The light year is a bookkeeping unit that has been promoted, by repetition and by the poverty of better language, into a cosmic speed limit. Both halves of that sentence are wrong in slightly different ways. A light year measures the distance a photon covers in one orbit of Earth around the Sun, and it measures that distance against the stage on which photons and Earths and Suns appear. We treat that stage as the bedrock of reality because every instrument we have ever built reports back from inside it. Our instruments cannot, by their nature, report from anywhere else. A fish with sophisticated sonar maps the reef in exquisite detail and concludes the reef is all there is. The water is invisible because the water is the medium of seeing.

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