The h Bar

PM 2Ring

00:33

In other news, the impact probability of asteroid 2024 YR4 in 2032 has dropped considerably. ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2024%20YR4

Solution date 2025-Feb-23_08:32:55 , observations: 418, cumulative Earth impact probability: 5e-5 = 0.005% — PM 2Ring 31 mins ago

naturallyInconsistent

00:47

@Feynmate im not sure of that? I mean, we have relativity, so the zero critical velocity is zero in some frame...

Allie

00:57

hi everyone <3

qwerty

@PM2Ring do we get a reprieve from asteroid and tsunami questions on the main site? xD

PM 2Ring

01:29

@qwerty I suppose so. As yet, there haven't been any questions about 2024 YR4 on Physics.SE and only one on Astronomy.SE.

qwerty

ah I assumed the recent one(s?) about mega tsunamis were about that

PM 2Ring

Oh, right.

From the current trajectory data, it seems that 2024 YR4 won't even be visible to the naked eye when it makes its close approach in 2032. Oh well. OTOH, we still don't know how big it is. And it will soon be too hard to see. We'll get more data in its next close approach in 2028.

Feynmate

02:28

@naturallyInconsistent the description of superfluids is non relativistic, though. Search Landau's criterion

[nii.ac.jp/qis/first-quantum/e/forStudents/lecture/pdf/qis385/… 5.1)

@qwerty wait until you start watching Franchino Er Criminale

naturallyInconsistent

02:45

@Feynmate NR descriptions still have Galilean invariance.

Also, there is no good reason why we cannot have an SR description of superfluids, other than mathematical difficulties.

John Rennie

06:14

@PM2Ring At one stage I saw figures as high as 3.2%, which was high enough to be worrying.

Feynmate

@naturallyInconsistent well, anyways, as the link shows, a zero critical velocity implies dissipation for any flow, so an ideal BEC cannot be superfluid

Ryder Rude

hi

naturallyInconsistent

06:31

@Feynmate oh miao miao finally sees your tongue-in-cheek joke lol

qwerty

07:39

@PM2Ring you say "oh well" like it missing us is a bad thing ;p

@Feynmate 👀I will look him up when I get home

I made it through my day on 3h sleep because I stayed up on the main site lol :')

PM 2Ring

@JohnRennie It is a bit concerning, especially when the initial low probability looks like it's growing. But the uncertainty when propagating an orbit over two periods can get pretty large. This site (from the Space.SE question) has a good explanation of how they estimate the probability. b612foundation.org/…

@qwerty A hit on the ocean or unoccupied land would be pretty awesome. Maybe. :) OTOH, the Tunguska event was pretty scary over a large area. But I was saying "Oh well" because it probably won't be visible. Unlike Apophis.

2

A: Where should I go in April 2029 for the best chance of naked-eye observing of 99942 Apophis (2004 MN4)?

This isn't a complete answer to your question, but it contains some relevant data. According to JPL Horizons, the next close approach of Apophis to Earth occurs on 2029-Apr-13 21:45 UTC, with a distance to the centre of the Earth of ~38,000 km. Here's a Horizons query URL for that data. The SPICE...

@qwerty Beware of posting too many good answers in one day. If someone upvotes them all, the system may treat it as serial voting and reverse the votes.

qwerty

@PM2Ring aw I didn't consider that!

I did a bunch because I figured out how to sort by my tags+newest+not closed

PM 2Ring

07:57

@qwerty It can also happen if you go back and make a bunch of edits to old answers, bumping them to the front page. It's not usually an issue on SO because it gets a lot of traffic. But it can cause problems on smaller sites.

qwerty

ah

PM 2Ring

The exact algorithm is secret. But I think doing 5 votes (or fewer) is safe.

naturallyInconsistent

I'm sure you already know all of these, PM 2Ring, but 3B1B's interview of Terry Tao on the cosmic distance ladder 2 parter is now finally both out, and there is even an FAQ on Terry Tao's blog for more funsies

qwerty

Terry Tao is awesome. he's so humble

PM 2Ring

@naturallyInconsistent I heard about it, but I haven't watched it.

naturallyInconsistent

08:01

Both 3B1B and Terry Tao are awesome indeed. It is very lovely to watch them both.

I'm sure they will tickle you nonetheless

qwerty

the fact he once said that 27 is a prime number on telly also makes him somewhat relatable

PM 2Ring

His fans should be called Terry Tao-lings.

qwerty

xD

I'm a Terry Taoist myself

omg the joke only just landed

PM 2Ring

:D

I've written a lot of prime sieves & primality testing code. So I've seen the primes <1000 countless times. But I still don't recognise them immediately, even though there's only 168 of them (IIRC).

qwerty

btw @PM2Ring does your username mean something?

