I recently made a flippant response to a question on the internet question-answer site, Quora. My answer was about extraterrestrials, and I was perhaps naively surprised to discover that my one line response then surfaced on Facebook.
Well, to set the record straight, what follows is a more considered response about the possibility of extraterrestrial life beyond our own world. To put it simply, I believe that the existence of extraterrestrial intelligence is extremely likely because of the abundance of galaxies beyond our own solar system.
We know from our own experience on earth that life is characterized by an extraordinary abundance and diversification. Life in an almost bewildering variety of different forms is exuberant and very resilient, and it thrives even in toxic environments.
Outside of our own solar system, there are an estimated 17 billion earth-sized planets in our Milky Way galaxy alone, and outside of our own galaxy, there are an estimated 200 billion more galaxies in the observable universe (although a recent German super-computer simulation puts that figure much higher, at 500 billion galaxies). It’s not unreasonable to assume that most of the stars in these galaxies would have planets, and many of these planets would be in the Goldilocks Zone of being able to support life.
Also, many of these galaxies are older than our own, so the chances are very high that, yes, the universe has the potential to be teeming with life, including intelligent life.
So, considering this prodigious potential for suitable planets in our own, as well as in other galaxies, it would be extremely unlikely if we were the only forms of intelligent life in the universe.
So, considering all the possibilities, why haven't we heard from ET?
The simple answer is that the vast distances involved make communication as we know it extremely difficult. For example, our own Milky Way Galaxy is about 100 to 120 million light years in diameter and 10,000 light years thick.
The speed of light is 300,000 kilometers per second. The maximum speeds that humans have been able to generate are puny in comparison.
For example, it took the Voyager 1 space mission 35 lunar years to pass beyond our solar system and reach interstellar space. Voyager 1 was launched in 1977 and it is still the only manufactured object to leave our own solar system (on September 12, 2013).
Voyager 1 is travelling at a relative velocity to the sun of 61,000 km/h, or 38,000 mph, or 17 kilometers per second. This speed sounds fast if you think of it on a terrestrial scale, but it's positively snail-like in terms of interstellar travel. Or, to put it another way, Voyager 1 is travelling at only 1/18,000th the speed of light.
At this velocity, it would take Voyager 1 nearly 80,000 years to reach Alpha Centauri (if Voyager 1 were pointed in that direction, which it isn't). Alpha Centauri is the closest star to our own sun, at a distance of 4.3 light years. And anyway, Alpha Centauri is a binary star that doesn't provide suitable conditions for planetary life as we know it.
Also, the time it will take Voyager 1 to reach the edge of our Milky Way galaxy will be approximately 643,000,000 years. That is, assuming that Voyager 1 will survive for this length of time.
Another indicator of extraterrestrial distance is the time it takes for rockets from earth to get to Mars. Mars is 34 million miles away, and it takes a terrestrial rocket around 7 months to get there - one way.
OK, well, communication signals can travel at the speed of light, yes? But the vast distances involved in space travel mean that a single message could take many decades to reach a destination, and then many more decades to receive a reply. That is, if we could recognize and unscramble such messages in the first place.
Also, life is unicentric in that it develops according to the prevailing conditions around it. So it would be extremely unlikely that exoplanets would replicate Earth exactly. Therefore, extraterrestrial intelligence would likely have evolved in ways very different from our own (check out speculation on this by astrophysicist Paul Davies in his book 'The Eerie Silence').
Most likely, ET would be either a combination of organic life and artificial intelligence, or the development of artificial intelligence unimpeded by biological limitations. Stephen Hawking believes that the development of full artificial intelligence “could spell the end of the human race" – although this transition need not necessarily be catastrophic, as per the Terminator movies.
However, existing human biology is an impediment to space travel. Life forms would have to be very different from our own to survive travel beyond our gravitational system. For example, during the relatively short time involved in our own extraterrestrial missions, we know that the human body quickly suffers muscle atrophy and bone deterioration without the effects of earth’s gravity. Also, ordinary cosmic radiation causes cognitive impairment, and the effects of intense bursts of radiation from solar flares are likely to be fatal.
And what about insterstellar space ships? Well, anything able to travel to another star would have to be far superior to anything we have so far designed – our own computer and mechanical systems have a maximum life of probably 40-60 lunar years. This is less than the blink of an eye in terms of interstellar travel.
Enabling physical objects to travel at the speed of light, or approach anywhere near the speed of light, is another problem again.
For example, according to our knowledge of physics, it's impossible for a physical object to travel at the speed of light because mass would become infinite and cause all of the universe to collapse around it. Then, as an alternative, we're into the realm of Star Trek and imaginative/wistful speculation about time travel, such as worm holes, black holes, parallel universes and antimatter drives.
Not to mention the prodigious financial cost involved in designing and building an interstellar spaceship (let alone anything intergalactic), plus the fact that even the nearest possibly habitable exoplanet is far beyond the limitations of our own life spans.
It would be impossible for the same human being who takes off for another solar system to be the same person who gets there. And it would certainly be impossible for this person to later return to earth. Science fiction likes to imagine interstellar travel in terms of some form of cyrogenics, where the body is somehow frozen in time during space travel, and then woken up at the moment of entry. Yes, but would you trust computer systems with your consciousness and your life?
The search for the nearest possibly suitable exoplanet has identified Gliese 581 d
This speculative home away from home is 20 light years, or 120 trillion miles away.
As pointed out in the above video, it would take Voyager 1 over 350,000 terrestrial years to get to Gliese 581 d, even if astronomers are correct about its existence. For a better grasp of the time frame involved in 350,000 years, it’s this far back in our own history when humans were first thought to have harnessed fire.
So, yes, the possibility of (intelligent) life on exoplanets is very high. But because of the enormous distance/time/technology problems involved, it's unlikely we'll hear from ET or meet ET any time soon.