How the TARDIS Travels in Time And Relative Dimensions In Space
Okay, this is going to be a total geekout, so don't complain that I didn't warn you. Not a tech geekout like when I talk about how OAuth works or why I like to use MD5 hashes as database keys. No… This is a Doctor Who Geekout.
For those of you who don't know (and I don't know where the heck you've been) The Doctor has been travelling Space and Time for over 2,000 years now in his TARDIS. The TARDIS (which stands for Time And Relative Dimension In Space) is often referred to as The Doctor's 'ship' -- it has a door, external sensors and makes a big noise when it's moving through Time and/or Space.
But I have a theory… The TARDIS is not a ship. It's a pocket universe slightly out of phase with our own universe. It doesn't travel through Time and Space in our universe, it just couples and uncouples a portal between the TARDIS universe and our universe.
This makes the whole, 'It's bigger on the inside' thing actually possible. It's not bigger on the inside, because… there is no 'inside' to the TARDIS.
My guess is that the Time Lords had something like the Large Hadron Collider and found they could create pocket universes by banging together particles in just the right way (probably involved a really hot cup of tea). Then they discovered a way to keep the door open between the pocket universe and our own.
Real estate solicitors were either devastated by infinite property being available or clamouring for rights to sell all this new property.
Except the property was probably useless. These pocket universes don't have to share the same laws of physics with our universe. You may find your atoms falling apart because you're in a universe with no strong force. Or you stop dead because the speed of light is 3 miles an hour and basic electrical/chemical reactions, like in the human (or Gallifreyan) body, won't work.
But assume they get around the Law and find a way to create universes with identical (or close enough) laws of physics to our own. No matter what, you need a life support system 'cause there isn't going to be any air in some neo-spawned pocket universe.
First thing you do is move in the hardware to control the connection between our universes. I'm sure this is big stuff and takes enormous power (apparently you can get that power from a black hole, which makes me wonder can you swallow a black hole with a pocket universe, but let's set that aside for a moment). Then you build a habitat around the Universal Kit so can now control the aperture between the universes from the TARDIS universe. Make the aperture big enough and put in an entire moonbase, or a planet if you want, after all you have a UNIVERSE of space to put stuff. It just has to have a self contained life support system.
Okay, so now you have a moonbase in a pocket universe and you can shrink the aperture down to the size of say, a Police Box. Seeing as you're manipulating universes anyhow, that whole 'Chameleon circuit' that changes the appearance of the TARDIS isn't a problem. Unless that bit of kit gets broken and you can't get new parts. Regardless, there isn't any real exterior to the TARDIS (the exterior we see in our universe, that is, not the moonbase in the TARDIS universe). It's just a manipulation of quantum forces to provide the appearance of a police box.
Now, the fun begins. Turns out time is just one of those things that happens in universes. Close the aperture and you're disconnected from our universe's rules of time and space. Do a bit of calculating, burn some of that black hole power and you can open the aperture in any point in time and space you want.
This solves a bunch of problems with the technology of the TARDIS -- it explains why it's bigger on the inside than the outside, it lets us actually travel in time and space, and that noise it makes is exactly the noise I would expect a ripping of two universes to make (but I've been watching Doctor Who for a very long time, so that might be a time loop making me think that).
'But what about other universes? Why does The Doctor make such a big deal out of not being able to connect to those universes?'
My theory is that it has something to do with the tuning of the aperture. That parallel universe with zeppelins over London was close enough to our universe that it even had the same people. But the laws of physics? Slightly different. Just slightly different enough to make it really hard for the TARDIS to operate (and make Micky a badass).
That even helps explain those 'fixed points in time' and why The Doctor is so big on not changing them, or maybe why he can't. The universe has to have a certain shape for the TARDIS to work. You can't map Time And Relative Dimensions In Space if you've lost all context because some minotaur was allowed to eat Winston Churchill. You have to keep all of time and space looking pretty much like it did when your TARDIS's pocket universe was created, or they fall out of sync.
It might even be part of the TARDIS's built in programming -- to find weak places in time and space where trouble might threaten the shape of space. Which is why The Doctor can't seem to have a relaxing holiday without something horrible happing.
Either that or he's just an adrenaline junky… and if you have two hearts that would be quite an addiction.