String theory makes predictions about physical systems that contain a large amount of energy, packed into a very small space. The energies needed for string theory predictions are so large that it might never be possible to construct a device able to generate that much energy and test the predictions.

The matter in our universe would never do anything interesting if it weren’t for the addition of energy. There would be no change from hot to cold or from fast to slow. Energy too is conserved, as discovered through the 1800s as the laws of thermodynamics were explored, but the story of energy’s conservation is more elusive than that of matter. You can see matter, but tracking energy proves to be trickier.

Kinetic energy is the energy involved when an object is in motion. Potential energy is the energy contained within an object, waiting to be turned into kinetic energy. It turns out that the total energy — kinetic energy plus potential energy — is conserved any time a physical system undergoes a change.

## The energy of motion: Kinetic energy

Kinetic energy is most obvious in the case of large objects, but it’s true at all size levels (large objects in comparison to particles, so a grain of sand and the planet both would be considered large in this case.) Heat (or thermal energy) is really just a bunch of atoms moving rapidly, representing a form of kinetic energy.

When water is heated, the particles accelerate until they break free of the bonds with other water molecules and become a gas. The motion of particles can cause energy to emit in different forms, such as when a burning piece of coal glows white hot.

Sound is another form of kinetic energy. If two billiard balls collide, the particles in the air will be forced to move, resulting in a noise. All around us, particles in motion are responsible for what takes place in our universe.

## Stored energy: Potential energy

Potential energy, on the other hand, is stored energy. Potential energy takes a lot more forms than kinetic energy and can be a bit trickier to understand.

A spring, for example, has potential energy when it’s stretched out or compressed. When the spring is released, the potential energy transforms into kinetic energy as the spring moves into its least energetic length.

Moving an object in a gravitational field changes the amount of potential energy stored in it. A penny held out from the top of the Empire State Building has a great deal of potential energy due to gravity, which turns into a great deal of kinetic energy when dropped (although not, as evidenced on an episode of MythBusters, enough to kill an unsuspecting pedestrian on impact).

This may sound a bit odd, talking about something having more or less energy just because of where it is, but the environment is part of the physical system described by the physics equations. These equations tell exactly how much potential energy is stored in different physical systems, and they can be used to determine outcomes when the potential energy gets released.