The h Bar

Nihar Karve

3:32 AM

The base space is a topological space, right? So what are the open sets?

(probably a dumb question)

RewCie

4:11 AM

Yes.

There are open space too representable through sets.

Nihar Karve

4:47 AM

@RewCie I honestly have no idea what you're trying to say

RewCie

@NiharKarve Open Spaces can be represented with sets. That's what I've said.

see this

Open set

In mathematics, particularly in topology, an open set is an abstract concept generalizing the idea of an open interval in the real line. The simplest example is in metric spaces, where open sets can be defined as those sets which contain a ball around each of their points (or, equivalently, a set is open if it doesn't contain any of its boundary points); however, an open set, in general, can be very abstract: any collection of sets can be called open, as long as the union of an arbitrary number of open sets in the collection is open, the intersection of a finite number of open sets is open, and...

Nihar Karve

Sorry, I should have been clearer - I was asking about the relation between the open sets in the total space and base space in a fibre bundle

RewCie

No idea about Fibre Bundle

Nihar Karve

5:03 AM

Oh never mind, I got it - the projection map bestows the quotient topology

Prateek Mourya

5:31 AM

How to control anger?

Sir Cumference

6:18 AM

@PrateekMourya meditation

Nihar Karve

A squeeze ball works wonders

RewCie

7:02 AM

@PrateekMourya Go to bath

Prateek Mourya

8:45 AM

Alright now after talking

user 6232128

9:09 AM

That's usually the best way to release anger.

let it all out

Mauro Giliberti

9:33 AM

Do you guys know about ScienceClic?

I loved their last video on QFT and would love to hear some other opinions

Quantum Field Theory visualized

►

ACuriousMind

I'm very skeptical of anything that contains the words "QFT" and "visualized" without a "cannot be" in between :P

4

Mauro Giliberti

I was too! But the video does its job, I think

Emilio Pisanty

man, SE is a strange place

Mauro Giliberti

It gives a slow, step by step entrance, of QM, what's wrong with it, and the foundations of QFT. It should probably mention that the images one sees in the video aren't to be taken literally, but it covers the concepts of symmetries, spin, and superposition pretty well.

Emilio Pisanty

10

Q: What does the dipole moment really represent?

Wikipedia gives the most general expression for the $n^{\rm th}$ moment $\mu_n$ of a physical quantity $\Lambda$ as: $$ \mu_n = \int {\bf x}^n \space \lambda({\bf x}) \space \rm d^3 x$$ provided that we know the spatial distribution density $\lambda({\bf x})$ of $\Lambda$. Here are some examples ...

this thing goes on HNQ

which I can completely understand

but then so does this one

6

Q: Why is a symmetric traceless tensor zero when averaged over all directions?

In page 168 in Ref. [1], the authors search for a suitable order parameter for the nematic phase in liquid crystal. If $\vec v^\alpha$ is the direction of a single molecule, than due to the inversion symmetry, both $\vec v^\alpha$ and $-\vec v^\alpha$ contribute to order so the order parameter mu...

condensed-matter tensor-calculus disorder

like, seriously?

it literally starts off with "In page 168 in Ref. [1], the authors ..." and it only gets more technical as you get on

Nihar Karve

9:47 AM

@EmilioPisanty how are the HNQ decided? I'm guessing it's a bit more complicated than just 'most upvotes'

user 6232128

ok

Emilio Pisanty

@user6232128 no, views are not counted

@NiharKarve it is indeed a bit more complicated than just 'most upvotes'

ACuriousMind

@MauroGiliberti Yeah, from the first few minutes I think the "visualization" there doesn't really do all that much except not being completely wrong, but the author clearly knows what he's talking about (even if he adopts the "particles are disturbances in the field" language which, while widespread, doesn't really correspond to anything in the formalism (particles are created by the fields in the operator formalism, they are not related to the value of the field))

Emilio Pisanty

with the literal meaning of "bit"

110

Q: What are the criteria for questions to be selected for Hot Network Questions?

I really like the new Stack Exchange home page, where certain questions from the Stack Exchange Network are presented, along with a hotness rating that is described as "arbitrary" in its tooltip. Such questions also appear randomly on the sidebar on questions across the network, under the heading...

support stackexchange.com hot-questions faq-proposed

(MIN(AnswerCount, 10) * QScore) / 5 + AnswerScore
-------------------------------------------------
         (QAgeInHours + 1) ^ 1.4

ACuriousMind

@EmilioPisanty 3 answers in a short timespan can push questions into HNQ pretty fast on our site. I do believe that we have so many "simple" and "speculative" questions getting into HNQ because there often are many users who can answer them or want to take a guess at it.

Emilio Pisanty

9:54 AM

Updating the Hot Network Questions List - now with a bit more network and a little less “hotness”! is also relevant - it details changes effected ~2 years ago.

@ACuriousMind yeah, that's probably what did it

Mauro Giliberti

@ACuriousMind I agree, but the concept of "void state" on which the operator acts to create a particle can be complicated and I don't think it gives a much deeper understanding (to the general public)

user 6232128

@EmilioPisanty how did they come up with the 1.4 exponent?

Emilio Pisanty

@user6232128 no idea

user 6232128

ok, thanks

Emilio Pisanty

that formula almost certainly traces its origin to the slightly-different one used on Stack Overflow proper

202

Q: What formula should be used to determine "hot" questions?

Right now the front page Popular tab is fairly broken -- it's a simple descending sort by views. As Joel said in podcast #18, it is "a self-fulfilling prophecy." But this is not intentional, it's only because we haven't had time to improve it yet! As I sit down to write a better algorithm, I thou...

discussion stack-exchange-button hot-questions algorithm

that one's got a ^1.5 exponent

and a link to a previous usage on Y Combinator

that's presumably where the (t+offset)^power formulation came from

the dial-down from 1.5 to 1.4 reduces the turnover by a smidge, which I guess was the intention

B. Brekke

10:22 AM

"Because of energy conservation, the time reversal operator commutes
with the Hamiltonian." My book says this, but it sounds wrong. I guess you could break time reversal symmetry by a magnetic field, but still have energy conservation?

