Undergrad General Relativity: Angular Momentum, Gravity & Questions

[manolo-mm]

Hi everybody

I saw quite a nice Youtube vid about general relativity and how gravity bends spacetime and therefor redirects angular momentum into the center of gravity. I thought the first time I begun to understand the concept but immediatly the questions poped up.

The video basically says that curved spacetime redirects angular momentum into the gravitational center. Ok there is no no difference in gravity on northpol to southpol. so all external angular momentum forces are redirected in full. And all objects in our solar system are moving in the same speed along with the sun through the universe. And there is no gravitational differences on other bodys in our solar system as far as we know. So they all redirect the angular momentum of traveling through the universe to their center of gravity. To why do we have different amounts of gravity on the different bodys in our solar system? The only answer i can imagine is that its not angular momentum that is redirekted through curved space time but another force. But that's just a guess. Anybody has a solid answer for me?

 

[PeterDonis]

how gravity bends spacetime

No, stress-energy bends spacetime; "gravity" is just another name for the effects of bending spacetime.

and therefor redirects angular momentum into the center of gravity

This sounds like nonsense to me. I have not watched the video but I am skeptical that it is a reliable source. [Edit: It looks like the problem is with the OP's misunderstanding something, not the video itself--see follow-up post below.] YouTube videos in general are not a good place to be looking if you want to learn actual physics. You should be looking at textbooks and peer-reviewed papers.

 

[PeterDonis]

The video basically says that curved spacetime redirects angular momentum into the gravitational center.

And now having watched the video, I don't see where it says this anywhere. What it does say actually looks OK to me, though of course it only barely scratches the surface of GR. But I don't see it saying anything like this. Where are you getting this from?

 

[manolo-mm]

Thats how I understood and interpreted this vid.

 

[PeroK]

Thats how I understood and interpreted this vid.

Which of the seven levels mentioned angular momentum?

 

[vanhees71]

YouTube videos in general are not a good place to be looking if you want to learn actual physics. You should be looking at textbooks and peer-reviewed papers.

I think, what are pretty reliable videos is if they come from lectures at well-known universities, often also appearing at youtube (e.g., the online lectures from MIT). It's of course easier to produce reliable videos on the natural sciences when addressed to students of the subject rather than to the public. To "explain science as simple as possible but not simpler" is among the most difficult tasks for a scientist!

 

[PeterDonis]

I think, what are pretty reliable videos is if they come from lectures at well-known universities, often also appearing at youtube (e.g., the online lectures from MIT).

Yes, these are something of a special case, since as course materials they have to meet certain standards that a random YouTube video does not.

 

[vanhees71]

Of course, the random Youtube video is more likely to be unreliable. It's a kind of 2nd law applied to "web content" ;-).