Yesterday, as happens occasionally, someone asked me what evidence I had seen that the Earth is billions of years old – my first answer, of course, is “you should really ask a geologist or astronomer that question, not a physicist”. But, keeping in mind that this is not my primary area of expertise, there are a couple helpful, brief things I can say from my own experience, at least about “old stuff” if not about “billions of years old stuff”, and here they are.
1. It has been my experience that young-earth creationist organizations seem to get the facts on dating objects half-right in a potentially dishonest way. It is quite true, and they do well to point out, that any kind of dating of old objects relies on some kind of assumption. When you date a tree by tree rings, you assume one ring = one year, and sometimes that assumption might be wrong (maybe a volcano exploded one year and made June really cold and dim, so that year got two rings, say). When you try to date something buried in sediment, you might have some assumption about the rate at which sediment accumulates. When you carbon-date a plant remnant, you assume that cosmic ray bombardment of the upper atmosphere has been roughly constant over time, keeping the amount of radioactive carbon in the carbon dioxide in the atmosphere roughly constant over time. (Or, if you think the amount has changed, you try to get a measure for that by some other method.) By themselves, at some point in the process any dating method is going to have some sort of assumption that we cannot *prove* is correct (since we lack a time machine), and these organizations will correctly point that out. What they usually neglect to point out, however, is that sometimes multiple dating methods with different assumptions can be used to date ancient objects, and when different methods give roughly the same date, that makes a good case for the assumptions themselves being correct. It also is not “proof”, but it’s very important data and they seem to not mention this fact when they’re talking about dating.
For example, here is an Answers in Genesis page that actually does a very good job describing how carbon dating works, but neglects to mention that objects dated by multiple methods lend support to the validity of that dating. Here is a nice and interesting article originally from Nature that talks about the intersection of four different ways of dating objects – recorded human history, tree rings, sediment deposition, and carbon dating.
2. In a sense, it’s hard to quickly provide evidence for an old earth, because “old earth” is really a paradigmatic position. It’s something that explains an enormous amount of data, and while you probably could explain this piece of data, or that piece of data, in some different way, “old earth” is the paradigm that seems to do the best job of holding all the data together. To give a simpler example – we have this idea that all the continents once formed a super continent called Pangaea, around 200 million years ago. It is interesting that when the idea was first proposed, we didn’t have any way to explain how the continents actually could have moved, but it was proposed anyway because of several other pieces of evidence. At first order, the continents appear to fit together in a reasonable way. There are bands of fossils and geologic features that seem to stretch across continents that are, today, separated by oceans, but which form nice contiguous groupings when the continents are connected. And some of these fossils show tropical plant life in places where tropical plant life can’t exist today, but which become a tropical location in the Pangaea picture. And then add to that our modern understanding of plate tectonics where we know, and can actually measure, the slow movement of the continents. So you take all that data and put it together, and the Pangaea explanation makes a lot of sense – you could try to explain individual pieces of data in other ways, and people did try. (There was a proposal of some kind of land bridge across the Atlantic connecting South America and Africa – one reason that explanation now seems wrong is because we know that continental crust is less dense than oceanic crust, so it shouldn’t just sink and disappear.) So the “Pangaea” paradigm holds all the data together – and so it is for a lot of the data supporting an old earth. It’s not “one thing”, it’s everything together. (This is also why it is difficult for non-experts, who don’t usually have a good grasp of the “everything”, to punch holes in these paradigms.)
3. One final comment – probably some people are inclined to say “but what about Genesis 1?”. I was going to write a little bit here about relativistic Physics and how the rate at which time passes for an observer (as measured by another observer) depends upon the relative observational situations of the two observers. But I actually don’t think that is especially relevant to the conversation, it’s just fun to talk about – more relevant would be something like this talk by Michael Heiser, where he essentially argues that how truth is communicated necessarily depends, to at least some degree, on cultural assumptions and pre-knowledge. And God did not see the need to try to make the ancient Israelites into a totally different people before revealing stuff about himself to them. And that’s OK. And property understood, that does not threaten (and may actually enhance) your view of Biblical inerrancy, which Heiser himself would identify as a proponent of.
That is all! I await correction or clarification from any disagreeing geologists or astronomers.