Re: on formally describable universes and measures

From: Wei Dai <>
Date: Wed, 27 Dec 2000 20:17:19 -0800

On Wed, Dec 27, 2000 at 04:50:42PM +0100, wrote:
> None of the quantum effects we observe forces us to give up the simple
> idea that our universe can be simulated on a classic TM, just like
> there is no evidence that forces us to assume the existence of complex
> and incomputable things such as uncountable sets.

I think we understand each other sufficiently on the other issues, so I'll
only follow up on this one. I agree that our universe can be simulated on
a classic TM. What I don't agree with is that our universe can be
simulated quickly on a classic TM, which is what a speed prior based on a
classic TM would predict. In other words, the speed prior predicts that we
will never observe any quantum effects that can't be simulated quickly on
a classic TM. I suggest that you talk about this prediction more
explicitly in your paper since it would have major consequences. Many
people are busy trying to build quantum computers, which would be a waste
of effort if this prediction is correct.

Even within classic models of computation, there seem to be significant
variations in speed. As far as I can tell from my theory of computation
book, moving from a multi-tape TM to a single-tape TM can cause a squaring
of running time for some problems, which would translate to a squaring of
the speed prior for some strings. So a similar question is, how do you
pick which classic TM to base S on?
Received on Wed Dec 27 2000 - 20:22:42 PST

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