What the Search for Extra-Terrestrial Intelligence (SETI) Says About Humanity's Chance of Survival

Estimated number of extra-terrestrial civilizations that we, in principle, could communicate with:

Estimated distance to the nearest one:

These estimates are found in my favourite paper on the subject. It was written in 2020 by Christopher Conselice, Professor of Extragalactic Astronomy at the University of Manchester, and Tom Westby, Assistant Professor at the University of Nottingham

CETI Civilisations

When talking about intelligent life, astrophysicists use the term ‘CETI civilisation’. CETI stands for ‘communicative, extra-terrestrial, intelligent’. The space of interest is restricted to our own galaxy, which gives us civilisations that we, in principle, could communicate with.

Any discussion on CETI civilisations is, of course, entirely hypothetical, and will remain so until any positive detection is made. Nonetheless, models like this are helpful in encouraging debate on the occurrence rate of CETI civilizations. If nothing else, we use them to learn more about our own existence, and whether the search for extra-terrestrial intelligence (SETI) is ever likely to bear fruit.

Astrobiological Copernican Strong Condition

It turns out that Professor Conselice is not a fan of the Drake equation, which is the usual method for estimating the number of CETI Civilisations. The Drake equation contains a number of probabilistic terms that some astrophysicists suggest are just as unknowable as the original problem it seeks to solve. Instead of using the Drake equation, Conselice and Westby base their number on:

- Galactic star formation histories,

- The distribution of metallicity, and

- The likelihood of stars hosting Earth-like planets in their habitable zones.

Importantly, they use very specific assumptions, which they describe as the ‘astrobiological Copernican weak and strong conditions’. These are based on the one situation in which CETI life is known to definitely exist – Earth. They looked at a number of different scenarios, with one extreme being the ‘weak astrobiological Copernican principle’, where a planet forms intelligent life sometime after 5 billion years, but not earlier. The other extreme is the ‘strong condition’ in which life must form between 4.5 to 5.5 billion years, as on Earth.

The estimated minimum number of CETI civilisations, , uses the assumptions of the strong condition (i.e. the strictest set of assumptions). So it’s a lower limit. It’s based on the assumption that the average life-time of a communicating civilization is 100 years (since we know that our own civilization has had radio communications for this time).

The number given for the estimated distance of the nearest CETI civilisation, , assumes these CETI civilisations are spread uniformly throughout the galaxy. Sadly, if this turns out to be roughly true, then detection of a CETI civilisation far surpasses our abilities for the foreseeable future, and interstellar communication remains impossible.

The Great Filter

I have to agree that this seems pessimistic. After all, life has expanded to fill every niche on Earth, so why shouldn’t it form more abundantly throughout the universe? The ‘Great Filter’ is a theoretical explanation to why we don’t see signs of intelligent life beyond Earth in nearby star systems or even in our solar system. It is the idea that, even if life does form abundantly, every CETI civilization will ultimately face a barrier to its own survival. This might come from external threats like an asteroid strike, or from self-made threats like all-out nuclear war.

Whenever I mention this to colleagues or family members, I’m often told to “cheer up”, or “it’s all just hypothetical”, or “please stop shouting at the neighbours”. The great hope, of course, is that humans can continue to survive for a very long time. If we can, then the estimated number of CETI civilisations increases, the estimated distance between CETI civilisation decreases, and the chances of detecting CETI civilisations become more positive.

Looking at this the other way around, if we do detect one or more CETI civilisations in nearby systems, then it will look likely that humanity can indeed survive long beyond our current and looming challenges. So I for one will be keeping one eye firmly on the work of the JWST. Good luck astrophysicists!

As the consistently cool Brian Cox put it on the Joe Rogan show: “We should consider ourselves extremely valuable”.

Source: I heard from Professor Conselice in January 2023, as part of my technical analysis of the James Web Space Telescope for Space Impulse. Professor Conselice and his team from Manchester are pouring blood, sweat and tears into a number of exciting workstreams related to the data from Webb. My original article is available on the Space Impulse platform.