The book begins by considering how particles communicate with one another. Conclusion: Most such communications result from the fact that all particles vibrate. (Exception: Sometimes particles communicate by direct contact.) In particular, electrons and protons vibrate-electrons at rest vibrate at a rate of 1.2*10**20 times per second; protons at rest vibrate at a rate of 2.3*10**23 times per second.In each vibration cycle, a particle emits a probe. This probe flies out at the speed of light, leaving a trail of mass-energy behind it, and goes until it runs out of mass-energy. It then reverses direction and flies back to its particle, recovering all the mass-energy it had left behind. This ends the cycle, but the particle immediately starts another cycle by emitting another probe in almost the opposite direction.On its outward journey, a probe may hit another particle. If so, the result is an electromagnetic interaction between the probe and the particle. This interaction usually involves the exchange of a tiny amount of mass-energy between the probe and the particle. (This is how light is carried from one particle to another.) Immediately after the hit, the probe returns to its particle. No more hits can occur on this cycle, which then ends.On its outward journey, a probe may hit another probe. If so, there is a gravitational interaction between the two probes. This causes a tiny attraction between the point of the hit and each of the particles from which the two probes come. These translate into a tiny gravitational attraction or repulsion between the two particles. It also causes a slight change in each of the probe directions. After the hit, both probes continue in their outward journeys.The accumulation of such gravitational attractions and repulsions results in gravity as we know it.This book shows why the planets and asteroids that orbit the Sun behave as they do. It is not because of general relativity, which turns out to be a pretty good approximation but is not exact. Instead, it is because the Sun is limited by its own gravity; in fact, its mass prevents the release of some of its gravitational potential.The book also shows that a misunderstanding of how light is transmitted led Edwin Hubble and others to use an incorrect formula to measure the relative velocity of stars with respect to Earth. The result was the concept of an expanding universe. A correct formula is given in the book, and implies a non-expanding universe.