Key Moments
How Imaginary Numbers Were Invented
VeritasiumKey Moments
Imaginary numbers, initially invented to solve cubic equations, are now fundamental to physics.
Key Insights
The quest to solve the cubic equation, a problem for millennia, led to the invention of imaginary numbers.
Early mathematicians resisted negative and imaginary numbers, viewing math as strictly tied to physical reality.
The cubic equation's solution required abandoning geometric intuition and accepting abstract mathematical concepts.
Girolamo Cardano's discovery of the general cubic solution, utilizing imaginary numbers, was published despite conflicting oaths.
Imaginary numbers, first seen as a mathematical curiosity for solving cubics, are now essential in quantum mechanics.
The use of imaginary numbers in physics suggests that reality is more complex than our direct sensory experience.
THE IMPOSSIBLE CUBIC AND THE BIRTH OF ABSTRACT MATH
Mathematics, initially a tool for quantifying the real world, faced a seemingly insurmountable challenge: solving the general cubic equation. For thousands of years, mathematicians from various civilizations attempted to find a universal method, but all failed. Luca Pacioli, in 1494, declared a general solution impossible. This impossibility stemmed from a deep-seated belief that mathematics must directly mirror physical reality, making concepts like negative solutions in geometric contexts nonsensical. The quadratic equation, easily solved geometrically, had already encountered resistance to negative solutions, which required a conceptual leap.
EARLY ATTEMPTS AND THE GEOMETRIC APPROACH
Ancient mathematicians visualized algebraic equations through geometry. For a quadratic like x² + 26x = 27, they imagined a square of side 'x' and a rectangle of 26x. By completing the square, they could find 'x'. However, this approach was limited by the need for positive quantities, excluding negative numbers as solutions because they didn't correspond to physical lengths or areas. Omar Khayyam, in the 11th century, identified numerous cubic equations, seeking solutions through intersections of geometric shapes, but still fell short of a general algebraic solution, leaving the problem for future generations.
DEL FERRO'S SECRET AND TARTAGLIA'S TRIUMPH
Around 1510, Scipione del Ferro discovered a method to solve 'depressed' cubic equations (those without an x² term). Fearing professional rivalry, he kept his solution secret. Upon his death, his student Antonio Fior was entrusted with the method. Fior, lacking his mentor's talent, boasted of his ability, leading to a mathematical duel with Niccolò Fontana Tartaglia. Tartaglia, skeptical but motivated, independently found a way to solve the depressed cubic using a three-dimensional extension of completing the square. He devised an algorithm, famously written as a poem, to solve these equations.
CARDANO, OATHS, AND THE GENERAL CUBIC SOLUTION
Gerolamo Cardano, persistent in his pursuit of the cubic solution, eventually persuaded Tartaglia to reveal his method under oath of secrecy. Cardano, a physician, saw the potential for a general solution. He discovered that by substituting x = y - b/(3a), any general cubic could be transformed into a depressed cubic. This breakthrough solved the problem for millennia, but Cardano faced an ethical dilemma due to his oath. He found a way around it by discovering del Ferro's prior work, allowing him to publish the general solution in his monumental work, 'Ars Magna'.
THE EMERGENCE OF IMAGINARY NUMBERS
While working on 'Ars Magna,' Cardano encountered cubic equations that, when solved using his method, produced intermediate steps involving the square roots of negative numbers. He could find real solutions (e.g., x=4 for x³ = 15x + 4), but his formula yielded complex expressions. He considered these 'imaginary' numbers useless, as they couldn't be reconciled with geometry. It was Rafael Bombelli who, a decade later, embraced these 'imaginary' quantities. By treating the square root of -1 as a new type of number, he showed how these terms could cancel out during calculations, leading to valid real solutions.
FROM ALGEBRAIC CURIOSITY TO PHYSICAL REALITY
Over the next century, mathematicians like François Viète and René Descartes formalized algebraic notation and popularized imaginary numbers, which were eventually named 'imaginary' by Descartes and symbolized by 'i' by Euler. These numbers, forming complex numbers, emerged from an abstract pursuit of solving cubic equations. Astonishingly, in the 20th century, Erwin Schrödinger incorporated 'i' into his foundational wave equation for quantum mechanics. This use revealed that, contrary to initial beliefs, imaginary numbers are not just mathematical abstractions but fundamental to describing the universe's true nature.
IMAGINARY NUMBERS IN QUANTUM MECHANICS
The appearance of 'i' in Schrödinger's equation puzzled physicists, who initially sought real-valued descriptions. However, the unique properties of imaginary numbers, particularly their connection to rotation in the complex plane, proved indispensable. Multiplying by 'i' represents a 90-degree rotation, and the function e^(ix) elegantly combines sine and cosine waves—the basis of wave mechanics. The derivative of exponential functions involving 'i' is proportional to the function itself, a property crucial for wave equations. This suggests that reality itself operates on principles described by complex numbers, a profound realization born from an abstract mathematical problem.
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Common Questions
Imaginary numbers were invented to solve the cubic equation, a problem that had stumped mathematicians for centuries. Initially, they seemed like a mathematical trick disconnected from reality.
Topics
Mentioned in this video
A mathematics professor at the University of Bologna who found a method to solve depressed cubic equations around 1510 but kept it secret.
Authored 'Summa de arithmetica', a comprehensive summary of mathematics in Renaissance Italy, which stated that a general solution to the cubic equation was impossible.
A mathematician who successfully solved 30 depressed cubic problems in a challenge against Fiore and later revealed his method to Cardano.
A polymath who learned Tartaglia's method, discovered the solution to the full cubic equation, and published 'Ars Magna'.
Introduced modern symbolic notation for algebra in the 1600s.
An Italian engineer who, around 10 years after 'Ars Magna', worked with the square roots of negative numbers and found a way to consistently solve cubic equations using them.
A polynomial equation of the third degree, which mathematicians struggled to solve for millennia.
Numbers formed by combining real numbers and imaginary numbers, crucial for various scientific fields.
A polynomial equation of the second degree, which had been solved by ancient civilizations.
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