**3. Fundamental background: the nature of matter, anti-matter, and light**

### **3.1 The nature of matter & anti-matter**

Matter is the "stuff" that you know. It's the material that you can touch and feel, that you can breathe. It's the material that makes you.

In a basic way, matter is composed of protons, neutrons, and electrons. Protons have a positive electrical charge, neutrons have no electrical charge, and electrons have a negative electrical charge. Protons and neutrons are heavy, and electrons are very light, but electrons are not weightless. **Figure 2** shows a "planetary model" of a helium atom, with electrons "orbiting" the nucleus of the atom.

Anti-matter is the mirror opposite of matter. In a basic way, it is composed of anti-protons, anti-neutrons, and positrons. ("Positron" is the "P" in "PET" scan, for example.) Anti-protons have a negative electrical charge, anti-neutrons have no electrical charge, and positrons have a positive electrical charge. Anti-protons and anti-neutrons are heavy, and positrons are very light. But, as with electrons, positrons are not weightless. **Figure 3** shows an anti-matter "helium" atom, a mirrored concept of a helium atom.

When our universe was first created, there were probably equal amounts of matter and anti-matter. For some reason, our universe ended up being composed almost entirely of matter. Nonetheless, when an electron and a positron collide, they are annihilated—reduced to nothing. No mass, no electrical charge. But they do expel two

### **Figure 2.** *Basic model of a helium atom (courtesy Mustang Publishing).*

### **Figure 3.**

*Basic model of an anti-helium atom (courtesy Mustang Publishing). (Note: the symbol "p+" is used to show the positron. It is used here for a general audience, for ease of understanding; it should be noted that the international scientific convention for positrons is "e+").*

**Figure 4.** *Electron/positron annihilation (courtesy Mustang Publishing).*

rays of light, called gamma rays ("γ"), in opposite directions, as well as a neutrino (or neutrinos—the exact number produced in an annihilation is unknown), a light, extremely fast particle with no electrical charge. The gamma rays produced in an electron-positron annihilation possess the energy of 511,000 electron volts, usually represented as "511-keV" [5]. **Figure 4** conceptualizes the collision of an electron and positron, resulting in their annihilation and the creation of two gamma rays; the resulting neutrino or neutrinos is/are not shown.

## **3.2 The nature of light**

In addition to matter, our universe contains energy, particularly light and gravity. Gravity is what holds you to the ground. Isaac Newton, famously, is said to have thought of gravity as the force that causes an apple to fall from a tree and hit the ground.

Light is the energy that lets you see, or makes a radio work, or makes an x-ray machine work. In a basic way, light can be thought of as a bit like the waves on a lake. Our understanding of light-as-a-wave began in 1803 with Thomas Young, who conducted the first experiment that we now call the "double slit" experiment [6]. It showed that light, traveling through two separate, but close, slits, creates a pattern that is just like the pattern made by waves of water after a splash hits near two piers.

But light can also be thought of as particles—a thing that goes from one place to another. The light from distant stars, for example, looks to us like a single point, without a wave aspect at all. Albert Einstein received the Nobel Prize in Physics in

**Figure 5.** *Light as a wave, light as particles (courtesy Mustang Publishing).*

1921 for his discovery of the "photoelectric effect," which demonstrates that light, in some ways, acts as if it is made of particles.

So light has a dual nature. In some ways, it's like a wave, and in some ways it's like a particle—like a golf ball hit by a golf club. Thinking of light as a particle helps one analogize matter/anti-matter rockets with chemical rockets. Instead of the gaseous particles that propel chemical rockets, matter/anti-matter rockets use light "particles" to propel the rocket forward. **Figure 5** visualizes our two ways of thinking about light: light as waves and lights as particles.
