As I have suggested, here and elsewhere - this is a special time to live, and that makes us ‘special observers’ - which mean the normal rules of random sampling don’t apply.
But the chance that we are special observers does depend on one factor - to ensure our privileged status, we must ensure that we do become an expansive galactic civilisation. I can imagine that every advanced civilisation faces a choice like this - do we stay on our home planet, and remain a population limited by our planetary priors- or do we expand into the galaxy, becoming a civilisation that can barely remember where our home planet is, and care less? Perhaps most civilisations choose to stay at home, and fade away when their Doomsday comes- we do not need to follow that example.
I doubt that they would either travel at near-light speed, or communicate with each other often.
They might communicate with the system that they launched from, which would have much larger receivers and transmitters than the ships. If two ships in interstellar space needed to communicate with each other they would probably send messages to the big receivers back home, which would then relay the messages by equally big transmitters. The ships themselves would probably only have relatively small transceiver dishes. This back-and-forth could take decades.
You’ve said that, but I’ve got no idea why you would believe that. In order to influence the probability to find ourselves in our current circumstances, there would have to be some factor that observers like us only exist in these special circumstances—i. e. some factor about our present situation that makes us us, such that we’re unlike other minds living at other times, and hence, not being sampled from the full set of minds. But there is no such factor.
As an analogy, consider cell phone buyers. Say that you’re one of the first to buy a cell phone. You could then claim that, well, since only five thousand people have so far bought cell phones, with a likelihood of 95%, there’ll never be more than a hundred thousand cell phones sold.
But the population of early cell phone buyers is saliently different from that of the total cell phone owners—they’re generally tech savvy, early-adopter types who would rush out to buy the latest gadgets. Given that you’re one of those, you can’t validly conclude anything about the total cell-phone buying population at large.
However, there’s no factor that makes us inherently ‘early’ minds, nothing that says something like us can only exist within the early, pre-galactic stage of civilization. Our knowledge (or lack thereof) of whether there are other civilizations, or whether we will make the jump to the stars, has no influence on the probabilities at all. After learning of the existence of other civilizations, and moving to the stars, the reasoning that says we’re with 50% likelihood among the final 50% of humans to ever exist is still just as valid; getting that data does not cause any updating of our probability assignments. You’ve still got access to the same data—you are mind number n—in the popular analogy, it took n draws from the urn to come up with you. From this, estimation of the total sample size works the same way in every case.
Because, as I have repeatedly said, every expansive galactic civilisation must go through a short period of time between the invention of probabilistic mathematics and the development of interstellar space flight, when they think they are part of a random distribution of entities confined to their home planet. Once their population starts to exponentiate once again, their special position at the start of that curve is exposed.
That is the factor that makes us necessarily ‘early minds’, and it does depend on the development of galactic expansion. I accept that we may never develop an expansive galactic civilisation, and in that case Carter would be correct - we would be doomed. That is an argument to strive towards galactic expansion, as far as I am concerned - galactic expansion is the best way to cheat Carter (for a while, at least).
What is an optimistic figure for the number of civilizations in the Milky Way advanced enough to engage in interstellar travel? 100? Let’s go with that figure. And let’s say those star systems are spread evenly throughout the galaxy. The closest to us is 2,000 light-years; the furthest is 100,000 light-years.
For the moment, let’s take Dyson spheres, self-replicating probes and wormholes off the table. But, let’s say most of them sent/send a reasonable number of bio-manned and AI-manned missions from their home planet for exploratory purposes. None have achieved FTL propulsion, but a few have achieved 0.5 c. Let’s say the first launch from the oldest civilization was 1-million years ago.
What are the odds, do you suppose, that we would see any evidence of advanced civilization given these parameters? What are the odds of any of those civilizations seeing evidence of intelligent life on Earth (our radio waves have traveled ~200 light-years so far)? What are the odds that any of them will visit us in the near future?
Bonus: Which headline would you rather see:
Proof of Extraterrestrial Intelligent Life!
Proof of Extraterrestrial Intelligent Life—They’re Heading Our Way!
I assume that all travel is rather slow, but vessels would still like fast communication. One message might be “Planet here suitable for terra-forming for the spider people of Tau-93. Please send platinum and uranium which are in short supply here.” Without light-speed communication, delays would double.
