печально я гляжу
Aug. 8th, 2020 12:41 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
juan_gandhiВот тут
https://www.youtube.com/watch?v=QcUey-DVYjk
"излагают гравитацию" на пяти уровнях: маленькой девочке; 16-летней школьнице; студентке младших курсов; аспиранту; завкафедрой физики.
Маленькая девочка в общем-то вполне врубается в ее уровень, хотя и довольно примитивно; известные мне дети все были гораздо более продвинуты; но в целом же Ей Было Интересно, и в общем-то нормально.
16-летняя, которая планирует стать физиком - уже швах. На половине рассказа ее глаза помутнели (я преподаю, так я вижу это сразу), глядеть она стала не в глаза астрофизику (женщине), а, извините, на ее грудь, и элементарных разъяснений она ни хера не поняла. По мне так контент был для третьего класса (советской школы).
Студентка, якобы физик, при этом китаянка - ну это был абсурд. Внимание она не ослабляла, китаянка все-таки. Но эта физика у нее на уровне седьмого класса (советской) средней школы. Ужас. Т.е. задачи на всякое там бросание шарика под углом она, по-моему, не решит.
Аспирант-физик, занимается нейтронными звездами. В черной дыре, говорит, space-time breaks down. Но это ладно; так-то вполне нормальный физик, но оба уже перешли на язык, "понятный народу".
https://www.youtube.com/watch?v=QcUey-DVYjk
"излагают гравитацию" на пяти уровнях: маленькой девочке; 16-летней школьнице; студентке младших курсов; аспиранту; завкафедрой физики.
Маленькая девочка в общем-то вполне врубается в ее уровень, хотя и довольно примитивно; известные мне дети все были гораздо более продвинуты; но в целом же Ей Было Интересно, и в общем-то нормально.
16-летняя, которая планирует стать физиком - уже швах. На половине рассказа ее глаза помутнели (я преподаю, так я вижу это сразу), глядеть она стала не в глаза астрофизику (женщине), а, извините, на ее грудь, и элементарных разъяснений она ни хера не поняла. По мне так контент был для третьего класса (советской школы).
Студентка, якобы физик, при этом китаянка - ну это был абсурд. Внимание она не ослабляла, китаянка все-таки. Но эта физика у нее на уровне седьмого класса (советской) средней школы. Ужас. Т.е. задачи на всякое там бросание шарика под углом она, по-моему, не решит.
Аспирант-физик, занимается нейтронными звездами. В черной дыре, говорит, space-time breaks down. Но это ладно; так-то вполне нормальный физик, но оба уже перешли на язык, "понятный народу".
Завкафедрой физики из NYU. Смотрит а астрофизика как на говно. Ну, этим разговором я насладился! Не знаю, как вы, я не физик. Но я насладился. Квантовая гравитация! (в изложении для лохов типа меня)
Но трехмерная голография!
И тут мы открываем матрицу: https://t.co/UMAeTFyEkT?amp=1
Хорошо эта астрофизик сравнила гравитацию с температурой.
Такие дела.
Re: Sturgeon law
Date: 2020-08-10 10:14 am (UTC)They both tried to choose words suitable for the large audience. Choice of words in such settings doesn't reflect someone's understanding. Most popular descriptions of modern physics sound sloppy or even incorrect, because otherwise they get very technical.
>So, when a star dies, if the star is massive enough, there's a huge explosion, called a supernova, and the stuff that's left behind, that doesn't get blown away collapses into a tiny compact blob called a neutron star.
~~~~~~~~
That statement implies that:
- Supernova explosion may result in the whole stuff to get blown away completely
No, there is no such implication here. If there's always some stuff left, we can address it as the stuff that's left behind, the stuff that doesn't get blown away, two wordings of the same thing.
> - Neutron star forms after supernova explosion. [That is not true, because a neutral star forms before the explosion, not after].
