The very first episode of Cosmos: A Personal Voyage discussed the Great Library of Alexandria and the tragedy of its destruction, so it is fitting that the final part of Cosmos: A Spacetime Odyssey should return there, to bookend the series. Tyson's description of the library complements Sagan's without very much repetition.
It acts as an introduction to the chief topic of this programme, how the limits of human knowledge are pushed back over time, but also how they can be lost again if the knowledge is not guarded carefully.
Tyson chooses a superb example to demonstrate the pushing back of the limits of human knowledge by beginning with Victor Hess discovering cosmic rays in 1912. He worked out that they were stronger when higher up in Earth's atmosphere, but didn't know why this was or what they were.
Later, in the 1930s, Fritz Zwicky proposed they came from supernovae, as well as being the individual who theorised neutron stars, gravitational lensing and dark matter.
Later still, his theories were confirmed, by Vera Rubin studying distant galaxies and observing that they were rotating at different speeds than those predicted by the laws of gravity as they were then understood.
Dark matter and dark energy make up 95% of the universe but, as Tyson explains, dark matter and dark energy are "code words" for these things because we still don't know what they really are. This is the frontier of scientific knowledge in astronomy as it stands today. Tyson emphasises that scientists are OK with not knowing more - yet - because admitting ignorance is simply better than pretending to knowledge, a (not very) veiled shot against the opponents of science.
From the frontiers of science, Tyson begins to wrap up the series by moving to the frontiers of human exploration. The Voyager 1 and Voyager 2 space probes will travel through interstellar space for a billion years, carrying with them the gold discs with messages from the Earth. Science is the language the inscription is written in, with the (universally constant) frequency of the hydrogen atom as the base unit. Music and greetings from humans (and whales) form the "sounds of Earth."
Voyager 1 also took the photograph known as the "pale blue dot" of the Earth as seen from beyond the orbit of Neptune, 6 billion km away, which was the idea of Carl Sagan and gave rise to one of his most moving and impassioned speeches, repeated here almost in its entirety.
A billion years ago the Earth was populated by single-celled organisms in the ocean and nothing lived on the land, so who knows how unimaginably different the Earth will be a billion years in the future? Tyson ties together the hopeful message of his Cosmos series in a final appeal for science to always belong to everyone, to prevent misuse by an elite and a recurrence of the fate of the Great Library, and that way to ensure the long-term prospects for humanity in a vast, uncaring universe.
In the final scenes Neil deGrasse Tyson appears on the beach location used by Carl Sagan in some of his most memorable Cosmos scenes. The Ship of the Imagination then flies off without him, symbolic of him bequeathing it to the future, or to us.
"Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.
The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity – in all this vastness – there is no hint that help will come from elsewhere to save us from ourselves.
The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand. It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we've ever known."
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