Part I: The Question
The most simple questions that we can ask often turn out to be the biggest. Physicist Enrico Fermi is testament to this with his big ask ‘where is everyone?’ Where is everyone in the vast universe, why do we hear nothing but silence when we scour the night skies, where are all the aliens?
When we look up a particularly starry night sky we are only observing a very small proportion of the stars in our galaxy. Which in itself is a very very small proportion of the stars in our observable universe (note. observable universe, and that’s just the part closest surrounding us that we can receive light from!) Let’s throw some numbers into the mix.
Part II: The Calculation
There are 100-400 billion (yes billion that’s 10^9) stars in our Milky way galaxy. Now astronomers believe that there are roughly 100 billion galaxies in the observable universe. If we take the lower estimate for the number of stars (100 billion) that means for every star out their in our galaxy there is a whole other galaxy full of stars. So together, in the observable universe, that comes to 10^22 stars [100 billion x 100 billion]. Let’s let that number blow our mind for a little bit. Actually scrap that, I can’t even comprehend that number it’s too big, 10^22 doesn’t mean all that much to me, let’s go a little closer to the scales our human mind can chew on.
So of these 10^22 stars, astronomers again estimate 5-20% are sunlike, i.e. they are the same size as the star at the center of our universe. If we take the lower estimate 5%, being conservative, that gives 500 billion billion (or 5×10^20) sun-like stars.
Next step. How many of these sun like stars are thought to have Earth-like planets orbiting them. Astrophysical studies say 20-50%. Let’s take the lower bound again, 20%. So that gives 100 billion billion earth-like planets! 100 billion billion earth-like planets in the observable universe.
Penultimate step. How many of these earth-like planets are though to harbour intelligent life? Re-cap, by earth-like we mean planets that are in the so called ‘habitable’ zone from their host star, not too far as to be too cold, not too close as to be too hot – just right in the goldilocks zone, a position to maintain an atmosphere, maintain liquid water. So let’s be ultra conservative, let’s say a lowly 0.01% development intelligent life – this would mean 10 million billion intelligent civilisations in the observable universe. Little comparison for you, the population of the Earth is 7 billion. For every person on the earth this model predicts, 1.4 million intelligent civilisations.
Final step. Divide this number by the number of galaxies in the universe (100 billion). This gives us the number of supposed intelligent civilisations in the our galaxy… answer to the calculation – 100,000 intelligent civilisations in our galaxy alone.
Now this calculation may be speculative but it isn’t at all ludicrous – it is based off experimental observations of the stars in our local universe and the types of planets found around these stars by exoplanetary scientists. These statistics are then extrapolated and why should the rest of the universe be any different to our local spec? We see the universe as homogeneous don’t we.
So 100,000 intelligent civilisations in our galaxy alone by this reasoning, we listen and we listen and yet nothing. Complete and utter silence – so where is everyone?
- Extraterrestrial life does not exist – hence why we don’t hear anything.
- Extraterrestrial life does exist but we don’t hear anything for multiple possible reasons.
So here goes – please comment below with which one (and the reason) would place your bets on and let’s make this post interactive!
Part III: The Reasons
1.THERE IS NO EXTRATERRESTRIAL LIFE
- Extraterrestrial life is rare or non-existent.
Complex life is considered extremely unusual or even non-existent apart from us. This is also known as the Great Filter hypothesis. This hypothesis states there is a point, a ‘wall’ that exists between simple life and complex life and it is very hard for civilisations to make this transition. This may just be because the transition is a biologically rare occurrence and it requires very finely tuned environmental conditions for it to occur – this is the ‘Rare Earth’ hypothesis. Life likes ours requires an improbable combination of astrophysical and geological events and circumstances (I won’t go into this in more detail as i’m not biologist, please see here for more details).
Or the Great Filter may not be a biological transition but a periodically occurring cataclysmic event which wipes out species before they can naturally make the transition from simple life to complex, making life extremely rare. This could be something like the probability of collision with an asteroid or a gamma-ray burst which would incinerate everything in the local vicinity. The booming field of exoplanetary studies, which discovers more and more earth-like planets every year, seems to be continually building statistical weight this line of thought but there it is, possibility one. We are alone in this universe.
2. THERE IS EXTRATERRESTRIAL LIFE BUT WE DON’T HEAR ANYTHING BECAUSE….
- It is the nature of intelligent life to destroy itself
This is the view that technological civilisations destroy themselves before (or shortly after) they develop the capability to control radio or spaceflight technology. This is due to the assumption that they have also developed other forms of harmful technology and the destruction comes in the form of things such as world wars, accidental environmental contamination (nuclear bombs), Artificial Intelligent mishaps (think i,Robot).
- It is the nature of intelligent life to destroy others
The most intelligent species out there is destroying other less intelligent species. Why? They are a super predator, just like humans are here to all the other species on earth. Why do we kill other animals, we kill animals in rainforests for expansion, we kill animals to eat for strength, we kill animals out of fear of threat. These motivates of expansion, strength or paranoia/aggression may well be fuelling the actions of the super predator as well. As such perhaps we should be quiet instead of broadcasting our signals, naively, stupidly giving away our location to the super predator. In fact perhaps other species like us know about the existence of the super predator and thats why they are smartly being quiet, perhaps everyone out their isn’t transmitting out of fear. Here’s Carl Sagan’s view on our broadcasting behaviours ‘the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.’
Quick rebuttal though – why don’t we hear signals from the super predator of other civilisations and their desperate pleas for help before they die? Can the super predator really be so stealthy in their universal massacres?