Why exactly do you think so? When the outer layers of the original star are still there, can you really call the collapsing core a star?
>Another sloppy statement from this Phd Candidate:
===
A black hole is sort of like a neutron star's big brother.
===
But, actually, a black hole does not have to have a neutron star inside.
He doesn't say there's a neutron star inside, you're making things up again. It's a "big brother" in the sense of being more massive, creating stronger spacetime curvature and often being literally bigger. And "brother" because they are born in very similar processes.
>~~~The event horizon of the black hole forms, which is the shadow, the curve that's so strong that not even light can escape.~~~~~~~~~~
A curve of a black hole is "strong"? Really?
She's talking about spacetime curvature. Yes, black hole curves it stronger (at the "surface"), the very definition of its event horizon has to do with how spacetime is warped there to not let light outside.
>-----
it collapses to a black hole, and those are so dense that space-time breaks down.
-----
What does "space-time breaks down" even mean?
I can think of at least 3 things these words may mean. The swap of signs in the signature of the Schwarzschild metric between time and radius, so radius behaves like time and vice versa. The fact that for a static outside observer the region behind the horizon is excluded from our space, it's in the infinite future, no point on or below the horizon is there "now", no event there is simultaneous to any event of our history (in the simple model of a static black hole). There's also a third sense coming from certain quantum mechanical or string theory descriptions, where there's literally no spacetime at all under the horizon for old enough black holes.
Why do you call "crap" things you've never heard about?
>========
the star's gone, that black hole is empty.
========
That means that Janna does not understand:
- That mass causes gravity.
- What "black hole" is.
No, it doesn't mean that. By Penrose–Hawking singularity theorems once the event horizon forms there's inevitably a singularity inside and all the matter quickly (by that matter's clocks) goes into that singularity and in some sense ceases to be in our spacetime, at least if we only follow the GR equations. All the volume under the horizon (singularity has zero volume) becomes empty space, so she's not incorrect, in pure GR sense. Gravity is not caused by just mass, in GR it's caused by energy and momentum, mass being a special case source of energy. Gravity itself also has gravity (see gravity self-interaction), so one may say the black hole is just the warping of spacetime having the same energy/mass as the matter that formed it. Of course these are more or less speculations based on following general relativity literally, but at least in this sense what she says makes perfect sense. If you don't see it, maybe you don't know what a black hole is? (we all don't know really, all we know is some indirect observations and our theories)
Role of gravity in a supernova explosion
Date: 2020-08-10 12:35 pm (UTC)There is an implication that [allegedly] supernova explosion may blow away the star completely.
That implication comes from our intuitive understanding of how explosion works (from bombs or balloons).
I know - because I was on the receiving end of such sloppy "Supernova explosion" explanation for decades, and this sloppy explanation created a strong impression of the possibility that the whole star may get blown away.
Furthermore, this sloppy "Supernova explosion" explanation created strong impression that the energy for such explosion came from the standard source of energy for stars (fusion) [which is incorrect].
> If there's always some stuff left, we can address it as the stuff that's left behind, the stuff that doesn't get blown away, two wordings of the same thing.
Yes, we can address that later. But why postpone mentioning the most significant effect of gravity on a neutron star (how gravity fuels forms neutron star and causes supernova explosion) -- if you already talk about neutron stars in the context of gravity?
Not only he postponed that explanation. He never mentioned direct connection between gravity energy and supernova explosion at all.
Which makes me think that he does not even know about what energy fuels supernova explosion.
> When the outer layers of the original star are still there, can you really call the collapsing core a star?
I can, but while the core is still collapsing -- I would, probably, prefer to call it a "collapsing core" (not a "neutron star" yet).
Then after star's core gravitationally collapsed -- neutron star is almost ready and only needs to "shed away" the remaining star shell in the following Supernova explosion.
>Another sloppy statement from this Phd Candidate:
===
A black hole is sort of like a neutron star's big brother.
