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Extraterrestrials Probably Haven't Found Us - Yet
kasparn writes "The Guardian today has a story about the Danish astrophysicist Rasmus Bjoerk, who recently conducted simulations on how long it will take to colonize the Milky Way. The basic idea is to send out probes in different directions (including various heights above the galactic plane). He estimates that it will take some 10 billion years to explore 4 % of the Milky Way. Since the age of the Universe is of the same order, his conclusion is that aliens can't have had time required to find us yet."
We will be in a lot of trouble if the Cylons find us first.
Why 1/10th c? Why not 99% of c? Why not faster than c? Granted faster than light travel is nothing more than theory and dreams at this point, but this article makes the assumption that other civilizations have not progressed in the field of physics any faster nor further than we ourselves have, to date.
I am, therefore you think.
Sheesh, talk about "proof by lack of imagination." This is supposed to answer the Fermi Paradox?
You can't explore a galaxy with a handful of probes. 72 probes??? First of all, if you're going to do it that way, you'd create hundreds of thousands of probes, if not millions of probes (mass production would reduce the cost). Second, you still probably wouldn't do it that way. You'd wait until you had the technology to make self-replicating probes, and the galaxy could potentially be explored in thousands of years.
Not impressed by this guy's argument.
Sometimes it's best to just let stupid people be stupid.
Come on. 4% is a hell of a lot better than your odds of winning the lottery and that happens *everyday*.
Plus he's not taking into account multiple alien races. So that's like double 4% which is almost 8%. Do that a few hundred times and you get 108%. This guy clearly doesn't understand math.
This figure of taking billions of years to explore the galaxy is utterly wrong. Actually, it only takes a few dozen million years to colonize the entire damn galaxy, which is a lot more effort than merely exploring it.
This figure is based on some very reasonable assumptions. Colony ships travel at much below the speed of light. Each colony gets a thousand years of development time from first colonization before it starts sending out its own colony ships. As you can see, even though it seems quite "slow", thanks to the magic of exponential growth, the entire galaxy is colonized in short order.
We won't merely be discovered if aliens exist - we'll be colonized. That's the most likely scenario for running into aliens. If they never spread beyond their home planet, they'll just be one star out of trillions - but if they do start colonizing, we'd find them everywhere.
Cyde Weys Musings - Scrutinizing the inscrutable
To paraphrase: But Sir! If we only send 8 probes it'll take billions of years to search a mere 4% of the Milky Way galaxy!
That's why you have to make the probes self replicating.. utilizing in-situ resources to make more probes at each star they visit, the growth becomes exponential and it only takes a few thousand years to search the entire galaxy. And seeing as we're visiting all these stars anyway, how about looking for planets that don't have life on them, but have nice suitable conditions for starting life on them. Cover a virgin planet with a wide variety of Earth lifeforms and fly on.
How we know is more important than what we know.
I need to introduce this guy to my next-door neighbor...
Insisting on "correct" English is like saying that there is only one, definitive recipe for chili.
Exactly, who say Aliens - if they exist - hasn't come up with some vastly superior way of travel (maybe instant teleportation, etc). And even if they do know about our presence, why would they care? There are most likely millions of other planets that are available, why bother fighting over one that has inferior beings on it, that will most likely destroy themselves within the next few centuries. We most likely have nothing of value to them, so what would be the purpose of them "contacting us"
...his conclusion is that aliens can't have had time required to find us yet."
Under what time frame? If an alien race has had advanced technology for 100,000,000 Trillion years, then they'd have plenty of time (and would probably have technology more advanced then sending out physical "probes"). It doesn't see likely from what we know, but I don't think we actually know that much.
Why is it that scientists think that only what we can achieve is possible? It's like us looking for aliens using our technology (SETI). Not that it's impossible, but I'd think other intelligent being could come up with other forms of communication than our own; even if it wasn't more "advanced".
Traveling at the speed of light, it would take a quarter million years to reach Andromeda. What's more is that if I went into statis now, the compound interest on my savings would pay for the journey.
What about probes that land and replicate on foreign terrestrial bodies? 1 probe lands and makes 10 or a hundred of itself. Send out 10 of these type of probes, and exponential growth will do your work for you.
Whatever his assumptions are that leads him to 4%... it seems that he is considering only the probability that any ONE alien civilization is looking. But in all likelihood there are many, if not millions of alien civilizations out there than may be search, so the probability that any ONE of those million will find us seems quite a bit higher than 4%.
