The h Bar

lol

6:05 AM

Those suckers thinking it's still a good idea talking to him

7:51 AM

"the category of smooth (topological?) manifolds is one of those categories where the objects are very nice but the category itself is terrible"

Rudi_Birnbaum

8:43 AM

@EmilioPisanty thanks a lot for the bounty. I think the answer we have obtained is very good.

10:06 AM

@Rudi_Birnbaum no worries.

Q: Why does space expansion not expand matter?

10:31 AM

Is there something that is not affected by the expansion of the universe? Will the expansion ever end? It's a bit hard wrapping your head around the idea that the big bang occurred everywhere at the same time. What does "the same time" mean considering relativity and time dilation?

John Rennie

10:58 AM

152

I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for. So here is my question: One often hears talk of space expanding when we talk about the speed of galaxies relative to ours. Why, if space is expanding, does ma...

@JohnRennie Matter doesn't expand because the force expanding is less than the force that pulls atoms together, right? If that so, what expands, is the distance between macroscopic objects right? Also EM waves?

11:54 AM

Matter expands very slowly, slowly enough that it does not perturb the actual bound systems it is made of

ie an electron is slightly further than usual from the nucleus, but due to the stability of the orbitals, it just gets closer again

you could calculate this exactly if you so wished

Consider the Dirac equation in curved spacetime

\begin{equation}
i \gamma^a e^\mu_a (\partial_\mu \psi - \frac{i}{4} \omega^{ab}_\mu \sigma_{ab} \psi - ie A_\mu \psi) - m \psi = 0
\end{equation}

Apply this for the FRLW metric and $A^\mu$ of a fixed point mass

Heck that's actually a good article idea

I should try it

danielunderwood

I mean you'll probably lose a lot of people at "Dirac equation in curved spacetime"

12:22 PM

@Slereah If you don't know yet, I'm not a big fan of complicated equations :P

Actually, I am.

but I just don't know that one :P

then see above

It's just that atoms are stable

ie : if you move the electron slightly from its orbit, it will just go back to it

1:11 PM

Computing spin connections isn't fun

It's a little bit funny

this feeling inside

It tastes like honey

I'm reading the Iliad

This is ground control to major Tom

The apple is falling on Newton's bum

Gravity gets invented

and my salary is incremented

The hbar is the best in the game

Tell this to the world when you get the fame

So more people can come in the room

Rennie will help them and buy a new broom

Harry Potter is coming to town

Tell this to Santa Claus

It's been a couple months since I've been single so

You can call me Novalium Claus minus the ho ho

I apologize to any newcomers reading this

1:33 PM

@Slereah im gonna give a GR talk in the grad student seminar

@RyanUnger what topic?

positive mass theorem

Neat

Hm

What would be Coulomb's law in an FRW metric

the four-current needs to be some Dirac distribution but also the conserved current must be $e$

What's global charge conservation like in GR

I guess I just need to apply Stokes theorem

Hm

this seems too complex to do at the office

best wait a bit

I think I need to split the current according to some time foliation first

Is total electric charge conserved in GR?

It is indeed not conserved

Hm

Do I pick a current that is in such a way that total charge is conserved, or just $\rho = Ze \delta(r)$

Which one would best represent a proton

2:35 PM

physics.umd.edu/grt/taj/675e/Bonnor.pdf

Dang it

seems Bonnor did it already

though not using Dirac it seems

projecteuclid.org/download/pdf_1/euclid.cmp/1103858128

but here it is!

He just picked $\phi = e/r$ for the potential

2:51 PM

"Therefore, in case of a static spherical space(Einstein Universe) the hydrogen atom cannot be used as a standardclock. Thus in general, the hypothesis that the clocks of the microphysicsrepresent always "good" standard clocks cannot be maintained."

user228700

3:12 PM

@SirCumference @Balarka @Blue What's up, y'all?

user228700

Remember me? x'D

user228700

I don't think I've popped in here and had a proper conversation with anybody in literally years.

user228700

Okay, more like *year, I guess.

