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Is their anything that would make Quantum computing obsolete in the future? I know a Matrioksha Brain is the most powerful theoretical computer; but it probably won’t ever be realized. Too large and hard to build. I know about hypercomputing but again that seems also too far-fetched.
So let’s say quantum computing becomes mainstream. What would be the next upgrade?
This is mainly a philosophical question and depends on your model of reality. Under some assumptions, time trevling ability will allow aditional speedup.... You can read about the CTC model, mainly introduced by Scott Aaronson, that deals with this subject. https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.scottaaronson.com/papers/ctchalt.pdf&ved=2ahUKEwih8a_Qz-LnAhUCGewKHUFlBZMQFjABegQIBhAL&usg=AOvVaw3TMaefh-UTmCsAHKRT6EhL
There are really only two types of physics - classical and quantum. In fact classical physics is actually the limit of quantum mechanics for large n, where n is the number of particles/atoms in the physical system. Because quantum computing obeys the laws of quantum physics which operate in an infinite dimensional Hilbert space, it is an exponentially more powerful paradigm than classical physics. So quantum computing is the final say in terms of computing power that obeys the currently known laws of physics.
I heard some time ago that if we were able to shoot a computer arbitrarly close to a black hole and retrieve it, by virtue of general relativity properties about the speeding of time near black holes we would retrieve an older computer, which would have thus computed for longer... Or alternatively we could get close to the black hole and we would see the calculations of a far away computer being made faster and faster. Note that in the first case this P and P-blackhole (the complexity class were you can shoot the computer to a black hole and retrieve it) would probably be equal, here's the reasonning. Given that in time t you can only harvest the energy contained in a sphere of size proportionnal to $t^3$ (we are restrained by the speed of light). Given that to get the computer $\epsilon$ close to the horizon of events and retrieve it you need an an energy polynomial in $1/ \epsilon$. Given that a computer $\epsilon$ close to the horizon of events experiences a speedup polynomial in $\epsilon$. We can deduce that P = P-blackhole.
In advance sorry if what I said is not an accurate description of the phenomena of general relativity, i'm going by memory and my limited understanding of the theory.
