Cosmos: A Spacetime Odyssey, Episode 13, “Unafraid of the Dark”
For their final episode, the authors of a show usually focus on their core message, on the one element that lies at the heart of what they want to say. The new Cosmos is no exception, and for the series finale, what they chose to emphasize was intellectual humility: the fact of our fallibility and the immensity of how much we still don’t know. In the vastness of the cosmos, there are many wonders yet to be revealed; but it’s only through science, through patient curiosity and the unflinching willingness to put every idea to the test, that we’ll discover them.
We began where the original Cosmos began, in a digital reimagining of the Library of Alexandria, though lusher and more realistic (it reminded me of Agora). Alexandria was a government that recognized the value of truth, spending money to copy every scroll on arriving ships. But the city’s philosophers considered this knowledge the purview of the elite and disdained the idea of sharing it with the masses, which may have meant the library had fewer defenders than it otherwise would have when it was put to the torch.
Although the knowledge of the ancients was laughably incomplete from our viewpoint, we shouldn’t congratulate ourselves too soon. There are plenty of things we don’t know either, which was brought home by a scene on the discovery of the high-energy cosmic rays that rain down on our atmosphere. Tyson introduces us to the astronomer Fritz Zwicky, who coined the term “supernova” and suggested that cosmic rays were produced by these colossal stellar deaths – which is now accepted to be part of the answer, although there are some even higher-energy cosmic rays whose origin is still a mystery to us. (Zwicky did get some things wrong, in particular his belief that cosmic redshifts were due to “tired light“, rather than the expansion of the universe.)
At times, it felt like the writers were reading my mind. In my review of the previous episode, I was concerned that the new Cosmos hadn’t mentioned major discoveries like dark matter and dark energy, but this episode answered that in a very satisfying way. Zwicky, again, was also one of the first to notice that the mass of galaxy clusters seemed insufficient to account for their motion. But it was a later scientist, Vera Rubin, who did the work to nail this conclusion down. The galaxies are rotating too fast; if the matter we could see was all there is, they’d fly apart. The most widely accepted explanation of this is that all galaxies, including our own, are surrounded by vast haloes of some invisible substance that only interacts through gravity.
This episode also addressed one of my earlier criticisms, that they’d left out the cosmic distance ladder, with an excellent discussion of type Ia supernovae and why they can be used as standard candles. Tyson shows how this led to the astonishing discovery that the expansion of the universe is accelerating, driven by a still-mysterious force called dark energy.
The terms “dark matter” and “dark energy” are, so far, only placeholders for our ignorance. We don’t yet know what these things are. What we do know is that all the luminous matter – all the stars, all the planets and moons, all the gas and dust, everything astronomers can see in their telescopes – accounts for only about 5% of the total mass-energy of the observable universe. In a haunting analogy, Tyson compared our situation to someone standing on a beach at night, watching whitecaps crash on the shore, and concluding that the phosphorescent foam on the waves was all there is. But what we don’t realize, we beings of spindrift, is that an entire ocean remains to be discovered.
In the emptiness of interstellar space, these records will survive for at least a billion years. Even so, the odds that anyone will ever find them are infinitesimal. But their existence says something noble about the species that created them: what we think of ourselves, and what we one day hope to be.
The final segment, which in retrospect was the only possible choice, went straight for the heartstrings: a screen-filling image of Earth, steadily receding and dwindling as the other planets whirl by, until at last it becomes the “pale blue dot” image taken by Voyager, while all the while we hear Carl Sagan reading the words of his greatest speech: a meditation on how small and fragile the Earth is, and how this knowledge should inspire us to rise above petty provincialism and treasure our planetary home. If you could listen to it without getting choked up, you’re a stronger person than I am.
Although this finale went a long way toward filling the gaps left by earlier episodes, when all’s said and done, there were some things I was sorry not to see. The series didn’t mention the revolution in the detection of extrasolar planets, for example, or the accomplishment of sequencing the Neanderthal genome, or what we’ve learned about the geologic history of Mars. It devoted much time to the danger of climate change, but didn’t pause to point out how lamentably long it’s been since human beings have walked on the Moon, much less made any more ambitious plans.
Still, I have to admit that any popular science show, even if it went on forever, would have to have omissions. (If for no other reason, new discoveries would happen faster than new episodes could be filmed.) I prefer to focus on what a monumental achievement this series was: a bold, dazzling and unapologetic encomium in praise of science, aired in prime time on broadcast television. I wish this sort of thing weren’t so rare, but they did exceptionally well with the time they had. In every sense, it was a worthy successor to Carl Sagan, and a tangible proof that the candle he lit is still shining out in the dark.
Other posts in this series: