The h Bar

user54412

00:52

@dmckee It is salaried. My reading of the legalese seems to be that (1) I don't pay SS taxes, (2) I don't get SS benefits if I retire or are disabled from this job, and (3) the university takes its own cut of my gross income and puts it in its own retirement fund.

DanielSank

@ChrisWhite The uni taking money from your pay and putting into a retirement fund is standard operating procedure.

Not paying social security tax is new to me though.

user54412

:(

0celo7

trying to apply for a job

user54412

Given that I'm still not making much more than minimum wage, I'd really prefer to have that cash to spend now.

0celo7

I can't find the fucking button "show vacancies"

argh

@ChrisWhite fed min wage or local min wage?

user54412

00:59

local - 13/hr

0celo7

@ChrisWhite Yikes. That's the wage for seniors here...

user54412

I really hope the tech industry collapses soon.

DanielSank

Hey... screw you!

user54412

oh... I mean....

user54412

:p

DanielSank

01:03

:p

ACuriousMind

lol

DanielSank

Bad @ChrisWhite. No cookie.

user54412

I'm sure google has installed plenty of secret cookies on my machine

user54412

@Kreia I actually didn't recognize that pic of you

ACuriousMind

@ChrisWhite Go find them, at least you have something to eat then ;P

DanielSank

01:06

rekt

ACuriousMind

@ChrisWhite People tend to not look beneath the hood :(

DanielSank

@ChrisWhite Who's Kreia?

ACuriousMind

My current avatar

0celo7

03:19

certainly a ball not centered at the origin is convex, right?

ball arising from a norm

this seems quite strange indeed

I can't get a proof

0celo7

03:41

ah, the proof is tricky

DanielSank

04:47

What's going on in here?

Everyone behaving?

Secret

Still digging through my theory of art forms book

DanielSank

o_O

@ChrisWhite ::still wearing colander on head... wondering when I can take it off::

Secret

https://blog.reedsy.com/live/worldbuilding-tips-editor-martian?utm_source=facebook&utm_medium=p&utm_campaign=worldbuilding#magic

I think based on what I read so far, there's a joke counter to quantum mysticism:
Quantum cannot be a source of mysticism as some people believed, because while magic itself is nebulous and has ethereal rules, it is perhaps the best known adherent to a well defined causality. Quantum systems, however are shown to be able to display the property of causality with no well defined order due to the superposition of processes, thus saying quantum is the source of myst…

user54412

@DanielSank The next time you make pasta.

DanielSank

@ChrisWhite That could be a while...

@Secret That is oddly similar to one of my favorite arguments against typical interpretations of various organized religions.

user54412

05:01

quantum mysticism is a thing?

Secret

I always want to understand how magic people think, because magic and science have very similar analytical ways of thinking, which (in a risky sense of a non mainstream idea) might have something that can contribute to finding novel ways to analyse a science problem. This is because worldviews, like soft skills, are often transferrable between disciplines

DanielSank

> magic and science have very similar analytical ways of thinking...

Wat?

Secret

They both try to chunk up the cause of a phenomenon as made of up something more fundamental

except scince is rigoruous, reproducible

and has maths as a universal language

whereas magic is more handwavy and less rigorous

and most importantly, not always reproducible

Science and magic then contrast with religion, where the phenomenon is ascribed to some deity

DanielSank

I'm confused.

Science is a system used for understanding Nature. Magic is a made-up thing in fantasy books.

I've never met anyone who practiced magic as a way of understanding the world around them.

user54412

who are "magic people"?

user54412

05:06

DanielSank

^ That

Secret

https://en.wikipedia.org/wiki/Magic_(paranormal)
Well, magic nowadays are pretty much fiction anyway, and as a scientist, I kinda doubt or at least agnostic about their existence

@DanielSank no, its more complicated than that, while the systematic way of thinking is simialr, the end goal of magic is more similar to religion, to either brought out a desired effect via some mystical forces, or for self actualisation and meaning

after all it kinda justifies in historical records since before the rise of the catholic church, magic used to be a bunch of other religions

Also back in medieval ages, alchemy is a way to investigate how various materials transmutate between each other (plus some spiritual stuff that I don't understand from wikipedia and other articles), and that is the precusor to the later development of chemistry

The magic we all saw in fantasy books, are based on magic as a social phenomenon back in the past (plus some exaggerations, distortion etc.)

