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I'm not a physicist.

It seems to me that a quantum computer needs to store and access all its quantum states.

Currently the universe is concerned with calculating the interaction of its particles and radiation. If, via a quantum computer, we gave it a problem requiring enormous bandwidth/information-storage, couldn't this interfere with the universe's everyday housekeeping causing a general slowdown?

Of course it might be supposed that if everything slowed down proportionally then this would be undetectable. Is there anything we could use to detect a ubiquitous slowdown in the universe's metabolism? Could such a slowdown be catastrophic?

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    $\begingroup$ Could you explain why do you think a quantum computer should slow down the universe? $\endgroup$ – Ofek Gillon Nov 16 '18 at 18:11
  • $\begingroup$ One can imagine a number that is bigger than the number of states in a finite granular universe. I am supposing that a large enough quantum computer might require more than that number of states in order to complete a very large but well-defined problem. Let's say factorising a prime number that can easily be defined but is larger than the number of states in the universe. $\endgroup$ – chasly from UK Nov 16 '18 at 19:43
  • $\begingroup$ That's where the speed-up in quantum computation comes from: In fact, it is the universe around it which is being slowed down! $\endgroup$ – Norbert Schuch Nov 16 '18 at 19:45
  • $\begingroup$ So you are confirming my guess? If so, can we detect the slowing down in any way? $\endgroup$ – chasly from UK Nov 16 '18 at 19:48
  • $\begingroup$ @ Norbert Schuch: How do you define the speed of the universe? $\endgroup$ – D. Halsey Nov 17 '18 at 0:51
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If the universe is actually obeying the laws of physics as we understand them, it's already doing as much computation as would be needed for it to perform a quantum computation.

It's only if we assume the universe is cutting corners and merely approximating the laws of physics as we understand them that it would need extra computational power to perform quantum computing.

Why would the universe only approximate the laws of physics?

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  • $\begingroup$ My supposition is that there are, in theory calculations that require more processing power than is needed to drive the whole universe. $\endgroup$ – chasly from UK Nov 16 '18 at 19:46
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"Currently the universe is concerned with calculating the interaction of its particles and radiation. If, via a quantum computer, we gave it a problem..."

A quantum computation is not performed by the whole universe, but only by the atoms, particles, etc, that make up the computer, and they are all it has to work with. It can't arbitrarily draw upon the resources of all the other particles in the universe when things get difficult.

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