Fermi's Paradox - Why Are We Alone?

The Universe is very old, at around 14 billion years and our galaxy has at a minimum 100 billion stars and as many as 400 billion stars. This raises the question, where are all the aliens or at the very least where are their robot probes or radio signals?

Even if interstellar travel were slow it seems apparent that long-lived species or their computerized probes would have at least visited us.

This was a question that bothered Frank Drake and so being an astrophysicist he decided to create an equation to try to estimate just how many advanced civilizations there should be out there.

(Remember without equations it's not science it's just screwing around.)

The equation he came up with was this:

I know that this may seem to some like a long and daunting equation but in reality it is just one scientist's way of starting a technical conversation on a topic.

Let's go through the terms in the equation one by one and you will see what I mean.

'R_*' means the number of stars formed in our galaxy each year. Astronomers have a good hard answer for this one and it turns out to be in the neighbourhood of 1.5 to 3 stars per year on average.

'f_p' means the fraction of stars that have planets. I do not think the field of exoplanet research was even started at the time Drake created his equation but the field is now very active and it has some good data. It is currently believe that almost every star will have at least one planet (not a planet with life, just a planet mind you).

'n_e' means the average number of planets per star that may be Earth-like. In this case astronomers tend to mean whether the planet is orbiting in the so-called Goldilocks Zone (not too hot and not too cold). If a planet is orbiting in too close to its star it will be too hot and there will be no water and life will die or not even form. If it is too far away then it will be too cold, chemistry will be too slow and all the water will be frozen.

The latest best estimate for this parameter 0.4 planets per star on average are in the habitable zone.

'f_l' means the fraction of those planets in the habitable zone that go on to generate life. Earth seems to have created life very quickly after it stopped being a lava ball so this suggests that f_l should be close to 1.

There is also the "mundane hypothesis" which is a technical opinion that there is nothing special about Earth or the life that can be found on it. Everything we seem to learn about the Universe seems to move us further and further from being the centre of the Universe to just being an average occupant of it.

It seems that many calculations just set this to 1.

'f_i' means the fraction of planets with life that go on to develop intelligent life. Speaking selfishly maybe we want this to be zero or close to zero. I mean who wants competition?

Anyways the phenomenon of the Cambrian explosion and the apparent predator-prey drive to become ever more clever seems to be a force that would tend to drive this parameter to be close to 1.

'f_c' means the fraction of intelligent aliens that reveal themselves to the rest of the galaxy either through radio waves, mega structures, neutrino communication , gravity waves or whatever you can think of.

The conversation around this parameter often revolves around the speculation that aliens may not actually want to reveal themselves in case there may be an even more advanced and aggressive species out there. Also, advanced species may be noisy at first with radio waves but then just naturally start using quiet communication technologies like fibre optics etc and so simply disappear right off of the radar screen.

On the other hand they stay quiet because they may not want our germs.

This parameter often gets set low.

'L' means the lifetime that a civilization either lasts or communicates out into space. The stockpile of declared nuclear weapons seems to stand at about 14,905 and is easily enough to kill each and every last human being. I speculate that the technological capability for developing a planet killing bioweapon might also be within technological reach of humans.

This is a really hard one to estimate. Once a civilization contacts another older and peaceful advanced civilization then it would learn from the older one and become more likely to last longer. Some advanced civilizations may simply become effectively immortal as they develop and perfect their technologies.

The Answer(s)

Applying this equation and trying out different values for the parameters gives a wide range of results because we simply do not have enough data for good estimates of these parameters.

The estimates range from N = 1 (just us humans) to N being very large.

Criticisms of Drake's Equation

The first criticism (and it is mine) is that the equation does not seem to help us to narrow down the answer very much. After doing all that math the range of answers is still all over the place.

Another criticism is that the equation might be too anthropocentric (human-centred). All we know about is the Earth and the life on this planet. There may be a vast array of other types of life configurations out there and different solutions for intelligence that we cannot yet conceive of.

Closing Words

The Drake Equation is a fun attempt at solving a difficult question but at this point in our knowledge the parameters are still mostly conjecture.

It is encouraging that there are many stars with planets and many planets in the Goldilocks zone so who knows? One day this question may eventually get answered.

Post Sources

Equations in this post created using the CodeCogs online LaTex editor: https://www.codecogs.com/latex/eqneditor.php

https://en.wikipedia.org/wiki/Fermi_paradox

https://en.wikipedia.org/wiki/Drake_equation