The bottomline is, even if magic is nonexistent, there used to be a social phenomenon based on that concept which has far reaching influence to the development of both religion and science

DanielSank

This is the magic I know about...

Secret

05:22

yes, that is a subset of the concept I mentioned above, and pretty much how most people think of magic nowadays

peterh

05:49

@Secret Laws of the nature are universal. At least the ones we know. But what if there are also laws, which aren't? What if there are laws of the Universe which work only once?

Secret

then like what DavidZ mentioned, it is an untestable hypothesis and it cannot be inquire by science

and we will need something more complete to find out the truth

user116211

07:43

Nothing I can tell about this; but sounding engineering or primarily opinion based; though can't say anything as I've no idea on this; still check it:

user116211

0

Q: Scattering of acrylic glass in ice

For some biological testing reason, I have to place some tubes in a frozen sea ice environment. I need to choose a proper material. I would like something that leaves the visible light field within the ice as undisturbed as possible. Therefore, no scattering, and no absorption (that could warm ...

David Z

09:08

@MAFIA36790 I think the only reason it sounds opinion-based is the short last paragraph, which could be edited easily.

peterh

11:50

5

Q: Product of one minus the tenth roots of unity

If $1$, $\alpha_1$, $\alpha_2$, $\alpha_3$, $\ldots \alpha_9$ are the $10$th roots of unity, then what is the value of $$ (1 - \alpha_1)(1 - \alpha_2)(1 - \alpha_3) \cdots (1 - \alpha_9)? $$ I am not being able to solve this. Please help!

complex-numbers roots-of-unity

Another wonderful solution to the homework problem.

Balarka Sen

Easier would probably be to take the limit of $(x^{10}-1)/(x-1)$ as $x \to 1^{-}$ but one needs a bit of justification there.

ACuriousMind

12:22

@Secret You're looking at "alchemy" through a modern lens here, imbuing it with your modern scientific notions of "investigating". That "spiritual stuff" is the core of what makes alchemy, and "magic", not science. They do not proceed by anything resembling the scientific method. Completely unfounded beliefs are freely mixed with results from experiments, and no distinction is drawn.

Crucially, the goal of alchemy also was not to understand something, but to do something specific (e.g. transmute lead into gold) when there wasn't even reason to believe that goal to be possible

peterh

My problem with the PSE close reasons, that they are too broad. :-)

ACuriousMind

I don't know where you got the strange idea that "magic" or alchemy are somehow equivalents of science, or have a "similar analytical way of thinking"

I also don't know what, exactly, you mean by "magic", so I restrained myself to the alchemy part.

0celo7

12:41

Lol ACM doesn't know what magic is

ACuriousMind

@0celo7 I find it surprisingly difficult to define "magic" other than in opposition to science, so I'm cautious about just assuming what others mean by it.

0celo7

12:57

@ACuriousMind spells and stuff

Secret

13:09

@ACuriousMind Ok, it seems other than the transmutate lead into gold, and making the philosopher stone, I got pretty much everything wrong. (I think I still don't fully understood magic, which is why I don't understand those spiritual stuff)

@ACuriousMind and as for that, I need to check my notebook. I got that idea from a past seminar of something which somehow get mixed with some other random concepts

Secret

13:31

ok I cannot locate that seminar, but from my logs, the 2nd nearest thing that reoinforce that weird idea is from a TVTropes page

tvtropes.org/pmwiki/pmwiki.php/Main/SufficientlyAnalyzedMagic

Section=Real Life

So based on what you said, it is highly possible that th alchemy example there are looking back alchemy through a modern lens

> Here's an extended example: A certain crystal has the properties of Oil of Vitriol. Another crystal has properties of Saccharum Saturni. You need, for an alchemical process, a crystal with the properties of both Brimstone and Lead, which are components in both crystals. You make use of a specially-prepared solvent to separate the crystals into their component properties, and then because of the strong affinities of Lead and Brimstone, compared to between Oil of Vitriol and Saccharum Saturni, a new crystal, Anglesite, forms that cannot be separated by this solvent, so you pour off useless …

So perhaps, what I mean by "analytic" might be trying to describe how they both have this kind of "step by step", "doing things in order" and "mixing various basic things to get some new thing" property (which I probably lack words to describe and misused the word "analytic"

ACuriousMind

@Secret 1. TVTropes is an endlessly fun resource, but it is not scientific. 2. They are not saying that alchemy was scientific. They are saying that once alchemy became a "Sufficiently Analyzed Magic", it stopped being alchemy and started being science.

