Engines of Our Ingenuity

No. 2631
THE DOUBLE SLIT EXPERIMENT

by Andrew Boyd

Today, a mysterious experiment. The University of Houstonís College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Physicist Richard Feynman was fond of saying that the mysteries of quantum mechanics could be understood from one experiment. Itís called the double slit experiment. The results leave us in awe. But the experiment is simple.

Imagine I take a baseball and dust it with a little chalk, so when I throw it at a wall, it leaves a white mark. Suppose I place a giant metal plate between me and the wall. I cut a slit in the plate wide enough for well thrown balls to get through, and I start throwing. My aimís not very good, so many of my throws bounce off the metal plate. But those that get through the slit will form a pattern of chalk marks on the wall, and theyíll look a lot like the shape of the slit. Itís like putting a giant metal stencil in front of the wall, and spray painting with chalk-covered baseballs. If I put two slits in the plate, I wind up with chalk marks of two slits.

Nothing very exciting so far. But letís try this same experiment on a smaller scale with electrons. Electrons are particles, with mass just like baseballs. When I throw them at a single slit, they act just like the baseballs and make a pattern that looks like the slit. But if I throw them at two slits situated next to one another, something very strange happens. They no longer show up just behind the slits. They show up everywhere, making complex wave patterns. Well behaved particles just canít do that.

Electrons passing through two slits and showing up in too many places          Pattern left by electrons passing through only two slits.                   
Image: Timm Weitkamp

To investigate further, we might try watching how the electrons got to the wall, not just where they hit. Thatís harder than watching baseballs, so weíll set up a camera at the metal plate. We wonít see an electronís whole trip to the wall, but at least we can watch which slit each electron goes through.

This experiment works just fine. Electrons are thrown, and we see them hit the metal plate or pass through exactly one of the slits. But hereís the really weird part. When I watch them, they no longer show up all over the place on the wall. They show up only behind the slits, just like weíd expect them to. It appears that the very act of watching the electrons changes the outcome of the experiment!

If this seems too strange to comprehend, it is. The experiment is easy enough to understand and repeat, and thereís good mathematics describing what happens. But an actual interpretation plunges physicists into the world of metaphysics. Niels Bohr believed the observer really did create reality. Stephen Hawking turns to an infinite number of parallel universes for an explanation. But the wonder of the experiment is that it takes us to the precipice of human understanding. The math may seem daunting, but the experiment and the questions it raises are open to anyone willing to ponder them.

I'm Andy Boyd at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Notes and references:

The double slit experiment, in various forms, is one of the most famous experiments in physics. As such, a wealth of information can be found on the Web. See, for example, http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html and http://physics.about.com/od/lightoptics/a/doubleslit.htm. A short, animated video can be found on Youtube. Accessed July 30, 2010.

The pictures are from Wikimedia Commons. The image on the right, http://commons.wikimedia.org/wiki/File:Double_slit_x-ray_simulation_0600mm_log-grayscale.jpg, was created by Timm Weitkamp.


The Engines of Our Ingenuity is Copyright © 1988-2010 by John H. Lienhard.