Seeing in the Dark: LISA 6

Bats' ears at AMNH

To really see in the dark, at some point you must abandon the light...

You can go far in faint light by using high efficiency detectors, large collecting areas and amplification;
but, as nature discovered a long time ago, to really find things in the dark you need to switch to an entirely different spectrum and look at vibrations.

This is also true if you want to look at Black Holes.
You can do a lot with light, but to really probe what is going on with black holes, to come close to the event horizon, to test relativity, to measure spins and test astrophysics of formation and growth you must give up the light and embrace gravitational radiation.

There are several gravitational wave detectors currently operating, on the ground, including several "resonant bar" detectors, ultracool metal bars which ping in resonance to incoming gravitational radiation, and some laser interferometers, L-shaped vacuum tubes with high power lasers in interferometer mode, stretching over hundreds of meters or kilometers in odd corners of the globe.

But the ground is noisy: trucks rumbling, air masses flowing, trees falling all disturb the extremely sensitive measurements of the vibrations of space-time; so to really make progress we must go to space (deliberate exaggeration for effect...).

Enter LISA the Laser Interferometer Space Antenna.

LISA is a three spacecraft interferometer, a joint ESA-NASA mission (MoU in Aug 2004), which the Europeans plan to launch in 2015, and the USA plans to launch sometime after 2021, but they are not quite sure when, or even if.

LISA is big:

The spacecraft form an equilateral triangle in an Earth-trailing orbit that is 5 million km on each side.
The orbit is such that viewed from the center of the triangle, which is on Earth's orbit about 100 million km behind the Earth, the three spacecraft do a slow cartwheel bobbing up and down and in and out about Earth's orbit.

The three pairs of lasers (one going each way along each side of the triangle) form a phase closed interferometer; a time delay interferometer - usually you want to hold the lengths of the arms of an interferometer fixed, but this time what is measured is the change in the lenghts of the arms; the precision of the measurement is measured in picometers - the change in spacecraft separation, on time scales of seconds to minutes, is measured to less than the width of an atom. The gross length of the arms fluctuates on longer times scales by thousands of kilometers.

What is measured are the separations of the payload, a small gold/platinum alloy cube, which is free floating in orbit about the Sun, and responding to ripples in space-time from black holes merging billions of light years away.

Right now the 6th biannual LISA meeting is being held at NASA Goddard, and there will be more when my laptop is recharged...

Have to say, a vibration sensor would have been most useful last night when the large retread dumped by some truck on the I-83 did its ping-pong off 6+ cars at 65 mph.
Joys of rural living.