The whole page is about the trope where magic is treated as science so much that it starts to become indistinguishable from simply being science in a world with different natural laws.

Secret

Ok that means things are more complicated actually. suppose we have 3 concepts, religion, magic and science. We knew that reproducibility and the scientific method is a qualia of what is science, the presence of one or more divine entity to carry out a purpose according to a will is a qualia of religion, then what is the qualia of magic? (paragraph continue on next post)

Danu

in Mathematics, 25 mins ago, by user 1618033

@JuanFran I revolutionized my mathematics in some sense, and I repet, in some sense, this is my opinion, not bragging, not anything like that.

This guy always triggers me

every time

Secret

For example, we knew in history the 3 branches of social phenomenon and movement are closely intwined. Like how I learnt from a PhiloSE question that magic used to be religion like before being outlawed by the catholic church, and how as seen from a modern lens, some science fields begain from mystical practices such as alchemy. Therefore:

ACuriousMind

@Danu Intentionally you or are you just annoyed with him? :P

Secret

13:45

The question becomes:
The 3 social phenomenon - mystical stuff = science
The 3 social phenomenon - understand something =magic and religion
magic and religion - ??? = magic?

Danu

@ACuriousMind I'm not actually in that conversation, so I meant just generally.

The way he's literally always talking about how great his "new discoveries" are...

ACuriousMind

@Secret Stop. You can't just assert that there are "3 concepts: Religion, magic or science". You obviously don't know what "qualia" means (the singular is quale, and it means the subjective mental state of experiencing something that is effectively incommunicable to other beings), and saying that "magic used to be religion" directly undermines you taking them as different concepts.

@Danu Does he actually show those discoveries?

Danu

@ACuriousMind No

ACuriousMind

lol

Danu

Also, they're all integrals of infinite series (and their solutions).

That's his only interest

(her?)

ACuriousMind

13:49

@Danu Oh dear

Secret

1) This is where I got the idea there are 3 concepts en.wikipedia.org/wiki/Western_esotericism. paragraph 1. 2) Sorry the word I intended to type is "quality" in the context of philosophy 3) I learnt the idea of "magic used to be religion" from the answer of this PhSE: philosophy.stackexchange.com/questions/32409/…

So based on what you said, I must have overlooked something

If I have not mistaken: Judeo-Christian religion give rise to modern chirstianity, Enlightenment rationalism give rise to scientific method, thus that leaves esoterics

ACuriousMind

@Secret So, uh, you call everything that's not Judeo-Christian religion or science "esoteric"?

You're not making any sense at all to me.

Instead of cobbling together different pieces from different sources without a connecting thread, try making a coherent argument for once

ACuriousMind

These gravatar issues are...unsettling. Has @Qmechanic turned green and spiky for anyone else?

Secret

I think the root of my arguement is based on that wikipedia article, which the phrasing of it gives a suggestion there are only 3 types of social movements (which I might be wrong to conclude that suggestion). Back when I first read this 2 years ago, I am trying to understand how is esoterics not science nor judeo christian religion and what is the key common feature that defines it

My thought process whenever learnign a new thing:That is, given 3 things {a,b,c}, in order to understand them better, I always try to look for what defines e.g. a by looking for a and not b and not c

ACuriousMind

14:04

@Secret You really have to learn reading. Saying "Esotericism is a name for certain traditions distinct from Judeo-Christian religion and Enlightement" is a completely different thing than "The only three traditions are esotericism, Judeo-Christian religion and enlightenment.".

Secret

Ok then I guess one thing that went wrong in my thinking is if I see "a is <insert description> that is different from b and c" $\rightarrow$ "There are only a,b and c". I will keep that in mind

ACuriousMind

Yes. It's always good to keep basic logical principles in mind when you are trying to make an argument others are supposed to understand.

Secret

Still, after reading this whole article, there does not seemed to be a common thread that connect all esoteric practices, thus I have no idea what esoterics is

It seemed to be not as clear cut as other concepts

0celo7

@Danu you're probably just jealous cuz you can't do hyperelliptic Jacobi integrals of the $\sqrt\pi$-th kind.

2

ACuriousMind

@Secret Some concepts are less clearly delineated than others, and no concept is so clearly delineated that you can't find some edge cases. That's always the case. What's your point? Why are we talking about the Wikipedia article for "esotericism" when this whole thing started with you asserting that "magic and science have similar analytical ways of thinking"?

