Sun, 27. October 2019
Are We Alone In A Hostile Universe?
"Are We Alone In A Hostile Universe?" That was the title of a recent lecture at Macarthur Astronomy Forum, given by Professor Geraint Lewis, an astronomer from Sydney University.
Thirty years ago I was an optimist about other civilisations living on planets orbiting other stars in the Milky Way. SETI leaders kept telling us that discoveries were imminent. Look at all those stars! Some of them must have planets and the chances of alien civilisations existing must be very high! After all, the Universe wasn't created just for us!
Whilst that was a commonly held public view, I began to lose my optimism. Over the last fifteen years I've written about my diminishing expectations but I never heard about professional astronomers examining the issue in any detail.
Until Geraint came and gave his talk.
Whilst we will never know for certain that we ARE alone, we've been searching for decades and have not been able to gather any evidence that we are NOT alone.
The "take-home" information, which Geraint used to hose down expectations, is summarised as follows, together with some of my own thoughts:
The Galaxy
The Solar System orbits in the galactic "habitable zone". The galactic core region is too active. The outer fringes of the galaxy are chemically poor.
There is no evidence of catastrophic astronomical events occurring in our galactic neighbourhood - gamma ray bursts, supernovae explosions, black holes etc. - so we've been lucky but such events do occur commonly in the galaxy, and even the galactic "habitable zone" can be a dangerous place.
No evidence that other civilisations exist in the galaxy. No radio signals despite decades of listening.
Solar System
The Sun is a stable G type star. It is not a variable star or other type of unstable star. A lot of stars can be ruled out. For instance, giant star types - O, B or A type stars, which emit radiation deadly for life as we understand it and which in any case would not live long enough for life to flourish before the star dies and goes supernova.
At the other end of the scale, M type stars (red dwarfs, the most common star type) are believed to be unstable and unlikely to promote life.
The Solar System is a single star system. Planets around double or multiple star system would exist in unstable orbits. Single stars are less common than double or multiple stars.
The Sun has an expected life-span of ten billion years, suitable for life to evolve.
Much of the debris of the early Solar System has been swept up, especially by Jupiter and cataclysmic events are rarer now.
By eliminating O,B, A and M type stars, together with double/multiple stars systems, white dwarfs, red giants and stars outside the galactic habitable zone, we have probably ruled out civilisations in 90% of the stars in the galaxy without even considering other factors!
Earth is Lucky
The Earth is in a stable orbit around a stable star.
The Earth is in the habitable zone around the Sun where water can exist in liquid state but not all bodies in the Sun's habitable zone are suitable for complex life to flourish. Venus and Mars are also in the habitable zone, as indeed too is the Moon. Only one body in four evolved advanced life.
The Eath's orbit is near circular.
The Earth's rotation is fast and not tidally locked like Mercury.
The Earth is a 'wet and dry' world with 70% water.
The Earth has a molten core which generates a magnetic field to protect us from harmful solar radiation.
Plate tectonics have helped form our planetary surface.
We have a large Moon relative to size of planet, providing a desirable tidal effect on the oceans.
Our atmoshere is finely balanced, with enough trace gases to trap the right amount of heat to keep the planet warm, without a runaway greenhouse effect like on Venus.
Evolution
The likelihood of life evolving beyond a primitive state may be remote. Even if it does, it is not guaranteed to progress to higher intelligence.
Civilisation
Civilisations, if they do exist, may face potential catastrophic conditions (called Great Filters), beyond which they cannot progress, e.g. over-population, degradation of environment, nuclear war, excessive religious intolerance, astronomical calamities, disease, rise of an adversarial species, etc.
How long can an advanced civilisation evolve and survive for? A hundred years? a thousand? a million?
As I mentioned in a previous article: assume, pessimistically, that 10 million advanced civilisations have existed in the Milky Way in the last ten billion years; and that the average duration of such civilisations is 1,000 years; then, spanning that 10 billion years, an average of just one advanced civilisation existed in the galaxy at any particular time. 10,000,000 civilisations could rise and fall without ever existing at the same time.
What actually defines an "advanced civilisation"? The ability to speak? or to read? or to communicate by radio? or to stop killing each other?
If alien civilisations exist in the galaxy, they are likely to be advanced enough to have already colonised the galaxy - but there is no evidence of this and they are not here.
As Geraint summarised:
- The Universe is a hostile place and we may truly be alone in the Universe.
- Even if life arises, it may never progress past slime.
- Even if higher life arises, it may never progress to intelligence.
- Even if intelligence arises, it might face a Great Filter.
- We might truly be alone in the Universe.
Whilst some of that is speculation, it is probably closer to the truth than many others would believe.
However, we should be cautious with words like 'believe'. I'm pessimistic where I was once full of optimism but the only truth, to date, is:
"We don't know."
😐