As a plot device, the TARDIS is sheer brilliance. Bigger on the inside, there is no need for expensive special effects of its exterior, and the exterior is easily moved from set to set to location to set etc. Able to move its exterior anywhere in space and time, it opens limitless possibilities for adventures. But could it actually exist? Can an object really be bigger on the inside than the outside?
Maybe, if you’re willing to think of things in more than three dimensions. And what’s more, such things could be natural phenomena, though the idea is pretty much just speculative at the moment. I mean, we know space can warp; it certainly does around massive objects. But a group of scientists at the University of Helsinki have come up with something they call the Cosmological Tardis Model, which suggests that the accelerating expansion of the Universe may not be the result of dark energy but actually an optical illusion caused by observing the universe through regions that are indeed bigger on the inside. The main problem is that light passing through such regions should warp, much as it does in other bits of bent spacetime, like the gravity wells of giant galaxies, and we don’t see that. But it’s an intriguing notion all the same, and it gives me the chance to put this in both the Space and Doctor Who categories. ;-)