[ExI] QT and SR

Lee Corbin lcorbin at rawbw.com
Fri Sep 12 07:02:48 UTC 2008

Stuart writes

> --- Stathis Papaioannou <stathisp at gmail.com> wrote:
>> The splitting can be said to occur while they were separated, but
>> should we say that they were in the same universe they are revealed
>> to be in when they meet even while they were separated?

Stathis, let me see if I am interpreting your question correctly.
First, I thought that the Alices and Bobs did stay physically
separated from each other, so that you probably mean at
a time before some signal went from a particular Alice event
A to a particular Bob event B, after which we do understand
them to be in the same universes, (i.e. A1 and B1 as henceforth
ongoing persons can reliably communicate with each other
and marvel that their photons did come out the same way,
way back when).

So let's say that A becomes A1 and A2, and B becomes B1 and B2,
and the question is, should we say that A1 and B1 are
immediately in the same universe, even though the splits
caused at A and B haven't yet had time to reach each other
and match up? I suppose that yes, we could say that A1 and B1
(pre-any signal having gone from any A to any B) are in the
same universe now, since they're fated to be anyway.
But I'd be agin it because

\                /
/                \

the splits that are (very roughly speaking) still
separated (in someone's reference frame) by the
distance ab and which are moving towards each other
don't make it clear *by definition* that the upper
slashes will turn out to be together in the one universe
and the lower slashes will turn out to be in the
other universe. That the photon that was measured
somewhere to the left of the diagram will have outcomes
exactly parallel to the EPR-entangled one measured
somewhere to the right of the diagram, is a deduction
or at least a mathematical consequence. 

>> It seems to me that the Alice outside Bob's light
>> cone who will eventually be demonstrated to be in
>> "same" world is just as unreachable, just as
>> causally isolated from him as the Alice in the
>> "other" world.

Yes, I agree.

>> Perhaps this could be dismissed as a mere
>> philosophical point, but it does emphasise
>> that no FTL communication is possible or
>> necessary.

Anything to help with that! But it may simply do,
to say that A1 and B1 *will* be in the same universe
after their splits join up, rather than announcing
that they're beforehand already in the same universe.

Stuart writes

> I don't see why people would have a problem
> with the possible FTL nature of a correlation
> in the EPR experiment, or wave-function collapse,

It's incomprehensible on the theory of SR, that's why.

> but have no problem with the idea of the entire
> universe being causally split in the mere seconds
> it takes for someone to make a measurement,

The whole universe never ends up being split in its
entirety unless it's of finite extent. The splits
start locally and speed outwards only at c.

> each and *every* time a measurement is made.

It *is* a horrible zoo; David Deutsch says on p. 213
of "The Fabric of Reality":

   "...rely on such things as solid matter or
   magnetized materials, which could not exist
   in the absence of quantum-mechanical effects.
   For example, any solid body consists of an
   array of atoms, which are themselves composed
   of electrically charged particles (electrons,
   and protons in the nuclei). But because of 
   classical chaos, no array of charged particles 
   could be stable under classical laws of motion.
   The positively and negatively charged particles
   would simply move out of position and crash into
   each other, and the structure would disintegrate.
   *It is only the strong quantum interference 
   between the various paths taken by charged
   particles in parallel universes* that prevents
   such catastrophes and makes solid matter possible.

So any solid object is making nearly infinitely
many measurements each nanosecond, and those "splits"
radiate away at c, so that the whole fabric of reality
is a seething jumble of massive interference everywhere.

> Perhaps one of the Everettistas could explain this to me.
> If the split starts at Alice and travels at c toward Bob,
> if Bob is space-like separated it won't ever reach him.

We have to be clear about the difference between an event
Bob-X and the ongoing entity Bob who retains the characteristics
of a physical object. Say the split starts at Alice-Y,
Alice is then hit by a truck, but the split goes on
(especially if it's evil, as the old saying goes) and
reaches not the Bob-X what was interviewed at a point
along his timeline where it was deemed that Alice-Y
was outside his light cone, but instead reaches Bob
when he's at a different point (say z, so it's Bob-Z). 

> If you imagine that the universe is a sheet of paper,
> it doesn't matter what dimension you split it in,
> whether you simply rip it in half or peel it apart
> along the plane, the split still has to travel at
> some finite speed.

Yes, and it travels at the speed of c. 

   "Although the points of measurement indeed are few"
   (remarked the late Professor Grew)
   "Instead of all four worlds created anew
   We only get but two!" [1]

> Or let's say it's simply the information in the
> universe that is splitting like a file being copied.
> The larger the file, the longer it will take to be
> copied, even if you don't edit the file to make sure
> that one of the particles had its spin reversed.

Yes, could be like that.

> Of course if MWI is *magical* or something, then I suppose the
> splitting makes sense.

Hopefully not any *instantaneous* splitting.
Not enough evidence to go against SR yet.


[1] With respect to Dr. Paine, who was of triangle poetry fame,
and who didn't, I suspect, really have a daughter Rachel by name,
(Damn, the little ditty used to be on-line, but I can't find it now.)
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