What counts as evidence in science?

Recently a young earth creationist asked me if I had looked at the evidence for a young earth and I said that I had and did not find any of it to be credible evidence for a young earth. He challenged my assertion that there was no evidence for a young earth and said, "you should be ashamed of yourself ... to say there is NO evidence rather than you just disagree with the conclusions of the data." He then listed many things he thought were evidences for a young earth. The problem was his list of "evidence" didn't actually contain evidence. They were attacks on the theory of the old earth, or attempts at finding uncertainty in what we know. There was nothing there that hinted at positive evidence for a young earth.

So what counts as evidence in science and why did I say there was "NO evidence" for a young earth instead of saying that I disagree with how to interpret the evidence? For that we have to look at what was presented as evidence.

One thing usually presented as evidence for a young earth is the argument that there is massive uncertainty in radioisotope dating. One of the ways we date rocks and meteorites is by using the half-life of radioactive isotopes to find out when a rock first formed. This is the most direct evidence that we have for the age of the earth. This of course presents a problem to anyone insisting that the earth is only 6,000(-ish) years old.

Since this counts as strong evidence for an old earth this obstacle must be removed if a young earth is to be proved. Thus the need to call into question the reliability of radioisotope dating. It is true that if the earth really was 6,000 years old then our method of dating rock really is unreliable and cannot be used as evidence for an old earth.

Whether or not you accept the reliability of isotope dating does nothing to provide evidence for a young earth. Unreliable rock dating only removes evidence for an old earth, it does not create evidence for a young earth. Young earth creationists will have to argue persuasively that it is unreliable, but it also means they cannot then use it to argue for a young earth. Without radioisotope dating the age of the earth could just as well be 21 million years instead of 6,000 years.

This kind of argument is presented as "evidence" for a young earth, but it does not provide evidence, it only removes the evidence for an opposing theory. Additionally it does not provide an explanation for why radioisotope decay is the way it is, it only attempts to undermine the reliability of it.

This then gives us an example of what counts as evidence in science. For something to be evidence it must increase our confidence in a particular theory and not just introduce uncertainty into our understanding. Many of the arguments for a young earth are of a similar form and do not actually provide evidence. They only seek to decrease the certainty of the evidence for an opposing theory.

Sometimes it is necessary to argue that certain evidence is not as certain as we think it is, but for that to then count as evidence for a different theory the uncertainty must be resolved in favor of the other theory. The purpose of science is to increase our confidence in how we view the workings of the universe, thus for something to count as evidence it must decrease our uncertainty.