Instead of presenting an argument that's supposed to convince me of anything, you've just given a host of unrelated and partly misunderstood references.

(Please don't answer the "why" part of that literally by recapping our conversation to me.)

Qmechanic

14:17

@ACuriousMind : Damn, I was trying to be incognito :) Nah, it's the same as this meta post.

Secret

The why: en.wikipedia.org/wiki/The_Librarian_(franchise) The librarian is the franchise that arose my interest in understanding the concept known as magic, because the magic featured in the franchise all have historical references and roots, thus not the "pew pew type" magic commonly seen in game, or almost completely fabriacated magical rules seen in Harry Potter. Seeing from The Librarian finally give a suggesstion to me that

ACuriousMind

@Qmechanic I know, I already linked that post under "gravatar issues" ;) So you're seeing yourself green, too? Because it seems not all users are equally affected, e.g. I see only pink at this answer

Secret

magic can finally be understood without its common reputation in worldbuilding that it is "all up to the author", I want to find out what it really mean for something to be magic. I then started chatting with various people from spiritual to art to science in order to better understood it. The chats so far lead to my (later found to be misinformed) assertion that "magic and science have similar analytical ways of thinking", that's how it all started

I even started paying attention to some artifacts tied to magical practice in e.g. Musee du Louvre in my recent trip to get a better idea

More generally, my mode of learning a new knowledge is that: I always interested in findnig out what makes A to be A, and not something else

ACuriousMind

@Secret My issue here is that the only support for your thesis that "magic and religion have similar analytical ways of thinking" was "alchemy became science". Knowing your personal motivation for looking into this doesn't do anything to convince me you're right - you have to give arguments.

This is a common and very frustrating issue with what you say for me: You rarely give arguments, you always retreat to "my mind just works that way". Well, that may be, but the way your mind works is not an argument for something to be right or wrong.

I have all sorts of ad-hoc associations too, but I don't go around stating them as facts.

Secret

I never say I am right, because I am not sure, I am more of trying to get more views on the topic in question. By combining all you guys viewpoints, I can then work out what is right

Qmechanic

14:26

@ACuriousMind : Now I see only green versions.

Secret

@ACuriousMind For that point, it is a side effect that I always thought I get the correct information (after talking to many people of diverse backgrounds and cross verifying), and thus which is why it is stated like facts, only to be pointed out wrong by the n+1 th person or worldview

so basically to me for this discussion, it was found I got the wrong assertion, and thus I need to re read and re analyse the situation again to learn better

ACuriousMind

@Secret Where is this "cross-verification"? Are you just taking what people are telling you at fact value without asking how they know what they say? If you are "cross-verifying" things, where is that evidence? Why don't you use that as arguments instead of telling me about some TV show?

0celo7

@Qmechanic Ah!

Qmechanic impersonator

@ACuriousMind tbh, your conversations with Secret seem to be getting weirder and more pointless all the time

Balarka Sen

Shit just got real in the mathematics room.

0celo7

let me quantum warp right over

Balarka Sen

14:31

And I wasn't there the whole while. Boo, could have been a good popcorn-passtime.

ACuriousMind

@0celo7 Well, I was tired of seeing all those weird and unconnected posts in the chat. And Secret also asked why it's so hard to understand him and how to do better. So I thought I would actually engage with his communication style and try to point out where it's flawed.

Secret

For me, I tend to ask people and books and internet source the question "How do you think of <my assertion>. Does it make sense in your view, how will you interpret about <assertion> based on your field of knowledge, what is missing, what is another way to think of it?"
By asking books, a more concrete example is e.g. in quantum, say, "do wavfunctions evolve determinstically", I then google or check a quantum text for the answer to that question

ACuriousMind

However, I seem to be getting nowhere.

0celo7

@ACuriousMind Sort of like how I think short people are really smug?

Secret

@ACuriousMind Ok, let me put this in another way: Specifically for this case, I initiallyshow you the sources on how I learn about ideas that lead to my assertion, you then point out what went wrong with them, As the discusison continues it becomes clear my assertion is wrong, thus there is nothing else to present. does this sound clear?

ACuriousMind

14:35

@0celo7 lol, yeah, I guess

0celo7

@ACuriousMind It makes me so mad thinking about short people

Balarka Sen

The trash-ed transcript is an amazing read.

