Bead of time
The above image shows a galaxy similar to ours. Let's pretend it's ours. It is hard to define its size, but an indicative measure is a radius of about 53,000 lightyears. Our sun is located in one of its arms, roughly 26,000 lightyears from the centre. In the above picture, the sun is at the centre of the red spot.
Nothing spreads faster than the speed of light (see my previous posts), including information. Assume that aliens have set up a SEPI programme, Search for Extra-Planetary Intelligence, and listen to outer space with radio receivers. Although Hertz transmitted the first radio waves in 1886-88, it took until 1920 for the first large distance radio transmission. Give or take, let's say we have been sending radio signals into space for 100 years. These first signals have ventured about 100 lightyears into space. The distance is indicated by the size of the red spot in the figure above. If aliens listen to us now, Earth is radio silent outside the red area.
If not radio waves, light from the surface of the Earth might make it unharmed (e.g., not scattered or absorbed by clouds / air) into space. Once it left behind our atmosphere, it ventured relatively unhindered into the depths of the galaxy. If aliens were located at the orange circle above, they would "see" light about 5000 years old. If they have the sensitivity and technology to gather enough information, they would witness humankind developing writing in Egypt and Sumer. Or they could observe the life at the settlement Skara Brae in Orkney, Scotland. The yellow circle marks about 20,000 lightyears. Twenty thousand years ago, the first humans arrived on the world stage. Artefacts found in Canberra, Australia, suggest an early human activity, and around this time, the earliest human footprints give testimony that humans started walking the Earth. No matter how sophisticated the alien technology is, outside the yellow circle above, they cannot know of the existence of us humans on Earth.
The Big Silence
A question, I daresay as old as humankind itself, is: are we alone in the universe? With modern technology, we are coming very close to an answer. Since about 1960, we have been intensely listening to radio signals of extraterrestrial origin, searching for intelligent life that might have generated them. It started with NASA's SETI (Search for Extra-Terrestrial Intelligence), with about 100 scientists and a profound public engagement.
The puzzling discovery is that there is none. In the wake of August 6, 1967, excitement was tremendous: Jocelyn Bell, a PhD student at the time, discovered radio pulses not too different from Morse signals, for which a terrestrial source was soon ruled out. Years of discoveries finally found that those signals are generated by rapidly rotating stars with a strong magnetic field. Those look like a beacon to us. Besides discovering what we call Pulsars, artificial radio signals have not been found until today.
Over the recent years, advances in astronomy led to a discovery of a multitude of planets orbiting other stars - Exoplanets. Our galaxy contains about 100 billion stars. The technology limits our glance to a few 1000 lightyears (remember, the radius of our galaxy is about 50,000 lightyears), but within this perimeter, about 4659 planets have been found. There even is a list of detected exoplanets. About 64 planets are potentially habitable.
This is a mystery: we start as a red dot in the top picture, but if we keep using radio waves for the next 100,000 years, our signals have traversed the whole galaxy. Suppose life evolves according to the same rules as on Earth. In that case, there is no reason not to believe that an alien civilisation has found radio waves 100,000 years earlier and has been happily transmitting ever since. Given 100 billion suns, a significant fraction of those have planets, and about 1.3% of these planets are habitable, the galaxy should be teaming with artificial radio signals, but none have been discovered.
A possible explanation of the Big Silence (also known as the Fermi paradox) is that civilisations use radio waves for a limited amount of time, let's say a few 1000 years. Their signal is like the spreading of a ripple on the water of a pond in which we tossed a stone. When the surge passes Earth, it might not coincide with our period of 1000 years when we are listening. Although the galaxy is teaming with life, we keep missing the signals from each other. Civilisations are living in their Bead of Time. An alternative explanation, which we cannot rule out, is that human life is unique indeed. The genesis of intelligent life, e.g. evolution, is such an unlikely process that we are alone in the universe. Research in the origins of life is thriving, and I am confident that in a not too distant future, we will have an answer (as we have now for the existence of exoplanets).
If we consider that civilisations are confined to their Bead of Time, a natural question is why civilisations stop using radio waves. A particularly chilling reason could be that it is in the nature of intelligent life to destroy itself. Given that we just (at historical time scales) went through two world wars and are at the brink to destroy nature's regulatory system for our climate, this explanation has its merits. A similarly dark cause is that intelligent life tends to destroy others - a widely explored theme of Military or Hard SciFi. More jokingly, it might be that all civilisations are listening and do not spend significant efforts on actually transmitting. An all-time classic is that the best evidence for intelligent alien life is that they avoid us.
A more moderate explanation is that, after using radio waves, civilisations discover more efficient ways of communication. A candidate technology would be the use of neutrinos, although it is fair to say that we do not have (yet) the slightest idea of how neutrinos can be controlled to the efficiency needed. It also could be that humankind is peculiar in its desire to communicate and explore.