one-way and two-way speed of light measurement.

[robert bristow-johnson]

suppose, at the outset, that we revert the definition of the meter to
pre-1960, so that the speed of light is something to be measured,
rather than defined.

i remember reading about Michaelson's original experiment that
measured the two-way speed of light using a reflector on a mountain 35
km distant. i'm pretty sure i understand the mechanics of that
experiment.

now, instead of the source and detector housed in the same building,
there was an observer located at a third location who is equal distant
from both the light source and the reflector. and this reflector is
turned to reflect the beam of light from the source to the observer.
but the source also sends a beam directly to the observer.

let's say that this beam is pulsed so that it's clear when it's turned
on and off.

so the observer (who is equidistant from both source and from the
reflector) first sees a pulse coming directly from the source and then
sees the reflection of a simultaneous pulse come at a shortly later
moment. if it is well established that the distances from source to
observer and from reflector to observer are equal, then doesn't this
difference in time between arrival of the two pulses represent the one-
way time of travel from source to reflector.

i understand that the path, as drawn, is a closed curve (an isoceles
triangle), but this measurement is a path-length-difference, not a
round-trip length. if all points are fixed, relative to each other
(same frame of reference), all distances known, then isn't this a "one
way" measurement of the speed of light? what makes this a two-way
speed of light measurement?

r b-j

[Jonathan Thornburg [remove -animal to reply]]

robert bristow-johnson <r...@audioimagination.com> wrote:
> suppose, at the outset, that we revert the definition of the meter to
> pre-1960, so that the speed of light is something to be measured,
> rather than defined.
>

[[...]]

> so the observer (who is equidistant from both source and from the
> reflector) first sees a pulse coming directly from the source and then
> sees the reflection of a simultaneous pulse come at a shortly later
> moment. if it is well established that the distances from source to
> observer and from reflector to observer are equal, then doesn't this
> difference in time between arrival of the two pulses represent the one-
> way time of travel from source to reflector.

The source-to-observer and reflector-to-observer *distances* are equal,
but you have to *assume* that the one-way speed of light is the same
for these two paths in order to infer that the transit times are equal.

--
-- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu>
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
on sabbatical in Canada starting August 2012
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

[Rich L.]

On Friday, April 26, 2013 4:44:00 PM UTC-5, Jonathan Thornburg [remove -animal to reply] wrote:
...

> The source-to-observer and reflector-to-observer *distances* are equal,
> but you have to *assume* that the one-way speed of light is the same
> for these two paths in order to infer that the transit times are equal.

...

> -- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu>
> Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA

This is certainly true, but it is interesting to note that the
calculated speed of light is not the same for observers at each end of
this experiment if one end is deeper in a gravitational well than the
other. From the bottom, the round trip transit time will be shorter
than that measured at the top, due to the gravitational red shift.

Rich L.