The double-slit experiment with electrons

The experiment that holds the whole mystery

Richard Feynman said of the double-slit experiment that it holds "the only mystery" of quantum physics. Carried out with electrons, it is the most striking demonstration of duality.

The setup

Electrons are sent, one by one, toward a wall pierced by two slits, behind which is a detector screen.

   source        two slits           detector screen
   of electrons       ||
                      ||             |
      e-  ------>     ||     ------> |  (each electron
                   ___||___          |   makes ONE dot)
                      ||             |
                      ||             |

What we would expect from simple particles

If the electron were a classical ball, it would go through one slit or the other. We would get two bands, opposite each slit:

   Expected for balls:              Screen
                                    |  ####     <- opposite slit 1
                                    |
                                    |  ####     <- opposite slit 2

What we actually observe

Instead of two bands, the screen is covered with alternating fringes — zones where many electrons arrive, separated by zones where none arrive. This is an interference pattern, the unmistakable signature of a wave:

   Observed:                        Screen
                                    | ##
                                    |     ####
                                    | ##########   <- central bright fringe
                                    |     ####
                                    | ##
                                    (interference fringes)

In passing through, the electron behaves like a wave that goes through both slits at once and interferes with itself.

The vertigo: one electron at a time

The most disturbing part: the electrons can be sent one by one, with long pauses. Each one indeed makes a single dot on the screen (particle aspect). But as they accumulate, the dots gradually draw the interference pattern (wave aspect):

   10 electrons :    . .  .   . .      (scattered, random dots)
   1000 electrons:   .:.: :.:: .:.     (a structure appears)
   100000 elec.  :   sharp fringes     (the wave was there from the start)

Each electron "knows" where it is allowed to land, as if it had interfered with itself. The wave pattern was therefore encoded in each individual electron.

The final blow: looking at which slit it goes through

If you install a detector to find out which slit each electron goes through, the interference pattern disappears: we get back the two bands of classical balls! Observing which way it goes forces it to "choose" a slit and destroys its wave behaviour.

   without "spy"  ->  fringes (wave)
   with "spy"     ->  two bands (particle)

In summary

Sent one by one toward two slits, the electrons arrive as isolated dots (particles) but collectively build an interference pattern (wave): each one interferes with itself. And if you try to find out which slit the electron goes through, the interference vanishes. This is the heart of wave-particle duality: the nature of the electron depends on the question you ask it.