===
But, actually, a black hole does not have to have a neutron star inside.
> He doesn't say there's a neutron star inside
He did not say it explicitly, but what else does "neutron star's big brother" mean?
I understand it as "bigger/special version of a neutron star".
> It's a "big brother" in the sense of being more massive
Yes: more massive neutron star.
If he wanted to say "more massive" and did not mean "neutron star" -- why did he use word "brother"?
Re: Role of gravity in a supernova explosion
Date: 2020-08-10 12:51 pm (UTC)Regarding "why brother", already told you: they are born very similarly. What makes someone your brother? Common parent. Here too, black holes and neutron stars have a common parent, a big collapsing star.
Re: Role of gravity in a supernova explosion
Date: 2020-08-10 02:16 pm (UTC)Yes.
When it was acceptable for me to explain neutron star as a result of supernova explosion [without mentioning that neutron star formation caused that supernova explosion] -- I had a misconception about the energy that produced that supernova explosion.
Now that I no longer have that misconception -- I do not allow myself to separate that gravitational energy source from the explanation that already includes neuron star and supernova explosion.
So now when I see such obsolete explanation (without mentioning gravity as a supernova explosion energy source) -- I assume that the narrator has a similar misconception that I once had.
Do you think it is unreasonable to make such assumption?
> black holes and neutron stars have a common parent, a big collapsing star
1) "A big collapsing star" is a source of material for a black hole (not a "parent").
2) Black holes do not have to have "A big collapsing star" as a source of material.
For example, a black hole may form from multiple stars merging with each other.
So that "Big brother [of a neuron star]" analogy is quite misleading in explanation of what a black hole is.
"The curve that's so strong"
Date: 2020-08-10 01:00 pm (UTC)What does black hole "curves"?
Geometrical shape?
Do you mean that the curvature is stronger at the "surface" of the black hole than inside of the black hole (closer to the center of the mass)?
Re: "The curve that's so strong"
Date: 2020-08-10 01:07 pm (UTC)Spacetime as a manifold in Riemannian geometry.
>Do you mean that the curvature is stronger at the "surface" of the black hole than inside of the black hole (closer to the center of the mass)?
No. I'm comparing curvature properties at the "surface" of a black hole and at the surface of a neutron star. One makes all light cones turn inwards, the other doesn't. In this sense the first is stronger.
Re: "The curve that's so strong"
Date: 2020-08-10 03:05 pm (UTC)Now compare it with: "The event horizon of the black hole forms, which is the shadow, the curve that's so strong"
Referring to "curve" right after "shadow" suggests to the listener that Janna Levin meant geometrical shape, not "a manifold in Riemannian geometry".
Even if Janna has a correct understanding in her head (which I doubt), her explanation is quite sloppy and misleading.
> I'm comparing curvature properties at the "surface" of a black hole and at the surface of a neutron star.
What if a black hole has a neutron star inside it -- would you say that curvature properties "at the surface of a black hole" are stronger than "inside of that black hole" (AKA "at the surface of a neutron star inside of that black hole")?
Re: "The curve that's so strong"
Date: 2020-08-10 03:31 pm (UTC)Neutron star inside a black hole would not exist, no matter inside BH can resist collapsing further towards singularity. If GR models of black holes are correct about what's inside, then the closer to the singularity, the stronger the curvature. The curvature scalar is proportional to 1/R^2.
Matter collapse inside a black hole
Date: 2020-08-10 05:46 pm (UTC)Under "BH" here do you mean, specifically, non-rotating black hole?
Do you mean that inside non-rotating black hole even individual neutrons collapse further into a soup of individual particles?
Re: Matter collapse inside a black hole
From:Gravitational singularity
From:Re: Gravitational singularity
From:Can't say anything about black hole?
From:Re: Can't say anything about black hole?
From:Re: Can't say anything about black hole?
From:Re: Can't say anything about black hole?
