As has been discussed in this chapter and Chapter 7, “empty” space is filled with pairs of virtual particles and antiparticles that appear together, move apart, and then come back together and annihilate each other. So, one can regard the pair of particles as a single particle moving on a closed loop in space-time. When the pair is moving forward in time (from the event at which it appears to that at which it annihilates), it is called a particle. But when the particle is traveling back in time (from the event at which the pair annihilates to that at which it appears), it is said to be an
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