Articles by HAS Members

Fun Fact - The Speed of Light

by Jonathan Park

This article appeared in the August 2023 HAS newsletter.

The speed of light in a vacuum is about 300,000 kilometres per second, or 300,000,000 metres per second. At this speed, an object would go around the Earth's equator about 7½ times in one second.

The EXACT speed of light (in a vacuum) is c = 299,792,458 metres per second (c is the symbol for the speed of light in a vacuum). There is nothing after the decimal place - it really is precisely this number. But there's nothing magical about this. The speed is exactly this because a metre is now defined as the distance light travels in 1/299792458th of a second.

Your next question might be, but don't we have to have an exact value for one second for this to work? And the answer is "yes". A second is now defined using a property of the caesium-133 atom. This gets rather complicated, being "the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom", which is a frequency of 9,192,631,770 Hz (cycles per second). So one second is 9,192,631,770 of these cycles! You can read more about this in the links below, but needless to say, this is very precise. The old way relied on how long the Earth takes to rotate, divided by 24, then by 60, then by 60 again - this is not precise because Earth's rotation is gradually slowing down due to tidal friction and other factors.

These definitions and others were updated in 2019. You can read more here:
https://www.measurement.govt.nz/metrology/si-units/
and here:
https://www.bipm.org/en/measurement-units/si-base-units