The Doctor's Time Machine
The Police Box that The Doctor travels around in is really a transport device capable of travelling through time and space. TARDIS was a name made up by someone who was close to the Doctor. The word TARDIS stands for Time And Relative Dimension in Space.
A TARDIS usually changes its appearance on each trip to blend in with its surroundings. The Doctor has stated that it was a broken chameleon circuit that caused the TARDIS to remain in the shape of a 1960’s London police box.
The TARDIS is dimensionally transcendental, meaning it’s bigger on the inside than the outside. The interior exists in a different, relative, dimension to the exterior.
The TARDIS’ outer physical structure or “Time Travel Capsule” is created through a cold plasma engineering process which makes it invulnerable to all forms of SDC attack, and mundane forms of MDC attacks are reduced to 1/10th damage. There are three separate force fields that can be extended around the exterior of the TARDIS; one is an outer shell, another is a redundant force field and the third is focused around the area of the door, with an environmental protection field that can be extended in a bubble or bridge some distance outside of the front or sides. The interior of the TARDIS is a pocket dimension over miles across at its widest point. A force field sits just inside of this as added protection and stabilization.
T.T. Capsule Main Body: 250,000 MDC
T.T. Capsule Primary Force Field: 100,000 MDC
T.T. Capsule Secondary Force Field: 100,000 MDC
TARDIS Interior Force Field: 100,000 MDC
A TARDIS is normally powered by a black-hole generator called the Eye of Harmony which exists on the planet Gallifrey. A “proxy” Eye of Harmony exists on each TARDIS, and the energy from the primary EoH is sent through the Time Web via the Time Vortex to these proxies. Occasionally the EoH proxy can experience a low power reserve after a period of continual use; this quickly recharges within a matter of hours, however it can be recharged manually by setting the T.T. Capsule down in the middle of a temporal distortion that can access the Time Vortex, such as a Temporal Rift or Dimensional Triangle. The Doctor’s TARDIS has had its EoH proxy removed, and now can only be run off of the reserve energy cells, which he recharges in the previously stated method.
The control system of the TARDIS and its ability to manipulate space/time is made possible by a living psychic entity which is incubated and grown on Gallifrey. These entities are marginally self-aware, but the Doctor’s TARDIS is particularly so. The piloting of a TARDIS is difficult, and made easier by a mental connection between the pilot and TARDIS entity via specialized Telepathic Circuits. The TARDIS entity resides within a contained temporal distortion which is effectively an open gateway to the Time Vortex itself, which the TARDIS entity is partially one with and connected to. The TARDIS entities feed off of this energy and can freely manipulate it. The TARDIS entity creates a breach in the fabric of time/space, allowing the T.T. Capsule to move through it into the Time Vortex, then neatly seals the rift behind it.
The Doctor’s TARDIS (Type 40) – Psionic Entity