The Fermi Paradox asks why we haven't found any evidence of aliens. Here are 13 potential answers to that question.
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- The Milky Way galaxy has billions of planets that could potentially host life. Yet despite scientists' efforts to monitor for and occasionally signal to extraterrestrials, we have not found any evidence that aliens exist.
- This conundrum is known as the Fermi Paradox, and it has inspired debate among researchers for decades.
- In his new book, "End Times," author Bryan Walsh discusses the many theories about why we have not made contact with aliens - and possibly why we never will.
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In the summer of 1950, physicist Enrico Fermi posed a simple question to his colleagues over lunch: "Where is everybody?"
Fermi was referring to alien life in the universe.
Arguably, he said, in the 4.4 billion years it took for intelligent life to evolve on our planet, the rest of our galaxy should have been overrun with similarly smart, technologically advanced aliens. But despite decades of searching the Milky Way for signs of extraterrestrials, we haven't found anything or anyone.
This conundrum has come to be known as the Fermi Paradox.
Scientists have offered myriad potential answers to the question, including that aliens are hibernating or deliberating hiding from us. Some researchers have also suggested that highly advanced technological civilizations destroy themselves before they have the opportunity to get in contact with other intelligent life in the universe.
Since Fermi first asked his famous question, space telescopes have searched the stars for Earth-like planets capable of hosting life.
But the chances that our telescopes could be coincidentally pointed at the exact right part of space at the right time to detect signs of extraterrestrial civilization are infinitesimal.
SETI's monitoring system is predicated on the idea that aliens are trying to message us — we just need to hear it.
Astrophysicist Frank Drake created an equation in 1961, now known as the Drake Equation, that offers a way to calculate an estimate of number of technologically advanced civilizations in the Milky Way.
The first and perhaps simplest answer to the Fermi Paradox is that Earth holds the only intelligent life in the universe.
Another possibility is that aliens want to talk to us but can't.
If other intelligent life in the universe has outpaced us technologically, it's possible those beings think Earth isn't worth contacting at all.
Or maybe our radio messages just haven't reached anyone yet.
But not all scientists thought sending messages into space was a good idea. Astrophysicist Stephen Hawking cautioned against attempting to make contact in 2010.
Aliens could also be deliberately hiding from us.
Aliens could simply be content to leave us alone until we become too greedy and pose a threat to the greater universe.
Or perhaps extraterrestrials are just hibernating, Walsh wrote.
Another hypothesis is that we're living in the "galactic sticks," on the outskirts of where intelligent life is located in the Milky Way.
But that answer to the Fermi Paradox has a problem: The Milky Way is old. Walsh argues that, given enough time, an intelligent civilization should have been able to find us by now, even if they were traveling slowly.
But perhaps a technologically advanced civilization just can't last long enough to be able to travel through the galaxy for millions of years.
Some scientists have argued that intelligent civilizations similar to ours could have gone extinct because of the same dangers that threaten humanity on Earth.
A study published last year points to climate change as the most likely "filter" that prevents a civilization from reaching other star systems.
Walsh suggests that we may never find anything in our search for aliens except their remains — evidence of extinct civilizations, in other words.
Planetary scientist Alan Stern, meanwhile, thinks it's possible that — unlike humans on Earth — aliens live in the interior of their respective planets, which is why we've yet to find signs of life.
One of the most recent responses to the Fermi Paradox, published last month, suggests that aliens have already visited Earth — just not recently enough for us to have noticed.
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