Directed energy weapons—such as lasers—use energy fired at the speed of light. These weapons can produce force that ranges from deterrent, to damaging, to destructive. Many countries, including the U.S., are researching their use. Because they use energy instead of bullets or missiles, directed energy weapons could be less expensive per shot and have virtually unlimited firing power. However, the long-term health effects of these weapons are unclear. They also generally have a shorter range than conventional weapons, and weather conditions—such as fog and storms—can make certain directed energy weapons less effective.