Standards and Units
Simple Explanation
A standard is the fixed, universally agreed reference that gives a unit its exact meaning. Modern SI standards are defined using fundamental constants of nature rather than physical objects — the second is defined by a specific number of vibrations of a caesium atom, the metre is defined using the speed of light, and the kilogram is now defined using the Planck constant.
Why Do We Need It?
Defining units from unchanging constants of nature (instead of a single physical object kept in a vault) means every laboratory in the world can reproduce the exact same standard independently, without needing to compare against one original object.
Worked Example
Use the modern definition of the metre
The metre is defined as the distance light travels in a vacuum in exactly 1/299,792,458 of a second. Use this to find how far light travels in exactly 2 metres worth of time.
Why Does This Work?
Because the speed of light in a vacuum is a fixed constant of nature, defining the metre in terms of a time interval and this constant fixes the metre exactly, with no ambiguity, anywhere in the universe.
Real-Life Example
Retiring "Le Grand K"
Until 2019, the kilogram was defined by a single platinum-iridium cylinder kept in a vault in France, nicknamed "Le Grand K."
Because that one physical object could very slightly change mass over time (through cleaning or contamination), the kilogram was redefined in 2019 using the Planck constant instead — a fixed number that never changes.
Practice
The SI second is defined using which of the following?
MediumCommon mistake
Assuming units are defined by a single physical object kept somewhere — modern SI standards are deliberately defined by unchanging constants of nature instead, precisely so no single physical object needs to be protected or compared against.
Quick Review
- A standard is the fixed reference that gives a unit its exact meaning.
- Modern SI units are defined using fundamental constants of nature (speed of light, caesium vibrations, the Planck constant).
- This makes the standards reproducible anywhere, without relying on one physical object.