0celo7

Juan Fran is a bit of a dick

Also he doesn't include geometry in his list of "serious mathematics"

Secret

I only continue on arguing about something if I knew my assertion is well supported. But if during the course of the presentation and it became more apparent that the assertion is unfounded, then I will say what I went wrong and stop defending the assertion

I suspect it might sound weird because most people will defend the assertion regardless of how much it is revealed to be unfounded

0celo7

@BalarkaSen Have you come to terms with flows?

Balarka Sen

14:47

Learnt the proof of their existence.

0celo7

Was it more than I said it was?

ACuriousMind

@Secret Nevermind, I guess I'm just frustrated with how often "what went wrong" seems to be a simple misreading of the source. It's not fun to have a discussion end with "You read that sentence wrong".

Balarka Sen

Nah, but one needs to be a bit careful to go from a local solution of the differential equation of the flow to a whole one-parameter family of global solutions.

But it's no biggie.

0celo7

@BalarkaSen Oh, I thought you wanted local existence

Secret

Not many people are willing to give up an assertion even if a lot of evidence against it. But I think that is a bad attitude, because too strong beliefs on something will prevent one from seeking the real answer

0celo7

14:50

but IIRC you only get global solutions if the vector field has compact support?

Balarka Sen

Dude. Compact manifold.

0celo7

:P

Balarka Sen

:)

Secret

This is one reason why I seemed to be not really arguing things because the argument can go unfounded in the middle and the discussion then ends with a conclusion of what has been learnt

0celo7

Some people have done Ricci flow on noncompact manifolds

Apparently all the equations blow up and it's a shit-show

Compactness is an unreasonably nice condition

Secret

14:52

I have been reading some logic from the topology notes I showed 0celo7 earlier, I hope that will help minimise my misreading sentence problem, that might help on having more fruitful arguments

0celo7

lol

Balarka Sen

So, if $M$ is compact, suppose $X$ is a vector field on it. You flow along that to some diffeomorphism $f : M \to M$, fixed points of which should be the places where the flow stops: namely, the zeroes of $X$. Index of $X$ at those zeroes should precisely be the Lefschetz number of $f$ at those points - need to check this. Once that's done, Lefschetz number of $f$ becomes the index of $X$.

0celo7

mathematical logic will not help you communicate better

Balarka Sen

But we obtained $f$ by flowing: that naturally gives a homotopy from the identity map - Lefschetz no. of which is $\chi(M)$ - so we'd have proved the Poincare-Hopf.

This is le idea.

0celo7

@BalarkaSen It's nontrivial to show that the flow has a fixed point iff the vector field does.

One direction is easy, nay, I dare say it's trivial.

Balarka Sen

14:54

The other direction isn't obvious to me, yes.

0celo7

Does GP prove it?

I once saw an MSE post about it and the answer was pretty complicated.

@BalarkaSen Yes, that seems like the correct idea.

Is that how GP goes about proving it?

I like my cohomological proof just fine :)

Balarka Sen

@0celo7 Da, but there they don't bother with flows. Only moving forward along $X$ at time = 0 is sufficient.

0celo7

Moving forward along $X$?

Balarka Sen

You don't need $X$ to be tangent to the flow at all time.

0celo7

...isn't that what a flow is?

@BalarkaSen Oh.

Balarka Sen

14:58

@0celo7 I am not sure if it'd be too hard.

I mean, we're moving along the integral curve for any given point, yeah?

0celo7

@BalarkaSen I feel like I should do all of the exercises in Chow, Lu, Ni's review chapter on Riemannian geometry

I should formalize my holonomy proof, for one

hmm

@BalarkaSen yes.

@BalarkaSen It's not true for noncompact manifolds btw.

Balarka Sen

If $X$ is nonzero at $p$, the integral curve of $X$ starting at $p$ shouldn't stay put. I don't know how to prove this.

0celo7

Flows can have fixed points without the vector field being zero.

Balarka Sen

Oh, maybe I do.

But that's not enough, is it? We can always find an $\epsilon$ arbitrarily small so that starting from the integral curve at $p$ and moving $\epsilon$-forwards flows me a nonzero distance.

But this $\epsilon$ need not be uniform.

Ah, this is why it's not true in noncompact manifolds, @0celo7, methinks.

0celo7

You can make it uniform on a compact manifold, no?

Balarka Sen

15:02

Yup.

0celo7

Are the fixed points of the flow necessarily isolated?

Balarka Sen

For compact domain, should definitely be.

