The h Bar

Terry Bollinger

03:33

@ACuriousMind thanks, that is an excellent good point about linguistic and conceptual shift over a period of more than a century. It reads like modern English, but context is of course where actual meaning derives -- and also why it's so blinkin' hard (still, but less so these days) to train neural nets for seemingly "similar" environments, e.g. for robots two deserts are not the same.

Still... just from a physics and math perspective: It is really the same thing to invoke any kind of infinitely detailed completed-infinity field (an idea not liked by the Greeks) as part of the foundation of mathematics, versus postulating as axioms that there are are only construction rules, and that those rules allow such fields to be elaborated when needed? Those statements are not equivalent... well, at least I don't readily see how they possibly can be.

(signing off for the night, cheers all)

B. Brekke

07:55

In a short presentation about "symmetry" for non-physicists, which pointers would you emphasize? I am starting out with some clichè opening line about symmetry being beautiful, and will end up talking about spatial groups in crystallograhy.

Physics Meta

-1

Q: What are the views of PSE community members on moderator deletion of upvoted answers?

I would like to hear the views of PSE community members on the deletion by ACuriousMind of my two upvoted answers to this question. I am especially interested in the views of ordinary (non-moderator) members of the PSE community Please note that the questioner already had the answer to their prob...

discussion

ACuriousMind

08:12

@B.Brekke I would try to give some sort of impression of Noether's theorem, because it connects the rather abstract and mathematical concept of symmetry to a concrete idea of a number being constant

Danu

08:33

@RyanUnger Hi, sorry if you didn't approve of me posting it. Should I remove it, now that the reference to your old username is gone, or not? In any case, congratulations again

Danu

08:43

@MikeMiller I left that as an exercise to the interested reader.

Secret

08:59

iopscience.iop.org/article/10.1088/1361-6382/aba4bc

Well that is kinda interesting

It appears given 4 parties, you can have a scenario where 2 parties does things leading to party 3 to deliver a spacelike signalling in one direction to 4, even when there is no CTC involved

so there is some acausal signalling stuff going on in a CTC spacetime even when the events are not part of a CTC

Roberto Valente

Guys, my last question went unanswered, but can someone help me make sense of this comment? — “As improbable as it sounds, I suspect that when dimensionally reducing from (4+1)𝑑 to (3+1)𝑑 space, there's a magical conspiracy such that after the initial positive wavefront passes, its infinite-duration negative wake is continuously exactly canceled by successive positive wavefronts arriving from sources that are ever further-away in the fourth spatial dimension”

What does it mean by “negative wake”? “Flipped vertically” or “going on the opposite direction”? Any simple answer would suffice! …

Charlie

10:47

Is the Fock space of a non-interacting QFT the same space regardless of which field we have quantised?

shelton Benjamin

11:13

Why is internal energy measured from centre of mass film of reference

ACuriousMind

@Charlie No, since the one-particle spaces are different for different spins and masses

(the representation of the Lorentz group is different and irreps are classified by spin and mass, cf. Wigner's classification)

Charlie

11:29

@ACuriousMind ahhh of course, got it, tyvm

Slereah

Roughly speaking the one-particle Hilbert spaces are the functions $\mathbb{R}^n$ to $V$ for the irrep $V$, with some inner product defined so that they are square integrable

ie to $\mathbb{C}^2$ for spinors and such

Charlie

Is this the "operator valued distribution" stuff?

Slereah

It's the one-particle Hilbert space, so no

Here we're just considering old fashioned particles

Charlie

ah

Slereah

The full field Hilbert space is the Fock space built on those

Charlie

11:36

When you say "the functions $\Bbb R^n$ to $V$" are we talking about $L^2(\Bbb R)$ space?

Slereah

Slightly more complicated, but yes

Because here, the "canonical" representation of the Hilbert space is a vector of function

So you need some kind of inner product defined, too

Charlie

I'm a bit surprised to see the target space of the wave functions described as the irrep of some group

Slereah

I mean why not

In the end your observable has to be similar to the classical field

so the EM field has to be a vector

Charlie

Well just that when I think of the representations of a group on a quantum system I think of the Hilbert space itself as being the irreducible representation

or, not necessarily irreducible, but still

oh wait are we talking specifically about fields

Slereah

No

For instance this is true of non-relativistic spinors

A spinor state in the Pauli equation is a function $(\psi(x), \chi(x))$

On which acts the infinite-dimensional spinor representation of the rotation group

Charlie

11:44

But the vector space in which the spinor lives is the representation of the group isn't it?

the functions $\psi(x)$ and $\chi(x)$ defined above just have the complex numbers as their target space, which isn't the representation space of su(2) in this context is it?

