Does energy have mass?
Note: That's just a theory, but today there are many new theories that contradict this in that they postulate that new Universes are being created and destroyed constantly, on a universe sized time scale. These universes can also collide so "new" matter/energy CAN be introduced in to our universe. Think black holes and white holes. Black holes suck matter/energy from our universe and white holes spit it out into new universes. So in essense the "big bang" was really just a white hole and every black hole in our universe is creating a new universe somewhere else. Or so that one theory goes.
False Statement: Under ordinary circumstances, there is no free interconversion of mass and energy. These events can occur only in the environment of tremendous heat and pressure which exists in a star, or, momentarily, in the detonation of a nuclear device.
That statement is completely false but I kept it to demonstrate how poorly the common man understands this concept. You yourself are converting energy to mass and mass to energy at this very moment. Any warm blooded being with some intellect should be able to figure that out.
A simple concept, is a good old fashioned fire in the fireplace. Wood burning turns to heat and light. Wood has more weight before being burned and there's not as much weight left in the ash, true there is some weight lost to out-gassing but it is negligible. So if, Weight equals mass times (the acceleration of) gravity and the weight of ash is less than wood with gravity being a constant, wouldn't you conclude that energy has mass.