The primary trigger causes the gun to emit a small discharge which emits energy to the targeted object. Like most other weapons in the game, the gravity gun has two trigger functions. The gravity gun significantly increases the player's ability to manipulate objects in the game. She explains to Gordon that she once found it useful 'for clearing minefields'. Alyx Vance explains that the gravity gun is designed for handling hazardous materials, but is mostly used for heavy lifting. Around a quarter of the way through the game, the player acquires the gravity gun, properly named as the ' Zero-point Energy Field Manipulator'. Although the player can pick up and throw objects early in the game, this ability is somewhat limited in scope. The physics engine within Source is derived from Havok, which opens up a wealth of possibilities for object interaction – particularly when later in the game, Freeman receives an energy-beam weapon that lets him move huge objects. Valve's Half-Life 2 made significant use of physics, powered by the Source engine.
