[Music] [Music] the earth our small rocky home floating in the vastness of space is thought to be 4.5 4 billion years old having formed nearly 10 billion years after the universe itself came to be in the Big Bang but how can we a species of organism which has existed for a mere two hundred thousand years possibly have the intellect and evidence to have come to such a specific conclusion it didn't happen overnight the story of how we figured out the age of our planet is a long one with many twists and turns controversy and debate but this journey illustrates the amazing power of persistence and perseverance and the true potential of the human mind nearly 2,400 years ago Greek philosopher Aristotle theorized that the earth was eternal having existed forever many years later during the 16th and 17th centuries biblical references were analyzed in prominent theologists and philosophers suggested a drastically different estimate of 6,000 years in fact James Ussher a 17th century Irish Archbishop took this a step further and declared definitively that the earth had formed on Saturday October 22nd 4004 BC at 6 p.m. just to be clear there's no place in the Bible where an age of the earth is offered the estimate of 6,000 years suggested by usher and others is based entirely on the genealogical tables in the book of Genesis assuming they were a complete record he deduced that four thousand and four years had passed between the creation of the earth and the birth of Christ with this began the ongoing debate between those who look to science to come to an answer and those who relied solely on the Bible and its teachings around the same time that usher was analyzing the Bible to calculate an accurate age natural ists like Nicholas steno and William Smith were digging through a different kind of record the rock record steno examined layers or strata within rock outcrops and the fossilized remains of living things found within those layers he proposed that the deeper a layer the older it is an idea that would become known as superposition he noted that the layers are originally laid down horizontally and that any strata that have been altered must have been exposed to forces at some point long after their original formation Smith added that if widely separated layers contained the same fossils those layers were likely the same age Smith's nephew John Phillips later estimated in age of the earth at nearly 100 million years based largely on the work of steno and Smith at the same time other scientists were making their own radical claims this included naturalist Mikhail Lomonosov who suggested that Earth had been created entirely separate from the rest of the universe and that this occurred several hundred thousand years before the universe itself came into existence for the first time laboratory experiments were conducted to attempt to answer the seemingly unanswerable question how old is the earth for example in 1779 George Louis Leclerc created a small globe that resembled earth in composition and then he measured its rate of cooling this led him to an estimated age of the earth of 75,000 years as you can see there was a wide range of ideas about how old our planet actually is well into the 1800s it was at this point that geologists like Charles Lyell and James Hutton proposed further theories about the evolution of our planet they specifically theorized that the changes that occurred on the planet were perpetual rocks were continuously forming eroding away and being redeposited these ideas about how both gradual and rapid changes have been occurring on earth throughout history eventually developed into an idea known as uniformitarianism which suggests that understanding the past is the key to understanding the present for example if we observe limestone forming at the bottom of a warm shallow sea today we can infer that all limestone found on earth likely formed in the very same way while certainly important in our evolving understanding of geology these ideas did not offer any further evidence or details about the Earth's actual age in 1862 physicist William Thompson published calculations that suggested in age of the earth at between 20 and 40 million years this was based on the amount of time he felt it would take for a fully molten mass to cool to its current temperatures unfortunately Thompson failed to acknowledge other factors which would impact how quickly the earth cooled down including things like radioactive decay and mantle convection shortly thereafter around the end of the 19th century John Jolley calculated the rate at which the oceans would have accumulated salt from the erosion of land materials and he determined that the oceans must be between 80 and 100 million years old unfortunately too many these estimates seem far too low particularly Darwinian biologists who believe that the process of evolution that resulted in the varied and diverse life forms on earth would have taken far longer to occur than was previously proposed in the early 1900's things changed drastically with the discovery of radioactivity the amount of heat given off as naturally occurring radioactive isotopes decay would dramatically alter the rate at which the earth would have cooled after its formation the idea that this could have happened in a mere 100 million years now seemed impossible but even more important than the implications for the rate of cooling was the realization that these radioactive isotopes decay not randomly but at a predictable rate in a process which now would become known as radioactive decay because it was predictable scientists could determine the age of Iraq based on the relative amounts of the radioactive elements that it contains discovered and confirmed by multiple scientists including Ernest Rutherford Frederick Soddy and Bertram B Boltwood in the early 20th century radioactive decay and dating would become widely known as the key to our new understanding of the age of the earth this work however was not widely accepted at first and as such it did not progress until another scientist would revisit it geologist Arthur Holmes used radioactive lead dating to estimate the age of several rock samples to be more than 1.6 billion years old an estimate that was far older than had previously been suggested Holmes later published the age of the earth and introduction to geological ideas in which he presented an age range for the earth of 1.6 to 3 billion years despite its promise radioactive dating remained questionable in the eyes of the larger scientific community it wasn't until 1956 that an age of 4.5 five billion years very close to today's accepted age was determined by Clair Cameron Patterson who used uranium lead isotope dating on several meteorites meteorites are solid chunks of debris from an object like a comet an asteroid or meteorite they originate in outer space and are able to survive the passage through the Earth's atmosphere reaching the surface of the earth now this may sound confusing why would we look at meteorites which originated out in the solar system and not here on earth to determine the age of the earth well it turns out that these meteorites offer a much more pure unaltered sample than the rocks found here terrestrial rocks have undergone differentiation into the core and the mantle in the crust and have then undergone a long history of mixing and unmixing by processes like plate tectonics and weathering any or all of which may have adversely affected our ability to effectively date them meteorites represent the primitive material from which the original solar system was formed the earth is widely believed to have formed at the same time as the solar system so dating the earth and the solar system are one in the same an estimated age of 4.5 4 billion years has since been confirmed dozens of times over based on the analysis of hundreds of rock and meteorite samples collected on the surface of our planet additional confirmation has come from analysis of the rocks collected by the Apollo astronauts who explored the surface of the Moon that brings us to today though it's not likely the end of the story as with so many scientific mysteries the question of the age of the Earth will never be unequivocally answered we will continue to explore developing more accurate methods for dating objects uncovering more and more clues along the way we can look forward to not only new answers but most certainly new questions as well such is the nature of science [Music]