The holodeck or holosuite serves both entertainment and training purposes. It combines transporter technology with that of replicators, by generating holographic images in 3D space as well as projecting force fields to give the objects the illusion of substance. It can be controlled from an exterior control or the interior arch control. This arch could be summoned at any time to change the parameters of a running program.
Holodeck walls can generate holographic images that appear to extend for an unlimited distance, seemingly much larger than its own dimensions. In doing so, however, the holodeck is aware only of its users; it does not recognize its own created objects. For example, if a person were to throw a holographic rock at the holodeck's walls, the rock would not be allowed to pass beyond the wall. It does this by continuously adjusting the projections of the force fields and the use of a forcefield ‘treadmill’. With this, an individual approaching a wall causes an instant shift away. The holodeck can change gravity in three dimensions, so occupants don't notice the change.
Holodeck matter can impersonate real matter even at the molecular level. Molecule-sized magnetic bubbles replace molecules in full- resolution holo-objects. The computer can manipulate them individually in three dimensions. The computer may use large magnetic bubbles to simulate surfaces and textures rather than create an object at the molecular level. However, Objects created within the holodeck could not survive beyond the holodeck itself, as they only exist as energy and matter.
Holograms can also be projected into space. They can be augmented with force beams to simulate solid, tangible objects or with replicator technology to create actual solid matter such as foodstuffs. All foods eaten on the holodeck are replications. No other type of simulation would survive outside of the holodeck
A holodeck also has the ability to create holodecks within holodecks, and holodeck programs are able to be saved to a cube that can be inserted into special devices with information to "last a lifetime".
Computers cannot duplicate the complexity of electron shell activity and atomic motions that determine biochemical activity in living creatures. This prevents replicators from duplicating life and resurrecting the dead. Advances in computer technology may allow this, permitting a person to live forever in any chosen environment while interacting with real people and objects visiting the holodeck
The energy matrix of a holodeck is incompatible with other ship systems. Failure of a holodeck's matter conversion subsystem can cause the loss of solid objects within the holodeck environment.
Among the viewing modes on a holodeck is objective mode, in which the user does not interact with the characters, and subjective mode, in which the viewer can interact with the characters as well as alter his or her surroundings.
The Operations Department is primarily responsible for the maintenance and programming of holodecks in Starfleet.
Due to the practical use of holographic technology in both recreational and work activities many starship set aside one or more holodecks solely to be used for official ship activities. These holodecks may not be used by off duty personnel or civilians and can only be used for work related activities.
Many departments have viable work related activities which either require or are made easier by using a holodeck. The most common is for training purposes. For example, in battle training it is simpler to recreate a holographic version of a starship than to divert resources away from the real version. The same applies to weapons practice or training medical personnel in difficult medical procedures. But there are practical uses beyond mere training. The engineering or science departments can create holographic models in order to perform tests without having to use any real resources. Even the counselling and medical department can use the holodecks in order to create more ideal environments to treat certain conditions.
See Medical Holodeck
By far the most common use of holodecks are their recreational properties due to their ability to create almost any environment.
In some cases a user will just create an environment and then interact with that environment as they wish. This can include anything from creating a ski slope to ski down or a beach to relax on. These tend to be the simplest programs as they do not require the complex coding needed to create holographic characters.
However, other programs can include holographic characters that will react to users in whatever way they are programmed to. This will allow a user to create an entire city full of people.
Another type of holographic program is the holo-novel. A holo-novel is a program in which a user is expected to take on the role of one of the characters in a story. The user will role play this character following the story that the program sets out before them.
Each member of a ships crew and its passengers are assigned a certain amount of time they are allowed to spend on a holodeck either by a weekly or monthly basis. This is to ensure that every body is given a fair chance to use a holodeck. A person must book their time on a holodeck well in advance to arriving so that clashes do not occur.
Also See: Holodeck Memory Alpha