@KaumudiH hi! guess you have a life now right? :)

user228700

@vzn Hi! :-D Haha, I suppose. How are you?

3:20 PM

@KaumudiH doing ok thx you? seem to recall youre studying a computer-related undergrad degree? hows college?

user228700

Yep! Computer Science Engineering, in fact. College is okay, I guess.

@KaumudiH CSE great choice! are you a sophomore this year?

user228700

I'm a junior this year, haha. Screaming intensifies. :-P

user228700

I'm sorry if you've told me this before and I forgot, but what did you do in your undergrad?

@KaumudiH (lol know the feeling!) so whats your favorite computer language?

@KaumudiH software engr. prob similar to yours...

user228700

3:24 PM

Nice!

:)

user228700

@vzn To be perfectly honest with you, I'm not exactly a pro-coder, or anything. It's only now that I'm in my junior year that I'm beginning to test the waters, sort of. Our course is focused more on theory than, y'know, software engineering.

@KaumudiH huh ok. theres a lot of theory in computer languages. what kind of theory?

user228700

There are a few subjects here and there, like Data Structures & Algorithms, stuff like that, but it's mostly been dry theory till now.

@KaumudiH di hi I am new here, are you an iitian

user228700

3:28 PM

@vzn Not in computer languages. I meant the computer. The hardware. We've had a couple of electronics subjects - stuff like Switching Theory, Computer Architecture & Organization, Microcontrollers & Microprocessors...

If old humans slowly developed language, what language were they actually speaking? Some kinda their own jibber jabber?

user228700

@yuvrajsingh I wish, lol. I gather that you're preparing for the JEE? I'm the real OG in that field around these parts. x-P

user228700

Also, sorry, hi!

Tests are the worst invention of humans in the field of education.

I think you won, t belive I am iit Ian @KaumudiH

3:31 PM

The education system of todays world is broken.

It's inefficient and tortures students.

user228700

@yuvrajsingh You are?

First year iit roorkee mathematics and computing

user228700

Nice!

stuDying what, when and how school makes you is torture

I want to get in iisc bangalore

So I am here

user228700

3:32 PM

@yuvrajsingh I saw you in the Problem Solving Strategies chatroom, so I assumed that you were preparing for the exam. My bad.

user228700

@yuvrajsingh Ahhh, nice. Good luck!

if(school == torture) { send("asdf"); }

asdf

So in which year you are

ScHoOl sUcKs

user228700

@yuvrajsingh In my 3rd year. Studying in a govt. engineering college in Kerala.

3:34 PM

@KaumudiH engineering is nice

user228700

I disagree, lol. x'-D

@KaumudiH why?

Engineering is amazing and has allowed humanity so much

user228700

Oh, absolutely. The field is cool and all, but the study. Especially in my country. Kinda sucks.

@KaumudiH what do you dislike... too theoretical so far?

OK @KaumudiH welcome bye

3:37 PM

@KaumudiH This confirms my theory that school is torture and makes people hate subjects they would usually love if they studied them on their own

user228700

@vzn Well, yeah. And we've got such an outdated syllabus, too. It doesn't at all feel like we're being prepared for "what's out there." And I don't even learn much in my classes. Not that I don't try. It's just that it feels like all we're doing is trying to score well in the exams, y'know?

user228700

Sucks the joy RIGHT out of learning.

user228700

@yuvrajsingh Sure, bye.

Schools are a bigger problem than climate change. Cuz if schools don't do their job right, dumb apes with phones will have to fix climate change

user228700

Speaking of exams though, I've got to run now. Got two exams tomorrow. I'll be back around this time, I guess. It was so nice speaking with you, @vzn! :-D Hope we can continue the conversation later.