I'll have to ponder on these. Handwaving is not enough.

0celo7

@BalarkaSen I have to get to class.

Balarka Sen

Flee, then.

vzn

EmDrive: Nasa Eagleworks' paper has finally passed peer review, says scientist in the know / ibtimes

Danu

15:51

@ACuriousMind Yup.

DanielSank

16:05

@vzn "Highly controversial"? Why do pop-sci writers say stuff like that?

0celo7

16:17

@ACuriousMind The eleven lord hath blessed me with another Problem-Set! All hail Au'riel!!

vzn

@DanielSank not following. you dont think its highly controversial? (why do experts constantly complain about popsci writing? ... nevermind...)

0celo7

This problem set is much harder than the previous one

Sir Cumference

16:40

@0celo7 So, uh, was anything I said helpful?

Also anyone here know GR? I've got a question I've asked ACM, but I'm looking for some other opinions too

John Rennie

@SirCumference I know GR - well, bits of it

Sir Cumference

And QFT?

My question involves Hawking radiation

John Rennie

I know next to nothing about QFT. But you could try asking. It's free.

Sir Cumference

All right, as I approach a black hole's event horizon, time dilation will increase to infinity, right?

John Rennie

Careful!

Sir Cumference

16:46

In other words, I would see the rest of the Universe speed up before my very eyes. So, before I reach the event horizon, will the black hole have already evaporated through Hawking radiation?

John Rennie

My understanding is that yes the black hole will evaporate before you can reach the horizon.

But I'm not sure the situation is definitively understood because, well, it's hard!

Sir Cumference

I'm not talking about the Schrödinger solution by the way, I'm talking about whatever solution approximates black holes as finite, evaporating objects

Okay, thanks. I'm trying to get other opinions on it

John Rennie

Schwarszschild you mean?

Sir Cumference

Oh jesus christ

I can't believe I said Schrödinger

I wrote that in one of my answers...

Yes, Schwarzschild

John Rennie

@SirCumference No, it was definitely Schwarzschild :-)

Sir Cumference

16:48

Haha

ACM said "we don't understand black holes well enough", some have said "yes" and others have said "no"

John Rennie

We don't have an analytic solution for an evaporating black hole.

We normally use a Schwarzschild solution and perturb it.

Sir Cumference

Funny enough, someone asked the same exact question I've had just yesterday

2

Q: Would a black hole evaporate via Hawking radiation before you fall past the event horizon?

It takes an infinite amount of time for something to fall past the event horizon of a black hole from the perspective of someone outside the event horizon. Black holes also evaporate after a finite amount of time from an outsider's perspective due to Hawking radiation. Does this mean that you ...

black-hole hawking-radiation

Though the answers are not what I'm really looking for

They don't seem to explain it well enough

John Rennie

The answers and comments to that question are largely rubbish.

Sir Cumference

@JohnRennie Hmm, I posted my own, reload

I deleted it since, as you could tell, everyone was dead certain on a "no"

ACuriousMind

@JohnRennie That's exactly why I was reluctant to say "yes" or "no". Hawking radiation is a "property of the horizon", but if you don't let stuff fall into a Schwarzschild black hole, but e.g. a "real" formed black hole like the Oppenheimer-whatshisname collapsing solution, I don't even know if we predict Hawking radiation from that thing.

John Rennie

16:52

Snyder

Sir Cumference

@ACuriousMind So, was my answer correct then?

@ACuriousMind Tolmon-Oppenheimer-Volkoff?

John Rennie

That's a pretty good summary but it's not an answer since it just says we don't know.

Sir Cumference

@JohnRennie Au contraire, it clears things up, even if I was just saying "we don't know"

0celo7

@SirCumference what did you say?

ACuriousMind

@SirCumference Well, "We don't really know" is the answer I gave to you. I am however certain that you'll find many people, especially those engaged in the information paradox debate, who are inceplicably certain they know the correct answer.

Sir Cumference

16:55

@0celo7 That stuff about senses

Being logarithmic

0celo7

What?

John Rennie

@SirCumference That's the limit on the max size of a neutron star isn't it?

Sir Cumference

@JohnRennie Yep, so I was guessing that's the name (never heard of any such solution before)

@0celo7 This

John Rennie

@SirCumference The simplest analytic solution for a collapsing star is the Oppenheimer-Snyder metric.

