The Doctor uses an “old” Type 40 TARDIS. IT travels in space and time. It’s bigger on the inside than the outside. It has a broken Chameleon Circuit which means it’s stuck in the form of a police box.
Doctor Who, like Star Trek, has it’s own sort of technobabble.
From the Whoniverse
The bulk of a TARDIS is made up of Block-Transfer Mathematics. This form of Distributed Cluster Algebra was first developed by the people of the planet Logopolis. It is based on the idea that the essence of matter is structure and the essence of structure is mathematics. Using the hexadecimal notations of Block-Transfer it is possible to create Space-Time Events through pure calculation. An event or object can be described by thinking the correct Block-Transfer equations and it will instantly become reality.
These calculations are powered by raw Artron Energy. Block-Transfer Mathematics can only be computed with an organic brain because the mathematics alter the nature of reality. This would cause a traditional computer to malfunction. The original equations can however be recorded in traditional non-organic storage media such as bubble memory
When I was reading a lot about Doctor Who (around the time of the FASA RPG) it was made plain to me that the inside of a TARDIS is a completely different dimension to the outside of a TARDIS and that indeed, the outside of a TARDIS was nothing but a doorway to the TARDIS. Which is why it can materialise and dematerialise – it’s just a mathematical construct. This also explains why a TARDIS is functionally indestructible and it’s doors impenetrable.
So, the question remains, how far can you push a TARDIS?
How big can the inside be?
How many doors to the outside can there be?
This is all leading up to a game I’m prepping for Graham, Jim and Rory. They’ll encounter a TARDIS which is planetary in size, with a million doors.