Cosmos: A Spacetime Odyssey, episode 4, A Sky Full of Ghosts

Logo for the television series Cosmos: A Spacetime Odyssey. Fox Entertainment

In this fourth episode from the series Cosmos: A Spacetime Odyssey, host Neil deGrasse Tyson presents the concept that looking at stars in the night sky is actually looking backward in time at light which left those stars years before.

Dr. Tyson frames the explanation through the story of William Herschel (1738 - 1822), a visionary scientist who was one of the first to realize that the stars were actually time machines, allowing us to look into the distant past.

The animation of Herschel is voiced by British actor Patrick Stewart, best known for his role as Captain Jean-Luc Picard on Star Trek: The Next Generation. Stewart's dramatic voice lends a certain gravitas to the story about Herschel discussing the mysteries of the universe with his son, John Herschel.

After explaining the basic concept that light takes time to travel to us, Tyson introduces the concept of the light-year, the astronomical unit of distance that represents the distance traveled by light in a year. Using this concept, he then walks the viewers through some notable astronomical landmarks ... and discusses that light from the Crab Nebula took about 6,000 years to reach us, and suggests that light from anything beyond it would be unable to reach us if the universe were only 6,000 years old. In my opinion, this is the single largest piece of scientific evidence in opposition to the concept known as young earth creationism, and Cosmos: A Spacetime Odyssey is daring in its effort to take this non-scientific worldview head on.

Returning to the story of Herschel, we learn that he discovered the existence of binary star systems, showing that even the most distant stars were bound together by the forces of gravity. But the problem was that Isaac Newton, upon developing the theory of universal gravitation, was unable to actually determine the mechanism by which gravity worked.

The narrative at this point fast forwards to the early twentieth century, when young Albert Einstein, on a vacation in Italy, began to think about the problem of gravity ... or, at first, about travelling at the speed of light. What, he wondered, would the world look like when one was moving this quickly? This line of reasoning eventually became formalized into his theory of special relativity. Gravity comes back into the story through Einstein's theory of general relativity.

This convergence of light and gravity leads Dr. Tyson to a detailed discussion of black holes, along with various speculations about what might happen inside a black hole. He should be commended for making it clear that this portion of the discussion is highly speculative, though that doesn't prevent him from discussing concepts such as wormholes along the way. (Nor should it! That is, after all, the whole reason to give such a disclaimer!)

One final interesting fact gets dropped along the way to the finale of the episode: Herschel's young son, John, goes on to invent the process of photography, which itself is a form of freezing time in place through light. (Or, at the very least, he has a large hand in the early development of the field.)

As with the previous episodes of Cosmos: A Spacetime Odyssey, this one does an exceptional job of balancing the scientific details with the historical narrative. I've given only the barest outlines above, though the links should help fill in the gaps, but the narrative of the show really does a good job of giving a sense about the scope of the phenomena that Dr. Tyson is discussing.

If you missed "A Sky Full of Ghosts," all episodes of Cosmos: A Spacetime Odyssey are available for free streaming online through the Cosmos website and also through Hulu ... at least at the time this is written.  The entire series is also available on blu-ray and DVD.