3:40 PM

@KaumudiH Tell the teacher to **** *******

@KaumudiH have heard india is very exam focused to get into colleges but maybe its very exam heavy while inside also, but US colleges seem like they are heavy exam focused also... agreed it can feel soulcrushing at times, remember that feeling... :o o_O

The Surprising Behavior of "Whirlpools" of Light

with the added bonus of a remarkably-on-topic contribution, at the top of the comments, by a former regular of this chatroom

Curio

If a gas occupies all the volume of a box, does entropy increase?

Well, I don't see any heat deltas

@vzn India competition, creates the top brain for different fields, Mark my words, the world who is cursing Indian exam system, will salute the upcoming researcher,

@KaumudiH You're back! :D

How's life

3:49 PM

@yuvrajsingh "the world"? it seems to be mostly indians cursing it... o_O

Curio

But we all know that it should increase, I'm confused

No, one who curse is weak, in wildlife also the fittest will survive, @vzn we are human being.

Any way hi @vzn I am sorry I sound rude, I do not mean that

@yuvrajsingh it doesnt sound rude to me but does sound raw/ brutal. aka ("social") darwinism. it is not unheard of in US either. maybe more with the economic system.

Right @vzn more developed the country greater the competition

In America there, people from every part of the world which make the situation different. @vzn

@yuvrajsingh aka "the melting pot" o_O

4:03 PM

Yup I think now world will not see Newton and Einstein again @vzn

@yuvrajsingh they are part of their times. genius will always show up intermittently... in new forms...

Q: Quantum entanglement faster than speed of light?

27

recently i was watching a video on quantum computing where the narrators describes that quantum entanglement information travels faster than light! Is it really possible for anything to move faster than light? Or are the narrators just wrong? Regards,

quantum-information quantum-entanglement faster-than-light quantum-computer

ugh

boy, does that thread need attention

Are you a coder @vzn

@yuvrajsingh yes!

@vzn I am doing of basic of c and c++

4:07 PM

... ideally from >20k-rep users who understand the role of their moderation privileges in keeping misinformation out of the site

say, @JohnRennie

@KaumudiH was thinking about you & how lively/ more cohesive past chats have been! glad you returned, yeah hope to chat more... ps think theres some kind of story in your new profile pic for sure :)

@yuvrajsingh so, computing? like hardware + software?

It is a degree mathematics with computer science and financial engineering. ...@vzn

@yuvrajsingh nice combination! :) they teach "financial engineering"? what does that entail?

4:35 PM

I finally got that GR book

Want to know how old and obscure it is?

It has a bloody punch card in it!

and it was borrowed 4 times in about 50 years

@Slereah More times than I'd expected!

But it does have a whole article on the topic of measuring the metric from actual experiments!

@ACuriousMind A shame really

It seems like a great book

But it is a conference book

and not a proper GR book

Actually I think the whole conference was on experimental GR

Boy oh boy

mysterious

4:53 PM

@Slereah I feel a relevant question here is what exactly the man is smoking

He is an Observer, @ACuriousMind

He needs to keep his mind sharp

To observe all things

@EmilioPisanty I was confused at first, but then I kept scrolling

I wonder who did the art of that book

@ACuriousMind It doesn't stop :)

"The publisher is pleased to acknowledge the assistance of Cecilia Duray-Bito, who produced the illustrations"

5:02 PM

@AaronStevens It eventually will, one way or the other :P

@ACuriousMind Based on previous questions, it looks like they have been sitting on this "proof" for years. Extrapolating, it looks like we will see a similar post in about a year or two

One post per year? How can we possibly manage?

I just can't imagine all of the angular momentum that will not be conserved in that time period

It could...spin out of control

What are you torquing about?

5:15 PM

I googled but, are centripetal and centrifugal forces real?

Depends on your definition of "real"!

Well, there is an actual force pulling inwards and outwards

what is "an actual force"

...

heheh

5:18 PM

help me formulate my question please

In an inertial frame of reference, there are only centripetal forces (i.e. inward forces that keep rotating bodies on their circular trajectories).

