Unless you're using a non-standard meaning of either "photon", "rest" or "frame".

And that many of the citations there point to recent papers by C. Wang seems a bit...biased.

Wait, all of these questions are cited with Wang's papers. And the page is largely the work of a single editor who hasn't contributed to anything else.

It would appear that Wiki page is more of a monograph than an encyclopedia entry :P

skullpetrol

13:59

@ACuriousMind interestingly, it appears that it depends on the noun

cc @EmilioPisanty =P

yuvraj singh

14:14

0

Q: Regarding asking a question

Actually when I was a new user to site I posted the homework type question. But as the time passes I learn, and understand how to ask a good question, as I learn a lot from the site. But now problem is that, if I have doubt, I can, t ask a new question, because the site banned from asking the ...

feature-request homework-and-exercises answers user-accounts post-ban

ACuriousMind

@yuvrajsingh You don't need to post your meta question here, there's a bot that periodically posts new meta posts to this room anyway.

yuvraj singh

Sorry @ACuriousMind I do not know, should I delete it.

ACuriousMind

No need to do anything, just for future reference.

Physics Meta

0

Q: Regarding asking a question

Actually when I was a new user to site I posted the homework type question. But as the time passes I learn, and understand how to ask a good question, as I learn a lot from the site. But now problem is that, if I have doubt, I can, t ask a new question, because the site banned from asking the ...

support homework-and-exercises answers user-accounts post-ban

skullpetrol

lol

yuvraj singh

14:23

Come on.... @skullpetrol

skullpetrol

How are you my friend @yuvrajsingh

yuvraj singh

Today I gave the exam, so relaxing in room.

skullpetrol

Good to hear. Try and get some well deserved sleep also.

yuvraj singh

Sure.

Have you read my meta question.

skullpetrol

:-)

yes I have

yuvraj singh

14:30

What you think, I genuinely show, my problem but I do not know why @ACuriousMind straight away closes the question.

skullpetrol

209

Q: What can I do when getting "We are no longer accepting questions/answers from this account"?

Do not repost the question you were about to ask until you have READ EVERYTHING WE ARE ABOUT TO TELL YOU. While trying to ask a question, one could get: We are no longer accepting questions from this account. See the Help Center to learn more. Likewise, for answers: We are no longe...

support faq asking-questions post-ban

ACuriousMind

@yuvrajsingh All the general advice we can give for people affected by the automatic question ban is in the meta post DavidZ links in the answer to the duplicate.

There's no point in letting someone post another answer to your question that just links to the same post.

"duplicate" closure doesn't mean "this is a bad question" but "we already have that question".

yuvraj singh

@ACuriousMind I agree with you, I understand, but if you carefully look at the question I don't think so I can do anything.

skullpetrol

@yuvrajsingh I think you should carefully read the above post after you get some sleep pal.

ACuriousMind

@yuvrajsingh If you don't think you can do anything, then you can't do anything - and neither can we.

JMac

14:35

@yuvrajsingh You might not be able to do anything... but the point is that all the information is in DavidZ's post, the attached meta post, and any message you got when the question ban occured. There's not much a new meta question and answer would do to change that.

skullpetrol

You have been through a lot and you must be tired @yuvrajsingh

13 mins ago, by skullpetrol

Good to hear. Try and get some well deserved sleep also.

The internet will still be here when you wake up :-)

JEEs are well known for burning students out @yuvrajsingh

Novalium Company

14:51

damn it I have school tomorrow

skullpetrol

how many classes do you have?

Novalium Company

6 (which is good)

I usually have 7

Abhas Kumar Sinha

@NovaliumCompany damn... me too.

Novalium Company

15:06

just 1 more year and a half, I can do this

skullpetrol

only 5 should have any real homework, right?

Abhas Kumar Sinha

@NovaliumCompany just 1 month for me. I'll take a drop for JEE

@skullpetrol no all 6 subjects carry equal weightages

Novalium Company

@skullpetrol I don't do my homework, I'm a gangsta. F*ck school.

skullpetrol

lol

Abhas Kumar Sinha

@NovaliumCompany That kid is learning from me.

Our school teachers suck, both by manners and knowledge, the only thing they are good at is EGO

Novalium Company

15:13

I don't even wanna talk about school honestly. It doesn't deserve my energy.

skullpetrol

\o @JohnRennie how are you sir?