PM 2Ring

08:09

I'm a fan of A. M. Turing.

We're similar in some ways, but different in others...

qwerty

ahhh cool

to me PM = prime minister, so are you prime minister Turing? :p given your domain over the primes

PM 2Ring

No, it's just AM vs PM, as in morning vs afternoon / evening.

qwerty

too many puns

PM 2Ring

Here's a primality tester in Python that goes up to 3317044064679887385961981 gist.github.com/PM2Ring/…

naturallyInconsistent

mew mew

Feynmate

08:32

@qwerty it's an Italian guy going around Italy and giving his opinions about traditional food. There is also the AI feature of English dub in more recent videos, although they are less expressive

@PM2Ring Oh, after a couple of years, I finally understand your username

Mate

qwerty

@Feynmate the AI dubs are not fun!

Feynmate

I agree. They make him sound too peaceful

Food is serious stuff, you don't discuss about food peacefully

qwerty

08:48

hahaha

Tobias Fünke

11:23

Hello everyone:)

qwerty

hi! :)

Tobias Fünke

The deutsche Bahn has serious problems. Every single time there is delay around Frankfurt (Main) :s

@qwerty how old was he when saying so? Hehe

@qwerty lol haha. But ikr... when there is an interesting thread or even several, I stay up much longer than I actually wanted to

qwerty

@TobiasFünke hahaha it was on the colbert show about 10yrs or so ago I think, but I never found the video online, just these sorts of threads reddit.com/r/math/comments/2m5gk3/comment/cm1btm1

Tobias Fünke

Ok hehe. Makes him much more "likeable" (is this the correct word?)

But when I see a 7 as a last digit, it just feels right to declare it a prime number hehe

qwerty

yeah, maybe :)

@TobiasFünke is it because of strikes? there's always train strikes where I am

Tobias Fünke

11:38

Nope. Business as usual haha

But no joke, most of the times I left or arrived from/at FFM at least 5-10 minutes late. Which can ruin the whole time schedule

qwerty

I thought German trains were known for punctuality! D:

Tobias Fünke

Haha noo that is really only a myth

I guess the problem in FFM is that trains can enter and leave the station only in one direction. Also, there the trains usually get cleaned a bit and the restaurant refilled... so even if you have delay, you will wait there some minutes

qwerty

I bet train nerds have compared international train punctuality and reliability data somewhere

maybe I should have a google

when @naturallyInconsistent asked the other day why buses would get randomly cancelled I tried to find out, but all i found were stats claiming this only happens 0.5% of the time (it feels like more)

Tobias Fünke

Yes the train company themselves collect the data

Switzerland is really really good

qwerty

epf.eu/wp/10929-2

i did not expect the french to be top 4!

Tobias Fünke

11:49

I guess the German trains are doing quite well overall, and compared to many other countries. But I think it is generally accepted here that it has to be much much better

qwerty

i wonder what's going on with Greece? :S

i'm looking at public data which says that buses here are on time about 96% of the time, trams were about 99% last year whereas trains are on time around 70-90% of the time. i'm glad I don't use the train that much.

fqq

the problem with long-distance trains in Germany is that they're not great even when they are on time :P significantly slower than comparable countries

@qwerty for long distance, all-or-nothing <5min late is not a great measure IMO. also the data is 10 year old

Tobias Fünke

@fqq you think so? The ICEs are really good IMHO

fqq

they are very slow compared to high speed rail in italy and france

Tobias Fünke

Sure

qwerty

11:58

@fqq yeah it was like the top link I found from a quick search. idk what other public data is compiled and out there

Tobias Fünke

they are comfortable. Much more than e.g. TGV, imho

fqq

the trains are alright, in my limited experience between the bad and good TGVs, better than italian ICs but worse than frecciarossa/italo

Tobias Fünke

I have, unfortunately, no real experiences with the trains in Italy

fqq

high speed are quite good, other long-distance hit and miss, local trains around the big cities quite bad

Tobias Fünke

I see

Feynmate

12:27

I have always wondered: why do people say that German trains are always late? Of course, it sounds like a sterotype, but there is also the stereotype that Germans are very punctual people, so I can wrap my head around it

Either Germans are so punctual that they have unrealistic expectations, or German trains are not scheduled by Germans :P

Slereah

They don't have Mussolini to make the trains run on time

Feynmate

12:42

Wait, is that joke about trains and fascism know abroad?!

qwerty

yes

qwerty

13:07

physics.stackexchange.com/questions/841130/… this surprises me a little since it was a demonstration given to the class by an optics prof when I was in first year at uni, and it was also discussed in Dirac's QM book.