So if your notion of a "real" force is one that can be observed in an inertial frame of reference, then the centripetal forces are real and the centrifugal ones are not

centripetal*

Why when we release the object, the ball continues forward? Is there a force there or just inertia?

Loong

You spin me right round, baby, right round, in a manner depriving me of an inertial reference frame. Baby.

@ACuriousMind Well, here's another fun thing

How do you define an inertial frame of reference

(I know I'm terrible)

@Slereah It is a frame of reference that is inertial

5:23 PM

@NovaliumCompany It's just no force + Newton's laws - a body in motion will continue in a linear trajectory with the same velocity unless a force acts on it

@Slereah It's a frame of reference in which there are no centrifugal forces :)

heh

that's the good thing about GR at least

No need to worry about which frame is the inertial one

"All frames are equal, but some are more equal than others"

nvm

5:35 PM

so no takers for that link above?

shame

I mean, just look at this glorious Pollock of a laser speckle pattern

People like to pretend that the CMB frame is a preferred frame

But the CMB isn't even homogeneous

therefore no such frame even exists!!!

5:53 PM

It’s almost homogenous

Well then how can you tell which frame is the CMB frame

It’s close enough

There are worse crimes in physics

Q: Are the criteria for an automatic question ban known?

I think that book may be the first thing I've read of wheeler outside of MTW

Physics Meta

0

I’m asking because I noticed a member who has asked five questions, all of which have been significantly downvoted (-7, -15, -5, -4, -6), but who has not received a ban. I thought that members who consistently ask badly-received questions got some temporary suspension of their right to ask more q...

discussion

6:30 PM

For you, @ACuriousMind

Ah, Zorn's genie :)

If you really had 3 choices now, what would you choose?

6:49 PM

I wish for the wish i would choose if I had perfect knowledge of the consequences

Swapnil Das

@JohnRennie Hi, I wanted some guidance from you. I want to study Fluid dynamics sufficiently enough to write papers, can you guide me through?

@JohnRennie After a week, I shall start studying from a book by F Charlton, I just want to know the formal roadway to study fluid dynamics.

Q: Superrotation equivalent definition

7:08 PM

@EmilioPisanty given the regular in question, I approach this remark with suspicion

bolbteppa

7:35 PM

-1

Just as supertranslations can be defined as certain diffeomorphic transformations such as angular-dependent time translations at the null infinity, what is the equivalent definition of superrotations?

diffeomorphism-invariance

7:46 PM

yo i'm no CS expert but is it a security concern if a website emails you your password whenever you create an account?

That has nothing to do with CS, but yes, passwords should never be transmitted in plaintext.

> i'm no CS expert

:P

I think I've succeeded in reinforcing that

CS = computer security, clearly /s

Ah yes @Semiclassical gets what I mean

Ultradark

8:32 PM

what happens when four closed strings collide at once?

I think they would combine to make a closed string enclosed in a larger closed string

@bolbteppa could be worse. could be saying it's trivial or inelegant. Them's real fighting words

9:07 PM

This book is so old

The bibliography is hilarious

It's entirely devoid of the usual GR refs

It's like Schrodinger and Weyl and Pauli's GR books

on that note, trying to read old math texts is a headache

especially when it comes to tracing "who came up with this idea first"

@Semiclassical I'm mostly impressed by its topicality

bolbteppa

9:24 PM

Apparently that post is not a troll

@Semiclassical the greeks probably

Oh gooood

One of the important reference is an unpublished thesis of the author

From 1959

Maybe I can contact the Princeton library and ask for it

worldcat.org/wcpa/ow/177661955

That's the one

"Those outside of the University network and those requesting theses from 1924-2012 may request a digital copy of a thesis for a fee by completing the Senior Thesis Order Form."

9:45 PM

@EmilioPisanty are we sure it’s not an accident

Because I feel like that’s the kind of thing he’d say even absent that context