Cosmos: A Spacetime Odyssey, Episode 4, “A Sky Full of Ghosts”
Any science show faced with time constraints has to strike a balance between breadth and depth. The process of science is always laborious and not always interesting; but if you don’t show it at all, it leaves the impression that the results just dropped out of the sky like a religious revelation. The second episode of Cosmos felt overstuffed with detail; the third was just right. This one, in true Goldilocks fashion, felt a little sparse.
It started out promisingly, with Neil deGrasse Tyson explaining cosmic distance scales and how, when we look out into the universe, we’re looking backwards in time. Starting from the moon, about one light-second away, to the sun (eight light-minutes), the farther planets (a few light-hours) and the nearest stars (a few light-years), all the way out to astronomical objects millions of light-years away. The glimpses of what the Earth looked like when some of that ancient light set out was a nice touch.
Along the way, Tyson takes another well-deserved poke at creationism, pointing out that the assumption of a 6,000-year old universe would “extinguish most of the light” we can see in the cosmos. He illustrated this by showing a widescreen view of the Milky Way, and then dimming out everything except a small sphere that would be the extent of our view. (He didn’t mention that you could, of course, get around this problem by postulating a trickster god who deceptively filled the sky with ghost images of things that never happened.)
This was good as far as it goes, but I would have appreciated an explanation of how we know how far away astronomical objects are. This would have been an ideal place to discuss the cosmic distance ladder: how we can directly measure the distance to nearer stars using parallax, like the way your finger seems to move back and forth in front of your face as you look at it with first one eye and then the other; how from there we can work our way up to the “standard candles”, objects of known intrinsic brightness whose distance can therefore be calculated by measuring their apparent brightness; and how we can leap from there to the measurement of cosmic redshifts.This episode’s historic figures were William Herschel, who deepened our understanding of gravity by observing the dance of binary stars orbiting each other, his friend John Michell, who remarkably predicted the existence of “dark stars” (what we now call black holes) long before Einstein, and his son John Herschel, one of the inventors of photography. However, a mention that his sister Caroline Herschel was a notable astronomer in her own right wouldn’t have gone amiss here. I recognize that there are historical reasons why famous scientists were mostly white men, but a series like this should try to counteract that bias whenever possible.
I did like the scene in New York City which illustrated what would happen if the strength of gravity were changed, and how at hundreds of thousands of g, the gravity crushes even fire hydrants into puddles. Another potent visual was the binary system Cygnus X-1: in visible light, it looks like a star whipping around an invisible companion, but in X-ray light, the companion shows up as a blazingly bright accretion disc, the radiation created by heated gas swirling in a vortex as it’s sucked out of this universe.
By contrast, the dramatic dive into a black hole added little or nothing scientifically. As Tyson said, everything from that point onward was pure speculation, but quite a bit of screen time was spent flying through psychedelic depictions of other universes, to no real benefit. (People on Twitter were upset that Tyson didn’t mention “spaghettification“, the phenomenon he’s famous for popularizing.) It ended up with a good point – the hypothesis that black holes give rise to baby universes, and that our own universe may have been born from a black hole in another cosmos – but it took longer than it should have to get there.
Other posts in this series: