I feel like I should have posted about this before, but I hadn't thought of it. This one is going to have some opinions in it that you might object to, for which I'll apologize if they do offend you, but they're relevant and required.
To start the explanation, let's make sure you know what a Half-Life is. It's most commonly used as a scientific term in relation to radioactive materials. There is no way to know when a single atom of a radioactive element, like plutonium, will become inert, safe. On the other hand though, we know about how long it will take for half the atoms in a lump to become safe. The Half-Life is basically the time period until 50% of the plutonium will be safe. A second time period later, only 25% of the total plutonium will remain, then 12.5%, and so on.
They also use this in medications, how long it will take for a drug to be absorbed into your body and how long for it to leave your system can also be measured in half-lives. It's how they know how often you need to take medication, so instead of just taking an entire bottle at once and being good forever, you take one pill 3 times a day or something.
Now that you understand a half-life at least a bit, we'll move on to the half-life of facts.
The actual number here depends on a lot of things, such as what you're counting in the set of 'knowledge'. The people doing a study about this basically set things into one of three different groups:
1. Things that are always true and have been true and are extremely unlikely to change. For example, humans have 5 fingers. This isn't likely to suddenly change anytime soon.
2. Things that are always changing, like what time it is or what you're doing. Those are 'facts' that are always going to change.
3. The middle ground. Things that we haven't always known but we think we know now. Like, DNA. We knew it existed, but not what shape, then we did. We knew what it looked like, but for a while, we didn't know how many chromosomes we had. Then we assumed that all life had the same, but it's different. We assume a lot because we think things match even if we shouldn't, like the number of chromosomes in humans and pigs, or because we make rules of the universe that are proved wrong when we discover something new-- like the belief that the world was flat, until we proved it was round.
Facts in the first group are unlikely to change and their half life is...never.
Facts in the second are rarely correct multiple times, so the half life is pretty small.
Facts in the third group...are what I'm mostly talking about with this.
A little bit of a subject change.
In the 1990's, a few scholars guessed that knowledge had doubled in the last 25 years, and at least some of those things meant that what they'd known before was wrong. By their count, it took 50 years before that to get to the same point, so it was growing faster.
By current studies, it depends on what subject you're looking into and how advanced in it you're looking, but facts have a half-life of about 5 years before half of the information will be wrong. This is part of why textbooks have to be reprinted. It's not just about new things being discovered, it's also about old things being changed or discovered to be false.
Before you start freaking out, this 'half-life' doesn't mean as much as it might sound. It means that half of the facts have changed, but there is no guarantee that it's the half that you remember, the half that is important right now, or even the half that you learned in the first place. You can read the dictionary and the half that could change in a decade could be 10% no longer being used, 10% new words being added, 10% new information about the history of words, and 20% changing definitions by adding slang meanings or new forms of words. You still know a lot of the words, you picked up a lot of the new ones in life, and you can still figure out what's going on, but it's shifted a lot. Then again, a lot of what might have changed could be a bunch of words you never bothered to read about leaving the book, so you might not notice the change at all.
What it boils down to is that things change and what is fact one day can easily be wrong the next because humanity as a whole is always adding to our body of knowledge.
Here's my opinion, so feel free to skip over it. Some idiots think that, because knowledge is shifting and changes, it means that Science can be ignored because it doesn't know what it's talking about. Things may change, but that doesn't mean you should ignore it all. Especially not the parts that haven't been proven wrong yet. Fine, ignore the new stuff until it's been studied a bit more, a lot of people wait for double-checking, but don't assume that just because a new study has proven something else wrong, means that everything is wrong. Do not assume that because they got things wrong about what family group a flower is in, that they also got it wrong that having it in a tea doesn't help your headaches any more than a placebo. They might not have known that dinosaurs had feathers, but they definitely knew they existed and a long time before humanity. A random dude might have decided that humanity started in the Caucus mountains, thus Caucasian, but that doesn't mean that it's wrong that they started in Africa. Especially when the new idea comes with a lot of new proof.
Feel free to doubt things that don't have a lot of proof yet, but please do not assume that everything is wrong just because science changes. They actually admit it, so it's one heck of a lot better than a lot of other parts of human life where they don't want to admit to being wrong or would rather double-down on a wrong idea.
Please.
Opinion part two; Your elders deserve respect and consideration, but this is one of the things that causes problems between generations because not everything that your grandparents were taught is the same for you. There is alot of new information that schools are trying to teach these days than when your grandparents went to school. There is a lot more research to consider, a lot more study done, and so on. I'm not saying they're always wrong, but don't trust everything they say. And, if you're going to tell them about this, make sure they understand that this isn't saying that everything they learned is bad now, but that what they were taught as fact might not be the same right now. When they say that there is no evidence for 'new-fangled' ideas like evolution or ecology or climate change, that might be because there was none when they looked 30 years ago, or it could just be that they didn't care to look. Either way, there is. Please look for it. I'm not saying you should do it to prove your grandparents wrong, but do it so you don't look like an idiot when you parrot their lines back to someone that might know different.
The main reason I wanted to bring this up here is that things change. I said yesterday that Morepork is the only known owl in New Zealand other than the Barn Owls that migrated there, but there might be another that pops up and I wouldn't be surprised. Some of the articles said Morepork and Boobook owls are separate, while others said they were one species, so that's something that changed. Scientists are learning all sorts of new things, so I will be wrong sometimes. I might trust the wrong source, I might read an old article, I might misunderstand what I read. That doesn't mean I'm not trying to get you the right information, but it'll happen. If it does, please tell me so I can fix it. I just also want you to know it happens, which is actually part of why I haven't been going into exacting detail about a lot of things. Those details are what are most likely to change.
Thanks for reading, I'm sorry for the wall of text here, but I wanted to make sure it gets said because knowledge shifts and changes and I don't like when people don't seem to understand that. Either way, I'll see you again soon, and I hope this wasn't too hard to read.
Sources;
National University of Singapore-- The Half-Life of Knowledge
FS-- Half Life of Knowledge
FS-- The Knowledge Project Podcast-- Samuel Arbesman, Future-proofing Your Knowledge
PubMed Central-- Does Knowledge Have A Half Life?
Times Higher Education-- Knowledge Has A Half-life
CIO- Thriving in a World Of Knowledge Half-Life
TUV Reinland Academy-- Is there a half-life of Knowledge?
