**4. Interactions between light and matter, including the Compton effect**

When we think of light as particles, we call these particles photons. The energies of photons constitute a continuous spectrum, from lowest energies to highest (and longest wavelengths to shortest), called the "electromagnetic spectrum": radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Photons interact with matter in a number of ways. Photons can be absorbed by matter. Photons can be reflected by matter. At higher energies, light can push matter. Arthur Compton won the Nobel Prize in 1927 for demonstrating that X-rays push the electrons of matter, depending upon the angle that the photons strike the electrons of the material [7]. At even higher energies, photons, in a process known as the Bethe-Heitler Process, can cause pairs of electrons and positrons to be ejected from an atom. It is generally understood that the electrons and positrons produced in the Bethe-Heitler Process arise from the "quantum vacuum" [8]. **Figure 6** shows

**Figure 6.** *Photon interaction with lead, as a function of photon energy [9].*

the interaction of photons at various energies interacting with lead; the photoelectric effect, Compton scattering, and Bethe-Heitler pair production are shown (Rayleigh scattering is not separately indicated).

The electron/positron pairs generated in the Bethe-Heitler Process, and the Compton effect, where gamma rays push matter, are the two physical processes that are at the heart of matter/anti-matter propulsion. As will be discussed more fully below, in a matter/anti-matter spacecraft, a high-energy laser will generate electron/ positron pairs in the Bethe-Heitler Process and then those pairs will separately be directed to a thruster where they will collide, annihilate, and generate gamma rays. When the gamma rays hit the inner shell of the thruster, the gamma rays will propel the spacecraft forward utilizing the Compton Effect and Newton's Third Law.
