Image via Guilherme Sagas on Flickr.
Ever wanted to know how the TARDIS from Doctor Who worked, but didn’t have the physics pedigree to take a stab at it? Well, that’s totally fine, because two fans of the show, who conveniently happen to be physicists, just did it for you.
In a pair of papers, one aimed at us layfolk and the other for those more comfortable with the Theory of General Relativity, Dr. Ben Tippett and Dr. Dave Tsang lay out how a TARDIS-like vessel would operate. Just looking at the second paper, “Traversable Achronal Retrograde Domains in Spacetime” gave me a serious case of blankbrain, so I opted instead for the plebian offering, “The Blue Box White Paper.”
Before delving into the nitty-gritty of what TARDIS travel might entail, Tippett and Tsang lay out the basics: what general relativity is, the significance of lightcones, and the ins and outs of spacetime geometry. Don’t be scared by the terminology. These two are quite adept at translating otherwise mega-intimidating topics into digestible brain food, a skill they’ve honed on their podcast, The Titanium Physics Podcast.
The Doctor's robotic canine companion K9 demonstrates how a curved surface causes his otherwise straight path to curve. This helps explain the curvature of spacetime in the paper.
With the fundamentals ringing in your head, the researchers address the concepts and limitations of other theories of time travel (Warp Drive, wormholes) before finally detailing the TARDIS scenario.
The TARDIS described by Tippet and Tsang is not the TARDIS you might be familiar with from the show. According to Tippett, they focused primarily on one aspect of the vessel’s functionality: the ability to travel between any two points in space and time. Making something that is “bigger on the inside” was too easy while approaching the issue of how something can just materialize or dematerialize, as the TARDIS does on the show, is “bananas,” Tippett told me. “I have no idea how to justify something ‘materializing.’”
There are other divergences, too. What the acronym stands for—Traversable Achronal Retrograde Domains in Space—is different and, according to Tippett, more descriptive. It is still a box, but unlike The Doctor’s TARDIS, it can only go in circles as it moves sideways and backwards in time.
Schematic of the TARDIS described in the papers as it travels through spacetime. The arrows describe the direction of time as it is defined inside and outside the vessel.
But it is possible, at least in theory, to make a more Doctor-y TARDIS by cutting up spacetime geometries and sewing them back together in non-circle shapes using the maths outlined in this paper. “The technique is really powerful,” noted Tippett, “because you can use the simple paper-pencil solutions, and glue them together to get a kind of simplified model of a more complicated system.” And that new Frankstein-spacetime will still work under the general relativity.
Ultimately, though, as you might expect, the TARDIS described is an impossible dream. In order to skew the spacetime geometry in such a way that the ship could function, we’d need exotic matter which is gravitationally repulsive and faster than light. Such matter simply does not exist in our universe.
Diagram showing the "strange and unphysical properties" of the sort of material required to maket his TARDIS.
Though I can only speak for the less complex paper, it happens to be a gleeful exploration in how to get someone to read about physics on a Friday morning. And I mean, really, what is science for if not to explain the fictional exploits of a hyper-intelligent alien who has the power to both go back in time to kill Hitler as well as travel to the year five billion to witness the death of Earth?