Slereah

well the full Hilbert space vector is those two functions together

Charlie

You might just be making a more subtle argument than I understand

yeah

Slereah

ie $\Psi(x) = (\psi(x), \chi(x))$

This vector transforms under the appropriate infinite dimensional rep which is uuuuuh

Charlie

oh ok I can see that

Slereah

Something like $$\exp(i\sigma_a x^a)$$

with $\sigma$ the Pauli matrices

It's not the finite dimensional representation of this group

you're not rotating a single spinor

Charlie

11:48

a spinor field right

Slereah

Ah, the rotation operator is $\hat U(\vec \alpha)=\exp(-i\vec \alpha\cdot\hat{\mathbf J}/\hbar)$

Secret

12:09

That looks like a PR box

where the input of a pair fixes the classical outputs of two other pairs

Slereah

you can check that this rotation operator leaves the Hilbert space vector invariant up to a phase

ACuriousMind

@Slereah what? The one-particle space is just the irrep

I'm talking about the infinite-dimensional unitary irreps from Wigner's classification directly on the Hilbert space, not the finite-dimensional irrep on the field target space

note that if you express the rep as wavefunctions with values in the target space, the infinite-dimensional representation is not just applying a matrix to the value, but also transforming the argument of the function

i.e. $\psi(x) \mapsto \Lambda\psi(\Lambda x + a)$, schematically

this is, once again, the confusion between the finite-dimensional rep on the target space and the unitary infinite-dimensional rep on the Hilbert space I've talked about several times now, @Charlie

Slereah

12:24

Sure, that's what the operator is there for

Charlie

I was slightly unclear on exactly what we were talking about, I think I mostly get the difference between the two representations at this point though

Slereah

@Charlie I'd advise maybe studying the Pauli equations a bit

It's the simplest case for this

No need to go full QFT to study this

Look at the non-relativistic hydrogen atom, it's the standard example for non-relativistic fermions

Charlie

Yeah I had to look them up and I remember having looked them up at some point. They don't seem to come up much in standard qm teachings so I haven't really had much reason to go out of my way to learn them yet

Slereah

Hence the hydrogen atom

it's where they pop up the most

Charlie

yeah

Slereah

12:35

Landau also has a chapter for like

Non-relativistic electrons in a magnetic field

(you can also just work with free electrons, too)

Charlie

on the long list of things I need to catch up on the hydrogen atom is on there somewhere :P

Slereah

Bold to do QFT before the basic QM examples :p

Free particle, hydrogen atom, particle in a well, particle scattering, quantum tunnelling

oh and the harmonic oscillator

Charlie

I've covered all of those except pretty much particle scattering

we did the hydrogen atom in chemistry but it's sort of a cartoon-ish model that doesn't really teach you good qm habits

so next to most topics in my long list of things to learn is a sidenote to forget everything I already know about it and start from scratch :P

Slereah

I hope you remember your spherical harmonics!

Charlie

You mean the big colourful pictures that show "where the electron lives"? of course!

bolbteppa

13:00

I can't even remember the normalization coefficient in front of the hydrogen wave function (for the discrete spectrum!) though I've derived it maybe 10 times :(

Jim

13:20

@bolbteppa be a theorist. Assume it's 1 until shown differently

Jack Rod

Hi! everyone

Jim

Hi Dr. Nick!

Jack Rod

@Jim to me?

bolbteppa

I have no choice but to do that :p

Jim

@JackRod it was a simpsons reference

Jack Rod

13:23

oh!

I was looking at the stat of data scientist job it is quite famous in European companies despite the fact uk has a very low number of the university which offer the courses on statistics
why is it so?

JMac

13:42

@JackRod What do you mean UK has a low number of universities with courses on statistics? I'd be surprised if basically any university with math/science degrees didn't have some statistics courses.

Jack Rod

the course which especially based on a stat like bachelor in stat program

JMac

I looked up "Bachelor of statistics UK" and I see a bunch of UK schools offering a BSc in statistics.

ACuriousMind

Also, many statisticians will just hold a general math degree

JingleBells

14:19

I can't believe the story of Theranos... lol I can't believe it actually happened

Investors were like: Stanford dropout, check. Sillicon Valley girl, check. Looks like an entrepreneur, check. Talks like an entrepreneur, check. Let's give her money and continue to give her money even though she hasn't introduced a plausible way to make her device work.

i mean, bruh

BLACK LIVES MATTER

salesmanship will take you far in this world

JingleBells

I guess I would have trusted her too. I mean, look at her, she looks like the perfect entrepreneur. The next Steve Jobs.

