“If you can Heal, you may eventually be able to exchange mass and energy if you conserve hadrons,” Derik told Roi. (from Homecoming.)

Exchange mass and energy? And what are hadrons?

Einstein’s famous equation—E=mc2—says that a very small amount of mass can be turned into a very large amount of energy or vice versa. If you could turn a gram of mass (that’s about the mass of a small gourmet jellybean) into energy, you’d get roughly 25 million kilowatt-hours. In practice, you have to conserve hadrons—essentially the total number of neutrons and protons in the cores of atoms, which make up most of their weight.

Thus it is possible, with enough heat and pressure, to force four hydrogen atoms (each with one proton and one electron) together to form one helium atom (with two protons, two neutrons and two electrons.) The four hydrogen atoms turn out to have about 1% more mass than the helium atom, and this extra mass, the binding energy, reappears as gamma rays and particles. This is the energy of the hydrogen bomb, and also the energy that powers the sun and most of the stars. The process is called fusion (coming together), but as of yet we cannot control it. (I don’t think you can call a hydrogen bomb controlled.)

A second way in which mass can be transformed into energy involves very heavy elements, such as uranium. It turns out that all of the protons and neutrons packed together in these elements aren’t really happy. They may split apart on their own, or because they are hit by some other particle, but the net effect is that the mass of the parts they split into is less than that of the original atom, so the split produces energy. This is nuclear fission (splitting.) This is the energy of the original atomic bomb, but it is also the energy that drives plate tectonics and volcanoes, and is occasionally tapped for geothermal energy. This one we can control to some extent, and it is the energy source for nuclear power plants—but those large, unstable atoms are rare in nature.

The middle-weight elements—such as iron—are stable. There is no way to extract binding energy from iron—it is sometimes called the nuclear ash of stars. (Note that many of the elements of all masses have isotopes in which an imbalance between neutrons and protons leads to instability, but these are very rare in nature. They are very important in man-made nuclear waste, however.)

Most of the energy we use today comes from sunlight past or present—fusion energy. Solar energy intercepted by the earth today fuels not only what is called solar energy but also wind power, hydropower, and biofuels. These are all “renewable” sources—we can use them at the rate present-day sunlight generates them. But they are limited by the amount of sunlight available.

Fossil fuels are storage for the sunlight of the past—and we are using them up far faster than they can be replenished. Only a tiny fraction of the sunlight falling on the surface of the earth is actually captured as biomass, and only a tiny fraction of that biomass is buried and eventually becomes coal, oil or natural gas. But ultimately, it is all nuclear energy.

So yes, mass can be turned into energy, and in fact almost all of the energy we use actually comes from such conversion. But there are limits.