Sir Cumference

@JohnRennie Ah, okay

John Rennie

16:56

grwiki.physics.ncsu.edu/wiki/Oppenheimer-Snyder_Collapse

ACuriousMind

@SirCumference John's "Snyder" was the correct answer, Oppenheimer and Snyder tried to describe gravitational collapse accurately.

Sir Cumference

@ACuriousMind John Synder?

Jeez, I can't believe I was saying "Schrödinger solution" instead of "Schwarzschild solution". I must be out of it

ACuriousMind

@SirCumference what?

JohnRennie said "Snyder" immediately after I said "Oppenheimer-whatshisname"

Sir Cumference

Oh, wait

No I'm just stupid

Ignore me

John Rennie

@SirCumference (c) 0celo7 - he'll be demanding royalties

Sir Cumference

16:59

@ACuriousMind But zephyr brings up a good points about black holes, in that question

> Besides, logic should indicate that if that were true, nothing could fall into a black hole and thus no black holes could every form or grow larger

Can we simply say "no" then? Or just "we don't know enough about black holes"?

John Rennie

Well strictly speaking no black hole ever forms. Though it depends on exactly what you mean by black hole.

Sir Cumference

@JohnRennie You can find a solution where a black hole forms from star collapse

John Rennie

See:

127

Q: Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he said. References: Article in Nature News: Stephen Hawking: 'There are no black holes' (Zeeya Mera...

black-holes quantum-gravity hawking-radiation event-horizon

ACuriousMind

@SirCumference This is a point of large confusion and no one is quite sure how to resolve it. See e.g. How can anything ever fall into a black hole as seen from an outside observer?

John Rennie

It takes infinite coordinate time to form, where coordinate time is the time as measured by an external observer at infinity.

Sir Cumference

17:01

@JohnRennie What solution are you talking about? Schwarzschild?

If so, then yes, black holes must be eternal. But the Schwarzschild solution doesn't explain how a star could collapse into a black hole.

ACuriousMind

I think the main issue here is the large amount of idealization done to model the situation, but people keep trying to ask thought experiment questions that more or less evidently violate one of the idealizations.

John Rennie

@ACuriousMind Rubbish! Every GR-head knows how to resolve it. The only problem lies in people inisisting that GR must conform to their preconceptions. Just like QFT really :-)

@SirCumference Oppenheimer-Snyder. In the OS collapse the event horizon take infinite coordinate time to form.

Sir Cumference

@JohnRennie I'm sure there is some solution that approximates black holes as something that forms in finite time

John Rennie

@SirCumference Wrong.

Sir Cumference

Really?

John Rennie

17:04

Unless you redefine what you mean by black hole of course.

Sir Cumference

Well, black hole can have multiple meanings really, depending on what solution you're using. Schwarzschild defines it as something that exists for all times.

John Rennie

Leaving aside the Hawking radiation, most of us use the term black hole to mean something that will inevitably form a horizon given infinite time.

Sir Cumference

@JohnRennie Absolute or apparent? Or both?

ACuriousMind

@JohnRennie If you mean the "obvious" resolution that horizons never form, then I don't understand why many "GR heads" are so bothered by things like the information paradox which seem inextricably linked with horizons

John Rennie

@SirCumference a true horizon not an apparent one.

@ACuriousMind The information paradox only troubles QFT heads. We GR heads don't care :-)

Sir Cumference

17:07

@JohnRennie Welp, how do you define an event horizon? The observable universe has its own cosmic event horizon, which is analogous to those of black holes

John Rennie

The universe doesn't have a true horizon at the moment. It will have, courtesy of dark energy, but it will take an infinite coordinate time to form.

ACuriousMind

@JohnRennie Mh. Okay, I'll let you weasel out of this one :)

Sir Cumference

@JohnRennie Our light cone will keep approaching the event horizon, though. After enough time, shouldn't objects begin to stop and redshift once they approach it?

Just like if they were touching the event horizon?

John Rennie

@ACuriousMind I take your point, and I honestly don't know if and how the information paradox is modified by the fact no true horizon ever forms. Though presumably that was Hawking's point I alluded to above.

Sir Cumference

Regardless, then if no GR solution describes how black holes form in finite time, then do we have an unsolved problem in physics?

John Rennie

17:12

@SirCumference I've never looked into what happens as the de Sitter horizon forms, but I imagine that yes we would see things slow and redden just as we would for things falling into a star undergoing OS collapse.

