From: Eliezer S. Yudkowsky (sentience@pobox.com)
Date: Mon Oct 15 2001 - 12:24:42 MDT
Mike Lorrey wrote:
>
> Sorry, I missed a comma. Entanglement creates violation of light speed
> limits. Just because I signalled someone instantaneously 1 light year
> away with an entangled pair doesn't mean that they can communicate with
> me in my past with the same signal. If they turned around immediately
> after I signalled them and sent it back instantaneously, their signal
> would still arrive after I had sent my first signal. No communicating
> with the past.
Ah, sorry, I missed the original context here. The problem is that
"simultaneously" means different things in different frames of reference.
If quantum entanglement or other instantaneous communication sends a
message to a distant spacelike-separated event according to the space of
simultaneity of the originating event, then you can use it to send signals
into the past.
> Andrew Clough wrote:
>
> > Next, have this second party change
> > their velocity be a Good Deal. This will change what is considered
> > "simultaneous" with them. (Actually, I believe non-local simultaneity is no
> > longer a valid concept because of this, much like absolute frames of
> > reference.) They can then be simultaneous with you past self, and can
> > relay the message to him or her.
>
> These ideas about communicating with the past via exceeding light speed
> are a misperception caused by too much Star Trek watching.
Andrew Clough is stating a recognized fact of modern physics. Signalling
across spacelike separation leads to global causality violation unless you
assume a single absolute space of simultaneity, which is even worse than
allowing global causality violation.
> There is an absolute frame of reference in at least one dimension. For
> example, we all know the Big Bang occured at some point in the past.
> Temporally speaking, therefore, we can all deal relativistically with
> each other in reference to the Big Bang event, so in the dimension of
> time, there IS an absolute frame of reference.
All observers agree that the Big Bang occurred a certain Minkowskian
interval ago on your worldline. They disagree about how much "time" it
took. Everyone agrees that the Big Bang is in the past light cone because
that's a Minkowskian fact. You can't use that fact to orient everyone to
a single space of simultaneity.
> We may experience
> more time than others due to our relative veolcities (even then, the
> rate of temporal passage occurs in reference to actual zero velocity.
Um... no, not really, although I can see how you're tempted to say so.
> This is an interesting question to ask: Just what frame of reference
> results in the fastest possible passage of time?
That depends on your frame of reference. The "fastest possible passage of
time" is always in the frame of reference identical to your own, at least
as you measure time. If you're on a ship travelling at .99c and we orient
the frame of reference so that you're motionless, then someone who leaves
the ship in a pod, decelerates to be at rest with reference to the fixed
stars, stays that way for a week in his reference frame, and then
accelerates to catch back up with the ship he left, will experience less
total subjective time than someone on board ship who didn't experience any
accelerations.
-- -- -- -- --
Eliezer S. Yudkowsky http://singinst.org/
Research Fellow, Singularity Institute for Artificial Intelligence
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