In "The Pandorica Opens" and "The Big Bang" the Doctor states that the Tardis' exterior is just normal wood to the Daleks. Does that mean the exterior of the Tardis changes material?
It's the Chameleon circuit of the TARDIS that make hit appear as normal wood. The exterior of the tardis is described as Outer plasmic shell, without any explanation what plasmic is. So, the exact materials composing the TARDIS are a mystery, it's even believed that the TARDIS may be Organic.
TARDISes were incredibly complex machines. The nature of their construction was such that they were said to be grown rather than constructed (DW: The Impossible Planet), thus simulating a biological process, though it is not clear whether this is indicative of the machine being biological in nature or simply so intricate and complex as to appear to mimic the processes of a biological entity.
I believe he was using the idea of wood figuratively. In other words:
- The TARDIS, because of it's stuck chameleon circuit, appears to always be made of wood.
- It is actually made of a mostly-unknown material that is far stronger than wood.
- This material is impervious to damage from almost every source in the universe.
- The Daleks, however, have been fighting TARDISes for centuries (e.g. via the Time War) and have weapons that are design to do so.
- So, the TARDIS exterior will be easily damaged by their weapons, as easily as if it had really been made of real wood.
Remember the TARDIS is not a single object as much as it is two linked objects- the plasmic shell exterior and the interior. The outer shell of a functional TARDIS is automatically morphed into an appropriate form via the chameleon circuit. Understand this technology is designed to fool human and non human detection methods so simple illusions are not enough, it must become the object it is imitating. Also remember this is technology from a culture that has conquered time. Simple transmutation of materials is child's play. The TARDIS exterior most like not only looks like wood, it probably IS wood and the exact wood that a 60s era police box is made out of. It also happens to be completely invulnurable simple wood due to the fact that is a both in the "normal" world and temporally out of phase. Add to that a layer of shielding commented on in the new season and anything that could penetrate or damage the TARDIS is a major threat indeed.
No. The TARDIS's exterior shell is made of Block-Transfer Equations. Pure Mathematics. Which is why it can change it's shape. The TARDIS being "just wood" is an example to show how weak the shielding is to the Daleks. For example, if you had the most powerful ship in the universe and your strongest enemy could get inside wouldn't you say, "Right now that metal is just a marshmallow compared to their strength"? It doesn't mean your ship's made of marshmallows does it? I think not....
My theory speculates of how a TARDIS is grown is that they grow the TARDIS with the coral after its fully grown to its full size they put a piece of the eye of harmony in it and build over its muscle just like how we have skin on ourselves and they somehow link the TARDISes entity and soul to its matrix connecting to the console and the interior so it can expand its inside and add or take off rooms