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The Question
(Submitted January 03, 2010)
Why can't we use the signals received from distant spacecrafts to
look for gravity waves?
Wouldn't their transmissions show spatial or temporal based shifts when
the time-space from transmitter to receiver is expanded or contracted
by gravity waves?
The Answer
The reason is that the amplitude of gravitational wave is tiny:
http://en.wikipedia.org/wiki/Gravitational_wave
Using 10-20 as the representative amplitude, and using a baseline of
30 astronomical units (roughly the distance to Neptune, say), we need
to measure the displacement by 4.5x10-8 m. That's the size of a
molecule.
The clocks on spacecrafts we have launched are nowhere near accurate
enough to measure such a tiny displacement, whatever techniques we might
want to apply. The transmitter on these spacecrafts are nowhere near
stable in frequency to allow measurements at such a demanding level.
That's why the proposed LISA mission is so technically challenging, and
the same goes for ground-based detectors such as LIGO.
http://lisa.nasa.gov/
Hope this helps,
Koji & Barb
for "Ask an Astrophysicist"
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