what is a half-life?
In: Physics
It’s a tool we use to describe an exponential loss of something.
For linear loss, you can just use the time it takes to reach zero because the rate of loss never changes, for example your phone’s battery life is X hours.
For exponential loss, the rate of loss actually slows down and approaches zero as the amount of whatever you’re measuring approaches zero. For example, most medicine is metabolised by your body faster at higher concentrations. In this instance, it’s not really useful to talk about how long Advil will remain in your body, because trace amounts can remain for days or weeks, so we go by half-lives instead. Aspirin has a half-life of about 3 hours. So after taking a 200mg dose, the amount left in your system after 3 hours is 100mg. 6 hours after taking it, it would be 50mg. 25mg at 9 hours, 12.5mg after 12 hours, 6.25mg after 15 hours, 3.125mg after 18 hours, etc, etc.
Edit: also it’s a common mistake people make to assume amount left is zero after two half-lives (“well if after 3 hours you lost 100mg, so you should lose another 100 after another 3” is a natural assumption to make). Don’t fall into this trap. It’s actually a quarter of the original.
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