This timeline is expressed in "millions of years" (Ga)
Current theory and observations suggest that the universe is 13.75 ±0.17 billion years old (W).
In 2007 the Keck telescope, a team from California Institute of Technology found six star forming galaxies about 13.2 billion light years (light travel distance) away and therefore created when the universe was only 500 million years old. (W)
As of 2004, the age of the oldest star in the galaxy yet discovered, HE 1523-0901, is estimated to be about 13.2 billion years, nearly as old as the Universe (W)
The Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.
let's extrapolate :
If this last figure seems small to you, considering the vast interstellar distances, remember that the Galaxy is only 100,000 light years accross - and with exponential growth to take into consideration, an average speed of 1/10th the speed of light should not be a major problem.
Since we are only talking about our Galaxy, the first 5,000,000,000 years do not count, and within our Galaxy, an additional 5,000,000,000 years are needed for first-generation stars to be born and die, and for second-generation stars to form. First generation stars do not possess heavy metals and cannot have planets, much less life on them.
We may of course consider that the first second-generation stars have been formed 6 or 7 billion years ago, but why not stick to the beautiful round figure of 5,000,000,000 years. Starting from then, all planets are on their own (I do not believe in extraterrestrial origins of life, but even so, the history of all civilisations begins 5,000,000,000 years ago).
5,000,000,000 years ago, evolution started on one star, and around the same time also on others. 5,001,000,000 years later, the life form that started on that star has colonised the entire Galaxy. Now we may say, why 5,001 ? Why not 4,937, or 6,285 ? We might say that evolution takes about that time to happen, but it is extremely unlikely that this 5,001 is a special number, and that at this moment in time, several thousands of intelligent races are about to take their first extrastellar steps. A universe with Humans, Vulcans, Klingons and Romulans, all at about the same point in their technological evolution, is a very unlikely event indeed.
Put this argument another way : at some moment in time, call it 0MY (million years), the first second-generation star, capable of evolving intelligent life, is formed. At some other moment, let's say 1,837MY, our sun is created. around 5 billion years later, life from our Sun has colonised the Galaxy, just put that at 6,928MY. If the same thing has happened on every of the 1 million earth-like planets in the Galaxy, don't you think the first of these colonisations must have happened earlier, say, at 5,246MY. That is 1,700,000,000 years ago! There was barely any life on our earth at that time!
Is the main point in my argument. If there are others, they should have visited us, and left some sign. If there are others, there are lots of them (maybe millions). Why, after all our searching (including decades of SETI), have we found not a single trace of any of them ?
Now some may think there is some "Prime Directive" at work. Aliens have visited us, seen that we show potential, and left us to our own evolution (and perhaps left a sentinel on the far side of the moon). All very nice, and precisely what we would do, if we found a planet with Ape-like animals on it.
But my main argument is that the first intelligence to wander out among the stars, must have found almost barren planets out there, and colonised them. If the main life-form on some beautiful planet you find is some moss, would you have ethical problems in colonising that world ? Especially if every single planet you find, has some different kind of moss on it ? You'd probably create a sanctuary for the indiginous life forms and terraform the rest of the planet.
Evolution takes 5 to 7 billion years to reach "intelligence", but 1 million years later that intelligence has colonised the entire galaxy, leaving no room for a second intelligence to evolve.
Or just the first? I conclude the latter. We have found no traces of Galaxy-wide civilisations, neither out there, nor on our own planet. But there must be life out there, and we are just the first to reach intelligence.
I don't believe so. If for some reason we are first (here at 6,998MY) and the second will come at some normal distribution rate, that would make it 7,013MY, or something. We might reach that planet and conclude that it will have a civilisation within the next 20,000,000 years, and leave it to itself. We just colonise all other 999,999 planets we find. So for all practical purposes, if we are first, we must also be alone. (I have elaborated on this argument in a separate piece on the statistics of my argument)
No it doesn't. Someone has to be the first. The second one does not get a chance to write this same essay. Since I am writing this, we can be the first.
No it isn't. I don't like to prove something by stating that the reverse would be impossible, if I cannot make at the same time the statement at least probable.
I have said that the evolution of life on earth is a natural phenomenon, which should happen on any of the 1,000,000 stars that are like our sun.
I don't believe in luck, not on this scale.
There must be something about our earth, or about our solar system, that is unique, or in some way "uniquer" than around similar stars.
Look up at night : the Moon.
Yes. The ratio of weight between primary and satelite is 1/81 in our system. The similar rate for any other body in our Solar system is very much larger (13,000 for Ganymede, 4,000 for Titan, 3,000 for Triton) Only the Pluto-Charon double planet is even closer (7), but that system is in such an eccentric orbit that it was probably formed through some form of collision. The earth and moon are in an almost circular orbit around the Sun, which seems to indicate that they were formed together, not the result of some later (near-)collision.
The fact that Science is not sure about how the Moon did originate also contributes to my belief that the earth-moon system is a very unlikely occurence. (Scientific theory seems to settle on a near collision with a Mars-sized object - also a quite unlikely event in my opinion)
Well, one solution is that we are here because of it. If the moon were not there, nor would I be, so I could not be wondering about it.
You would not believe that the Moon could directly be involved in the evolution of intelligence. But there is no need for it to be directly involved. As I said, we need not be alone, we need only be the first.
Maybe there is some direct (inverse) relation between the size of a planet's major satelite, and the time it takes for intelligence to evolve on it. Then if we have the biggest satelite in the Galaxy, this explains why we are the first, and thus able to write this.
So a better question is :
Well, I can think of several ways :
I am not saying that these are conclusive arguments, but they do help in making acceptable my opinion that we are alone.
Well of course many of these ideas have been mentioned in Science Fiction stories, of which I have read quite a few.
There are only a limited number of variations on the theme of extraterrestrials :
I am a great fan of all of these, but that has no bearing on my personal beliefs :
I have received two reactions so far, which I present here and here
Herman De Wael - Antwerpen, Belgium
written 1999-02-23 - last adapted 2000-04-06