There was a similar thread some months back
. I'll just repost what I said in there:
The study of Astronomy is like history: the study of things past. Only with astronomy we have no idea what the current situation is because of the vast distance and time required (thousands, millions, billions of years) for light and sound to travel to our mini planet.
Unless faster than light (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. 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 5,000 light years away would need to have peaked technologically at least 5,000 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 10,000 years beyond that (think of the two planets separated by 5,000 (light) years each travelling along parallel lines of time (upward for future let's say). Their signal leaves at A and arrives at Earth 5,000 years later in which time both have moved 5,000 years along the timeline. We respond with a signal back that takes another 5,000 years to arrive at A. Not exactly instant communications.
So do I think they saw UFO's? They certainly saw something and because they can't identify it that means it's an Unidentified Flying Object (UFO). Does that mean this object was "Extraterrestrial"? No. Just regular air crafts and the mind playing tricks. Nothing more.
Does that mean there aren't other lifeforms out there somewhere? It would be arrogant to think that we are alone in this vast space. But those on other planets are probably struggling with their own limitations for space exploration, just like we are. Will we ever meet with one another? Doubtful unless we can each overcome the limits of time and space travel.
For those that don't know about "light years":
|A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More precisely, one light-year is equal to 9,500,000,000,000 kilometers.
Now if a place is 10 light years away, that is more than 100 trillion km! It would take you 10 years travelling at a constant speed of 300,000 km/second to reach that destination.
Then there is the whole "mass" problem
The more you accelerate towards the speed of light, the more massive and the more energy required to propel the mass. That's why the only thing nearing light speed is, well, light (massless particles). By the time you're moving 80% the speed of light you'll need a couple million spare stars for power.
Longetivity, modes of transport, take-it-all-with-you are all speculative unfortunately. The problem with the latter 'entire biosphere' ideology is that, for instance, Earth isn't a closed system. It has been fed by asteriods, comets, dust, solar radiation, and extra-solar radiation for billions of years. Life on Earth is dependent completely on the Sun - ignoring vent life and subsurface microbes. So, you'd essentially want to take your entire solar system out for a spin - what would you use for propulsion?
I know I sure wouldn't want to come face to face with any species that has the ability to throw their solar system around like a football. And where would they park it? I'm sure that we would notice if a solar system was parked in our back yard,