According to Dr. H. Paul Shuch who is an expert on this area, our current technology could detect earth-like TV and radio leakage signals from an extra-solar planet 1,000 light years away:
It is entirely possible that technologically advanced species exist on other planets and already have visited us. And decided we just ain’t ready yet.
What, we need to be fattened up?
Yea, and we’ll be soon ready.
There are a lot of possibilities. It is certainly possible that someone like this is in orbit right now, but invisible to our sensors.
However, when it only takes one aggressive/expansionist species a few million years to span the galaxy, I would still interpret the lack of evidence for intelligent species to mean that there just aren’t very many. (Especially so when natural selection favors aggressive expansionist species to such a large degree. That should be true on any planet.)
He doesn’t specify what kind of man-made radiation he is talking about, but probably the most detectable emissions would be various kinds of radar, especially the tight-beam radar that is used to map asteroids. Of course this sort of radiation is very directional, so you’d have to get lucky. Another highly directional emission would of course be deliberate messages aimed at likely candidates; I don’t know if any alien species would be daft enough to do this, however.
The payload might be the size of the spacestation, but the rocket would probably be a lot larger. To get between the stars, a self-repping system might need to use very large, and very bright, rockets. We might detect such rockets at significant distances, especially if there are lots of them.
“Energy based beings” are a staple of sci-fi, but what does that even mean?
I was going to post an analysis like this on why I suspect we’d be able to see alien probes - heat signatures, if nothing else, because getting rid of waste heat is not easy for a spaceship - but there’s so much we don’t know about what is possible. Anything we can theorize from current knowledge is very likely to be detectable by our current technology.
Still… I do acknowledge the possibility that HP Lovecraft was right and we’re continually surrounded by fourth-dimensional Cthulhu-spawn. 
A very good book on the subject: Webb, Stephen (2015). If the Universe Is Teeming with Aliens… Where Is Everybody? Seventy five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (2nd ed.). Copernicus Books. ISBN 978-3-319-13235-8. (I only have the 2002 edition which only contained 50 solutions.)
The meaning is perfectly clear, even if the physics is totally speculative. One can easily imagine a self-sustaining “living” reaction in a complex of magnetic and electric fields. Self-focusing impulses of light actually do exist; a whole blort-load of these, interacting with each other, might accumulate into a “photonic computer” of some sort, and thus generalizable to a “life form.”
It’s also perfectly legitimate for such a speculative notion to permit a substrate of matter; a glowing ball of plasma, maintained in a self-organizing manner, would be an “energy being” as much as a “matter being.”
You’re right that the idea cannot be precisely modeled…but, c’mon, we all know what it “means.”
13.8 billion years sounds like a long time, but compared to how old the universe is going to get, it’s still a baby. Assuming that until relatively recently the universe was a very dangerous place full of GRBs etc, it’s possible Earth is among the early generation of worlds with technology. It’s still an open question whether we’ll ever leave our solar system much less colonize it. We haven’t yet.
My personal favorite WAG is if there are spacefaring races they’ve probably spent so much time in interstellar space that’s their preferred habitat and we may not find them until we ourselves wander out that far.
I’d be pissed if I found out they are out there watching and haven’t popped by to give us some suggestions. What kind of jerk do you have to be to see an emerging species still suffering from war disease and famine and not help out? Dicks.
The thing I find ridiculous about the Fermi Paradox is that according to it, the human race doesn’t exist. We haven’t colonized the galaxy. I guess if someone on another planet comes up with their own version of the Fermi paradox, they’d conclude we don’t exist. Also, it usually only takes into account the Milky Way and excludes the billions upon billions of other galaxies that may harbor planets and life. The distances are just too vast to say for certain that “Welp, because we haven’t seen complete colonization of our lone Milky Way galaxy, life must not exist!” I think it’s idiotic.
You’re misreading the issue somewhat. The paradox doesn’t conclude that other civilizations don’t exist: it asks the question, “Why have we not seen evidence of them yet?”
Essentially, you’re transposing an implication into its converse, which is bad logic.
“If other civilizations exist, we should have evidence of it.”
This is very, very different from, “Because we have no evidence of other civilizations, they must not exist,” something that no one has said, suggested, or claimed.
None of that follows. There is nothing in the Fermi Paradox that requires us to have already colonized the galaxy. And I have no idea what you mean by saying, “it only takes into account the Milky Way.”
The Fermi Paradox is simple: It says that if we are not unique, then there should be billions of other planets with life on them. And since we’ve come along many billions of years after life should have been possible (i.e. solar systems like our existed), there is no reason to believe that we should be the first ones to reach an advanced technological level of civilization. And given the rate at which our technology is advancing, it therefore stands to reason that there should be alien civilizations so powerful and capable of manipulating so much energy, and capable of sending self-replicating probes through the galaxy, that their presence should be visible to us.
