How We Know The Earth Is Ancient

in astronomy we talk about billions of years like it's no big deal but how can we be sure about time scale so far beyond the capacity for human intuition our discovery of what we now call a deep time is very recent as recent as our discovery of the true special vastness of our universe and it came as scientists tried to measure the age of the earth what they found was as shocking and humbling as anything ever seen through the telescope how long has the world been here the question is surely as old as human civilization and perhaps older the earliest written record we have of a cosmogony an origin story for the cosmos came from the Sumerians followed soon by the Babylonians there were floods dismemberments of gods all that good stuff that was later adopted by other traditions but the main thing was this notion that the world had a beginning the early Jews adopted this idea and in turn was picked up by the early Christians and the idea of a single unique creation event became a cornerstone of some of the earliest religious texts like the book of Genesis the Genesis story was so influential that many people used it to try to figure out the age of the world probably the most famous guess came from the Irish official James Ashley in the early part of the 17th century I shall looked not only at the Bible but at hundreds of other ancient texts trying to align the different and often conflicting histories that they presented uh sure eventually concluded that the world began at 6:00 p.m. on Saturday October 22nd 4004 BC and oddly precise prediction given the source material and obviously a bit lower than the value given to us by science the French naturalist George Louie LeClair Comte de Beaufort was among the first to use what we call scientific methods to calculate the age of the earth publishing his result in 1778 it was ingenious really he assumed the earth started out as a ball of molten rock which subsequently cooled down to its current temperature reforms spent six years measuring the cooling rates of materials in his lab and then calculated that the earth was seventy four thousand eight hundred and thirty two years old and yes that's also a weirdly precise number both uh sure and before a docked half a point for excessive significant figures before premise was clever if flawed his seventy five thousand ish years was too small and we now know why and I'll come back to that at around the same time as before we're staring at warm months of iron the Scottish geologist James Hutton wandered Britain pondering the ages of its rock formations he became convinced that they were formed when molten rock pushed through the crust from Earth's interior this directly gave us igneous rocks like granite and basalt while sedimentary layers resulted when these materials eroded and were deposited on ocean floors to be later pushed up again in an endless cycle Patton's biggest breakthrough was that he reasoned that these processes were driven by the same forces operating in the world today so-called uniformitarianism but those processes were excruciatingly Lee slow and so harden realized the earth must be unthinkably old we can thank Hutton and his 1788 book the theory of the earth for opening scientific eyes to the possibility of an ancient planet and the idea of what we now call deep time the initial temporal vertigo was shocking Hutton's collaborator John Playfair put it well the mind seemed to grow giddy by looking so far into the abyss of time Hutton didn't propose a beginning for the earth instead he assumed a series of infinite cycles this was also the notion of the great German philosopher Immanuel Kant a few decades earlier Kent wasn't afraid to throw out some numbers based on pure speculation to quote there has mayhap flown past a series of millions of years and centuries etc etc creation is not the work of a moment by the way Kant was also the first to have speculated about the of galaxies beyond the Milky Way Island universes as he called them deep time and deep space we'd known since Copernicus and Galileo that is just one planet among several in our solar system astronomers now swear by the Copernican principle that Earth is not in a privileged position in the universe and so the laws of physics also work the same everywhere Hutton's uniformitarianism is a sort of temporal Copernican principle for geology the idea was further developed and popularized by another Scottish geologist Charles Lyell in his principles of geology published in the 1830s while talks about millions of years of geological processes he speculates on a true beginning for the earth but doesn't try to date it instead he imagines that it must be vastly older than geology can yet see and he draws an astronomical analogy that the universe must be vastly larger than what was visible at the time Lyle's work was incredibly influential and not least to a young scientist named Charles Darwin Darwin read Lyles book during his fame as a voyage on the a TMS Beagle Lyle's ideas gave Darwin the millions of years he needed for his theory of evolution and it's painfully low mechanism of natural selection but Darwin was also into geology in his Origin of Species he estimates a