Last Saturday I had a chat with Anthony from http://alienonamission.blogspot.com. He's a young earth creationist, putting a maximum age of the earth at about 10,000 years but believing the real figure to be closer to 7,000. He's a pleasant, polite and interesting chap and while we disagree profoundly on the fundamental nature of the planet we both inhabit I found I quite enjoyed the chat. He'll be making a podcast available at some point in the future (to be announced), but in the meantime I'll be running a series of articles on the age of the earth and how we know it. I'm going for shorter articles, hopefully one a day.
I'd like to thank Anthony for the opportunity. I'd also like to thank his friend Jim for buying the tea and biscuits.
Anyway, on to the article:
Lee Segall once said "A man with one watch knows what time it is; a man with two watches is never quite sure." Accurate, but not as profound as Segall hopes.
What of a man who carries three watches? Early oceanic navigators found themselves in this boat (pardon the pun) when navigation depended on accurate timekeeping to judge one's longitude. By having three timepieces you could identify any discrepancies and put your trust in the two agreeing clocks.
But why stop there? why not have a wall adorned with a variety of clocks, all from different manufacturers, all built on different mechanisms, and all agreeing? More is better when unanimity points towards accuracy, and each clock serves as an independent witness to the accuracy of the others. Some might only have a second hand, some might only measure the hour, but by taking a unified picture and confirming that they all agree we can be quite certain of the time. The navigator would be happy. The owner would be in perilous debt, and the poor cabin boy would be kept awake at night with all the ticking.
Thankfully these are nautical issues and do not trouble us greatly.
So, let's see what clocks can we consult when we wish to judge the age of the earth.
How long is a second? Stop contemplating your navel, the question was not designed to promote meditation. Basically, a second is defined in terms of the frequency at which an atom of cesium resonates at sea level. Sounds complicated, but gives us a clock that is accurate to one second every 138 million years. We now have very reliable methods of measuring time. And what of the past? Much of recorded history is replete with diaries, newspapers and dated documents of human construction. No-one quibbles over our ability to reliably chart the last millennium. But what will take us back further?
We all learn in school how tree rings are indicators of a tree's age. A new ring is added each year's growing season - good years are characterized by thick rings, poor years by narrower. This sounds very useful for judging the age of a single tree. We can even use core samples to judge the age of trees without having to cut them down, like Methuselah, a 4,842 year old bristlecone pine. But we can go further back. Each tree keeps in their rings a sort of diary, letting us know what years were good, what years were bad and what years were middling. We can compare these notes in much the same way we could compare diaries, and by obtaining enough samples in one area we can build on this shared record of good and bad years and travel back in time. My parents have a statue of Jesus carved from bog wood that predates the gospels by over a thousand years. This is small change. Using this science of dendrochronology we can form a great unbroken chain that travels back over 11,500 years.
The more astute among you will notice that 11,500 years subtracted from 4.54 billion years leaves a considerable amount of change. After this point we lack an unbroken record of completely preserved trunks. There were of course trees on this planet 12 thousand years ago and earlier - they just rather inconsiderately decided not to stay in the same spot in one long, unbroken chain of preserved wood. The advantage of dendrochronology is only not that it proves beyond doubt that the earth is older than 7,000 years, it is that it gives us a useful yardstick by which to measure other clocks.