Locality, determinism and causality

[sharp_blue]

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

Comments

[sharp_blue]

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.

[sharp_blue]

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.

[kalashnicough]

You demonstrate a faster-than-light time machine!?

[sharp_blue]

Well, I show that if you give me a faster-than-light engine then I can make a time machine. At least I didn't use the phrase "constructively demonstrate".