diff --git a/physics/hamiltonian.py b/physics/hamiltonian.py new file mode 100644 index 000000000000..ccae924a3db8 --- /dev/null +++ b/physics/hamiltonian.py @@ -0,0 +1,98 @@ +""" +The Hamiltonian is a central concept in both classical and quantum mechanics. +It represents the total energy of a system and is used to describe how that +system evolves over time. + +Classical mechanics: + H = T + V + where T is kinetic energy and V is potential energy. + +Quantum mechanics (1D, finite-difference form): + The time-independent Schrodinger equation, H|psi> = E|psi>, can be solved + numerically by discretizing space into points and approximating the second + derivative with the finite-difference method. This turns the continuous + Hamiltonian operator into a matrix: + + H[i][i] = hbar^2 / (m * dx^2) + V(x_i) + H[i][i+1] = H[i][i-1] = -hbar^2 / (2 * m * dx^2) + +References: + - https://en.wikipedia.org/wiki/Hamiltonian_mechanics + - https://en.wikipedia.org/wiki/Hamiltonian_(quantum_mechanics) + - https://en.wikipedia.org/wiki/Finite_difference_method +""" + + +def classical_hamiltonian( + mass: float, velocity: float, potential_energy: float +) -> float: + """ + Compute the classical Hamiltonian H = T + V for a particle, + where T = 0.5 * m * v^2 is the kinetic energy. + + >>> classical_hamiltonian(2, 3, 5) + 14.0 + >>> classical_hamiltonian(1, 0, 10) + 10.0 + >>> classical_hamiltonian(2, -4, 0) + 16.0 + >>> classical_hamiltonian(-1, 2, 5) + Traceback (most recent call last): + ... + ValueError: mass must be positive + """ + if mass <= 0: + raise ValueError("mass must be positive") + + kinetic_energy = 0.5 * mass * velocity**2 + return kinetic_energy + potential_energy + + +def quantum_hamiltonian( + num_points: int, + potential: list[float], + mass: float = 1.0, + hbar: float = 1.0, + dx: float = 1.0, +) -> list[list[float]]: + """ + Construct the Hamiltonian matrix for a particle in a 1D potential + using finite-difference discretization of the time-independent + Schrodinger equation. + + >>> quantum_hamiltonian(3, [0.0, 0.0, 0.0]) + [[1.0, -0.5, 0.0], [-0.5, 1.0, -0.5], [0.0, -0.5, 1.0]] + >>> quantum_hamiltonian(2, [1.0, 2.0], mass=2.0, hbar=1.0, dx=1.0) + [[1.5, -0.25], [-0.25, 2.5]] + >>> quantum_hamiltonian(3, [0.0, 0.0]) + Traceback (most recent call last): + ... + ValueError: potential must have length num_points + >>> quantum_hamiltonian(0, []) + Traceback (most recent call last): + ... + ValueError: num_points must be positive + """ + if num_points <= 0: + raise ValueError("num_points must be positive") + if len(potential) != num_points: + raise ValueError("potential must have length num_points") + + diag_term = hbar**2 / (mass * dx**2) + off_diag_term = -(hbar**2) / (2 * mass * dx**2) + + hamiltonian = [[0.0] * num_points for _ in range(num_points)] + for i in range(num_points): + hamiltonian[i][i] = diag_term + potential[i] + if i > 0: + hamiltonian[i][i - 1] = off_diag_term + if i < num_points - 1: + hamiltonian[i][i + 1] = off_diag_term + + return hamiltonian + + +if __name__ == "__main__": + from doctest import testmod + + testmod()