# How Cartesian Coordinates Mapped the World and Paved the Way for Quantum Mechanics

### Discover how a 17th-century Breakthrough Revolutionised Mapping, Spatial Exploration and laid the foundations for Quantum Mechanics

René Descartes is best known for his philosophical works. If the following statement rings a bell, then you already are familiar with some of his most popular works.

I think, therefore I am. - René Descartes

But did you know that the most profound contribution that Descartes made was not in philosophy but in mathematics? Yes, that’s right. Apart from being a prolific philosopher, René Descartes was also a savvy mathematician who made major contributions to the field.

It is not far-fetched to say that most modern day mapping systems from tracking geolocation on earth to mapping the orbits of planets in the solar system have been made possible thanks to the Cartesian coordinates system introduced by Descartes.

Cartesian coordinates also played a crucial role in developing and describing the foundational ideas of Quantum mechanics and continue to be a major tool in the Quantum exploration toolkit today.

## How Descartes discovered the Cartesian Coordinates System

In the 17th century, René Descartes made a remarkable discovery while sitting in his study. As he watched a fly buzzing around the room, he noticed something intriguing. The fly moved along the ceiling, then landed on the adjacent walls.

Curiosity sparked within Descartes. He wondered if he could map the fly's position using a simple system. Grabbing a pen and paper, he drew two perpendicular lines, one horizontal and one vertical. The point where they intersected became his starting point.

With this simple coordinate system in place, Descartes recorded the fly's position as it moved. He realised that by assigning numerical values to the intersecting lines, he could precisely locate any point in the room.

This groundbreaking insight led to the development of what we now know as Cartesian coordinates. Descartes saw the potential of this system to map not only flies but also objects of all sizes and locations. His work laid the foundation for analytical geometry and revolutionised mathematics.

The name "Cartesian" comes from Descartes himself. He Latinised his name to "Cartesius," and the coordinate system became known as Cartesian coordinates in his honour.

## A formal definition of Cartesian Coordinates

Cartesian coordinates, in mathematical terms, refer to a system used to specify the position of a point in space or on a plane. It involves representing each point by a unique ordered pair or triplet of real numbers, corresponding to its distances from two or three mutually perpendicular reference lines, respectively.

In two-dimensional Cartesian coordinates, also known as the *Cartesian plane*, points are represented using ordered pairs *(x, y)*, where *'x'* denotes the distance from the vertical reference line (often called the x-axis), and *'y'* denotes the distance from the horizontal reference line (often called the y-axis). The intersection of the x-axis and y-axis is the origin, denoted by the ordered pair *(0, 0)*.

In three-dimensional Cartesian coordinates, points are represented using ordered triplets *(x, y, z)*. The *'x'* value represents the distance from the *yz-plane*, the *'y'* value represents the distance from the *xz-plane*, and the *'z'* value represents the distance from the *xy-plane*. The origin in three dimensions is denoted by the ordered triplet *(0, 0, 0).*

## Why Cartesian Coordinates matter

The Cartesian coordinate system allows for the precise representation and analysis of points, lines, curves, shapes, and objects in mathematical and geometric contexts. It forms the basis for many mathematical concepts and calculations, facilitating computations involving distances, angles, areas, and volumes.

By utilising Cartesian coordinates, mathematicians, scientists, and engineers can describe and study the relationships between various points and objects, enabling the development of mathematical models, simulations, and analyses across a wide range of fields and applications.

### Precise Positional Representation

Cartesian coordinates provide a precise and standardised method for representing the position of points in space or on a plane. This allows for accurate communication and analysis of spatial relationships, enabling scientists, engineers, and mathematicians to describe and study the physical world with precision.

### Analytical Geometry

Cartesian coordinates form the foundation of analytical geometry, a branch of mathematics that combines algebra and geometry. Analytical geometry allows for the study of geometric properties and relationships using algebraic techniques, facilitating the exploration of curves, lines, conic sections, and other mathematical entities. It provides a bridge between algebraic equations and geometric concepts, aiding in the understanding and manipulation of complex mathematical relationships.

### Coordinate Systems in Physics

Many branches of physics heavily rely on Cartesian coordinates. From classical mechanics to electromagnetism and quantum mechanics, Cartesian coordinates enable the precise description and analysis of physical phenomena. They allow physicists to track the motion of objects, model the behaviour of waves, and understand the behaviour of particles in various systems.

### Navigation and Mapping

Cartesian coordinates are essential for navigation and mapping. They provide a common reference system that allows us to locate and navigate through physical spaces accurately. From GPS navigation systems to aerial surveys and satellite imagery, Cartesian coordinates play a crucial role in modern mapping and spatial analysis.

### Data Visualisation and Analysis

Cartesian coordinates are widely used in data visualisation and analysis. By mapping data points onto a coordinate system, patterns, trends, and relationships can be visually represented and analysed. This aids in making informed decisions, identifying correlations, and deriving meaningful insights from complex datasets.

## The role of Cartesian Coordinates in Quantum Mechanics

In quantum mechanics, Cartesian coordinates play a fundamental role in describing the position and spatial properties of particles and systems. I have listed below some major uses-cases for Cartesian coordinates in quantum mechanics. Of course, this is not an exhaustible list as Cartesian coordinates are used in a myriad of different ways in quantum mechanics. But I have avoided adding some of the more complex uses-cases for the simplicity of this article.

