There are two major ways we can look for alien life:  look for signs of biology or look for signs of technology.

SETI includes searches for the latter—technosignatures. These might include big bright obvious information-rich beacons (in the radio or with lasers, for instance) , or they might be passive signs of technology like waste heat, pollution, or leaked radio emission from radar and communications.

I have often seen the argument that this is nice to look for but must be less likely to work than searches for biosignatures. The flaws in this argument have been pointed out and analyzed for as long as SETI has been a thing (most of them were hashed out in the ’60’s). But that discussion isn’t actually familiar to most astronomers and astrobiologists, and so working with the CATS collaboration led by Adam Frank (for Characterizing Atmospheric Techonsignatures) I’ve written a paper summarizing them.

In this series of posts I’ll break down the argument, following the argument in our new paper in Astrophysical Journal Letters.

The Role of the Drake Equation

One part of the argument goes back to the Drake Equation.

Let’s look at why:

If we just count up the parts of the Drake Equation that lead to some kind of life to be found, we might end up with something like

where R* is the rate of star formation, multiplied by the usual fraction of stars with planets and the mean number of planets per planet-bearing star that can support life, and the fraction of those planets on which life arises.

Here, L_b is the lifetime of detectable biosignatures. N(bio) is then the number of biosignatures there are to find out there.

We can also rewrite the full Drake Equation in a similar manner for any technosignature:

Here, we’ve added f_t representing the fraction of planets where technology arises, and now L_t is the lifetime of detectable technosignatures.

Based on this reasoning, SETI looks like a terrible idea compared to searches for biosignatures! It’s a tiny, tiny subset of all possible ways to succeed, because (this reasoning goes):

Why? Because f_t <= 1 by definition, and since you need to have non-technological life before you can have technological life, L_t<L_b. This would seem to justify the huge imbalance in time and money that NASA spends on astrobiology in general compared to SETI (which has been almost zero until recently).

This is the abundance argument against technosignatures, and it is wrong, for many reasons! Let’s take a look at why.

Abundance

First of all, let’s think about the Solar System.  N(bio) is, as best as we can tell, exactly 1.  If there are other biosignatures in the solar system, we have not noticed them yet, so they must be very hard to detect.

And what is N(tech)?  Well, based purely on what we can detect with our equipment it’s at least 4!  Earth is loaded with technosignatures, but we also detect them from Mars all the time, and Venus and Jupiter also have them. We also have several active interplanetary and interstellar probes, and many many more derelict objects are out there too.

This gives us our first clue about how the reasoning above fails: if technology can spread through space, then one site of biology can give rise to many sites of technology.

And this, of course, has been appreciated by SETI practitioners for decades. It’s the basis of the Fermi Paradox, which asks why alien life hasn’t spread so thoroughly through the Galaxy that it isn’t here right now. Drake’s equation is based on the idea that it’s easier to communicate via radio waves than to travel via spacecraft, but of course one doesn’t preclude the other, and if both are happening, then N(tech) could be much larger than the equation says.

This is not really a major failing of the equation, whose original purpose was to justify SETI.  After all, if you can conclude there is something to find in the absence of spreading, that’s a sufficient condition to go looking.  The equation is often misinterpreted as foundational, like the Schrödinger Equation, as if you can calculate useful things with it.  Instead, it’s best thought of as a heuristic, a guide, and an argument.

So, how large could N(tech) be? Well, in the limit of the Fermi Paradox reasoning, it could be upwards of 100 billion, even for a single point of abiogenesis!  We’ve written about this before, for instance here.

So, the argument isn’t that this will happen, just that N(tech) has a higher ceiling than N(bio).  This long tail out to large possibilities (both in the sense that we are ignorant of the right answer, and in terms of a distribution among all of the alien species) means that it is not just possible but plausible that SETI is much more likely to succeed than other life detection strategies.

Next time: The second reasons to do SETI: technosignatures may be long-lived.