Despite what UFO enthusiasts might claim, virtually every scientist agrees that humanity is yet to receive a message let alone a visitor from beyond our planet.
But in the vast scale of the universe – containing an estimated 2 trillion galaxies – scientists say there is a ‘100 per cent chance’ that there is life somewhere apart from Earth.
This raises a intriguing question: If alien life truly is common in the Universe, why haven’t we heard from them?
From the ‘Dark Forest Hypothesis’ to the inevitability of nuclear war, the answer to this question may offer a chilling glimpse into the future of our own civilisation.
Professor Frederick Walter, a galactic astronomer from Stony Brook University says: ‘Life is a biochemical process, it’s going to happen, but as you go further down the chain things become more uncertain.’
Despite the high probability of alien life existing humanity is yet to encounter any real evidence of life beyond the stars. The Fermi Paradox is a puzzle which asks why this might be the case
Despite the likelihood of alien life, none of the Earth’s satellite arrays such as the Very Large Array in New Mexico, have detected any signs of intelligent life
Professor Walter says he would guess there is a ‘100 per cent chance’ that life exists in some form in the Universe.
Yet, on the other hand, scientists also face the absolute absence of any evidence of the existence of life – intelligent or otherwise.
The so-called ‘Fermi Paradox’, named after physicist Enrico Fermi, asks how we can balance the likelihood of life’s existence with our lack of evidence for it.
Or, as Fermi famously asked his friends one afternoon in 1950: ‘Don’t you ever wonder where everybody is?’
Everyone is hiding
Given that the Universe is 13.8 billion years old, if only a handful of civilisations emerged they would have plenty of time to become extremely advanced.
When Fermi’s paradox was first presented, some scientists argued that the age of the Universe suggests there should be super-advanced civilisations in our galaxy.
And while science fiction like Star Trek imagines that civilisation may become more peaceful as it advances, some scientists disagree.
Dr Walter says: ‘It could be that the Universe is teeming with life but everybody’s hiding because we’re scared.’
To understand why this could be the case, imagine that you are a hunter creeping through a dark forest.
Scientists suggest that the conditions on Earth which allowed life to develop are not unique, this means there is a very high chance that alien life should exist somewhere
If you saw another hunter lighting a fire, would you light your own to say hello?
You don’t know who else is out there, whether they are armed, or what their intentions might be.
However, what you do know is that if you stay quiet and don’t make any light, no one else is going to be able to find you.
The so-called Dark Forest hypothesis argues that, if you really want to survive, the only smart decision is to stay quiet and shoot anyone you can see.
Popularised by Liu Cixin’s Three Body novels and their recent Netflix adaptation, this theory suggests that the universe could be in a similar state with civilisations taking the role of hunters.
Stephen Hawking (pictured) warned about the dangers of contacting aliens which could be far more advanced than our own civilisation
So, while humanity is doing its best to reach out to the stars and make contact, other more powerful forces might be waiting silently beyond our sight.
If this is the case, it might explain why our efforts to communicate with any other civilisations have so far gone unanswered.
Dr Walter says: ‘You have no idea if other civilisations are friendly or hostile and remember, you’re the new kid on the block.
‘They’ve been around for thousands of millions of years and are probably much more advanced, you don’t know what to expect so the safest thing is to remain silent.’
In Netflix’s Three Body Problem (pictured), it is suggested that the universe is like a dark forest in which civilisations are quietly hiding to avoid being killed
No one wants to speak to us
However, the picture of the universe painted by the Dark Forest Hypothesis may be a little too pessimistic for some to believe.
Professor Walter points out that, while the theory is popular with science fiction authors, he isn’t ‘that paranoid’.
Instead, if we want to explain why nobody is responding to humanity’s messages, we might have to accept that we are the problem.
If life is as common in the Milky Way as some calculations suggest, aliens could be spoiled for choice for planets to try to communicate with.
In 1977 the ‘Golden Record’ was sent out on the Challenger spacecraft as a sign of humanity’s intelligence. But some experts suggest that aliens simply might not be impressed enough to speak with us
As 2022 paper argues, if this is true then humanity simply might not be interesting enough for aliens to bother with.
Rather than trying to speak with every planet where life could exist, aliens are more likely to be looking for signs of technology and intelligence.
Although life started on Earth almost as soon as the planet cooled enough for liquid water to form, humanity only developed radio technology in around 1880.
Even then, it wasn’t until 1974 that scientists would send the first deliberate high-power broadcast to aliens with the Arecibo message was sent to the globular star cluster M13.
This means that Earth may simply not have given off any signs of intelligence that an alien civilisation would consider worth investigating further.
Humanity’s use of radio waves only began in earnest during the 20th century with the construction of structures like the Wardenclyffe Tower. That might not give aliens enough time to detect our intelligence
On the other hand, aliens might be very well aware of humanity’s level of technology and still not want to talk with us.
Dr Gordon Gallup, a biopsychologist at the University of Albany, has argued that aliens might be scared of visiting humans.
In an paper published in the Journal of Astrobiology, Dr Gallup wrote: ‘If [alien life] exists it may have found us by now and discovered that humans are dangerous, violent and ceaselessly engage in endless bloody conflicts and war.’