They want all of our "base"?
1. Probes sent by extraterrestials cannot travel faster than our probes.
2. The ET search is not targeted.
3. The ETs are not much closer to Earth and found us by luck, early in their search.
At any rate, while the math is interesting, it just shows that we're not likely, as in snowball's-chance-in-hell likely, to have been found already. From a logical point of view, though, one cannot say that we haven't been found yet.
As far as we know for certain, the Vogon construction fleet could be circling our system as we type these responses... though the chance of that being the truth is small enough that we could very well see an Improbability-driven ship come in for a landing at JFK or LAX.
"Trolls they were, but filled with the evil will of their master: a fell race..." -- J.R.R. Tolkien on Olog-hai
Also, this simulation was about colonization. It's a lot easier to find something than to colonize it, especially in places that aren't very conducive to supporting life.
Terrible idea. The Sylandro had one of them, and look what almost happened! Never trust a Melnorme.
Wow, it's almost as though you're quoting from my post that already existed at the time you hit reply ...
8 &cid=17668728
http://science.slashdot.org/comments.pl?sid=21759
*scratching head*
I am, therefore you think.
Bitten by the ole RTFA bug eh. The quote from the article is
He found that even if the alien ships could hurtle through space at a tenth of the speed of light, or 30,000km a second, - Nasa's current Cassini mission to Saturn is plodding along at 32km a second - it would take 10bn years, roughly half the age of the universe, to explore just 4% of the galaxy. His study is reported in New Scientist today.
No mention of colonization there.
Plus
Mr Bjork confined the probes to search only solar systems in what is called the "galactic habitable zone" of the Milky Way, where solar systems are close enough to the centre to have the right elements necessary to form rocky, life-sustaining planets, but are far enough out to avoid being struck by asteroids, seared by stars or frazzled by bursts of radiation.
So there's that too. Looks like you should have taken a look at the article first.
You'll have that sometimes...
Speculations like this are complete garbage. Even assuming aliens would have to build a craft to travel here is too much. Who is to say aliens search, travel, or think anything remotely like us? It is like Christopher Columbus saying no one would EVER travel to the moon because sailing there would take more than one person's lifetime.
"At first, we thought it was just another snake cult."
72 probes??? First of all, if you're going to do it that way, you'd create hundreds of thousands of probes, if not millions of probes (mass production would reduce the cost). Second, you still probably wouldn't do it that way. You'd wait until you had the technology to make self-replicating probes, and the galaxy could potentially be explored in thousands of years.
:)
Hmm . . .
1.- self replicating probes... check
2.- enuff "intelligence" to determine something it sees/feels/etc is an actual lifeform... check
3.- humanity's own history making buggy, security lax software... check
4.- throw in some polymorphic stuff in the software so the probe can better itself...check
5.- an "easter egg", timebomb prank from a bender-obsessed hacker (MUST KILL HUMANS)... check
Possible end result? == The cylons
We are terribly limited by our own ignorance. We barely have an understanding of space travel, dark matter, string theory, time-and-space and many other things. I recall reading something once that said people in the early 20th century believed the human body would shake apart if we traveling faster than 25mph. The knowledge and intelligence of an alien civilization could be so far beyond our comprehension and knowledge that it's almost futile to even speculate. Right now, we think nothing can travel faster than the speed of light, however it wasn't too long ago people also believed the world was flat. I guess we can only make assumptions based on our current knowledge levels, but we must also take into account that there may be ways of doing things that we've simply not discovered yet, or cannot comprehend.
boycott slashdot February 10th - 17th check out: altSlashdot.org
I agree. He's only basing his assumptions on our current capabilities and applying them to an unknown alien civilization. Great that he's making these assumptions but his final conclusion, We have not yet been contacted by any extraterrestrial civilizations simple because they have not yet had the time to find us. Searching the Galaxy for life is a painstakingly slow process., is just jumping to conclusions, perhaps invalid for the work he did.
No one knows what aliens are going to look for in a planet. Our planet could be written off as an inhabitable nitrous sphere. They might be non-carbon based life forms. They could have progressed technologically much faster than we did as you suggested. By assuming aliens match our capabilities, he made an unstated assumption that was key to actually understanding the conclusion.
A more fitting conclusion from his work would be that it would take US 10 billion years to search a small portion of the Milky Way for life at our current technology levels.
We will be in a lot of trouble if the Cylons find us first.
