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photon, the other. The point where the three lines join—the event that emits the photon—is the vertex.
    Here is a way to view a Feynman diagram as a short “movie.” Get a square of cardboard a few inches on a side and make a long thin slit about one sixteenth of an inch wide. Now place the square over the Feynman diagram (first fill in the dashed lines) with the slit oriented in the horizontal direction. The short line segments showing through the slit represent particles. Start the slit at the bottom of the diagram. If you now move the slit up, you will see the particles move, emit, and absorb other particles and do all the things that real particles do.

    The vertex diagram can be turned upside down (remember, past is down, and future is up) so that it describes an electron and a photon approaching each other. The photon gets absorbed, leaving only the lone electron.
Antimatter
    Feynman had a purpose in mind when he put little arrows on the electron lines. Each type of electrically charged particle, such as the electron and proton, has a twin, namely, its antiparticle. The antiparticle is identical to its twin, with one exception: it has the opposite electric charge. When matter meets antimatter, look out! The particles and antiparticles will combine and disappear (annihilate), but not without leaving over their energy in the form of photons.
    The antiparticle twin of the electron is called the positron. It appears to be a new addition to the list of particles, but according to Feynman, the positron is not really a new object: he thought of it as an electron
going backward in time!
A positron propagator looks exactly like an electron propagator except that the little arrow points downward toward the past instead of upward toward the future.
    Whether you think of a positron as an electron going backward in time or an electron as a positron going backward in time is up to you. It’s an arbitrary convention. But with this way of thinking, you can flip the vertex in new ways. For example, you can flip it so that it describes a positron emitting a photon.

    You can even turn it on its side so that it shows an electron and a positron annihilating and leaving only a single photon or a photon disappearing and becoming an electron and a positron.

    Feynman combined these basic ingredients, propagators and vertices, to make more complex processes. Here is an interesting one.

    Can you see what it describes? If you use the cardboard-with-slit to view the diagram, here is what you will see: initially, at the lower part of the diagram, there are only an electron and a photon. Without warning, the photon spontaneously becomes an electron-positron pair. Then the positron moves toward the electron, where it meets its twin, and together they annihilate, leaving a photon. In the end there are a single photon and a single electron.
    Feynman had another way to think about such diagrams. He pictured the incoming electron as “turning around in time” and temporarily moving toward the past, then turning around again toward the future. The two ways of thinking—either in terms of positrons and electrons or in terms of electrons moving backward in time—are completely equivalent. Propagators and vertices: that’s all there is to the world. But these basic elements can be combined in an infinite variety of ways to describe all of nature.
    But aren’t we missing something important? Objects in nature exert forces on one another. The idea of force is deeply intuitive. It is one of the few concepts in physics that nature has equipped us to understand without consulting a textbook. A man pushing a boulder is exerting a force. The boulder is resisting by pushing back. The gravitational attraction of the earth keeps us from floating away. Magnets exert forces on pieces of iron. Static electricity exerts forces on bits of paper. Bullies shove wimps. The idea of force is so basic to our lives that evolution made sure that we had a concept of