The Fermi Paradox (after physicist Enrico Fermi) briefly goes as follows. Extraterrestrial intelligences with advanced technology and interstellar spaceflight capability exist. Sub-light interstellar spaceflight violates no laws of physics. Adopting the mantra of quantum physics, 'anything that's not forbidden is compulsory'. The time it takes to explore every nook and cranny of our Milky Way Galaxy via sub-light interstellar spaceflight is a tiny fraction of the age of the Galaxy. There's at least one universally valid reason to boldly go - species survival. No star, no solar system lives forever. We (Planet Earth) can't hide from alien exploration and/or colonization. So, where is everybody? [By analogy, terrestrial life forms like bacteria, ants and cockroaches, birds, and of course humans, have explored and colonized Planet Earth in tiny fractions of the time that Earth itself has existed.] So again, where is everybody?
I can hear screams of 'objection, objection' now. It's obviously too far and takes too long to get from there (wherever that is) to here. Well, life wasn't meant to be easy! Seriously, if you think about it a while, methinks you protest too much!
Firstly, aliens could have a very long natural lifespan relative to us carbon-based terrestrial bipeds. There's no natural law that confines intelligent life forms to an existence of just three score and ten.
Secondly, advanced extraterrestrials may have perfected various hibernation techniques. Put your spaceship on autopilot and sleep the long journey away.
Thirdly, there's that way old sci-fi chestnut, the multi-generation interstellar spaceship. While I feel that's an unlikely concept, especially for exploration, it might not be quite so far out if the objective is interstellar colonization.
Then there's bioengineering, turning an organic body into something that's more machine than flesh and blood, perhaps akin to Doctor Who's Daleks. Given advances in artificial body parts for humans, albeit it hip replacements or dentures or even mundane tooth fillings, that's certainly a valid possibility.
Fifthly, why stop there? Send 100% machines - artificial intelligences in the form of cybernetic 'organisms' or robots or androids or tiny nanotechnology machines. One obviously things of Data from 'Star Trek: The Next Generation', or something akin to the original 'Battlestar Galactica' Cylons. Think of the savings in not having to provide life support and other life essentials for biological organisms. We've made a start already down this path. There's nothing different in principle between a Cylon and our Pioneer 10 & 11; our Voyager space probes. It's just that a Cylon is a lot more sophisticated. The day will come when our Pioneers and Voyagers will morph into something approaching a Cylon, or any one of multi-dozens of similar 'beings' in the sci-fi literature. Since AI is nearly immortal (relative to flesh and blood), that takes care of travel time arguments, and the possible environments fit for relative easy exploration (colonization?) are expanded greatly.
Lastly, maybe, just maybe, a sort of warp drive, faster-than-light ship is possible. Aliens whose science is thousands of years more advanced than ours just might have gotten around Einstein's speed limit. I wouldn't want to wager any money on it, but I'd be less than open minded not to admit the possibility, however remote. Add to that, theoretical but allowable 'gateways' between distant points of our Universe, maybe even to other universes - wormholes and Black Holes. Maybe, just maybe, an advanced alien civilization has the ways and means to manipulate such objects and forces to facilitate easy travel in space (and time too maybe). An excellent hardcore science based sci-fi novel that doesn't rely on pseudo techno-babble that illustrates this is Carl Sagan's "Contact".
So yet again, where is everybody?
Answers include (but aren't really limited to) general concepts that suggest that...
They don't exist; never have and never will. What's wrong with that? Well, given the vastness (100,000 light-years across) and timelessness (over 12 billions of years minimum) of our Milky Way Galaxy's entire expanse, the odds that we are the proverbial IT, the one and only, is extremely unlikely. It's a massive violation of the Principle of Mediocrity or the Copernican Principle.
We're the first kids on the block, not the new kids on the block. What's wrong with that? Again, the odds that in all the vastness of our Milky Way Galaxy we should happen to be the first, is unlikely in the extreme. Our Solar System is but 4.5 billion years old; our Galaxy is way, way, way, way older than that.
They exist but don't care to explore space, to seek out new life and new civilizations. They don't want to boldly go or seek communications. They want to be left alone - isolationists. What's wrong with that? That might be true for one, or several alien civilizations, but to extrapolate and suggest that that applies across the board to each and every extraterrestrial civilization is illogical.
They boldly go, but haven't come our way yet. What's wrong with that? Again, it doesn't take that long to explore the entire Galaxy. It would be a fluke if we hadn't of been noted and logged in some other civilization's database.
They're here, but leave us alone. What's wrong with that? Again, that might be true for one, or several alien civilizations, but to extrapolate and suggest that that applies across the board again strikes me as illogical. There is such a thing called the Zoo Hypothesis to explain the Fermi Paradox. It's both a Star Trek 'Prime Directive' concept combined with that of a zoo. Aliens (the zoo keepers) don't interfere with us (though of course every now and then the zoo keepers have to interact with the animals (humans) in the zoo), don't allow others to interfere with us, yet probably wouldn't allow us to escape the cage (meaning probably the confines of our solar system - I mean we have been allowed to travel to the Moon).
They're here and interact with us and our environment - UFOs anyone? What's wrong with that? Absolutely nothing!
UFOs are a perfect answer to the Fermi Paradox!
Some recommended readings on the Fermi Paradox:
Hart, Michael H. & Zuckerman, Ben (Editors); Extraterrestrials: Where Are They?; Pergamon Press, N.Y.; 1982:
Hart, Michael H. & Zuckerman, Ben (Editors); Extraterrestrials: Where Are They? [2nd edition]; Cambridge University Press, Cambridge; 1995:
Verma, Surendra; Why Aren't They Here? The Question of Life on Other Worlds; Icon Books, Cambridge; 2007:
Webb, Stephen; Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life; Copernicus Books, N.Y.; 2002:
Science librarian; retired.
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