Ryder Rude

13:26

meditation brings out a calmer version of you

when u do it, dont just sit there. meditation is not about forcing urself to sit with eyes closed

it is about sitting and observing ur thoughts and make them go away whenever u want to

when u have this control, u r doing it right

when u make the thoughts go away, the mind should stay empty for a few seconds

DIRAC1930

16:00

@RyderRude What are the most profound physics books that you have read?

Ryder Rude

16:43

@DIRAC1930 i havent finished any physics book except shankar's principles of QM. it is an extremely profound book

other than that, i like Wald's and Carroll's GR books

i also like Nakahara's Geometry, topology Physics. but not the physics chapters. i liked the algebraic topology chapters

i havent encountered great QFT books yet. Schwartz was a pain to read

and Weinberg felt too dense

i am looking forward to reading quantisation of gauge systems

User198

17:14

@RyderRude I want to read Shankar's QM, but all the pdf s I found are pretty bad. And the book is kind of expensive 90€ + shipping

Ryder Rude

@User198 oh

User198

There is not a Dover edition right?

Ryder Rude

i read it on pdf

@User198 idk...

@User198 do u mean the characters are thick

i cant link pdfs here... it is against the rules

User198

No, its ok you don't have to

@RyderRude I mean its sometimes blurry but readable

Ryder Rude

yeah

u probably already have the pdf i read it from, if u downloaded them all

User198

17:20

I really like the feeling of having the book rather than PDF.

Its a whole different story when you have a book

Tobias Fünke

yeah

Ryder Rude

the pdf is not the best but i have nostalgic feelings toward it

User198

@RyderRude Yeah, I will just pick one that looks the best

Tobias Fünke

perhaps your university library has a copy

Ryder Rude

cuz i really loved reading this pdf

User198

17:21

@TobiasFünke I don't think soo. I will check tho thanks

Ryder Rude

the characters were blurry, yes

it gave the feeling of hogwarts books

i havent read paper books in long, so i might be biased. but i like the feeling of pdfs

User198

@RyderRude Hm for me its the opposite. I read pdf only when I don't have the book available. Books > PDFs for me :)

Ryder Rude

yeah. i would still choose paper if i had the option tbh. but it is also nice to have a whole library on one device

DIRAC1930

17:42

@RyderRude I don't think I've ever finished a book either

naturallyInconsistent

@qwerty Its kinda both. The classical description works perfectly if you just use only waves. However, the quantisation of the detection into photons is equally insane and non-classical. i.e. that experiment straddles both classical description and quantum, depending upon what aspect is being emphasised.

Feynmate

Does anyone else find AI generated videos extremely unsettling?

There is something about the way people move in those videos that makes me want to tear out my eyes

naturallyInconsistent

uncanny valley is where they all belong

Feynmate

Isn't this question more suited in the history of science SE?

Ah, a comment says that

naturallyInconsistent

and its neumaier

Feynmate

17:51

I know that :P

I mean, I've heard his name before on SE

naturallyInconsistent

sadly, he doesnt know his lattice qcd

Feynmate

I don't have enough lore to understand :P

naturallyInconsistent

with minimal explanation, you'll get it

Feynmate

Not lattice QCD, I mean your claim about Neumaier and lattice QCD :P

DIRAC1930

Arnold Neumaier comments and posts are amongst the most helpful on the whole of the stackexchange

He understands QFT, QED etc very deeply and has insights that are difficult to find anywhere

naturallyInconsistent

17:58

@Feynmate again, with a bit of explanation, it will be obvious

Tobias Fünke

I don't get it (either?)

Feynmate

either :P

naturallyInconsistent

or

good night~

PM 2Ring

This Arnold Neumaier? arnold-neumaier.at

naturallyInconsistent

yes

Tobias Fünke

18:09

+++

PM 2Ring

@Feynmate I try to minimise my exposure to GenAI output, to reduce the brainrot.

in Tavern on the Meta on Meta Stack Exchange Chat, Nov 1, 2024 at 16:20, by PM 2Ring

With a year or two of intensive feedback training, LLMs can become smarter than humans. However, some humans refuse to submit to the training...

Feynmate

It's odd enough, though. Videogames or animated movies are not necessarily realistic either

Maybe it's the ever-mutating background? I can't explain what I consider unsettling

Tobias Fünke

@Feynmate yeah. it looks odd!

