Here exists the long standing question, to find the answer is currently the historical “holy grail” and motivating desire of many branches of science and pseudo-science—shared by competing teams from different sectors of planet earth-a question posed by many thinking people everywhere on Earth
Running in the background, is the question that from time to time may gnaw at us all: "Are we living on the only rock in the universe that supports life and is Earth just a solo comic accident, an old catastrophe that underwent hundred million years of genetic trials, and beat odds in the trillions to one to bring us forth? Here are some ideas that follow such thoughts:
The distance from our galaxy to the next is probably a greater distance than even the best minds of science can actually conceive. Some say the journey would be exactly 4.5 light-years from Milky Way to the closest star in Andromeda, our nearest sister Galaxy. Such long distances present a fine food and logistics problem to visitor pilots of craft moving at the "slow" cosmic velocity of even 17,500 miles per hour. Be forewarned though, Galaxies are either coming toward us or moving away at incredible rates. Andromeda is said to be on a move to collide with Milky Way far in the distant future.
I speak here of distances in light years-not of light "hours": Light travels at 186,000 miles per second- a light minute is about 18 million kilometers of distance. The light hour is many times farther, but a light year is so far away that we have no conceptual framework to understand it-traveling 4.5 light years is a distance that boggles our best minds and therefore may be considered nearly infinite to us.
Traveling near light speed, it would take a huge ET craft moving at light speed , the top universal speed limit, more that 100 years to arrive in our neighborhood. It follows that it would take ET a long time just to locate Earth –even by tracking our radar and television waves. This may be a big problem if their galaxy is not very near our own Milky Way. We may be visited by their inconceivably huge craft. If the ship wasn't simply “beamed up” (dreamed up?) by transporter to us. Not really probable if you really look at it. Some say they must needs travel at the speed of a decree or pronouncement of mind-instantaneously one place then-voila!—arriving at another point in the next instance-all too nebulous to a scientific mind. But it could be real in our universe—especially since we are only forming a theory of “everything”.
Still our faith is strong, some of us, and we want so vehemently to in believe in technology that today is seen to verge upon magic. To the skeptic, it's a scientific "cop out" to inject such extreme science fiction into serious scientific inquiry.
Distance would force ET to depend on well planned multiple missions sending food on ahead to orbits that could be calculated precisely and lined up for the much welcome inter-galactic rendezvous—full of much needed refreshers-say about fifty thousand of them, set at way points to restock and supply new fuel, metals, and chemicals etc.
And if the alien life cycle is similar our own, the time of travel would need many generations (for human analogue types) and if real, the ET crews might wishfully be endowed with high tech longevity secrets. Cryogenics today, for example, will not even perfectly preserve a cadaver-let alone living human bodies.
These complexities should leave us well concentrated on planets within our own galaxy. Let's set aside "heuristic science"-unproven hypothesis such as "Parallel Universes" and advanced ducking in and out of time and space-such as through theoretical "worm holes" (such travel could bee extremely dangerous). There are more than 95 million stars-with too big, too hot, too weak, too dangerously situated planets to support evolving life forms using a similar time dependent evolutionary processes to our own.
Notwithstanding, the prize of finding such a unique mate to our own planet has attracted astronomers who are on the verge of announcing that finds like Gliese 581 (See Wikipedia Encyclopedia Giliese 581) both are planets from outside our galaxy-also thought to exist as "water planets "uniquely located in "safe zones" near somewhat similar sun stars. Problem here is that they need to have a wobble stabilizing moon such as our Moon to act as a “dynamic stabilizer” and provide water currents to circulate oceans and keep them oxygenated.
Water planet Gliese has some large brother and sister planets to run interference from destructive asteroids and meteors, but it wobbles in its orbit, way too fast for many kinds of life. Life can't usually take too climate variation-like ultra high / ultra low rapid temperature differentials
ET would also need to come from a star system (or two) located safely away from the black hole near the center of their (and all) galaxies, and they would need to live near a Jupiter-like massive gas-giant planet to catch Comets, Gamma ray and X-ray bursts. They would need to be somewhere close enough to the Jovian gravity to limit collisions with space rocks, planetoids, and cosmic ray storms and the like-A kind of necessary big brother.
Our exhaled guests would be from a planet that provides stabilized weather similar to our own and maintains somewhat mild conditions for many millions of years to evolve their similar human-like forms. The alien’s planet needs a magnetic field to shield their rock from steady cosmic radiation and particles emitted by its own sun star. Human mammalians relay upon lower life forms in a great diverse biosphere for the food and phytochemistry and beneficial bacteria most probably analogous to the needs of ET. Like Humans they are likely dependent on bacteria and even insects like the ones we depend upon-even the common house fly or something similar- for long term survival.
