Historical question: Equations of motion from lagrangian

In summary, the field equations for general relativity were derived by Hilbert from a Lagrangian density. The first solutions, including the Schwarzschild metric, were found soon after by solving non-linear partial differential equations. The Einstein tensor is derived from an action that is extremised according to the least action principle, and the Schwarzschild metric can be derived using the spherical symmetry and the requirement that the Einstein tensor be zero. The equations of motion can then be found from this solution using the Euler-Lagrange method.
  • #1
solveforX
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Hey, in general relativity, essentially I am asking how any metric (I.e. schwarzschild metric) was found. are the metrics derived or are they extrapolated from the correct lagrange equations of motion? If there is a derivation available, please provide a link.

thanks
 
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  • #2
Hilbert showed how to derive the field equations for GR from a Lagrangian density. Soon after Einstein & Hilbert the first solutions have been found - but forgetting about the physicalcontent this is nothing else but solving non-linear partial differential equations; of course it can be done but there is no complete theory as for linear partial differential equations
 
  • #3
solveforX said:
Hey, in general relativity, essentially I am asking how any metric (I.e. schwarzschild metric) was found. are the metrics derived or are they extrapolated from the correct lagrange equations of motion? If there is a derivation available, please provide a link.

thanks

The Einstein tensor comes from an action which is extremised according to the least action principle.
http://en.wikipedia.org/wiki/Einstein_field_equations

The Schwarzschild metric can then be derived using nothing more than the spherical symmetry and the requirement that the Einstein tensor be zero.
http://en.wikipedia.org/wiki/Deriving_the_Schwarzschild_solution

Given a solution of the EFE the equations of motion can then be found using Euler-Lagrange method for the Lagrangian of a body moving in curved space.
Any textbook on GR has this derivation.
 

1. What is the Lagrangian in the context of equations of motion?

The Lagrangian is a mathematical function that describes the dynamics of a physical system. It is derived from the principle of least action, which states that the path of a system between two points in time is the one that minimizes the action, a quantity related to the energy of the system.

2. How are equations of motion derived from the Lagrangian?

The equations of motion can be derived from the Lagrangian using the Euler-Lagrange equations, which relate the partial derivatives of the Lagrangian with respect to the system's variables to the forces acting on the system. By solving these equations, one can determine the trajectory of the system over time.

3. Can the Lagrangian be used for any physical system?

Yes, the Lagrangian approach can be used to describe the dynamics of any physical system, from simple objects moving in a straight line to complex systems such as planets orbiting around the sun. However, the Lagrangian may be more useful for certain types of systems, such as those with constraints or varying forces.

4. Can the Lagrangian be extended to include non-conservative forces?

Yes, the Lagrangian can be extended to include non-conservative forces, such as friction or air resistance. This is done by adding additional terms to the Lagrangian that account for these forces, allowing for a more accurate description of the system's dynamics.

5. How does the Lagrangian approach differ from Newton's laws of motion?

The Lagrangian approach is a more general and elegant way to describe the dynamics of a system compared to Newton's laws of motion. While Newton's laws focus on the forces acting on a system, the Lagrangian takes into account the entire path of the system and minimizes the action. This approach also allows for more complex systems to be described, as it does not rely on specific force laws.

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