From:Speed of gravity in a black hole
From:Re: Speed of gravity in a black hole
From:"Space-time breaks down" crap
Date: 2020-08-10 01:56 pm (UTC)> I can think of at least 3 things these words may mean.
That multitude of possible interpretations means that the explanation is bad.
Explanation needs to be clear.
> The swap of signs in the signature of the Schwarzschild metric between time and radius, so radius behaves like time and vice versa.
Latitude coordinates South from the equator are on "S" scale, while latitude coordinates North from the equator - are on "N" scale.
But we do not say that at the equator "latitude breaks down".
Similarly, "the swap of signs" is not a reason to use "space-time breaks down" term.
> The fact that for a static outside observer the region behind the horizon is excluded from our space,
Black holes are not excluded from our space.
Black holes just have special behavior and limited visibility.
> it's in the infinite future,
What is "it" that "is in the infinite future"?
> no point on or below the horizon is there "now", no event there is simultaneous to any event of our history (in the simple model of a static black hole).
That is incorrect.
Black holes reliably (and timely) interact with the surrounding material through the gravitational force.
Therefore we can connect black hole events to the events in our history.
For example, in case when a black hole acretes material -- that black hole increases gravitational force on the surrounding objects.
> There's also a third sense coming from certain quantum mechanical or string theory descriptions, where there's literally no spacetime at all under the horizon for old enough black holes.
What is the use of such "no spacetime" mental model?
> Why do you call "crap" things you've never heard about?
I use word "crap" in the sense of "extremely poor quality".
These "explanations" do not help me to reason about the real world, and therefore are "extremely poor quality".
>> the star's gone, that black hole is empty.
> ceases to be in our spacetime, at least if we only follow the GR equations.
It reminds me solipsism.
Allegedly, if I
only follow the GR equationsclose my eyes, then all the world around me disappears.You are forgetting that these GR equations is only a tool that helps us to model the reality.
These GR equations is not even a full model.
Furthermore, vast majority of Janna Levin's audience is not able to follow these GR equations anyway, so [implicit] references to these GR equations is just a waste of audience time.
Re: "Space-time breaks down" crap
Date: 2020-08-10 03:23 pm (UTC)>Latitude coordinates South from the equator are on "S" scale, while latitude coordinates North from the equator - are on "N" scale.
But we do not say that at the equator "latitude breaks down".
Unlike positive-negative latitudes, when time and space exchange signs in the metric signature this is expected to mean they exchange roles, this is something qualitatively more significant. But you're right that it might not exactly deserve to be called "breaks down". Maybe they had some other effects in mind.
>> The fact that for a static outside observer the region behind the horizon is excluded from our space,
>Black holes are not excluded from our space.
Geometrically they are, at least in certain frames of reference, like one of a static outside observer. I you define space of current moment as a set of events (points in 4D spacetime) that are simultaneous to you now, then the events behind the BH horizon are not included in this set. That's how GR sees it, at least.
>> it's in the infinite future,
>What is "it" that "is in the infinite future"?
The region (set of events) behind the horizon.
>> no point on or below the horizon is there "now", no event there is simultaneous to any event of our history (in the simple model of a static black hole).
That is incorrect.
Black holes reliably (and timely) interact with the surrounding material through the gravitational force.
Now you're just showing you're not familiar with general relativity.
>> There's also a third sense coming from certain quantum mechanical or string theory descriptions, where there's literally no spacetime at all under the horizon for old enough black holes.
>What is the use of such "no spacetime" mental model?
I don't know, I'm not familiar with those particular model. It's supposed to solve (or try to) certain paradoxes of quantum information vs. black holes, see "the black hole war".
>Furthermore, vast majority of Janna Levin's audience is not able to follow these GR equations anyway, so [implicit] references to these GR equations is just a waste of audience time.
