Here’s a reasonable working definition of life – a process which produces localised entropy decrease. Intelligent life doubly so.
Think about it like this: light from the sun hits the moon and turns to heat. No life involved. Light from the sun hits Venus and creates an enormous amount of turbulence in the atmosphere, also heat, eventually the turbulence generates heat too. So there’s a more complex process turning low entropy energy (light) into high entropy energy (heat). On Earth the process is, in places, even more complex. The sunlight might hit the leaves of a plant which turn certain chemicals into other chemicals, later eaten by a herbivore then shat out onto the forest floor and breaking down into, once again, heat.
So what’s involved? There’s a complex system – life being localised order bought at the cost of generally increasing disorder. In our case the concentration of energy in the Sun makes an entropic gradient on Earth, between sunlight and heat. That gradient generates order. Not just any order. The storms on Venus are both very simple in their general description and utterly chaotic in their detail. We can say that winds form at such and such a speed in weather patterns like x. But the movement of any particular set of particles within that doesn’t have a pattern, it’s simply a stochastic process. Look at life and you get patterns from top to bottom, from cells and DNA up to migrations of species and evolution of behavioural traits over time. Everywhere are patterns, the intersection point of simple order and simple chaos, maximal information density.
But life arises and exploits that entropy gradient. Thermodynamics notes a variable beta = dS/dE, the rate of change of entropy with respect to energy. Energy will tend to flow from systems with small beta to systems with high beta. As it does so the total entropy of the combined system will increase. Actually temperature can be defined as 1/beta, or dE/dS. Thus for constant pressure and volume dE is TdS. Greater rate of change in energy, in other words more energy available as work, where there are high temperatures and the rate of change of entropy is high.
For this reason we would expect to find life in steep entropy gradients and high temperatures. But life arises as complexity and high temperatures make structure unlikely. True, there are weather patterns in sun spots and they seem complex, they might be good places to look for life. By the way I’ve always thought that the red spot on Jupiter is a good candidate. But prima facie we should be looking in entropy gradients where complex structures are more easily possible. It’s all about that border between chaos and order.
Here’s my proposal for SETI. If we’re not finding attempts to communicate perhaps we should look for large-scale exploitation of entropy gradients at moderate temperatures. Perhaps these can be detected through observation of complexity, but it would be enough to see heat signatures indicating unexpectedly slow entropy changes in places which should have steep gradients. Or piles of waste heat being pushed out of the way so as to maintain the steep gradient and high exergy. If this sort of thing exists at the scale we can see from far off with crude instruments then I’d start to think we’re seeing intelligent life. Life that can get outside its initial ecosystem. Wouldn’t that be lovely?