Some of the reason given for belief in aliens were absurd, of course, but, in contrast to a belief in gods, there is sound logical argument for thinking that life, and thereby possibly intelligent life, would exist elsewhere in the universe.
The maths is relatively simple: leaving aside Earth, there are so many, maybe half a trillion, galaxies in the universe, each with somewhere around a trillion stars, that the probability of several, maybe very many, stars having a planet on which life evolved approaches certainty.
However, because of the distances involved and, for all practical purposes, the limitation on speed imposed by relativity, the probability of us coming into contact with any of them approaches zero.
How do we get to that conclusion?
For the planet to contain the right elements for life to evolve it would need to form from the accretion disc formed by a second or third generation star. This means a star formed from the remnants of a first or second generation star, which in turn means it's unlikely to have formed before the universe was about 5-10 billion years old. Given that we know it took about 3.5 billion years for intelligent life to evolve on Earth to the point where we could even consider space travel, let alone undertake any beyond the nearest planets in our own star's system, there is no reason to suppose life on any other planet would have evolved technology any more quickly and no reason to suppose they could have overcome the limitation of Relativity, so they will not be travelling at anywhere approaching the speed of light.
Within our own galaxy, the nearest star is Proxima Centauri, a star which is gravitationally associated with the binary star system known as Alpha Centauri consisting of Alpha Centauri A and Alpha Centauri B. Alpha Centauri B has a single planet, the Earth-sized Alpha Centauri Bb. It is possible that this is the closest planet on which intelligent life could have evolved, though there is no evidence for this one way or the other and it's likely surface temperature probably rules it out.
This system is about 4.37 light years from our sun.
This means that light reaching Earth left the Alpha Centauri star system 4.37 years ago. A spaceship travelling at the speed of light would have needed to set out 4.37 years ago to arrive here tomorrow. In practice, of course, even if it could attain a speed of half that of light, it would take some 9 years to reach us, not allowing for acceleration and deceleration. Travelling at the sorts of speeds our spaceships currently achieve, it would need to have set out tens of thousands, even millions of years ago to get here tomorrow.
And this is the closest body which could conceivably be a location for extra-terrestrial life. Most of the likely candidate planets will be many millions of light years away. So, it doesn't take much imagination to work out that inter-galactic travel, and even intra-galactic space travel, is for all practical purposes, impossible. Quite simply they wouldn't have had time to get here yet.
Never-the-less, it is interesting to speculate what we would discover about such an alien civilisation, were we ever to contact one.
We can be sure of both these things because we know that religions always diverge and splinter, never converge on the truth but always lead to violent conflict over which is the right truth because there is no factual basis by which differences can be settled and no objectivity by which disputes can be decided. We know that the same religion has never arisen twice independently despite all their supporters believing theirs was given to them by all-knowing, all-powerful deities. We also know that science always converges on the same answer no matter how or where it arose and no matter what the beliefs of the culture from which the scientists come. We know the reason for this is because science looks objectively at evidence and is always prepared to re-examine, revise and replace opinions when the evidence demands it; that science is an open-minded process for discovering and moving ever-closer to the truth. And we know that religion is a closed-minded psychological process of self-deception and delusion used by the ignorant and intellectually indolent and by the charlatans who parasitise them.
1. ReligionOne thing we can be sure about, given our own experience of contacting other civilisations consisting of members of our own species on this small planet, is that, if they were religious at all, their religions (there is no reason to suppose they would only have one, if they had any at all) would be nothing like any that we have on Earth. None of their creation myths, if they have such things, will bear any resemblance to our creation myths. If they have gods, they will be different to ours, their gods will have different powers and will demand different rituals to ours, just at ours differ one from the other. Some of their religions may have many gods, some may have one, or at least a supreme one. Some may have no gods at all.
However, if they are religious, we can be sure their history will be littered with wars between followers of different religions and with examples of different priesthoods assuring their followers that they are killing unbelievers for good reasons demanded of them by gods who love them or to rid their world of the evil followers of evil gods intending to do them harm because they hate them.
If they have or have had religions, their history, like our's, will not have been a peaceful one.
2. ScienceWell, we can be as certain that their science will be exactly like ours as we can be certain that their religions will not be. We can be sure they will know the velocity of light in a vacuum, and that it will have the same value we have discovered. We can be sure they will know about atomic structure and will have identified the same elements and isotopes we have identified and will have discovered the same number of the same elementary particles in them that we have discovered, though they will have different names for them. They will have discovered the same half-lives of the same radioactive elements as we have discovered. Their electrons will occupy the same energy levels surrounding the atomic nuclei that our have and will take exactly the same amount of energy to raise them to a higher state as our do. They will know how catalysts work to speed up chemical reactions.
They will have discovered electrons and electricity and their electrons will have the same charge relative to a proton as ours have. They will know about protons and neutrons and that a neutron is a proton bound to an electron. They will know about photons and about wave-particle duality because they will know about quantum mechanics. They will use a form of the Feynman equation and will understand quantum entanglement and decoherence at the macro level. They will also know about bosons and gluons and quarks in all their different flavours and they may know more than we do now about the underlying fabric of reality and how (and if) quantum mechanics and Relativity can be unified.
They will know how stars and planetary systems are formed and it will be the same way we understand it. They will know about the uncertainty principle and the Planck length. They will know about the cosmic microwave background radiation, the red shift and the rate of recession of bodies in space. They will know about the Big Bang and how long ago it happened and they will agree with us on all the important measures, though they may know them with greater accuracy.
They will know about gravity and how bodies move in a gravity field. They will use the same mathematical formulae to calculate trajectories and the rate of acceleration due to gravity. They will know the laws of thermodynamics and how energy can change form but not be created nor destroyed. They will know the gas laws and the chemistry laws of constant composition and multiple proportions. They will know Avogadero's number and how to calculate the volume of a given weight of gas at a given temperature and pressure.
They will understand Archimedes principle, how gears and levers work, how to calculate the length of the hypotenuse of a right-angled triangle and that the ratio of the circumference of a circle to its radius cannot be expressed rationally in the same units as the radius. They will know that e = mc2.
They will know about evolution by natural selection and how it leads to biodiversity, change in complexity and fitness for survival in any suitable environment, and how accumulated change over deep time pushes an organism up an improbability gradient towards peaks in the possibility landscape. They will almost certainly understand the genetic code, which will very probably be based on DNA and RNA. They will come from a planet which has biodiversity and life forms which can be arranged into hierarchical trees based on morphology, anatomy, physiology, genetics and differences between body proteins, and they will all show descent with modification from common ancestors and all these trees will be nearly identical.
They will know all these things. The only difference will be the terms and the basic units of measurement they use and maybe the base of the maths they use, otherwise our science will be identical to theirs, though it may not be so advanced. They will know all these things because science is a truly universal concept and deals with truly universal realities.