On 13 January 2014 05:20, John Clark <johnkcl...@gmail.com> wrote:

> On Fri, Jan 10, 2014 at 4:47 PM, LizR <lizj...@gmail.com> wrote:
> > "Retro-causality" (time symmetry is a better term) only exists at the
>> quantum level.
> Why? Where is the dividing line? And with a Schrodinger's Cat type device
> a quantum event can easily be magnified to a macro-event as large as
> desired, you could connect it up to an H-bomb.

The dividing line appears to be roughly where decoherence occurs. Basically
anything above a single quantum entity engaged in a carefully controlled
interaction is liable to get its time symmetric properties washed out by
interactions with other particles. I'm not sure exactly where the dividing
line is, but once you get above the scale of coarse-graining at which the
entropy gradient becomes manifest, you are going to lose any easily
measurable consequences of time symmetry. Only in carefully controlled
situations (like EPR experiments) can we remove the effects of influences
from the rest of the universe to a great enough extent that we can see
time-symmetry operating in a detectable manner (to, for example, violate
Bell's inequality, at least if Bell is to be believed).

> > The laws of physics are time-symmetric, but constrained by boundary
>> conditions.
> And that is exactly what I've been saying over and over, and that is why
> the second law is almost always true and that is why time has a direction.

Yes, I've been saying this over and over, too. So we agree. The second law
is almost always true, and only in special cases like EPR experiments can
we easily see the effects of time symmetry -- even though we *know* most of
the laws of physics are time-symmetric (insofar as we "know" anything, of

> > There is a very influential boundary condition in what we call the past,
>> namely the Big Bang, plus less influential ones in the future,
> Exactly.

This is why it's so hard to get our heads around the consequences of time

>  >> And by the way, if time is symmetrical then there is no point in ever
>>> actually performing an experiment because you would remember the future as
>>> clearly as you remember the past, so you would already remember the outcome
>>> of the experiment just as clearly as you remember setting up the
>>> experimental apparatus.
>> >I assume you're not so stupid as to think that's what I've been
>> claiming, so I can only assume this is a deliberate attempt at mockery,
> Yes sometimes I mock people but I promise you that was not my aim this
> time. It's just a fact, if time were symmetrical then you'd be just as good
> at predicting the future as you are at remembering the past, so you'd know
> the outcome of an experiment before you performed it just as well as you
> remember setting up the apparatus. But this is not the way things are
> because the second law exists. And the second law exists because of low
> entropy initial conditions. And I don't know why there were low entropy
> initial conditions.

OK. So the above statement of yours about predicting the future is still
false, and hopefully you now understand why. To recap briefly -- the laws
of physics are time symmetrical, and most particle interactions are
constrained by boundary conditions. Almost everything in the universe is
constrained by the boundary condition of the Big Bang (+ cosmic expansion).
This creates an entropy gradient (or rather what we perceive as one, as
Brent explained the entropy of a system doesn't change at the quantum
level, but we exist above the level of coarse graining at which the 2nd law
emerges). This prevents us measuring the results of any future experiments
that involve anything above the level of coarse-graining, i.e. above the
level of a few carefully prepared particles.

And since we don't use EPR type experiments for our memories, we can't
remember the future.

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