Comments | sharp_blue: Locality, determinism and causality

sharp_blue

Locality, determinism and causality

Jun 03, 2006 22:20

A physical theory is local if it doesn't allow faster-than-light influences.

A physical theory is deterministic if a total knowledge of the current state of a system whose behaviour is governed by the theory exactly determines the future states of the system (or, more weakly, if the current state determines the outcome of all possible future ( Read more... )

physics quantum

Comments 24

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sharp_blue June 3 2006, 22:12:25 UTC

In what way aren't you with me on causality?

It seems to me that we might be able to relax causality in moderately benign ways if we really need to. For example, I'd be quite happy with a theory that was built arond Novikov's ideas of consistent histories (in which causal loops can exist as long as they are entirely consistent with themselves). Consistent causal loops might do weird things to our ideas about free will, but they don't bother me nearly as much as other approaches involving parallel universes or radical rewrites of history.

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sharp_blue June 4 2006, 15:00:04 UTC

Actually, I'm not sure you need to understand what I mean by "causality" at all, as the Bell argument is about locality and determinism and I only included causality too for completeness.

Definining causality in the general, philosophical sense is pretty hard - see, for example, Dennett's Freedom Evolves - but physicists generally use a much weaker version. In this case, it's good enough for causality to mean something along the lines of "an absence of time machines" or "no (invariantly) backwards in time signals".

There are various kinds of time machines that are at least semi-respectable in physics. One is the "closed timelike curve" of general relativity. Another is the faster-than-light time machine in special relativity (which I demonstrate in "Relativity, FTL and Causality"). Generally speaking, physicists think that causality is such a strong requirement and relativity so well supported experimentally that faster-than-light travel and closed timelike curves will be ruled out by some as yet unknown (or semi-known) principle. ( ... )

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kalashnicough June 4 2006, 14:33:24 UTC

I think most lay people prefer non-deterministic theories as they allow us to have free will.

sharp_blue June 4 2006, 14:46:03 UTC

Possibly, but on the other hand I don't think modern physics has ever included theories that allow free will. The non-determinism of quantum mechanics, as you well know, is of a purely random kind and I don't see how randomness is any better than determinism as a substrate for free will.

kalashnicough June 4 2006, 16:51:58 UTC

Well, assuming you don't follow Walker's somewhat hippyish hypothesis that "consciousness is the hidden variable", then non-deterministic theories are seemingly totally random. However, you then get into all kinds of crazy-assed metaphysical territory about what constitues an 'observer' - Einstein's mouse and all that jazz. Personally, I suspect that we were created by a supreme being who gifted us with free will and...

Actually, I'm undecided on the whole issue, but I think most people at least like to have the illusion of free will - and that means that they will reject any superdeterministic theories on the basis of belief, rather than evidence.

Anyway, some Dutch physicist (whose name escapes me at the mo) recently published a paper in which he mathematically 'proved' that there's a purely deterministic sub-quantum layer to reality.

Hi rich! vacuum_dentata June 11 2006, 10:56:51 UTC

You sound like my Art history prof. I haven't seen you in years. Been getting my act together, school, meds, etc. I have manic depression and lithium has helped a lot. I wanted to find you an apologize for what a whack job I was in my late teens and offer current friendship (if you want it). I tried logging into Mirc but my puter as of right now is a piece of garbage and can only really handle basic irc chats and emails/text messaging ( ... )

Re: Hi rich! sharp_blue June 11 2006, 11:04:32 UTC

*hugs you for a good long time*

Re: Hi rich! vacuum_dentata June 11 2006, 16:15:06 UTC

Thank you. :) I'll email you later today.... with a more personal update/explanation. This means we can still be friends? *huge smile*

And I AM very sorry for my behavior of a few years ago. You deserved more than that, and if you give me the oppurtunity, I think you'll find I have changed a lot and that I will be a much more fun friend and more stable, healthy and upbeat one too!

*hugs you back*

Lex

funny websites vacuum_dentata June 11 2006, 16:38:23 UTC

Oh yes... for when you're down.. websites that'll hopefully interest you/make you laugh ( ... )

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