Its like you are thirsty so you build an intercontinental jet plane from scratch using parts from around your house to fly to Japan to steal a single Coke from a 7-11.
All such spacecraft would have to do to cause massive devastation is aim at Earth and not stop, though it's likely that more advanced civilizations could come up with more effective means of destruction.
Alien civilizations could have advanced scientifically for thousands, hundreds of thousands, or even millions of years. It is unlikely that we can accurately put limits on their capabilities.
 - https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propu...
But as you point out, anything that can travel between the stars can trivially wipe out all life on Earth without warning. They also know we exist already thanks to the changes in the chemical composition of our atmosphere. If they want to kill us then we're dead.
The flaw in this analogy is that the you with these capabilities has different, and possibly incomprehensible desires, compared to the you who wants a Coke.
The natives of Central America and Mexico were not aware of how excited those Spaniards would be about Gold and Silver.
Such a strike might not even be that expensive or need FTL technology: just send out a few one-way nuclear-armed suicide drones at 0.12 c , via a path that obscures their origin (e.g. via a gravitational slingshot or three).
 Perhaps with a design like https://en.wikipedia.org/wiki/Project_Daedalus, https://www.damninteresting.com/the-daedalus-starship/, which is probably even within our technical capabilities. We also have weapon designs that could render the a planet uninhabitable (e.g. https://en.wikipedia.org/wiki/Salted_bomb).
Let's say an advanced civilization around Sirius wants to take us out. A chunk of metal (or anything really) of about 1000 metric tonnes would be completely invisible until it were very close. Travelling at 99.99% of the speed of light, it would impact with the energy of 13 times the meteor that wiped out the dinosaurs. The light of the projectile being "fired" would reach us in 10 years, and the projectile itself would arrive just 8 hours later. There is no foreseeable way to defend ourselves against that.
An overly cautious civilization with that kind of technology might just decide to start sterilizing all the planets in its surroundings without having to leave their solar system, just in case.
This used to be a common SciFi staple, that a civilization advanced enough to travel the stars must have solved their social issues first or they would have destroyed themselves with their own technology. Basically as the cost of ending all life on the planet drops you need to advance your ethics fast enough to prevent it from happening. Eventually everybody is carrying around pocket antimatter batteries and either everybody is well adjusted or everybody is dead.
An active weapon could be targeted at a solar system too far away for an accurate preparatory survey, then identify and target rocky planets and large artificial structures once it arrives. It could also perform course-corrections in transit, and remain substantially dormant for much of the trip for stealth and surprise.
Do you let the plane in? Do you stop it from traveling at all? Do you glass the surface of America with nuclear weapons? How do you know they won't do it to you?
Plus, it doesn't even make sense to try to "protect" yourself because their first move if they even thought like this would be to nuke everything flat with no warning. There's no defense against it. You can't hide either, the signals are already out there, they know where you are.
It makes no sense to insist that aliens would think as you think they should. That doesn't even make sense for humans in most cases.
> You can't hide either, the signals are already out there, they know where you are.
That's arguably not true.
We are on the cusp of being able to detect the biological markers of life on distant worlds by spectroscopy. An alien civilization that can traverse the enormous gulf between solar systems can very easily build widely spaced telescopes that can monitor all nearby solar systems for changes to atmospheric composition. Anybody who is paranoid enough to need to kill any life around them would obviously build such an array. There is no hiding from them, the technology is much too simple compared to what you need to travel between solar systems.
Maybe they can't themselves, but don't like the idea that someone else eventually may be able to.
I think you're overconfident of the safety of actively advertising our presence to the universe because you're making a lot of assumptions and only considering a small number of possibilities. The problem is that there are so many unknowns and so many shades between different possibilities that no one can really be confident about anything, except that the risk could be anywhere between zero and the maximum.
But you can imagine a situation where like we’re looking for life on Mars and in doing so inadvertently destroy unknown life.
That's how he justifies the broadcasting of signals, but take the next logical step: It also indicates either that nobody is listening, or that they're unable or unwilling to contact us, or that they're already in the process of trying. In any case, it's hard to imagine the METI messages would change anything. By that same argument. And I mean why would the intentional/contrived message be any more persuasive than all our "honest" and un-self-conscious messages to each other over the years? Anybody who was curious could have learned everything by now.
