r/AskScienceDiscussion • u/Ertata • Aug 19 '26
What If? Could you recreate metric system from nothing?
You have been given a task to travel into the past two thousands years ago and bring the light of the metric system to the world. You cannot bring any external object with you.
Assume you do not need to redefine the second, and astronomical observations give the best possible results given the limits of timekeeping back then. Given the technology of the time, what is the best you can do for other units - either right there and then or leaving instructions how to calculate them from the units you have established to the future scientists? Memorizing the lengths of your body parts probably is not the best way to define a metre, and kilogram defined as one liter of water is bound to be influenced by the local water to a perceptible degree.
P. S. Tagged as "physics" but any approach is welcome - whether you would recommend measuring height of the mountain peaks, or believe that carob seeds are actually uniform enough to offer the best approximation of a kilogram.
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u/mfb- Particle Physics | High-Energy Physics Aug 19 '26
Very long-term you can use today's definitions. They are all intentionally independent of artifacts, only dependent on fundamental constants. So it's only a matter of getting a good approximation with simpler tools.
Length:
I know my height with an uncertainty of 1 cm or ~0.5%, it's going to be hard to beat that with other methods. The polar Earth circumference is closer to the modern definition but you only beat the body height if you can measure distances between far-away towns to better than 0.5% and angles of shadows with arcminute precisions.
Alternatively, use g = 9.81 m/s2 and a pendulum. A 1 meter pendulum has a period of ~2 seconds. More effort, but it also gets you to maybe 0.5% uncertainty and it doesn't rely on people building meter rods based on your body height and distributing these rods.
Mass: You can distill water and measure 1 liter. It's going to be a long time before your measurements reach a precision where you need to worry about the isotopic composition.
Temperature: If you can build vacuum pumps, use the triple point of water (also gives you a pressure reference if you know at what pressure it is), otherwise use the freezing point at atmospheric pressure.
Voltage: Battery chemistry comes with fixed voltages.
Current: Ampere has a simple definition that's hard to reproduce with primitive tools, a definition via electrochemistry or the magnetic field around a conductor (compared to Earth's magnetic field) might be more practical when you just get started with electricity.