Veritasium - How To Measure The Tiniest Forces In The Universe

Short Summary

Main recommendation: The video's central message is that measuring tiny forces and masses depends on redefining the kilogram from a physical object to a fixed physical constant (Planck's constant), and on specialized instruments such as the Kibble balance, electrostatic force balance (EFB), and atomic force microscope cantilevers that can measure from milligrams down to femtonewtons. The video also shows how mass standards are subdivided and why industry and science need such precision.

Key Points

• Mass standards were historically based on a platinum-iridium kilogram cylinder in Paris; over about a century, replicas were found to diverge from the original by up to 75 micrograms, making a changing physical reference unacceptable.
• In 2019, Planck's constant was fixed to an exact value in joule-seconds, which together with the definitions of the meter and second now defines the kilogram.
• Subdivision (comparing smaller masses against a kilogram in steps) works, but each step increases uncertainty; at the milligram level the uncertainty is about one part in 10,000, or roughly 0.01%.
• The Kibble balance balances weight against an electromagnetic force, with electrical quantities tied to Planck's constant for direct traceability.
• The electrostatic force balance (EFB), a smaller cousin of the Kibble balance, uses a capacitor to measure milligram-range masses in a vacuum, in a temperature-controlled underground lab.
• Small-force sensors such as fused silica parallelogram flexures can detect femtonewton changes, and a laser pointer produces about seven piconewtons of force.

Details Worth Noting

• Concrete instructions: Mass subdivision is done by making two 500 g masses equal on a balance, then comparing them together against a kilogram; the same halving approach continues down (two 250 g, then 125 g), with smaller masses such as 500 mg, two 200 mg, and 100 mg used to sum to the target. Test weights should be kept clean by handling with tweezers, avoiding fingerprints, and brushing or wiping with a lint-free cloth and brush, since dust can affect results.
• Concrete instructions: A simple calibration check is to place a reference test mass (for example 50 g, 10 g, or 1 g paper clip) on a scale and see whether the reading matches; if it does, the scale is reasonably well calibrated.
• The EFB lab keeps air at a constant 20°C and measures in a vacuum to avoid air currents and buoyancy.
• Gravity at the lab was measured by the USGS using an absolute gravimeter at 9.81 m/s².
• Industrial uses include measuring diesel exhaust particulates (about 50 micrograms for environmental standards), calibrating laser power, and pharmaceutical microgram dosing.
• The video notes that the 10-microgram test weight is roughly the size of a hair or smaller than an eyelash, and a 24 mm macro lens was needed to film it.

Bottom Line

Precision measurement has moved from relying on a single physical kilogram to defining mass through fundamental constants, enabling measurements far below what was previously possible.