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A TARDIS (or more properly a Gallifreyan TT Capsule) is a time-travel vehicle commonly used by the Time Lords, including the Doctor, on the television programme Doctor Who.

The Name

The name TARDIS is an acronym for Time And Relative Dimension (or Dimensions) In Space, and it is typically written in all uppercase letters. Whether the "D" in TARDIS stands for Dimension or Dimensions is an open question. The singular form was used in the first episode of the program. Since then, however, the Doctor has used both versions almost interchangably.

In the first episode of the "classic" series, "An Unearthly Child" (1963), the Doctor's granddaughter, Susan, claims that the name TARDIS was her own invention. The idea that TARDIS is not the official designation for Gallifreyan time machines is supported by the fact that Time Lord officials usually refer to them as "TT Capsules." However, over the course of the programme, other Time Lords have used the TARDIS designation as well. Some of the spin-off novels written since 1989 have used this fact to suggest that The Doctor and Susan may have once been highly influential Time Lords, but the canonicity of such spin-off materials is hotly debated.

Appearance

The external appearance of a TARDIS is controlled by a device called a "chameleon circuit." The chameleon circuit changes the external appearance of a TARDIS each time it materializes at a new location in time and space. As the name of the device implies, the chameleon circuit attempts to choose a form that will be suitably inconspicuous at the new locale.

Police boxes such as this one were once a common sight on the streets of London

During a visit to 1963 London, the Doctor's TARDIS assumed the form of a London police public call box - a common sight on the streets of London at the time. However, The Doctor's chameleon circuit malfunctioned upon his departure from that location and his TARDIS has remained stuck in this form ever since.

The internal appearance of a TARDIS can be quite surprising to newcomers. Gallifreyan TT capsules are "dimensionally transcendental," that is, larger on the inside than on the outside. While the exterior of a TARDIS may be no larger than a phone booth, its inside may include vast mazes of rooms and corridors. Most often, the room immediately inside the front door is a large control room dominated by a hexagonal console used to pilot the TARDIS. However, the internal appearance of the TARDIS also appears to be variable. The Doctor has been seen to modify the configuration of rooms in the TARDIS from the central console, including deleting rooms to convert their mass into energy for emergency power, and the appearance of the console room itself has changed (sometimes dramatically) over the years. The Doctor winkingly refers to this as "redecorating."

Defenses

The external shell of the TARDIS appears to be invulnerable to most attempts at penetration. Most attempts to physically damage it or penetrate it have been completely unsuccessful. The only exceptions to this rule appear to be the exercise of truly god-like power. For example, Egyptian god Sutek was able to send a mental projection into the TARDIS interior in Pyramids of Mars, but the Doctor commented that mental power of that magnitude was inconceivable. Another example could be seen in 2005's Bad Wolf, in which a transmat beam penetrated the TARDIS. However, the Doctor observed that this beam was thousands of times more powerful than an ordinary one.

The TARDIS also maintains a shield that keeps the air inside when in airless environments, even if the door is open.

Power Source

Until recently, all Gallifreyan technology was powered by a black hole. Gallifreyan stellar engineer Omega created this particular black hole by collapsing a star. Planetary leader Rassilon then harnessed the power of this black hole and the complete power system became known as The Eye of Harmony. Some of the Doctor Who novels suggest that transdimensional engineering was used to enclose the black hole itself in a pocket universe beneath the capital city. But, the canonicity of such spinoff materials is uncertain.

In addition to there being an Eye of Harmony on Gallifrey itself. There is also a device called the Eye of Harmony on board every TARDIS. It has not been established whether the Eye of Harmony on board a TARDIS is a separate device from the one on Gallifrey or an interdimensional extension of the same device. However, given the fact that the Doctor was no longer able to use the eye in his TARDIS after the destruction of Gallifrey, the latter explanation seems likely.

The Eye of Harmony that was present in the Doctor's TARDIS appeared to be a large spherical vortex shielded by a pair of heavy eyelid-like doors. It has been established that leaving these doors open for any length of time could have catastrophic consequences for the planet on which the TARDIS was currently located.

Since the destruction of Gallifrey, the Doctor appears to be unable to draw power from The Eye of Harmony. He instead resorts to charging the TARDIS batteries from latent energy sources he comes across in his travels.

Huon Particles

According to the 2006 episode, The Runaway Bride, huon particles are a rare substance that have been removed from the universe, except that they still exist in the heart of the TARDIS. Since these particles have a strong attraction to one another, the Doctor was able to use a mass of them to call the TARDIS to him. The Doctor explained that the heart of the TARDIS is only place in the galaxy that Huon particles lived and that the only way to make huon particles is to grow them with in a living being.

Are they alive?

The idea that a TARDIS is in some sense alive goes back as early as the third televised story, 1963's "The Edge of Destruction and it is supported as recently as the 2006's "The Impossible Planet" in which the Doctor comments that the TARDIS was grown, not built. However, the vast majority of televised stories simply refer to it as a machine and strongly imply that all TARDISes are merely constructs built by the Time Lords for their own purposes. It was not uncommon in the classic series to have no mention of any intelligence in the TARDIS for hundreds of consecutive episodes.

However, it is clear that the TARDIS does have some sort of rudimentary intelligence and that it sometimes decides to act on its own. For example, in 1985's "Attack of the Cybermen" it apparently refuses to change shape from the familar blue box despite the fact that the Doctor has repaired its chameleon circuit. There are also several mentions in the series history of the TARDIS appearing to be hesitant about materializing in a particularly dangerous location.

Nevertheless, the intelligence exhibited by TARDISes seems to be at more of an instinctual level than that of conscious thought. Significantly, the Doctor's TARDIS doesn't seem to be able to communicate even the simplest ideas to him without a great deal of inference on the Time Lord's part. In "The Edge of Destruction," the TARDIS's attempt to warn the crew of impending danger telepathically, initially confused the crew rather than informing them. In 1980's "Logopolis," the TARDIS had to resort to ringing an emergency warning system known as the 'cloister bell' when attempting to communicate the much more complex concept that its own deteriation due to extreme age was becoming a severe problem.

Another indication of the TARDIS's intelligence level comes from K-9. When attempting to interface with the TARDIS in 1978's "The Invasion of Time," K-9 comments (in typically blunt fashion) "you are a very stupid machine."

More to come

Until this article is completed, please see the main Doctor Who article for more details on the TARDIS.

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