You awake, get up and walk out of your hut in the jungle of Lonopelemaumau, a small island about 1600 miles ESE of Fiji. You glance at the clear-ish morning sky and notice a tiny spark tracing out a long thin straight line of cloud, then go on about your day. Not having contact with western civilization in this remote place, it means nothing to you, and you have seen these all your life but not been particularly curious about them.
You go down to the lagoon to collect some crabs for breakfast. The beach is littered with plastic trash, but you merely view it as a nuisance that must come from some deep sea creature or plants growing on some other island.
Which is to say, we might be looking right at what we are looking for and not realize it. The out there is an incredibly strange place, in which we seek the ordinary, which is not resolvable to our tools.
And, of course, there is no place at all like Lonopelemaumau on Earth. The very real possibility exists that, while life is likely almost everywhere, we could be the most advanced creatures in the western spiral arm. As noted above, any one of many different natural setbacks can have befallen a distant civilization, leaving it back in the Stone Age for millennia, or just wiped out completely. By the big bang theory, the universe is barely 3 times older than the Earth, and for a great fraction of that time, some of the vital elements needed for life as we know it simply had not formed and spread across space in sufficient abundance.
If we ever develop interstellar travel, we could realistically be the first in our galaxy. If so, I hope we embark with due diligence.
On its face this seems plausible, but I don’t think it is on further scrutiny.
Using Earth as a guide, the gap between having used up most of the readily available oil, and being a space-capable species, is little more than a century. While humans might get blasted with a gamma-ray burst between now and developing interstellar tech, this does not seem to be a likely enough set of events that we would expect it to have happened to the majority of civilizations.
And a disaster happening prior to available oil being used would be neither here nor there. The millenia it would take for a civilization to get back on its feet is an accounting error in the grand scheme of things.
Well it certainly looks that way, given the lack of evidence of any extent interstellar species.
It’s just a pity we don’t know why that is the case yet.
Assuming conditions for life, and even intelligent life, are common across the universe, I think the possibility that alien civilizations should have become galactic or intergalactic has just diminished with the recent discovery of clusters of black holes lurking in the dark space between galaxies. I’ve recently read a couple of articles on this but unfortunately I don’t have the titles right now. (I read them on my phone, while I’m on my laptop right now.) However, I have managed to find this article where it is stated that dark matter actually consists of black holes. With the black holes comprising five times as much matter as the visible universe, I think intelligent civilizations will find it hard to successfully venture outside their galactic havens.
Right, but the argument is being made that at this point, we should have condos being built on our beach. Hard to not notice.
An important one of those is phosphorous. This is in pretty low abundance, and seems pretty important to life as we know it.
If this is the case, it still doesn’t mean that there is like a wall of black holes. Space is big, intergalactic space even better.
Hypothesized, maybe. It kinda explains some of the properties of dark matter that we have inferred through the observation of gravitational lensing of very distant galaxies.
The reason that primordial black holes keep being doubted is because, unless there is a mechanism that determines a minimum size in their formation, then we should be seeing them blow up on a pretty regular basis.
Fascinating link, although I don’t quite get it.
The original reason for postulating dark matter was that stars within galaxies were rotating faster than they should be, or could be (they should be ejected), were visible matter the only thing holding them together.
Black holes outside of galaxies aren’t going to hold galaxies together.
And black holes inside of galaxies would certainly be detectable in the masses required.
Also black holes within galaxies that are close to the center eventually merge into one supermassive black hole. The supermassive black hole at the center of the milky way is not big enough to account for our galaxy’s rotation, and is consistent with being formed from a dense stellar region…Sagittarius A doesn’t seem to suggest there are / were lots of black holes in our galaxy.
So color me confused. IANA astrophysicist but I don’t see how this model could account for more than just a fraction of dark matter.
Sorry, I think I get it now.
The model is that outside of each galaxy is a whole bunch of black holes, of stellar mass or lower. They could even be arranged in spiral arms.
I can’t think of any argument against this model, which is a relief – as a non-scientist, if I think I’ve seen an obvious flaw in a paper published by actual scientists, it’s a red flag that I’ve misunderstood something.
