In the many worlds idea discussed on this wiki, every action causes a new world, but let's say we go back in time, all the way to one cell organisms, to when life first evolved. Could there be creatures that are so vastly different than us in appearance, in culture, in ideology, in technology, in literally every aspect that if they came to this dimension they would be viewed as alien, despite the fact that they're earthlings from another dimension?
If the divergence started back when unicellular life first appeared, then the degree of difference would have extremely large. Essentially this means an alternative Earth with a possibly entirely different evolutionary history.
Stephen Jay Gould considered that if you could rerun the evolutionary history of planet Earth you would end up with was an entirely different biosphere. We would recognise some organisms because their body shapes were moulded by environmental factors, for examples, dolphins and icthyosaurs have similar body forms but otherwise are entirely different kinds of creatures.
One thing you do need to consider is that we don't know how likely it is for sapient life to arise on a planet like Earth. Assuming there are vast numbers of alternative Earths all with utterly different evolutionary histories there still could be precious few where sapience had developed and further along a technological civilisation to go with it. This is something that could be used in a story.
If they came to us, then they have sapience, technology and everything that goes with those characteristics. While they would be Earthlings there is an extremely good chance they would be remarkably alien Earthlings indeed.
Considering that in the many-worlds interpretation of quantum mechanics the world splits into 10^1,000 worlds per second, this means over the span of evolutionary history which goes back possibly four billion years there would be a massively astronomical degree of divergence (as life on planet Earth seems to have started quite early after the planet settled down). Just work out how many seconds there in four billion years and the result will be an utterly unbelievable number of worlds. You can make your other dimensional earthlings as alien as you like. Have fun with it!
The short answer is that we don't know, so imagine away.
I'm not sure why you would need to go back to the beginning of time to imagine strange creatures, or why they would have to originate in different dimensions.
There are more stars in our galaxy than there are grains of sand on any beach you care to name. Each of those suns has planets revolving around it. For that matter there are more galaxies in our universe than grains of sand on said beach as well.
The statistical probability of us being the only life in our galaxy, let alone universe, are vanishingly small - basically to the point of impossibility.
Would some of those life forms look utterly alien to us? You betcha.
Would some of those life forms be sentient, and perhaps vastly more intelligent than us (and thus able to travel to our planet)? Again, you bet. As Neil DeGrasse Tyson (a very famous astrophysicist) put it, the genetic difference between us and a chimpanzee is less than 2%, yet look at our level of intelligence vs them. Now imagine an alien race that is 2% different from us in the same way that we are from chimps. To then we would be nothing more than brain damaged children, and our technological advances would seem like a big joke.
To cut to the chase, take anything you can imagine, and by sheer statistical probability, that creature has, does, or will exist somewhere in our universe. In fact, it's more unlikely that we would meet an alien race and have something in common with them, than that they will be utterly alien, and we won't be able to get along.
Definitely, sort of. It is easy to imagine different evolutionary paths that could be successful. But they will probably still be carbon based organisms and even though they would look alien, they will also look familiar. That, I guess will depend on your definition of alien. Some probable results would be flying trees and intelligent blob like water borne creatures.
Umm, yes? That is to say, it takes relatively little difference to call someone an alien - plenty of earthlings are called that now (often with a prefix of legal or illegal, to be sure). The word itself pretty much just means "foreigner".
Of course, the term is also used to refer to space aliens, which is a very different thing with all sorts of other assumptions included. It's still possible for someone to be considered alien with relatively small differences - look at the rubber forehead aliens that weave through our stories, it doesn't take much to give an alien feel. It would be pretty easy for any visually exotic people to be considered "alien from outer space" if combined with a few cultural differences.
If the world in question split even just as recently as when homo sapiens first evolved, they might look like rubber forehead aliens to our eyes - an exotic change might be roughly equivalent to racial differences if it's visible enough, and the human race evolved those in the in that time (blue, or furred, or tailed are even genes present in our population, and they might have succeeded in the alternates). Split a little further back, and they would be genuinely alien, a sapient species that is not human - and depending on how far back you go, they might be really physically, chemically, evolutionarilly different.
Of course, the other major factor in being considered "alien" is the act of labeling. If the rubber forehead alternates called themselves "human" or "earthling" from earth, or identified themselves as from an alternate timeline, they wouldn't be considered "space aliens", just the regular kind.
If they used different linguistic terms or labels (after all there's no reason the language would be the same if the appearance isn't), they might easily be believed to be a different species. If no one knows they're from an alternate version of Earth, it would be easy to assume they're from a different planet, whether or not the planet is known to be in a different universe or not (and maybe no one notices it's actually an alternate timeline, how could we tell?). Maybe people will assume they're from a different planet, somehow handwavey-transported through space instead of dimensions. Maybe they'll think the planets are different in their respective dimensions.
So, yeah. You can get aliens from other dimensions, and maybe never know they're from a version of earth. It just takes a few assumptions.
There are three basic classifications for beings which visit Earth. Interstellar: the one many people assume is the ONLY type.
Time travelers: a difficult thing to accept, but both aliens and the American military possess technology which allows them to travel through time in both directions. This technology is used daily. Strange but true.
Interdimensional: Many alien craft are what are known as "jumpers", meaning they originated in another dimension and have the ability to jump between that dimension and ours. This is why UFO's are so often reported to suddenly pop into sight as if from nowhere or to completely vanish from sight--they are not moving through space, but between dimensions.
It is entirely possible for alien races to belong to two of these classifications, or even all three. They can originate in a different solar system and a different dimension and also travel through time to arrive here at the present moment. Most aliens do appear to fit into at least two of these categories, though not all do. There is also at least one non-human intelligent race which appears to be native to Earth and to live deep underground in cavern and cave systems which have been enlarged over time.
If you doubt the time travel aspect--as I did myself, before seeing it used personally--have a look at the cards from a game which was published in 1995 called the Illuminati Card Game. On the cards you will see drawings which are exact representations of the Twin Towers and the Pentagon on 9/11, including impact points at precisely the right locations, the Fukushima event, Julian Assange and other things besides. They had knowledge of these things when the card game was designed.
These things are out of box, they are hard to accept as true. But the bottom line is, out of the box is what it's all about when you're dealing with highly-advanced alien beings. If it's not out of the box in some way, it probably has nothing much to do with aliens.