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Operators and States: Understanding the Math of Quantum Mechanics

September 12, 2023
Charlotte Anderson
Charlotte Anderson
United States
Quantum Mechanics
She is a distinguished mathematician and physicist known for her expertise in the fascinating realm of quantum mechanics. With a Bachelor's degree in Mathematics from Harvard University.
Quantum mechanics, the branch of physics that deals with the behavior of matter and energy at the smallest scales, is a fascinating and complex field. It has given us a profound insight into the nature of the universe, challenging our classical intuitions and opening up a world of possibilities for technological advancements. At the heart of quantum mechanics lies the concept of operators and states, which form the mathematical foundation of this theory. In this blog, we will delve into the fundamentals of operators and states in quantum mechanics without delving into complex mathematical formulas and expressions. To excel in understanding the complex principles of Quantum Mechanics, it's essential to complete your math assignment effectively, as it forms the foundation for solving intricate quantum problems.

The Quantum World: A Different Reality

Before we dive into the mathematics of quantum mechanics, let's briefly set the stage by understanding why quantum mechanics is so different from classical physics.

Understanding the Math of Quantum Mechanics

Classical physics, which includes Newtonian mechanics and Einstein's theory of relativity, describes the behavior of objects in the macroscopic world. These theories are deterministic, meaning that given the initial conditions of a system, you can predict its future behavior with certainty. In contrast, quantum mechanics deals with the microscopic world, where particles like electrons and photons behave very differently.

In the quantum world, particles do not have well-defined properties, such as position and momentum, until they are measured. Instead, they exist in a superposition of possible states, and the act of measurement collapses this superposition into one of the possible outcomes. This inherent uncertainty and the bizarre phenomena associated with quantum mechanics, such as entanglement, have captured the imagination of physicists and the public alike.

Operators: Mathematical Tools of Quantum Mechanics

In quantum mechanics, operators are mathematical entities that represent physical observables. These observables can be properties like position, momentum, energy, or angular momentum. Operators act on quantum states to provide information about these properties when measured.

Think of operators as mathematical tools that help us extract information from quantum states. The most common operators in quantum mechanics include the position operator (x), the momentum operator (p), the energy operator (H or E), and the angular momentum operator (L). Each of these operators corresponds to a physical quantity that we might want to measure.

Operator Algebra

One of the fundamental principles of operators in quantum mechanics is that they do not commute, meaning the order in which you apply two operators matters. This non-commutative behavior is a departure from classical physics, where you can interchange the order of mathematical operations without changing the result.

For example, the position operator (x) and momentum operator (p) do not commute, as given by the Heisenberg Uncertainty Principle. This non-commutative property is a key feature of quantum mechanics and is a source of the inherent uncertainty in the quantum world.

Eigenstates and Eigenvalues: Foundations of Quantum Mechanics

In the intricate world of quantum mechanics, the concepts of eigenstates and eigenvalues are pivotal for understanding the behavior of quantum systems. These concepts provide us with a structured framework to predict and comprehend the outcomes of measurements and observations. Let's explore them in more depth.

Eigenstates: The Unchanging Quantum Identities

Eigenstates, often referred to as "eigenstates of an operator," are quantum states that possess a rather remarkable property. When a mathematical operator, such as the position operator (x), momentum operator (p), or any other observable, acts on an eigenstate, it doesn't transform that state into something entirely new. Instead, the eigenstate remains essentially unchanged, up to a normalization factor.

In simpler terms, if we have a quantum system described by an eigenstate |ψ⟩, and we apply an operator (represented by a mathematical symbol like A) on it, the resulting state is also |ψ⟩ (or a scalar multiple of it). Mathematically, this relationship is expressed as:

A|ψ⟩ = λ|ψ⟩


  • A represents the operator (e.g., the position operator, momentum operator, etc.).
  • |ψ⟩ is the eigenstate of the operator A.
  • λ (lambda) is a scalar, known as the eigenvalue corresponding to the eigenstate |ψ⟩.

Eigenstates are, in a sense, the "preferred" states of a quantum system when observed with respect to a particular observable. They are the states where measurement outcomes are most predictable and least subject to uncertainty.

Eigenvalues: The Quantized Measurable Outcomes

Eigenvalues are the numerical values associated with eigenstates when an operator acts on them. In other words, they represent the possible outcomes (values) that the operator can yield when measuring a specific observable on a quantum system. Each eigenstate corresponds to a unique eigenvalue, and this pairing is fundamental to quantum mechanics.

Mathematically, eigenvalues are represented as λ (lambda) in the eigenstate equation:

A|ψ⟩ = λ|ψ⟩

Eigenvalues are quantized, meaning they can only take on specific discrete values rather than a continuous range of possibilities. When you measure an observable associated with an operator A on a quantum system in an eigenstate |ψ⟩, the probability of obtaining the eigenvalue λ is determined by the square of the coefficient of |ψ⟩ in the superposition.

In summary, eigenstates and eigenvalues form the bedrock of quantum mechanics, providing a structured and systematic way to predict the outcomes of measurements in the quantum world. These concepts allow us to navigate the inherently probabilistic nature of quantum systems and gain valuable insights into the behavior of particles at the smallest scales.

