Have aliens ever visited Earth? There are those that believe they have, but what does the science say? Is it even possible for a physical being to transverse the galaxy? The answers may not be so clear, and in fact, they may surprise you.
How Would Aliens Get to Earth?
Before we can even address whether beings from another world have indeed visited the Earth, we must consider how they would have gotten here in the first place. Based on the fact that we have never detected extraterrestrial life in our own solar system it is safe to assume that they would have had to travel from a distant solar system. Even at nearly the speed of light, it would take decades to make the trip.
Or would it? Is there a way to travel the incredible distances of the galaxy faster than the speed of light would dictate? Well, yes and no. There are several theories (explained in great detail here) that would allow for such travel to exist. But under close scrutiny, it does not seem possible. Or if it was, a living being would not survive the trip. Of course, this statement assumes our current level of technology and understanding of physics. Until the 20th century it was thought that the Universe only extended to the edge of the Milky Way Galaxy, the concept of other galaxies was foreign to science. And it wasn't all that long ago that rational people would have thought of landing men on the Moon as an impossible proposition.
So is it impossible? No. But at this point, our understanding of the Universe would indicate that it is extremely unlikely. At the vary least it seems that interstellar travel will involve science and technology that man has yet to even dream about.
Is There Evidence That We Have Been Visited?
Let's assume for a moment that it is somehow possible to transverse the galaxy in a reasonable amount of time. After all any alien race that were to visit us would clearly need to be more advanced (at least technologically), so perhaps they simply have discovered technology or understand science that is beyond our current understanding. What evidence do we have that aliens have actually arrived?
Unfortunately much of the evidence is anecdotal. The pictures of UFOs are usually very grainy, or at the very least lack the crisp details that would stand up to scientific scrutiny. Most of the time, since the images are usually taken at night, the photos and videos are nothing more than lights moving in the night sky. But does the lack of clarity in the images and videos mean that they are fake (or at the very least useless)? Not exactly. Photographs and video may shed light onto phenomenon that we can't explain. And who knows, perhaps someday a crisp image may give us the detail we need to actually have proof.
What about physical Evidence? There have been claimed discoveries of UFO crash sites and interactions with actual aliens (dead and alive). However, the evidence is still inconclusive at best. Most of the physical evidence lacks corroboration or any witness at all. Some things can't be explained, but that does not necessarily mean that they are alien.
However, it is interesting to note the evolution of the evidence. Specifically, in the early 20th century nearly all stories of alien space-craft described seeing something resembled a flying saucer. And as for the appearance of alien beings, they looked mostly human. More recently however, aliens have taken on a more, well, alien appearance. And spacecraft began looking far more advanced. As our own technology advanced, the design and technology of UFOs increased proportionally.
This is something we can't ignore. It appears that the description of aliens and their craft are correlated with our bias and belief of what they should look like. As our understanding of science and technology evolve, so does the evidence. The simplest explanation for this is that our societal and environmental influences are causing us to see things as we want to see them; they fit our expectations. Had we actually been visited by aliens our perception and description of them shouldn't have changed. Unless of course the aliens themselves have changed, and had dramatic increases in technology, over time. This seems rather unlikely. Ultimately, there is a lack of any conclusive proof that we have been visited by alien beings. In fact the odds seem heavily stacked against it.