Actually the "cylons" will find us first, it is far cheaper to send robotic explorers out. Then if anything interesting is found send the "manned" missions.
He covers these issues. The article summary is misleading.
Self replicating is ruled out due to risk. That sounds fairly silly since computers are computers. They do what we tell them to and not a thing more. But I suppose a few worrywarts are a good thing.
The number of probes is more like 2.08 million probes, if i'm reading him right, as his simulation was done at 1/260000 scale.
HBI's Law: Frequency of calling others Nazis is directly correlated with the likelihood of the accuser being Communist.
If you've ever played "Spaceward Ho" you'll recognize that the author has proposed an asinine strategy for exploring the galaxy. Indeed, if you try to play Spaceward Ho by that sort of probing you'll rapidly get your tail kicked.
A more rational approach is exponential: You colonize a solar system. Then from that system you launch probes at anything reachable. Then you colonize everything reachable that qualifies. Rinse and repeat.
The main disc of the galaxy is about 100,000 light years across. Assume 10% light speed for probe travel time, light speed for information return and 50 years for each new colony to build infrastructure to a point where they can launch probes. You'd have 90% of the galaxy explored in three or four million years -- almost 4 orders of magnitude less than this fellow's estimate.
Moderating "-1, Disagree" is simple censorship. Have the guts to post your opinion.
Worse than that- the researcher assumes:
1. That they can't develop PROBES that travel faster than 1/10th the speed of light.
2. That probes of this form that would keep running long enough would be so massively expensive that even the most ambitious race would only be able to build 8 of them (He does address this complaint, and also considers 200 probes instead of 8, and von Neuman machines instead of static probes, neither of which drop the figures below 4x10^6 years to explore a mere 4% of the Galaxy).
3. He doesn't even consider non-material, photon-based probing methods, which would increase the rate of exploration by a factor of 10.
SJW: a person who perceives an injustice, and while correcting it, commits a greater injustice.
Come on, they haven't visited us yet? There were yet another case of alien abduction as reported by the World's Weekly last week.
One of these Turing machines reached Earth about 4 billion years ago. It first had to start by building very simple amino acids, then it graduated to proteins, then to RNA and then to DNA, and then these DNA machines built bodies around them and started using natural selection to evolve into more and more capable organisms. The final aim of these DNA structures is to build powerful radio beacons and send the information back to the original aliens who created these molecules and scattered them to the (solar) wind.
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
IT's not the ACCELERATION, it's the DECELERATION. Even if you could apply some force to slowly accelerate a massive space ship, once you got it up to that speed wouldn't it take K^2 (squared) units of fuel to slow it down it again? So let's say it takes a million tons of some super fuel to get your space ark up to speed. Wouldn't it take a million million tons to park it again?
We are currently broadcasting the galactic equivalent of "Eat at Earth" sign. Remember we consume "lesser" lifeforms for food. I do love a good steak! Who knows if the aliens who find Earth will consider us as equals or as appetizers.
I am sure their galactic physicians will recommend they don't eat too many humans from the Northwestern Continent due to cholesterol or something, but that they can eat all the yellow humans from the east they want, even if they will be hungry again in a few parsecs.
I only look human.
My mother is a halfling and my dad is an ogre, so that makes me an Ogreling
I'd hold off on criticizing others for a lack of imagination. Don't you realize that self replicating probes will doom us? We will be galactic spammers, the aliens will wipe us out as a nuisance. Or our probes will harvest the planet they pray towards, the aliens will wipe us out as heretics and blasphemers. At a very minimum the probes will be crossing the border without proper documentation, the fines and impound fees could leave us in "debtors prison" for millennia.
"photon-based probing methods"
You mean looking at stuff through a telescope?
They could have swept by, mapped the planet fully, dropped to look around, grabbed some soil samples wrote their catalog entry, and left to the next one during 99.9% of human history and we wouldn't have noticed.
Outside of human history - which is just a sliver of whole earthtime - there has been a lot of time on this planet where not much was going on, intelligence-wise. "We've found another planet of ferns, sir."
Or they could just not be particularly impressed with us. We seem to behave as though we are certain that we are best-looking girl in school so any available boy who doesn't ask us out must be gay or afraid. Take a look around - as a species we fight and squabble endlessly over dirt, water, bizarre ideas and myths. The top quarter of the race could give a crap that the bottom quarter endlessly suffers and dies when there's plenty of food and cures around for all. Maybe instead of sweeping in as benevolent parents to uplift us, they just see us as yet another batch of troublemakers who would not make good company. Above all, a people who definitely do not need a warp drive to take our ways on tour. To them, we could be just another example of a type that either grows out of this stage or eventually kills itself off. When we're worth talking to - and far less likely to shoot them or other folks - they may decloak/pull off their masks/come back.
