5X5 Episode 73

The Age of the Earth
S: This is the SGU 5x5 and tonight we are talking about the age of the earth. Scientists currently estimate the age of the earth to be around 4.55 billion years, but how do we know how old the earth is and how certain are we of that date?

E: That's been a question that scientists and philosophers and lots of people have asked since the, well, beginning of civilisation almost. And for quite a long time it was believed, or widely held, that the earth was perhaps only a few thousand years old, as various books of the Bible tell you. Right up until the 1700s, that was the preferred view until the age of science and reason started to come along, which started to collect data about the earth and we realised well its quite a bit older than that. Estimates ranged in the 1700s from as low as, say, 75,000 years old up to several billions of years old. So there was a whole wide variety of opinions at that point as we were collecting the data. And once we got into the 1800s, we started to get that refined a little bit. We knew it was at least hundreds of millions of years old, perhaps billions of years old but it really wasn't until the turn of the twentieth century, when become available to us and we started to really get a much better sense of the age of the earth.

B: The age of the earth is like so much in science&mdash;there are so many independent lines of evidence that converge on roughly the same value. If the accepted age were wrong by a significant amount, much of geology, physics and other sciences would need drastic revisions. Some of our best evidence, though, includes different methods of radiometric dating, as you mentioned, Evan. Radiometric dating rocks found on earth and from space. This depends on the decay of unstable isotopes into more stable isotopes over vast stretches of time. By looking at the ratios of these parent isotopes compared to the daughter isotopes, we can determine how much time it took to produce them. The oldest earth rocks are about 3.8 billion years old, and this is the lower limit, though, since plate tectonics pretty much have destroyed any rocks older than that. Now if we make the reasonable assumption that the earth formed at the time our solar system did, then dating meteorites provides the most direct method of dating the earth. Similar dating techniques used on meteorites from different locations of the solar system all produce consistent ages of the solar system and earth at about 4.55 billion years.

S: That's right. Interestingly, there is one particular meteorite, the, that is considered to be the most reliable single meteorite to date because of its composition. It allows for more precision in determining the ratios of the parent and the daughter particles that&mdash;essentially, they use lead dating on this and this one comes up with a date of 4.53 to 4.58 billion years. And as you say, Bob, that is based on the assumption that when the disc, the solar disc of material around the sun formed and things solidified, that's when the planets formed, so meteorites formed at the same time the earth was forming.

R: Of course, not everybody dates the earth to billions of years. There are, of course, the Young-Earth Creationists, who decided to reject all science and evidence and instead get the age of the earth from the Bible by literally adding the lifetimes of people mentioned, adding those 7 days at the start and coming up to a figure of around 6,000 years. Although, some do give it a little bit of give and say could go up to 10,000 years old. Claims that are so ridiculous as to almost defy explanation as to how someone could believe something so silly, since literally all evidence shows that the earth is so very much older than that.

S: Yeah, they get into a lot of problems when they have to explain things like the Grand Canyon.

R: Right, the Grand Canyon is, according to them, the Grand Canyon was formed by a flood, which has been showed again and again to be a completely untenable theory. A lot of processes that would take millions of years, including evolution, of course&mdash;they come up with new and more ridiculous explanations for.

J: You know an interesting thing that creationists say is that God created the earth to actually look like it was created a much longer time ago, including inserting dinosaur bones into the soil. But it's an untestable theory.

S: Right. It's the unfalsifiable hypotheses so they have just exited the realm of science. Some try to make more sciencey types of arguments for a young earth. Two common arguments that are put forward for a young earth: one is that at the rate at which dust is accumulating on the moon over billions of years, there should be a hundred or hundreds of feet of lunar dust and then the Apollo landers would have sunk down into this dust when they landed on the moon. But of course, they just get the math wrong and they use erroneous figures in order to come up with that. The other argument is that the rate at which salt is collecting in the oceans is such that over millions of years, far too much salt would have accumulated. But again, they are getting the math wrong. They are neglecting geological processes that are removing salt from the oceans. We are pretty close to steady state, in terms of not necessarily exactly year to year, but the amount of salt in time going into the oceans and coming out is roughly in steady state. And of course, we would have noticed if salt were accumulating in the ocean over the last couple of hundred years at the rate that those creationists say it is accumulating in order to come up with their figures. So, we can say empirically that they are wrong without having to just extrapolate from what we can observe today. So those types of arguments for the earth having to be very young all collapse under close inspection.