The quantity 1/e is one that I've seen quite a bit in physics. Especially when describing 'lifetimes' of various things (ie radiative lifetimes). I'm curious about why this value is used. I've heard explanations about how it's a probabilistic thing or that it's just a sort of 'standard candle' for measuring these quantities. To put my question in another form: The value of 1/e is close to that of 1/3. So why don't we just use 1/3 instead. What is it that makes 1/e more useful or preferred. Thanks!