Fifty years of time travel
The Doctor, the charming scientist who travels through time and space, is the eponymous character in the British Broadcasting Corporation's cult classic program, Doctor Who. Originally intended to have an educational bent, Doctor Who is set in altered versions of the past and imagined versions of the future, allowing opportunities to teach history and science. It is the longest-running science fiction television series in the world, with 798 episodes in the 50 years since the airing of the pilot episode, "An Unearthly Child," on 23 November 1963.
Doctor Who is best described as science fantasy. Although classic themes from science fiction and speculative physics—such as parallel universes and time travel—recur through the storyline, full scientific explanations are frequently evaded.
The Doctor regenerates when critically injured. Each new incarnation possesses a distinct personality, quirks, and fashion sense. From left to right: the First Doctor (William Hartnell), Second (Patrick Troughton), Third (Jon Pertwee), Fourth (Tom Baker), Fifth (Peter Davison), Sixth (Colin Baker), Seventh (Sylvester McCoy), Eighth (Paul McGann), Ninth (Christopher Eccleston), Tenth (David Tennant), and Eleventh (Matt Smith). CREDIT: BBC
For example, while explaining the complexities of time travel during the episode "Blink" (original air date 9 June 2007), the Doctor begins promisingly with, “People assume that time is a strict progression of cause to effect, but actually from a nonlinear, nonsubjective viewpoint,” before abandoning science completely to end with, “it's more like a big ball of wibbly-wobbly, timey-wimey stuff.” And yet, between the narrowly averted catastrophes and teasingly nontechnical terminology, Doctor Who manages to include plenty of genuine science.
It would take—and indeed has taken—a book to tackle all the science embedded in 50 years of Doctor Who. In his survey, The Science of Doctor Who (2010), Paul Parsons locates numerous instances of scientific plausibility in the series. In August 2012 BBC America devoted an entire documentary to the science in the series. And earlier this month, the UK's BBC2 enlisted particle physicist and science popularizer Brian Cox for its own documentary as part of the lead-up to the 50th anniversary special, “The Day of the Doctor.”
This small article of celebration looks instead at the science in Doctor Who through prism of astronomy.
The Doctor has been played by 11 actors, each one portraying a new incarnation regenerated from its predecessor. Although the Doctor's core identity is unchanged by regeneration, his appearance, apparent age, and personality are noticeably transformed. Over the span of the series, the Doctor's views on cosmology have evolved as the science itself has advanced.
In "The Invasion of Time," (original air dates 4 February–11 March 1978), the Doctor considered what might have existed before the universe was created. But in "The Satan Pit" (original air date 10 June 2006), he listed as one of his guidelines for his traveling companions: “Rule 54: Anything can come from outside of the universe, but nothing can come from before the universe."
In "Castrovalva" (original air dates 4–12 January 1982), the Doctor is incapacitated by a regeneration gone awry. The TARDIS, his time-traveling spaceship disguised as a police box, has been hijacked by his nemesis and sent hurtling back to the first event that started it all. With dawning horror, and a small bit of astrophysical confusion, his companion Nyssa shouts, "The creation of the galaxy out of a huge inrush of hydrogen. We're heading straight into the biggest explosion in history!" Although wobbly on the details—the universe contains many galaxies, the initial plasma was a sea of particles combining into helium and lithium in the first three minutes, and the Big Bang was not so much an explosion as a sudden appearance of space—the general concept aligns with current cosmological theory and experiments.
An earlier episode, "Genesis of the Daleks" (original air dates 8 March–12 April 1975), casually uses the expansion of the universe, first reported by Erwin Hubble in 1929, as a rhetorical device. A messenger responds to the Doctor’s scold by sanctimoniously sneering, "We Time Lords transcended such simple mechanical devices when the universe was less than half its present size."
In "The Doctor and the Silurians," we learn that Earth’s original bipeds were intelligent reptiles. CREDIT: BBC
The beginning of the Universe isn’t the only deep-time event the Doctor observes. In "The Runaway Bride" (original air date Christmas 2006), the Doctor and the titular bride Donna travel back to the Hadean Eon to watch the accretion disk of the Solar System coalesce into proto-planetoids, including our Earth. The Doctor explains to Donna, “Eventually, gravity takes hold. Say, one big rock, heavier than the others, starts to pull other rocks towards it. All the dust and gas and elements get pulled in.” After watching, he puzzles, “But the question is, what was that first rock?” Here, the fantasy steps into the science: The original biggest rock pulling our planet together was no rock, but an alien spacecraft.
"Doctor Who and the Silurians" (original air date 31 January–14 March 1970) tells of a race of intelligent reptiles that evolved during the Hadean Eon. The Silurians buried themselves to hide from an enormous impactor, predicted to evaporate the atmosphere and destroy all life. Instead, as the Doctor explains, "millennia later, that small planet was drawn into the Earth's orbit and became the moon. Your catastrophe never happened.”
Most current theories of the Moon's formation would side with the Silurians, not the Doctor: The moon most likely came into being as a result of Earth's violent collision with a Mars-sized object and not as the result of Earth's capture of a Moon-sized object that left our planet unscathed. That said, even if they had buried themselves, the Silurians and all contemporaneous life would have been obliterated by the impact, which broke apart Earth. The only conceivable trace of pre-impact life would be in microfossils preserved in rocks that were ejected from the system entirely.
The Doctor returned to early Earth in the Archean Eon long after the passing of the Silurians to witness the start of life as we know it. In "City of Death" (original air dates 29 September–20 October 1979), the Doctor begins, as usual, with scientific description that conforms to current theories. Toying with sludge he finds there, The Doctor describes it as, "the amniotic fluid from which all life on Earth will spring, where the amino acids fuse to form minute cells. Cells which eventually evolve into vegetable and animal life."