Huntred
I think the real debate should be about self-replicating probes. Is the author assuming that every civilization capable of building these is automatically freaked out by potential doomsday scenarios, to the extent that none will be built? Even if it is foolish, I found that it pays to expect more foolishness in the universe rather than less.
A scientist saying things are a certain way based on a theory cannot be right until the theory is proven true and absolutely correct
Theories are rarely (if ever) "proven to be true" as it's a lot easier to show that something is false rather than absolutely 100% true and correct. Science is more about finding the best model to fit the data than a quest for certainty. Even experiments don't prove theories, they just add to the evidence that a model is the best explanation for a certain phenomenon.
At some point, somewhere, the entire internet will be found to be illegal.
More puzzlingly, he assumes these probes can repair themselves for and keep running for billions of years, but they can't self-replicate. Really? If the probe can repair every potential internal probem on its own, the capacity to self-replicate should come almost for free.
Thanks for the summary. You must be new here.
I think we've got something, sir. The report is only a fragment from a probe droid in the sol system, but it's the best lead we've had.
We have thousands of probe droids searching the galaxy. I want proof, not leads!
The visuals indicate life readings.
It could mean anything. If we followed every lead...
But, sir, the sol system is supposed to be devoid of humaoid forms.
That's it. The humans are there.
There are so many uncharted worlds...
That is the system! Set your course for the sol system. General, prepare your men!
Ben Hocking
Need a professional organizer?
We've seen this in STTM... V'ger came home and destroyed the sending race in a futile attempt to contact the "creator."
Face it, we're not going to meet aliens, because they've already been destroyed by their own creations.
Interested in a Flash-based MAME front end? Visit mame.danzbb.com
The study in question does not even address the Fermi Paradox in any meaningful sense, much less "resolve" it. In fact, if this study is being offered as a resolution of the Fermi Paradox then it suggests the researcher does not understand why the Fermi Paradox is a paradox at all.
The fundamental difficulty with any explanation offered for the complete absence (so far) of any sign of other intelligent life in the universe is that the proposed explanation has to be universally valid.
The span of time for colonization, or dispersal of replicating probes, or of building vast telescopically detectable artifacts is so great that even one single exception from any proposed explanation would be capable of generating ubiquitous evidence in a tiny fraction of the life of the Universe.
Simply describing some model for exploration, and then arguing that this model won't do the job says nothing about other models. This study apparently does not consider the geometric growth that occurs with any exploration program that uses some form of replication of explorers, for example. If replication is thought to be impossible then the study would have the high hurdle of convincingly demonstrating this. (The material evidence of life on Earth seems to argue persuasively against it though.)
Arguments that "interstellar travel is impossible" would qualify for explaining why alien artifacts aren't being found locally (but do not address communication signals or telescopically detectable artifacts), but require convincing arguments that this is indeed true. On the contrary, physics does not seem to make this impossible at all, just very costly and slow. Too costly and slow for anyone to bother? Not even one single civilization?
The Fermi Paradox seems to be telling something important about the Universe. If only we knew what it is...
Starships were meant to fly, Hands up and touch the sky - Nicky Minaj
That's why I laugh when people spot human-sized UFO craft. If there are UFOs here, they're microscopic.
Assumptions are just that, assumptions. You can laugh all you want, but to me, it just shows one more scientific dogma. The attitude of "knowing it all" is sadly very prevalent here on Slashdot, and probably why so many spend time writing here, instead of discovering new stuff.
The problem is lack of creativity. In 0.5 seconds, I thought of nano-UFOs. Send one, or trillions of those, and let them dig into a moon or planet to rebuilt itself into a fully fledged macro-sized "UFO". Or, maybe if you want to "recreate yourself in your own image", why not send out organic "bombs"? Etc. etc. There are so many possibilities when you dont restrict your mind.
Just because you cant think of it, doesnt mean it isnt possible or thinkable. Please free your mind! There is so much more to know than we already know! And instead of giving focus to more effective ways to kill people, why not science of life?
http://www.debunkingskeptics.com/
Did the scientist consider "electromagnetic radiation" in his calculations about "Probes seeking out only likely solar systems?" We have 50+ years of radio emissions streaming out at the speed of light from our Solar System. So that once a society reaches the industrial age, they would most likely be a lot more noticeable.
