On Tue, Jan 7, 2014 at 7:57 PM, LizR <lizj...@gmail.com> wrote:

> On 8 January 2014 12:53, Jesse Mazer <laserma...@gmail.com> wrote:
>> On Tue, Jan 7, 2014 at 4:35 PM, LizR <lizj...@gmail.com> wrote:
>>> On 8 January 2014 08:59, Jesse Mazer <laserma...@gmail.com> wrote:
>>>> Well, most physicists already agrees physics is time-symmetric (well,
>>>> CPT-symmetric, but the implications are the same for Bell's inequality and
>>>> thermodynamics),
>>> Yes, they do, but it doesn't appear to be taken into account when
>>> discussing Bell's inequality.
>>>> but I don't see how this alone can explain violations of the Bell
>>>> inequality.
>>> No, you need to work out the consequences mathematically, and I dare say
>>> that is quite difficult. This is simply a *logical* demonstration that
>>> Bell's inequality can be violated while retaining locality and realism,
>>> which is otherwise impossible.
>> As I said in another comment, if you allow information about the state of
>> complex systems like detectors to flow back in times as well as forwards,
>> it's not clear that this really counts as preserving locality.
> Nothing is flowing either way in time. (Assuming a block universe, nothing
> *can* flow in time - the notion doesn't make sense).

I think you're reading something into my talk about information "flowing
back in time" that I didn't intend. I certainly didn't mean to deny the
block universe view of time or suggest that an imaginary observer viewing
all of spacetime "from the outside" (like we might observe 2D flatland)
would see spacetime itself "changing" (as opposed to just a set of frozen
worldines as one would expect in a block universe). But the notion of
information flowing from one point in spacetime to another doesn't imply
any such denial of the block universe, for example David Deutsch, who
argues forcefully for the block universe view in his book "The Fabric of
Reality", wrote a paper at http://arxiv.org/abs/quant-ph/9906007 titled
"Information Flow in Entangled Quantum Systems". The notion of information
flow just requires some type of logical ordering, with one point in
spacetime being the event of the information first being "transmitted" and
another point being the event of it being "received".

There might be no way to define transmission vs. reception of a message at
a fundamental quantum level, but if we're dealing with macroscopic
transmitters and receivers of some kind that have a local thermodynamic
arrow of time, there shouldn't be a problem distinguishing them...for
instance, the transmitter has to *first* compose the message (perhaps after
observing some local event, like the outcome of an election, that the
message is supposed to convey information about) and *then* transmit it,
relative to the local thermodynamic arrow of time, while the receiver
*first* has a sequence of bits come in which are *then* made sense of. So
as long as we can locally define transmission vs. reception in this way, my
talk of information flowing backward in time just means that the
reception-event is on the past light cone of the transmission-event, and
information flowing forward would just be the ordinary case of the
reception-event being on the future light-cone of the transmission event.
The point is that if both sorts of information-transmission are possible,
then I should be able to transmit a message back to be received by a relay,
then the relay can transmit a copy of the message forward to a friend
light-years away from Earth, such that the friend receives the message on
the same day I sent it (relative to our mutual rest frame), despite being
light-years away. It's not obvious that a universe where such things were
possible would really count as one that respects "locality", even if each
individual message travelled at the speed of light.

Locality is preserved so long as no physical objects travel faster than
> light.

I don't think physicists use such a narrow definition--if the equations of
QM were modified so that the EPR experiment could be used to transmit
*information* FTL, then even if no measurable particle or wave was observed
to move FTL this would still probably be seen as a violation of locality.
And the pilot wave in Bohmian mechanics is arguably just a sort of rule for
coordinating the behavior of distant particles rather than a "physical
object", but its ability to coordinate them instantaneously is typically
seen as a violation of locality. Unfortunately I have not been able to find
any very precise definition of locality that would give a totally clear
answer about cases like this, it tends to be stated in terms of imprecise
terms like "effects" and "influences".


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