We have to assume that life exists
We have to assume that life exists elsewhere, since earth alone has millions of species. A planet with adequate conditions promptly produces life IMO. Now we could argue what those conditions are, debate the Drake equation, etc., but after all that discussion we will end up just having to assume it exists and live without the proof. We will never make contact. Here's why:
- passive radio beacons will not be detected because of the inverse square law of radiation, they will be many orders of magnitude too weak.
- lasers must be aimed with an accuracy of 1 billionth of an arcsecond to target the inner orbit of a nearby star. Less accuracy and it could miss the entire solar system. The beam will also become wider with distance so therefore must be focused with the same accuracy, 1 billionth of an arcsecond, otherwise the power level (watts per square meter) at the receiving end will be far too weak to detect.
- a true beacon would have to split a single laser beam millions of ways, sending each beam to a different star system. Each beam would require optical tracking systems 6 orders of magnitude more accurate than those of the HUbble Space Telescope, capable of aiming and confining the beam within 1 billionth of an arcsecond. Since the laser beam is split so many times the original beam would have to be proportionally more powerful, perhaps on the order of trillions of watts. The beacon would have to be maintained and operate continuously for however long it takes, perhaps thousands or 10's of thousands of years before giving up hope for a response from a particular star.
- detecting an alien beacon would be equally improbable.
- an optical telescope capable of seeing city lights on a planet 74 lightyears away would have to have a diameter of 414.7 kilometers. The number of stars theoretically within this distance of Earth is about 800,000, and most have not been discovered yet. Such a telescope is impossible to build currently, although optical interferometry is in the works with much less ambitious goals under NASA's Origins Program. None of the planned projects could see artificial light sources on another planet even in theory.
- interstellar travel is impossible. The simplest problem to define is survival of the spacecraft under constant bombardment from micrometeorites. At any reasonable speed, a tiny 10 gram meteorite would impact with the energy of many tons of TNT. No spacecraft could be built to survive and still carry enough propellant to get it up to speed.
So you see, whether alien life exists out there we will never know. We must assume it does, just as life exists on Earth. But we need to understand that life is both isolated and confined to its native solar system. Perhaps this is a temporary condition and there is some higher plane of existence, but that is a taboo subject for the science community. I tend to think there is but do not pretend to understand it.