Introduction

This document is written to help fifth graders (and adults!) understand how the definition of a meter changed in 1983. We’ll explore what changed, what stayed the same and whether the word “meter” changed its part of speech.

Before 1983

Meter as a Measured Thing

Before 1983, a meter was a physical object or something you could measure with light. You could hold a meter stick or count waves of orange-red light from an atom. It was a noun, a thing.

After 1983

Meter as a Definition

In 1983, scientists decided to stop measuring the speed of light and instead define it as exact. Now they say a meter is how far light goes in exactly 1/299,792,458 seconds. So, instead of using a stick or counting light waves, they use a time number and assume light is always the same.

Did the Word “Meter” Change its Job?

No, the word “meter” is still a noun. It’s still a thing, a unit, a measurement. But how we define that noun changed. It used to be defined by something we measured. Now it’s defined by a number based on something we can’t see (light “traveling”), which we assume instead of test.

Simple Way to Think About it

Imagine we used to measure a “step” by watching someone walk. Now we say, “A step is how far you would go if your heart beats 72 times in one minute”. It might be the same length, but we’re no longer using steps to define steps, we’re using something else entirely.

Produced by The Lilborn Equation Team:

Michael Lilborn-Williams

Daniel Thomas Rouse

Thomas Jackson Barnard

Audrey Williams