Taken from: Wikipedia - Holodeck
Introduction
A holodeck is a simulated reality facility located on starships and starbases in the fictional Star Trek universe. An
episode of Star Trek: The Animated Series, "The Practical Joker", formed the groundwork for the idea in the 1970s by
portraying a recreation room capable of holographic simulations. The holodeck was first seen in the pilot episode of Star
Trek: The Next Generation, "Encounter at Farpoint". The concept of a holodeck was first shown to humans through an
encounter with the Xyrillan race in the Star Trek: Enterprise episode "Unexpected".
In recognition of a holodeck's potential usefulness as a real world device for combat training, research is underway to
create such a facility.
Features
The holodeck is depicted as an enclosed room in which objects and people are simulated by a combination of replicated
matter, tractor beams, and shaped force fields onto which holographic images are projected. Sounds and smells are simulated
by speakers and fragranced fluid atomizers, respectively. The feel of a large environment is simulated by suspending the
participants on force fields which move with their feet, keeping them from reaching the walls of the room (a virtual
treadmill). Self-proclaimed "nitpickers" have criticized one of Picard's logs in "The Big Goodbye" for being motivated not
so much by the story as by a need to explain to viewers that the holodeck is a very realistic simulation.
Most holodeck programs shown in the episodes run in first person "subjective mode", in which the user actively interacts
with the program and its characters. The user may also employ third-person "objective mode", in which he or she is "apart"
from the actual running of the program and does not interact with it (all of the program's characters will ignore the user
as if they were not there—this was shown in the Enterprise episode "These Are the Voyages...").
Matter created on the holodeck ("holomatter") requires the holoemitters to remain stable and will quickly disintegrate if it
is removed from the holodeck without a mobile emitter to sustain it, although this principle has been overlooked in some
episodes, and nitpickers have identified most of these instances. Writer Phil Farrand has often pointed out how in many
episodes matter from the holodeck that gets on a real person still exists when the real person exits the holodeck. In
"Encounter at Farpoint", Wesley Crusher falls into a holodeck stream, but is still wet after exiting the holodeck. In "The
Big Goodbye", Picard has lipstick on his cheek after encountering a holodeck simulation of an old girlfriend. In
"Elementary, Dear Data", Data and Geordi LaForge exit the holodeck with a piece of paper that originated in the holodeck.
This could be explained using replicated rather than holographic matter.
In most episodes, the holodeck is controlled by voice commands, though physical controls have been shown in a few
episodes.
The holodeck includes safety protocols to protect the users. Viewers have criticized many holodeck-oriented episodes in
which safety protocols are predictably disabled in order to create suspense when the show's main characters are trapped in
the holodeck, or when holodeck simulations pose a threat to the ship as a whole; nitpickers have pointed out that the
frequency with which accidents happen on the holodeck would surely preclude their use as an entertainment device.
Some users may develop an addiction to the holodeck, leading to them spending unhealthy amounts of time there and
personifying artificial characters. This was demonstrated in the Voyager episode "Pathfinder" and The Next Generation
episode "Hollow Pursuits".
Applications
Starfleet personnel use holodecks for both recreation and training. They are used to recreate or simulate settings and
events for analysis, such as to explore the forensics and logistics of a crime scene for law enforcement purposes, or for
scientific experimentation. In general, the holodeck "functions as a cultural repository of narrative possibilities that
would normally be excluded from the ship's own sociohistorical moment" and "allows the Enterprise community to include even
that which it excludes by containing the excluded within a proper, controlled place which in no way intrudes upon the
everyday space of the ship."
The Emergency Medical Hologram on ships such as USS Voyager applies holodeck technology to present a single "character"
within the otherwise natural environment of the ship's sickbay. In one episode, Captain Janeway and Harry Kim mention having
used a holodeck as children. However, the holodeck was referenced in the first season of Star Trek: The Next Generation as
being fairly new technology.
An example of the holodeck's recreational functions are the holosuites that are owned and rented out – often for sexual
purposes – by Quark on Deep Space Nine. It has been suggested that if the holodeck came to exist in real life, it would be
used mostly for sexual purposes.
Writing stories and plotlines for the holodeck is an activity pursued by people known as holonovelists. It was the chosen
profession of Lt. Tom Paris of the USS Voyager, and he pursued it when the ship finally returned from the Delta Quadrant in
the show's finale.