For centuries, physical imaging tools have been opening new frontiers in biology. The discovery of the cell nucleus by Scottish botanist Robert Brown was made possible by early-19th-century light microscopes, and DNA was unveiled by mid-20th-century x-ray diffraction imaging.
During his observations in the 1820s, Brown made another discovery, which has come to bear his name. He was startled to see the jittering, lifelike motion of small particles enclosed in pollen grains. He used control experiments with dust particles to rule out the notion that the movers had to be living “animalcules.” In the early 20th century, Brownian motion became the subject of theoretical investigations by Albert Einstein, Paul Langevin, Marian Smoluchowski, and others.
Now once again, another connection between biology and physics is being forged, this time by a new imaging technique called single-molecule spectroscopy.1 Tracking individual molecules or small tracer particles in living cells yields insight into...