FM, do you by chance enjoy(ed) the series "Love Death and Robots"?

or anyone else here :)

PM 2Ring

It's definitely an Uncanny Valley effect. It's very close to realistic, but AI gets it wrong in ways that a (sane-ish) human artist never would.

Uncanny valley

The uncanny valley (Japanese: 不気味の谷, Hepburn: bukimi no tani) effect is a hypothesized psychological and aesthetic relation between an object's degree of resemblance to a human being and the emotional response to the object. The uncanny valley hypothesis predicts that an entity appearing almost human will risk eliciting eerie feelings in viewers. Examples of the phenomenon exist among robotics, 3D computer animations and lifelike dolls. The increasing prevalence of digital technologies (e.g., virtual reality, augmented reality, and photorealistic computer animation) has propagated discussions and...

Tobias Fünke

Oh, I've never heard of this effect, I think. Thank you for the link

Feynmate

18:33

@TobiasFünke I'm afraid I don't know it, but I will baccalà alla ghiotta for dinner

Oh so, uncanny valley (@naturallyInconsistent) wasn't a meme word, there is a deeper meaning

@PM2Ring that reminds me a lot of the manga and anime Kiseijuu

[en.m.wikipedia.org/wiki/Parasyte](here)

It's not formatting properly (?) Anyways, thank you both for giving a name to my fears :P

PM 2Ring

Here's a recent example. It's pretty creepy, IMHO. youtube.com/shorts/rDn-uwsPve4

@Feynmate's link Parasyte

Feynmate

19:01

What was wrong with mine?

Tobias Fünke

the other way around

with the brackets

Feynmate

Oh right ahahahh

I don't know why ever so often, when I'm really tired, I swap them and I feel so sure the order is right

Well, actually the order is, it's just that I conflate the order of brackets with that of their arguments

Tobias Fünke

yes, sorry

Feynmate

19:46

Why are you sorry for my confusion? Did you drug me?!?!?!

skullpatrol

Knowing what mistakes one usually makes when they get tired is useful.

User198

From this $\psi(t)=U(t) \psi(t_0)$ I get this:

$\psi(t_0)=U^\dagger(t) \psi(t_0)$

But I am confused now, if the LHS is constant, how can the RHS be a function of $t$ ?

Tobias Fünke

20:08

@Feynmate no, but I said that the order of brackets is wrong while you reversed the arguments, no?

Feynmate

@User198 What is constant with respect to what? In that sense $\psi(t)$ is constant for a fixed $t$, it's the value of the wavefunction at that time

Tobias Fünke

@User198 what you write makes no sense

Feynmate

That being said, the evolution operator is $U(t,t_0)$

Tobias Fünke

$\psi(t)=U(t)\psi$ implies $U^*(t)\psi(t)=\psi$, and the RHS is time-independent as it should be.

Feynmate

@TobiasFünke But your message can be interpreted correctly depending on the picture, passive or active

Tobias Fünke

20:09

lol haha

Feynmate

Tobias, you have the fastest "lol/hehe" reaction time

2

Tobias Fünke

hehe?

hehehehe

Feynmate

I'm laughing right now, why am I so broken by sleep deprivation WHYYYY

skullpatrol

Get some well deserved sleep 😴

User198

@TobiasFünke Hm. RHS is time-independent as it should be, yes. It follows from the definition of the unitary transform.

Idk it is confusing that one side of equation is time inpendant and the other is time dependant. Maybe it should not be confusing.

Tobias Fünke

20:22

no why you say one is time-dependent?

2=1+x+1-x

both sides are $x$ independent

User198

Ah, I guesss the $(t)$ notation confused me

Ok thanks

2.) When deriving I got that
$A(t) = U^{\dagger} A U$

Now, since states flow in the opposite way than observables

Feynmate

Tobias, in case you are trying to understand User198's point of view (and not just giving hints), the mistake here is the same as conflating $t\mapsto f(t)$ with $f(t)$

User198

For the density matrix (that must be in the Schrodinger picture) I should just write $\rho(t)=U\rho U^{\dagger}$

Feynmate

I don't understand if the doubt has been solved or not, though

User198

@Feynmate Yes, Idk why I tought of it as a function

Feynmate

20:27

@User198 The density matrix is not an operator in the proper sense

User198

But it evolves as $\rho(t)=U\rho U^{\dagger}$ no?

Feynmate

@User198 You were thinking of the function mapping $t\mapsto\psi(t)=U(t) \psi(t_0)$, yes

@User198 Yes, because it is basically a state

User198

@Feynmate Yes, my bad

Feynmate

No one's bad

It evolves in the Schroedinger picture and it doesn't in the Heisenberg picture

User198

@Feynmate Yes ok thanks

Tobias Fünke

20:30

@Feynmate it is an operator

Feynmate

@TobiasFünke On the space of states?

