I did not assume that "physics and engineering are 'done'". I assume instead that the known laws of physics which seem to prevent faster-than-light travel hold and are inviolable, despite how smart any species may be.

The analogy with the the argument 500 years ago doesn't apply because 500 years ago we did not have the understanding of physics and chemistry that we have today. Of course we still have a lot of physics left to discover, but it's extremely unlikely that whatever new physics we discover will violate special relativity.

It could be as simple as a hedging your bet strategy. You might not even need faster than light travel. The weapon of our destruction in the distant future could have already been launched, making its way from its factory at 99% the speed of light as we speak, simply because the characteristics of our planets suggest a nonzero possibility of life arising. If the bet means your species has no potential predator in the future, then committing all the resources you can to such an effort would be a decent strategy.

I hope I am wrong of course.

Everyone seems to be overlooking that the costs for this are insanely high. This has to be funded by tax dollars. Why should we use taxpayer money to try to annihilate every other single living organism in the galaxy?

The reasoning that "there's a non-zero chance that if we don't, someone else will do it to us" is practically useless reasoning, especially from a policy perspective. That's like saying that the laws of quantum mechanics prescribe a non-zero probability to a planet-sized nuclear bomb materializing next to Earth and instantly exploding, demolishing all life here, yet we don't make policies based on such vanishingly low probability events.

Plus is every sentient species going to be this genocidal by nature? Humans do have a history genocide, but in civilized society today, we do everything we can to avoid war. The United States clearly has the military might to invade any number of nations in the world and easily conquer them, yet we don't for a whole host of totally valid and logical reasons. Heck, we pass laws (I assume for good and justifiable moral reasons) to protect indigenous people even though we have the might to completely obliterate them.

Everything about the Dark Forest is completely impractical when thought through with the smallest amount of sensible reasoning.

I was framing it more like the aliens are the ones launching these weapons, not us, we can hardly get our act together for such farsighted planning. And sure, not every species are genocidal, but some certainly are by nature. We hesistate fighting other humans because they are humans, and we value ourselves more than any other lifeform. But people don't hesitate swatting a mosquito, or dumping bleach on a colony of mold in their shower. My cat doesn't care about slowly and painfully killing the insects it catches. The tree doesn't care if its shade blocks other plants from taking advantage of the sun. The invasive species doesn't care if it displaces the native ones. The virus doesn't care if it kills its host.

Alien life might realize this, that the probability is very high that other life forms really don't care about the continued reproductive success of other life forms, unless there is some lopsided benefit involved. Maybe they aren't even consuming their own resources to act on this goal of wiping out the possibility, however low, of a danger to their species emerging. Maybe they've already sent out some AI controlled probes, that are on distant metal rich asteroids far from their homeworld, autonomously mining, building weaponry, identifying targets, and launching these weapons that will hit their targets millions or billions of years in the future. Such implementation could even outlast the species that initially created it.

There are so many practical problems with doing such a thing. The first is that it's incredibly economically expensive to do so. If the aliens aren't spending their resources, whose resources are they spending? Clearly it must be theirs.

Secondly, the thought of unleashing a civilization-destroying self-replicating AI into the galaxy is so dangerous that I doubt any civilization would do it. Imagine the dangers of that. All it takes is for the AI to misidentify you as "another" civilization and wipe you out, or for these AI to be taken over by another party and turned against you, or for the AI to actually just misidentify you and kill you. The dangers of that far outweigh the benefits in the miniscule chance that the Dark Forest is in fact real.

We don't even understand gravity entirely.

It's entirely possible we will come up with a different theory which is largely consistent with much of what we do know, while still allowing for things that we currently think are impossible.

We know so very, very little.

You don't need faster than light travel to cover the entire galaxy. Using it as a roadblock doesn't make sense.

It does when traveling interstellar distances. Even traveling at 10% the speed of light, it would take a ship one million years to cross the galaxy. The energy required to accelerate to that speed (and then decelerate to rest) is absurdly high. Once you realize how large the costs become for interstellar travel, simple economics makes most of these arguments moot. The outcome is not worth the cost for almost anything you want to achieve.

