In the Tenth Doctor story “Planet of the Dead” (2009), the story features a distant planet with mysterious alien sand, three Suns, and a wormhole.
“How do you decide whether tomorrow’s technology includes time travel? Where do you look for evidence that our descendants have discovered the means of temporal voyaging? If time travel is a one-way process forward, there is no way we can know. If, as the new physics suggests, it is possible to move back in time, then the evidence we are searching for will present itself as anachronisms. Human beings are careless. They drop things they shouldn’t, like the metal tubes found in Saint-Jean de Livet in France. They are also vulnerable. Whatever safeguards are in place, sooner or later someone will be trapped in a time period other than their own and die there. If the time period is historical, their death will leave no anachronistic trace, but if we examine the depths of prehistory, it becomes possible to trace the series of temporal disasters, which left a trail of corpses where they decidedly should not be.”
—J. H. Brennan, interview, Time Travel: A New Perspective (1997)
Picture this: the Doctor is standing at the Crucifixion. Captivated and awestruck, he or she can’t help but study the scene. One of the most famous in all of history, at least on this world. Here is one of the benefits of time travel. Experiencing, firsthand, history in the unfolding, even if it is parochial to this planet. Just a few points to remember. He or she must do nothing to alter history. (Note to self: no messing with the Blinovitch Limitation Effect this time!) And when the crowd is asked who should be saved, she should join in with the call, “Give us Barabbas!”
Suddenly, the Doctor realizes something weird about the crowd. Not a single soul from 33 AD is here at the scene. The mob condemning Jesus to the cross is made up of time travelers from the future. And some of them from Mars!
But wait. The scene isn’t passively littered with folk from the future. They’ve actively changed the outcome of history itself, by being present at the Crucifixion. The time travelers think they know the way history is meant to go. Rather than Jesus being set free, the crowd is meant to choose Barabbas, the bandit. But the decision only goes that way because travelers from the future are witness to the scene. Would Jesus have been set free instead, if they hadn’t interfered? Bang goes the Blinovitch Limitation Effect again.
The above story is one of my favorite ways of introducing the idea of time travel and its contradictions. The tale can be told a little differently each time, depending on context. But credit must be given to the wonderful story upon which this introduction is based. This nifty time-travel scenario (not including the Doctor Who twist, of course) was dreamt up by Garry Kilworth in his 1975 science fiction story Let’s Go to Golgotha. It’s typical of the kind of conundrum that the Doctor Who writers have to reckon with when they tell a tale about tampering with time.
As you may know by now, the most scientifically credible way to achieve time travel is through the use of wormholes. Possibly. What do wormholes look like? You remember the title sequence at the beginning of many programs of Doctor Who? You get the bit where they tell you the name of the episode, along with the cast and production members who worked on that program. Then, as the unforgettable Doctor Who theme song spooks out of your speakers, you see the TARDIS falling through what looks like a sinkhole in space. Or like a giant plughole is sucking the TARDIS into oblivion. That sinky, pluggy thing is a wormhole, as the Tenth Doctor might say.
They’re called wormholes because they’re a little bit like the tunnels that worms make in apples when they eat their way through. The journey through the apple tunnel is shorter and quicker than going over the apple’s surface from one side to the other. That’s how wormholes work in space too. Einstein figured there would be regions of space where there are shortcuts, linking vastly separated regions with a much shorter tunnel.
Famous American science-fiction writer John Campbell was the man who invented such tunnels or “space warps.” In his 1931 story Islands of Space, Campbell used the idea as a shortcut from one region of space to another. And in his 1934 story The Mightiest Machine, he called this same shortcut “hyperspace,” another now-familiar phrase. A year later, world famous Nobel Prize–winning scientist Albert Einstein (him again) along with colleague Nathan Rosen, came up with the fact behind the sci-fi invention of time travel. Einstein and Rosen worked out the scientific theory that explained the notion of these “bridges” through space. It was much later that we all started calling these bridges “wormholes.”
So that’s why you often see the swirly cosmic tunnels of wormholes in many movies. It’s meant to show when a spacecraft is on a journey through space and time. Some character in the movie might even do you the favor of saying that a wormhole has at least two mouths, connected to a single throat, or something like that. They may even grace you with the fact that scientists really do believe they exist. At least in theory. And, as that theory is Einstein’s, people take it seriously. Especially sci-fi writers. Stuff may “travel” from one mouth to the other by passing through the wormhole. We haven’t found one yet. But the cosmos is immense, and we haven’t really been looking for very long. Please also bear in mind that American cosmologist Lawrence Krauss, author of the 1995 book The Physics of Star Trek, suggested on NBC’s What Einstein and Bill Gates Teach Us About Time Travel in May 2017, “Most physicists now working would bet against the possibility of time travel, not merely because of the practical difficulties of generating the necessary conditions to allow it, but also because of the implications of time travel if it becomes possible.”
So how could wormholes help the Doctor travel in this Universe? Imagine we create a wormhole. As we have said, a wormhole is a region of space-time that’s warped. That is, a “shortcut” in space and time through which to travel. The trouble is, according to science fact, the Doctor would not be able to travel back in time to a date before the wormhole was created. For example, if the Doctor managed to create a wormhole on April 1, 1999, he or she wouldn’t be able to go back in time before 1999. So, in order to travel back further, some splendid Time Lord in the distant past would have had to conjure up a wormhole to get the whole thing going.
Here are two wormhole recipes for the Doctor. Recipe one: the Doctor takes a dash of exotic matter. He makes sure this matter is made up of particles that have antigravity properties. He pops them into the throat of a wormhole. (Note: he avoids black holes, which are one-way journeys to oblivion. The Doctor’s wormhole should have two mouths, an exit and an entrance. His challenge will be to keep the wormhole’s throat open by using a force opposed to gravity, kind of antigravity, if you like.) Geronimo! The wormhole throat stops imploding, and the Doctor has made a wormhole. The stuff of sci-fi has become fact.
Recipe two: the Doctor manages to make a wormhole that has two mouths side by side. He plonks one of the mouths into the TARDIS. (It should be easy enough to take a wormhole mouth into the TARDIS, as wormholes are areas of extremely warped space with huge gravity fields, which would be attracted to the TARDIS if he could convince the wormhole that the TARDIS was really massive.) Then, when the TARDIS goes worming away at great speeds, the time-frame of the TARDIS mouth has slowed down a lot, compared to the mouth of the wormhole he’s left behind (that’s because “moving clocks run slow,” according to Einstein, as we know!). Now, if the TARDIS brings the traveling mouth of the wormhole back to the other mouth, let’s say ten years later, then the TARDIS mouth would have effectively jumped ten years into the future!