What Is a Random Number?

Random · LESSON 1

A random number is a number nobody can predict. But there are actually two kinds of them!

True random

Natural random

Nobody can predict what nature does next. That is why nature makes "true random numbers."

LightningNobody knows when or where it strikes

RaindropsWhere they hit the glass is anyone's guess

Lava lampThe blobs shift endlessly, never the same twice

Leaves in the windWhich way they drift changes every time

A lava lamp is a great real-world source of natural randomness. It is a decorative lamp holding soft wax and a watery liquid in a glass jar. A bulb below heats the wax so it floats up, then it cools and sinks again, over and over. The shapes keep changing and never repeat exactly.

One company actually makes true random numbers from these ever-changing lava lamps. Cloudflare, a large U.S. internet company, lines a wall of its San Francisco lobby with about 100 lava lamps and films them nonstop with a camera. The unpredictable motion of the blobs becomes true random numbers.

The lava-lamp wall in Cloudflare's lobby · Source: Cloudflare blog ▶

The lava-lamp wall in Cloudflare's headquarters lobby

But natural random numbers are slow to make and hard to gather in bulk.

Random No.

Pseudo random

Random numbers a computer makes fast, by calculation. ("Pseudo" means "close to.") Not truly natural, but they come out just as evenly.

Why is it needed? If millions of people enter a draw, you need millions of random numbers. You can't wait for millions of lightning strikes. So computers use pseudorandom numbers made quickly instead. Making that safely and well is the goal of this "Random Numbers" section.

Try it — roll the dice

It rolls using computer (pseudo) random numbers. Roll many times — do the results come out as evenly as natural random?

Total rolls 0

The more you roll, the more even the bars get. That is what "even = fair" means.