Young's double-slit experiment

From Wikipedia, the free encyclopedia
(Redirected from Double-slit experiment)
Jump to: navigation, search
The slits; distance between top posts approximately one inch.

The double-slit experiment in quantum mechanics is an experiment invented by physicist Thomas Young. It shows how light has both a wave nature or characteristic and a particle nature or characteristic, and that you cannot get rid of either one. So light is said to have a dual wave-particle nature. The same is true of electrons and other quantum particles.

The experiment[change | edit source]

Same device, one slit open vs. two slits open (Note the 16 fringes.)


This experiment is very simple. It requires only a double slit device like the one in the picture, something to hold the double slit device still, and a good laser such as the kind that is used by workmen to "draw" straight lines when they are building. The laser is supported so it can only be moved on purpose. It is aimed at the center point between the two slits from a point about one half meter away. Something like a movie screen or a smooth white wall is put up on the other side of the double slit device several meters away. When everything is fixed, a pattern of light and dark bands will show up.

J is the distance between fringes. J = Dλ/B "D" = dist. S2 to F, λ = wavelength, B = dist. a to b[1]

Lasers are devices that will produce one or more photons when given a certain amount of electricity. The photon or photons come out of a very small hole within a well known period of time. The speed of light is known, so the time the photons appear on the screen can be predicted. When photons are produced one at a time, what shows up on the screen are individual spots of light. If photons were waves then we would expect them to spread out as they travel and wash across a wide area of the screen, but that never happens. If photons were particles then we would expect them to appear at two points on the screen connected to the laser through the two slits in the middle. But that is not what happens either. When Thomas Young first did this experiment, he could only understand it by imagining light to be like waves of water. He thought that light waves moved out from the light source like waves spreading out from a pebble dropped into a pond, and that when the wave fronts hit the double slits, then the original wave gets through at the two slits and there are two different waves from then on. It was easy to figure out how two waves would interact to produce the bright bands and dark bands (often called "fringes") on the screen.


But there were big problems. Light did not show up at the screen as waves that washed across it. Light came to be understood as swarms of photons that individually hit the detection screen. And, very surprisingly, a single photon could interfere with itself as though it were a single wave that fit the old wave description. It split into two waves at the double-slit device, and they then combined at the screen.

Importance to physics[change | edit source]

The double-slit experiment became a classic thought experiment for its clear explanation of the central puzzles of quantum mechanics.

Importance to philosophy[change | edit source]

The double-slit experiment has been of great interest to philosophers, because the quantum mechanical behavior it shows has forced them to rethink their ideas about classical concepts such as:

  • "particles",
  • "waves",
  • "location",
  • "movement from one place to another" and
  • "observation".

Experience in the micro world of sub-atomic particles forces us to re-conceptualize some of our most ordinary ideas.

Young+Fringes.gif

Moving picture showing how one series of waves hits a double slit and produces two series of waves that interfere with each other.

Copenhagen interpretation[change | edit source]

The Copenhagen interpretation is what one group of the pioneers of quantum mechanics thought -- that it is not useful to say anything that goes beyond the math, the equipment, and the real results that let us to get some idea of what goes on at the atomic scale.
Note that we really only see anything when we are shooting off a photon, an electron, etc., and when one of them shows up on the screen at the other end of the experiment.
If by "object A exists" we mean "we see object A at point x, y, z, t," then this object "exists" only at the point of emission (shooting it off) and at the point of detection (seeing it show up on the screen). In between times it is completely out of the "ordinary world" that we experience at human size. We can know what happens at the beginning when we touch the switch on a laser. We can know what happens at the end, when we see a flash of light on the screen. We cannot know what happens in between.

Other websites[change | edit source]

References[change | edit source]

  1. Philipp Frank, Philosophy of Science, p. 200f.