Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox
Introduction
The Fermi paradox, or more properly the Fermi question, consists of the apparent discrepancy between assigning a non-negligible probability for intelligent life emerging, the size and age of the visible universe, the relative rapidity with which intelligent life could expand across space or otherwise make itself visible, and the lack of observations of any alien intelligence. Put more simply, why don't we see the aliens that should be out there [1]?
There are about stars in the Milky Way and about stars in the visible universe. Planetary systems appear to be relatively common. Assuming the mediocrity principle, which states that Earth is not special, but merely a typical planet, it would appear that life and in particular intelligent life would have developed on some of these worlds. Even if a very small fraction such worlds developed intelligence, e.g. , it would imply hundreds of intelligent species in the Milky Way. These civilisations would be likely considerably more advanced than us, since the Earth is a latecomer: the median age of terrestrial planets is 1.8±0.9 billion years older than the age of the Earth [2], [3].
Even at a relatively slow spread using subrelativistic starships, such species could colonise the galactic disk within 50 million to one billion years [4], a very short time compared to the galactic age of years. However, we neither see any evidence of past or present visitations in the solar system nor any signs of technological activities elsewhere. This is potentially worrying for the future of the human species. If the explanation for the Fermi paradox is simply that intelligent life is hard to evolve (an ‘early Great Filter’), then there are no implications for us. But if the explanation is that intelligent life tends to destroy itself before becoming spacefaring (a ‘late Great Filter’), then we have reason to worry about our future [5], [6]. Gaining information about the relative weights of these possibilities is relevant for policy, for example whether to focus far more on reduction of existential risk.
The contribution of this paper is to sharpen the Fermi paradox by demonstrating that intergalactic colonisation is a feasible task for a civilisation capable of interstellar colonisation—in fact, intergalactic colonisation is not far beyond our current capabilities today. So the apparently empty skies hold a much greater puzzle than before: not only are we not seeing all the alien civilisations that might exist in our galaxy, but we are not seeing all the alien civilisations that might exist in the millions of nearby galaxies and that could have crossed over to ours.
We will not speculate much in this paper about resolutions to the Fermi paradox. But this sharpening does increase the likelihood of some explanations (e.g. some sort of cosmic disasters typically wipes out intelligent or pre-intelligent life) while diminishing the likelihood of some others (e.g. alien life is quite common, but by chance we are living in a galaxy without any).
Generally one can argue for a likely expansion by going into details about some particular design for how it could be accomplished [7], [4], [8], and showing that method would produce a galaxy-wide colonisation on a time-scale far shorter than evolutionary or astronomical time scales. Our paper takes a similar tack, but using a far higher fan-out than is usual and intergalactic colonisation instead. We argue that it is rational and possible for a civilisation to send a large number of replicating probes to great distances, and that the resources demanded (time, cost and energy) are small on an astronomical scale. It is very much in a “Dysonian SETI” vein, using exploratory engineering to study possible macroscale projects and their SETI implications [9].
Extragalactic SETI has not been studied much [9] (with the exception of [10]), perhaps due to a bias towards radio communications and against the Kardashev Type III civilisations that would be the only observable intelligences at this scale. Similarly there is limited prior studies on intergalactic colonisation (the exception may be [11]), likely because interstellar colonisation is deemed sufficiently challenging. Expansion timescale arguments are hence content to deal with just a single galaxy to show that there is a timescale discrepancy. However, the main difference between interstellar and intergalactic travel is merely a longer time until the destination is reached. If the contents of the colonising probe are inert over long timescales (as they would need to be for many forms of interstellar travel) it is likely that they can be made inert over the longer flights to other galaxies. Given recent advances in our understanding of the large-scale structure of the universe it behooves us to investigate its feasibility and implications.
The structure of the paper is as follows. Initially, we will look at whether humanity could attempt a mass colonisation of the reachable universe, and on what time scale. To this end we will first delineate a potential replicator probe design, and tackle how such probes could decelerate upon arrival (four different scenarios will be considered, from speculative anti-matter drives to reasonable fission engines). We will calculate how many duplicates need to be sent out to avoid collisions with intergalactic dust particles. We will analyse the necessary launch systems, and delve into some details as to how they could be powered, calculating what portion of the universe is reachable under different designs.
This human-based analysis serves simply to estimate the potential ease with which putative alien civilisation could have reached the Milky Way in time for us to have noticed them today. Thus we will look back into time, and see how many galaxies could have reached us by the present time. We will end with a discussion of the motivations that could cause us or some other civilisation to embark on a colonisation of the reachable universe, and on the implications of this model for SETI.
Section snippets
Exploratory engineering
Our paper will by necessity consider technologies that do not exist yet: What is the rational approach to analyse them?
Exploratory engineering is the art of figuring out what techniques are compatible with known physics, and could plausibly be reached in the future by human scientists [12]. Fig. 1 illustrates this: there are known physical limits, and known engineering techniques quite far off from them. Exploratory engineering looks at results that are beyond what is currently possible, but
The probe and replicator design
Before any other considerations, we need to look at the design of the probe itself: the colonisation machine that will be sent out to the other galaxies.
The launch phase
We will need a large source of energy to power the multitude of launches needed. Barring some exotic way of directly transforming matter into energy, the most likely source of energy is the sun itself, in all its glory. The concept of the Dyson sphere [32] is a staple in science fiction: a collection of solar captors partially or completely surrounding the sun to capture a large proportion of its radiated energy. How would we actually go about building such a mega structure? And what
Total energy and time requirements
So for each of the modes of deceleration – no deceleration, matter–antimatter, fusion, fission – we have a mass for the probe, a speed to be accelerated to, and hence a number of galaxies to be aimed for. For the matter–antimatter powered probes, we will also need to manufacture the antimatter to put on board (the energy costs of furnishing the other probes with hydrogen or uranium 235 are negligible in comparison with other costs).
We recall the assumptions: the probes will be carrying either
Reaching us
The main purpose of the forgoing was to show that, given certain assumptions, it was possible for humans to launch the colonisation of the entire reachable universe on scales of time and energy that are cosmically insignificant—only requiring about two replication stages to reach every star we could ever reach, with a rapid launch phase. If human civilisation could achieve this, then it is highly likely that any star-spanning alien civilisation would be capable of doing so as well. We can run
Discussion: What does this imply?
We have shown that, given certain technological assumptions, intergalactic colonisation appears to be possible given known natural laws and the resources within a solar system. This process could be initiated on a surprisingly short timescale (decades)—well within timescales we know some human societies have planned and executed large projects. A star-spanning civilisation would find the energy and resources requirement to be so low that they could do this project as an aside to their usual
Acknowledgements
We are very grateful for comments, support and help from Milan Ćirković, Pedro Ferreira, Eric Drexler, Nick Bostrom, Timothy Clifton, Toby Ord, Robin Hanson, Daniel Dewey and Seán Ó hÉigeartaigh.
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