Since the dawn of the Space Age, when it became apparent that traveling to other planets was technologically possible, humans have looked up and wondered if there were other intelligent species out there.
Yet in all of human history, we haven't found any inarguable evidence for the existence of alien civilizations — and not for lack of trying, nor for lack of solar systems to check out. Indeed, we know there are so many trillions of stars in the universe, many of which are circled by inhabitable planets. So where are all the aliens?
This conundrum is called the Fermi paradox. Named for the Nobel Prize-winning physicist Enrico Fermi, it essentially boils down to math. There's plenty of opportunity out in space for advanced civilizations to emerge — so why don't we see them? Or, if they are out there, why haven't they contacted us?
It could be that Earth appears boring, or that we just aren't very smart compared to everyone else out there. Now, a new paper published on the arXiv preprint server, which is awaiting peer-review, argues just that: perhaps Earth hasn't given off enough signs of intelligent life, and aliens may be totally uninterested in contacting us. This research could have some intriguing implications for SETI, the Search for Extraterrestrial Intelligence.
The paper's author, Amri Wandel is an astrophysics professor at The Hebrew University of Jerusalem who has previously published research on what makes certain planets suitable for life to develop. There are many of these extraterrestrial worlds, which are called exoplanets, but so far, humans haven't found any hard evidence that life exists anywhere but planet Earth.
In the paper, Wandel explains that this could be due to having a weak "technosignature" — meaning an indicator that life not only exists on a planet, but that it's technologically advanced. It can include everything from artificial light to pollution to radio waves that spill out into the cosmos.
For example, earlier this year, scientists discovered an exoplanet that seems a lot like Earth, nestled around a star about 105 lightyears away. Dubbed LP 890-9c, it's slightly larger than our home world, but it seems to have liquid water, which most astrobiologists agree is necessary for life to exist.
That's it, however. There is no evidence LP 890-9c has life, let alone intelligent creatures that build spaceships or send radio signals into space. The universe is likely filled with trillions of planets like this. And it would be difficult for a planet like Earth to stand out; therefore, aliens might not think we're interesting enough to contact, let alone visit.
"Presuming biological life is common, Earth's biosignature would not be outstanding," Wandel writes. "In other words, to extraterrestrial civilizations would not consider Earth as special, since there are probably many biotic planets closer to them."
That brings up another major issue: the unfathomable distances between our solar system and others. Even if we were to look attractive enough for aliens to send a message, or vice versa, it could take dozens, if not hundreds, of lightyears to reach us. In contrast, any substantial technosignature emanating from our planet, such as radio or television waves, has only been ongoing for a short while.
Even if life on other planets evolves the ability to send information through radio waves, they might kill themselves off via nuclear weapons or climate change, or go extinct due to galactic hazards.
"The first short-wave radio transmissions that could penetrate the ionosphere and leak to space were broadcasted in the 30's, less than 100 years ago," Wandel writes. "Therefore, the maximal distance of a civilization that could detect Earth's radiosphere and eventually send back a message that would reach Earth at present is approximately 50 [lightyears]."
That is a very narrow window of time in which extraterrestrials would need to notice us. So to any off-world civilizations that have peeked in our direction, we might just look like another dumb, wet rock that might have life, but no one worth talking to.
Additionally, we'd need to have been able to detect and receive any messages that might be sent to us. For these circumstances to line up, Wandel estimates we'd have needed to be using advanced telecommunication infrastructure for "a few hundred to a few thousand years." Too bad the Ancient Romans were too busy crucifying people instead of building radios — we could be talking to E.T. by now.
There have been many attempts to explain the Fermi paradox. One main theory is that advanced, intelligent civilizations tend to have a short shelf life. Even if life on other planets evolves the ability to send information through radio waves, they might kill themselves off via nuclear weapons or climate change, or go extinct due to galactic hazards like gamma rays and black holes.
Another explanation, posited by paleontologist Peter D. Ward and astrobiologist Donald E. Brownlee, is that Earth truly is special. So special in fact, that we're essentially alone out here in our wing of the universe. In their 2000 book, "Rare Earth: Why Complex Life Is Uncommon in the Universe," Ward and Brownlee argue that "complex life—animals and higher plants—is likely to be far more rare than is commonly assumed."
"I used to be very sanguine, believing that microbial life would be almost everywhere. And I'm less and less sure about that," Ward said in an unpublished portion of a recent interview with Salon. "I would bet my life that we're not the only intelligent species in the cosmos. The numbers are too great. How could we be? But boy, it might be just here and there, and so far apart, you can never communicate."
So maybe the problem isn't that humans aren't advanced enough to be worth talking to, but that we're just statistically unlikely to exist in close proximity to anyone who would notice our specialness. We'll never know unless we keep looking.