I also would like to call out the typical assumption of a "technically-advanced race of aliens" and whatnot. The aliens always seem to be technically advanced, don't they? In other words, like us, but more so. That's a mirror you're looking at, and it's how you recognize something man-made. Homo sapiens is the one with the fetish for techne. It seems the aliens we imagine are always either similar to us (which we like) but better, or else they're dissimilar, and predictably worse/more evil (and tend to look like combinations of creatures from THIS planet that we don't like... insects and reptiles figuring prominently). That's more a mirror of our dislikes, but still a mirror. Aliens are always created in our image, and that shows an embarrassing narcissism & lack of imagination. We're just projecting our hopes & fears. Keep your mind open.
One more nitpick: "We won’t know for about 25 years, assuming the communique is sent at the speed of light."
Pretty good assumption, given that it's not possible to send EM waves at any other speed!
"Humans, Vakoch said, have been using radar, which can purportedly be detected 70 light-years away, since World War II. Television and radio signals would long ago have signaled our presence to malevolent space ruffians, he said."
> "Humans, Vakoch said, have been using radar, which can purportedly be detected 70 light-years away, since World War II. Television and radio signals would long ago have signaled our presence to malevolent space ruffians, he said."
I think the key word there is "purportedly." Has anyone done an analysis about what kind of equipment would be required to detect those old signals at 70 light years? Has there been any analysis of how for out they could propagate until they were totally overwhelmed by noise?
Personally, I'm skeptical that those signals were transmitted powerfully enough or focused narrowly enough to likely be detected. This article (https://www.nature.com/articles/461316a) makes it sound that way:
> Indeed, SETI is marked by a hope, bordering on faith, that not only are there civilizations broadcasting out there, but that they are somehow intent on beaming their signals at Earth. An alien SETI project relying on a similar faith in Earth would be sorely disappointed. It's true that a random mix of radar and television signals has been expanding outwards from Earth at the speed of light for the past 70 years. But there have been only a few short-lived attempts to target radio messages at other stars — with each attempt arousing concerns over alien reprisals.
(Since someone will probably bring it up, no, I do not believe extinction events are aliens trying to wipe out life, on the grounds that if intelligent, hostile aliens intended to wipe out life on Earth, I'd expect them to succeed, and not merely inconvenience life briefly, but actually totally and permanently sterilize Earth. Any culture that can fling either mass or energy at early Earth in any reasonable period of time is already very near the necessary energies to do that. Across billions of years I'd expect someone to have managed.)
Another reason why I tend towards the "yeah, we are non-trivially alone" myself. Maybe the entire universe isn't empty, but I don't think there's any reason to believe there's anything nearby.
Even if they're out to destroy all life, they might prioritize killing intelligent life first, as it could be a threat (or for some other reason).
The amount of energy available to even a single solar system, let alone all the advanced galactic civilizations that are supposed to be out there, dwarfs what it would take to sterilize Earth. And don't think I'm underestimating that quantity; it's larger than any impact real Earth ever sustained, and it has sustained some doozies. But what's in even a small star, even in a small star's yearly output, is orders of magnitude larger. And this is with the relatively expensive brute force method too; if you're willing to roll the dice on the target not having intelligent life a few steps above where we are now, you could probably put together a gray-goo-esque replicator that would conclusively eliminate any civilizational threat from a given planet permanently, for the cost of delivering a few kilograms of payload to the target system, which is presumably trivial for our advanced civilizations.
As I said, I'm still pretty much on the "yeah, we're probably effectively alone" side, because when you really treat the "the universe is full of advanced intelligent life" idea seriously, you just don't get what we see in our sky. Not least of which is the wide variety of ways in which we simply shouldn't exist, because our ecosystem should have been wiped out one way or another, be it by raw destruction, or simply interaction with some other much more advanced ecosystem that simply steamrolled Earth's primitive life the same way we steamroll an ant hill, a billion years ago. Just about the only way to save the hypothesis is that we are the "Progenitors" and the universe is full of intelligent life if you look at a random place and time, it's just that the "full of intelligent life" hasn't happened yet. If the universe as a whole is full of intelligent life, there still has to be a first set of civilizations, and while every individual in those civilizations may rationally say "the odds of me being here when the universe is empty are as good as zero", there still have to be some "lottery winners".
