When I thought of the speed of light, I imagined it as a measured quantity in m/s units, as anyone would. Rookie mistake. Speed of light, in the modern world, is ‘defined’ rather than ‘measured’. Physicists started with measurements, and the measurements kept getting better, but everything changed in 1983.
Since 1983, the speed of light is ‘defined’ as c = 299,792,458 m/s. Fixed. That’s it. No more changes allowed. It sounds absurd at first, but then it aligns with the fact that speed of light is constant.
A second is ‘defined’ as 9,192,631,770 vibrations of the caesium-133 atom. Fixed. That’s it. No more changes allowed.
A metre is then ‘derived’ as the distance that light travels in 1/299,792,458 seconds.
What does all of this imply? If today I measure the speed of light to be 299,792,457 m/s (that is 1m/s slower), I will be told that my stopwatch is slightly too fast or that my metre stick is slightly too long! The fault in my scales.
Now that we’re on it, let’s also get over with the kilogram. That’s what a fat physicist would say. Till 2019, a kilogram was ‘defined’ by the mass of a platinum-iridium cylinder kept in Paris. What’s with Paris anyway? But again, that’s not a fundamental definition. 2019 onwards a kilogram is ‘derived’ from the Planck’s constant, the metre, and the second.
Things can easily get complex in the exploration of space physics. So let’s focus only on the light-stuff first. You see what I did there😏?
💡Fun fact – The study of measurement is called ‘metrology’.
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