You Could Make A Measurement System Based On The Speed Of Light

Idle sci-fi worldbuilding idea: A measurement system based on the speed of light would make a reasonable amount of sense. It would still be nailed to Earth units of time, but if we assume that a healthy human sleep/wake cycle is 24 hours long regardless of what the lighting situation is (has anyone actually studied this?), then we’ll carry that length of day in our biology even as we leave behind the planet that evolved it into us. And light distances mostly conform really well to easily managed 1-100 scales for most human use cases.

A light nanosecond is almost the length of one foot. That means it’s a good length for measuring the size of things that are close to human scale. Human height is measured in a single digit number of light nanoseconds. It is slightly annoying to express small lengths like we usually use inches or centimeters for as hundreds of light picoseconds, but people who use meters have the reverse problem where you have to use decimal points all the time because most humans are between one and two meters tall and there’s never any riots demanding a return to imperial measurements.

A light microsecond is a really good unit of distance for overland travel. The average human can walk a light microsecond in a couple of minutes, and can walk about sixteen light microseconds per hour, so human walkable distances largely go from a scale of one (for something that can just barely be described as going for a walk rather than going down the street) to one hundred (for something that will require several hours, approaching the limit of what a reasonably fit but untrained human can walk if they set aside the day for it).

Car speeds largely go from 100 microseconds/hour for residential distances to 500 microseconds/hour (US speed limits usually cap out around 430 microseconds/hour, but most people end up going ~450 anyway, and it’s easy to imagine a society built around this measurement system using 100-500 as “car speed”). This isn’t quite as snug as nanoseconds to microseconds because now instead of a 1-100 scale that keeps commonly used distances to double-digits. If you’re in any kind of hurry, you’ll take the car for anything that would take more than 5-10 light microseconds, and the upper limit of a day trip in a car is well over a light millisecond. A light millisecond is about 186 miles, which is far past the limit of being a quick car ride but well under the limit of how far a car can take you in a day.

On the other hand, light milliseconds do lend themselves really well to measuring the distances for flights. A distance of less than one millisecond is generally too short to be worth going to the airport for, an NYC to Singapore flight is about fifty milliseconds, and the circumference of Earth is about 134 milliseconds. It’s hard to doublecheck the shortest commercial flight distance because the low end of that scale is taken up by trips that are well within driving distance but there’s water in the way and trips are infrequent enough that it’s not worth it to build a bridge. London to Paris is slightly more than one light millisecond and they built a tunnel under the English Channel because people were getting annoyed at having to take a plane for that distance. It’s definitely rare for any single-trip distance on Earth to be anywhere near the 100 milliseconds at the top of a 1-100 scale, on account of the Earth is a sphere so if you’re going 80 milliseconds it’s basically guaranteed that you could go the other way around and get there in 60 or less.

One full light second is about the distance of the Earth to the moon, and intrasolar distances are frequently measured in light minutes in the inner solar system and light hours in the outer solar system because those are already the most convenient units to use. The inner solar system keeps to 1-100 light minute scale while the entire solar system is about 22 light hours across, so even in the outer solar system where the distance from one object to another might be, depending on orbits, hundreds or even low thousands of light minutes away, distances in light hours are always manageably low numbers.

As you get into sci-fi intergalactic distances, you quickly run into the problem that the galaxy is a heccin chonker 100,000 light years across, and that’s not unusably large the way trying to measure galactic distances in miles or kilometers is (although metric has the advantage of being able to scale up to petameters and exameters), but it does massively exceed the 1-100 scale I’ve been trying to keep to. Of course, to actually use that space you must necessarily be using FTL travel of some kind, at which point you are making up how fast things go and may as well invent some hypertech excuse why light speed travel is measured in kiloyears on major trade spines but goes down to lightyears per hour (or per day or per week, depending on how isolated you want solar systems to be) outside of the main hyperspace routes.

Fractions of the speed of light are a futuristic, science-y kind of measurement tied to a fundamental law of physics and a unit of time that, while arbitrary, is probably pretty deeply tied to human biology, and it works really well for measuring the size of things at roughly human scale in light nanoseconds, walking distances in light microseconds, flight distances in light milliseconds, distances within solar systems in light seconds, minutes, and hours depending on the exact region, and distances between solar systems in light years (although that last one’s a freebie because the propulsion systems and frequency of landmarks worth caring about is up to the author anyway).

It’s annoying how car distances straddle the microsecond/millisecond line, but it’s otherwise very usable and fairly easy for readers to translate, easy enough that it might not be frustrating in use – provided that your plot doesn’t deal with the ugly car distance microsecond/millisecond overlap, because while I think people can quickly grok “walking distance is measured in microseconds, in-atmosphere flights are measured in milliseconds, and they both go on a scale of 1-100,” the hundreds of microseconds to whole milliseconds scale of car travel means they’ll start trying to convert to miles or kilometers in their head or treat the distances as white noise. And while you can make atmospheric flights stretch up to 100 milliseconds by making planes faster (while the bottom of the scale remains the same because it’s controlled by the point at which a car is slow enough to justify a plane, not the point at which the plane is too fast) and just not use the top 50 points of the scale because Earthlike planets aren’t that big and it’ll still work, you can’t do the same for cars because humans aren’t getting any faster so you cannot make a 300 microsecond trip reasonable walking distance, but cars are already too fast to reasonably be capped at sub-millisecond distances. Getting around that would require significant worldbuilding oriented around making this distance system reader-friendly enough to be usable.

I’m not really going anywhere with this, I have no plans for this measurement system and am not working on anything remotely Star Trek-ish enough to bother using it. So this is one of the random research project posts.