Zero Point Energy Field Manipulator

"The Zero Point Energy Field Manipulator is not a toy, Alyx!"

- Judith Mossman

The Zero Point Energy Field Manipulator (ZPEFM), most commonly known as the Gravity Gun, is a tractor beam-type weapon that was originally designed for handling hazardous materials, but, as Alyx stated, was primarily used for heavy lifting. At its core is a substance that appears to be a Xen crystal. Introduced to the player by Alyx Vance shortly after arrival at Black Mesa East, the Gravity Gun soon becomes an invaluable tool in Gordon's arsenal and is considered one of his trademark weapons.

Overview
The gravity gun has two basic modes. The primary fire releases an energy blast which punts the targeted object with tremendous force. This is useful for clearing out barriers and moving heavy objects. The secondary fire picks objects up and holds them just ahead of the gun. These objects can then either be dropped by pressing the secondary fire button a second time or punted with the primary fire. The secondary fire can only pick up lighter objects, mainly those which Gordon himself could carry, but the primary fire is capable of punting much larger objects even if the secondary fire cannot affect them. The gun cannot affect most organic material, though there are a few exceptions.

The combination of the gravity gun's secondary and primary fire modes allow it to be used as a powerful weapon. Since it uses physics objects rather than conventional ammunition, there is virtually no limit to how many times it can be fired; however, this advantage is balanced by the need to "reload" after every shot by grabbing a new object. Depending on the object fired, the gravity gun is potentially one of the most damaging weapons in the game. Heavier objects like metal barrels and propane tanks can kill even Overwatch soldiers in a single hit, but conversely things like plastic crates and wooden pallets will do very little damage due to their lesser mass. Explosive barrels in particular are a highly devastating weapon when combined with the gravity gun, but this has the potential to backfire if the enemy shoots them while they are being held. The gravity gun is most effectively used as a short to mid-range weapon, since the physics objects it uses for ammunition are affected by gravity.

In addition to its use as a weapon, the secondary fire of the gun can be used to pull objects over from a distance, allowing Gordon to access supplies or other objects that would otherwise be out of reach. It can also be used to grab larger objects and hold them up as makeshift shields. Though normally unable to drag heavier objects, it proves capable of doing so in a select few areas, mainly as a way to create bridges. It should be noted that the secondary fire of the gravity gun does not automatically tractor in targeted objects. The object must be brought within a certain distance for the field to capture it, during which time it is still subject to gravity. The gun must also be constantly trained on the object to maintain the effect until it is captured.

In addition to its primary uses, the gravity gun can be used in more esoteric ways. Manhacks, City Scanners and Shield Scanners can all be held and punted by the gun; the first, in particular, can be held by the gun for use as a makeshift chainsaw. All three are heavy enough to deal significant damage if punted, and will be destroyed if punted into a nearby wall. However, if punted over greater distances, they will stabilize their flight. It can also be used to grab Hopper Mines and remove Rollermines from vehicles, holding them safely. Hopper Mines grabbed in such a manner will automatically switch to recognise the player and their allies as friendly when planted again, while a Rollermine held with the gravity gun will be reprogrammed by Alyx, if she's around, to attack hostile targets instead of friendlies. Both types of mine can also be punted into enemies directly. Combine Sentry Guns can also be manipulated by the gun.

Though the gun cannot affect most organic matter, Headcrabs and Antlions are vulnerable to the primary fire. Three punts from the primary fire will kill any headcrab. Antlions are harder to kill in this manner, but being hit by the gun causes them to flip over, exposing their weaker undersides.

Dark Energy Field Manipulator
When exposed to the Confiscation Field in the Citadel, the Gravity Gun is "supercharged" rather than being vaporized as other weapons are. It also shorts out the field in the process. Once supercharged, the Gravity Gun emits blue light rather than orange, quakes in Gordon's hands, and creates small bolts of energy across its muzzle. Its powers are vastly enhanced, enabling it to manipulate objects of far greater mass than it could normally, as well as pull and repel them with greater force. Its most significant enhancement, however, is its ability to manipulate living matter. Any Overwatch Soldier punted by it is instantly killed, and their body retains a charge that will kill others if it strikes them. If grabbed by the gun's secondary fire, they are likewise killed. It also has the side effect of vaporizing any weapons the soldiers were carrying.

The first time the Gravity Gun is supercharged is in Half-Life 2's thirteenth chapter, Our Benefactors, when Gordon's pod deposits him into a Confiscation Field and remains so until the end of the game. During this time, Gordon's HEV Suit also becomes 'supercharged' to the point of harboring 200% energy, and power chargers restore Gordon's health as well. At the start of Episode One, it is no longer supercharged, but becomes charged again after Gordon and Alyx re-enter the Citadel to stabilize the core when Gordon accidentally enters another Confiscation Field. It remains supercharged until Gordon reactivates the core's containment field, at which point the Gravity Gun reverts to its normal state.