Ok, yes, it is a sum of projectors

I phrased it badly. I meant that it should be regarded as a state, so you don't expect it to evolve in the Heisenberg picture

Tobias Fünke

yes indeed!

User198

@Feynmate Oks I understand yes

But what if I don't want to just "Reverse the order, because its flows in the opposite way"

I want to derive it from the definition of
${\rho =\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|}$

${\rho =|\psi _{j}\rangle \langle \psi _{j}|}$

Feynmate

Great, now: you're in the Schroedinger picture

What does that entail?

User198

I want to get $\rho(t)=U\rho U^{\dagger}$

Feynmate

20:34

I didn't ask for the conclusion. I'm guiding you :P

User198

@Feynmate That states evolve and operators don't

Feynmate

So, the states $\psi_j$ evolve

User198

Yes

Feynmate

Use $\rho(t)=\sum_j p_j\lvert\psi_j(t)\rangle\langle\psi_j(t)\rvert$

And use the time evolution of states

I'm starting to mentally read my messages with a Russian accent

User198

@Feynmate Haha, me too. Like you are some Russian general guiding a mission strategy

Feynmate

20:38

I was thinking more of "tensor is like watermelon"-like mantras @qwerty

That being said, it's time for me to go

We must away, ere break of day

User198

Cheers

Tobias Fünke

@Feynmate start from FM's expression, now just take out the time-evolution operators out of the sum

it is as easy as that

good night :) sleep well

qwerty

haha hi/bye ;)

Feynmate

I will catch up with Daima first. Spoilers loom and this week it ends. Bye!

User198

I got from:
$\rho(t)=\sum_j p_j\lvert\psi_j(t)\rangle\langle\psi_j(t)\rvert$

this:

$\rho(t)=\sum_j U(t)|\psi _{j}\rangle \langle\psi_j\rvert U^\dagger(t)$

Tobias Fünke

20:42

yes

now just take them out of the sum

User198

Hm

I can just disregard the sum and I get $\rho(t)= U(t)|\psi _{j}\rangle \langle\psi_j\rvert U^\dagger(t)$

But I want $\rho(t)= U(t)\rho U^\dagger(t)$

Tobias Fünke

no

User198

$|\psi _{j}\rangle \langle\psi_j\rvert =\rho$ ?

Tobias Fünke

$\sum ( A a_i B)=A(\sum a_i)B$

...

You are mixing up several things, I am afraid

You said: "I want to get $\rho(t)=U\rho U^\dagger$

What do you mean by that?

the equation is meaningless if you do not specify what the symbols mean.

User198

Yes ok right.

I get this:
$\rho(t)= \sum_jU(t)|\psi _{j}\rangle \langle\psi_j\rvert U^\dagger(t)$

$\rho(t)=U(t) \sum_j(|\psi _{j}\rangle \langle\psi_j\rvert) U^\dagger(t)$

$\rho(t)=U(t) \rho U^\dagger(t)$

Tobias Fünke

20:47

good

User198

That is good?

Tobias Fünke

Just to be sure: $\rho$ means the initial density matrix, and as you can verify $\rho(0)=\rho$ (or $t_0$ instead of $0$, depending on your conventions)

@User198 it depends on what you mean with this equation

qwerty

@naturallyInconsistent I may have heard your perspective before, but DanielSank said there was "absolutely nothing quantum'" about it and "a shame" it's used... which was a surprise to me

User198

@TobiasFünke Yes

@TobiasFünke Ok thanks. I think it is clear now

Thank you

Tobias Fünke

welcome

you can also see it just as an axiom

i.e. that the time-evolution of the quantum state (here the density operator) is given by what you've written

User198

20:53

this:
$\rho(t)=U\rho U^{\dagger}$

$A(t)= U^{\dagger}A U$ ?

@TobiasFünke Yes, and than do QM dynamics from there.

Ok I see thanks.

Tobias Fünke

yeah. I mean you either postulate the evolution of observables or of states

User198

Got it

Tobias Fünke

then you can introduce the corresponding other picture

nice

User198

Upvote +1

Tobias Fünke

hehe

Ryder Rude

23:06

@DIRAC1930 i also think it is useless to finish books. one should only refer to discussions or chapters

DIRAC1930

Ideally a book should contain no superfluous information. There are some books that are only 150 pages long that only make sense if you read it all (or up to the place that you need)

Silly Goose

23:44

superfluous is relative though

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