Cosmos: A Spacetime Odyssey Episode 4: A Sky Full of Ghosts — This week we learn about William Herschel, gravity, spacetime, black holes, and throw shade at those who think the universe is only 6,500 years old.On this week’s episode of Cosmos: A Spacetime Odyssey, Neil deGrasse Tyson highlights the work of William Herschel. Herschel was the first to articulate the concept of spacetime as it relates to light traveling through the universe. This concept, in conjunction with Newton’s laws of gravity and motion, paved the way for other thinkers (i.e. Einstein) to revolutionize our understand of…everything.
Stars are ghosts. Herschel’s articulation of space/time/light involves stars. By the time light reaches Earth, the star that gave off that light may have already died. Here is a less morbid example. It takes eight minutes for light from the sun to reach Earth.1 Therefore, the sun we see in the sky is actually the sun of eight minutes ago. Neptune, the planet farthest from us in the solar system, is about four light-hours from us. So if you spot Neptune in a telescope, you are seeing Neptune on a four-hour delay.
The Library is Open. NdGT throws major shade toward those who believe the universe is only 6,500 years old by using the above explanation. If the above is true, we would not be able to see anything beyond the Crab Nebula, which is approximately 6,500 lightyears away. Thanks to the Hubble telescope, we have a record of light that is approximately 13.4 billion years old. Considering the projected date of the Big Bang is just shy of 14 billion, that’s pretty dang impressive.
Everything is in motion. The concept of ghost stars dovetails with another concept introduced by Herschel—”invisible partners” or binary stars. The gravity of large objects interacting with one another means that everything is in orbit of something else. Since everything is in motion, measuring cosmic distances (and the effects of things like light and gravity within those distances) can only be done with one object relative to another. Enter Einstein and his theory of relativity.
Black Holes (why it gotta be black?) One of Herschel’s contemporaries was a man by the name of John Mitchell. He theorized a star so massive that its gravity would suck in everything, including light. Though he did not coin the term, this concept became commonly known as a Black Hole. NdGT says it isn’t a “mythic vacuum” cleaner dead-set on absorbing the universe, but it is recommended that you keep your distance. Einstein’s theory of relativity demonstrates that space and time are connected, which means that the gravity of a black hole theoretically has a level of distortion which could upend our current conception of physics. This is where the time travel/wormhole concept in sci-fi originates.
Time Machines. NdGT describes a telescope as a time machine, as we now know any object viewed appears on cosmic delay.2 Photography also functions as a time machine of sorts, using light to capture an instance. And the man who coined the term photography: John Herschel, William’s son. Oooo, cosmic.
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