Detecting a virus or any nanosize particle usually means fixing it to a substrate or attaching a fluorescent probe to it. Neither method is practical for detecting particles in real time. Now University of Rochester physicists have assembled a simple system for doing just that. They split a laser beam in two, sending one half to a sample. When the light hit a small particle, it scattered back and recombined with the reserved half of the laser beam, producing a detectable interference pattern only when a moving particle was present. The method works where others do not, the researchers say, because it relies on the light's amplitude rather than intensity. The amplitude is the square root of intensity, so it decays much less than intensity as particles get smaller. The investigators have so far detected single particles as small as seven nanometers across. Peruse their findings in the January 13 Physical Review Letters.