Amazing device on the Enterprise in Star Trek: The Next Generation which has the ability to create completely realistic holographic simulations of anything you can imagine, even to the point of giving the simulations real solidity (using complex matter manipulation and transporter technology) and even, in many cases, artificial intelligence. Some of the coolest ST:TNG episodes involved the holodeck. Then again, some of the worst episodes also involved the holodeck. If I had a holodeck, I'd go in and never ever come out.

Information from the Star Trek:TNG Technical Manual.
(pgs 156 and 157)
(in my own words, and with quotes)

The Holodecks are the familial descendents of the cassette tapes and newspaper headlines that were passed up to astronauts during the flights of the mid-twentieth century. These devices, which utilize forcefield generators, replicators and imaging devices allow Starfleet ship crews to survive the rigors of extended missions without giving up the psychological benefits of a walk in a park, or a night under the stars.

"The Holodeck utilizes two main subsystems, the holographic imagery subsystem and the matter conversion subsystem. The holographic imager subsection creates the realistic background environments. The matter conversion subsystem creates physical "props" from the starship's central raw matter supplies. Under normal conditions, a participant in a Holodeck simulation should not be able to detect differences between a real object and a simulated one."

The holodeck's software is thus given a choice to make an object out of forcefields, or out of matter, and to quickly switch between the two states as the need arises. Characters in holodeck simulations are usually composed of solid matter arranged by transporter based replicators and manipulated by highly articulated tractor beams. Objects created on the holodeck that are purely holographic images cannot be removed from the Holodeck. Those objects created using replication can be removed, but they are no longer controlled by the Holodeck program.

"The basic mechanism behind the Holodeck is the omnidirectional holo diode (OHD). The OHD comprises two types of microminiature device that projects a variety of special forcefields. The density of OHDs is 400 per cm^2, only slightly less than the active visual matrix of a multilayer display panel, and powered by standard medium-duty electro plasma taps. Entire walls are covered by OHDs, manufactured in an inexpensive wide-roll circuit printing process. A typical Holodeck surface comprises twelve subprocessing layers totaling 3.5 mm, diffusion bonded to a lightweight structural cooling tile averaging 3.04 cm thick. The primary subprocessor/emitter materials include keiyurium, silicon animide, and superconducting DiBe<2>Cu 732. Each single OHD measures 0.01 mm. The optical data network mechanism by which OHDs are sent impulses is similar to that for smaller display screens, though complete walls are broken down into manageable high speed segments, each 0.61 m^2. Dedicated high speed subsections of the starship main computers drive these room sized displays."

The OHDs also manipulate force fields in three dimensions, allowing users to feel holoprojected objects. The tactile feedback is perfected by algorithms approximating the actual surfaces of the objects being simulated. Other senses, such as sound and smell, are taken care of in more traditional ways, with atomizers and speakers. These can also be incorporated into replicated objects and characters for a more realistic experiance.

The shaped forcefields and background imagery allow a participant to scroll through the hologenerated landscape. In this way, areas much larger than the Holodeck itself can be experienced.

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