Artificial gravity

Artificial gravity is employed by the UNSC, Covenant and the Forerunners, to generate gravity of varying strengths in an otherwise zero or low-gravity environment.

Centrifugal force
A number of UNSC ships employ rotating sections of the hull to provide centrifugal force on the interior of the hull to allow comfortable movement in "gravity." On certain ships, the rate of rotation can be adjusted, allowing more or less gravity, and a number of other ship classes, such as Halcyon-class cruisers, also used rotating sections before the introduction of more advanced gravity plating.

The Halo Installations rotate, and may use centrifugal force to provide at least part of their Earth-like gravity, possibly employing the same gravity generation techniques used by the Covenant, or another method, to generate the rest.

Covenant
Almost all Covenant vessels employ some sort of artificial gravity mechanism that does not involve rotating hull sections or acceleration. Instead, their method allows for the generation of actual gravity.

The most likely explanation is that the Covenant have developed some form of directional gravity emitter that generates a beam of gravitons. This would mean the Covenant are able to generate gravity without mass. It is possible that the Covenant have figured out a way to convert energy into graviton particles and direct them in much the same way as a beam of light; this seems to be the basis for the Covenant's gravity lift. The production of "antigravitons" would create a repulsive effect, which may be the basis for some of their antigravity drives. All this suggests the Covenant are able to produce gravity and antigravity as easily as humans can produce positive and negative electric current.

Human
While some human ships used rotating centrifuges to mimic gravity, humans did possess a form of artificial gravity even prior to their contact with the Covenant. This is demonstrated by the DCS freighters enabling artificial gravity through "anti-gravity units" and the orbital platform Tiara having artificial gravity without the use of rotating sections. Additionally, many ship classes commissioned before the Human-Covenant War, including Halcyon-class cruisers and Phoenix-class colony ships were known to have a form of artificial gravity other than spinning sections. Typically, artificial gravity systems are found on older craft, because newer craft rely on the more effective and, ironically, lower-tech rotating carousel system.

Artificial gravity systems consume power at a tremendous rate, so much so that a ship's fusion reactors typically cannot keep them running for more than a day and often far less if the ship is in combat or expending energy on maneuvers. As a result, during routine operations, rotating carousel sections spin to generate gravity. In combat or crisis situations, the carousels are locked in place and artificial gravity systems are activated. Some Captains eschew artificial gravity altogether. Using the technology allows high-gravity maneuvers which would otherwise put stress on the crew. This allows combat teams to focus on battle rather than the uncomfortable conditions aboard the ship. However, many naval technicians maintains that the technology wastes too much energy in extended combat.

It may be that antigravity plates were related to tests with artificial gravity. Since the antigrav plate had a crushing or repulsive effect, and it was described as generating a beam, it most likely generated directed antigravitons.

UNSC starships clearly have a form of anti-gravity technology, demonstrated by their ability to hover in planetary atmosphere without rocket propulsion being directed below. It's possible that only very large power plants can generate the power needed to produce antigravity, and the antigravity plate may be an early attempt to miniaturize the system found on starships. This would mean the human and Covenant technology were not that far apart in this regard, with humans lacking the technology needed to miniaturize gravity/antigravity generators.

Forerunner
Most Forerunner ships or installations possessed artificial gravity without the use of centrifugal force. This was achieved through the use of energy fields known as buffer fields. The science behind these fields, or whether Covenant and human artificial gravity devices operate by a similar principle, is unknown.

Trivia
In all of the Halo games, all human vessels or stations seem to employ sophisticated, unseen artificial gravity generators to simulate gravity, as opposed to rotating sections. Originally, it was widely assumed that the UNSC gained their current artificial gravity technology from the Covenant, and had to use rotating centrifuges to mimic gravity prior to this. According to Halo: The Fall of Reach, human ships were still undergoing tests with reverse-engineered artificial gravity by 2552, while newer sources such as Halo: Contact Harvest portray artificial gravity present even on automated freighters.

Internal

 * Anti-gravity technology

External

 * Gravity on Wikipedia
 * Artificial gravity on Wikipedia