Professor Anne Sickles is currently teaching a laboratory class at the University of Illinois in which her students will measure what happens when two photons meet.
What they will find is that the overlapping waves of light get brighter when two peaks align and dimmer when a peak meets a trough. She tells her students that this is process called interference, and that—unlike charged particles, which can merge, bond and interact—light waves can only add or subtract.
“We teach undergraduates the classical theory,” Sickles says. “But there are situations where effects forbidden in the classical theory are allowed in the quantum theory.”