Maybe in a few decades, we will learn that we need to be more circumspect, and try and hide better from alien races.
Until then, a probe doesn't need to stumble upon us, it might be able to see patterns in radio transmissions. Who knows, if a race can figure out how to migrate through space, it might just know how to detect life from a distance (which I find highly likely).
They should mod this topic as "speculation."
>> And on Slashdot, I have the right NOT TO READ THE ARTICLE, before giving my valuable opinion.
>>"ad space available -- low rates!!!"
I reckon theres a Slashdot survey on the best way to explore:
How would you prefer to travel?
a. A blue Police Box that can traverse space and time, with a hot British former 'teen star' that is obviously in love with your weirdness.
b. A big ancient ring that can take you anywhere where there is a corresponding ancient ring, but you keep bumping into Egyption dog people who try to kill you.
c. A large dinner shaped spaceship that does warp factors, but you get to shoot at klingons and make sexy time with green chicks (remember its all about the Journey!) Just dont get assimilated by Bjork!
d. Travelling with the Robinson family and a stupid robot that shouts "Danger" long after it stopped being funny. Oh and a pedophile.
e. In a ship that can make the Kessel Run in less than twelve parsecs - With a great big hairy Wooky and a gay robot.
e. Spending time on the only ship to have survived an attack by robots with KITT in their face, where it is a daily battle to stay alive.
f. On a moon that was flung out of orbit by a massive thermonuclear explosion initiated by the build up of magnetic radiation, which there is much debate as to it being caused by global warming.
g. Traveling across universes with a guy that looks like Mike Moore, where each new universe you 'slide' into is exactly like being on LSD.
h. On a ship with a dorky hologram an evolved cat, a computer with an IQ of 6000 and a very stupid robot, but every day is hilarious!
I. The space shuttle. (yawn)
I think most people would agree that papers like this are based upon so many assumptions that they are pretty much worthless, regardless of which cosmology you believe in. It's just a product of our affinity for math and our desire to feel like we have more confidence in some sciences than we can actually achieve in the absence of input-output experiments (ie, to varying degrees, astronomy, geology and archaeology).
But it's interesting to note that the biggest single assumption in this type of logic is that the universe is not infinite in time and space. In a static electric universe, without a beginning to base your calculations upon, chances are high that neither stars nor galaxies have determinable ages. The entire system is essentially "transient" and papers like this are completely meaningless. As painful as it is to imagine it, aliens could have started seeding the universe an infinite amount of time ago. It's possible that not even they could tell you when they started. This is of course no more painful though than imagining what happened before the Big Bang.
I've also seen it mentioned amongst people who are aware of Electric Universe Theory that the more you understand plasma, the more the plasma of the universe appears to constitute a living organism. The fact that plasma can form double-layers to "protect" its charge suggests parts of a living entity. And if Chip Arp is correct, the notion that spiral galaxies can "spit" out quasars might be the process by which the organism spreads out of its original domain. The stars are the organism's cells and mobile charged particles act as the nutrients for the plasma, which would ironically be like the organism's blood. Within this context, the rocky planets are a rare, harmless non-plasma pocket where we humans, like tiny viruses, can multiply and possibly expand.
Taking the idea one step further, another strange curiosity of EU Theory is that all of the plasma phenomenon within the universe we've observed thus far are actually electrical loads and transmission lines. Once you've become acquainted with the theory, you begin to wonder what is in fact the *source* of the power. You'd have to conclude that we're likely not in range to view the source, but this is a very interesting question. It's the EU Theory version of asking how old the Big Bang Universe is.
Weird shit. Once the public starts to learn more about plasma, I think it's inevitable that it will become a popular topic for strange ideas like this.
"A man cannot begin to learn that which he thinks he already knows." --Epictetus, 1st Century A.D.
this just in, 1000 telephone sanitizers landed at Kennedy Space Port.
Sounds like a version of Intelligent design.
Which I don't say is impossible -- just that counting on ID in a classroom, where you have to teach science is pointless and merely to make Fundies happy.
