The most famous of Einstein’s equations, E=mc² says that energy (E) and mass (m) are equivalent. In other words, mass can be converted to energy and vice versa. The conversion factor is the speed of light (c) squared, an enormous number when you consider that the speed of light itself is a whopping 186,000 miles per second.

       This means that a small amount of matter can be transformed into a huge amount of energy. That’s the secret of stars, where high temperatures and densities permit lighter atoms to fuse into heavier ones. Each heavy atom weighs less than the combined weight of the lighter atoms that formed it, and that difference in mass becomes the energy that keeps stars shining. The process also works in reverse: Energy can be transformed into mass. Cosmologists think that’s how the matter in the universe arose—in the first second following the Big Bang, photons of incredible energy collided with one another, creating pairs of particles and antiparticles.