I'm going to stick with physics on this one: interstellar travel is impossible.
Add common sense: even if an alien species was capable of it, it would be such an effort that they wouldn't come here for just for a picnic, there'd be a lot of evidence found of them.
Add Occam's Razor for good measure: there's a lot of much more propable explanations than aliens for this, many mentioned in this thread already.
Therefore, not an alien. Next mystery? :hehe:
No doubt there are intelligent alien species out there, but they are stuck in their solar systems like we are in ours.
This.
As far as I understand it the closest known planet that
could be habitable, Gliese 581 g, is a good 20,5 light years from Earth, and with spaceships not being capable of going even at the speed of light (granted we can get pretty close, however due to our mass it would be impossible to truly reach that speed without disintegrating into tiny particles) it takes even longer to get there. And even with that this planet only
could support life and even if it had life then what would the chances of it having any sort of truly intelligent life on it be ?
It could be many hundreds of light years to the next planet with intelligent lifeforms and even if it was only a single hundred light years, then who would be willing to spend his or her entire life traveling there ? Heck even if aliens had longer lifespans then it would still be something not to be desired since it would still be 100+ wasted years just traveling to a planet (and let's not forget the trip home...) which no one would put themselves through especially for simple reasons such as just visiting, if an alien had ever come here it would be on a mission of a larger scale than fickle feeling of curiosity and a need to visit new places.
Even if we think about them being able to freeze themselves until they've finished the trip it would still be a bad decision, unless they could somehow persuade the entire rest of their planet to do the same... imagine going to sleep and waking up 100 years later when everyone you know is either dead or has forgotten about you.
And so one can go on, basically the gist of it is the following. With the world being so massive there probably are aliens out there that can match or even out-perform the intelligence of humans, but no matter how smart they are they can't change the laws of physics, only use them for their purposes like we do, so it would take far too long to travel between planets for it to be worth it. So even with aliens to match us in intelligence we would never travel to them and they would never travel to us. So there have never been any aliens here on Earth, there are no aliens here on Earth, and there never will be any aliens here on Earth.