# The Drake Equation

1. Nov 22, 2015

### Andrew Jacobson

Hi, I'm doing a presentation about the fermi paradox and I use the drake equation:
$N = R_* \times f_p \times n_e \times f_I \times f_i \times f_c \times L$
Where the symbols have their usual meanings. I don't understand why this would give the number of civilisations whose EM emissions are detectable. Surely it would give the number of civilisations created in L years whose EM emissions are detectable? If somebody could explain why the L (length of time which such civilisations release detectable signals into space) would give the N value that would be much appreciated, thanks.

2. Nov 22, 2015

### Chronos

The motivation for thd L term in the Drake equation is to weed out civilizations that no longer exist. The objective is to approximate the number of currently detectable intelligent civilizations, not the total number that may ever have existed.

3. Nov 23, 2015

### newjerseyrunner

It is assumed that advanced species will have several logistical hurdles that they have to get through to remain active. We've already past one of these hurdles: the development of nuclear weapons. If our species were just a little bit more aggressive, we may have used them and set our civilization back another thousand years. AI will likely end up being an existential threat that'll make nukes look like child's' play, and there are probably more hurdles ahead of us that we haven't even dreamed of yet. That's why L is an important (an unknown) factor.

4. Nov 23, 2015

### Smattering

AI's would also be observable sentient species. This, replacement of a biological species by an AI species would not change the numbers.

5. Nov 23, 2015

### Ken G

Perhaps it would help to reframe the equation in the following way. The key part that you are asking about is the R*L, so let us instead say R=N/t, where t is the lifetime of a star (since there are many types of stars, with their own lifetimes and formation rates, we just add up terms like this for each type), and N is the number of stars in the galaxy (or that particular type, if we are keeping track). Then say R*L = N*L/t, and notice the ratio L/t = P can be viewed as the probability P that any civilization that appeared at that star is actually there now (where we needn't worry about the time of flight of the light, just use a lookback time in place of "now", it's no big deal). So instead of R*L, we have N*P-- the number of stars N that have life on them now, where P is interpreted as a fraction of those stars.

6. Nov 24, 2015

### AgentSmith

The Drake equation is not a law or observed relationship, but an attempt to rationally guess the number of detectable civilizations. It includes some assumptions, and the range of some of the terms are large, so the answers you get range from about zero to millions. Hopefully all the exoplanets we've found can narrow down some of the terms.