Observatory · the double slit
Wave and particle
Shine light through two narrow slits and the screen behind shows stripes: where the waves from both slits meet in step they add up, where they meet out of step they cancel. That settled it in 1803: light is a wave. Then it turned out light also arrives in lumps, photons, each landing in one single spot. Send them one at a time, so there is never more than one on its way, and the stripes still build up, dot by dot. Every photon seems to go through both slits at once. Look which slit it takes, and the stripes are gone.
One photon at a time
Fig. I. The two-slit apparatus, the screen, and the tally of where each photon landed.
- Photons
- 0
- Wavelength
- What you see
Each flash is one photon leaving the lamp, and each dot is where one landed: one spot, never spread out. At a few a second they look random. Turn the rate up and stripes appear, the same stripes a wave makes, drawn on the right as a count. Show the wave draws that wave: rings spreading from both slits, adding up into bright lanes and cancelling into dark ones, and the dots only ever land where the lanes hit the screen. Redder light has longer waves, so its stripes lie further apart; bluer light, closer together.
Close one slit and the stripes vanish: just one broad patch. Strange enough, opening a second slit makes some places on the screen darker, places a photon could reach through either slit alone. And watch puts a detector at the slits to see which one each photon takes. It always finds it in one slit, never both, and from that moment the pattern is two patches added up, no stripes. Knowing the path destroys the interference. Richard Feynman called this experiment the heart of quantum mechanics.
Who worked it out
Where it already matters
Thomas Young, 1803. Young showed the Royal Society the stripes he got by splitting a thin beam of sunlight with the edge of a card, and measured from them how long light waves are: about half a thousandth of a millimetre. In 1909 Geoffrey Taylor made the light so faint, the equivalent of a candle more than a mile away, that only one photon at a time could be in the apparatus, and after months of exposure the stripes were still there.
Every camera and solar panel. In 1905 Einstein explained why light knocks electrons out of metal in lumps of h f, no matter how bright, the work his Nobel prize was for. A camera sensor counts photons pixel by pixel in exactly that way, and a solar panel turns each photon into, at most, one electron's worth of energy.
Electrons and molecules too. Electrons make the same stripes; the Hitachi team of Akira Tonomura filmed them building up one electron at a time in 1989. Because their waves are thousands of times shorter than light's, electron microscopes can see single atoms. In 1999 a group in Vienna sent buckyballs, footballs of 60 carbon atoms, through a grating and got stripes, and the record now stands at molecules of about 2,000 atoms.