The biggest ships would never visit solar systems and, anyway, wouldn’t transmissions originating near a star require high power to be distinguishable from the star’s own emissions?
A culture that only sends out exploration ships and never establishes any colonies will probably never meet anyone else.
But even if we do not allow the development of exponentiating, self-replicating probes, these 100 civilisations might exponentiate in a different way - by sending out daughter colonies from their original colonies. If each colony sends out 2 daughter colony missions on average, and takes 10000 years to do this (on average), they would end up filling the galaxy in much less than a million years.
Yes, you’ve repeatedly said that, the problem is just trying to figure out why you think it matters. Again, both before and after the jump to the stars, each mind has access to the same data—the approximate number at which it arose—and thus, before and after the jump to the stars, each mind can apply the same reasoning, and each will just as correctly conclude, for instance, that with 50% probability it’s within the final 50% of humans to ever live.
There’s no ‘cheating’ the doomsday argument. It’s not some fixed cataclysm that somehow lurks within our future, unless we make the jump to the stars. It’s a bit of probabilistic reasoning, which takes as its sole input that there’s a finite number of total humans to ever live, and that you’re one of them. A priori, you will either be within the first fifty percent of humans, or within the final fifty percent. Furthermore, fifty percent of all humans are within the first fifty percent, and fifty percent are within the final fifty percent. Hence, considering yourself to be within the final fifty percent has a fifty percent chance of being right. Whether you’re born before the expansion to the stars, or afterwards, simply doesn’t matter.
The same goes for all other percentages. 95% of all humans are within the final 95% of humans to exist. Each human being, thinking to themselves, ‘I’m within the final 95% of humans’, thus has a 95% chance of being correct about that. 5% of humans thinking that will be wrong, since they’re in fact about the first 5%.
After the jump to the stars, the same reasoning will apply. The doomsday argument won’t just somehow become wrong due to our moving to the stars, because the same conditions will hold then as they do now—you’ll be the nth of a total of N humans to ever be born.
Take the following reasoning. You’re born. You estimate that n humans have been born before you. You consider yourself to be within the final 95% of all humans. 95% of all humans will be right about being within the final 95% of all humans. Hence, you conclude that, with 95% probability, no more than 20n humans will ever be born.
What, in this reasoning, depends in any way on whether you’ve been born before mankind ever made it to the stars?
I discussed this with my daughter, who is a mathematical biologist. She says that generalisations like Carter can only really apply where we have no modifying information. I submit that successfully expanding to the stars would be a modification to Carter’s simple scenario, whereby we would suddenly exist at the start of an explosion of population that could have a final number measured in quintillions. We do not have sufficient modifying data to fine tune our population estimates yet.
This is where the Fermi Paradox is another, frustrating lack of useful information- we haven’t found any civilisations to compare ourselves to, and to interrogate for information about galactic population curves. We could, with luck, gain extra modifying data to constrain our future population by comparing our expansion with the past expansion of other, alien civilisations; these would presumably display a range of numbers, some large, some small, which should give us some idea of the possible limits to our future expansion. If we did have access to this information, it seems entirely likely that the modifying data available to a civilisation before it starts expanding and before it makes contact with other civilisations would be entirely different to the data we would have access to after these events.
In short, we are in a special position of ignorance.
Which is completely true, of course—see the cell phone example I gave above. It’s just that knowing whether we make it to the stars doesn’t modify anything; it’s simply irrelevant to the reasoning. Making it to the stars, or not, does not cause any update of the probability estimates we have.
That would be right, if making it to the stars would somehow actually tell us how many people ever will live; but as it is, it makes the problem actually more acute: after expanding out in the galaxy, we should expect that many more human beings will be born than have existed. But that’s exactly the problem, because then, any typical human ought to find themselves at some point to the far future—independently of where we are, right now.
Again, this isn’t relevant data for this problem. You need data that will shift the reference class—i. e. puts us in a class of observers that’s not that of ‘all humans ever’, but some small subset.
We will always be in that same ‘special’ position, as we will never know, a priori, the total number of humans to ever live. The universe could always end tomorrow due to a vacuum metastability event. Hence, we will always reason that we’re 95% likely to be among the final 95% of humans to ever live.
