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1. Why do we call quantum physics an ab initio model?

Ans- A mathematical model about a sufficiently large set of natural phenomena is called an “ab initio model” or a “first principles model” if the starting point of the model cannot be further reduced to more fundamental concepts (at least not within the language of the considered natural phenomena).

Quantum physics is an ‘ab initio model’ because it deals with a system starting from it’s Schrödinger equation which involves the basic Hamiltonian of the system and doesn’t include any empirical parameters.

2. Why do we call classical mechanics an ab initio model?

Ans- Classical mechanics deals with a system using the basic principles such as Newton’s laws of motion, law of gravitation, laws of conservation of energy and momentum. These principles are completely general and don’t involve any specific fitting parameters.

3. Is classical electromagnetism an ab initio model (and if it is one, what are its first principle) ?

Ans- Classical electromagnetism is also an ‘ab initio’ model where every electromagnetic system can be explained using the basic Maxwell’s equations which can be considered as the first principles of classical electromagnetism.