I'm continuing my series on creation / evolution questions, where I explain why I'm genuinely unsure about how old the Earth and the universe are.
Let's just explore the idea of an Earth that was created suddenly, without a Big Bang or accretion disks or anything, and think about what it would be like. We need to do this so that we can test it as an idea against what we observe the Earth to be like, and see if they fit.
Suppose that Adam decided to dig down underneath the Garden of Eden. What would he see? Well, eventually I guess he'd see rock. And rocks on Earth are classified as igneous, sedimentary or metamorphic. But we observe all three of those being formed today, and all three therefore have inferred histories - we look at them and say "this rock used to be part of a volcano" or "this rock used to be a bit of sand at the bottom of an ocean" or something. In other words, if a geologist had been there looking at the rocks, he wouldn't have been able to tell that the Earth was new (well, not just by looking at the rocks).
What about trees in the Garden of Eden? If Adam had cut a tree down, would he have seen rings? I guess so. But again, when we see tree rings, we infer a history from that. What about horses' teeth? Did horses in the Garden of Eden have rings in their teeth? (I don't know much about horses, but apparently they have rings in their teeth - like trees rather than bulls' noses).
What about limestone? Limestone today is formed by lots of dead sea creatures (or their shells) getting squashed by huge pressure. Would there have been limestone on Earth when it was created? I don't see why not, but if there was, then a geologist there would infer that there had been lots of sea creatures millions of years before, which had died.
I don't think there's any way to escape the fact that if Earth was created suddenly in the last 20 000 years, then it probably in some sense had to have the appearance of age, even down to having the appearance of previous organisms having died.
That's really annoying in a way for trying to work out how old the earth is, of course, because it means that we need to be very careful with dating techniques.
Some people might well point out that this could be said to lead to (the unscientific and unfalsifiable) "Last Thursdayism" - the belief that the whole universe was created last Thursday, with the appearance of age and everyone having memories. But the difference is that there's no good reason to believe Last Thursdayism but some people say there is good reason to believe in a sudden creation of the universe during the last 20 000 years. I think they're probably exaggerating the evidence, but that's a different story...