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Physics · Document 13

Quantum Mechanics

Wave functions, operators, and the foundations of quantum physics

1. The Wave Function

In quantum mechanics, the state of a particle is described by a wave function psi(x, t). The wave function is complex-valued and contains all information about the system.

i * h-bar * (partial psi / partial t) = H * psi
Schrodinger Equation
The Born Rule: The probability of finding a particle at position x is |psi(x)|^2. The wave function must be normalized: integral of |psi|^2 dx = 1.
🎮 Interactive: Particle in a Box
xpsi|psi|^2
Energy Level
E_n = n^2 h^2 / (8mL^2)
Nodes
0 nodes
Wavelength
lambda = 2L/1
Born Rule: The probability of finding the particle at position x is |psi(x)|^2. Notice higher n states have more nodes where P(x) = 0.

2. Operators and Observables

Physical quantities are represented by Hermitian operators. The eigenvalues are the possible measurement outcomes.

ObservableOperatorEigenvalue Equation
Positionx-hat = xx|x> = x|x>
Momentump-hat = -i*h-bar * d/dxp|p> = p|p>
EnergyH = p^2/(2m) + VH|E> = E|E>
Angular momentumL = r x pL_z|l,m> = m*h-bar|l,m>

The Uncertainty Principle

Delta x * Delta p >= h-bar / 2
Heisenberg Uncertainty

This is not a measurement limitation but a fundamental property of nature! It arises from [x, p] = i * h-bar.

3. Spin and the Pauli Matrices

Spin is intrinsic angular momentum with no classical analog. Electrons have spin-1/2, meaning they can be in states |up> or |down>.

🎮 Interactive: Spin-1/2 on the Bloch Sphere
|up>|down>
Qubit State
|psi> = cos(theta/2)|up> + e^(i*phi)sin(theta/2)|down>
P(|up>)
100.0%
P(|down>)
0.0%
Expectation values:
<sigma_z> = 1.00, <sigma_x> = 0.00
Spin-1/2: A rotation of 360 degrees gives |psi> = -|psi>. Only a 720 degree rotation returns to the original state! This is the signature of a spinor.

The Pauli Matrices

sigma_x
[0 1]
[1 0]
sigma_y
[0 -i]
[i 0]
sigma_z
[1 0]
[0 -1]

These satisfy sigma_i * sigma_j = delta_ij * I + i * epsilon_ijk * sigma_k

4. Wave-Particle Duality

The double-slit experiment reveals the fundamental mystery of quantum mechanics: matter exhibits both wave and particle properties.

🎮 Interactive: Double-Slit Interference
SourceSlitsScreen
Interference Condition
Maxima: d*sin(theta) = n*lambda
Wave-Particle Duality
Each particle interferes with itself!
The mystery: Even when particles are sent one at a time, the interference pattern builds up! The particle goes through "both slits" as a wave, but is detected at one spot as a particle.

De Broglie Wavelength

lambda = h / p

Every particle has an associated wavelength! For an electron at 100 eV, lambda ~ 0.1 nm.

5. The Path Integral Formulation

Feynman showed that quantum mechanics can be formulated as a sum over all possible paths:

<x_f|x_i> = integral of exp(i*S[path]/h-bar) D[path]
Path Integral
Every path contributes with a phase e^(iS/h-bar). The classical path dominates because nearby paths have similar phases that add constructively.
Z = integral exp(-S_E) D[fields]
This extends to quantum field theory as the partition function

6. Connection to Z^2 Framework

Quantum mechanics provides the foundation for understanding the Z^2 framework:

|psi|^2 = probability
The Born rule connects complex amplitudes to observable probabilities
[x, p] = i*h-bar
Commutation relations define the quantum algebra
SU(2) for spin
Spin-1/2 transforms under SU(2), the double cover of SO(3)

Why Quantum Mechanics Matters for Z^2

  • * Complex amplitudes: The fundamental description uses complex numbers
  • * Spin: Fermions require spinor representations of the Lorentz group
  • * Path integrals: The language of quantum field theory
  • * Symmetries: Quantum numbers are eigenvalues of symmetry generators

Exercises

  1. For the particle in a box, show that the energy levels are E_n = n^2 * pi^2 * h-bar^2 / (2mL^2).
  2. Verify that the Pauli matrices satisfy [sigma_x, sigma_y] = 2i*sigma_z.
  3. Calculate the de Broglie wavelength of an electron with kinetic energy 100 eV.
  4. Show that sigma_x^2 = sigma_y^2 = sigma_z^2 = I (the identity matrix).
  5. For a spin-1/2 particle in state |+x>, calculate the probability of measuring spin up in the z-direction.