ACuriousMind

@B.Brekke Yeah, time reversal is a discrete symmetry and hence not among the continuous time translation symmetries related to energy conservation by Noether's theorem

B. Brekke

10:37 AM

@ACuriousMind Right. How about the other way around, if we have time reversal symmetry, must the energy be conserved?

ACuriousMind

I don't think so

ah, might be, see physics.stackexchange.com/a/44637/50583

B. Brekke

@ACuriousMind I guess I could suggest an example where the energy goes as $\cos{\omega t}$, but I don't know any physical example

ACuriousMind

Qmechanic shows that if you have time reversal symmetry around every point in time, you also have translation symmetry, hence energy conservation

B. Brekke

It would be interesting to find an example which has time reversal around at least one point but not all, and I guess this would have to be an example where energy is not conserved

Slereah

10:56 AM

Having time reversal at one point isn't too hard

just add an even function in $t$

B. Brekke

@Slereah Ah, of course

RewCie

Just In: BREAKING: THE FIRST INTERIM DATA ANALYSIS OF THE SPUTNIK V VACCINE AGAINST COVID-19 PHASE III CLINICAL TRIALS IN THE RUSSIAN FEDERATION DEMONSTRATED 92% EFFICACY : Source 2: Sputnik V vaccine demonstrates 92% efficacy, claims Russia Via - Official Sputnik V Handle/ Business Insider India

RewCie

11:25 AM

Just In: 11th November as Chinese Singles' day

Singles' Day

The Singles' Day (Chinese: 光棍节) or Double 11 (Chinese: 双11), originally called Bachelors' Day, is a Chinese unofficial holiday and shopping season that celebrates people who are not in relationships. The date, November 11 (11/11), was chosen because the number "1" resembles a "bare stick" (Chinese: 光棍; pinyin: guānggùn), which is a Chinese Internet slang for a single man who doesn't marry and thus can't add 'branches' to the family tree. The four "1"s also abstractly refer to the demographic group of single people. The holiday has ironically also become a popular date to celebrate relationships...

Nihar Karve

11:39 AM

Again, how is that "Just In"?

ACuriousMind

11:53 AM

At least it relates to today, I guess? :P More seriously, @RewCie, this chat is not a news channel. If you post something here that's not of obvious relevance (i.e. about physics or a currently ongoing discussion about something else), it would be nice if you at least commented on it rather than just posting the link. What do you think about it/why are you posting it? Are you seeking other people's opinions on it? Are you exhilarated/frightened/amused by it?

Physics Meta

12:04 PM

0

Q: What is the maximum limit of suspension in physicis stack exchange?

Today I came across an account which had been suspended for a very long time. Is there any other instances for such suspensions? What are the probable reasons of very long time suspension? Do anybody thinks that it's worthy or important to create a system set up which will generate some auto...

discussion support suspensions

RewCie

12:26 PM

@ACuriousMind I was disturbed by the sounds of silence in the room so I posted it here to start a discussion.

Emilio Pisanty

12:56 PM

22

Q: Reference request: Examples of research on a set with interesting properties which turned out to be the empty set

I've seen internet jokes (at least more than 1) between mathematicians like this one here about someone studying a set with interesting properties. And then, after a lot of research (presumably after some years of work), find out such set couldn't be other than the empty set, making the work of y...

reference-request ho.history-overview research

JingleBells

I'm struggling to understand this analogy of how ensemble methods work

The probability of the majority of tosses being heads is always 51%, isn't it?

RewCie

Law of large numbers

In probability theory, the law of large numbers (LLN) is a theorem that describes the result of performing the same experiment a large number of times. According to the law, the average of the results obtained from a large number of trials should be close to the expected value and will tend to become closer to the expected value as more trials are performed.The LLN is important because it guarantees stable long-term results for the averages of some random events. For example, while a casino may lose money in a single spin of the roulette wheel, its earnings will tend towards a predictable percentage...

@JingleBells A slightly biased coin means it is a special coin which doesn't has 50-50 probability

JingleBells

Hmm, so with every flip of the biased coin, what exactly approaches 100%?

"the probability of obtaining a majority of heads" - what does that mean? I can feel it but I can't explain it :D

As I toss the biased coin more and more, I'm going to see the 51-49 ratio clearer with every toss.

RewCie

1:12 PM

Honestly, that line doesn't make sense to me.....

@JingleBells The convergence approaches to 100% as number of trials $\rightarrow \infty$

That's what LLN states

But I don't understand what does the text mean....

JingleBells

What approaches 100%?

The ratio of heads? $\frac{heads-tossed}{all-tosses}$?

RewCie

Convergence. It means that as the number of trials are increased, the number of heads/total trials will increase more towards 51%

@JingleBells yes, it'll reach 51%

So, convergence will reach 100%

But, I'm totally confused what the text is trying to say/what LLN has to do here...

JingleBells

What's convergence? $\frac{ratio-of-heads}{51}$?

RewCie

Wait till someone else who knows this shows up here.

$\frac{\text{ratio of heads}}{51}\times 100$

^convergence where the total outcomes is 2 in sample space

JingleBells

ye, got it

thx, all is clear now

Emilio Pisanty

1:39 PM

hey, folks

did \newcommand just break on the site?

never mind, I messed up the syntax

ACuriousMind

\newcommand is always broken on the site :P

if you get two posts trying to define the same command on the same page it gets ugly

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