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kikie
A chance to find Earth in the Space (or even in our Milky Way galaxy only) without exactly known coordinates is much less than a chance to find a certain ant in a forest: ~1:150 billion. And this is just the top of the problems iceberg.
It depends, how many are looking and how are they searching?
Besides, there are many other factors that make it impossible for life forms from different systems to meet (in case of using a vehicle): time (life is short; although the time slows for accelerated objects, it stays the same for static ones, in other words: before anything reaches us, the humankind will possibly end its existence already)
Assuming the Sun has 5 billion years left, and nothing else kills us, that would allow a single spacecraft at current speed to cross the galaxy in a straight line 500 times (estimated from 90,000 mph of Juno probe).
On the problem of single organism life spans, you can use multiple generations, or some kind of preservation method, which have been successful with simple lifeforms.
plenty of dangerous objects and forces in the Space - meteors, comets, debris, dust, hydrogen atoms (that can wipe everything flying close to lightspeed like a sandblast), gravitational fields that can blast an incredibly fast object in atoms (if the Earth gravity is pretty strong, then we can say only one thing about massive stars (or, say, the Galaxy) - their gravities are VERY strong, and the aliens will have to calculate their route to avoid being caught in a gravitational field of some massive star, and there are 150 billions of those stars, so they are just planning their route yet (won't they fly at random?), and they've made only 0,1% if they begun when the Egyptian pyramids were being built.
[Hydrogen] atoms aren't much of a threat, unless you mean things like gas clouds or nebulas, then they might be.
What do you mean with the part about gravity blasting something in atoms?
Gravity is important because you would use it to slingshot and save fuel. You only want to avoid some bodies, others are very helpful. Think of space travel like ship travel, you're not moving fast relative to distance, you do have time to react to things.
The deeper we are in physics, the less abilities of travelling in the Space we can suppose
I think it's the opposite honestly.
I'm not saying that we have been visited by aliens, I'm only saying that we can't possible know what kind of technology they have if they exist at all. We are all thinking in our limited way about e.g. space travel.
I repeat; for all we know, they have some kind of technology to travel vast distances in a short amount of time.
Going with this actually makes things harder, because we have more to look for. It adds credibility to any crazy story that someone cooks up since the story might be true as a result.
Alien technology may be insanely advanced, but most probably it obeys the physics we know.
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