 There must be records somewhere of satellites that have fallen to Earth, and I really, really hope that they're called orbituaries. While I was traveling in Italy recently, I got to visit the Galileo Science Museum in Florence. They have a ton of cool stuff there, including some well-crafted apparatus that Galileo had built for physics demos, this incredible model of the geocentric universe for the Medici's, and three of Galileo's fingers. There was also this telescope that Galileo famously used to observe the orbits of moons about Jupiter. That observation directly conflicted with Aristotelian and Ptolemaic cosmology, which everyone, including the Pope, pretty much accepted as gospel at that point. Galileo believed, quite loudly, that the moon's orbits lend support to a previously unpopular theory published 50 years prior by Nicholas Copernicus, that the Earth was just another planet, and that it went around the sun, not the other way around. Copernicus' theory, which got Galileo into a bunch of trouble with the Catholic Church, wasn't complete by any means. It still used perfectly circular orbits instead of ellipses, and it still required the use of epicycles, these weird little loopy suborbits. But it wasn't just heresy or incompleteness that made the Copernican model of the universe so unpopular in academic circles, it's also vastly unintuitive. Yeah, Earth's planethood is pretty obvious now that we've taken selfies from space, but right when telescopes were first becoming popular, that's a hard sell. I mean, it really doesn't look like we're moving, right? It doesn't look more like the sun and all the stars just sort of spin around us. Don't we call it sunrise and sunset? And the Earth doesn't really look like these little pinpoints of light, at least not from here. Equating those and this, that's quite a mental leap. Copernicus made that leap and provided some testable mathematics to back up that startling idea that our position in the universe wasn't special. Rather than just going along with Aristotle, who thought that the Earth was clearly the most important thing in the cosmos, the anchor about which the sun, the moon, and the stars all spun, he suggested that maybe it's just another thing, like those things in the sky. This general assumption that our observations of the universe aren't made from a privilege or unique position has been instrumental to the development of science as a discipline. It spawned several sub-theories in different contexts, like the cosmological principle or the mediocrity principle, but in general, it's known as the Copernican principle. It crops up in a ton of different places. Edward and Hubble observed that all of the stars that we can see appear to be moving away from us. Either our solar system is special and everything else in the universe is trying to get away from it, or everything in the universe, including us, is moving away from everything else. Maybe we're not special. We can observe natural processes and environmental pressures shaping the development of living things. Either humans are unique and didn't form according to those processes, or we share the same origin as all other species. Maybe we're not special. That's pretty cool. But there's also an interesting practical application of the principle, which borrows heavily from Bayesian statistics. It was first developed by J. Richard Gott and described in his book, Time Travel in Einstein's universe, which is actually a pretty fantastic read. Gott's version of the Copernican principle is used to estimate the remaining lifespan of anything, and it's actually pretty simple. You start with the same basic assumption only applied to time. Assume that the period in which you are observing something is not unique or privileged. Let's take me, for example. I know I just said that the universe doesn't revolve around me, but let's just run with it. Assume that the period in which you are observing me is not special. So what does that mean? Well if I were to die tomorrow, and this was the last that anyone saw of me, you'd be seeing a pretty unique period in my life right now, somewhere in the last 1%. So let's apply the Copernican principle and say that this period in my life that you're seeing right now isn't special. In fact, let's say that this probably isn't the last 5% of my life. I mean, I hope. Let's also say that this probably isn't the first 5% of my life that you're seeing right now. But this isn't some weird winding up period before I ascend to demigod status and then outlive the sun and the stars, because that would also be pretty weird. So assuming that I've been around 29 years and assuming that this isn't the first nor the last 5% of my life that you're seeing, we can apply the Copernican principle, crunch some numbers, and get a remaining life estimate. On the low side, if this is about the 94% of my life you're seeing now, then I could be expected to live another 1.5 years. And the way that I drive, that's totally feasible. On the high side, if this is just the 6% of my life that you're seeing right now, I'll live to be 580, given that seems a little ridiculous, but with the advances of medical technology, if I start exercising and eating right, who knows? So using the Copernican principle in this weirdly specific way, we can bookend what kind of hypothetical longevity I can expect. Yes, the range is large, but that's only because I picked a relatively conservative estimate of what counts as not special. One which would only theoretically be wrong 10% of the time. If I wanted to, I could take a gamble and say that you're probably not seeing me in the first or last 10% of my life. If I wanted to be more conservative, I could say that you're probably not seeing me in the first or last 1%. Now, hang on a second. Before you go off estimating how much longer the human race has to live, or how much longer your marriage will last, there are some serious caveats to this method. First, and most importantly, the Copernican principle lifespan estimation depends a lot on the assumption that the period at which you're observing something is actually a random sample from its lifespan. And that could be hard to justify. I mean, it's pretty obvious that you shouldn't watch the grand opening of a business and then look at your watch and shout, you've got two minutes left. But frequently, judging that the period at which you're viewing something isn't special depends a lot on intuition. It's also a method that has a built-in margin of error. It's totally possible that a jet engine will flatten me while I'm filming and I was just unlucky enough to be in the last 0.1%. And then I wake up in a parallel dimension where I didn't actually die and I'm haunted by some entity that's dressed in a giant bunny costume. You know how this goes. So it's far from perfect. If you have any data to base an actual longevity estimate on, then you should probably use that. But it's an interesting little thought experiment. And if you don't have anything else to go on, it's actually not bad. Copernicus wasn't the first to think of a heliocentric solar system. And he was wrong about the circular orbits. But he did make a crucial advancement in thought that would be part of the ignition for a huge revolution in science. His theory raised the possibility that the entire universe operated on principles in which human beings were not the most important figures. That maybe we weren't the center of the universe, but merely working parts of it. That may seem intuitive in retrospect, but most paradigm shifts are. As someone who used to be a stupid teenager, I can tell you, realizing that you're not the center of the universe, is actually harder than it sounds. If you've got a minute, do a Copernican principle lifespan analysis on something that's lasted so far, and post it in the comments. Does it seem reasonable? Let me know what you think. Thank you very much for watching. Don't forget to subscribe, like, share, and don't stop thunking.