On 3/14/2011 3:04 PM, Jim Lux wrote:

Not precisely true. Interplanetary space slightly dispersive. Emphasis
on *slightly*.

Kenelm Philip predicted a difference back in 1957

Modern estimates for electron density in interplanetary space of 1E6 to
1E10 per cubic meter.

dTau = e^2*Ne*L/(2*pi*m*c) * (1/f1^2 - 1/f2^2)

e= charge on an electron 1E-18 Coulomb
m = mass of an electron at rest (9.11E-31 kg)
c = velocity of light (3E8 m/s)
L = propagation distance
Ne = electron density (pick a number between 1E6 and 1E10)

f1 and f2 are the frequencies (in Hz) (assumed relatively closely spaced)

To bound the magnitudes.. for 1000 light year and 1 and 2 GHz, the
dispersion is about 1 nanosecond.


-- if you're interested in optical as opposed to RF
http://ipnpr.jpl.nasa.gov/progress_report/42-65/65I.PDF

For his idea of how things work I was plenty close enough. He's
thinking in digits of percent.

tom
K0TAR