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## Gott’s Principle

Filed in Ideas
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HOW TO PREDICT THE LIFETIMES OF (SOME) THINGS

In 1969, John Gott looked at the Berlin Wall and asked himself how long it would stand.

The wall was 8 years old at the time.

He could be witnessing something that would stand many years to come or something that would fall down tomorrow.

In certain domains, you can make the inference that you are observing a thing at a random point in its lifetime, and this is what Gott did.

He figured that if he’s witnessing it 50% of the way through its lifetime, then it would stand another 8 years (Because it has been up 8 years and is halfway through its life).

He figured that if he’s witnessing it 5% of the way through its lifetime, then it would stand another 152 years. (Because if 8 years is 5% of its life, then 160 years is 100% of its life. Since it has been up for 8 years, it has 152 years left).

He figured that if he’s witnessing it 95% of the way through its lifetime, then it would stand another 5 months. (Because if 8 years is 5% of its life then 8.42 years is 100% of its life, Since it has already been up 8 years, it has .42 years, or five months, left).

In this way, Gott came up with a principle for estimating confidence intervals for the lifetimes of certain classes of things. A 95% CI for the lifetime of the Berlin Wall would be 8.42 to 160 years, which contains the age of the wall (about 28 years) when it came down.

Gott’s principle was also used to predict the predict the closing dates of 44 Broadway and Off-Broadway shows, and was about 95% correct.

So there you have it, a heuristic for predicting the lifetimes of things. It doesn’t apply everywhere (e.g., it doesn’t work on human lives), but it’s kind of fun.

REFERENCES
Gott, J.R. (1993). Implications of the Copernican principle for our future prospects. Nature, 363, 315–319.

Gott, J.R. (1994). Future prospects discussed. Nature, 368, 108.

Photo credit: http://www.flickr.com/photos/siyublog/1982035178/