печально я гляжу

[juan_gandhi]

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.

Comments

Re: "In the future"

[dennisgorelik]

> Now you're arguing definitions.

... only because the definition you used was confusing.
~~~~~
The region (set of events)
~~~~~

I want to understand you correctly, so I showed my understanding of what you wrote and my confusion about it.

If you meant "events only" then why you did not define it as "All events inside of black hole"?

Re: "In the future"

[thedeemon]

Ok, back to basics: spacetime is a 4D manifold where each point has 4 coordinates: 3 of space and 1 of time. Such point in spacetime is what's called "event" in relativity. So a region is a set of events.

4D time-space model

[dennisgorelik]

> 4D manifold

What is the advantage of modeling black hole in terms of 4D manifold as opposing to modeling black hole events on an isolated time scale [+ 3D space]?

Does this 4D model help you to reason?

I find 4D model almost useless as a reasoning tool most of the time, because time and space behave quite differently from each other.

Re: 4D time-space model

[thedeemon]

Relativity theory is all about geometry, it's just a very convenient and visual way to understand it and see the answers to many questions. For instance, the famous "twins paradox" is immediately solved as a simple triangle inequality. The way gravity works in GR is also very geometrical, free falling things following their geodesics. Do you know there is no gravitational force in GR? That 4D geometry is basically the language of relativity.
I'm not surprised by your attitude towards isolating time from space, it's only natural if you never got past Newtonian physics. However one of the basic ideas in special relativity (that only gets more interesting in GR) is that simultaneity is relative, it's dependent on a frame of reference, there is no absolute "now". Switching between reference frames is very much like rotation in spacetime, so treating this geometrically helps immensely.

Re: 4D time-space model

[dennisgorelik]

> "twins paradox" is immediately solved as a simple triangle inequality

It does not look simple to me:
https://www.askamathematician.com/2010/09/q-how-does-the-twin-paradox-work/comment-page-1/

This geometrical representation (triangle) still does not explain why Alice is getting 1 side and Bob is getting 2 sides, even though Alice is moving relative to Bob.

> Do you know there is no gravitational force in GR?

I do not know that.
Does "no gravitational force" model make it easier to reason about the real world problems that have gravitational force in it?

Re: 4D time-space model

[thedeemon]

It might not look simple for a newcomer, but after learning some basic rules it becomes pretty simple. That triangle looks structurally the same in any one inertial frame of reference. If Bob turns at some point, his frame of reference is not inertial from start to finish, a turn is a change of frame of reference that changes the relative coordinates of Alice relative to Bob in both space and time, so for Bob it looks more complicated. But as long as you're looking from any frame of reference that remains inertial the whole time, it's simple.

In GR you can use not just inertial frames of reference but accelerating ones too, however spacetime in them may look warped even when it's empty and not distorted by massive bodies.

>Does "no gravitational force" model make it easier to reason about the real world problems that have gravitational force in it?

Sometimes. Not many people's "real world problems" require anything beyond Newton's mechanics. But GR provides a certain worldview, a framework with certain explanatory power.
People took atomic clocks and flew them on planes around the world in opposite directions: East and West. How to see why one clock ticked less time than the other? With a geometric approach it becomes obvious once you learn how to use it.

Re: 4D time-space model

[dennisgorelik]

> his frame of reference is not inertial from start to finish, a turn is a change of frame of reference that changes the relative coordinates of Alice relative to Bob in both space and time

Thank you - I will try to remember your view on 4D time-space model.

> Not many people's "real world problems" require anything beyond Newton's mechanics.

That is an important point too.
Digging dipper at this time seems a bit impractical for me, because learning to reason within 4D time-space is quite time-consuming, and I am not a professional physicist, so there is no compelling reason to prioritise learning in that "4D time-space modeling" direction.