It’s a hypothosis that makes a model that fits the data slightly better than the current models.
Could be right, probably isn’t.
The biggest reason that it is unlikely is that, unless there is a mechanism that creates primordial black holes, but only above a certain size, then we should be seeing the small ones blowing up right now.
Or there is something else that we don’t know.
A scientist will never tell you that they are sure of anything, only that they have done their best to rule out everything other than their hypothesis.
The intracluster medium contains something like ten times as much mass as the galaxies that define the cluster. And it is relatively hot and active. If there were black holes between the galaxies, even as sparse as the matter is (there is quite a bit more space between galaxies as there is in them), I should expect that we would see accretion disks out there. Because, that is what black holes do. Accrete.
Yeah I’m back to thinking the physics of this don’t make sense to me.
My hunch now is that the article is exaggerating the size of this effect i.e. it could account for some, but not all, dark matter.
Again, IANA astrophysicist, but I’d be prepared to make a wager.
And, if predictions are correct that there’s only ~1 hydrogen atom per square meter of intergalactic space, how would self-replicating probes gather enough matter to replicate and travel millions of light years to reach us?
Of course you don’t need energy to drift in space, so probes don’t need to gather resources during that journey.
The self-replicating probes idea is mostly talking about civilizations within a given galaxy spreading to other stars within that galaxy. If there is no hyper-advanced species in our galaxy, then the observation that no such probes exist makes a lot of sense. But of course we’d still have plenty to discover in what filter(s) are responsible for there being zero hyper-advanced civilizations per 200 billion star systems.
For one, what you are saying is an average, that doesn’t mean that that is all that is out there. There are stars between galaxies, just spread further apart.
And there is also the fact that you would not be replicating and fueling your probe in the middle of nowhere, you would build and gas it up at a place that does have stuff, and then send it out across the void.
Your point that extragalactic space is sparse does not in any way prevent this from happening.
You asked: “how would self-replicating probes gather enough matter to replicate and travel millions of light years to reach us?”, and I gave an analogy as what I thought would be the best answer, to explain that you don’t have to gather resources where there are few, but that you would gather them where they are plentiful.
Also, even at one hydrogen atom per cubic meter – how good are your ramscoops? I know some people don’t believe ramscoops can exist at all, so that’s one end of the spectrum-of-discourse. But where’s the other end?
Yes, but if the probes don’t replicate along the way, how many would they have to launch from their galaxy, which to us is just a point in the sky, in order to have any chance of intercepting our solar system? I’m guessing it’s an unfathomably large number.
Yes, I understand the self-replicating probe idea is meant to be an intra-galactic thing. I was simply addressing those who believe inter-galactic travel is a viable option and should be factored into the Fermi paradox. I believe there are some universal impossibilities that all civilizations must abide by (no matter how advanced), and inter-galactic travel is one of those.
Which, again, leaves us with the ~250 billion star systems in our galaxy.
Ram scoops were a staple of 70’s sci-fi, especially Niven.
Somewhat recently, people did math, and found that, no matter how efficient your fusion was, you couldn’t get more thrust than you we losing due to gathering the hydrogen.
People sulked for a minute, thinking that they lost a cool toy, then they suddenly realized, “Hey, that means we have brakes!”
Grin! Thanks for setting me straight; I hadn’t heard that update. Pity, but, hey, every now and then a beautiful idea caroms against the brick wall of reality. (Cavorite!)
Well, a few hundred billion. That’s a large number, and it is unfathomable to mere mortals like us.
To a K3 civilization? Peanuts.
Self replicating probes would start by filling up just one galaxy. That is something that would be obvious to observers from hundreds of millions or even billions of light years away.
I keep seeing this assertion, but other than it being unimaginable to you, I’m not seeing any reason for it. Once we have figured out how to have any form of self sustaining asteroid colony, we will end up filling up the local cluster in fairly short order. Everything reachable will eventually be ours, with the expansion of the universe taking things away from us being the only limiting factor.