I believe when talking to an expert ther were allowed to refer to their knowledge of advanced theories, otherwise how would their dialog be different from a dialog with a child? They tried not to be too intimidating for the viewers but there's a conflict between clarity for general audience and technical correctness. So whatever they say you'll always attack them for being either not technically correct or being too unclear for an outsider.
Black hole gravity
Date: 2020-08-10 06:24 pm (UTC)> Now you're just showing you're not familiar with general relativity.
What mistake do you see in my claim?
Do you mean that if black hole does not have gravitational force that pulls surrounding objects toward the black hole?
"In the future"
Date: 2020-08-10 06:26 pm (UTC)> The region (set of events) behind the horizon.
You [incorrectly] imply here that "The region" is the same as "set of events [in that region]".
But these are not the same.
"The region" includes black hole content as well.
Even if we cannot observe events that happen with the content inside of a black hole, we still can observe [through gravitational force] that the content of the black hole is still inside of the black hole currently (and that content did not jump to the future).
Re: "In the future"
Date: 2020-08-10 06:52 pm (UTC)Re: "In the future"
From:Re: "In the future"
From:4D time-space model
From:Re: 4D time-space model
From:Re: 4D time-space model
From:Re: 4D time-space model
From:Re: 4D time-space model
From:Re: "Space-time breaks down" crap
Date: 2020-08-10 06:34 pm (UTC)It is ok to explicitly mention General Relativity or some of General Relativity concepts.
It is not ok to use concepts [such as "curve" that is "strong"] that implicitly refer to the complex calculations deep inside General Relativity formulas.
Re: "Space-time breaks down" crap
Date: 2020-08-10 07:04 pm (UTC)Re: "Space-time breaks down" crap
Date: 2020-08-10 07:42 pm (UTC)Is this a funny question about physics and gravitation? I did enjoy it.
Re: "Space-time breaks down" crap
Date: 2020-08-10 07:53 pm (UTC)Re: "Space-time breaks down" crap
Date: 2020-08-10 07:56 pm (UTC)Because you prove by analogy.
How about crossing the equator the latitude stops being degrees of angular distance, and becomes degrees Celsius, and ambient temperature becomes angular distance.
Re: "Space-time breaks down" crap
Date: 2020-08-10 08:19 pm (UTC)In this case I may say that coordinate system changes [crossing the equator].
I still would not say that the coordinate system breaks down.
Do you, personally, think in terms of "Space-time breaks down" [at a black hole event horizon]?
Re: "Space-time breaks down" crap
Date: 2020-08-10 08:30 pm (UTC)I enjoy your discussion, guys.
Re: "Space-time breaks down" crap
Date: 2020-08-10 09:21 pm (UTC)How about acceleration flipping from m/s^2 to s/m^2? No? Not weird? Take that as an illustration only.
Re: "Space-time breaks down" crap
From:Re: "Space-time breaks down" crap
From:Re: "Space-time breaks down" crap
From:Re: "Space-time breaks down" crap
Date: 2020-08-10 08:32 pm (UTC)As that woman said in the 5th part, gravity is like temperature.
Re: "Space-time breaks down" crap
Date: 2020-08-10 09:41 pm (UTC)But I do get the idea how "force" is just a label to a "thing" that we demand to be the cause of the change of velocity. Sometimes we have to invent these "things" to fit the framework of causes and consequences - like the Coriolis force pops out of thin air to explain the curved trajectory of a ball as observed from a merry-go-round.
Suitable words
Date: 2020-08-10 02:48 pm (UTC)Yes.
But they failed to choose suitable words at the large degree.
On one hand, they used over-complicated and confusing concepts that most of their audience cannot understand (e.g. "space-time breaks down").
On another hand, misconceptions they introduced are too strong and are not really helpful (e.g. "big brother of neuron star").
> Most popular descriptions of modern physics sound sloppy or even incorrect
Yes - "ninety percent of everything is crap".
But good explanations (e.g. Richard Feynman explanations) still manage to stay informative, simple enough, and correct (or almost correct).