ACuriousMind

@SirCumference What would that unsolved problem be? We have never observed a true horizon, so there's no reason to expect the to be a finite-time black-hole forming solution

John Rennie

@SirCumference I'm not sure I see what you're getting at. Black holes don't form in a finite time.

So how is the fact we have no model for them forming in a finite time a problem?

ACuriousMind

In fact, I believe that a horizon forming in finite time would often amount to a topology change that's explicitly forbidden, but that's more of a question for @0celo7 ;)

Sir Cumference

I'm asking, if black holes can't form in a finite time, how would that explain all the black holes that exist today?

John Rennie

@SirCumference No black hole exists today.

Sir Cumference

17:14

@JohnRennie Okay. What do you define as a black hole?

John Rennie

Something with a true horizon.

Though this is a somewhat purist definition of a black hole.

Sir Cumference

I'm talking about those astrophysical objects in space, at the center of most massive galaxies or sometimes in the center of globular clusters

John Rennie

Those are objects so compact and dense that will inevitably form a true horizon given infinite time.

(and no Hawking radiation)

Sir Cumference

@JohnRennie Huh? I thought an absolute horizon forms before an apparent horizon?

John Rennie

But as of right now they have no true horizon.

Sir Cumference

17:15

Especially if we take the Oppenheimer-Snyder model

The event horizon forms first followed later by an apparent horizon that is at, or interior to, the event horizon.

John Rennie

What do you mean by absolute horizon?

Sir Cumference

Apparent horizon

In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward. Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from event horizons. Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole. It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in...

Absolute horizon

In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again. An absolute horizon is the boundary of a black hole by definition. In the context of black holes, the absolute horizon is almost exclusively referred to as an event horizon, though this is often used as a more general term for all types of horizons. The absolute horizon is...

John Rennie

OK absolute horizon = true horizon = event horizon

Sir Cumference

Excuse my typo earlier, I meant an absolute horizon forms before an apparent horizon

@JohnRennie There's more to it than that. Both describe how light will interact.

> Within an apparent horizon, light is not moving away from the black hole, whereas in an absolute horizon, light cannot escape from the black hole.

> It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in the future will not be able to escape (e.g. because the mass of the black hole is growing) and therefore inside the absolute horizon.

0celo7

@ACuriousMind Yikes, no questions about dynamical black hole formation please

Geodesics balls, yes.

Black hole formation devolves into a mess of functional analysis and PDE real quick

John Rennie

17:22

I can't remember how the position of the event horizon is described in the OS collapse. I'd have to go away and read up on it, which I'm not going to do right now.

I think the true horizon radius is a function of time. It starts with radius zero and grows outwards to the Schwarzschild radius in finite proper time but infinite coordinate time.

Sir Cumference

@JohnRennie Au contraire, as far as I know, the absolute horizon is essentially where the apparent horizon will eventually meet

So it is essentially the future location of the apparent horizon

John Rennie

Right now a cold beer and my latest SciFi book are calling ...

@SirCumference I'll defer to your knowledge of the Oppenheimer-Snyder metric.

0celo7

@JohnRennie "today" is a dangerous word in GR...

Sir Cumference

@JohnRennie Don't take my word for fact though. I'm still learning GR, this is just what I think I understand

Someone like @ACuriousMind is more fit to describe them than me

0celo7

@SirCumference No one in this chat has the background necessary to understand black hole formation.

ACuriousMind

17:27

@SirCumference Uh, my knowledge of GR is thin beyond the basics and what I remember from random talks I heard

Sir Cumference

@JohnRennie And really, I don't know nearly as much as I would like to.

I've just read this information around

0celo7

@ChrisWhite is your best bet, but I doubt he understands the precise theorems.

Sir Cumference

@ChrisWhite Where are you when we need you most?

ACuriousMind

@0celo7 Well, I'm not sure about @ChrisWhite . Maybe Dr. White can take a break from worrying about his financial future to enlighten us

Ah, I see you had the same thought

0celo7

@ACuriousMind By understand, I mean the analytic nitty-gritty.

Maybe you with your vast amount of Sobolev-space knowledge could ;)

ACuriousMind

17:30

@0celo7 I'd also just take someone knowing what certain results there are

I don't want the detailed proof of those, at least not currently

@0celo7 Ahahahaha. Ha. Ha...

Fucking Sobolev spaces...although, with a bit of distance, that lecture wasn't as bad as I complained back then

0celo7

@ACuriousMind Sadly our functional analysis course here is all operator theory and QM

I would gladly trade with you

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