Some numbers are very special. They never end. They do not repeat a pattern. You can find them in circles. They are in many places. We use them to measure things. Do you like to find patterns?
Some numbers are very strange. They are called irrational numbers. 
Most numbers can be written as a simple ratio. This means you can split them into equal parts. But irrational numbers cannot do this. They never end when you write them out. They also do not repeat a pattern.
You can find these numbers in a circle. The number pi is one of them. 
Long ago, people in Greece found these numbers. They used shapes to study them. These numbers showed that some lengths are hard to measure. They are part of our world.
Most numbers are easy to write. You can turn them into a ratio. A ratio is just two whole numbers compared. For example, one half is a ratio. We call these rational numbers.
But some numbers are different. We call them irrational numbers. These numbers cannot be written as a simple ratio. If you write them as decimals, they never end. They also never show a repeating pattern. One famous example is pi. Pi is the ratio of a circle's edge to its width.
Long ago, Greeks studied these numbers. A man named Hippasus found a problem. He looked at a special right triangle. He found a length that could not be a ratio. This discovery was a big shock. It broke many old rules about math.
Later, a thinker named Eudoxus helped solve this. He made a new way to look at sizes. He said some things are numbers. Other things are magnitudes, like lines or areas. This helped math move forward. Today, we know that almost all real numbers are irrational.
Most numbers we use are easy to write as a ratio. A ratio is just a way to compare two whole numbers, like one half or three quarters. We call these rational numbers.
Irrational numbers have a special property in geometry too. If you have two lines, their lengths might be incommensurable. This is a big word that means they share no common measure. No matter how tiny a ruler you use, you cannot measure both lines perfectly using only whole numbers of that unit. One famous example is the square root of two. This number appears when you look at the sides of a special right triangle. All square roots of natural numbers are irrational, unless the number is a perfect square.
History tells us that these numbers were a huge shock to the ancient Greeks. A man named Hippasus, who was likely a Pythagorean, discovered them in the 5th century BC. He was studying the sides of a pentagram and a special triangle. He proved that the longest side and a shorter side could not both be whole numbers. This discovery was very upsetting to his fellow mathematicians. Some legends say he was thrown overboard at sea for this discovery. Others say he was simply sent away into exile.
After this shock, other thinkers had to fix the rules of math. A man named Zeno of Elea argued that quantities are continuous rather than made of tiny, separate units. He showed that you can always split a line in half, forever. Then, Eudoxus of Cnidus created a new theory to help. He made a distinction between numbers and magnitudes. He said numbers jump from one value to another, like 4 to 5. Magnitudes, like lines or areas, can change continuously.
Many different cultures studied these strange numbers over a long time. In India, mathematicians like Manava believed some square roots could not be exactly determined as early as the 7th century BC. Later, Indian thinkers like Brahmagupta and Bhāskara I made great progress. Between the 14th and 16th centuries, the Kerala school discovered infinite series for numbers like pi. In the Middle Ages, Muslim mathematicians used algebra to treat these as objects. They merged the ideas of number and magnitude into the real numbers we study today.
Irrational numbers are a specific type of real number. They are defined by what they cannot do. You cannot express an irrational number as a ratio of two integers. A ratio is a comparison of two whole numbers, like one divided by two. Because of this, irrational numbers are quite different from rational numbers.
To understand how these numbers behave, we can look at their decimal notation. Every real number can be written as a decimal. For an irrational number, the decimal expansion has two unique properties. First, it does not terminate, meaning it never ends. Second, it never settles into a repeating sequence of digits. Take the number pi as a famous example. It begins with 3.14159, but the digits continue forever without a pattern. You can never write down the exact value of pi using a finite number of digits. Conversely, any decimal that ends or repeats must be a rational number.
Irrational numbers also create unique situations in geometry. When the ratio of two line segments is irrational, they are called incommensurable. This means the two segments share no common measure. No matter how small a unit of length you choose, you cannot use it to measure both segments perfectly. You could never find a single unit that fits an integer number of times into both lengths. One common example is the square root of two. This number appears when comparing the sides of certain triangles.
The discovery of these numbers caused a massive crisis in Ancient Greece. A mathematician named Hippasus, likely a member of the Pythagorean school, discovered them in the 5th century BC. He was studying the sides of a pentagram and an isosceles right triangle. He used a method called reductio ad absurdum to prove his point. He assumed the hypotenuse and a leg could be expressed as integers. Using the Pythagorean theorem, he showed this led to a mathematical impossibility. He proved that both numbers would have to be even, which contradicted his initial assumption. This discovery shattered the Pythagorean belief that all things could be reduced to whole numbers.
This revelation was so upsetting that it led to dark legends. Some stories claim Hippasus was thrown overboard at sea by his fellow Pythagoreans. They were angry because his discovery denied their core doctrines. Other legends suggest he was simply sent into exile. Regardless of the legend, his work forced mathematicians to rethink the universe. It highlighted a conflict between discrete objects and continuous quantities. Zeno of Elea helped explore this by questioning if quantities were made of finite units. He argued that quantities are actually continuous. He showed that you can always split a line segment in half, infinitely.
To solve these problems, Eudoxus of Cnidus developed a new theory. He created a vital distinction between magnitude and number. Numbers were seen as discrete values that jump from one to another. Magnitudes, such as lines or areas, were seen as entities that vary continuously. This allowed mathematicians to work with incommensurable ratios without needing to turn them into simple numbers. Eudoxus also developed the method of exhaustion. This was a logical way to establish proofs through deductive reasoning. This method eventually became a foundational step toward the creation of calculus.
Different cultures contributed to our understanding of these numbers over centuries. In India, mathematicians addressed square roots as early as the Vedic period. By the 7th century BC, Manava believed some square roots could not be exactly determined. Later, the Kerala school of astronomy and mathematics made huge strides. Between the 14th and 16th centuries, they discovered infinite series for pi and other values. In the Middle Ages, Muslim mathematicians used algebra to treat irrationals as algebraic objects. They merged the ideas of number and magnitude into the general concept of real numbers. This work helped build the complex mathematical systems we use today.
🖼️ Images & Media (2)
More to explore
✨ What else?
Related topics you might enjoy
🪜 Step back
Simpler topics to build understanding
What is Nepedia?
A free, ad-free encyclopedia for children. Every article is written at five reading levels, so the same page works for a five-year-old and a fifteen-year-old — use the level switcher above to see this one change. No account needed to read.