After more than 50 years and 13 incarnations, the Doctor is back — Doctor Who has returned for Season 12 with Jodie Whittaker at the helm. Surely there will be new threats, new environments, and maybe a new companion or two. New questions will certainly be asked, but it's the old questions, still unanswered, that niggle at the mind.
We're embarking on new adventures with the universe's favorite Time Lord (or Time Lady, I'm unclear on how noble titles work on Gallifrey), and, considering that time is, after all, wibbly-wobbly, there is no greater moment than right now to ask the decades-old question …
HOW IS THE TARDIS BIGGER ON THE INSIDE?
It's a basic fact of the show, taken almost at face value: The Doctor, on all their escapades, utilizes a piece of advanced alien technology, a blue phone box called the TARDIS, in order to traverse the vast expanses of space and time.
The TARDIS — standing for Time and Relative Dimensions in Space — is more than just a prop, though. It's a character in its own right; it has a life, a personality, and a will all its own. When the Doctor changes, so does the TARDIS. But some things are always consistent.
During its more-than-half-century run, Doctor Who has developed a few running gags. One is the name of the titular Doctor themself ("Doctor? Doctor who?"); another is the exclamation uttered by everyone who enters the hallowed space of the Doctor's mobile home: "It's bigger on the inside!"
Given the Doctor's many talents, it might be easy to hand-wave the existence of that little (from a certain point of view) police box. But there's a possible explanation, at least theoretically speaking.
We already know that Gallifreyans achieved considerable mastery not only over their own bodies, conquering death through fantastical and emotionally compromising displays, but also over space and time. There's no known way, at least with human technology, to create an object which is bigger inside than it is outside. Yet Time Lords take it as a given.
Anyone who's familiar with even elementary-level geometry knows that the area inside an object is directly correlated to its exterior measurements.
Simply put, given a perfect square, one centimeter by one centimeter, the interior area is one centimeter squared. Two by two, the area is four square centimeters. Four by four: sixteen. It's simple multiplication.
The numbers are easy to follow when examining objects in two-dimensional space. And there's good reason to start here, as it makes explaining an additional dimensional axis easy to follow.
Imagine yourself a Time Lord, living in a 2D world. You hail from a three-dimensional reality but are slumming it with the circles, squares, and rhomboids of Flatland.
You arrive on 2D Earth, a slightly ovular, but mostly circular object, upon which several billion flat little people are living their lives. You arrive on the surface of this world in a flattened police box, roughly 285 centimeters in length and 137 centimeters in width.
Then you find a flat companion, because of course you do. You can't very well go off fighting the flat Daleks and preventing the flat apocalypse without one.
The flat people see your 2D TARDIS as an object with an area of approximately 39,000 square centimeters. Certainly large enough for a citizen of Flatland to enter and call for help, but not large enough to travel comfortably through space for any extended period of time.
That is, until they walk inside.
Suddenly, a third dimension opens up. Now they're attempting to comprehend a space of more than 5.3 million centimeters and their mind just cracks in half. It's bigger on the inside and they are incapable of understanding how or why.
When asked about it, you explain that it just is. How do you explain a three-dimensional space to a two-dimensional person? There are closets full of colorful scarves, and there's a pool. Oh, and a library! Just accept it and let's go exploring. Allons-y.
We, living here in our three-dimensional reality, would endure a similar experience when encountering a four-dimensional object.
Carl Sagan explains it pretty well in the video below.
In short, there might be ways we could infer the existence of extra dimensions without ever being able to experience them for ourselves.
String theory seems to suggest that there are, in fact, additional spatial dimensions in our universe. They're just wound up so tightly we can't see them.
Moreover, the universe could be bound up in spatial configurations we're not able to deduce.
According to recent research, our universe is flat. It seems difficult to square with what we know of our own experience, but it's most probably true.
The basis for determining if an object (like the universe) is flat is fairly simple. All you need to do is send two parallel lines off into the distance and see how they behave. You can replicate this pretty simply.
Grab a scrap of paper and draw two parallel lines. As they move toward the edge of the paper, if they remain parallel, you know the paper is flat. Try the same thing on a ball. Those two lines will eventually intersect, even though they continue straight on.
You can achieve the opposite by drawing parallel lines on something saddle-shaped ... like a saddle. Your lines will diverge, despite never shifting their trajectory.
Measuring the behavior of lines is a reliable way to tell the shape of something. And when we measure the behavior of lines in the known universe, it leads to a consistent conclusion: Our reality is flat.
That doesn't mean, however, that there aren't additional dimensions we can't see.
It's possible that the universe isn't actually flat. I know … I know I just told you that it was. But science never makes definitive statements, it only declares what we think we know, based on the best available evidence. It's entirely possible that the universe is simply much larger than we think and that, on a large enough scale, we would find some curvature.
Though even that isn't necessary in order for there to be additional dimensions to our reality.
Take that same exercise, drawing lines on a piece of paper, and then bend that paper into a cylinder. Those lines will continue to remain parallel, despite circling back to where they started. Geometrically, that cylinder is flat, but it definitely has an extra dimension that would be impossible to perceive by any person living atop it. You can do the same thing on a donut, or a Mobius strip. There are many ways the universe could be shaped while still remaining flat, all of which allow for extra dimension within, which another type of being (a Gallifreyan, for instance) could operate.
If there's one thing we know unequivocally, it's that we don't know very much at all. Every discovery, every push we make into the great expanse of possible knowledge, only reveals more of the mystery of reality.
We are, in a sense, flat creatures trying desperately to understand a three-dimensional world.
Maybe one day we'll get there. Until then, we can hope for the tell-tale whooshing of an extra-dimensional visitor and the appearance of a little blue box.