There's a common notion floating around, due to Karl Popper, that scientific theories are characterized by the fact that they are falsifiable. The idea is that it is never possible to verify a scientific theory (i.e. the sun always comes up) because one day it might not happen. But it is possible that the sun might not come up some day, and then the theory is falsified. It must then be rejected, and replaced with something more complicated.
Now, let me confess right away that I have not gotten this idea by reading any of Popper's writings. It is an idea which has been popularized in the scientific community. You see, everyone knows what Popper said without having read any of it ourselves. It could be that if I actually read Popper's books, my idea of what he said would be falsified. So let me confine myself in this post to discussing Popperism as commonly understood.
If a theory is unfalsifiable (that is, if no experiment you could possibly perform would rule it out, then according to Popperism it is not a scientific theory. Among those who subscribe to Scientism, this is usually assumed to be A BAD THING™. (The way some people talk, if a theory is unfalsifiable, that means it is false!)
People often characterize bogus pseudoscientific ideas as unfalsifiable, because of the tendency of people who believe in them not to subject them to rigorous scrutiny. But this is clearly an oversimplification. True, there is such a thing as mystical Woo-Woo from which no definable predictions can be made, either because the ideas are not precise enough or because they don't relate to any actually observable phenomena. But many psuedoscientific ideas, such as homeopathy, reflexology, or astrology, can be tested experimentally, it's just that the people who believe in them don't like the results when people do!) I've heard people refer to Young Earth Creationism (YEC) as unfalsifiable. I think their reasoning must be the following:
1. YEC is unscientific and wrong.
2. I've been taught that when ideas are unscientific, the reason is because they are unfalsifiable.
3. Therefore, YEC is unfalsifiable.
In fact, though, the real problem with YEC is that it IS falsifiable, and in fact has been falsified many time over. If the universe were created about 6,000 years ago and we have to get all of the layers of fossils and rock from a single planet-wide Flood about 4,500 years ago, then there are a gazillion problems with observation. It contradicts the results of almost every branch of science which tells us anything about the past. (Adding bizarre extra ideas, like God created the earth with fossils in it in order to trick us into believing in evolution, may make YEC unfalsifiable, but it might be better to characterize this as pigheaded refusal to accept reasonable falsification.)
[Fun fact: if you interpret all of the genealogies in Genesis as being literal, with no gaps—which of course I don't—then it follows that when Abraham was born, all of his patrilineal ancestors were still alive, back to the tenth generation (Noah)! (This is using the Masoretic Hebrew text that omits Cainan, who is included in the Septuagint Genesis and Luke.)]
All right, digression over.
Clearly there is something right about the idea that theories ought to be falsifiable, yet not confirmable with certainty. Major scientific theories usually deal with generalities: they make predictions for a large (perhaps infinite) number of different situations. Normally, it is not possible to verify them in all respects, because even if it works well in many cases, it could always be an approximation to something else.
On the other hand, I think there are some scientific ideas which are verifiable but not falsifiable. Here's an example:
Ring Hypothesis: Somewhere in this universe or another, there exists a planet with a ring around it.
I submit to you that: 1) our observation of Saturn verifies the Ring Hypothesis, 2) when scientists verify a proposition by looking through a scientific instrument, that counts as Science, and 3) no possible observation could have falsified the Ring Hypothesis. (Even restricting to the Milky Way, eliminating planets with rings would be a tall order, impossible with current technology.) Therefore, there are scientific propositions which are verifiable but not falsifiable.
On the other hand, even if an experiment "falsifies" a theory, it could be that the experiment rather than the theory is wrong. As Einstein once said "Never accept an experiment until it is confirmed by theory". This witticism may seem to turn science on its head, but nevertheless it has a bit of truth to it. A while back, there was an experimental observation which seemed to suggest that neutrinos travel faster than light. Soon there were many papers on the arxiv trying to explain the anomaly. But it turned out, not surprisingly, that there was an error in the measuring devices. Usually, when a well-tested theory is in conflict with an experiment, and the anomaly has no particularly good theoretical explanation, it is the experiment which is wrong. Not always, but usually.
What this means is that we need a more flexible set of ideas in order to discuss falsification and verification. In particular, we ought to accept that falsification and verification can come in degrees—observations can make an idea more or less probable, without reducing the probability to exactly 0 or 1. The accumulation of enough experimental data against a theory should make you reject it, but it may be able to withstand one or two anomalous measurements.
The quick answer is that one ought to use Bayes' Theorem instead. This is a general rule for updating beliefs, taking into account both our prior expectations and observation. This goes not just for Science, but also for everything else. The only thing that makes Science special is that, due to a number of special circumstances, the process of testing through observation is particularly easy to do.
Even though falsification is not the best way to think about Science, it still works pretty well in many cases. In a later post, I hope to explain the connection between Bayes' Theorem and falsification. Usually we should expect good theories of the universe to be falsifiable, but in certain situations they don't have to be. Bayes' Theorem can be used to understand both the general rule, and why there are exceptions.