#### Alladi Ramakrishnan Hall

#### Action Principle for Gravity, Null Surfaces and Thermodynamic Consequences

#### S Chakraborty

##### IACS, Kolkota

*In the first part of my talk I will discuss the variational*

principle for general relativity and some associated subtle facts. In

particular, I will try to arrive at the correct action principle for

general relativity in presence of null boundaries. Possible generalization

to Lovelock theories will also be presented. In the second part of my talk,

I will discuss the thermodynamics of null surfaces. In particular, I will

show that three different projections of gravitational momentum related to

an arbitrary null surface in the space time leads to three different

equations, all of which have thermodynamic interpretations.

Done