Why couldn't extraterrestrials be visiting Earth?

While few people seem to doubt the possibility of extraterrestrial life in some form, a far more contentious topic is that of extraterrestrials actually visiting Earth.  The primary argument against extraterrestrial visitation seems to stem from what we know about the vastness of the universe and the limitations imposed by physics.  What I'll present here are a few ideas that may have you thinking again about whether or not ET could visit us.

If we assume that every star in the sky has a planet just brimming with life, some of it intelligent, then those beings are going to have to travel for hundreds or even thousands of years if their technology is equivalent to ours.  Even moving at the speed of light it would take them quite a while, depending on which star they were coming from.  There are a few counters to this though.  If the civilization is older than ours (and it need not be drastically older) then it may have discovered the secrets of faster-than-light travel or perhaps even instantaneous travel between two points in space.  We tend to fall into the trap of thinking we know everything about the universe and how it works when we're actually just scratching the surface.

These beings may also not be biological at all; they may have advanced to a point that they have merged with machines.  This would make time of travel less of an issue for them.  Even cryogenics could make it less of an issue, depending on distances.  Take into account the time dilation present when traveling near the speed of light and they buy themselves even more time so they could actually make the journey (and possibly the journey back) within their lifetimes.  Or these beings could live and die on an entirely different time scale than we do.  They could have lifespans on the scale of thousands or tens of thousands of years (or more).

We tend to think of our solar system as being pretty barren with the exception of Earth.  What if ETs are coming from other planets in our solar system though?  Then the technology doesn't have to be nearly as advanced.  There could be civilizations hiding out on the Moon, Mars, Venus, and possibly the moons of the outer planets, potentially in those underground bases we hear so much about.  Their origins aside, if they are coming from some place close then it doesn't seem that improbable anymore.

With all of these ideas thrown out there, some people might ask "Why would they come here at all?"  Why wouldn't they come here?  If they're anything like us then they're probably very curious.  We can't really pretend to know the motives of extraterrestrials though as they are most likely very different from us.  The question of "Why?" might have to remain unanswered until we can ask them ourselves.