# The Postulates of Quantum Mechanics

#### ByYogev

Sep 2, 2023

Quantum mechanics, a cornerstone of modern physics, introduces a paradigm shift from classical mechanics. This chapter delves into the postulates that underpin the quantum framework, illuminating the fundamental principles governing the behavior of particles at the quantum scale.

Postulate 1: State of a Quantum System

At the heart of quantum mechanics lies the concept of a quantum state, a mathematical representation of a particle’s properties. The state vector, often denoted as |ψ⟩, captures information about the particle’s position, momentum, energy, and other observable quantities. This postulate asserts that a quantum system is completely described by its state vector.

Postulate 2: Observables and Operators

Quantum mechanics introduces a novel approach to observables – physical properties that can be measured. Each observable is associated with a mathematical operator. For instance, position is associated with the position operator, and momentum with the momentum operator. The eigenvalues and eigenvectors of these operators yield measurable outcomes and describe the state of a quantum system after measurement.

Postulate 3: Measurement and Projection

Measurement in quantum mechanics is a transformative process. When an observable is measured, the system’s state “collapses” to an eigenstate of the corresponding operator. This postulate outlines how the act of measurement projects the system into one of its possible eigenstates, yielding a specific measurement outcome.

Postulate 4: Superposition and Linearity

Superposition is a hallmark of quantum mechanics. It stipulates that a quantum system can exist in a linear combination of multiple states simultaneously. Mathematically, if |α⟩ and |β⟩ are valid states, then so is their linear combination α|α⟩ + β|β⟩. This postulate captures the intricate blending of possibilities inherent in the quantum realm.

Postulate 5: Quantum Evolution and Schrödinger Equation

The evolution of a quantum system is dictated by the Schrödinger equation. It describes how the state vector changes over time, governed by the system’s Hamiltonian operator. This postulate provides the foundation for predicting the behavior of quantum systems, from electron orbits to wavefunction dynamics.

Postulate 6: Quantum Entanglement

Quantum entanglement, a phenomenon defying classical intuition, is encapsulated in this postulate. When particles become entangled, their states are correlated in such a way that the measurement of one particle instantaneously influences the state of the other, regardless of distance. This phenomenon underscores the non-local nature of quantum interactions.

Postulate 7: The Principle of Quantum Measurement

The act of measurement perturbs a quantum system. This postulate stipulates that after measurement, the system collapses into an eigenstate of the measured observable. Additionally, the probability of obtaining a particular measurement outcome is given by the squared magnitude of the corresponding coefficient in the state vector’s expansion.

Summary: Navigating Quantum Principles

The journey into quantum mechanics unfolds through the lens of its foundational postulates. We’ve explored the state of quantum systems, observed the role of operators and observables, unraveled the intricacies of measurement and superposition, and encountered the evolution of quantum states and entanglement. With these principles as our guide, we embark on a profound exploration of the quantum universe’s mysteries.