‘It would also be obvious, that as a byproduct of increasing pollution, habitat destruction, coupled with endless wars, pillage, death, destruction and the desire for conquest, that humans pose an unparalleled and unprecedented risk not only to other life forms on Earth but to life on other planets.’
If Dr Gallup’s arguments are true, it may be possible that the Galaxy is full of life but that humanity has made itself a social outcast of alien society.
Humanity’s reckless pursuit of nuclear weapons (pictured) might have also put aliens off from ever wanting to speak with us
We’re too late
One of the core assumptions of the Fermi Paradox is that the development of alien civilisations is fairly common on the cosmic scale.
And many scientists do agree that it is likely for some communicative alien life forms to have emerged.
In fact, one 2016 study estimates that humans are only likely to be alone in the Universe if the odds of civilisation developing on a habitable planet are less than about one in 10 billion trillion.
However, that impressive statistic obscures a very important caveat: civilisations might have emerged in the past but that doesn’t mean they will stick around.
Instead of assuming that aliens are staying silent for some reason or another, the ‘Great Filter’ theory suggests that we might have already missed them.
During the Cuban Missile Crisis, humanity came extremely close to destroying itself with nuclear weapons. The Great Filter theory suggests that all civilisations might ultimately destroy themselves in a similar way
To understand how this might happen, we need look further than the short history of our own civilisation.
‘Ever since we’ve developed technology we’ve been on the brink of wiping ourselves out,’ Professor Walter explains.
During the Cold War in particular, humanity seemed to teeter on the edge of nuclear annihilation on more than one occasion.
As technology advances and planetary resources diminish, perhaps nuclear war and self-destruction are the fate of all sufficiently advanced civilisations.
And, if nuclear weapons don’t seem dangerous enough, there are plenty of other ways that civilisations might destroy themselves.
Recent years have seen the shockingly fast development of AI systems that are becoming increasingly integrated into every aspect of our lives.
SpaceX CEO Elon Musk (pictured) has warned that AI could pose an existential threat to humanity
But in a paper published this year, Professor Michael Garrett of Manchester University argued that AI ‘presents a formidable challenge to the survival and longevity of advanced technical civilisations, not only on Earth but potentially throughout the cosmos’.
Since AI develops so quickly, it might always get out of control before species have a chance to become a multi-planetary civilisation which could survive the challenge.
This means that advanced civilisations may have already destroyed themselves long ago.
‘As Professor Walter puts it: ‘Our capabilities exceed our wisdom, and maybe other civilisations do that too.’
Experts warn that the rapid development of AI through companies like OpenAI could outstrip our ability to control, creating a great filter which ultimately destroys our civilisation
We’re too early
Compared to the 14 billion years the Universe has been around, humanity is extremely young.
And, like a toddler on Christmas Day, some scientists suggest that humanity might be up and running about before the adults have woken up.
If there are very old civilisations out there, they may have already spread out into huge galaxy-spanning empires.
But in a paper published in 2017, researchers from the Future of Humanity Institute argued that these massive civilisations might be patiently waiting out of sight.
Since quantum computers like this one need to be extremely cold, a smart alien civilisation would be better off waiting for the universe to cool to start using its resources
So why could aliens be potentially biding their time like this?
Researchers point to the fact that computers get more energy-efficient as they get cooler.
Quantum computers, which exploit the strange behaviour of quantum particles, need to be even colder – requiring temperatures close to absolute zero.
If you’re an advanced race looking to get the most computing power out of your energy, you need to get things as cold as you possibly can.
Rather than spending lots of resources creating vast cooling systems, the researchers suggest that aliens would be better off waiting for the universe itself to cool down.
That could explain why we can’t see any of these massive civilisations that the Fermi Paradox suggests should be out there.
There might be alien civilisations out there among the stars, but they could be in a state of hibernation and will remain hidden until just before the end of the universe
There’s no one to talk to
However, Professor Walter explains, the ‘simplest answer’ to the Fermi Paradox is simply that nobody else is out there.
The reason many people think there may be intelligent alien civilisations out there for us to speak with comes back to a formula called the Drake Equation.
Proposed in 1961 by Frank Drake, the formula is a way of reaching a very rough estimate of how many communicative advanced civilisations could exist in the Milky Way.
Depending on what your assumptions are for various factors, Professor Walter says the number of civilisations you reach ranges between ‘zero and millions’.
But whatever the number is, the vast majority of life in the universe could be unintelligent.
In our own solar system, some scientists hope to find traces of ancient life in Mars’ Jezero Crater (pictured) which is believed to be a dried-up lakebed. But even then the best hope is merely to find microscopic life.
‘Is there life out there? Yes. But when people ask “Is there life out there” they’re imagining something different,’ says Professor Walter.
‘There’s life but it may be bacterial, microscopic life; slime mould and things like that.’
And, to make matters worse, even if there is intelligent life out there it doesn’t necessarily mean that we could communicate with it.
Professor Walter says: ‘There are lots of species on Earth that can communicate, at least with each other, but they don’t build radios.
‘Dolphins might be more intelligent than we are but they live underwater, they don’t have hands, they can’t work with electricity; so you’re not going to communicate with a creature that’s on a water world.’