light (and matter, too) has a dual nature. The particles we call “photons” and “electrons,” as well as all the other particles of matter, are Janus-faced. Sometimes they appear as particles, but they can also appear as waves. This is one of the strangest and most counterintuitive findings of quantum theory. Even stranger is the finding that what makes the difference about whether a particle is in the wave or particle state is the role of an observer—if we try to observe the particle in its wave state, it becomes a particle. But if it is unobserved, it remains in the wave state.

light (and matter,
too) has a dual nature. The particles we call “photons” and “electrons,” as
well as all the other particles of matter, are Janus-faced. Sometimes they
appear as particles, but they can also appear as waves. This is one of the
strangest and most counterintuitive findings of quantum theory. Even
stranger is the finding that what makes the difference about whether a
particle is in the wave or particle state is the role of an observer—if we try
to observe the particle in its wave state, it becomes a particle. But if it is
unobserved, it remains in the wave state.