So if we've been using radar since WWII, and radar has been capable of signalling our presence to alien civilizations, then we don't need METI. Alternatively, if METI is proposing a signal that can reach alien civilizations in a way that TV and radio signals cannot, then the original objections to METI still stand, and the TV/radio signal argument is a red herring.
But really, it's just common sense. Everything we know about the universe tells us that complex organisms exist at the expense of simpler organisms. As a nascent space-faring civilization, we are the minnows in the ocean, and signalling our presence to the bigger fish is the kind of mistake that a minnow only makes once. What exactly is the perceived positive outcome of making contact with a more advanced civilization?
Also, this is 70 years of noise, a blip on the galactic timescale, getting weaker the farther it goes. We can't stop what's gone out already but that doesn't make us obligated to keep advertising our presence forever.
In Carl Sagan's Contact, we were only able to determine the signal came from the star Vega because it was the only star that would have been at the right distance to receive our earliest radio signal and transmit it back to us - also Vega is relatively close by compared to other stars, close enough that our parallax created by our orbit around Sol was sufficient to get a read on signal source.
A deliberate attempt at active signaling would remove all doubt for an extrasolar observer.
Liu Cixin's The Dark Forest explores this scenario in depth. I would explain more but don't want to reveal some important spoilers.
Absent any form of antenna technology we don't know about, all of these signals are going to be extremely dispersed and very near to the rate of noise, for any receiver that is even moderately far away. It is still possible to detect these signals, but only you know exactly what you are looking for. Not only the direction and frequency of the signal are needed to know, also amplitude, polarization and any form of modulation pattern. Otherwise, the signal might just be interpreted as noise.
Also 70 ly are "not even beyond our backyard" so to speak. 70 ly is such a small distance, compared to other possible distances in our galaxy. It is a far too small patch to be taken as a basis and extrapolate to the rest of the 27000 ly of galaxy ahead of the signal. Maybe the space ruffians are 80 ly away from us, or 800...
The METI guys are working on more powerful emitters specifically so as to send additional data that will be coherent over larger distances.
Naw... if there is any alien apocalypse to worry about it's that some advanced civilization saw a life friendly Earth through their telescopes, perhaps millions of years ago, and decided to seed the place with their own bio-material and without any regard to what life might be there. It won't be a marching army, but a "Terra forming" project that may well already be underway if its happening at all, I would expect.
Humans routinely drive other species on Earth to extinction, our own cousins. We also damage our own planet's environment.
It may be logical for an ET to just destroy us the way you might destroy a harmful bacteria, which might otherwise spread unpredictably. It wouldn't be about wanting slaves or raw resources (although as someone said, Earth itself might be considered somewhat valuable).
True, but my argument is that their intentions fundamentally don't matter. The real costs of acting on ill-intentions through interstellar space would probably give any real life Klingons or Romulans pause.
Meanwhile we fly, sometimes even into space, drive, walk outside, stay inside, eat/drink things that we shouldn't (but enjoy), and a thousand other risks which are substantially more real on an everyday basis... and many of those risks also come with rewards that can make life much more full and rich.
I think making contact with aliens would most likely just 1) confirm we weren't alone or 2) might actually be of benefit as we exchange knowledge and ideas, and/or 3) tell us that evolution tends to creates fearful, xenophobic entities where it also develops intelligence... in that order and with no further consequence than the knowledge.
And even if the aliens are convinced that humans are 100% alien-friendly, it would still make sense to kill us all because there's a chance we'll develop uncontrolled recursively self-improving AI ("intelligence explosion"), which would be a threat to the aliens without us intending it.
I'm suggesting that it doesn't matter and that there are other, more significant factors that render this concern moot.
"Humans have a non-zero probability of growing powerful enough to be an existential threat to the aliens, so the safest option is to preemptively eliminate them."
Non-zero, but near zero. The same sorts of constraints on resource coordination and how much one can likely play with the physics involved not only constrain our would be destroyers, but constrains us as well. Any aliens advanced enough to hear our signals and have the realistic ability to contemplate a move against us can probably do the math: the likely threat of any alien civilization is from a practical perspective, zero, to them or us. The costs are too high, the barriers are too high, the risks are too remote, the rewards are too low.
But here's a different perspective. While hand-wringing about the risks, we completely ignore the possible rewards of actually making contact.