John Rennie

@skullpetrol Hi :-)

I'm about to start eating lunch, so I'm goooooooooooood! :-)

Abhas Kumar Sinha

@JohnRennie what is today at lunch? ...

John Rennie

@AbhasKumarSinha quiche! :-)

skullpetrol

pitty about your rugby team

Abhas Kumar Sinha

15:17

@JohnRennie ah okay, never heard that before, would try once I'll travel to europe.

John Rennie

@skullpetrol it happens :-) They did really well to reach the final, but I'd have to concede that South Africa were the better side.

@AbhasKumarSinha I don't know of anything quite like it in Indian cooking.

I don't think Indian cooking uses shortcrust pastry much.

Abhas Kumar Sinha

@JohnRennie yes, we don't use shortcrust pastry much here, most of the related dishes in India are french...

I defenestrated my girlfriend because she wouldn't cwtch me. Now all she does is floccinaucinihilipilification about our relationship, calling me agathokakological. Though I remain panglossian about it.

John Rennie

cwtch is a Welsh word, not English!

Abhas Kumar Sinha

@JohnRennie well informally it's english, we use it here.

John Rennie

@AbhasKumarSinha Really? It's used in India? Does it have the same meaning i.e. a cuddle?

Abhas Kumar Sinha

15:26

@JohnRennie yes, something like that.

@JohnRennie textbook type of word, rarely used in conversations.

John Rennie

@AbhasKumarSinha I wonder if it's in my English dictionary ...

Abhas Kumar Sinha

@JohnRennie are you welsh?

is welsh inside UK or UK is inside welsh? google probably showing wrong map images.

Country of the United Kingdom ah, welsh is a country inside the UK.

John Rennie

@AbhasKumarSinha My mother's family were originally Welsh, though I think they left Wales around a hundred years ago. Wales is one of the four countries that make up the UK.

Abhas Kumar Sinha

oh okay...

John Rennie

A lot of Brits have mixed blood. My father's family are from Northern Ireland and my mother also had a Scottish grandparent.

Abhas Kumar Sinha

15:32

@JohnRennie oh okay... here too there are people with mixed languages, (1000+ languages in total), my mother and father both have different language.

If you'll come to India, you'll be amazed to see even 10-year-olds speaking more than 3-4 languages.

@JohnRennie so where you live? Welsh or Scotland or london?

@JohnRennie The only British famous here is Bear Grylls, everyone is his big fan here. Everyone knows him.

indiatoday.in/television/top-stories/story/…

John Rennie

This is quiche:

user image

Egg with ham and leeks in shortcrust pastry.

Abhas Kumar Sinha

@JohnRennie woooooow! I like it...

QUICHE

John Rennie

It's one of my favourite meals.

Abhas Kumar Sinha

Is it pronounced like QI-CHI or QUICK-E

John Rennie

keysh

Abhas Kumar Sinha

15:41

@JohnRennie seems very good, especially the color variations tell, it's well made.

John Rennie

key as in the thing you use in locks, then sh

Abhas Kumar Sinha

@JohnRennie oh okay...

@JohnRennie got it.

@JohnRennie I want to know how you were as a student? Were you a topper in your class to whom seniors came to ask doubts or a silent kind of student.?

John Rennie

At school, or college?

Abhas Kumar Sinha

school

John Rennie

I was top of the class in school, but it was a small school. When I got to Cambridge I found I was no longer top of the class! :-)

Abhas Kumar Sinha

15:45

@JohnRennie Did you always complete your homework regularly then?

John Rennie

@AbhasKumarSinha yes.

Abhas Kumar Sinha

@JohnRennie You must be your teacher's favourite student then?

John Rennie

But studying for A levels in the UK is nothing like as intense as studying for the JEE in India.

Or at least it wasn't in 1977, and I'm fairly certain it still isn't.

ACuriousMind

@JohnRennie It's always funny that the proportion of adults claiming this is far higher than the proportion of students I knew doing it ;P

Abhas Kumar Sinha

@JohnRennie But, the environment here is a bit ruined, British students are typically well mannered, good and disciplined ones.

@ACuriousMind How were you? 95% above bright student?

John Rennie

15:50

@ACuriousMind :-) Honestly, it was a small village in a small city in rural Somerset. There wasn't that much competition :-)

ACuriousMind

@JohnRennie I meant the "doing your homework" thing, not the "top of the class"!

