37.0 - Introduction
Richard Feynman (1918 í 1988) was an American physicist best known to the
general public for his leading role in the commission that investigated the
destruction of the space shuttle Challenger in 1986. His bestselling memoirs
delighted millions with his irreverence and tales of bongo drum-playing.
In the physics community, Feynman was known for his insight into quantum
mechanics and the brilliance of his lectures. The “Feynman Lectures in Physics,”
still in print today, show his timeless ability to clearly and memorably explain
fundamental aspects of physics.
We start this chapter by giving you a chance to conduct a thought experiment that
Feynman described as “impossible, absolutely impossible, to explain in any
classical way, and which has in it the heart of quantum mechanics.” In this thought
experiment, Feynman emphasized the shortcomings of considering matter solely as
composed of particles, and the need to adopt a vision where matter has both a
particle and a wave nature. This is known as the wave-particle duality.
Feynman first asked the reader to consider classical particles, that is, those for
which the physics of Newton completely predicts the motion. Feynman used bullets in his experiment; we will be slightly more pacific and ask
you to imagine throwing baseballs at a picket fence. The picket fence is missing two of its slats. The missing slats provide gaps that the
baseballs can pass through. The balls will stop when they strike a wall behind the fence.
If you conducted this experiment yourself, you would find that most of the baseballs hit the wall directly behind the missing slats. A few
baseballs would hit slightly to the sides of those areas, corresponding to the balls that went through the gaps at an angle. You would see two
piles of baseballs that accumulated behind the gaps in the fence.
Feynman used this first thought experiment to remind his audience how they expect the world to “work” based on everyday experience. This
experiment provided the contrast with another experiment, one you can conduct for yourself using the simulation to the right.
In the simulation, you are conducting a similar experiment, but with particles of far smaller scale. Instead of baseballs being fired, electrons are
fired one at a time toward a barrier with two slits. The slits are very narrow and very close together.
The electrons pass through the slits and reach a screen, which is a photographic material that records where they land. The electrons cannot
be observed as they move from the source to the barrier, or from the barrier to the screen. Only the final position is marked, by using black
dots.
Press FIRE to launch a single electron. Then, hold the FIRE button down to fire a stream of electrons so you can see the pattern of where they
land.
Look at the pattern of where the electrons accumulate. Instead of accumulating in two piles, like the baseballs did in Feynman’s first thought
experiment, you see regions where many electrons accumulate, alternating with regions where very few electrons land. The pattern should
remind you of the dark and light fringes that are created on a screen when light shines through a pair of slits.
In the theory of optics, physicists explain the pattern of light and dark fringes by modeling light as a wave, with wave-like properties such as
frequency and wavelength. This simulation shows that something which you have solely considered to be a particle, an electron, also displays
wave-like properties in a similar experiment. This is the essential point of Feynman’s second thought experiment: Particles such as electrons
have wave-like properties. A single particle can travel from a source to a screen and demonstrate interference effects due to the presence of
the two slits. The wave that is associated with a moving particle is called a matter wave.
Now you can use the simulation again to see another fundamental aspect of quantum mechanics. Reset the simulation. Press the FIRE button
a few times and note the locations of the first three or four electrons. Then press RESET again, fire a few more electrons, and note their
locations. In your two experiments, did the first three or four electrons show up at the same location each time?
The answer to the question is “no”. In both this simulation, and in the real experiments that this simulation is recreating, the location of a single
electron cannot be predicted. Although the overall pattern of light and dark fringes as you fire hundreds of electrons can be predicted, where a
particular electron will strike the screen cannot be determined in advance.
This is the second point of the animation: The locations of the electrons can be stated in terms of likelihoods, like the probability of drawing an
ace from a deck of cards. The pattern of light and dark fringes provides a “map” of where an electron is likely to strike the photographic paper
behind the slits. However, you cannot predict in advance where any one electron will land, any more than you can state with certainty that one
particular card will be an ace.
You are witnessing a major point of quantum mechanics in this simulation. Scientists have performed the experiment with electrons, and the
results have been exactly as depicted. Electron after electron can be fired through slits, and the interference of their matter waves will create a
pattern like the one you see here.
The simulation shows the wave-like properties of particles. Scientists like Einstein had also postulated the particle-like nature of light, which
had been considered a wave. This chapter will start with that topic, the particle nature of electromagnetic radiation, before returning to the topic
that the experiment in this section illustrates.(^688) Copyright 2000-2007 Kinetic Books Co. Chapter 37