Where is everybody? With billions of stars in our galaxy, many of which are thought to harbour habitable planets, surely there should be signs of life. Yet after decades of searching, we鈥檝e found nothing.
The mystery of this great silence is known as Fermi鈥檚 paradox, after physicist Enrico Fermi. Some have used it to argue that the search for extra-terrestrial intelligence (SETI) is doomed.
But a new mathematical analysis of SETI activity by Jason Wright at Pennsylvania State University and his colleagues shows this is far from the case. The team claim that the basic assumption of Fermi鈥檚 paradox 鈥� that there鈥檚 nobody out there 鈥� is false. In fact, we鈥檝e barely begun looking.
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Wright鈥檚 team analysed the many variables involved in SETI 鈥� what to look for, where to look, how often and for how long 鈥� and ended up with an eight-dimensional model. They then devised an equation that computes the fraction of the galaxy searched so far. 鈥淚t lets you build the haystack, then calculate how much of it you鈥檝e looked at,鈥� says Wright.
They claim that the volume of our galaxy searched so far is roughly equivalent to a bathtub of water in the world鈥檚 oceans.
鈥淵ou don鈥檛 have to do a calculation to say we鈥檝e only just started,鈥� says Duncan Forgan at the University of St Andrews, UK, who is a member of the UK SETI network. 鈥淏ut they鈥檝e done a nice job of showing the huge scale of the problem mathematically.鈥�
Solving Fermi鈥檚 paradox
As well as putting SETI in context, the equations can help researchers see which search techniques have been used less than others. For example, we only developed the technology to monitor higher radio frequencies quite recently. 鈥淏ut we鈥檙e getting better at that, so that variable will now shrink,鈥� says Forgan.
But advances in technology will only take us so far. Certain variables, such as how often an alien message might be repeated, cannot be changed. A signal sent once a year can only be listened out for once a year. 鈥淭here are things we can do better and things we can鈥檛,鈥� says Forgan. 鈥淲e just have to sit back and wait for the universe to do its thing.鈥�
Forgan has written a textbook that discusses 66 potential solutions to Fermi鈥檚 paradox 鈥� explanations for why we haven鈥檛 found alien life yet. These include that the Earth is somehow odd in harbouring intelligent life, or perhaps intelligence or technology are rare. On the other hand, perhaps they are common but civilisations have a short lifespan. 鈥淕enetic or nuclear disasters might wipe you out,鈥� he says.
There鈥檚 also a chance aliens are hiding from us. They may live in parts of the galaxy we struggle to see, such as deep in the galactic centre around the supermassive blackhole.
Or they may simply be keeping quiet. They might not use radio waves at all, says Forgan. Some have suggested that advanced aliens could be sending deep-space messages using gravitational waves, for example.
If we search for gravitational waves, maybe aliens will start appearing, says Forgan. 鈥淚t would be a way of ensuring whoever joins the club is technologically advanced. Being able to detect gravitational waves will be your membership card.鈥�
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