Clarification: Encoding the Past

From: Amara Graps (
Date: Tue Jul 03 2001 - 18:14:06 MDT

Hi Extropes,

A few weeks ago I wrote a post titled "Encoding the Past", and I see
from comments and questions publically and privately by Mitch, that
I may not have been clear enough. In case others are similarly
craving clarification, here, I will try to describe in a little
more detail how dust can encode some parameters from the past.

First his questions:

>Is recovering information from planetary or interstellar, or intergalactic
>dust potentially possible?

It depends what you mean by 'recovering'. If you mean by recovering,
"extract some information regarding the dust particle's source and
the environment through which the particle travels", the answer
is 'yes'.

>by asking if its hypothetically conceivable, that
>personalities and memories may be, indeed, recoverable, potentially, by some
>vast, Matrioska-Brained technology, albeit, astronomically, distant time in
>the future?

No. Dust doesn't have DNA, i.e. it has no blueprint for making
a living entity.

Think of dust like you would a photon. Physics tells you alot about
the source of that photon, as well as the environment through which
the photon travelled. My bias (but that's mostly because I'm a dusty
astronomer) is that dust carries more information than a photon. Dust
is a multidisciplinary topic, those fields include astronomy, plasma
physics, solid state physics, quantum physics, statistical physics,
electrodynamics, classical physics, fractal math, nonlinear physics,
and some chemistry, for starters. BTW, the physics involved is not
particularly new. Most of the physics in my simulations is directly
from Maxwell and Newton, and the other equations which depend on the
particle's optical properties and the currents generated, are about
20 years old.

I said previously:

"Once detected, it's an inverse problem to determine what processes
brought that encoded object (dust) to the detector. The particle
wouldn't have reached the detector unless its initial motion,
material properties, the intervening plasma and magnetic field held
particular values. Slightly changing any of these parameters gives
drastically different dust dynamical behavior (via the charge in the
Lorentz force, which is, _by far_, the dominant force acting on
submicron circum/interplanetary and interstellar dust particles), so
then we know quickly about where that object came from, and what is
(in) the intervening medium."

To clarify the above paragraph.

Assume that a dust particle was detected at the detector. To bring
that particle to the detector, we need to assume Forces, so then
some ordinary physics enters here. The forces are, _at least_:

* Planet(s)' gravitational force
* Light Pressure force
* Lorentz force
* Solar gravitational force

Plus, the dynamics must have:

* The initial conditions for the forces are position and velocity.
* The value of the magnetic field must be known/assumed (for B in the
Lorentz force).

The other free parameters are, _at least_,

1) dust particle's density and radius
2) radiation pressure scattering coefficient (this is essentially
the dust particle's optical property),
3) parameters for calculating currents:
   a) plasma parameters for the ion/electron collection current
   b) photoelectric energy distribution and yield,
   c) secondary electron emission and yield.

The items 2), 3) are coupled with the particle's density and
radius. I'm assuming spherical particles, which is not at all
accurate, but it's the best that we can do for now.

The last item, (3), refers to the specific currents generated on the
particle's surface, and so then, here we need to know the (plasma
conditions+mag field) through which the particle is travelling.

Explanation of the Currents: (Gritty details, so skip if not interested).

3a) The ion/electron collection current is simply the ions/electrons
that collect on the surface of the particle as it's sampling the
plasma. (depends on the charge potential already carried by the
particle and its velocity)
3b) The photoelectron current, which is positive, is when a photon
impacts onto the dust particle and releases electrons,
3c) The secondary electron emission current is when a high-energy
electron or ion from the plasma impacts onto the dust particle and
releases secondary electrons. The smaller the particle, the more
electrons that can be released because the electron can travel
quicker to the other side of the particle. The secondary electron
emission current is a positive current too, because electrons are

The secondary electron emission current is an interesting current,
and one of which many astronomers are not aware. I say
'interesting', because it depends directly on the dust's material
properties (similar to the photoelectron emission current). A SiO2
particle will require a higher energy electron or ion to release "x"
electrons than a SO particle. The values for the electron or ion
energy and number of released electrons are found in lab
experiments, and published in places like the _CRC Handbook of
Chemistry and Physics_. Apparently the lab experiments are diffcult
to repeat exactly and that is one reason why dust charging processes
are a "rough" science.

After we sum up all of the currents generated on the particle, these
currents go into the "Q" parameter of the Lorentz force. For
submicron particles, the Lorentz force is about 100 times larger
than the planetary gravity, therefore, knowing the charge Q of the
dust particle is important to determine a particle's trajectory.

Now you know some of the physics details showing how the dust
particle's environment (plasma+mag field+radiation) and particle
properties (for optical, charging terms), and dust particle's source
(for initial position and velocity) enters into the inverse problem.
The particle wouldn't have reached the detector unless its initial
motion, material properties, the intervening plasma and magnetic
field held particular values.

Frequency analysis can reveal information about the dust particle's
source, as well. For example, Io's frequency of orbital rotation is
~42 hours -- this period appears often in the Galileo dust detector
data from the last 5 years, while _inside_ of Jupiter's magnetosphere.
In addition, last summer, both Cassini and Galileo, while travelling
_outside_ of Jupiter's magnetosphere, detected a dust 'storm': my
frequency analysis showed that Io was the major source during this
storm too.

Another example of how we can learn something of the dust particle's
source and environment is the detection of interstellar particles
coming into our Solar System. The particles impacting on the Galileo
and Ulysses dust detectors give us speed, mass, direction. How do we
know that the particles were coming from outside of the Solar
System? The interstellar dust particles' trajectories showed that
they were on unbound orbits to the Sun, moving with the flow of
interstellar gas. Their dynamics revealed a specific orientation of
the Sun's magnetic field (which flips over every 11 years), and
intervening plasma. Other instruments such as the radar station:
AMOR in New Zealand have even extrapolated thousands of interstellar
dust particle's trajectories towards a source: the star system: beta

So you see, Mitch, no magic physics and no Jupiter brain necessary.
And remember what I said to Robert: the lifetimes of the particles
are on the order of tens or hundreds of thousands of years, so it's
not really possible (yet) to extrapolate back in time before that.
Plus, once we figure out better how to handle fractal-shaped
particles, the problem gets considerably more complicated.

Hope that this clarified things a bit.


Amara Graps email:
Computational Physics vita: finger
Multiplex Answers URL:
"Whenever I see an adult on a bicycle, I do not despair for the
future of the human race." -- H. G. Wells

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