3:33 AM
@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)

4 hours later…
7:55 AM
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.

-1

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...

8:12 AM
@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

8:33 AM
@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

8:43 AM
@MikeMiller I left that as an exercise to the interested reader.

8:59 AM
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

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!

2 hours later…
10:47 AM
Is the Fock space of a non-interacting QFT the same space regardless of which field we have quantised?

11:13 AM
Why is internal energy measured from centre of mass film of reference

@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)

11:29 AM
@ACuriousMind ahhh of course, got it, tyvm

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

Is this the "operator valued distribution" stuff?

It's the one-particle Hilbert space, so no
Here we're just considering old fashioned particles

ah

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

11:36 AM
When you say "the functions $\Bbb R^n$ to $V$" are we talking about $L^2(\Bbb R)$ space?

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

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

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

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

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

11:44 AM
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?

well the full Hilbert space vector is those two functions together

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

ie $\Psi(x) = (\psi(x), \chi(x))$
This vector transforms under the appropriate infinite dimensional rep which is uuuuuh

oh ok I can see that

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

11:48 AM
a spinor field right

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

12:09 PM
That looks like a PR box
where the input of a pair fixes the classical outputs of two other pairs

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

@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

12:24 PM
Sure, that's what the operator is there for

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

@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

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

Hence the hydrogen atom
it's where they pop up the most

yeah

12:35 PM
Landau also has a chapter for like
Non-relativistic electrons in a magnetic field
(you can also just work with free electrons, too)

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

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

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

I hope you remember your spherical harmonics!

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

1:00 PM
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 :(

1:20 PM
@bolbteppa be a theorist. Assume it's 1 until shown differently

Hi! everyone

Hi Dr. Nick!

@Jim to me?

I have no choice but to do that :p

@JackRod it was a simpsons reference

1:23 PM
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?

1:42 PM
@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.

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

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

Also, many statisticians will just hold a general math degree

2:19 PM
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

salesmanship will take you far in this world

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.

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.

@JohnRennie Totally agree.

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

2:37 PM
Yeah it was a bit crazy

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

Just a 90-minute long cat fight honestly

The ability for them to not stay on topic was pretty astounding
I wonder if Trump and Biden are certain users on this site

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

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

2:44 PM
I suppose that would have been interesting to see, although I would have preferred actual discussion about the issues that were brought up.

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

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

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

Definitely caught Biden with the covid vaccine stuff

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

2:46 PM
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

You know that's not what he said

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"

iirc Trump has publicly promoted the vaccines-autism link before

Who knows what anyone promotes anymore

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

2:49 PM
Really an unbelievable tweet

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

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

Why wasn't Kanye on stage last night?

He was in spirit

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

2:56 PM
@BioPhysicist here I was procrastinating away from my astro assignment. Looks like I’ve got to get back

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.

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.

Or a spray bottle

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

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

3:09 PM
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.

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

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.

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.

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

@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.

3:14 PM
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.

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".

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

I think I turned it off before then

@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.

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

3:43 PM
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.

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

@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...

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

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.

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

3:51 PM
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.

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

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.

If I could just cast a reopen vote I would.

@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.

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.

3:57 PM
@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.

@JMac there is (for moderators), but closing also removes it
(I've reopened it btw)

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.

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

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

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

4:00 PM
What types of questions don't deserve to be on HNQ even though they are valid PSE questions?

@PM2Ring
Isn't he active here anymore?
or what?

@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

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?

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

No, it wasn't

4:05 PM
@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

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

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

Yeah, for sure

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

@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...

4:09 PM
@JMac Do you have any idea?

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

@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?

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

@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

@JMac Ice Cream is already very cool.

4:10 PM

@ACuriousMind agreed :-)

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

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.

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

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?

4:13 PM
I wish there was a way to filter by close votes. It feels satisfying knowing you're the final vote haha

:-)

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

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.

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

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

4:19 PM
Where is PM2 Ring?
I needed to ask him a question :(

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

Okay....

why four dots after 'Okay'?
odd number for dots, even though it's even :P

I needed a review on CLRS...

4:33 PM
"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

Kapoor..........

5:17 PM
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

2 hours later…
7:06 PM
I hate it when I write an answer and at the end realize I should've chosen different notation but then I'm too lazy to actually go back and change everything :P

8:05 PM
Honestly, while I’m not that hot for one candidate I cannot tolerate the constant insults of the other. See en.m.wikipedia.org/wiki/Paranoid_personality_disorder#ICD-10

8:22 PM
@ACuriousMind ah! then you change the notation and realize it still doesn’t work and you either change it again or go back to the original one...

1 hour later…
9:51 PM
@ACuriousMind I’ll probably start reviewing once I get into the groove of this semester. Right now the assignments are just beginning to pop up. So hopefully I can get my workflow smoothened out soon.

10:49 PM
why in the world does ds9 have reversed dragging controls
i'm dragging my mouse right to move my map left

11:06 PM
Well, this is probably my last day as a contributor on Physics SE for a long time, or any StackOverflow or SE site for that matter. I cannot in good conscience support a site which enables the absolutely atrocious moderation that I see on Politics SE. I learned a lot by writing the answers I did, but I guess it's time to move on, rather than support a broken status quo.
So, bye everyone; hopefully we'll meet again, somewhere else on the internet.

11:33 PM
@probably_someone Of course the decision is yours but our moderators are doing quite a good jobs so is it wise to withdraw your contribution because of moderating excesses elsewhere?
Don’t get me wrong: if our own moderators expressed dissatisfaction I would bail faster than $c$.
Anyways I have to admit I’ve been examining my own level of contribution. It is sad to see another more senior contributor go.
Maybe my turn will come soon enough.