eli5: What does the measurement “half life” mean and what does it represent when measuring things such as radiation?
In: Chemistry
I had it explained to me with the feeling of caffeine in coffee.
Once you drink coffee the caffeine takes about 4 hours for HALF of it to leave your body
After 4 hours “half” the caffeine that was in your system will remain in the 5th hour
So the 5th hour you will have half the caffeine of the previous 4 hours
Then in the 6th hour, you will have half the caffeine of the 5th hour in your system
And so on and so on till it’s all out of your system.
Because after the 4th hour, that’s when about half the caffeine is gone so caffeine in coffee has a “half life” of 4 hours
Sorry for grammer errors and the redundancy … Its 4am and I’m about to sleep
If I’m wrong and I could be, I’m sure some helpful redditor will correct me
Radioactive stuff decays, falls apart into other components. Half life is the time it takes for half of the amount you start with to fall apart.
The half life of a radioactive sample is the time it takes for half of the sample to decay. For example, if you start with 1 kg of material, after 1 half life elapses you will have 500g left. After 2 half lives, 250g, after 3 half lives, 125g, and so on.
m_f=m_i*(0.5)^(t/t_1/2)
Where m_i is the initial mass, m_f is the final mass remaining, t is total elapsed time, and t_1/2 is the half-life of the substance.
Radioactive things spit out radiation, but that decreases over time because they’re breaking down – spitting out parts of themselves – in order to do that. Half-life is how long it takes for the radiation to decrease by half. After multiple half-lifes, the object may not be totally inert but close enough, because of the effect of halving repeatedly.
100 / 2 = 50
50 / 2 = 25
25 / 2 = 12.5
12.5 / 2 = 6.25
6.25 / 2 = 3.125
3.125 / 2 = 1.5625
As you can see from the progression, 50 radiation drops to 1.6 radiation over only 5 half-times. The time progression is linear, but the change in radiation is exponential. This makes it relatively easy to date things (as in carbon dating) because the numbers are so different from one period to the next.
Related, nuclear waste with a short half-life is often left to sit for 10 half-lives, after which the radiation is so low as to be close enough to inert, and it can be disposed of normally. (following the example above, by 10 half-lives, it has gone from 50 down to 0.049).
But different isotopes have different half-lives, so while some decay quickly, others take a long long time. Generally lighter things have less stuff to spit out, and therefore shorter half-lives, than heavy things, which have more stuff to spit out.
Because radiation is in a way unpredictable, we can not say when decay happens, we can only measure a probability for it.
Half life is basically the point where there is 50% chance for the atom to have decayed. After that point, no matter how many atoms you have, it is more likely that half of them have decayed than not.
Now there is nothing that says that there must be. They could all decay at once or last for an eternity.
And since probabilities get really hard to work with after a while. It is easier to consider half life steps. Each step is half of the last. So half, fourth, eight.
The fundamental problem is that an individual atom is unpredictable. But law of large numbers, make a mass more predictable. So half life is just the point, where it is 50% chance for decay to have happened. Therefor it is the point where in a mass 50% probably have decayed.
From this we can predict the by products and energy release, and be fairly confident about it. There is nothing saying that it must have happened. It could all decay at once or not at all, it is just very improbable.
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