- There occurs periodic extinction by natural events
There occur runaway heating or cooling events on earth-like planets [such ice ages or volcano eruptions] or astronomical events [such as meteorite collisions or gamma ray bursts] which happen on such a frequent basis that it is very likely life is destroyed before it can advance to a stage where they are able to send interstellar signals. This is similar to the argument stated above in second argument for the Great filter but it is just slightly tweaked in that periodic extinction events occur less frequently. As such life isn’t extremely rare, it does develop across the universe, but keep getting blocked before it can reach the stage where it can communicate. Perhaps we are just lucky enough to have avoided such an event so far, it is statistically unlikely we should have done and all too soon a meteorite will collide with Earth and wipe us out. (The monitoring of meteorite collisions seems to be neglected in our scientific community, though I guess there’s not much we could do anyway if it occurred… we should work on that.)
- Resource depletion occurs by the time of sufficient advancement
Industrial evolution that occurs on planets by the time sufficient technology advancement occurs may lead to a sustainability crisis on the planet, as we can all foresee in our future be it fossil fuels, food, clean air… This is a rather plausible scenario though perhaps there is the hope that other species may not be as greedy as us. Industrial evolutions are in large part driven by capitalism and is this trait a species-general trait or that of human beings?
- Intelligent civilisations are too far apart in space and time
Perhaps the civilisations are simply too far apart for meaningful two-way communication, the universe is enormous after all, maybe we are just tucked away in a tiny isolated corner. Perhaps civilisations become extinct before meaningful dialogues can be established. Perhaps we are receiving broadcasts but to us they are meaningless as without being able to establish two way communication we cannot ask further questions in order to de-code their messages properly (like receiving a code without a cypher). Or perhaps their signals simply have not arrived yet due the distance they have to travel. Remember the fastest a signal can travel is the speed of light and it takes a year for light to travel the distance of a light year (sorry if i’m being obvious). The point is, our nearest star, Alpha Centauri is 4 light years away, so a signal would take 4 years to reach us at quickest. But what is the probability in this vast universe, intelligent life lives on the next star over. The closest galaxy, Andromeda, is 2.5 million light years away, so civilisations within our galaxy could be hundreds of thousands of light years away. Now how long have humans been around – 200,000 earth years, let alone those with technology to manipulate electromagnetic signals… 100 years.
Now this sounds like a plausible one right? But there’s a caveat. This assumes that humanity’s advancement occurred the transition in time when the Milky way galaxy went from being empty (with no other civilisations) to a state where civilisations have already sent signals on their way but had not arrived (as otherwise there would already by signals already around us by the time we had developed the technology to listen). Instead they have to still be on their way. Humanity has been been around for a very small blip of time, relative to the age of the universe and this transition is also quite a small epoch so the likelihood they have coincided is rather small.
- We are not listening properly or we cannot comprehend
Either our probes are not advanced enough as the signals extraterrestrial use may have evolved beyond broadcasting in the electromagnetic spectrum, which is what we search for in programs like SETI, instead they may be communicating through technologies not yet developed by us, for example neutrino signals. Or perhaps we are unable to probe far or wide enough to be able to search effectively for signals being sent. We would need to be listening in the appropriate range of frequencies and be looking in the region of space in which the beam is being sent. Perhaps we’re just off with our detection, like in a very unlucky game of battleships.
Or maybe we simply cannot comprehend. Perhaps we simply to primitive to perceive their presence. Famous physicist Michio Kaku (this guy) has this to say on the matter. “Let’s say we have an anthill in the middle of the forest. And right next to the anthill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?” Perhaps we are the ants. Perhaps it would be like trying to get an ant to understand how to use an iPad.
- The Earth is deliberately not contacted (we are being watched)
Like a universal scale of The Truman Show, we are being observed, watched, analysed. This is known as the ‘Zoo Hypothesis’. Extraterrestrial beings are observing us, and allowing us to evolve naturally and uninterrupted either as part of their peaceful nature or an experimental study.
This argument breaks down a little under the idea that if there are a large number of extraterrestrial civilisations all it would take is one to break the rules and make themselves known. The probability increases with the number of civilisations, one naughty schoolchild in the class who lets a tadpole escape the tank.
- They are here undetected or perhaps just unacknowledged
This is the idea that if a civilisation is advanced enough to master interstellar travel they are likely to be advanced enough to be here on Earth and conceal themselves successfully. Or perhaps they are just unacknowledged, there have been many claimed UFO sightings over the years, just go ahead and google UFO (at your own risk of getting fixated and then suddenly realising 5 hours have passed). Finally there’s the good old government conspiracy theory that governments are suppressing publications of alien evidence for their own security of economic interests.
Now I love a good government conspiracy theory as much as the next person but I have an inkling that the reason transcends the personal interests of the human race.
- They are so advanced they no longer live in the physical reality
They have mastered artificial intelligence and technology to the point that they no longer live in their previous physical form, or the physical reality as we know it. They have uploaded their minds into a virtual environment and live, immortally, in ‘the matrix’. So the idea of the physical reality and sending physical signals that we can detect is well… rather beneath them.
- We are insignificant to them
Finally perhaps they aren’t trying to communicate with us because we simply aren’t that important to them. They don’t bother sending out signals in our direction because they have nothing to gain from establishing contact with us and they need not any of the resources on our tiny blue planet. Perhaps we think too highly of ourselves.
Right there we we have it, reasons why, if extraterrestrial life exists, we don’t hear anything. (I’m sure there are other reasons, but these are the ones best known to me.)
So either we are alone in the universe or we aren’t. Are we rare, special, naive, stupid, isolated… who knows. What we do know is that we don’t know much about our place in the universe at all, but we should keeping asking those all important big questions so we can get closer to finding out.
As I said please comment below which theory you would place your bets on (of course if you have a different reason i’d be eager to hear it). I’ll reveal mine after i’ve heard back from a few of you!