[FoRK] Inertial Mass != Gravitational Mass?
jbone at place.org
Sat Jun 19 05:35:37 PDT 2010
Sent from my iPad
On Jun 19, 2010, at 5:08 AM, silky <michaelslists at gmail.com> wrote:
> Obviously, you're assuming that time travellers go back into our time.
> Which may not be the case.
This is actually a reasonable point worth addressing.
There is a whole class of speculative "time machines" that have the following property: they permit travel to the "past" but only to points in the historical light-cone subsequent to the initial construction / operation of the device. An even more plausible subclass of these creates a closed time-like loop of fixed length or anchored origin / endpoint, meaning at any future point subsequent to their own creation they only permit travel a fixed distance into the past, or (slight variation, different 4-geometry) to a particular arbitrary point in the past, probably the first operation of the device.
Another conjecture I've raised occasionally relates to this. In an unconstrained multiple-worlds interpretation of quantum mechanics, you literally have within the entire phase-configuration space every possibility, however implausible. But the dimensionality of the configuration space is drastically constrained if you introduce closed time-like causal curves; they serve as strange attractors of probable events and dramatically prune the space of possibilities to those most probable ones that intersect one or more CTCs. From the embedded observer's perspective, under these constraints, the universe conspires to ensure certain high-impact events; e.g. travel back to prevent Lee Harvey Oswald from doing the deed and some guy you've never heard of does it instead. From the perspective of the observer, time "flows" like a river; drop a rock in the middle and it will flow around it, eventually converging on the path it otherwise would have taken absent the rock.
Cramer's transactional hypothesis takes this intuition to its ad absurdum conclusion. IMHO it is an embarrassment of riches. Only a few CTCs anywhere / anywhen / anyhow will do the trick....
Hawking takes the opposite view: nature abhors a time machine and conspires to defeat their construction. This seems to me arbitrary and rather unfounded, and provides no means of presenting the opposite embarrassment of implausible riches necessitated by unconstrained MWI.
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