minimum age for the earth based on the erosion timescale for a chalk formation in southern England his figure of 300 million years was actually too high for that formation but we now know much lower than the true age of the planet of course geology and evolutionary biology are now intimately connected we can trace the progress of evolution by mapping the fossil record to the geological clock the ordering of the appearance of ancient species is found when we date the rocks in which their fossils are found we've now found fossils as old as 3.5 billion years but to understand how we can possibly know that age we have to turn from geology to physics the discovery of radioactivity by Marie Curie and others in the eighteen my teas would open up a new world within the atom and yield new tools for probing vast stretches of time Ernest Rutherford discovered that certain elements released energy from radioactive decay at an ever decreasing rate and that some of those elements would slowly leak that energy over thousands or even millions of years we now know that radioactive decay is responsible for keeping the interior of the earth hot and explains why earth is much older than before 75 thousand year estimate based on the passive cooling of a ball of molten rock but the discovery of radioactivity also gave us our most accurate way to figure out the age of chunks of Earth through radiometric dating unstable atomic nuclei decay into lighter nuclei by splitting or by ejecting particles the rate of decay is expressed in terms of half-life which is the amount of time for a given radioactive nucleus to have a 50% chance of decaying or equivalently the amount of time it takes for half of a large number of radioactive nuclei to decay in principle if you know how much of the stuff there was to start with you can figure out how long the radioactive material has been decaying figuring out the initial content is typically impossible at least directly instead there are some clever workarounds perhaps the most well-known example is carbon dating in this case we're interested in the decay of carbon-14 that's the version or isotope of carbon with eight neutrons it's radioactive and decays with a half-life of 5730 nice a' topes are stable and so much more abundant carbon-14 exists on the surface of the earth because it's produced when cosmic rays hit nitrogen in the atmosphere resulting in a constant proportion of c14 within the atmospheric co2 the carbon is incorporated then from the atmosphere into living organisms by photosynthesis but when the plant or whatever ate the plant die is the c14 content gradually drops by measuring the current c14 content relative to the stable C 12 and C 13 contents the age of once living material including fossils can be determined but this is only accurate to around 10 half-lives or 50,000 years more useful for measuring the age of the earth is uranium-lead dating uranium decays on much longer timescales 710 million years for the u-235 isotope and 4.5 billion years for u-238 we've both decaying into different isotopes of lead you can figure out how much uranium a given sample had to start with by looking at the proportion of uranium to lead this only works if you can be sure that no lead was in the sample to start with and in some cases you can be sure some crystals like zircon tend to incorporate uranium atoms into their crystal structure when forming while at the same time repelling lead so when he led that you find in those crystals came from uranium decay by looking at the ratios of each type of uranium to the corresponding isotope of lead you get two independent ages which will match up if the crystal has not lost any of its lead but the comparison of the two ratios in something called a Concordia diagram allows you to correct for any lead loss the other cool thing about this technique is that the different half-lives of each uranium isotope means this radiometric technique is useful between hundreds of millions of years to many billions of years by the 1920s British geologist Arthur Holmes would declare that the earth was between 1.6 to 3 billion years old based on his radiometric dating this was around the same time that astronomers proved that Immanuel Kant Island universes were indeed other galaxies many millions of light years away the world simultaneously got a lot older and the universe a lot bigger older rocks were discovered pushing back the earth further and further but beyond a few billion years it starts to get tricky there aren't many patches of land left from way back then most of the original crust has been recycled into the mantle one of the few remaining OG patches is in Western Australia their circle crystals have been found as old as 4.4 billion years based on uranium lead radiometric dating but to go beyond that date we have to look beyond the earth we believe that the moon formed at the same time as the earth both coalescing after a giant planetary impact in the early solar system now the moon is tectonically inactive so rocks on its servicenow were there when it formed the Apollo missions brought back lunar specimens that have been radiometrically dated to 4.5 billion years that jibes with our measure for the age of the solar