The ET life form would have to be very tough and capable of greater brain power that we humans enjoy or more time to use what they have, and their environment have to have been relatively safe and stable even longer than our requisite 100MM years, and they might have had to survive more than the two extinction events such as the ones we have suffered- that's for toughness.
Then consider that there might be a problem of saurian life versus mammals. Would their gentleness with their young equal to or better than ours? Is kindness necessary to some degree to have civilization and high offspring survival-to live safe and relaxed enough to create and experiment for many millions of years to evolve the technological culture capable of inventing a craft advanced of anything we can currently conceive. There are many ideas being bandied about that claim that ET might sport aggressive ways too dangerous to attempt communication-hoping that we might obtain a shortcut to wild and incredible technologies. It's not surprising that inventors always say that fuel driven rockets or even solar wind or ion drive machines are "dinosaurs in terms of advanced propulsion". Let them come up with better. The latest postulate is something that warps space out ahead of it and therefore pulls the craft along-nice idea-show us!
The alien life form would have needed eons of time to develop intelligence and to have deployed advanced "team play", and would have to have been( or be) a predator or hunter-gatherer that was hunted itself- to develop a quick response to challenges great enough to need to preserve education, share insights, language development, reading and writing or libraries or an equivalent means of storing and advancing alien science, philosophy, psychology, ethics, technology-the evolved and improved knowledge that would help them advance beyond of our human evolution.
Another condition must be met: basic necessities of the alien life form and needed internal chemistry may needs be transported from the alien planet for aliens to constantly refresh the needs of their bodies by rendezvous with cargo on their horrendously long journey.
Many statistics and narratives seem to support the probability that ET exists somewhere; but such life may take inconceivable forms or even ultra primitive forms and to result in nullifying any practical use of conversation, we would also have to be hyper-intuitive or ethically and morally enlightened intuitive observers to do anything but fight over the evidence, material items or whatever being we might encounter. Human greed being what it is, the phrase “human nature" has usually denoted adequate greed, would the discovery or encounter with ET be shared by its discoverer or would it become a jealously guarded secret—information or an item to be sold in an underground, shadow market to draconian buyers.
Do we not need emissaries who are visionaries with great patience, hearts of gold and cool minds to represent humankind in an all important encounter of this magnitude? And would the lucky ambassador to ET need the manners and self control of a great leader or to be wise and courteous enough with strange minds that are not culturally similar- nor adequately reverent. Our alien beings might not revere the sanctity of life as we should! What if they have no concept of self-control and think killing our hopeful representative would be interesting sport? Humans have done such despicable acts in similar situations. Would they?
Our communication method for exchange of technology and knowledge exchange would have to be advanced, and their tech might not be easily understood. For the experience to be safe enough to be mutually beneficial, we might have to retain some of them, but what if they were beyond retention in some unknown way? How then, would we improve our tech or data mine—reverse engineer their craft?
The problems of meeting with ET and cautiously achieving a world changing advance technology transfer are many. Should we banish the thought?
Some science fiction writers propose that receiving advanced tech of many kinds could possibly wreck the economies of Earth. Example: I once imagined a hand held device that when aimed at a heavy object would convert it to something like a hologram, and the object could be moved-regardless of weight-without effort to any location that the device is aimed, and then set down effortlessly. In cyberspace we do the drag and drop. Imagine what a real high tech tool as the above might do to our concept and values relative to gravity-in other words: think not only of the work we could handle, but the jobs that would be nullified. The “auto-lift” was contrived in a novel of mine about two young girls who liked to hunt wild mushrooms who found...oh well, I might not be the first to have thought of it! Such a thought compelled me to throw six months of effort away. That was back when Stephan Spielberg came out with "Close Encounters of the Third Kind" and the computer was, at that time, not quite as user friendly.
Healthy skepticism is part of the scientific method. The method is evolving and improving-as we speak. Science has it’s beliefs and concepts that may be limiting, but scientific method is designed to remove a lot of preconception and dogmatic bias from the discovery process and it’s the best we have, and better than many other kinds of belief systems for it’s insistence on observable facts and provenance—as it seems to me.
* Science Channel: Stephen Hawking Videos: Paradox etc. See also Edwin Hubble "Big Bang" discoveries