# Are we alone after all? Understanding the recent re-evaluation of the Drake Equation

The Quartz.com article We may have answered the Fermi Paradox: We are alone in the universe says:

Researchers of Oxford University’s Future of Humanity Institute have another answer. It’s likely intelligent life doesn’t exist at all, outside of Earth.

In a paper submitted to the Proceedings of the Royal Society of London (it appeared online this month on the pre-publication site arXiv), the researchers write that there is “a substantial ex ante probability of there being no other intelligent life in our observable universe,” and we shouldn’t be surprised if we fail to detect any signs of it. In other words, there is no need to speculate about the fate of aliens. It’s likely they’ve never existed, they assert in the paper, titled “Dissolving the Fermi Paradox.”

And according to the Tech Times article Fermi Paradox Reassessed: Humans Likely The Only Intelligent Species In Observable Universe:

The number of scientific uncertainties derived from their calculations was highly distributed and those uncertainties lead them to a conclusion that humanity is most likely alone in the galaxy.

"We found that even using the guesstimates in the literature (we took them and randomly combined the parameter estimates) one can have a situation where the mean number of civilizations in the galaxy might be fairly high - say a hundred - and yet the probability that we are alone in the galaxy is 30%," explained Sandberg.

This does not mean, however, that humans should stop pursuing the quest for the extraterrestrial life. In fact, according to Sandberg, the level of uncertainty they calculated was tremendous enough to need the intervention of astrobiology and SETI to reduce those uncertainties of the parameters.

I've heard about the Drake Equation but not very familliar with it.

From what I understand so far, they've primarily widened the uncertainties in each of the terms. A broader distribution of probabilities would increase the probability that a values much higher as well as much lower than the most likely value are also possible.

Question: But is that all that's going on here, or does their new evaluation of the Drake Equation also include a shift downward in the overall expected results?

• – Russell Borogove Jun 25 '18 at 23:54
• @RussellBorogove Thanks, I've clarified the last sentence. – uhoh Jun 26 '18 at 0:31

A broader distribution of probabilities would increase the probability that a values much higher as well as much lower than the most likely value are also possible.

But is that all that's going on here, or does their new evaluation of the Drake Equation also include a shift downward in the overall expected results?

Both.

They (properly) point out that many people look at the Drake equation $N = R_* f_p n_e f_i f_C L$ and interpret it as a product of expectation (mean) values. But when each factor is uncertain, what you should really be looking at is the composition of the distributions: In what fraction of possible cases do you get interesting answers?

Their figure 1 captures this idea with common literature values:

Figure 1: (A) The uncertainty in the research community represented via a syn- thetic probability density function over N — the expected number of detectable civilizations in our galaxy. The curve is generated by random sampling from the literature estimates of each parameter. Direct literature estimates of the num- ber of detectable civilizations are marked with red rings. (B) The corresponding synthetic cumulative density function.

The top is using a Monte Carlo method to work out the distribution (again, from literature values) of possible number of civilizations in the Universe. The peak is about 100-1000 (hard to be sure with such a complicated scale), but there's a long "nobody home" tail on the left. The bottom is the cumulative: "What's the fraction that have less than this?" It shows that there's about a ~10% chance, even with those values of "alone in the universe" (only 1, e.g. us), and a ~30% chance of "alone in the Galaxy"

Bottom line: Even if those numerical values look like there's a good chance of neighbors, it may also play out that we're alone.

They then look at "conditioned credence" in the numbers, which is a technical exercise about how uncertainty has been represented, how measurement sampling was done, how factional cases and some other statistical issues are handled, etc.

Figure 3: Effect of updating on the Fermi observation for different scenarios. Random sampling: no observations among 1,000 studied stars, spatial Poisson: no ETI within 18 pc, no K3 civilization: effect of the $\hat{G}$-survey if $P_{K3} = 0.5$ and $P_{succ} = 0.01$, no settlement: no interstellar civilization has yet spread to our location in the galaxy.