Planck Time
Planck time is sometimes referred to as the "natural unit of time" for the universe. One unit of Planck time - a quantum of time - is, according to modern physics, the smallest amount of time that makes "sense". The concept of Planck time is interesting because of the way that it relies on both quantum physics and relativity as well as one of the simplest equations of classical mechanics.Planck Length
Quantum mechanics is all about the very small. Below a certain size, the "common sense" laws of the universe cease to hold and quantum effects dominate. The size at which this occurs is known as the "Planck length".The Planck length is the shortest unit of measurement that has any meaning. Below this scale any measurement becomes meaningless.
Now, Einstein's relativity theory tells us that nothing can travel faster than the speed of light. Since speed, distance and time are all related (d = st) we can use this to calculate the temporal equivalent of the Planck length: Planck time.
Planck time is the time that it would take a photon travelling at light speed to cross a distance of Planck's length. Since this would represent crossing the shortest possible distance at the fastest possible speed, this gives us the smallest meaningful unit of time.
Plugging the numbers into the equations, Planck time works out as around 5.39 x 10-44 seconds.
The concept of Planck time has an interesting ramification for cosmologists: it is meaningless to try to consider what "happened" to the universe at an age less than Planck time. To all intents and purposes, the universe came into being with an age of 5.39 x 10-44 seconds.