Its Bigger On The Inside
One of the biggest mysteries of Doctor Who is how the Tardis can be bigger on the inside than it is outside. My idea is that in the future we could well achieve just that, no I am not mad and I will explain why!
We all know we live in 3 dimensions as well as that of the fourth dimension of Time. We only know this because we can see in 3d.
I was born with a problem with my left eye and therefore I can only see in 2D and so really live in a 2 Dimensional world.
Imagine for example that the world were 2D and as flat as a pancake and something 3D were inserted, well not only would you see it appear just like the Tardis out of nowhere, but if only the front bit were protruding it would appear as if the object were smaller. On entering however you would go into the 3rd Dimension and it would look like the small object contained something bigger.
If something managed to exist in another dimension then it would have the same effect and thus you would think on entering that object that you were stepping from say a small box into a very large space.
Scientists are already working on this and it is called quantum physics were the known laws of physics cease to be.
In Doctor Who the Doctor always says it is Dimensionality Transcendental, which is just words given in a script. However, the truth of the matter is that is exactly what it is and what he is saying to based on fact.
The Fourth Doctor tries to clear up the art of trans dimensional designing to his friend Leela. (Television: The Robots of Death)
He did this by holding up two cubes and saying which one was larger? She picked the first cube and he then moved that cube further away and the previous cube nearer to her line of sight. Of course this was to explain to people watching and was a good explanation of the full concept.
Hartnell's Doctor explained the concept in An Unearthly Child in simple terms, using Television as an example of how a larger building could be seen in a small space. Just like an optical illusion it always relies on a trick of the light or senses. The Brigadier In The Three Doctors thought it was an optical illusion.
Rory Williams annoyed the 11th Doctor in Vampires of Venice when he ready up on how the dimensions are bigger and is rebuffed by the Doctor who says “I like it when people simply say, bigger on the inside, I always look forward to that! “
All of the Doctors like to show off and they hate it when one of the companions suddenly gets a bit too clever
The connection between the inside and the outside of the Tardis can break down and this was shown in the War Games. Also in the Meddling Monk the Doctor fixes the Monks Tardis so that it becomes small again.
What he uses in order to fix his Tardis is a Dimensional Control Circuit. This was shown in The War Games with Patrick Troughton, where the Doctor almost gets crushed when the controls are manipulated.
In some cases when a Tardis is dying, as it is a living thing, the dimensions start to collapse and as the eleventh DoctorWho once said everything starts to leak out and it grows. This happened to the Doctor on Trenzalore. In an alternative timeline, DoctorWho was never granted any new regenerations and the Tardis dies with him and serves as his tomb.