Much ado about Stephen Hawking's admonition we Earthlings should not loudly advertise our existence to aliens because they might not be very nice to us.
Given the solar system was formed in roughly the last third of the age of the Universe, 4.5 billion years ago, it would seem that any evidence of past colonization efforts over the last 4 billion years would be observable, say, on Mars, which has a bedrock surface that has not changed much since its early formation, minus a few big, extinct volcanoes.
The question of visitation is a question of archaeology, not listening for signals from space. Planetary archaeology, thus far, shows not a single sign of any habitation or visitation anywhere. Lunar mapping can now show objects on the Moon’s surface as small as the Apollo lunar module. But no signs of any other “craft.” No other “craft” in 4 billion years?
While photo resolution on Mars is not as good as the Moon, still, there are no signs of anything except entirely natural features. Eric von Daniken aside, not a single bit of evidence of visitation on Earth has ever been found, even though our fossil record goes back more than a billion years.
Given the past observable record, it seems ‘visitation’ that leaves permanent, observable marks has been non-existent in our solar system for millions of years. So it seems a very slim chance that now, suddenly, “they” will appear, coincidentally just as “we” are technologically advanced enough to look for “them.”
Problem: Space is 99.9999999998 percent Space
The problem with space is that it's well named. It's almost all blank, empty space. Just space. Lots and lots and lots and lots of empty space. Empty. Space. It's like a one act play where the play writer includes a stage instruction which says, "pause and look expectant toward stage right for 1 million years, then deliver next line."
But that's in part because our little solar system is in a sparse part of the outer arm of the Milky Way galaxy. There are other parts of space that have a lot more stuff than space as compared to where we are.
One example is the middle of a giant globular star cluster like Omega Centauri. As NASA notes at its Hubble Space Telescope site: "The average distance between any two stars in the cluster's crowded core is only about a third of a light-year, roughly 13 times closer than our Sun's nearest stellar neighbor, Proxima Centauri."
A third of a light year is "only" 2 trillion miles. For comparison, Pluto is 3.6 billion miles from Earth. That's about one three hundredth of a trillion, or the length of one football on a football field. So even if we were in the center of one of the densest star clusters in the Milky Way galaxy, the nearest next star to us would be 650 times farther than from Earth to Pluto. That's the length of one football on two football fields with the space for the band thrown in. Another more stuffy and less spacey spot is smack in the center of our own galaxy, but the dimensional difficulties described above are still there, if not worse.
So if we were in the center of a tight globular star cluster or the center of the Milky Way, there would be a lot more stuff than space compared to where we are now. We're kind of in a galactic Podunk. Or Easter Island. The last place to get strung up for cable. That's us. And there's nothing we can do about it. As the French say, il est teh suxxor.
First Contact: A Rorschach Test for Homo Sapiens
Lacking a scintilla of scientific evidence for any past or present contact or existence of any space aliens evarr, the concept is like eating cotton candy. Or as Detmar Schnitker said, "When you have no data, you can speculate all you want." And so we humans do. I cannot resist.
But precisely because there is no data to weigh, except in the negative, what you get is a Rorschach Test of each person's ideas about themselves and the world as they perceive it, including all their innermost fears, insecurities and ambitions. Asking the question, as Stephen Hawking did, of "how would aliens treat us if they found us" is really an exercise in holding each respondents' concept of themselves and humanity up to a carnival mirror. The only data to weigh is how people think of themselves as filtered through how they predict aliens would think of us. These comment threads give a good statistical cross-section.
Astrophysicist Ethan Siegel labels Hawking's statement "cowardly." Bad Astronomer Phil Plait agrees with Ethan, in the sense that he wants to side with his optimistic side, saying in essence that any alien advanced enough to travel light years to Earth would be advanced enough to have overcome any avaricious, rapacious tendencies it once may have had. This is all internal projection, of course, since no data exists to show either side is right or wrong. Without actual aliens, actually expressing their intent, or any evidence they even exist (or ever existed), it's a wide open playing field. It's like a poker game without rules where you can invent your own winning hand just by saying the cards you have is the "best" hand. But the other guy can too. It's like a board game where the rule book is missing so everyone can invent their own rules.
But Let's Play Anyway ...
If we accept the speed of light as a barrier, then "robot probes" to our solar system from other stars would be ineffectual, since they could not report back data for centuries, and most likely could not even do that because the signal strength would be too tiny and would be lost in the background. So, the only rational purpose for any visitation would be colonization, with the colonists having no hope of ever making a return trip home. And for any carbon-based, DNA based life, Earth is the only habitable planet in the solar system, or anywhere near it.
Earth is a pretty unique place in the neighborhood of 10 light years diameter from us. It is the only habitable place for carbon-based life forms. Nothing else even remotely fills the bill.
So colonization and inhabitation would be the only purpose for a visitation. And it would most likely not go well for us, since our visitors would be massively more technologically endowed.
Hawking for the win.
Now we need to stop destroying the one and only inhabitable planet known in the Universe.
Wait ... I'll take Bristle Cone Pines to block !!!
The Bristle Cone Pines of the Panamint Mountains around Death Valley U.S. America are the oldest living creatures in the known Universe. They can live to be 5,000 years or older. [The story of how the Methuselah Tree was cut down by a couple of idiot biologists in the 1960s to see how old it was, and then, after killing it, they found it was the oldest living thing in existence until they killed it, simply proves Stephen Hawking is correct at multiple levels.]
Bristle Cone Pines can live for 5,000 years or more. This is 50 to 100 times older than any Earth mammal. We don't really know how long whales can live, since we killed them all, but we know at least a few survivors of our harpoon insanity can live far longer than humans. But even whales can only live at best 10 percent as long as Bristle Cone Pines.
Now if an intelligent alien could live as long as a Bristle Cone Pine, then a long space journey would not be a big deal. For a creature that lives 5,000 years or more, interstellar space travel at sublight speed is not a long trip. It's only a real long time because us humans have a very short life span compared to trees.
Was Earth First Invaded by Alien Trees?
It's possible. Trees are, by far, the longest living creatures known in the Universe. And longest-living is the key requisite for interstellar space travel. You've got to be alive when you get there. And you can't be impatient during the ride.