

When Time Flows Backwards - DiabloD3
http://motherboard.vice.com/read/when-the-future-helps-determine-the-past

======
tomp
> And watching the experiment progress forward through time, it remains
> impossible to say beyond 50/50 odds what the actual final state of the
> particle will be in. But, if you follow the experiment backwards, following
> the particle's timeline from its future to its past, Murch and co. found
> that they could up their odds to 90 percent correctness. The implication is
> that everything that happened to the particle/state after the strong
> measurement, influences the strong measurement itself ... in the past.

I'm not a quantum physicist, but I don't get this. Maybe it's just explained
in a silly way, but this seems to imply correlation, not "causation" (i.e. the
future influencing the present). Correlation isn't that surprising, obviously
- if I flip a coin, a friend looks at it, then I look at it, I can predict
what my friend saw (in the past) with 100% accuracy.

Obviously I'm missing something.

~~~
lisper
It's explained in a silly way because explaining it in a non-silly way is not
so easy. Here's my best shot:

Measurement in QM is not an all-or-nothing process (and hence there is no such
thing as "wave function collapse"). You can measure "just a little bit" by
carefully controlling the entanglements of the system you are "measuring".
These are called "weak measurements". When you make weak measurements, the
probabilities of various outcomes can be non-intuitive because of how the
quantum math shakes out, similar to the Bell inequality, except that the
entanglements in this case are across time instead of across space. It's cool
experimental work, but there's nothing fundamentally new here. It was already
well known that the quantum wave function is timeless. This experiment just
confirms it.

~~~
tjradcliffe
It is worth emphasizing that weak measurements only (and can only) apply to
ensemble averages. They work by making weak measurements on many copies of the
same system (or equivalently, many identically-prepared systems) and then
drawing inferences about "the system" from this. But it is not, in fact "the
system" but the average over many copies of a system.

Bohr would likely have made a big deal over this, because there is no actual
measurement on any single system that corresponds to the retrodicted
inference, and that matters a great deal in Bohr's quantum ontology.

------
matheweis
Much better article (and from the source):

[http://news.wustl.edu/news/Pages/future-affects-past-
quantum...](http://news.wustl.edu/news/Pages/future-affects-past-quantum-
state.aspx)

~~~
lisper
And the actual paper:

[http://arxiv.org/abs/1409.0510](http://arxiv.org/abs/1409.0510)

~~~
pontifier
Future me probably understands this...

------
darkmighty
How does this compare to the classical electrodynamics interpretation of
[http://en.wikipedia.org/wiki/Wheeler%E2%80%93Feynman_absorbe...](http://en.wikipedia.org/wiki/Wheeler%E2%80%93Feynman_absorber_theory)
?

I mean, the former is not hard to believe for me, as a model for classical
eletrodynamics (due to the sucessful consistency of the theory). Why should
the quantum equivalent be hard to believe?

~~~
fizx
The interpretation this is based on is usually called
[http://en.wikipedia.org/wiki/Two-
state_vector_formalism](http://en.wikipedia.org/wiki/Two-
state_vector_formalism).

------
vorg
The 90-10 odds of "retrodiction" as opposed to the 50-50 odds of prediction
suggest it could in reality be 100-0 given better tools, and the initial 90-10
odds are the result of macroscopic "interference". Could this then mean
quantum decoherence isn't so much the result of measurement as it is the
result of time running backwards at the quantum scale? This would mean time
runs forwards at the gravitational scale but backwards at the quantum, and the
collapse of quantum states is where the two time directions smash into each
other, so to speak.

Perhaps the existence of consciousness is also the result of time running
backwards, somehow reaching up from the quantum to the macroscopic level. It
would be just as good as the weak anthropic principle to explain why we humans
seem to be the only intelligent life in the Universe. If the rise of
consciousness was somehow planned and executed from the future, it would
perhaps even _require_ only one instance of consciousness evolving somewhere
given how difficult it might be for those in the future to execute it.

One hint this is actually what's happening is the fact the distribution of the
planets in the solar system appear to be fine-tuned for inter-stellar space
travel. There's only about 10 other solar objects with gravity comparable to
Earth's and hence suitable for long-term civilization. The Moon has already
been walked on, and humans will one day easily build large contained cities
there using telepresent robots with a 1 second response time. Then they will
be experienced enough to tackle Mars and Mercury with semi-intelligent robots
to cater for the much longer response time. Mercury will be a good candidate
to build a huge particle accelerator around its equator because its very slow
rotation allows human workers aid in construction, and its highly elliptical
orbit around the Sun would give many gravitational variances for experiments.
Jupiter's 4 large moons would provide humankind with experience of much longer
travel times, a single large moon of Saturn eliminates any initial choice of
target, then the huge gap skipping Uranus's lack of sizable moons to Neptune's
Tritan gives an even longer travel distance. Concurrent with all this, humans
will spend probably thousands of years to terraform Venus, the ultimate in
hostile environments. By then humankind will have trained up, apparently
serendipitously, for inter-stellar travel using knowledge from terraforming
Venus, travelling huge distances to Tritan, and whatever's learnt from the
Mercury particle accelerator.

Perhaps humans will build a silicon-based consciousness in each star system
they colonize. Each planet-based consciousness in the galaxy could then
communicate with one another to build a one-off galactic consciousness by
structuring the gravitons flowing between the star systems, though its speed
of thought would be far slower than that of humans. Ditto for each galaxy
humans eventually reach in the observable Universe, and then the whole
Universe. This Universal Consciousness based on gravity then "begins" to reach
back in time using quantum retrodiction to ensure fine-tunings so one planet
in one galaxy 14 billion yrs after the Big Bang evolves monkeys then humans
who then mutate as they colonize the galaxy and build silicon-based AI and
eventually a single Universe-wide gravity-based consciousness. If the Universe
continues expanding and never comes to an end, then such a Universal gravity-
based consciousness need never finish being built, it only needs to be
continually being built. If a graviton-based consciousness can indeed reach
back in time through retrodiction, not only would the Universe increase in
entropy over time, but also increase in intelligence. Perhaps there's a law of
physics saying that as entropy increases, so does intelligence.

~~~
sritrisna
Think about it this way: Everything you’re doing at this present moment has
already happened. You’ve already reached your destination. All you’re doing
right now is letting yourself (go through the) experience (of that moment).

In the next 30 years, this will be much better understood, with our
(scientific) tools and "consciousness" evolving to a point that we can
measure, proof and present this as fact.

Go back to the middle ages and try not to fall for the common belief that the
“The Black Death” is a punishment by God for your sins, simply because there
were no tools to locate and fight “Yersinia pestis”.

Go back to when it was a common belief that the earth was flat, until someone
ventured out across the (then) vast oceans and prove otherwise.

And don’t forget, it already happened. Your just in for the ride.

------
stuartd
Thiotimoline

