#3 Nambu-Goto action
Given a -dimensional manifold with a metric , a string is a one-dimensional object moving inside . The Nambu-Goto action describes the dynamics of these objects. It’s strongly inspired in the relativistic dynamics of a point particle.
A point particle describes a curve when it’s moving through space and time. Such curve (its trajectory) depends on a single parameter, usually the proper time. Likewise, a string describes a two-dimensional surface called world sheet, and depends on two parameters . Even though the role of both parameters is arbitrary, one can think of as the proper time, and, for any given , would label each one of the points that the string is made of.
#2 – Hilbert – Einstein action – Part 2
Now we compute . First, the Riemann tensor has the following definition:
so
Self-portrait
#2 – Hilbert – Einstein action – Part 1
The Hilbert-Einstein action is:
Einstein equations (the evolution equations of the metrig ) can be found if we set .
#1 – Proof: variation of a determinant
By definition:
so
Thus:
If is a metric tensor, so we have:
Equating terms proportional to :
Finally: