I've tried Google, but couldn't figured out exactly whether its Newton, Einstein or Galileo. What is the criteria for appointment of father for a particular subject?
Terms like "Father of Physics" (or "Father of Modern Chemistry", or "Mother of Computer Science") aren't official titles, they're just subjective opinions. No one appoints them.
Calling Einstein the Father of Physics is like giving your dad a "Best Dad in the World" mug--it's a nice gesture, but no one ever did a survey of every dad in the world to conclusively prove that he is indeed the greatest father in recorded history.
Newton, Einstein and Galileo could all be considered "Fathers of Physics," because they contributed immensely to human knowledge.