Geons have gravity, but no mass. Could they really exist?

John Archibald Wheeler is a physicist who coined a lot of terms. You may have heard of a few of them. Wormhole. Black hole. Quantum foam. And geon. Wait, geon?

J A Wheeler was one of those people who seemed to be in exactly the right place, in exactly the right time, with exactly the right brain. After studying physics he collaborated with Albert Einstein, in Einstein's later years. This was not an uncontroversial thing. Einstein spent much of his later life working on unified field theory - a "theory of everything" that many now dismiss. (Even those who don't dismiss it don't believe Einstein should have worked on it, given the data he had at the time.) Wheeler continued Einstein's work, and seems to have made good use of it. He came up with a ton of ideas, some of which populate the physical universe and some of which populate science fiction stories. He coined the terms for black hole and wormhole.

He also came up with the term "geon." A geon is tough to find in reality, and won't help a star ship crew flip themselves across the universe, so it hasn't gotten the same amount of press as Wheeler's other ideas. Basically, it's an electromagnetic wave, and a very energetic one. As Einstein showed, mass and energy are equivalent, which means that energy should have its own gravity, the same way mass does. In Wheeler's mind, a certain amount of energy, all in one place, might generate enough gravity to keep itself together. So a geon is a wave of energy, kept in one "ball" and in one section space.

One of the big debates about geons is whether or not they are "stable." Gravity, Einstein showed, radiates outward at no faster than the speed of light. If a geon is radiating gravitational waves, it should be losing energy. Once it loses enough energy, it should lose the gravity needed to keep it together and dissolve. So a geon could be a very ephemeral thing.

Top Image: Tony Hisgett

Via Harvard.