Even though we still struggle with finding a satisfactory definition of life, that doesn’t mean that we can’t think about ways that life might be so different, so alien, that we would also struggle with noticing its existence.
In the past I’ve played around with this idea in a variety of ways. For example, considering life so different (or advanced) that its fundamental substrate is radically unlike ours. A suggestion that life could exist in dark matter made for great tabloid fodder. But the basic argument was, I think, an okay one: If dark matter has microstructure it could conceivably maintain the necessary complexity for living systems. And if dark matter does exist it must also constitute the majority of matter in the universe and therefore represents an awful lot of juicy real estate.
Similarly, what if complex, thinking biological life is fleeting on the cosmic scale but its machine progeny are more robust and more widespread? Such entities might also be very hard for us to recognize as such, either whizzing around at high velocities between the stars or massively encrypted in their fundamental design and their communication strategies.
Are there other forms of extreme alienness though? Qualities that would make life extremely hard for us to notice or understand?
I suspect that one characteristic might be to do with our perception of the passage of time. We are biased to be sensitive to a relatively narrow range of timescales for events. Even though we can build devices capable of operating with phenomena at the scale of femtoseconds (10-15 seconds) or even attoseconds (10-18 seconds), we’re nowhere near the theoretical limit of physically meaningful timescales at yoctoseconds (10-44 seconds). At the other extreme, while we can gaze across 13.8 billion years of cosmic history, we’re very insensitive to slight shifts in phenomena that might operate across such a long period. While it’s true that we have limits on quantities like changes in the proton-to-electron mass ratio, which is seen to be steady across billions of years to at least one part in a million, we could be missing more subtle variations in this or other fundamental quantities. Admittedly a variation would be at odds with the Standard Model, but that model also doesn’t really justify the expected stability.
So what about alien life? We know that on Earth other species work to different beats. Insects are fast moving and fast thinking. Plant life is painfully slow to our senses, but if you've ever watched a timelapse movie of plants growing, feeling, exploring, it’s pretty obvious that they’re up to all kinds of mischief under the cover of a different timescale.
And these examples may not represent the real extremes. Perhaps there is life (even here on Earth) that goes about its business at much faster or slower rates. Chemistry itself may limit the quickest biological life, but chemistry may not be the energy transduction system for all living systems in the universe. Electrons can be moved around by light, and even by gravity.
On longer timescales I think the possibilities are many. Complex interactions that form a living system could take place over thousands, millions, billions, or trillions of years. Interactions of matter that are extraordinarily subtle by our standards might play out over very, very, long periods.
I’m not suggesting that long-lived objects like planets, stars, or galaxies are part of a cosmic organism (although it’s tempting to jump to that place for fun). Interesting things could be happening on much more modest physical scales. For example, could the messy chemistry we see in fossil fuels on Earth – a smorgasbord of organic reactions, a seemingly tarry chaos – be simply a short-term view of a living system that functions across hundreds of millions of years?
Or consider a chunk of complex rock, a mixture of minerals and carbon chemistry. It may be bathed for a billion years in cosmic rays and indigenous particle radiation. It changes over that timescale, electrons are freed and captured, slow, slow chemistry and structural variation happens. Your pet rock might be just that, except you’re living too fast to notice.
Of course, rather frustratingly, to make proper hypotheses for these options we need a robust definition of life, but to make that robust definition we may need to first know the extent of options for life in the universe.
That’s why we need to both look for life as we know it, and at the same time keep track of the things lurking right in front of us. The trick is to maintain scientific rigor and skepticism while thinking out of the box.