### Position Representation

Quantum mechanics describes the behaviour of particles in terms of wave functions, which provide a mathematical representation of their quantum states. Cartesian coordinates provide a natural framework for expressing the position space representation of wave functions. In this representation, the wave function depends on the coordinates *(x, y, z)* of the particle, and its magnitude squared gives the probability density of finding the particle at a particular position.

The position space representation of a wave function in three-dimensional Cartesian coordinates is:

**Ψ(x, y, z)**

Here, *Ψ* represents the wave function, and *(x, y, z)* represents the Cartesian coordinates of the particle's position.

### Quantum Operators

Operators in quantum mechanics play a central role in the analysis and calculation of physical quantities. Many fundamental operators, such as position, momentum, and angular momentum, are expressed in terms of Cartesian coordinates. For example, the position operator in three dimensions is represented by the triplet of operators *(x, y, z)*, which act on the wave function to yield the corresponding position information.

**Position Operator:**The position operator in Cartesian coordinates is represented by a vector*(x, y, z)*:**^r = (x, y, z)**This operator acts on the wave function

*Ψ(x, y, z)*to yield the position information of the particle.**Momentum Operator:**The momentum operator in Cartesian coordinates is given by:**^p = (-iħ∂/∂x, -iħ∂/∂y, -iħ∂/∂z)**Here,

*ħ*is the reduced Planck's constant, and*∂/∂x, ∂/∂y, ∂/∂z*represent partial derivatives with respect to*x, y,*and*z,*respectively. The momentum operator acts on the wave function*Ψ(x, y, z)*to determine the momentum of the particle.**Angular Momentum Operator:**The angular momentum operator in Cartesian coordinates is expressed as:**^L = (^Lx, ^Ly, ^Lz)**Where:

*^Lx = -iħ(y∂/∂z - z∂/∂y) ^Ly = -iħ(z∂/∂x - x∂/∂z) ^Lz = -iħ(x∂/∂y - y∂/∂x)*Here,

*∂/∂x, ∂/∂y, ∂/∂z*represent partial derivatives with respect to*x, y,*and*z,*respectively. The angular momentum operator acts on the wave function*Ψ(x, y, z)*to determine the angular momentum of the particle along the*x, y,*and*z*axes.

### Schrödinger Equation

The Schrödinger equation is a fundamental equation in quantum mechanics that describes the time evolution of wave functions. When using Cartesian coordinates, the Schrödinger equation takes the form of a partial differential equation, where the Laplace operator (the sum of the second partial derivatives with respect to each coordinate) acts on the wave function. This equation allows us to calculate and understand the behaviour of quantum systems.

The Schrödinger equation is expressed as:

**ĤΨ(x, y, z, t) = iħ∂Ψ(x, y, z, t)/∂t**

Here, *Ĥ* represents the *Hamiltonian operator*, which is the total energy operator of the system. *Ψ(x, y, z, t)* is the wave function of the particle, which depends on the Cartesian coordinates *(x, y, z)* and time *t*. The symbol *∂/∂t* represents the partial derivative with respect to time, and *i* is the imaginary unit. *ħ* is the reduced Planck's constant, which relates the energy and frequency scales in quantum mechanics.

### Potential Energy Surfaces

In quantum mechanics, the potential energy surface describes the interaction potential between particles in a system. Cartesian coordinates are used to parameterise this potential energy surface, allowing for the analysis of energy landscapes and the determination of important features such as energy minima, barriers, and transition states. This information is crucial for understanding chemical reactions, molecular dynamics, and the behaviour of quantum systems.

### Quantum Harmonic Oscillator

The quantum harmonic oscillator is a fundamental model in quantum mechanics that represents a particle subjected to a restoring force proportional to its displacement from an equilibrium position. In Cartesian coordinates, the Hamiltonian of the quantum harmonic oscillator is expressed in terms of the position and momentum operators *(x, p)*, which are Cartesian coordinate-based observables. Solving the quantum harmonic oscillator problem provides insights into the quantisation of energy levels and wave functions in various physical systems.

## Conclusion

Cartesian coordinates have proven to be an invaluable system in various fields, ranging from mathematics and physics to astronomy and engineering. By providing a straightforward and intuitive way to represent points in space, Cartesian coordinates have revolutionised our ability to map and understand the world around us.

Through their use in the position space representation of wave functions, Cartesian coordinates have enabled us to explore the *probabilistic nature of quantum systems*, paving the way for advancements in quantum mechanics.

The ability to describe particles' positions and their spatial relationships has allowed researchers to study phenomena such as *wave interference, particle scattering, and quantum entanglement*. Cartesian coordinates provide a framework for defining important operators like *position, momentum, and angular momentum*, which are essential in quantifying physical quantities in quantum mechanics.

Beyond quantum mechanics, Cartesian coordinates have facilitated the mapping of the Earth's surface, the navigation of spacecraft, the design of complex structures, and countless other applications. By providing a standardised system for describing locations and distances, Cartesian coordinates have enhanced our ability to analyse and manipulate the physical world.

A sagacious foundation in Cartesian coordinates is absolutely essential if you are getting started with quantum mechanics or any other field that requires a deep understanding of the position and movement of objects. In the coming weeks, we shall be exploring in-depth, the uses-cases of Cartesian coordinates in quantum mechanics. If you’d like to join us in this journey, don’t forget to subscribe!

Props to Descartes. When you think about the smartest people who ever lived, his name should at least be considered alongside Einstein's, Newton's, and Aristotle's.

Loved the background history of Cartesian coordinates. Also, the uncomplicated language makes it an easy read. I know how difficult it is to not complicate things with all the extra information.