The most abundant form of free energy is the photon. The energy of a photon can range from virtually nothing up to about a Joule (see Planck). It provides us with heat - infra-red, light, even fast food - microwave. The photon's half-life is somewhere in the region of 6.5 billion years, which makes it pretty stable. At the lowest level, every atom absorbs and emits photons, and all of these photons travel at exactly the same velocity, without exception. This velocity is the same as electromagnetic force, would figure as electromagnetic force is down to the photon. It also has the same velocity as Gravity. When light strikes a surface, the photon imparts a force of approx 0.002 newtons. Here's the conundrum. The mythical particle, the Graviton, must be as prolific as the Photon, yet no one has ever managed to detect one, or any similar particle. The Graviton has the same velocity as the photon and presumably the same mass, or lack of. So, if a photon can excert a force on an object in the direction it is travelling, what is behind the photon ? If there exists a force behind the photon, it would be in the opposite direction to the photon's direction of motion. It would also be as prolific as the photon - bit obvious that. Could the illusive Force of Gravity be hiding behind the Photon, hence why it has never been detected, yet why it is so prolific.