Science Fiction and Fact
1803
The Double-Slit Experiment
By the early 19th century, most physicists agree with Newton's
well-established theory of light, which says that light takes the
form of particles—what we now call photons. But English
scientist Thomas Young isn't convinced, and in 1803, he designs an
experiment to test the status quo. Young aims a beam of light at a
barrier that has two slits. If light is made of particles, he
reasons, those particles should travel in a straight line through
the slits, projecting two distinct lines of light on the screen
beyond the barrier. Instead, Young sees a series of dark and bright
lines on the screen, a pattern that could only be produced by
waves of light interfering with each other. And yet other
experiments, both before and after Young's, convincingly show the
particle nature of light. Physicists are left with the unsettling
conclusion that light—and, as they later find, electrons
(matter)—has a dual nature: Sometimes it takes the form of
particles and sometimes the form of waves.