Well, this is what happens when you "fake it till you make it" for too long.

John Rennie

I can believe that Elizabeth Holmes started off thinking she could do it i.e. she didn't set out to create a fraud. But then things got away from her and she found she was riding a tiger.

JingleBells

@JohnRennie Totally agree.

BioPhysicist

Any Americans (or even non-Americans) watch the presidential "debate"?

bolbteppa

14:37

Yeah it was a bit crazy

BioPhysicist

I turned it off after an hour (made it over half way!). Nothing of real substance was really being said by either side IMO

Charlie

Just a 90-minute long cat fight honestly

BioPhysicist

The ability for them to not stay on topic was pretty astounding

I wonder if Trump and Biden are certain users on this site

bolbteppa

If they stayed on the issues both of them would expose themselves and it's not worth it with Trump anyway so at least Biden was able for the schoolyard bully routine

Charlie

Anyone even remotely articulate could have used every scandal over the last 4 years to make Trump look like a complete fool. I think Sanders would have embarrassed Trump, he is far better at speaking

BioPhysicist

14:44

I suppose that would have been interesting to see, although I would have preferred actual discussion about the issues that were brought up.

Charlie

Honestly a lot of the derailing with Trump making snide remarks while Biden was talking that caused the entire conversation to deteriorate

But Wallace was an excellent moderator

BioPhysicist

I think Trump could have had some good points to make about the Supreme Court, but the conversation moved to health care

I loved the moderator haha. He had some pretty good questions

Charlie

I did see him grill Biden about the supreme court for a few seconds, but apart from that there wasn't much

BioPhysicist

Definitely caught Biden with the covid vaccine stuff

Charlie

Wallace is a really good journalist in general, he grills pretty much everyone

What specifically? I watched it at 5am last night when I woke up

I remember wincing a few times when Biden misspoke and said million instead of thousand or something

BioPhysicist

14:46

Biden was essentially saying to trust the scientists, but he has also said in the past that we shouldn't trust a vaccine right away made by those same scientists

bolbteppa

You know that's not what he said

BioPhysicist

Oh yeah, he does that a lot haha. I think Biden was an astronomer in a past life, "Eh, just off by a couple of orders of magnitude, no big deal"

Charlie

iirc Trump has publicly promoted the vaccines-autism link before

twitter.com/realDonaldTrump/status/449525268529815552

BioPhysicist

Who knows what anyone promotes anymore

Charlie

It's important to know if they're the president :P

bolbteppa

14:49

Really an unbelievable tweet

BioPhysicist

I think that is the summary of Trump's twitter though haha

Charlie

Well he is the living embodiment of "never being held accountable for what you say"

BioPhysicist

Why wasn't Kanye on stage last night?

bolbteppa

He was in spirit

BioPhysicist

I would normally say I can't wait for the election to be over, but either way the craziness will not stop after that haha

Superfast Jellyfish

14:56

@BioPhysicist here I was procrastinating away from my astro assignment. Looks like I’ve got to get back

BioPhysicist

It's a sign haha

On an unrelated note, yesterday I heard the song "Harmony Hall" by Vampire Weekend for the first time, and I haven't been able to get it out of my head.

JMac

They need an airhorn at the next debate to get them to shut the hell up when the other person is talking... mostly needed for Trump.

BioPhysicist

Or a spray bottle

JMac

I wouldn't have been surprised if Chris told them to shut the f up at one point there. It was getting so bad.

BioPhysicist

Ugh, so frustrating

Someone on the radio this morning said they need Samuel L. Jackson to moderate to do exactly that

JMac

15:09

That would have been at very least much more amusing than last nights debate. I'm not sure if people would have gotten more answers; but at least we would get to see Samuel L. Jackson yelling more.

BioPhysicist

I watched the debate to be entertained, not for answers haha

John Rennie

Neither of them are fools. They behaved that way because they think it will go down well with the voters. If they are right that is the really scary thing.

JMac

I'm Canadian so it's mostly amusement for me, but it wasn't even that amusing watching two old men try to yell over each other.

BioPhysicist

Yeah, that's why I turned it off before it ended.

JMac

@JohnRennie Yeah that's what worried me too. Like it seemed like Biden tried to be respectful at first, but as it went on he seemed to be getting more Trump-like; since the alternative didn't really seem to do much when you can't get a point out.

