Given the estimated 1 billion Earth-like planets orbiting Sun-like stars, some of them in Goldilocks-like conditions, chances are that there are other intelligent aliens living 'among us'... somewhere in our universe.
So where are they all, and why haven't we encountered them?
Conspiracy theories aside, let's take a simple numbers approach to better understand the scope of this so-called paradox.
#1. The Universe is VERY BIG
Billions upon billions of galaxies 'big'.
The Milky Way galaxy itself has about 100 to 400 billion stars (ONE of which is our Sun), and at least as many planets.
Even assuming there were 1,000 technologically advanced E.T.'s within our galaxy, the chances of finding one in the right star system would be one in a million.
The proverbial needle in a galactic haystack.
#2. Speed is of the essence
What's more, our Milky Way spirals across 100,000 to 180,000 light years. (A light year is the distance light travels in a year, roughly 9.5 trillion kilometres.)
To traverse 'quickly' across the galaxy, you'd need to get close to the speed of light. (Light speed is believed to be the speed limit for all matter in the universe: approx. 300,000 kilometres per second.)
Our current best speed? Less than 15 km/s, or 0.005% the speed of light.
Even if a technologically advanced species were able to travel at 50-75% of the speed of light, it would take them 133,000 to 200,000 years to get from one end of the galaxy to the other. (Not counting the time it would take to drop in and have a visit.)
That’s about as long as homo sapiens, or modern humans, have existed on Earth!
#3. Right place, right time
Which brings us to the question of timing.
At 4.6 billion years old, Earth is roughly a third of the age of the universe. It may well be that our planet has been visited by aliens in that time.
Scientists believe that early bacterial life began 3 billion years ago, but it was only in the last 550 million years that life as we know it 'exploded' on Earth. Dinosaurs ruled the planet for over 160 million years before the first human species winked into existence, a mere 2-3 million years ago.
Modern humans (our ancestors) appeared only in the last 200,000 years. Perhaps our first notable 'technological' civilisation was the Persian empire of 2,400 years ago. And it was just 250 years ago when electricity came to mainstream society.
Which 'moment' would our alien visitors have spied? And would we have been worth a revisit, in say 10,000 years?
Clearly, it matters when they come visiting.
#4. Patience, young grasshopper
Since we began emitting powerful radio waves in the 1900s, our earliest signals have travelled roughly 100 light years (or 0.1% of the way across the galaxy).
Within 150 light years from Earth are an estimated 6,000 to 8,500 stars... a very tiny sampling of the billions of stars in our galaxy. Even if 'someone' did send back a message from 100 light years away, it would be a while (100 years at light speed) before we received it.
And what if the nearest space faring aliens were 1% of the galactic diameter away (1,000 light years)? Then we might not be hearing from them for another 1,800 years.
So where are the aliens?
I think it's very likely that primitive life could be found throughout the galaxy. Indeed, the basic building blocks of life (amino acids, which form DNA) have been discovered on asteroids and comets in our solar system.
But what percentage of that would have evolved into intelligent life...within the time and place of our existence? It took the Earth 4 billion years, and a confluence of 'lucky' events, for simple life to evolve into a technological society capable of space travel and communication through radio waves.
Now, suppose a technological civilisation only had an average lifespan of 10-100 thousand years. Our window of opportunity for meeting another intelligent and space-faring civilisation could be very small indeed.
A blink of an eye, really, compared to the age of the universe.