Tyson starts off with, and repeatedly brings us back to, a walk on the beach in 1802 on a clear, starry night in England. Astronomer William Herschel is taking is son, John, out for a father/son stroll, and talking, oddly enough for a man of science, about ghosts. Sure, there are ghosts, William tells his son. But not the kind in stories. William Hershel had the theory, since proven, that the stars are really suns in their own right, and their light has required eons to reach us, meaning we see them as they were when the light shining from them started its long journey to our planet and our eyes. Why, William said, some of them could already have burned out, meaning that we're seeing a star that was, as it was. In other words, the sky is full of ghosts.
Tyson will return to William Hershel and his son for two reasons: William is voiced by Patrick Stewart, and you don't waste that; also, William Hershel had theories about more than ghosts in the night sky. Hershel also theorized, correctly, that gravity worked as far out into the universe as they could 'see'. Or not see. And it gives us evidence for what we cannot personally witness, much like footprints tell us someone walked where we are before us. Or that a star in the constellation The Lion is actually two stars revolving around each other. BTW, Patrick Stewart makes William Hershel sound like the coolest dad ever.
Science class on the beach!
Since we know the speed of light (186,000 miles per fucking second), once we know approximately how far away a star, nebula or galaxy is, we can figure out old the light we see from it is. The moon- well, every time we see the moon, we're actually seeing it 1 second ago. Think of it as a astronomical time delay.
So what's a light-year? It's the distance light can travel in a year. Yep, 186,000 miles per fucking second, for a whole year. That's just under 6 fucking trillion miles. So when you hear a light-year, that's not a time measurement. Multiply that number by 6 fucking trillion, and that's how many miles away it is. Kind of see why we use light as the basis for measuring distance in space, right? Making it easy to say the number isn't the only reason.
The sun is eight light-minutes away, so we see the sun as it was 8 minutes ago. And speaking of our sun, we don't actually see it rise. I'm not just talking about the optical illusion that is the sun rise created by the Earth's rotation. When we see the sun over the horizon, we're actually seeing light from the sun bent by atmospheric gases enough that we will see the sun over the horizon before it actually is. Our horizon isn't really an edge, even though we perceive it as a line, as it's just the point where our planet arcs out of our sight.
In fact, there's a larger "horizon" that isn't an actual edge either- the cosmic horizon. Since our universe is only about 13.8 billion years old, we can only see objects that have been issuing light for 13.8 billion years. That means that we can only ever perceive light that has traveled 13.8 billion years to our planet. It's not an actual edge. There might be farther away objects, in fact there probably are, but their light hasn't had time to reach us. This 13.8 billion light-year radius extends equally from us in every direction, meaning that we're at the center of our known universe. But so is everything else. Every other object in the universe is at the center of it's own known universe. The cosmic horizon isn't real, it's a mathematical construct so we can explain the limits of what we can see. Or as Tyson says, "Space ends where time begins."
Tyson then goes on to demolish the whole young-earth theory, one of the few times he'll mention creationism in any way. The Crab Nebula is 6,500 light years away. That means that light from the Crab Nebula has taken about 6,500 years to reach us. If young earth creationism were true, or even close to it, the Crab Nebula would be at or near our cosmic horizon. But it's actually pretty close to us in the list of things we can see. We can 'see' to the center of our own galaxy, which is about 30,000 light years away. And we can see past our own galaxy, to other galaxies much farther away. The oldest galaxy we've seen: 13.4 billion light years away. That light has been travelling 2 million times longer than outright biblical creation believers think the universe has existed.
Light is more than a timekeeper. It also creates the highest speed known in the universe. In fact, nothing can be faster than light. How do we know? Tyson turns to a friendly motorcyclist for help. Unlike a ball thrown from a person standing still or a person running, we have measured the speed of light and realize that it always moves at the same speed. Whether the light comes from something we perceive as moving or still, or moving quickly or moving slowly, the light coming from that object will always travel at the same speed. Always. You can't add the velocity of the object it's coming from. Light will never exceed the standard speed of light. Which is a reason it's so handy for measuring distances- it will never vary or mess up our distances.
Light always travels in style
For every other object, we must measure its speed relative to something we think is stationary. Police measure your car's speed relative to a laser they're holding still, or a camera mounted to a still traffic light. The moon is revolving around an Earth that's stationary relative to it. The Earth moves around a Sun that is stationary to it. The Sun moves through a galaxy that's stationary relative to the Sun. And our galaxy is literally bounced around the universe from the pull of other galaxies' gravity. And gravity makes our galaxy go round. Literally.
William Hershel named his son John after John Michell, who first theorized that a star could be invisible, totally dark, emitting no light, but still exist. He called them 'dark stars', but we know them as black holes. The gravity of the collapsed star in a dense, 64km diameter form, is so high that not even light can escape them. We can only know of them by the effect their insanely high gravity has on stuff around them. We sort of discovered the first one, Cygnus X-1, by using x-rays. It's hard to know for sure, because we sure aren't sending anything there, since we can't be sure it will even be able to broadcast a radio signal (traveling at the speed of light) back. This point of know return, where you won't be able to leave the black hole's gravity, is called the event horizon. Relativity predicts their existence. And we have reason to believe there's one at the center of our own galaxy, creating our galaxy's spin. No one can even really know what it would be like to cross the event horizon, only that approaching it would completely stretch out time, making things appear to go more slowly. In other words, it would screw with our notions of past and present.
Your vacation at a black hole would literally last forever
John Herschel himself went on to become a pioneer in photography, which is really capturing light on a receptive surface, and transforming the negative image to a positive image that will last as long as the photograph, or it's original negative are preserved. In other words, using light, we can bring the past to us. Just like the stars do.
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