juan_gandhi | печально я гляжу

Вот 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".

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.

Flat | Top-Level Comments Only

From:

dennisgorelik

> Yes, black hole curves it stronger (at the "surface")

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)?

From:

thedeemon

>What does black hole "curves"?
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.

From:

dennisgorelik

> Spacetime as a manifold in Riemannian geometry.

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")?

From:

thedeemon

Sloppy wording and not an explanation, yes.

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.

From:

dennisgorelik

> no matter inside BH can resist collapsing further towards singularity

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?

From:

thedeemon

Any black hole. Rotating one also has a singularity, it's just not a point but a zero-volume circle.

I'm not sure what exactly neutrons collapse into (or maybe they remain neutrons), but as long as GR works there, they inevitably all come to the singularity.

From:

dennisgorelik

1) "Remain neutrons" hypothesis is not really compatible with "all come to the singularity".
"Gravitational singularity" implies that neutrons collapse too.

2) Most likely there is some limit to that gravitational collapse into singularity. At some extremely small size [a black hole content is squeezed into], one of [poorly understood] forces will be able to counteract gravity and prevent further collapse.

From:

thedeemon

Yep, I was talking about the particles as they're still falling (but already not holding up the neutron star), not at their final stage where I can't really say anything about their state. And yes, some people expect some quantum effects beyond GR to prevent the total collapse to a point, but anyway that's not a neutron star anymore.

From:

dennisgorelik

> I can't really say anything about their state.

1) We still can observe that some specific mass still exists inside of a black hole, right?

2) We still can hypothesize that this mass collapsed into a very small size [that is similar to theoretical gravitational singularity from our perspective].

From:

thedeemon

1) We can observe that a region of space slightly larger than calculated black hole size has the gravity of that mass.

Here's a question for you: if you're in a ship hovering a million km away from a black hole and you fire a torpedo that moves at million km / hour towards the BH, how soon, by your clock, will the torpedo reach the black hole horizon? When will you see it?

2) Yes.

From:

dennisgorelik

> how soon, by your clock, will the torpedo reach the black hole horizon?

The torpedo is likely to accelerate due to gravitational pull from the black hole.
So the torpedo will reach the black hole horizon faster than in 1 hour.

However I will not see how torpedo reaches that black hole horizon, because the light informing me about the torpedo movement will come to me slower and slower [depending on how close that torpedo is to the black hole event horizon].
When torpedo is almost at the event horizon, the light from that torpedo will be moving back to me at almost zero speed, so I will have to wait almost indefinitely in order to see it.

From:

thedeemon

In the world of Newtonian gravity the torpedo would indeed get there in less than an hour of your (and its) time.
In GR there is gravitational time dilation that makes things more interesting. According to GR in your frame of reference the torpedo will first indeed accelerate but then closer to the BH it will slow down, basically halting near the horizon. In GR, by your clock, the torpedo will never reach the horizon. Not in a day, not in a year. Literally never in the simple model of a static BH.
And because time ticks differently depending on where you are and how you move, it's not so obvious how to define the set of events simultaneous to you now. A common approach is to say an event N light seconds away is simultaneous to you now if light from that event will reach you in N seconds. Because of time dilation near massive bodies, coming from some places will take more time than from others of the same distance, so if you carefully trace such set of simultaneous events it won't be a plane, it will be a non-flat hypersurface that goes around the BH never touching it. No wonder since light cannot leave the BH horizon.

From:

dennisgorelik

> not so obvious how to define the set of events simultaneous to you now

Why not use gravitational signals for that?
The speed of gravity is the same as speed of light in vacuum, right?
And the speed of gravity is not dilated near/in black holes, right?

That means we can measure gravitational changes in order to measure when a gravity altering event happen.