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From: richard ruquist <yann...@yahoo.com>
Date: Wed, Oct 10, 2012 at 9:24 AM
Subject: Fw: the physics arXiv blog
To: "yann...@gmail.com" <yann...@gmail.com>

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*From:* Technology Review Feed - arXiv blog <ho...@arxivblog.com>
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*Sent:* Wednesday, October 10, 2012 8:13 AM
*Subject:* the physics arXiv blog

   the physics arXiv blog <http://www.technologyreview.com/>
  The Measurement That Would Reveal The Universe As A Computer
 Posted: 10 Oct 2012 04:08 AM PDT
If the cosmos is a numerical simulation, there ought to be clues in the
spectrum of high energy cosmic rays, say theorists
 One of modern physics' most cherished ideas is quantum chromodynamics, the
theory that describes the strong nuclear force, how it binds quarks and
gluons into protons and neutrons, how these form nuclei that themselves
interact. This is the universe at its most fundamental.
So an interesting pursuit is to simulate quantum chromodynamics on a
computer to see what kind of complexity arises. The promise is that
simulating physics on such a fundamental level is more or less equivalent
to simulating the universe itself.
There are one or two challenges of course. The physics is mind-bogglingly
complex and operates on a vanishingly small scale. So even using the
world's most powerful supercomputers, physicists have only managed to
simulate tiny corners of the cosmos just a few femtometers across. (A
femtometer is 10^-15 metres.)
That may not sound like much but the significant point is that the
simulation is essentially indistinguishable from the real thing (at least
as far as we understand it).
It's not hard to imagine that Moore's Law-type progress will allow
physicists to simulate significantly larger regions of space. A region just
a few micrometres across could encapsulate the entire workings of a human
Again, the behaviour of this human cell would be indistinguishable from the
real thing.
It's this kind of thinking that forces physicists to consider the
possibility that our entire cosmos could be running on a vastly powerful
computer. If so, is there any way we could ever know?
Today, we get an answer of sorts from Silas Beane, at the University of
Bonn in Germany, and a few pals.  They say there is a way to see evidence
that we are being simulated, at least in certain scenarios.
First, some background. The problem with all simulations is that the laws
of physics, which appear continuous, have to be superimposed onto a
discrete three dimensional lattice which advances in steps of time.
The question that Beane and co ask is whether the lattice spacing imposes
any kind of limitation on the physical processes we see in the universe.
They examine, in particular, high energy processes, which probe smaller
regions of space as they get more energetic
What they find is interesting. They say that the lattice spacing imposes a
fundamental limit on the energy that particles can have. That's because
nothing can exist that is smaller than the lattice itself.
So if our cosmos is merely a simulation, there ought to be a cut off in the
spectrum of high energy particles.
It turns out there is exactly this kind of cut off in the energy of cosmic
ray particles,  a limit known as the Greisen–Zatsepin–Kuzmin or GZK cut
This cut-off has been well studied and comes about because high energy
particles interact with the cosmic microwave background and so lose energy
as they travel  long distances.
But Beane and co calculate that the lattice spacing imposes some additional
features on the spectrum. "The most striking feature...is that the angular
distribution of the highest energy components would exhibit cubic symmetry
in the rest frame of the lattice, deviating significantly from isotropy,"
they say.
In other words, the cosmic rays would travel preferentially along the axes
of the lattice, so we wouldn't see them equally in all directions.
That's a measurement we could do now with current technology. Finding the
effect would be equivalent to being able to to 'see' the orientation of
lattice on which our universe is simulated.
That's cool, mind-blowing even. But the calculations by Beane and co are
not without some important caveats. One problem is that the computer
lattice may be constructed in an entirely different way to the one
envisaged by these guys.
Another is that this effect is only measurable if the lattice cut off is
the same as the GZK cut off. This occurs when the lattice spacing is about
10^-12 femtometers. If the spacing is significantly smaller than that,
we'll see nothing.
Nevertheless, it's surely worth looking for, if only to rule out the
possibility that we're part of a simulation of this particular kind but
secretly in the hope that we'll find good evidence of our robotic overlords
once and for all.
Ref: arxiv.org/abs/1210.1847: Constraints on the Universe as a Numerical

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