Roadmap to understanding the quantum mechanics

July 16, 2021 3 min read

In my university days I used to attend a course of quantum chemistry/mechanics, which was given in a typical post-soviet education style. All formalism, no essentials. As quantum computers are getting closer and closer to the reality by the day, I had a practical reason to finally improve my understanding of the theory. Here is my roadmap to understanding the quantum mechanics.

Lagrangian mechanics

Action and principle of minimal action: Leibniz, Maupertuis, D’Alembert’s and Hamilton’s

More of a philosophical then practical origin. Leibniz.

“Nature is parsimonious.”

S=t1t2L(t)dtS = \int \limits_{t_1}^{t_2} L(t) dt, where SS is the total action, and L(t)L(t) is the unit of change of action through an infinitesimal period of time.

Another pretentious name for the subject is “principle of stationary action”, which indicates that the first derivative of the action turns 0, but this, generally speaking, does not guarantee that it is a minimum - it could be a saddle point or maximum. Let us not be distracted by this technicality.


Configuration space and generalized coordinates for constrained optimization

Remember, Lagrange was known for the method of Lagrange multipliers.

Example: Body, attached to a wire.


Calculus of variations



S=x1x2L(f(x,x˙,t))dtS = \int \limits_{x_1}^{x_2} L(f(x, \dot{x}, t)) dt

L=TUL = T - U

Why Lagrangian was chosen the way it was chosen? 4 criteria.


Noether’s theorem

Every law of conservation (e.g. conservation of momentum, energy etc.) corresponds to some symmetry (e.g. translational symmetry of time, space etc.).

See: gauge theory (in Russian)


Hamiltonian mechanics

  • Phase space vs configuration space, coordinates and their derivatives vs coordinates and momenta, Legendre transform
  • Hamiltonian vs Lagrangian
  • n-forms, exterior forms, differential forms, symplectic manifolds

Basics of physics for quantum mechanics

  • Planck constant as quantum of action


Boris Burkov

Written by Boris Burkov who lives in Moscow, Russia, loves to take part in development of cutting-edge technologies, reflects on how the world works and admires the giants of the past. You can follow me in Telegram