Quantum States: Building the Foundation

To understand measurement and observables in quantum mechanics, it's essential to grasp the concept of quantum states. Quantum states are the fundamental building blocks of the quantum world, and they play a central role in the measurement process.

State Vectors

Quantum states are represented by state vectors, often denoted as |ψ⟩, where "ψ" is a placeholder for the state's name. These state vectors belong to a vector space called a Hilbert space. In simple terms, Hilbert space is a mathematical framework that allows us to work with quantum states and perform operations on them.

State vectors are unique in that they are not like classical states with definite properties. Instead, they describe the probabilities of obtaining different outcomes when measuring physical properties. The state vector |ψ⟩ represents a superposition of possible states, each with its associated probability amplitude.

Superposition and Entanglement

Superposition is a fundamental concept in quantum mechanics. It allows quantum systems to exist in a linear combination of multiple states simultaneously. For instance, a particle's state can be a superposition of being in two different positions at the same time. The coefficients of these states in the superposition determine the probabilities of measuring each position.

Entanglement is another intriguing phenomenon in quantum mechanics. It occurs when two or more particles become correlated in such a way that the state of one particle cannot be described independently of the other(s). This implies that measuring one particle's state instantaneously influences the state of the other, even if they are separated by vast distances. Einstein famously referred to this as "spooky action at a distance."


Wavefunctions are mathematical representations of quantum states. They provide a way to describe the probability distribution of a particle's position or other physical properties. The square of the absolute value of the wavefunction at a given point represents the probability of finding the particle at that location.

In the context of operators and states, wavefunctions can be used to extract information about a quantum system's properties. For instance, the position operator acting on a wavefunction gives the expected position of the particle.

Measurement and Observables

Measurement is a fundamental aspect of understanding and interacting with the physical world. In classical physics, measurements are straightforward, providing precise values for various properties like position, velocity, and energy. However, in the realm of quantum mechanics, the act of measurement takes on a unique character, giving rise to intriguing phenomena and challenging our classical intuitions. In this blog, we'll explore the concepts of measurement and observables in quantum mechanics, shedding light on the fascinating aspects of this quantum world.

Measuring Quantum States

In quantum mechanics, the process of measurement is a crucial aspect of understanding the behavior of quantum systems. When a measurement is made on a quantum state, the state collapses into one of its eigenstates, and the corresponding eigenvalue is observed. This collapse of the wavefunction is often referred to as the "measurement postulate."

For example, when measuring the position of an electron, the act of measurement causes the electron to be found at a specific location with a probability determined by the square of the coefficient of the corresponding eigenstate.

Observables and Hermitian Operators

Observables in quantum mechanics are physical properties that we can measure, such as position, momentum, or energy. Each observable is associated with a Hermitian operator, which is a special type of operator that has real eigenvalues.

Hermitian operators play a crucial role in quantum mechanics because their eigenvalues correspond to measurable quantities. When we measure an observable, we are essentially finding one of the eigenvalues of the associated Hermitian operator.

The Schrödinger Equation: Evolution of Quantum States

The Schrödinger equation is the fundamental equation of quantum mechanics, describing how quantum states evolve over time. It connects the concept of operators and states to the dynamics of quantum systems.

Time-Dependent Schrödinger Equation

The time-dependent Schrödinger equation is given by:


In this equation:

  • ∣ψ(t)⟩ represents the state of the quantum system at time t.
  • H is the Hamiltonian operator, which represents the total energy of the system.
  • ℏℏ is the reduced Planck's constant.

The Schrödinger equation tells us how the state of a quantum system changes with time. It describes the evolution of quantum states, which can be thought of as the system's "wavefunction."

Time-Independent Schrödinger Equation

In cases where the Hamiltonian operator does not depend on time (i.e., H is a constant), we have the time-independent Schrödinger equation:



  • ∣ψ⟩ is a stationary state of the system.
  • E is the energy eigenvalue associated with the state ∣ψ⟩.

The time-independent Schrödinger equation helps us find the allowed energy levels and corresponding wavefunctions for a quantum system.


Operators and states form the backbone of quantum mechanics, allowing us to describe, analyze, and predict the behavior of particles at the quantum level. While the mathematical formalism behind quantum mechanics can be incredibly complex, the fundamental concepts of operators, states, and measurements are essential for gaining an understanding of this enigmatic branch of physics.

In this blog, we've explored the role of operators as mathematical tools for representing observables, the concept of quantum states as superpositions with associated probabilities, and the intriguing phenomena of superposition and entanglement. We've also discussed how measurements are made in quantum mechanics, the significance of Hermitian operators, and the Schrödinger equation as the equation governing the evolution of quantum states.

Quantum mechanics challenges our classical intuitions and provides a unique perspective on the nature of reality. It continues to be a rich source of discovery and technological advancement, with applications ranging from quantum computing to quantum cryptography. Understanding the mathematics behind operators and states is a crucial step toward grasping the profound intricacies of this captivating field.

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