On 3/18/2014 5:07 PM, LizR wrote:
On 19 March 2014 12:47, meekerdb <meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote:
But in general that would mean knowing the state of everything the system
had
interacted with in the past, since it is now entangled with them. So even
if you
suppose there is no collapse of the wavefunction, decoherence has the same
effect.
I was only asking about the theoretical possibility, given unrealistically perfect
information about the state of the system.
The universe (assuming unitary QM) is reversible. In fact from the standpoint of QM there
is no arrow of time - it's deterministic, just like Laplace's universe. So, as always,
when the word "possibility" is used there has to be some context. To *calculate* a
history of the universe from it's present state would require knowing its *complete*
present state, including your mental state. Is that "theoretically possible"? I think it
involves a paradox of self-reference.
To put it another way, in the Game of Life, even with perfect information, you can't
trace the state of the system backwards because it loses information. So even the laws
of physics couldn't work backwards in a universe based on the GOL. QM, I'm informed,
doesn't lose information, so (very much in theory) you could work backwards - or (less
in theory) the laws of physics could.
Yes the universe doesn't lose information like the GoL. But relative to any point it
loses information across spacetime horizons. So there's no way to gather that information
up into a calculation unless you have some God's eye view from outside the universe, in
which case you could see the past anyway.
There's a couple of nice papers about this by Yasunori Nomura: arXiv:1205.267v2 is a
popular exposition and arXiv:1205.5550v2 is a more technical paper.
Brent
I wasn't asking whether I could build a chronoscope and watch the past
happening on TV.
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