A partial answer to your question is that there’s a minimum amount of heat necessarily radiated when doing computation, given by the Landauer principle.

It's not a given that Landauer's principle is an absolute threshold - the Wikipedia article describes challenges, and there are attempts like Reversible Computing which can potentially work around it.

Furthermore, I also do not think that we will detect dyson spheres, because if a civilisation wishes to hide, they won’t radiate heat uncontrollably by extracting all possible energy, but rather send that energy elsewhere, for example by dumping it into a black hole. But I could be wrong and such a civilisation might care more about energy than remaining undiscovered.

Fully agree that such an advanced civilization will most likely want to hide, and stop any infrared radiation to the largest part.

[–] [email protected] 1 points 7 months ago (1 children)

Reversible computing can not work around it because one simply can not extract information without irreversibly affecting the system. This is a fundamental constraint due to how, in quantum mechanics, once an observer entangles themselves with a system they can never unentangle themselves. I believe that from that single fact one can derive the impossibility of reversible existence.

[–] FooBarrington 0 points 7 months ago (1 children)

Better go tell the theoretical computer scientists who waste their time writing papers on the topic! Could save them a lot of trouble if they had just asked you.

[–] [email protected] 1 points 7 months ago* (last edited 7 months ago)

This comment tells me that you do not fully understand reversible computing, thermodynamics, nor what I am trying to say. The snark does not motivate me to be patient or pedagogical, but I'll still give it a shot.

By interfering with a closed system as an entity outside of that system (for example by extracting information by performing a measurement on any of its component subsystems such as the position or momentum of a particle), you are introducing a dependency of that formerly closed system's state on your state and that of your environment. Now, by state I mean quantum state, and by interfering I mean entangling yourself (and your environment) with the system, because our reality is fundamentally quantum.

Entanglement between an observer and a system is what makes it appear to the observer as if the wave function of the system collapsed to a (more) definite state, because the observer never experiences the branching out of its own quantum state as the wave function of the now combined system describes a superposition of all possible state combinations (their (and their environment's) preceding state × the system's preceding state × the state of whatever catalyst joined them together). The reason an observer doesn't ever experience "branching out" is because the branches are causally disconnected, and so each branch describes a separate reality with all other realities becoming forever inaccessible. This inaccessibility entails a loss of information, and this loss of information is irreversible.

So there you have it. You can never extract useful work from a closed system without losing something in the process. This something is usually called "heat", but what is lost is not merely "heat": it is the potential usefulness of the thing of interest. But it really all boils down to information. Entropy increases as information is lost, and this is all relative to an observer. Heat dissipation represents "useless information" or "loss of useful/extractable energy" as it concerns an entity embedded in a quantum wave function.