This is a bit of an old wives’ tale.  I’ve found it repeated elsewhere as well: people going, “My mother said I should never put bananas in the fridge or I’m going to die!”  Basically, this is not true.  Bananas are not poisonous and they do get refrigerated along their journey from wherever they grow, tropical places to you. Bananas produce a gas called ethylene or ethene and this is used to ripen fruit. So if you’ve got some hard fruit and you want to ripen it up stick it in a bag with a ripe banana and it will ripen nice and quickly.

One thing that will happen with bananas in the freezer is that they will go black.  Bananas have really sensitive cells in their skin, and so get damaged very easily below about 12 degrees centigrade.  They release enzymes and this is what causes that black oxidation.  The banana inside will be fresh and lovely!

They key to carbon dating is that the carbon isn’t the carbon that’s been on Earth ever since the Earth was formed.  The carbon that’s in carbon dating is carbon that’s been newly made.  Where that comes from is when cosmic rays - high energy particles from the sun -  hit the Earth’s atmosphere they interact with atoms and send neutrons flying around.  when one of these neutrons hit a Nitrogen-14 (¹⁴N) atom, it knocks out a proton, and the ¹⁴N becomes Carbon-14 (¹⁴C).  This then circulates in the atmosphere but because this process is happening roughly at the same rate continuously the amount of carbon that’s in the atmosphere is roughly continuous.  Most of it ends up in the atmosphere as carbon dioxide so you have ¹⁴C carbon dioxide. Plants then pick that up in their process of photosynthesis and they turn it into sugar. You then eat the plant and all the time that you are alive you’re gaining radioactive carbon in your body which you incorporate into your body. The level in your body will be roughly constant because you’re taking it in at a roughly constant rate from the environment. The ratio of radioactive to non-radioactive carbon should be the same all the time you or a plant are alive.

But when you die you stop adding new carbon-14 to your body and the ¹⁴C you’ve already got starts to break down to ¹⁴N because it’s radioactive.  The half-life is about 5500 years or so.  When you find an ancient specimen all you have to do is to compare how many ¹⁴C atoms are in it to the number of ¹²C atoms.  The ratio tells you how long it was since it was last alive and this gives you a ballpark figure for its age.

This does make the assumption that the production of ¹⁴C and incorporation into the food chain is the same now as it was thousands and thousands of years ago.  This assumption but it’s assumed to be a fairly reasonable and accurate way to do it.

The guy we have to credit is Willard Libby who discovered carbon dating in the 1940s, got the Nobel Prize for it actually.

There’s quite a few, all of which are types of radioactive dating.  They include potassium-argon dating, that’s useful for rocks over 100,000 years old.  There’s also uranium-lead dating, which has an age range of 1-4.5 million years old.  It can be used for such long time spans because the half-life of uranium turning into lead is billions of years, in the order of the age of the Earth at 4.5 billion years.

Mike, from Cambridge, also called in to remind us about thermo-luminescence which can be used in pottery,  also obsidian hydration and uranium trail dating when you observe the trails left behind by uranium decomposition.

It’s horrible, isn’t it?  The reason for this is not 100% known but there is a hypothesis.  The idea is that the kind of frequencies that you get when you get the fingernails-down-the-blackboard experience is similar to the kind of frequencies that animals release when they’re in distress.  It’s a kind of distress signal and therefore we are pre-programmed or galvanised into action by those particular choice of frequencies.