system nearly 4.6 billion years we get the same number from radiometric dating of solar system meteorites and also from our calculations of the age of the Sun I should add that science took its time getting to these large numbers for the age of the world the Mayan Long Count calendar includes cycles of 63,000 years Hindu tradition has a cyclic cosmology with nested epochs lasting millions of years in fact a single day of Brahma is 4.32 billion years oddly close to the age of the earth although it looks like we survived at least one day of Brahma and the last long count transitions so I'd say we're good for now so the picture has come together there's consistency in that 4.5 billion each year age across several independent measures that gives us some confidence in the results the confidence in the number may be justified but perhaps we've become a bit too comfortable with it these days we stare down at the unfathomable gulf of the past and shrug and we forget that the entire span of human civilisation is the width of a single hair on our heads compared to the stratified depths of the Grand Canyon not that our ancient earth cares it's revolved around the Sun 4.5 billion times through deep space and of deep time and it'll do the same again into what I guess you could call deep space time bring the comments from my apartment once again the New York Cove in nineteen locked down cannot be allowed to interfere with the important work of talking about black holes and last episode we went all the way through the black hole to talk about the speculative parallel universe on the other side your own speculations were impressive a few of you asked whether the Big Bang could be thought of as a white hole well the answer is that there are similarities but they're probably not the same first the similarities the Big Bang is a singularity it's the point of infinite density when you rewind the expansion of the universe so there's zero distance between all points in the universe from that infinitely dense point all of this energy and space expanded from the point and time flows away from that point the difference is that in the Big Bang the singularity was comprised of the entire universe there were infinite points at zero distance it encompassed all of space and so the Big Bang happened everywhere at once and it didn't expand into anything the white hole on the other hand would be an actual point within a larger universe and so it's expanding into something this is the stock response based on a standard mathematical description of the Big Bang but this probably also deserves a lot more depth and its own episode Clem Cao asks why some sources place the einstein-rosen bridge at the top of the Penrose diagram leading to a new white hole instead of to the side of the Penrose diagram as we saw in that episode well those diagrams that you're thinking of a Penrose diagrams of a rotating or a Kerr black hole the original einstein-rosen wormhole was for a non-rotating ausfahrt shield black hole in the Kerr black hole the central singularity is a ring and passing through the ring and back out appears to transport you to the interior of a white hole that ejects you into a separate region of space time that can be interpreted as a new universe and yeah you don't need superluminal speeds to do that now we'll also explore the detailed nature of wormholes and the interior of the Kerr black hole in their own episodes well I really get the best episode ideas from your comments a few of you asked about the painting behind me I'll quote canook monkey 13 who finds it utterly fascinating what is it who made it is it unique well actually it is an original but don't worry I don't earn nearly enough to cover my apartment in giant gold leaf paintings I'm lucky I can afford the wall behind it actually this was painted by my incredibly talented partner Behar and no I don't just date her for the art but it's a big plus Neal Kyle and Cory came to the comment section to say hi from Fraser Kane's channel where I did a live stream last Monday it was a fun conversation and worth a listen but even more worthwhile checking out the rest of phrases amazing Channel oh and you'll get to see another corner of my apartment now is a zaps - requested a full tour maybe after a serious cleanup right now it's a maximally extended space filled solution I was actually amazed at how many people notice that acronym in the last episode but there was an even more brilliant black hole acronym joke from serif from the spinning black holes comments panic at the inner most stable circular orbit I have high hopes you guys will get that joke you

Channel: PBS Space Time

Views: 788,149

Rating: 4.9003167 out of 5

Keywords: Space, Outer Space, Physics, Astrophysics, Quantum Mechanics, Space Physics, PBS, Space Time, Time, PBS Space Time, Matt O’Dowd, Astrobiology, Einstein, Einsteinian Physics, General Relativity, Special Relativity, Dark Energy, Dark Matter, Black Holes, The Universe, Math, Science Fiction, Calculus, Maths, Holographic Universe, Holographic Principle, Rare Earth, Anthropic Principle, Weak Anthropic Principle, Strong Anthropic Principle

Id: vAgiHreswj0

Length: 17min 24sec (1044 seconds)

Published: Tue Apr 07 2020