Quote - Read Larry Niven's "Protector".  You may not think trying to contact aliens is such a great idea after that. :)

Wasn't it Stephen Hawkings who also had this type of reservation on making such contact?

I think that we're pretty safe.  Unless FTL or forms of ultra-space traversal are possible - despite what all of the SciFi genre promotes heavily - there is little chance of any two sentient species from different planets meeting unless the planets reside in the same solar system.  My calculations of the time required just to jump from one galaxy to the next at light-speed is in the millions of years (!).  That pretty much rules out extra-galactic invasion without FTL travel.  Heck, the travel time to the nearest star at light-speed is something like 4 years (at more realistically possible speeds it may take 20 to 40 years).

Interestingly, the situation is bleak just by merit.  Like evolution on Earth that took a long time (and stable conditions) for a sentient species to evolve (4.5 billion years), I think that the universe operates in similar ways.  It is very probable that conditions weren't stable enough for so many billions of years (of the estimated 15.5 billion) for planetary systems to evolve.  Also, theory shows that elements of more complex types required long times to accumulate by the dying of many stars.  In other words, sentient lifeforms may not have been possible more than 5 or 10 billion years ago which would mean that sentient lifeforms may only 'now' be popping up in the universe due to stability and a rich source of elements.  And since the universe is like a history machine, each sentient lifeform would be practically isolated by several factors: expansion of the universe and speed at which light (EM) travels.

For instance, a sentient lifeform 5000 ly away would need to have peaked technologically at least 5000 years ago (our time) for us to receive any transmissions.  And for them to receive our transmissions they would need to have survived technologically for 10000 years beyond that (think of the two planets separated by 5000 (light)years each travelling along parallel lines of time (upward for future let's say).  Their signal leaves at A and arrives at Earth 5000 years later in which time both have moved 5000 years along the timeline.  We respond with a signal back that takes another 5000 years to arrive at A.  Not exactly instant communications. :)

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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