But, like the methane, ammonia, and water vapor mixtures mixed in laboratory flasks by Stanley Miller in 1953, chemical ingredients alone are not enough. “There's no life in it yet," the Doctor observes. "It's waiting on a massive dose of radiation." But instead of Miller’s jolt of electricity to transform the mix into amino acids, the Doctor offers an alternative theory: The requisite radiation came from an exploding alien spaceship.
It would seem that quite a few aliens have difficulty navigating near our planet, since an exploding spaceship in the upper atmosphere during "Earthshock" (original air date 8–16 March 1982) is blamed for the mass extinction of the dinosaurs. When one of his companions questions how a single impact could cause so much devastation, the Doctor provides a textbook description, "The impact would have caused millions upon millions of tons of earth and rock to be fragmented and then hurled up into the atmosphere, enveloping the whole planet."
To the Doctor and his companions in "City of Death," the precursor to life of Earth is a thoroughly unimpressive black sludge. CREDIT: BBC
Asked in the episode if the massive dust storm would block out the Sun, he replies, “To such an extent that the surface temperature would have dropped radically.” His companion mournfully concludes, “Poor old dinosaur.” Although subject to heated academic debate, a major impact and the related atmospheric disruption is certainly a viable and popular theory to explain the mass extinctions (including dinosaurs) at the Cretaceous–Paleogene Boundary.
Other story lines have touched on the inevitable consequences of living so close to an active thermonuclear reactor, otherwise known as the Sun. As we have become better able to monitor the tumultuous nature of our nearest star, we have also become more worried about the impacts of massive solar storms. In the fictional future of "The Ark in Space" (original air date 25 January–15 February 1975), humanity is less concerned about the damaging impact of a solar flare on the power grid or other trappings of civilization than about the radiation itself. The TARDIS lands on a space station in orbit around the Earth. On it, they find humans in hibernation. A revived passenger Vira explains, “Our scientists calculated it would be five thousand years before the biosphere became viable again,” so the entire population fled the planet and went into suspended animation to wait out the catastrophe. Unfortunately, a system malfunction kept them in hibernation too long, aliens infiltrate the spaceship, and another adventure begins.
Given enough time, all the hydrogen in the Sun's core will fuse into helium, at which point the core will destabilize and contract, leaving the outer shell of hydrogen to expand and cool. Although current models cannot say if Earth will be consumed by the gaseous shell as our Sun transitions into a red giant, the commander of the evacuation spaceship in "The Ark" (original air date 5–26 March 1966) is more pessimistically confident. “In a short time," he laments, "[the Earth] will burn and be swallowed in the pull of the Sun.”
Later episodes prove him temporarily wrong. A future version of the UK's National Trust preserves the Earth with gravity manipulators to protect it from the expanding Sun, and even uses fanciful technology to reverse plate tectonics and preserve the classic 20th-century continental positions. The Doctor brings his new companion Rose on her first off-planetary adventure in "The End of the World" (original air date 2 April 2005) to watch the demise of the National Trust's preservation project. Gazing over the doomed planet, he tells her, “But now the money’s run out, nature takes over,” a truth applicable to science experiments of any scale.
In "The End of the World," the Doctor and Rose look on Earth in its restored 20th-century tectonic layout shortly before the planet is engulfed and destroyed by an expanding red giant, the Sun. CREDIT: BBC
From then on, according to astrophysics, the Sun will fuse all the helium in its core, transitioning from red giant to a white dwarf, then radiate its remaining energy away until only a cold black dwarf remains. The runaway expansion of our universe—flat or open—will keep expanding space itself, even accelerating the expansion with the aid of dark energy. The most recent cosmological experiments say that while our universe started with a bang, it will die a long, slow, cold "heat death." The second law of thermodynamics inexorably demands that entropy of a closed system must forever increase to a state of maximum possible disorder.
In "Logopolis" (original air dates 28 February–21 March 1981), the heat death of the universe is at a critical stage. The priest-like mathematicians of the planet Logopolis have been forestalling heat death by means of what they call “charged vacuum emboitments,” which circumvent the laws of thermodynamics. "We opened the system by creating voids into other universes," the system monitor explains. But even this solution effectively only creates a larger closed system of multiple universes, all driven by the relentless growth of entropy, and it, too, will eventually be overcome. Once that state of thermodynamic equilibrium is reached, the universe will have zero free energy to perform any further work. Light and life will be gone.
The far future—the era of a dying universe—is where the Doctor, his companion Martha, and stowaway Jack visit in "Utopia" (original air date 16 June 2007). Stars have burned out, galaxies have evaporated through stellar collisions or hungry black holes, and only cold, bare planets drift in the dark. Even so, the universe still has a lot of stored energy. In the episode, the Doctor witnesses the Degenerate Era of proton decay evaporating the last dead stars and cold planets, followed by Hawking evaporation transforming black holes into lonely particles.
In the past 50 years, Doctor Who has covered a lot of ground, both physically and metaphorically. As the series adventured across time and space, it has stayed remarkably true to its educational mission. When the science is explained, the details can be a bit, or even way, off. Yet the description of the black hole in "The Satan Pit" is astrophysically accurate.
As we humans keep learning, exploring, and experimenting, the boundaries of where science fact meets science fiction grows ever more interesting. Until then, in the words of the Doctor in "The Robots of Death" (original air date 29 January–19 February 1977), "To the rational mind nothing is inexplicable, only unexplained."