Peter Saulson
The 27 graphics from
Saulson's talk are here in 7 web pages. In the narrative below you can
click on any subject to go to its
page. Click here for a
full
PDF version.
**pg. 1**
It is essential but difficult to provide explanations that are both
correct and intelligible to
those who support GR research, e.g.the general public and their
representatives. Saulson offered two such explanations of how LIGO's interferometer is to detect
gravity wave -- a heuristic, mostly correct, explanation and a
more elaborate one that is always correct.
His heuristic example starts
with a picture of a set of freely falling masses.
**pg. 2** He
shows what happens when a gravity wave
passes through the set of masses. The
detection strategy
uses
interferometry to sense the relative motion of free masses that are
kilometers apart. These small differences of displacement are converted
to
relative brightness.
**pg. 3 **The more
correct calculation
measures distance by
round-trip travel time of light. This requires generalizing
the spacetime
interval of special relativity with the appropriate
GR metric.
**pg. 4 **Then gravity
waves appear as small
additional
terms in the flat space GR metric. The waves have two
independent polarizations -- plus and
cross. Now solve for
variation of light travel time
in each interferometer arm.
**pg. 5** The
result is the
difference between the travel times in the x and y arms. This
difference can be expressed as a
phase difference that
the transponder turns into a brightness at the
output.
What does the calculation mean?
1) Do gravity waves move the test masses?
2) Do gravity waves stretch light waves?
3) If so, how can you use light waves to detect gravity waves?
**pg. 6 **Answers:
1) It depends on your
coordinates.
2) Again, yes and
no.
3) Light is a rubber ruler
participating in the distortions being measured, but it is not a
static ruler and over the time it travels through an interferometer arm
there is a response that builds to the size found with the heuristic
calculation. It will work.
**pg. 7**
You can do the math and not understand the results.
Professionals as well as non-professionals need
heuristics. We need them for
clear explanations.
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