How do scientists think they know the age of the earth?
I don’t know where you got 8.6 billion from – the Earth is approximately 4.56 billion years old. We get that number from the dating of radioactive elements like uranium. They decay in a very predictable fashion, and there are a number of these elements on Earth and we’ve found them in meteors as well. Science works the same way every time. We don’t have to have been there to know what happened – just like we don’t need someone to have witnessed a murder to convict the murderer based on many lines of evidence. Carbon dating is only useful for things a few thousand years old because carbon has a relatively short half-life. Uranium on the other hand has a very long half-life, so statistically it’s much more useful for dating very old (billions of years old) objects.
Some of the better answers above list some of the ways to measure the age of things in our environment. The basic idea is that you hunt for some process that takes time to occur and that also leaves evidence behind that it happened. If you can recreate the process right now and measure how quickly or slowly it happens, then you can work backward from the historical evidence you find to estimate how long the process has been operating. There are some assumptions you must make, like figuring that the process worked the same way in the distant past as it does now, and that something else you aren’t taking account of didn’t foul up the evidence. So you need multiple ways of checking your work. When several different lines of inquiry lead to the same conclusion, you have good reason to believe the conclusion is true. A physical theory is not just an idea out of the blue — it’s like a great tapestry of interwoven ideas, experimental tests, and logical reasoning. That’s what gives science th
Radiocarbon dating, or carbon dating, is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 (14C) to determine the age of carbonaceous materials up to about 60,000 years.[1] Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years “Before Present” (BP), “Present” being defined as AD 1950. Such raw ages can be calibrated to give calendar dates. Modern geologists and geophysicists consider the age of the Earth to be around 4.54 billion years (4.54 × 109 years ± 1%).[1][2] This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples.
Radioactive dating with long-lived radioactive elements. Here’s a simplified explanation—the numbers are changed a bit to make it simpler, but the basic idea is the same. We know from laboratory experiments that the half-life of U235 is about a billion years, and the half-life of U238 is much longer. Suppose you have a little nugget of Uranium, and it’s initially half U235 and half U238. Then you wait, and watch it decay: 0 years: 0% lead 50% U235 50% U238 1 billion years: 26% lead 25% U235 49% U238 2 billion years: 39% lead 12% U235 48% U238 3 billion years: 46% lead 7% U235 47% U238 4 billion years: 51% lead 3% U235 46% U238 5 billion years: 54% lead 1% U235 45% U238 If you search all over the Earth for nuggets of Uranium, you find that many of them are 52% lead, 2% U235, and 46% U238. Conclusion: those nuggets were formed 4.5 billion years ago. There are other, similar dating methods that give similar results. Our sun, for example, acts like an ordinary star that has burned about