John Rennie

@ACuriousMind ah :-) I don't remember getting that much homework.

Abhas Kumar Sinha

@JohnRennie Sometimes people from small places do big works.

John Rennie

I don't think the school regarded homework as that useful a teaching device.

Abhas Kumar Sinha

@JohnRennie sounds like a disciplined and well-mannered school too. :P

John Rennie

15:52

@AbhasKumarSinha I think we were pretty well behaved.

But then we had much less pressure than Indian students do.

Abhas Kumar Sinha

@JohnRennie Yes, here students are well behaved too, but not of the type the teachers would prefer. Students always break rules, ask questions, very energetic and they dream to change the world.

ACuriousMind

@AbhasKumarSinha Basically the same as John - best in class at school, then good-but-not-best at uni. The former is less an indication of how smart I was and more of me not finding any of the subjects boring (in contrast to some other smart people who simply couldn't be bothered to do poetry or philosophy or whatever)

Abhas Kumar Sinha

@ACuriousMind well I love literature... Sometimes I get fed up with science, I take up literature. You'll find me strange, but I like Shakespeare. My favourite ones include Leo Tolstoy.

@ACuriousMind do you like uni?

adios Amigos

ACuriousMind

@AbhasKumarSinha I guess I liked it well enough

Abhas Kumar Sinha

@ACuriousMind ah okay...

ACuriousMind

16:01

@JohnRennie It's not about the quantity, it's about the principle ;)

Novalium Company

Does anyone understand basic tensorflow in python?

I'm building a basic nn on mnist dataset with digits and I'm outputting some kinda probability densities in a very weird array format. For some reason doing np.argmax on the outputs spits out the number that the model predicted the image shows. This is where I'm confused. How argmax turned those probability density array of outputs (10) into the answer.

nvm got it

i googled :P (what argmax does xD)

Sir Cumference

morning

Abhas Kumar Sinha

16:21

@NovaliumCompany yes

@NovaliumCompany argmax is just threshold for regression/reinforcement.

depends on your situtation.

@NovaliumCompany Try AmazonAWS it's much better for newbies

sagemaker I mean, it's Aamazon's

Sir Cumference

@JohnRennie oh can i ask you a cosmology question?

John Rennie

@SirCumference i'd have guessed you know more cosmology than me by now ...

Sir Cumference

i'm still an undergrad and we've only got one cosmology course, which i'm taking now :/

John Rennie

You're welcome to ask

Sir Cumference

welp lemme see if I got this straight. I took the Friedmann equation $H(t)^2 = \frac{8 \pi G}{3c^2} \varepsilon(t) - \frac{k c^2}{R_0^2 a^2}$ and saw that for a flat universe, we'd have $\varepsilon(t)=\frac{3c^2}{8 \pi G} H^2$. so from what i can tell, the critical energy density is proportional to $H^2$

so in a static universe where $\dot{a} = 0$, would we have $H=0$ and therefore $\varepsilon_c = 0$?

John Rennie

16:39

You'd certainly have $H=0$, but I don't think that means $\varepsilon=0$

Sir Cumference

sorry, meant to write $\varepsilon_{\text{crit}} = 0$

John Rennie

A flat universe can only be static if a cosmological constant is present, and then you need to include it in the equation.

Sir Cumference

crud, right. although i guess i could modify that to say $\varepsilon_{\text{total}}(t) = \varepsilon_{\Lambda} + \varepsilon(t)$ in the Friedmann equation

since from what i understand, the cosmological constant is an energy

John Rennie

yes, it's an energy density give or take a few constants

Sir Cumference

OK, so factoring in dark energy, I get the total critical energy density in a static universe as being zero

wait i'm trying to remember what my confusion was...

welp i forgot my question, i'll see if i remember later. thanks though

John Rennie

16:55

OK :-)

3 hours later…

ABC

20:16

Hi guys, I have a doubt about complex numbers. I'm studying Poynting theorem about a non-dissipative object. I get a clear equation.
Now I'm going to represent my field with the complex formalism. I get the same reports as before more than the others, WHY? Being a writing equivalent to the complex one, shouldn't I have the same information? Here I get more information.

Novalium Company

21:00

Anyone know how to convert the shape of a png image of (28, 28, 3) to (1, 28, 28)? (in python)

Hooman Neshat

@JohnRennie No!

Novalium Company

nvm

I googled :P...

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