What's the tiniest length of time you can think of? Maybe milliseconds come to mind. Try again, but this time think like a physicist. Physics uses units of Planck measurements, which are extremely tiny. The Planck time is the smallest conceivable length of time. Its definition might make your head spin: One unit of Planck time equals the time it takes light to travel a distance of one Planck length in a vacuum. Keep in mind the Planck length is the smallest conceivable length.

German physicist Max Planck, the founder of quantum theory, proposed Planck units in 1899. They were invented, as reported by Universe Today, as a "means of simplifying the particular algebraic expressions appearing in theoretical physics, especially in quantum mechanics." Getting back to Planck time specifically, here's an easier way to imagine just how small it really is: There are more units of Planck time in one second than all the seconds since the Big Bang. Learn more about Planck time in the video below.