The Von Neumann Probe issue is an important one. We are probably on a trajectory to have the ability to build such a probe within hundreds or maybe thousands of years. That’s a blink of an eye in cosmic terms. And if we can do it, you’d think someone else would have already done it. And if they did, even if those probes only travel at a fraction of the speed of light the exponential nature of their replication means they should be swarming through the galaxy.
Unfortunately, the Fermi Paradox is called a paradox, and that’s not accurate. It’s more like food for thought. What don’t we understand? Why are we not seeing this? What are we missing? Those are the questions the Fermi Paradox tells us to think about.
I’m not sure we should view Von Neumann probes as an inevitability, or put much significance in their apparent absence. We’re a long way from building them and who knows what our views about them will be when we finally could build one. Human babies make good self-replicating probes too. Either way, your probes are phoning home and potentially telling the whole galaxy where you live, and we may decide that’s not prudent.
When we say it would only take a million to 10 million years to colonize the galaxy, aren’t we admitting we’re a dangerous infection? How are infections generally treated?
Well… the very first infection isn’t treated. Put one bacterium in a pot of chicken broth and within hours, you have a pot full of bacteria. Now if you add a new bacterium, there is a conflict to see whether one wins or whether they reach an equilibrium, but the first one is going to colonize the whole pot in short time.
Intelligent species make the outcome a little less predictable. They could choose not to colonize the galaxy, but it still seems like a given to me that the first species that wants to colonize the galaxy will do so on a time frame that seems long to people, but is short on geologic/evolutionary scales.
Any species that wanted to stop this overrun would have to overrun it themselves, if only to monitor other species that might be more aggressive.
Doesn’t matter how many Earth-like planets there are, or how many civilizations. They’re just too far away to reach.
Even if we built a spaceship that flew at the speed of light (which is preposterous in and of itself), the overwhelming majority of space would still be much too far to reach in any practical way - by the time our brave pioneers made first contact, empires would have risen and fell down here, and once more before they make it back, rendering any contact irrelevant. The same naturally applies to the putative aliens.
I’m not sure I agree with you about that.
First, if you had light-speed travel, the time dilation means that the trip would be near-instantaneous from the passenger’s perspective. Kingdoms might fall back home, but for you and your family of colonists, it was a two-year trip to what might be a whole planet full of unclaimed land. Some people (perhaps crazy religious cults, unfortunately) would find that very attractive proposition no matter what happened back home. Here on Earth, plenty of groups have made what are essentially one-way moves to uncharted territories.
Second, a self-replicating probe would be a lot like the Voyager probes. There was a short-term goal, but they are still providing useful information decades later. We’d be looking at perhaps 100 years to get to the nearest stars (assuming 5-10% of light as a more realistic speed), but then it would produce data on a continuous basis forever.
Third, humans have a relatively short life span. There’s no reason an alien species couldn’t live 10,000 years and have a very different perspective on what “too long” means.
Fourth, by the time a race gets to Dyson-sphere technology, they can’t live entirely on their planet anymore. That much energy would fry the planet. So they probably live on space stations of some sort. Once you’re used to living on a space station, it’s not such a stretch to put engines on it and send it off to some other star. Even if you know it will be your grandkids who arrive, the impact on your personal life is pretty small because you were always going to be living on that space station. Only the view out the windows has changed.
So on a scale of millions of years, galactic colonization is just not that big of a hurdle. It’s certainly appears to be easier than getting intelligent life in the first place.
Perhaps the universe is teaming with super-intelligent civilizations capable of FTL travel. If they exist, they’d realize they weren’t the *only *super-intelligent civilization in town and they’d take measures to assure they aren’t easily detected by any super-duper-intelligent civilization capable of knocking them out, like Mike Tyson against a 5 year old kid. I’m pretty sure such civilizations would operate in stealth mode (i.e. no leaky signals of any type traversing time and space announcing their whereabouts).
We, on the other hand, have *not *evolved to the point of realizing we’re playing with fire transmitting episodes of I Love Lucy out to the Great Beyond. We’re like fattened beefsteak prancing around singing, “dinner’s ready, come and get us.” I have no doubt there’s at least a few squadrons of interstellar spaceships manned by super-duper intelligent space-crew closing in on our solar system right now, wielding knives and forks…and bottles of A-1 sauce.
IOW: If advanced civilizations exist beyond our solar system, they’d probably want to detect us before we detect them and they most likely have the wherewithal to assure that happens. Let’s hope they prefer pork.