Catastrophe Engines: A possible resolution to the Fermi Paradox

Seconded. So many layers of specificity, one of which is "exotic physics" ... I have a hard time seeing why it's worth entertaining this idea over any of the other unlikely but less specific theories one could devise.

The observation of the Fermi Paradox call for some explanation and this is one albeit constructed case.

I mostly agree with you, but we may disagree on the implausibility of exotic physics. Do you consider all explanations which require "exotic physics" to be less plausible than any explanation that does not? If you are willing to entertain "exotic physics", then are there many ideas involving exotic physics that you find more plausible than Catastrophe Engines?

In the domain of exotic physics, I find Catastrophe Engines to be relatively plausible since are already analogues of similar phenomena to Catastrophe Engines in known physics: for example, nuclear chain reactions. It is quite natural to think that a stronger method of energy production would result in even greater risks, and finally the inherent uncertainty of quantum physics implies that one can never eliminate the risk of any machine, regardless of engineering. Note that my explanation holds no matter how small the risk lambda actually is (though I implicitly assumed that the universe has infinite lifetime: for my explanation to work the expected life of the Catastrophe Engine has to be at most on the same order as the lifetime of the universe.)

It is also worth noting that there are many variants of the Catastrophe Engine hypothesis that have the same consequences but which you might find more or less plausible. Perhaps these Engines don't have "meltdown", but it is necessary that they experience some kind of interference from other nearby Engines that would prevent them from being built too closely to each other. You could suppose that the best Matrioshka Brains produce chaotic gravity waves that would interfere with other nearby Brains, for instance.

Personally, I find explanations that require implausible alien psychology to be less plausible than explanations that require unknown physics. I expect most higher civilizations to be indifferent about our existence unless we pose a substantial threat, and I expect a sizable fraction of higher civilizations to value expansion. Perhaps you have less confidence in our understanding of evolutionary biology than our understanding of physics, hence our disagreement.

For the sake of discussion, here is my subjective ranking of explanations by plausibility:

1. There are visible signs of other civilizations, we just haven't looked hard enough.
2. Most expansionist civilizations develop near light-speed colonization, hence making it very unlikely for us to exist in the interval between when their civilization is visible and our planet has already been colonized
3. We happen to be the first technologically advanced civilization in our visible universe
4. Most artifacts are invisible due to engineering considerations (e.g. the most efficient structures are made out of low-density nanofibers, or dark matter).
5. Colonization is much, much more difficult than we anticipated.
6. Defensively motivated "berserkers". Higher civs have delicate artifacts that could actually be harmed by much less advanced spacefaring species, hence new spacefaring species are routinely neutralized. It still needs to be explained why most of the universe hasn't been obviously manipulated, hence "Catastrophe Engines" or a similar hypothesis. Also, it needs to be explained why we still exist, since it would be presumably very cheap to neutralize our civilization.
7. Some "great filters" lie ahead of us: such as nuclear war. Extremely implausible because you would also have to explain why no species could manage to evolve with better cooperation skills.
8. "Galactic zoo" hypotheses and other explanations which require most higher civilizations to NOT be expansionist. Extremely implausible because many accidentally created strong AIs would be expansionist.

I ignore the hypothesis that "we are in a simulation" because it doesn't actually help explain why we would be the only species in the simulation.

EDIT: Modified the order

I think the biggest reason we have to assume that the universe is empty is that the earth hasn't already been colonized.

We've only been emitting radio for around 100 years

We'we been emitting the presence of oxygen in our atmosphere for about 1 billion years. Every nontrivial alien would notice this.

Regardless of whether ETs are sending signals, presumably we should be able to detect Type II or Type III civilizations given most proposals for how such civilizations should look like.

The second civilization would just go ahead and build them anyways, since doing so maximizes their own utility function. Of course, there is an additional question of whether and how the first civilization will try to stop this from happening, since the second civ's Catastrophe Engines reduce their own utility. If the first civ ignores them, the second civ builds Catastrophe Engines the same way as before. If the first civ enforces a ban on Catastrophe Engines, then the second civ colonizes space using conventional methods. But most likely the first civ would eliminate the second civ (the "Berserker" scenario.)