Behind the scenes

 * As heard in sound files featured in the playable Half-Life 2 Beta files, Eli was to explain to Gordon at Black Mesa East that the Gravity Gun's "grand daddy" was used to manipulate hazardous materials in the Combine's portal reactors, suggesting the Gravity Gun uses Combine technology.


 * Exactly how the Gravity Gun works is unknown, but some conclusions can be made. It is possible it uses a Xen crystal, as the emitter is almost identical to a sample being studied in Eli's lab in Black Mesa East. When the gun is supercharged by a Confiscation Field, Overwatch describes it as a "counter-resonant singularity device". Because of this, it probably contains negative mass, or exotic mass. Negative mass is believed, by scientists, to have inverted gravity and repel normal mass instead of attracting it. The gravity gun may possibly control the gravity attribute of the crystal, making it able of both attracting and repelling objects. How the weapon should be able to concentrate the gravitational force at a single object is, however, unknown.


 * In physics, zero-point energy is a quantum mechanical phenomenon where the lowest of a system has non-zero energy. This has been measured, for instance, in the Casimir Effect, where it was shown the "empty space" is not in fact empty and that quantum fluctuations are real. There have been various hoax claims of ways to harness zero-point energy but no known successes. Zero-point energy does not violate any laws of physics.

Trivia

 * The Gravity Gun appears to be quite heavy, as Alyx Vance and other characters that attempt to use the gun have some trouble lifting it. It also has significant recoil, as demonstrated when Alyx uses it to punt a door away. With his HEV suit, Gordon Freeman can use the device with ease.
 * The Gravity Gun bears a vague resemblance to the Anti-Mass Spectrometer from Half-Life, as well as the Portal Gun from the spinoff Portal. The gun was actually seen inside a lab in Aperture Science Enrichment Center prior to GLaDOS killing its inhabitants.
 * The supercharged Gravity Gun can be activated in normal use by entering the console command "physcannon_mega_enabled 1", which is now available in every Half-Life game that runs on The Orange Box engine (As of the May 26, 2010 update).
 * If this console command is enabled, the NPC's weapon will disappear, even outside Citadel, regardless what weapon it is.
 * When dying with the supercharged Gravity Gun, it disintegrates.
 * In Episode One, during the sequence where the supercharged gravity gun is used, enemies killed by Alyx will still lose their weapons as if hit by the gravity gun. This is likely intentional, as the purpose of the gravity gun destroying enemy weapons is to prevent the player from taking them during the sequence.
 * Using console commands or exploits, if the player picks up a Combine APC or a Scout Car using the Dark Energy Gravity Gun, the game will either slow the framerate down dramatically, or crash the game. The muscle car should not be picked up due to faults in the physics engine.
 * The Gravity Gun is present in Garry's Mod as a starting tool. However, it cannot be supercharged by any means. If the player enters a Confiscation Field, all weapons will be destroyed as normal, except the Gravity Gun which just sits on the ground. To get around this, the player can use several console commands which control the behavior of the weapon such as "physcannon_pullforce". These commands are native to the Source engine; they can increase (or decrease) the power of the weapon to meet the supercharged version, however it still won't affect people. This might be intentional, as the supercharged Gravity Gun's skin is used by the Physgun or Physics Gun, a beta weapon resurrected in Garry's Mod as an integral tool.
 * The film District 9 features an alien weapon influenced by the Gravity Gun, built into a powered armor suit. It is able to pick up almost anything, including a pig, and hurl it with incredible force. It appears to have a much stronger field since it is able to stop bullets in flight, collecting and holding them just ahead of the muzzle, somewhat similar to a zero point energy field. It apparently also possesses some means of dictating the flight path of objects it launches, as it seen scattering a mass of bullets it has collected and simultaneously hitting several people standing in different positions around the room.
 * When shooting any invincible NPC with the supercharged Gravity Gun the corpse spawns, but the NPC remains unaffected.
 * The Gravity Gun was originally called the Zero-Point Gun.
 * In the book "The Areas of my Expertise" by John Hodgman, the Gravity Gun was briefly mentioned while naming types of sword canes.
 * It appears that Aperture Science had a Zero-Point Energy Field Manipulator; one can be seen on a table in Portal 2: Lab Rat along with a charger.
 * In Episode 2, the HUD calls it a "Zero-Point Energy Gun"

List of appearances

 * Half-Life 2
 * Half-Life 2: Deathmatch
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Lost Coast
 * Half-Life 2: Episode One
 * Source Particle Benchmark
 * Half-Life 2: Episode Two
 * Portal
 * Portal 2: Lab Rat
 * The Final Hours of Portal 2