We don’t need to think of Dyson spheres as all-or nothing structures (i.e. if they exist they block all the light from a star). Instead think of the concept more as ‘any set of artificial structures collectively large enough to occlude enough of the star’s light that it is detectable from Earth’.
The likeliest evolution of such a structure would be like this: A planet is short on energy, so it starts putting up solar power satellites in orbit. Eventually, planetary orbit is too crowded, so they start building huge solar power satellites in solar orbit. Self-replicating systems break down asteroids and moons to supply raw materials, which are constructed in solar orbit by more robots. Eventually, you have so many things orbiting the sun that perhaps 5% of the light is captured and turned into energy.
We would see this as a star that’s dimmer than it should be, but with a bias towards the infrared because of the relatively warm satellites in orbit. If the system is still being built out, then we would see a gradual dimming of the star over centuries, with a corresponding increase in infrared.
Tabby’s star would have been a reasonable example, except we discovered that the characteristics of the occlusion indicate dust clouds of small particle size. But we were capable of detecting that cloud, so we should be capable of detecting a ‘Dyson Swarm’ out several thousand light years. So far we haven’t seen one, but the odds could be low - even with optimistic interpretation of the terms in the Drake Equation we shouldn’t expect to see a technological civilization more than once every few thousand light years.
The next generation of telescopes will improve our detection capabilities dramatically.
My personal opinion is that we will detect biosignatures of life elsewhere in the next 20-30 years, but signs of technology might elude us for a long, long time even if they are out there. And if we do, it will likely be by detecting pollution in an exoplanet’s atmosphere or some indirect detection like that.
I would wager that any alien life capable of getting here also has the tech to do it completely undetected. Even as “space tourists”. And there’s good reason to remain undetected. The apes at the zoo don’t have nuclear weapons.
Another point to ponder: If our universe is indeed infinite and FTL speed is possible and instead of expanding toward the Big Chill we were contracting toward the Big Crunch, do you suppose we would be visited by aliens on a daily basis? If so, it’d be time to hang a no solicitation sign around our solar system.
Also, what if they’re not interested in humans at all? What if it’s starfish they want to communicate with? Or dolphins? Or tortoises? Or sloths? It’s awfully presumptuous of us to assume that human beings are the only life form on this planet worth getting to know.
Fully immersive vr would have the ability to be orders of magnitude more interesting to users than expending the resources to explore the stars. Basically infinite possibilities. Why would a civilization want to limit itself to the drudgery of interstellar travel?
Anyway, I’ve never understood the simulation hypothesis or kardeshav scale for civilization. It seems they both are pretty arbitrary. How about the McMechanic hypothesis that states: if it’s possible for cats (yeah, cats again) to ever time travel, then we must be living amongst cats from the future.
Well, every way of getting from one star to another using known physics uses mind-boggling amounts of energy, so we would probably see them coming several months in advance. To cut down on these energy requirements, the portion of the ship that actually arrives would probably be as small as possible, so the arriving aliens would probably be cramped and discombobulated from a journey lasting decades or hundreds of years. They may well need our help more than we need theirs.
Why is it the set of minds that could ask this question?
Why not the set of minds that do ask the question, or just the set of all minds?
The whole thing is completely arbitrary.
Let’s imagine I have a magic device that could erase the Doomsday argument from the minds of every human for the rest of time.
If I had such a device, just prior to pressing the Go button, can I now logically conclude that I now belong to a small set of humans aware of the doomsday argument? And further, that I now could conclude that nothing can be inferred about the probability of humans reaching the stars?
Either a “yes” or “no” answer looks problematic here.
This is incorrect. I said a proposition that is at least some of the time gives a demonstrably false answer – not close to the right answer, 100% wrong. And, for all we know, may *never *give a correct answer. This is a very different thing from error bars on a model that gives demonstrably useful empirical data.
No, but you’ve ignored the point being put to you.
The point is, the Doomsday argument is being applied to make the claim that mankind is unlikely to visit the stars. But, following its own logic through, it cannot be used to make a claim about the real universe since it naturally leads us to conclude we are not viewing the real universe.