The Planck time is the unique combination of the gravitational constantG, the special-relativistic constantc, and the quantum constantħ, to produce a constant with units of time. Because the Planck time comes from dimensional analysis, which ignores constant factors, there is no reason to believe that exactly one unit of Planck time has any special physical significance. Rather, the Planck time represents a rough time scale at which quantum gravitational effects are likely to become important.[clarification needed] The nature of those effects, and the exact time scale at which they would occur, would need to be derived from an actual theory of quantum gravity. All scientific experiments and human experiences happen over billions of billions of Planck times, making any events happening at the Planck scale hard to detect. As of May 2010[update], the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10−17 seconds), about 2.2 × 1026 Planck times.