I think that humans, in a few more decades, may very well want to "seed" nearby planets with modified earth DNA. The compulsion to do so will be hard to ignore. We could create food or useful organic crops on Mars and Venus -- or just experiment without ecological disaster on earth (or test ways to fix ecological disasters). There will be a lot of protest at first, but history shows that we ALWAYS do something that provides a profit -- whether or not it benefits people or any temporary form of ethics (worrying about Stem Cell, is just a ruse to get patents in the private domain, for instance).
So, I don't know any way we could disprove that Aliens have not visited earth or manipulated genes in some way. The debate against ID is more about good science -- not trying to disprove every possible explanation.
We also might be a creation of the Flying Spaghetti Monster. Who knows if that 90% "junk DNA" encodes for Meatball + a delicious sauce?
>>"ad space available -- low rates!!!"
He doesn't even consider non-material, photon-based probing methods, which would increase the rate of exploration by a factor of 10.
Doesn't matter. Light only travels so fast, and we've only been here, what, 10,000 years? Nobody further than 10,000 light years away could have possibly found us yet. And a 10,000 light year sphere is well less than 4% of the galaxy.
This whole study is kind of dumb, because it doesn't matter that you can explore 4% of the galaxy in 4 billion years when we've only been here for 10,000 years. Even if they did come to earth, it's almost certain that when they were here, they found either nothing or some bacteria and kept going.
paintball
Let us roll back the clock, say, 200 years: A person up to date with the technology of the time would have no knowledge of airplanes, cars etc would make the some silly statement that it would be impossible for a person to ever cross USA in one day. They'd also say that it is very unlikely to find a particular quote in some random book within three months of searching, Google etc changes that. Change the technology and understanding of physics and we'd laugh at anyone saying something as stupid as that now.
But won't people 200 years from know laugh at our pathetic understanding of technology and physics? If there is intelligent life (I don't think so personally), it might just be a couple hundred or thousand or whatever years ahead of us and would thus not be bound by the limiting assumptions we make today.
Engineering is the art of compromise.
...Aliens have not visited earth or manipulated genes in some way.Average weight of humans up... sounds like and interstellar Hansel and Gretal
Help test the
1. It's incredibly stable. It's launching an exploration program using probes that are going to take billions of years to get a result back to the original civilization. It expects to be around to pick them up.
By the same logic:
2. Individual members are incredibly long-lived, or the society is static and conservative enough that individual goals are submerged. They expect that the people around in a few billion years still care about the stuff they're doing, AND they care about the people who'll be around then.
The technology he's postulating is also very advanced.
3. Large scale space-based industry is routine enough for them to build probes capable of refuelling themselves using the raw materials in an as-yet-unexplored solar system, with surplus fuel to launch and recover the sub-probes. If they can do that, they can do the same thing in their own solar system.
If the probes are cheap by their standards, there's no reason not to keep building them indefinitely. So let's say they're expensive. Let's say it takes this civilization a hundred years to build a probe. Why do they stop after 800 years? They're long-lived, stable, conservative, so assuming they have the will to do it in the first place why would they stop building probes? As the author notes, probes break down.
So what happens when you add another probe into the search every century, indefinitely? Well, after a million years you've got 10,000 probes out there. Now you're looking at a search time measured in millions rather than billions of years, and it only takes millions of years to do it.
But why are they doing this? Looking for planets to colonize, perhaps? If they're just looking for civilizations they'd do much better depending on "signal intelligence".
But if they've got the ability to send out colonies, even the most conservative long-lived space-based civilization is going to figure out eventually that they don't actually need habitable planets to support a permanent colony. It's riskier without habitable planets, but even if the planetless colony is 10 times less stable than the home system you're still better off with your civilization in two baskets. And before long (in the terms of this civilization) you've got a roughly spherical shell of colonized star systems, expanding as fast as they can reach new systems. At 0.1C colonizing (not just exploring) the galaxy is going to take mere millions of years.
On the other hand, what if the self-replicating probes are members of the designing species themselves?
So either this level of technology is impossible to achieve, or we're back to the question of why no species has done it yet. There's lots of plausible answers, of course, but this paper sheds no light on them.
"we should expect alien technology to be roughly the same as ours, give or take a century or so"
Not really.
You need to allow for the mass extinctions in the Earth's past.
If the mass extinction 250 million years ago (Permian-Triassic) had not occurred then intelligent life may have evolved on this planet over 200 million years ago.
It would be quite possible for an alien race to be hundreds of millions of years more advanced than us just due to luck.
You could look for an oxygen/carbon-dioxide atmosphere, but then you're just making assumptions about what sort of life you're looking for...