At 9:32 AM -0800 3/28/02, Wei Dai wrote:

>
>Perhaps you're not familiar with the history of this mailing list, but
>Juergen Schmidhuber is one of the first authors to explicitly state the
>idea that all possible universes exist in a published scientific paper,
>and that paper is cited in the public invitation to this mailing list.

Good for him. It's an interesting idea.

>I
>have to say it's very inappropriate to publicly "plonk" him when it seems
>to be you that doesn't understand the issue at hand.

I believe that I do understand the issue at hand. It seems that 
Juergen and I are using "predictive power" in different senses.

>I agree with Juergen that AP does not predict anything by itself. You need
>at least a probability distribution or measure of some sort in addition to
>AP. There has been extensive discussion on this list over exactly what
>measure should be used, and what the measure is over.

I think that if this is the issue then we are looking at different 
interpretations of "predictive power", as I said. My POV is Bayesian. 
I am not thinking in terms of what you can say about ensembles of 
universes, but about what you can predict about *this* universe, 
given the background information that we are here, and other 
background information that we may already have (about physics for 
example). One can then write down things like P(a certain nuclear 
energy level is between x and y|we are here & nuclear physics works 
in the way we have learned that it does)=1, which is a prediction of 
where we will observe that nuclear energy level to lie when we get 
around to observing it (which hadn't been done when Hoyle wrote his 
paper). That in my book counts as a real prediction of physics, and 
it is a consequence of the AP...and of other things, I agree. If 
Juergen's point is that you may need other information than just the 
AP (such as how particles will behave in certain physical 
situations), then I will stipulate that. But that's a trivial point. 
All theories require background information, and a Bayesian is 
required to include all background information when making inferences 
(predictions).

For me (and I believe most scientists) a theory has predictive power 
if it allows us to accurately predict verifiable facts about *this* 
physical world, facts that we did not already know. That's exactly 
what Hoyle did, and it involved the AP, since (as Barrow and Tipler 
point out) we would not exist (certainly in our present form) if 
there were no carbon.

I will point out again, what you have not addressed, that people like 
Barrow and Tipler, and Tegmark, seem to regard the carbon prediction 
as a prediction of the AP.

>Have you read Juergen's paper "A Computer Scientist's View of Life, the
>Universe, and Everything"? If not please do so before making any more
>posts. It's at http://www.idsia.ch/~juergen/everything/html.html.

Yes, I have read it. He's talking about ensembles of universes, which 
isn't what I am talking about. I don't really disagree with anything 
I saw in that paper, but it's a different topic from what I was 
addressing.

Since Juergen and I seem to be talking at cross-purposes about 
different sorts of predictions, I will put him back on my list to 
read. I apologize to him for getting annoyed. But I wish that he 
would agree that there is predictive power in the AP as *I* am using 
the term, even if not as *he* is using the term. In other words, that 
his original statement was painting with too broad a brush.

The AP does have predictive power in the sense that I (as a natural 
scientist) usually mean it.

Bill

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