Gravity gun

A gravity gun is a concept in video games, particularly first-person shooters using an advanced physics engine, whereby players can directly manipulate objects in the world, often allowing them to be used as projectiles against hostile characters. A gravity gun is usually manifested as an in-game weapon, available from the player's inventory or weapon selection. The concept was first popularized by the gravity gun found in Valve Software's Half-Life 2; however, a similar concept was used by developers id Software during the production of the earlier game Doom 3, eventually leading to the introduction of a physics-based weapon in the expansion pack Resurrection of Evil. Later games have been influenced by the success of these physics-based weapons, adopting their own styles of comparable abilities or weapons.

Half-Life 2 gravity gun


Valve Software's Half-Life 2 made significant use of physics in the game, powered by Havok physics engine. Although the player can pick up and throw objects early in the game, this ability is somewhat limited in scope. Around a quarter of the way through the game, the player acquires the gravity gun, properly named as the "zero-point energy field manipulator". Alyx Vance explains that the gravity gun is designed for handling hazardous materials, but is mostly used for heavy lifting. She explains to Gordon that she once found it useful "for clearing minefields". The gravity gun significantly increases the player's ability to manipulate objects in the game. Like most other weapons in the game, the gravity gun has two trigger functions. The primary trigger causes the gun to emit a small discharge which emits energy to the targeted object. The distance which the object is forced is dependent on its weight and distance from the gun. The secondary trigger attracts the targeted object to the gun and holds it in midair a few inches away, negating its weight and allowing the player to carry it with them. Using the secondary trigger again will drop the item, while the primary trigger will launch it with considerable force.

By combining these functions, players can use the gravity gun to scale barriers and obstacles, create cover against enemy characters, or launch the objects at enemy characters, causing them considerable damage. Certain types of objects, such as saw blades, fuel barrels and hydrogen tanks are intentionally designed by Valve to be used as "gravity gun ammunition". The gravity gun, however, cannot manipulate heavier objects and enemy characters until the late stages of the game, when the device becomes temporarily infused with dark energy meant to destroy it.

The gravity gun was very well received by critics, who considered it one of the defining features of Half-Life 2's entertainment value. Planet Half-Life calls the gravity gun "the next level in interactive gaming." Electronic Gaming Monthly describes Half-Life 2's gravity gun as the "thinking man's death tool," which lets players "toy with gravity to kill foes with everyday objects." Call of Duty series military adviser Hank Keirsey stated that "the weapon is not very practical". He does, however, discuss its historical precedents, further stating that "The ancients learned very early how to use gravity to their advantage — but this usually involved rolling rocks down hills or pouring boiling oil down the castle walls. Those that failed to respect gravity suffered."

Doom 3 grabber
Although Half-Life 2 was the first game released to feature a gravity gun, id Software had previously conceived a similar idea during the development of the earlier title Doom 3. id Software designer Matt Hooper noted that "we actually used it as a tool throughout development where we'd grab physics objects and place them around the world". The tool was used to create "damaged" rooms in Doom 3; instead of constructing a ruined room, the designers would code a pristine room and use the device to "damage" it realistically. Although used to assist the development of Doom 3, the gravity gun was not implemented in the final game. Hooper explained that "we talked about that quite a few times, but we had such a big arsenal of weapons, and so many other cool things going on, that it was just one of those things that never made it in". However, Nerve Software revived the code for the weapon five months after the release of Half-Life 2 in Doom 3's expansion pack, Resurrection of Evil.

The device is noted in the Doom 3 storyline as an "ionized plasma levitator", created by the Union Aerospace Corporation for moving hazardous materials and a forerunner to tractor beams. Usually referred to as the "grabber", the player obtains the device early on in the course of Resurrection of Evil. The grabber operates differently from Half-Life 2's gravity gun, using only a single trigger function. Once the grabber is aimed at an appropriate object, it locks on, allowing the player to lift the object with the trigger. When the player releases the trigger, the object will be propelled forward with force, turning it into an impromptu weapon. One key ability of the grabber is its capacity to lock on to the fireball projectiles cast by some hostile non-player characters, allowing players to turn the attack against their foe. However, unlike the gravity gun in Half-Life 2, the grabber cannot hold objects for as long as the player wishes; if they wait too long to launch the object, the grabber will start to overload and disengage, dropping the object gently on the ground. Critics often compared the grabber directly with Half-Life 2's gravity gun, some noting that the device was far more combat-focused in operation than the gravity gun; in particular, the ability to turn projectiles cast by enemies against them was praised. However, the grabber was considered somewhat "awkward" to use, requiring a "finesse" that "is rarely something the player has time for in a close-quarters situation".

Influence on later games
Various later video games have included gameplay features that allow players to use the game's physics to their advantage in combat. In some cases, these are manifested as weapons or devices. For instance, Portal (also created by Valve Software) features the Aperture Science Handheld Portal Device which displays a limited capacity to move objects around the game world, while Crytek's Crysis allows the player to throw objects and enemy characters considerable distances through the use of an experimental nanosuit.

In other games, however, it can be represented in a different manner. In Arkane Studios' Dark Messiah of Might and Magic a psychokinesis spell allows for similar functions as Half-Life 2's gravity gun, Destroy All Humans! portrayed a similar gravity usage, known as PK, while 2K Games' BioShock displays the concept as a telekinesis plasmid that the player uses to alter their character's DNA. In EA's third-person horror shooter Dead Space, the player character acquires a 'Kinesis' module, which allows the player to grab and throw objects similar to Half-Life 2's gravity gun.