Initial Value Equations out of ADM formalism

In summary, the conversation discusses the derivation of MTW's Equation (21.90) and (21.115). The speaker has received positive feedback on their previous post and has moved on to trying to produce (21.115). However, they are struggling to find a way to produce certain terms, specifically terms involving ##N^{|ij}## or ##N^{|m}{}_{|m}##. They mention using the term ##[N^{|i}(\gamma)^{\frac{1}{2}}]_{,i}##, which gets them close but still results in an extra bonus term. They inquire if there is something they have missed or if there is another route to solving their problem. The attachment
  • #1
TerryW
Gold Member
211
17
Homework Statement
Use the ADM formalism to derive dynamic and initial value equations
Relevant Equations
MTW (21.115) - See attachment
In my earlier post, I demonstrated a way to derive MTW's Equation (21.90),

##16π\mathfrak{L}_{geom} = - \dot π^{ij} γ_{ij} - N\mathcal{H} -N_i\mathcal{H^i}-2(π^{ij}N_j-½N^iTrπ+γ^½N^{|i})_{,i}## MTW (21.90)

I received my first two 'likes' for this which made me really happy - Thanks guys!

I've moved on a bit now and have reached page 525, (four whole pages - wow!) and I am now trying to produce MTW (21.115) - see attachment.

MTW suggests that this can be derived from:

##I = \frac{1}{16π}\int[ \dot π^{ij} γ_{ij} - N\mathcal{H} -N_i\mathcal{H^i}]d^4x + \int\mathfrak{L}_{Field}d^4x ## MTW (21.95)

I've managed to produce nearly all of (21.115) but I cannot find a way to produce the terms I've underlined.

Instead of the term ##(π^{ij}N^m)_{|m}## I have ##(N^iπ^{jk})_{|k}##

I can't see any way to produce terms with ##N^{|ij}## or ##N^{|m}{}_{|m}## from any of the terms in 21.95, but if I go back to (21.90), I can use the term ##[N^{|i}(\gamma)^{\frac{1}{2}}]_{,i}##, which nearly gets me home, but I end up with

##(\gamma)^{\frac{1}{2}}N^{|ij} - \frac{1}{2}(\gamma)^{\frac{1}{2}}g^{ij}N^{|m}{}_{|m}##

and a bonus term## -(-g)^{\frac{1}{2}}\frac{1}{N^2}N^{|i}N_{|i}##

(I have a copy of the original ADM paper and interestingly, they do use the full (21.90) to derive (21.115), ie without dropping the divergence!)

Is there something I have missed that would enable me to show that the bonus term somehow resolves my problems, or is there another route altogether?
Regards

Terry W

Attachment:

MTW 21.115.png
 
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  • #2
MTW (21.115): ##16π\mathfrak{L}_{geom} = - \dot π^{ij} γ_{ij} - N\mathcal{H} -N_i\mathcal{H^i}-2(π^{ij}N_j-½N^iTrπ+γ^½N^{|i})_{,i} + (π^{ij}N^m)_{|m} - \frac{1}{2}N^{|ij} - \frac{1}{2}N^{|m}{}_{|m}##
 

FAQ: Initial Value Equations out of ADM formalism

1. What is an initial value equation in the context of ADM formalism?

An initial value equation in the context of ADM formalism is a mathematical equation that describes the evolution of a physical system over time, based on its initial conditions. It is an important tool in general relativity and is used to study the behavior of spacetime.

2. How is an initial value equation derived from the ADM formalism?

An initial value equation is derived from the ADM formalism by using the Hamiltonian and momentum constraints, which are equations that relate the spatial and temporal derivatives of the metric tensor. These constraints are then solved to obtain the initial value equation.

3. What are the main applications of initial value equations in physics?

Initial value equations have various applications in physics, particularly in the study of general relativity and black holes. They are used to model the evolution of spacetime, understand the behavior of gravitational waves, and analyze the dynamics of black holes.

4. Can initial value equations be used to study systems other than spacetime?

Yes, initial value equations can be applied to study systems other than spacetime. They can be used in classical mechanics to describe the evolution of a physical system based on its initial conditions. They have also been used in quantum mechanics to study the behavior of quantum systems.

5. What are the limitations of initial value equations in ADM formalism?

One of the main limitations of initial value equations in ADM formalism is that they are only applicable to systems that can be described by a metric tensor. This means they cannot be used to study systems that do not have a well-defined metric, such as particles or fields. Additionally, they may not accurately describe the behavior of highly dynamic systems, such as those involving strong gravitational fields or extreme speeds.

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