When you use a tape measure, you're participating in a chain of trust that tells you how long one meter is.

We rarely agree on things because they're true—we agree because disagreeing is too expensive.

Once a social agreement is locked in, we treat our own arbitrary rules like facts of nature.

Say you need to tell someone over the phone how long something is in your room. It’s easy: You take a ruler, measure the thing, and read the number to the person on the other end of the line. Scientists and much of the world use the metric system for this sort of thing.

This all sounds very precise, but how does anyone know how long a meter is?

Initially, the definition of a meter was tied to the Earth’s circumference. In the 1790s, the French defined it as exactly one ten-millionth of the distance from the equator to the North Pole. That’s about as objective as it gets, but is almost entirely useless in daily life: A tailor cannot triangulate the globe every time they need to cut a piece of fabric.

To make this whole measuring thing more practical, expeditions spent several years surveying the Earth, did the math, and forged a literal metal bar to represent that distance. For over a century, that was the “meter”: a platinum-iridium bar locked in a vault in France. If your wooden ruler matched the metal bar in Paris, you had a meter.

But a metal bar isn’t perfect either. It can expand in heat, contract in cold, and sustain microscopic scratches. As industrial technology and global communications demanded a higher degree of uniformity, the definition of a meter had to change again. First, it was pegged to the radiation emitted by........

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