I call it the Great M&M experiment, but you can also do it flipping coins or anything else.M&M’s are delicious, and only have an “M” printed on one side. With small classes I’ll do 30, with large classes maybe 10.Teaching about radioactive decay and geochronology has its challenges.
I show both the results for each step (average and standard deviation), and the “theoretical” relationship. But what is important I think is to demonstrate where the exponential decay curve comes from.When I used this many candies, I wasn’t surprised at how well it worked, however since this run I’ve started using fewer and fewer total candies. It isn’t magical or made up, but is the natural result in a system that is governed by probability.It is the same concept, for example, that explains why Las Vegas exists, and why the house always wins.We typically bombard them with concepts like “half-life,” which are often intended to simplify things, but in my experience rarely do.So it you take a Dixie cup full of them and turn it over on a lab bench, about half of them will land with the “M” up, and about half will land with the “M” down. With really large classes I use a variation that uses pennies, I’ll explain that later**. Have them turn the cup over onto a desk or bench, making sure not to dump candy on the floor.
This isn’t news, but it turns out that if you have enough M&M’s, you can easily create a decay curve. I usually have them put down paper to they can eat things when they are done. If an M&M lands with the “M” side up, we say it has decayed, and we remove it to a second Dixie cup. Count the number of candies that didn’t decay and record that number. Place the candies that didn’t decay back into the tossing cup, and repeat steps 2-4 until all of the candies have decayed. Compile the numbers up front, counting the number, or percent, of the total candies left undecayed after each step. These are the results from the largest experiment I ever ran, where 40 students each had 100 M&M’s.Even for the first investigation, there was a possibility of using radiocarbon dating to determine the age of the linen from which the shroud was woven.The size of the sample then required, however, was ~500cm, which would clearly have resulted in an unacceptable amount of damage, and it was not until the development in the 1970s of small gas-counters and accelerator-mass-spectrometry techniques (AMS), requiring samples of only a few square centimetres, that radiocarbon dating of the shroud became a real possibility. The shroud was separated from the backing cloth along its bottom left-hand edge and a strip (~10 mm x 70 mm) was cut from just above the place where a sample was previously removed in 1973 for examination.Many students think that the concept of a half-life is goofy, and is something that has been imposed upon a decay system.I’ve started taking a different approach, one that tries to explain the basic concept using something they all understand and then transitions to the complex systems geoscientists deal with.The results provide conclusive evidence that the linen of the Shroud of Turin is mediaeval.