What's the Fastest Thing on Earth?

A cheetah? A race car? A rocket? All great guesses. All completely wrong. The fastest thing on Earth is something you can't hold in your hand... and understanding it means understanding lightyears, the speed of light, and why space distances break normal numbers.

Dr Matt Agnew runs the ultimate speed race from Usain Bolt all the way to the speed of light, and explains what a lightyear actually is along the way. Spoiler... most adults get it wrong.

What you'll learn:

Usain Bolt tops out at around 44 kilometres per hour... the slowest in this race by a long wayA cheetah sprints at 100 to 120 kilometres per hour... basically a furry rocketAn F1 car reaches around 350 kilometres per hour... blink and it's goneNASA's Parker Solar Probe is the fastest human-made object at 690,000 kilometres per hourLight travels so fast it feels instant... and in one second it can circle the entire Earth about seven timesA lightyear isn't time, it's distance... how far light travels in one whole year, about 9.5 trillion kilometresThe closest star to Earth after our Sun is Proxima Centauri, just 4.2 lightyears away

Key Science Ideas:

Speed of light: About 300,000 kilometres per second... the ultimate speed limit of the universeLightyear: A unit of distance, not time... how far light travels in one whole yearProxima Centauri: The closest star to Earth after our Sun, 4.2 lightyears awayParker Solar Probe: NASA's spacecraft and the fastest human-made object ever builtAstronomical scale: Why scientists need special units like lightyears to measure enormous distances in space

Fun Experiment: The Lightyear Myth Buster

Ask three people in your house or school what a lightyear is. Most will say it's a unit of time. They're wrong... it's distance. Explain that a lightyear is how far light travels in one year, about 9.5 trillion kilometres. You've just done your first myth-busting science experiment, and you didn't need a single piece of equipment.

Why Though? The show for little scientists who love asking big questions. Follow or subscribe so you never miss an episode.

Why Though? The show for little scientists who love asking big questions. Follow or subscribe so you never miss an episode.

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