The Essential General Relativity

[little_monkey]

[BrianSoddingBoru4]

Just published a new blog on The Essential General Relativity. I wanted to condense a 400 page textbook on the subject in a 1 page blog. I know, that is totally crazy, but I think I managed it.


Comments would be greatly appreciated as I can edit it to make it better.

I was with you right up to 'What are the equations needed to go from Newtonian Physics to General Relativity?', then my vision went foggy and the room started to spin.

I'm sure it's a wonderful article, but I had to take 'Basic Physics' (a requirement for my degree) three times before I passed.

Boru

[Alex K]

Very nice summary! Upon first reading, apart from a few minor grammar mistakes, I have just one comment/question concerning the field equation. The special property of the einstein tensor, that it is covariantly conserved, seems only to be linked to local energy conservation once one has decided that the other side of te equation should consist of the energy momentum tensor. Before that its just *some* conserved current. In your text this order is reversed and I'm not sure whether the argument shouldnt better start with T for that reason...

[little_monkey]

Very nice summary! Upon first reading, apart from a few minor grammar mistakes, I have just one comment/question concerning the field equation. The special property of the einstein tensor, that it is covariantly conserved, seems only to be linked to local energy conservation once one has decided that the other side of te equation should consist of the energy momentum tensor. Before that its just *some* conserved current. In your text this order is reversed and I'm not sure whether the argument shouldnt better start with T for that reason...

Thx. My main goal in writing that blog was to show that GR's development was a mix of brilliant insights and a few lucky guesses. Einstein developped GR between 1905 and 1915, literally from scratch. The whole concern about energy being conserved arose from applying GR to an expanding universe, not something that Einstein would have known at the time. He did though find that using only the Ricci tensor, his equations were at odds with energy conservation. He tagged on the Ricci scalar to get that. Also, initially, the theory was torsion free. Einstein tried to include the torsion tensor and linked to electromagnetism without success. That idea was reprieved later on, with the torsion tensor linked to spin. There are those who believe that a pseudotensor torsion is needed to make the theory energy conserved. But that`s another story.

[ThePinsir]

Opened page. Saw this:
(A) Gμν = 8πGc-4Tμν

Closed page.

[little_monkey]

Opened page. Saw this:
(A) Gμν = 8πGc-4Tμν

Closed page.

A little more persistence and you would have read the three thought experiments that guided Einstein in his quest. There is little math involved, and they are quite brilliant, considering what was known at the time.

[Alex K]

Opened page. Saw this:
(A) Gμν = 8πGc-4Tμν

Closed page.

That's indeed all one needs to know, very efficient approach!