Given that the benefits for all involved in knowledge transfer could well be non-zero and is much more likely to be accomplished. The only issue there is that it's likely to be very, very slow. So maybe they have knowledge about achieving commercial scale fusion energy production. Maybe they help us to understand the dangers of recursive AI (which I also think are overblown). Maybe they can help us understand how to colonize our own solar system (much more realistic than interstellar colonization). Maybe they can help us escape our tendency to indulge in distinctly emotional responses, especially when confronted with the unknown, and rely a little more on reason and rationality...
If I were to gamble on this topic, I'd bet on the more possible, optimistic outcome than on the less likely doomsday scenario. And in the meantime, I'm going to hop into my car (and thereby taking a real risk) and pick up my son from school. Cheers!
This argument reminds me of economic theories that assume all humans are so-called "rational actors".
Even humans, which are in most ways like us and who we have some hope of understanding, do all sorts of things which most of use would consider "irrational", insane, self-destructive, and otherwise incomprehensible.
What justification could there be for assuming alien intelligences think in terms of costs, risks, or rewards? Or that they are in any way "practical" from some human's point of view?
Your argument also ignores the possibility that they have technologies or capabilities that might seem impossible (or even unimaginable) to humans of today (or perhaps of any time). Something that seems undoable to us, like FTL travel, might be possible or even easy for them. We don't know what we don't know.
Earth, as far as we know, is an unusual planet, and obviously so. Either we're wrong, and there's Earths all over the place, or we're alone, or ET has been keeping an eye on us for a long time already; that we're "hidden" is hard to imagine.
I think we should worry more about AI taking over rather than aliens. Its actually much closer to home. On the other hand the only alien intelligence which has the luxury of interstellar travel is the one which has no sense of time and that would be alien AI again.
In an indifferent universe, two optimizers optimizing different things will be in conflict. Can we build machines that optimize what we want to optimize? Can we align our optimization with aliens before it becomes optimal for one of us to annihilate the other?
An interstellar journey means you need a ship that can survive indefinitely in deep space, where you don't get free energy from solar panels or have easy access to comets to mine for materials. An in-system habitat has to be a solved problem before you can even consider building one. But at that point you have effectively unlimited living space, energy, and materials so why bother?
Even if your sun is turning into a red giant you can move your habitats to the optimal distance to sustain life. It's really hard to imagine a time when Humans have used up the entire mass of the other planets in the solar system and need to look outward for resources. You could build an almost unimaginable number of O'Neill cylinders using only the mass of Jupiter.
I fully support a careful approach to combing the cosmos.
Edit: May I ask why this is being downvoted?
The point is not some specific fate, but that anything could happen. And anything includes loads of bad outcomes , most of which we are probably unable to imagine.
Besides that, our solar system is otherwise quite rich. Middle-life, medium-sized star has a lot of mileage left. Being a single-star system, the planetary orbits and climates are naturally stable. Mercury has an enormous core of iron. Venus has a tremendous amount of energy stored and is a prime terraforming target for a civilization with the right technology.
We have 4 enormous gas giants, one of which forms a whole planetary system in and of itself, and could conceivably be collapsed into its own star.
The moon Titan is a giant ocean of liquid hydrocarbons.
This doesn't even get into the rich material wealth offered by the asteroid belt, Ceres, and Saturn's rings.
I'm sure there are better systems than ours from this perspective, but we're far from the bottom of the barrel. I'd sure as hell move here.
Another point is this, perhaps the potential of humans themselves presents a threat to another spacefaring civilization. What if they've seen species like ours rise up and conquer, and they'd prefer to nip it in the bud instead of waiting around for us to become more powerful?
Another word for caution noun
Careful forethought to avoid harm or risk:
calculation, care, carefulness, chariness, gingerliness, precaution, wariness. See fear
And I mean the perfect lack of knowledge is impossible, yes if I definitely did not know anything about something I would not be afraid of possible problems with that thing - how could I?
But we always know a bit even about the 'unknown' even if the thing we know is that there might be something in that unknown thing that would be dangerous to life, as many things in the Universe are like extreme heat, cold, releases of kinetic energy, too much water, or the presence of elements of one sort or another in more abundance than is healthy for us, or life that is similar to life on this planet since much of life on this planet is focused on killing other life on this planet for various reasons. I'm walking in an unknown environment on this planet I might still be afraid because I don't know what large carnivores it has, but I do know it might have large carnivores.
My original post was probably too broad to be useful. This topic requires nuance that I didn't express.