I'm sure you've heard of radiation before - the emission of energy and its transmission through space or matter. When radiation has enough energy, it can break the chemical bonds within molecules and atoms and release particles at very high speed. This is known as ionising radiation and it can be quite nasty to organic life like plants and animals.
One way to measure radiation (or radioactive decay more specifically) is by its half-life - the time it takes for half the energy of some unstable substance to break down and be released as radiation.
The half-life of oxygen is infinity - it is stable and doesn't normally decay.
The half life of Uranium-238 is nearly 4.5 billion years while the isotopes it decays into, Thorium-230 and Radium-226, have half-lives of 1000's of years - hence the concern about nuclear waste.
And the half-life of Oganesson-293 (first synthisised in 2002) is just 5 milliseconds - half of it decays every 1/200th of a second.
So what does this have to do with the price of tea in China - or even critical thinking for that matter? Quite a lot actually.
The growth of knowledge is compounding
In 1950, there were about 60,000 different academic journals in publication. Today there are over 1,000,000. The amount of new knowledge produced is growing exponentially at roughly 5% per year meaning the amount of new knowledge in the world doubles about every 15 years.
Some of this new knowledge is novel - it concerns things we didn't know about before. But much of it represents the reversal of old knowledge.
Remember Pluto (the celestial body, not the cartoon dog, nor the philosopher)? When I went to school it was a planet. Now it isn't.
Things that we once thought were true, turn out to be not so true after all. And sometimes, they even become true again.
The faster new knowledge is created, the faster old knowledge becomes out-dated.
This is what has become known as the half-life of knowledge - the time it takes for half of a body of facts to be superseded or found to be untrue.
This phenomenon occurs across all domains of knowledge - from anthropology to zoology and from engineering to psychology. Especially psychology!
Like radioactive decay, we don't know when or which particular fact (or atom) will decay. But we do know how long it will take half of them to decay.
Textbooks may age but expert teaching is timeless
The average person lives for about 80 years. The half-life of knowledge is about a quarter of that. By the time you die, only a half of a half of a half of a half (<7%) of the things that were true at your birth will still be true when you pass away.
Knowledge isn't static. It isn't some eternal collection of facts that can be chiseled into stone. Rather, knowledge is highly dynamic as what is 'true' now will likely change in the future.
The unfortunate consequence of humanity's rapidly expanding knowledge is this - most of what we learn over our lifetimes will turn out to be wrong. That also means much of the content that we teach will turn out to be wrong too.
This would be a major problem if we viewed teaching as merely transferring knowledge to students and our role as teachers as "sages on the stage" rather that "guides on the side".
Sequencing and present content matter is the job of a textbook - teaching demands much more than this. As educators, we need to teach our students how to think, not what to think.
The pedagogical expertise of teaching involves understanding how students think about subject matter, and of the interaction of the content with the student's present needs and capabilities.
So is Pluto a planet? Maybe. But by helping students understand the competing definitions of different astronomers, and having them create and apply their own criteria for planethood, we can answer more confidently no (and at the same time teach them to think more critically).
Are carbs bad for weightloss? Maybe. But by helping students understand the nature of scientific inquiry, the provisional nature of most scientific truths, and how to survey scholarly literature to find meta-analyses on numerous randomised controlled experiments, we can answer more confidently ... maybe.
By teaching our students how to think critically, how to effectively analyse and evaluate claims, how to reason collaboratively, and the importance of the values and virtues of inquiry, we can make sure that their education ages better than we do.
Arbesman, S. (2013). The half-life of facts: Why everything we know has an expiration date. Penguin.
Ioannidis, J. (2005) Why Most Published Research Findings Are False. PLoS Med 2(8): e124.