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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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