o sung wu,

" considering the immensity of deep space, which were are told, goes on indefinitely?"

Please note that these authors are only talking about the observable universe. They've come up with two numbers:

1. The chances of life in our galaxy - the Milky Way. That is life within 100,000 light years.

2. The chances of life within the observed universe. They make no claims about what might or might not exist in places we can't see and aren't even sure actually exist.

So, its possible to suppose that some civilisation exists in some far off (over 14 billion light years away) galaxy. But since we can't see such a place and don't know it exists, its futile to speculate about it.

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SteeleRedux,

If the chances of another civilisation existing in the Milky Way are low, the chances of such a civilisation being at a similar leave of development as us, are less likely still by orders of magnitude. Imagine where we'll be in 2000 years or 200000 years - a mere blink in time on a cosmological scale.

Current thinking is that, within the next 2000 years (and probably earlier), given how far we've come so far*, we'll be able to head off to colonise the galaxy. Guesstimates are that those endeavours will result in every habitable part of the galaxy being occupied within 250000 years.

So if a civilisation had evolved say 1 million years prior to us, that civilisation would have already reached the point where it could have and probably would have, been here.

Now I know there are all sorts of assumptions that they might be hiding or might not be interested in us or in expanding beyond their little patch. But, if the earlier Drake numbers are even close to true, that there are up to 1000 civilisations in our galay alone, then surely at least one of them would have evolved like us to expand out and colonise. But we can be pretty damn sure they haven't.

The galaxy is ours for the taking. And we'll probably take it.

/cont

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* my grandfather (born 1899) could remember his mother reading him a story about how these two Americans had been able to fly a plane for a couple of hundred yards. He was still alive when Armstrong walked on the moon. Such strides in one lifetime. Imagine the strides that could be made in the next 50 lifetimes.

Umm, why are we assuming that alien life will have the same biochemistry as life on earth? Unless the panspermia hypothesis is correct, that doesn't seem like a terribly sound assumption.

Loudmouth said- "Meanwhile, back in the real world, we have enough resources to feed everybody, if it were shared around. There are regions on this planet which are barely developed, given with our latest technology, if people could get hold of it. So as world population growth slows down, perhaps to ZPG by 2100, and as efficient productive technology becomes cheaper for developing countries across the world, the need to go to some other planet will diminish (like, say, growing tomatoes at the South Pole: possible but a bit expensive), along with the realisation that we are effectively alone in the multi-verse."

Answer- I think the UN says that Zero Population Growth won't occur till 2200 (not 2100) topping out at something like 11-12 Billion- but there is a range of possible outcomes- anything could happen before then- just look at the changes in the last 150 years. But it's always about power- a Confucius scholar said that the institutions of the community are critical for its stability- ie. if the institutions are irrational then the world will become irrational. There needs to be a way of encouraging countries to engage in sane policy- the UN was supposed to do this- but being malformed- won't achieve it.

Dear mhaze,

I hardly think our carbon based forms will be exploring the galaxy but our silicone imprints quite possibly.

In my opinion it is this technology which will allow our sentience to shoot for the stars;
http://www.sciencealert.com/scientists-put-worm-brain-in-lego-robot-openworm-connectome

Just imagine your entire neural network being transferred to a chip with sensors replicating your senses. Far more robust for the huge acceleration/deceleration forces involved in efficient space travel.

It is far better than trying to shift huge amounts of food and water vital in keeping a human alive. In many ways our bodies are just vehicles for our minds. There may come a time when the type of vehicle matters little.

However we should be clear that intelligence and interstellar awareness are not the same. Imagine intelligent life forms on a planet with constant high level clouds blocking out the heavens, or even a water world. How much of human advancement has been determined by the ability to view the night sky?

So when we ask are we alone are we are generally talking about intelligent life forms. What is your assessment of the likelihood of life on other planets?

Toni Lavis said- "Umm, why are we assuming that alien life will have the same biochemistry as life on earth? Unless the panspermia hypothesis is correct, that doesn't seem like a terribly sound assumption."

Answer- With respect- perhaps the issue isn't about biochemistry but more about the complexity of the coding. How would the DNA/ coding be constructed through random natural selection for "a cell created inside another cell". Once this occurred it could be possible to modify the code for outer cell to create specialist cell walls and other structures specific to different functions such as nerve cells, epithelial cells, etc- required for higher animals. There would be presumably a path from the first single celled eukaryotic cells to more advanced structures to flora and fauna. There is talk of Gonium Pectorale a 16 cell algae (eukaryot) one of the first multicellular organisms. Eukaryotic cells appear to allow greater specialization in the outer cell- one of its functions could be as an interface between the nucleus and the outside environment.