Pathfinder Goals
What does LISA Pathfinder actually measure?
Perfect free-fall!
How does LISA Pathfinder measure relative distances?
What is the phase difference of the two beams?
Which of these are the 4 interferometers in the Optical Metrology system?[X2,XR,X1,X12,XF,XB]
X1,X12,XR,XF
How is LISA pathfinder a small meteoroid detector?
When junk hits the spacecraft it moves WRT to the test masss
Why can LISA not detect gravitational waves?
The distance between the masses is too small
Why are two test masses needed? (Bonus 100 points: Why can the satellite itself not be used for the purpose of the quiet test mass?)
To measure an acceleration, a reference point is needed.
In theory the satellite could be used but it is too noisy of a reference point for the precision level of accelerations that LISA Pathfinder is trying to measure.
Why is LISA Pathfinder at a Lagrange point?
It is there in order to mimize the effects of gravity and other sources of noise in space. This lagrange point is one of the quietest points in space.
Why is only one of the test masses being measured independently?
Because only 1 mass is in free-fall
Why is perfect free-fall required to detect gravitational waves?
Measured signal must be due to gravitational waves and not the effects of interaction between the two masses or one mass and the satellite/something else
Which of the four interferometers measures the relative distances between the two masses and how does it work?
The X12 interferometer measures the relative distances. The measurement beam is steered with mirrors so that it hits both test masses. The other beam remains on the optical bench and the phase of the measurement beam will provide information if the relative distance between the masses has changed.