And at the same time it seemed like neither wanted to get many points out.

BioPhysicist

15:14

I knew it was going to be bad when the first question was about the Supreme Court and all Trump talked about during his first 2 minutes was how amazing Barret is. Like come on, you know the issues here, there are much better points to be made here.

JMac

One of the scariest parts was near the end when Trump basically refused to condemn white supremacy and basically dodged it by saying "What about antifa".

Charlie

Why would he condemn them, then they won't vote for him

:/

BioPhysicist

I think I turned it off before then

JMac

@Charlie Obviously that's the problem. He's clearly trying to get the backing of white supremacists, and it's terrifying that a presidential candidate and sitting president would be unwilling to condemn that just to get their support.

BioPhysicist

They should implement a shock collar to use when a question is dodged

Biden dodged the question about packing the courts. Of course that's not an egregious moral issue like white supremacy is, but it still has implications for the future of the country

JMac

15:43

Yeah Biden was definitely dodgy about that. TBH I don't think either of them looked that great in the debate. I just have a very poor opinion of Trump already so the dumb things he did stood out a lot more.

BioPhysicist

Understandable. Certainly was a far cry from a respectable discussion. Oh well.

JMac

@ACuriousMind There was already some substantial comments on this question about why it possibly wasn't a dupe when John Rennie dupe-hammered it. That whole discussion is gone but now it was mod duped again...

BioPhysicist

Yeah, I reopened it because it is asking a different question

I put my explanation back into the comments

The duplicate is arising from a misunderstanding that Newton's third law is about forces acting on different objects

The more recent post already knows this; it is correctly stated in the question. However, they are instead just missing the other forces acting on the system

JMac

Yeah I don't think they are the same either. I cast my reopen vote on this; but I'm not sure how likely it is to get enough reopen votes.

BioPhysicist

And I also do not understand why I am getting all of the upvotes when all of the answers are essentially saying the same exact thing haha

@JMac I also didn't understand the controversy your answer garnered

JMac

15:51

You answered like 40 seconds before me, and I really confused people by talking about rigidity; though I guess I helped OP. Wasn't expecting that to be a HNQ though.

I guess most people don't think of ropes when they hear "horse tethered to a cart"; but it was all I could think of so I felt the need to clear it up.

BioPhysicist

I think it got HNQ because it is an easy answer, and users failed to read other answers before putting in their own answer, thus attracting a lot of answers to put it to the HNQ

I get really annoyed when users post answers that do not have any new content compared to answers that were already there though

JMac

TBH I almost deleted mine because yours covered most of the key points; but I was almost done when yours was posted and I wanted to make the point about cars.

The 4 top answers all answered within like 5 minutes, so for those it was probably a case of starting to answer before any were posted.

BioPhysicist

If I could just cast a reopen vote I would.

ACuriousMind

@JMac @BioPhysicist I'm not a fan of having one HNQ per possible misunderstanding of the 3rd law, but I guess it's really not the same question. I hadn't seen it had been closed/reopened (both times via dupe hammer) before, I just saw two close votes on it and one comment disagreeing.

My expectation would be that people that understand the answer to the "duplicate" also understand this, but it's not fair to impose that expectation on everyone.

BioPhysicist

They are very similar, but the recent question doesn't involve a misunderstanding of Newton's third law. It looks like the OP correctly understands it.

JMac

15:57

@ACuriousMind I figured you hadn't seen any of that, and honestly I'm not super amped about this being a HNQ either (did you read that comment chain?); I just don't think it's really fair to call it a dupe. If there was a "Remove from HNQ" option I would pick that.

ACuriousMind

@JMac there is (for moderators), but closing also removes it

(I've reopened it btw)

JMac

I know closing removes it, I just think there's a decent case to be made that they aren't the same question, so closing doesn't seem like a fair way to remove from HNQ to me.

ACuriousMind

if you see a question that you think shouldn't be HNQ but also not be closed, feel free to flag it for mod attention

we don't remove every question flagged that way but it helps us gauge what the users think should(n't) be HNQ

BioPhysicist

I guess I had never considered the possibility of a question not deserving to be closed but also not being worthy of being on the HNQ

JMac

Should I flag this for tracking purposes, or is this conversation enough?

BioPhysicist

16:00

What types of questions don't deserve to be on HNQ even though they are valid PSE questions?

Azmuth

@PM2Ring

Isn't he active here anymore?

or what?

