One Planck time is the time it would take a photon traveling at the speed of light in a vacuum to cross a distance equal to one Planck length. Theoretically, this is the smallest time measurement that will ever be possible, roughly 10−43 seconds. Within the framework of the laws of physics as we understand them today, for times less than one Planck time apart, we can neither measure nor detect any change. 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 3.7 × 1026 Planck times.
Analysis of the Hubble Space Telescope's deep field images in 2003 led to a debate about the physical implications of the Planck time as a physical minimum time interval. According to Lieu and Hillman, speculative theories of quantum gravity "foam" where there are space–time fluctuations on the Planck scale predict that images of extremely distant objects should be blurry. However, blurring was not seen in the Hubble images, which was claimed to be problematic for such theories. Other authors have disputed this, in particular Ng et al., who stated that the blurring effect was overestimated by Lieu and Hillman by factors of between 1015 and 1030, and thus the observations are very much less effective in constraining theory: "the cumulative effects of spacetime ﬂuctuations on the phase coherence of light [in certain theories of 'foamy' spacetime] are too small to be observable".