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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."
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.
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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.
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.
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.
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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...
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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.
As science advances, we learn more and more about the forces that drive nature, and the laws they abide by. Those examples you gave us don't violate any of the known physical lays. I find it a bit disturbing that the advancement of science is taken to mean that everything will become possible. Instead, we better know the posibilities and certainly the impossibilities. Maybe we will find a way around these laws, but I highly doubt it.
I for one would really like to explore the universe and make contact with alien species. Unfortunately, my just wishing this is the case doesn't make it so.