ACuriousMind

@BioPhysicist That's why we want the flag to gauge what people think, see physics.meta.stackexchange.com/q/11146/50583

the answers give both abstract and concrete examples of things the authors think shouldn't be HNQ

BioPhysicist

I see. So questions that seem to be more click bait like

Like that recent one about the Styrofoam box falling out of the truck and then bouncing back into it

Was that one eventually closed?

ACuriousMind

we don't have a particular policy on this so we usually only use that mod privilege rarely and on a case-by-case basis

BioPhysicist

No, it wasn't

ACuriousMind

16:05

@BioPhysicist no, hovers at 3 close votes and the review isn't completed, physics.stackexchange.com/review/close/314244

we really could use more close vote reviewers

that's not a new thing, we've always had too few but still

BioPhysicist

I haven't been in the close queue for a while because I tend to use up my close votes being the first vote on questions I see on the main site

Or I use my close votes in the other queues

ACuriousMind

yeah, the problem is in general that too few of the 3kers use the privilege at all

BioPhysicist

Yeah, for sure

ACuriousMind

I'm still the top 4 close reviewer of all time and I haven't done a single close review since I became a mod

JMac

@ACuriousMind I saw that question when browsing and didn't really feel it was really worthy of closing to me. It's another one that I'm not sure is a good fit for HNQ, but it seems like they are asking about the physics to me...

Azmuth

16:09

@JMac Do you have any idea?

John Rennie

I find myself voting to close less these days. I must be mellowing in my old age.

JMac

@Azmuth Yeah I have an idea. What if we took all the ice cream in the world, and put it in the same spot. Do you think it would look cool?

ZeroTheHero

I'm looking for a pedagogical and insightful derivation of Liouville's theorem... undergrad level. If you have sources please let me know.

ACuriousMind

@JohnRennie well, the thing about reviews is that you can also vote to leave open! I'm less saying people need to close more than more saying they need to vote more on what we close

Azmuth

@JMac Ice Cream is already very cool.

BioPhysicist

16:10

@JMac Lol. I think Azmuth is asking about PM2Ring

John Rennie

@ACuriousMind agreed :-)

ACuriousMind

right now it's about 10 regular reviewers + mods who do most of the close decisions

John Rennie

Though the system works against voting to leave open as you only get 20 goes at the close queue.

I try to reserve the 20 votes for questions that really need closing so I end up skipping a lot so I don't use up all my goes.

ACuriousMind

see, if there were enough reviewers you wouldn't have to worry about that

"let's leave open the slightly bad stuff to make sure we get the worst" isn't the kind of quality we want to aim for

JMac

I'm not very active on the main site lately in general, so I don't do much work in queues TBH. That said most votes that I cast on the site are for closure, so I guess that's something?

BioPhysicist

16:13

I wish there was a way to filter by close votes. It feels satisfying knowing you're the final vote haha

John Rennie

:-)

ACuriousMind

I have no idea how the queue sort its entires, actually

John Rennie

It has always seemed bizarre to me that I get 25 close votes but I'm only allowed to review 20 items in the close queue.

BioPhysicist

Just a little extra to use outside of the close queue I suppose

ACuriousMind

Well, if you have 5 people voting to close 20 questions in the queue and 5 outside of it, then that's 25 completed close reviews (i.e. 25 questions with 5 close votes each)

kinda neat

not sure if that's the idea

Azmuth

16:19

Where is PM2 Ring?

I needed to ask him a question :(

ACuriousMind

@Azmuth Last seen on chat 41 days ago, you probably won't summon him just by complaining here.

Azmuth

Okay....

BioPhysicist

What is your question?

ACuriousMind

why four dots after 'Okay'?

odd number for dots, even though it's even :P

Azmuth

I needed a review on CLRS...

BioPhysicist

16:33

"three dots means ‘to be continued’, four dots is a typo, but five dots means 'Whoa, do not make me say what I want to say, baby, but if I did, it would blow your mind, dot, dot, dot, dot, dot.'" - Kelly Kapoor

Azmuth

Kapoor..........

Secret

17:17

https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc
That's clever, the process function took account the problem that you cannot define a global spacelike surface in CTC filled spacetimes, hence a straightforward evolution from past state to future states
this is because it captures how much the input and output and all their dependence with other local regions of spacetime disagree with each other
But then, physics approach to problem tend to do that alot in their modelling
finding the boundary conditions, and capturing all the changes with a few operators. The same approach is use…

But again, if there really is inconsistent time travel scenarios, it is probably too chaotic to model

The paper thus expand the understanding of how other nontrivial correlations in time can arise even if the events are not threaded by a CTC

ACuriousMind