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Coffee and (DARK!) chokolate

Good morning!
I forgot it takes forever to get from Charles de Gaulle airport, north of Paris, to the lab in Orsay, south of Paris... Normally I prefer to fly Norwegian to the Orly airport, which is just between the city and the lab, but this time Air France was the only company with reasonable flight times. Anyway, by the time I finally arrived at the lab, I was starving, and had to get something to eat. Then there was catching up to do, and suddenly it was 10:30 PM and I had to go to sleep because I had signed up for the morning/day shift today. I´ve been at the lab since 7 AM, and heavy doses of chocolate and coffee are needed 😉
(Didn´t have time to put on my make up this morning btw., so I look a little...hmmm...don´t really know...:P )


So I told you yesterday that I was embarrassed, and that I would tell you about it - was planning on doing it last night, but now you know how come that didn´t happen. I will tell you now instead:
Last Friday I was on the radio, and one of the things I was talking about was the amount of radioactivity that was present at earth 3.5 billion years ago (when life started). So I was talking about the half-lives of thorium-232, uranium-238 and uranium-235. 
Thorium-232 has a half-life of 14 billion years, so the amount of thorium here on earth is more or less the same as it was 3.5 billion years ago. The half-life of uranium-238 is 4.5 billion years, so around half of what was here 3.5 years ago is gone.
So far, so good...
But then I talked about uranium-235, which has a half-life of "only" 700 million years. This means that 3.5 billion years is more or less the same as 5 half-lives of uranium-235. Then I did the mistake: I said that since 5 half-lives of this uranium isotope has gone by, there is one fifth of it left - in other words, 3.5 billion years ago there was 5 times as much uranium-235 on earth, as it is today.
WRONG! 5 half-lives doesn´t meen that there was five times more :/ *feeling stupid*
1 half-life of something means that after that time half of it is gone. Two half-lives means that 3 quarters is gone, three half-lives means 7/8 is gone, four half-lives means 15/16 is gone, and five half-lives means 31/32 is gone - 1/32 is left. 1/32 is around the same as 3%, and 3.5 billion years ago there was 32 times more uranium-235 on earth...!
My excuse is that I was really stressed the day before, and something did NOT feel right (e.g. I didn´t get the right percentage of uranium-235 to have fission and a chain reaction going on in nature - and now I know why 😛 ); but this was a "rookie mistake" - and I shouldn´t have done it 🙁

Hope you can forgive me <3 <3 <3

Tomorrow is an important nuclear anniversary - anyone that can guess what it is? I will tell you tomorrow 🙂

3 kommentarer til “Coffee and (DARK!) chokolate

  1. hyperbolsk

    As someone who makes simple math errors on the whiteboard while teaching high school math, and has given students impossible to solve problems on physics tests, I feel a sliver of your pain and, more importantly, think your slip of the mind is perfectly understandable.


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