Subject: Re: How To Create A Time Machine.
From: devens@uoguelph.ca (David L Evens)
Date: 15 Nov 1996 11:02:01 -0800
Warren G Anderson (anderson@phys.ualberta.ca) wrote:
[Truly vast amounts of unneeded quoted text removed by moderator - jb]
: Believe it or not, serious research has been done into "time travel" within
: the context of general relativity. There exist, in relativity, solutions
: that have what are called "closed timelike loops".
As I understand it, these closed time-like loops can occur arround a Kerr
black hole with a high enough angular momentum. If so, then time travel
might be a naturally occuring phenomenon.
---------------------------+--------------------------------------------------
Ring around the neutron, | "OK, so he's not terribly fearsome.
A pocket full of positrons,| But he certainly took us by surprise!"
A fission, a fusion, +--------------------------------------------------
We all fall down! | "Was anybody in the Maquis working for me?"
---------------------------+--------------------------------------------------
"I'd cut down ever Law in England to get at the Devil!"
"And what man could stand up in the wind that would blow once you'd cut
down all the laws?"
------------------------------------------------------------------------------
This message may not be carried on any server which places restrictions
on content.
------------------------------------------------------------------------------
e-mail will be posted as I see fit.
------------------------------------------------------------------------------
Subject: Re: Occam's razor & WDB2T [was Decidability question]
From: kenneth paul collins
Date: Fri, 15 Nov 1996 19:13:42 -0500
Torkel Franzen wrote:
>
> kenneth paul collins writes:
>
> >There are no epicycles in the Copernican view. Indeed, Copernicus argued
> >in favor of the moving-Earth, heliocentric view precisely on the basis of
> >its being free of epicycles:
>
> You are sadly misinformed. The system presented by Copernicus
> required epicycles, and plenty of them. Please note, in the
> quotation from Kuhn, the qualification "at least qualitatively".
> The qualitative charm of a heliocentric model was, naturally, not
> enough to recommend it to people who wanted actual astronomical
> quantitative data.
Torkel, you are correct. Thank you for your persistence. Copernicus did carry
epicycles forward. It was those who extended the work of Copernicus that
eliminated the epicycles, not Copernicus.
Part of the problem was that the data in Copernicus's day was quantitatively
inaccurate, so any calculation that incorporated that data would have to have
incorporated, and reflected, such errors. Such errors were introduced by
astronomers trained in the Ptolemaic tradition, pretty much requiring the
perpetuation of the Ptolemaic methods.
This is some of the behavioral inertia that I addressed in my initial post in
this thread.
I'm grateful to you. I've been incorrectly stating things with respect to the
work of Copernicus for a long time. Cheers, ken collins
_____________________________________________________
People hate because they fear, and they fear because
they do not understand, and they do not understand
because hating is less work than understanding.
Subject: Re: Occam's razor & WDB2T [was Decidability question]
From: radford@cs.toronto.edu (Radford Neal)
Date: 16 Nov 96 17:31:49 GMT
>< Radford Neal:
><
>< One often sees people using priors that are such that the
>< effective complexity of the model increases as the amount of
>< data increases. This makes no sense - it amounts to using a
>< prior that one knows is going to be contradicted by future
>< data.
Neil Nelson wrote:
>... Of course the difficulty here is the
>determination of the prior probabilities and algorithmic
>relation, for which our only effective recourse is an analysis
>of the previously and currently available data. This implies
>that our prior probabilities and algorithm may change depending
>on any increase in the available data; or more simply, we would
>not want to hold to our previous judgment if new information
>indicated we were previously in error.
This is not the case for a full Bayesian analysis, since the prior
decided on before any data is collected will implicitly contain all
the revisions of judgement that would be prompted by any possible data
set.
In practice, a Bayesian is likely to use a model and prior that do not
contain certain possibilities that seem very unlikely at first, simply
because formalising all these possibilities is too much work. If the
actual data indicate that these possibilities need to be considered,
then the Bayesian might revise the prior and model, perhaps adopting a
more complex one.
However, I think that this scenario has little to do with the usual
reasons why people think that you can't use complex models with small
datasets. The usual reasons are not compatible with a Bayesian viewpoint.
Radford Neal
----------------------------------------------------------------------------
Radford M. Neal radford@cs.utoronto.ca
Dept. of Statistics and Dept. of Computer Science radford@utstat.utoronto.ca
University of Toronto http://www.cs.utoronto.ca/~radford
----------------------------------------------------------------------------
Subject: Re: Occam's razor & WDB2T [was Decidability question]
From: radford@cs.toronto.edu (Radford Neal)
Date: 16 Nov 96 17:31:49 GMT
>< Radford Neal:
><
>< One often sees people using priors that are such that the
>< effective complexity of the model increases as the amount of
>< data increases. This makes no sense - it amounts to using a
>< prior that one knows is going to be contradicted by future
>< data.
Neil Nelson wrote:
>... Of course the difficulty here is the
>determination of the prior probabilities and algorithmic
>relation, for which our only effective recourse is an analysis
>of the previously and currently available data. This implies
>that our prior probabilities and algorithm may change depending
>on any increase in the available data; or more simply, we would
>not want to hold to our previous judgment if new information
>indicated we were previously in error.
This is not the case for a full Bayesian analysis, since the prior
decided on before any data is collected will implicitly contain all
the revisions of judgement that would be prompted by any possible data
set.
In practice, a Bayesian is likely to use a model and prior that do not
contain certain possibilities that seem very unlikely at first, simply
because formalising all these possibilities is too much work. If the
actual data indicate that these possibilities need to be considered,
then the Bayesian might revise the prior and model, perhaps adopting a
more complex one.
However, I think that this scenario has little to do with the usual
reasons why people think that you can't use complex models with small
datasets. The usual reasons are not compatible with a Bayesian viewpoint.
Radford Neal
----------------------------------------------------------------------------
Radford M. Neal radford@cs.utoronto.ca
Dept. of Statistics and Dept. of Computer Science radford@utstat.utoronto.ca
University of Toronto http://www.cs.utoronto.ca/~radford
----------------------------------------------------------------------------
