# Geodesic and MiSaTaQuWa equation of motion

1. Feb 29, 2012

### ngkamsengpeter

I am new to General Relativity and confused by the geodesic equation and MiSaTaQuWa equation. Most of the book saying that the geodesic equation is the motion of a particle in curved-spacetime. However, I read somewhere about this MiSaTaQuWa equation of motion. What is the difference between geodesic equation and this MiSaTaQuWa equation?

If I want to describe the motion of a particle in curved-spacetime, which one should I use? The geodesic equation or the MiSaTaQuWa equation?

Thanks for the help.

2. Feb 29, 2012

### Mentz114

The geodesic equations describe the motion of a 'test particle' which is assumed to be very like a point. The MiSaTaQuWa equations are for a small extended body which itself has an interaction with the field.

From Samuel E Gralla and Robert M Wald (2008) Class. Quantum Grav. 25 205009

3. Feb 29, 2012

### atyy

Last edited: Feb 29, 2012
4. Feb 29, 2012

### Passionflower

In that case I would think it better to say that it co-determines the field as opposed to interact with the field.

5. Mar 4, 2012

### ngkamsengpeter

Does that means that geodesic equation describe the motion of a particle that doesnt interact with the field and the MiSaTaQuWa equation describe the motion of particle that interact with the field? Since any particle with mass will interact with the field, so does that means that geodesic equation only for massless or at least negligible particle?

6. Mar 5, 2012

### Sam Gralla

yep. Think of a real, finite-sized body that is small compared to its surroundings, and "Taylor expand" its motion in [size of body] / [scale of variation of external universe]. At lowest order you get geodesic motion, which is accurate enough for almost everything. The first correction is the gravitational self-force (described in a particular gauge by the MiSaTaQuWa equation).