​We are not Alone, Except that We Are...

As Douglas Adams has famously pointed out, the universe is big. Really, very, incredibly, biggly. This is common knowledge today, but we only learned this fact about 100 years ago. We named the coolest scientific instrument ever invented after the discoverer: Hubble. We actually have no idea how big the universe is. It could go on forever. The universe we can see is at least 25 billion light years across. Even though the universe might be finite in size, fear not, there is no chance of falling off the edge. Just like the spherical Earth, there is no edge. This idea of “the ends of the Earth” edge persisted despite the fact the Greeks knew the Earth was roughly spherical since about 300 BC.
Light is the universal speed limit. Practically all of physics relies on this fact. Certainly the physics of the very large (big stars and stuff) and the very small (atoms) are peppered with references to “c”, the symbol for that speed. (It is a curious fact that the speed of light has almost no influence  at our scale. About the only time you might be aware of it is during live satellite communications due to the substantial distances involved.)
For the record, that speed is 300,000,000 meters per second. Very large distances are often measured using the time it would take for light to make the journey. Hence the previously mentioned “light-year”. There is absolutely no reason to believe that this speed can ever be exceeded. UFO fans often believe otherwise. They are misinformed. Kip Thorne (who just won the Nobel for physics), in his book Time Warps and Black Holes, speculates on how the cosmic speed limit might be beaten. It usually involves getting from point A to point B without going through the intervening space. He speculates that this might be possible, but his speculations rest on fantastically exotic and largely hypothetical forms of matter and energy. To master such forces would require machinery built on a god-like scale. I discount these fanciful ideas for the same reason that I do not believe in god.       
While the actual universe is fantastically large, our bit of it is relatively small. We live in the Milky Way galaxy. The nearest galaxy to us is around two million light years away. Two million years is much longer than the time homo Sapiens have even been talking to each other (about 40 times larger). It is hard to imagine us ever talking to beings in another galaxy, the distances being so huge. Just to “hello” and get an answer would take 4 million years. Ordering Andromeda take-out is out of the question.   
Our galaxy is still pretty big though. One hundred thousand (100,000) light years across and includes 100 billion plus stars. This is still a daunting size. The fastest object we have ever launched—Voyager— is moving at around 60,000 km/h. This is only one 18,000th of the speed of light. The nearest exo-planet to the Earth that we know of is about 10 light-years away, making it about 180,000 years away at Voyager speed. Of course, we could launch faster vehicles if we want, but it isn’t cheap and we would need a good reason to do so.
It is a classic first year physics problem to calculate the energy required to move a man on a spaceship from a standing start near Earth to close orbit around the nearest star. The trip must be at one gee all the way, thus making for a comfortable ride for the astronaut. One gee of acceleration for the first half of the trip and one gee of deceleration to come to a stop near the target star. One Earth gravity, or one gee, means going about 10 meters per second faster, for every second. After just one day, the ship will be going more than 900 km per second… much faster than Voyager. Making ridiculously generous additional assumptions (infinite supply of weightless fuel, weightless engines and so on) it would still take all the power mankind can master, or ever has mustered to pull this trip off. In other words, if we add up all the wood, coal, oil, A-bombs, hydro, nuclear reactors, etc ever used throughout history, it would still not be enough energy to do the job. And when we get there, we would probably find nothing.
The upshot of all this is that if we ever expect to find company in the universe, we had better use light (or rather, radio waves) moving at the fastest speed there is, but even then, the conversation will be excruciatingly slow. 
How far away could the hypothetical exo-civilization be in theory? It depends. If the conversation were one way only, it could be quite far away. As long as the signal from the other civilization was bright enough, we could listen in. If the other civilization’s “ears” were really, really sensitive, they might be able to listen in on us. But how about chit-chat with a civilization just like ours? I did a back-of-envelope calculation on this subject. It involves a unit of electromagnetic flux called a Jansky. If the other civilization’s “ears” (actually huge radio telescopes) are equivalent to ours, they would have to be within about 400 light years of Earth for us to both chit and chat. This is essentially our back yard…. less than 1% across the galaxy. IMHO, it is unlikely that this relatively small bunch of stars and planets contain such a civilization. We have recently discovered that many stars have planets. We always suspected as much but confirmation has only come in the last few years. That is the good news. The bad news is that it is becoming more and more apparent that planets come in many shapes and sizes… and most of them are nasty. The Earth’s characteristics, like being in the Goldilocks zone (see below), are rare. How rare we do not know yet. Even being in the zone is no guarantee. Venus is much like Earth, and in the zone, and it is a hell hole.
Aside: The “Goldilocks zone” is the distance from the star one should be to allow for water to be in all three forms on a planet: liquid, gas and solid. It is just one of many criteria that are thought to be necessary for life.
If we were to find a civilization similar to ours inside this sphere, it would be truly amazing. The conversation would be slow, and perilous: we cannot say if “they” are friendly. But the conversation would be possible, and we would know that we are not alone. In fact, if we found one that close to us, it would not be a huge leap to imagine a galaxy riddled with life and intelligence… a la Star Trek. I love the idea, but again, I suspect it unlikely.
So get used to loneliness. The universe may be crowded. In fact, I think it must be. Even at one civilization per galaxy, we have a universe with billions of intelligent species. But our ability to talk with them is constrained by the cosmic speed limit, and those constraints make it unlikely that we will find a sister civilization anytime soon. Another curious fact is that the cosmic speed limit actually opens the door to sight-seeing the universe because of something called “time dilation”. But that is another topic.
Carl Sagan had it right. Our Pale Blue Dot is it. We can toodle about the Solar System all we want, and no doubt we will, but our only home is on the Earth, and if we want to be here for a while longer, we had better take care of it.