[extropy-chat] AAAS conference on retrocausation

[scerir]

Robert Bradbury:
> ... whether retrocausation could be how 
> the universes that don't work out 
> jiggle the probabilities in the ones 
> which are more interesting ... 

... close to another recent speculation by Hawking ...

The leading explanation for the observed acceleration 
of the expansion of the universe is that a substance, 
dark energy, fills the vacuum and produces a
uniform repulsive force between any two points in space -
a sort of anti-gravity. Quantum field theory allows 
for the existence of such a universal tendency. 
Unfortunately, its prediction for the value of
the density of dark energy (a parameter referred to 
as the cosmological constant) is some 120 orders 
of magnitude larger than the observed value.  
In 2003, cosmologist Andrei Linde of Stanford
University and his collaborators showed that 
string theory allows for the existence of dark energy, 
but without specifying the value of the  cosmological 
constant.  String theory, they found, produces
a mathematical graph shaped like a mountainous 
landscape, where altitude represents the value of 
the cosmological constant. After the big bang, 
the value would settle on a low point somewhere
between the peaks and valleys of the landscape. 
But there could be on the order of 10^500 possible 
low points - with different corresponding values 
for the cosmological constant - and no obvious
reason for the universe to pick the one we observe 
in nature. Some experts hailed this multiplicity 
of values as a virtue of the theory.  For example, 
Stanford University's Leonard Susskind in his
book  "The Cosmic Landscape: String Theory and 
the Illusion of Intelligent Design," argues that 
different values of the cosmological constant 
would be realized in different parallel worlds - 
the pocket universes of Linde's "eternal inflation" 
theory.  We would just happen to live in one where 
the value is very small.  But critics see the landscape 
as exemplifying the theory's inability to make useful 
predictions.  The Hawking/Hertog paper is meant 
to address this concern.  It looks at the universe 
as a quantum system in the framework of string
theory. Quantum theory calculates the odds a system 
will evolve a certain way from given initial conditions, 
say, photons going through a double slit and hitting 
a certain spot on the other side.  You repeat your experiment 
often enough and then you check that the odds you predicted 
were the correct ones.   In Richard Feynman's
formulation of quantum theory, the probability that 
a photon ends up at a particular spot is calculated 
by summing up over all possible trajectories for the photon. 
A photon goes through multiple paths at once and can even 
interfere with its other personas in the process.
Hawking and Hertog argue that the universe itself must 
also follow different trajectories at once, evolving through 
many simultaneous, parallel histories, or "branches." 
(These parallel universes are not to be confused with those 
of eternal inflation, where multiple universes coexist in 
a classical rather than in a quantum sense.)
What we see in the present would be a particular, 
more or less probable, outcome  of the "sum" over these histories. 
In particular, the sum should include all possible initial 
conditions, with all possible values of the cosmological constant.
But applying quantum theory to  the entire universe - where the
experimenters are part of the experiment---is tricky. Here you have
no control over the initial conditions, nor can your repeat the
experiment again and again for statistical significance.   Instead,
the Hawking-Hertog approach starts with the present and uses what we
know about our branch of the universe to trace its history
backwards. Again, there will be multiple possible branches 
in our past, but most can be ignored in the Feynman summation 
because they are just too different from the universe we know, 
so the probability of going from one to the other is negligible.  
For example, Hertog says, knowledge that our universe is very 
close to being flat could allow one to concentrate on a very small 
portion of the string theory landscape whose values for 
the cosmological constant are compatible with that flatness. 
That could in turn lead to predictions that are experimentally 
testable. For example, one could calculate whether our universe 
is likely to produce the microwave background spectrum 
we actually observe.  (Physical Review D, upcoming article).

-from PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 781,  June 19, 2006