We are about 30k light years from the center and the milky way radius is about 50k light years. So if we sent out probes in every direction we could hit 100% of the galaxy in 800k years.

A 100kg probe at 10% of the speed of light is about the energy of a 20 megaton nuke. Another 20 megatons to slow it down.

It seems very likely that the technology necessary will be developed in the next 100-10,000 years.

Let's say we send 10,000 probes out, each weighing 1,000kg at 10% the speed of light out into the Milky Way. We would do this with the sole intention of destroying an entire planet once we find any hint of even bacterial life? There isn't enough time to phone home to decide what to do with this information, so the probe would have to decide for itself whether or not to commit immediate genocide.

How could any government possibly convince their citizens that this is the best use of trillions of tax dollars (if not quadrillions of dollars), not to mention how can we create a 1,000kg device that can categorically annihilate an entire planet? If the Dark Forest theory is indeed true, why haven't we seen evidence of such galactic levels of destruction? The Milky Way has been around for over 10 billion years. Surely by then there must've been at least one civilization advanced enough to do this in that time. Yet we see no evidence for this whatsoever.

The more you spell this out, the more absurd it becomes from any practical perspective.

I find it amusing that your assertions about what aliens may or may not do completely ignores the fact that the one intelligent species we can observe does all the things you claim no species would ever do. Really drives home the point you are making with your satirical take.

I don't think that's true at all. Humans used to invade foreign lands and decimate the native population, but we did that in pursuit of resources, not as the primary goal (when Columbus sailed off from England, he wasn't sent out with the dictum of "go find other intelligent species and annihilate them"). Furthermore, that is clearly not the case anymore in modern times. As I've stated, we have the technology and military might to wipe many nations off the face of the Earth, yet we choose not to.

Our destruction of the environment is different also. Cutting down the Amazon is a side-effect of our pursuit of natural resource. We do not go there with the intention of killing all of the species there. That is just an unfortunate consequence of our hunt for resources.

The Dark Forest is entirely different. The entire thesis of that theory is an advanced civilization would have the singular goal of sterilizing all other life in the galaxy as the goal in and of itself. There is simply nothing analogous to that in human history.

Lets see, in the last 2 centuries humans have:

Devoted years worth of total resource production to kill each other by the millions over a few square miles of resource poor land.

That was such a popular hit that we threw a sequel that was even more costly.

As a result of the sequel we had several major empires devote trillions of dollars to making sure that we could annihilate our own species just in case one of the others also tried to annihilate the species first. This caused the collapse of at least one empire. People regularly call for the use of these annihilation weapons and it's sheer dumb luck that it hasn't happened yet.

Speaking of annihilation - there have been several large scale attempts at exterminating all humans that fit xyz criteria in the last century. Mostly these are not based on anything other than "exists with DNA slightly different than our own". It seems likely that "exists with way different DNA" and even "exists with a different building block than DNA" could be used as criteria.

If you don't limit yourself to intelligent species, humans have carried out large scale attempts at exterminating several native species. Some I'm aware of in the last couple centuries:

Wolf eradication programs paying bounties per dead wolf.

Same for the american bison. (this one is special because it was also intentionally an attempt to genocide native folks by starvation).

The "smash sparrow" campaign in china - kill all the sparrows because of a misunderstanding of what food they eat.

I guess I could buy your premise that humans wouldn't participate in the dark forest for logical reasons is semi-believable IFF we ignore the things humans actually do.

As I stated, people tried to annihilate each other in pre-modern times. In modern times, we literally try to do exactly the opposite, and we go so far as to pass to laws to prevent this even against countries that stand no chance against us whatsoever.

Give me an example of modern times and now we're talking.

All of the examples you said are so limited in scale to what the Dark Forest actually requires: complete sterilization of all life in the galaxy. To go from "killing bison" to exterminating all forms of life is a gargantuan leap.