Scientists just measured the shortest unit of time ever. It is a zeptosecond - a trillionth of a billionth of a second. It is a decimal point followed by 20 zeros and then a 1. The scientists used special tools to measure how long it takes a photon to cross a hydrogen molecule. It took 247 zeptoseconds. This is too fast for us to see.

In 1999, a scientist measured how fast molecules change their shape. This was one femtosecond – a millionth of a billionth of one second. One millisecond is a thousandth of one second. The shortest unit of time it may be possible to measure is one Planck. This is a decimal point followed by 44 zeros and a 1.