It think, though, that if there is intelligent alien life out there, we need to be careful with it. Look at our own planet, when the Europeans and the early Americans met, entire populations were wiped out by diseases for which some humans had no immunity, but others did. What would be case with alien diseases? Some wicked virus that is harmless to an alien would perhaps wipe out the human race......perhaps it's best to keep a low profile.

There are some interesting bounds estimates one can make. Personally I think life is probably extremely common, order of 1 per 100 to 1 per thousand star that will have one or more planets with enough free energy and interesting chemistry to make life a good bet, advanced life (structured animals) moderately rare (only certain planets will have the right temperature conditions and physical structure to lead to sufficiently competitive evolution), intelligent advanced life extremely rare (as it may not even be stable -- it seems as likely to destroy itself as to persist based on human history so far). I think one could work out the odds a bit better if we could pin down the kinds of processes and the environment that led to abiogenesis originally, although there may well be more than one that would work -- life elsewhere may not resemble our life. Robert Forward's book: Dragon's Egg and its sequel Starquake and David Brin's Sundiver suggest some of the ways life might be very strange indeed and arise in unexpected environments that have a lot of free energy and complexity but are nothing like Earth.

The most important bound estimate is obtained from looking at Earth as what would almost certainly be a highly desirable piece of real estate, at least for carbon/water based life forms that were anything like us. It has lots of water, a moderate temperature, lots of land, lots of oxygen. If intelligent life were commonplace AND if interstellar travel (with existing or as yet unknown physics) were feasible, the probability of our being visited sometime over the last billion years or so would be quite great. And if we were visited, I'd expect the visitors to have moved right in. Habitable planets are either rare and valuable to also rare intelligent lifeforms or they are common as dirt, and so in intelligent life. Either way our NON-colonization is a significant data point all by itself. It suggests that either intelligent life is rare or that interstellar travel really is all but impossible (or both). With a slightly weaker possibility that this is not true but there is a complex galactic society that protects indigenous populations from contact or exploitation (ethical aliens!).

I'd rather see our understanding of physics increase, our human civilization to stabilize, our solar system colonized, and for US to consider moving out into the galaxy as a whole, a human horde as it were to goldly blow where mon has never been a bore. It would certainly increase our overall probability of long term survival in the Universe -- right now one big rock falls from the sky, our sun gives one little hiccup, a nearly star gives one little hiccup -- and there we are. Dead and gone, not even bones left behind.

Of course running into another civilization that finds us very tasty could have the same effect, but I'd rather take my chances on that, I think, especially after a bit of diaspora.

rgb

Earth is only good real estate to life that evolved here. Science fiction would have us boldly going from one planet to the next, without a space suit if it has a breathable atmosphere. However, we are intimately entwined with the biology of this planet: the essential bacteria in our digestive tract; the proteins we need; our immune system evolved and trained for our cradle Earth, to name but a few.

Aliens likewise would have the same links to their planet. It would be a massive job of planetary engineering to sterilise Earth and make it safe and then seed it with their own biology at the same time as ensuring their own systems did not evolve out of control. We would face the same problems moving out into the galaxy.

Also, it would not solve any overcrowding problems on the home world, you just can’t move that many people.

While biology is a huge problem, an even bigger one is technology. Physicist Bob Park says each year he gets his new intake of students to calculate the cost of transporting people and their life support systems to the nearest star. It is just astronomical (forgive the pun). Also, it’s a one way trip taking generations to get there and no help once there. Again aliens have the same problems.

I agree with others here, other life is very likely in our galaxy, simply because of its mind boggling size and the length of time it’s had to evolve (many times over even). But I can’t see it as a threat to us for the many reasons outlined above.

We should, and probably will, move out into our solar system but as for going further I’m very sceptical of its probability and practicability. We have become too used to science fiction: there is no ‘warp drive’, no ‘transporter’ and no instant communication with ‘Starfleet Command’. These are just tools to move the plot along at a good pace.

I have no real arguments with that -- it is one of the sad things about becoming a physicist. I still enjoy SF, but I do have to work a lot harder to suspend disbelief these days...;-)

However, there is always the possibility of new physics. Go back 150 years, and ask the man on the street (or for that matter most scientists) about the possibility that one could fly through the air from New York to China, and when you got there drop an object the size of a living room sofa and utterly destroy Beijing with it, then fly back home (all without touching down upon the ground) and they would laugh hysterically or put you in Bedlam. And that's been a concrete possibility for fifty years now -- it isn't even new news.

As for the biological problems -- most of the pathogens that attack the human body co-evolved to attack the human body, or other bodies like the human body. Perhaps aliens would be susceptible to human diseases a la H. G. Wells. More likely IMO by the time we are able to fly to another star between breakfast and dinner (if this is ever possible without the aforementioned expenditure of resources and energy) dealing with antagonistic biology will be a piece of cake.

And even if the biology was extremely hostile, eliminating planetary biology altogether from space is easy. You get this really, really big rock (100 km in diameter would do nicely, although using a few dozen 10 km rocks would work better) from elsewhere in the star's system and drop it/them. Bang, instant sterilization of the land via pyroclastic flow, combined with first boiling, then freezing (nuclear winter) the oceans. Come back in 100-200 years and seed your own bacteria, plant your own crops, move right in. If the rocks you drop were e.g. nickel-iron, you have the next million years of steel immediately at hand to use to build with, all cooled off and ready to forge.

I fully realize that just because enormous paradigm shifts that have brought about changes in civilization so profound that what we now take for granted would have gotten you burned at the stake a few hundred years ago have happened -- frequently -- in the past, there is no guarantee that they'll happen in the future. Our current understanding of physics may well be converging to completeness, with no radically new stuff left to be discovered. It may well be that there really is no way to move economically from star to star, that our genetic destiny is bound irrevocably to Earth for a distinctly finite time, that even the planets here will prove to be too expensive and hostile to settle and populate.

This would make me sad, I have to confess, at least if I were around to watch it all unfold that way. In my blessed ignorance, however, I can nurture at least a dream that it will turn out to be otherwise, even though to me personally it will probably not matter a whit.

rgb

I too like the dream, but it's a bit like religion isn't it? Wishfull thinking and then you want to believe. :o)

It is a lot like it. Which is one reason that I think that religion can be quite useful, as long as you keep a decent bit of perspective. If we didn't have mad impetuous dreamers who refused to believe what "everybody knows" is true, we'd still be bashing antelopes in the head with rocks to collect dinner. Iconoclasts rule!

rgb