LISAPathfinderAMA

To answer your first question, I was lucky enough to do my undergraduate studies at the University of Glasgow in Scotland. I was studying astronomy and physics, but had not settled on a field. But in Glasgow there is a large group who have worked on Gravitational Wave research for many years, and I was luck enough to do a summer research project in that group. I was utterly captivated by the blend of instrument science, technology and astrophysics and cosmology. It seemed to me like I could study all the things I'm interested in by working on Gravitational Waves, and I was hooked. I did my PhD in the Glasgow group, moved on to working on the GEO600 Gravitational Wave detector in Hannover, and from there to the LISA Pathfinder mission, with the promise of building and 'hearing' the Universe through the ears of LISA!

For the second question, LISA can estimate the position of sources in the sky, to around 1 arc second for the 'brightest' sources. LISA has a response equivalent to 2 Michelson interferometers like those used in the LIGO detectors. Just like the LIGO detectors, we can't, however, 'aim' or 'point' the detectors, but rather we 'hear' all directions in the sky all the time, just like an omnidirectional microphone. However, because the LISA 'microphone' is moving in its orbit around the Sun, we can determine the change in the signal over time via doppler shifts in the signal, just as you can locate a police car as it passes by, even with your eyes closed. Also, in LIGO and in LISA, we can time the 'arrival' of the signal in the two detectors (two LIGO detectors on ground, two equivalent Michelson detectors in LISA), which gives us a triangulation of the source position.

M. H.

Gravitational waves are ripples in the fabric of spacetime. In Einstein's theory of general relativity, gravity is perceived as curvature in spacetime (think of a trampoline canvas as an analogy of spacetime - when you place a heavy object at the centre of the trampoline, the canvas curves into a funnel shape: the heavier the object the more deep the funnel - this is similar to the way gravity affects spacetime). Now assume you have two objects orbiting each other on the trampoline....the funnels move, and the trampoline surface oscillates. This is the same principle of gravitational waves propagating through the Universe.

What are the implications: One source of gravitational waves comes from the most powerful events in the Universe - the merger of black holes. When supermassive black holes merge, they produce enormous amounts of energy in gravitational waves, allowing us to observe them throughout the entire Universe, all the way back to the earliest stars and Galaxies. Using gravitational waves we can start to build a a picture of how the large scale structure of the Universe came to be.

Put simply: gravitational waves gives us a new sense on how to observe the universe. [PMN]

Thank you. The goal of the mission is to understand the physics of a free floating test body (in our case, a gold platinum cube). So the main reason for the mission is to understand new physics.

In terms of GR, we will not place new constraints with the LPF mission. However, future gravitational wave detectors will place very stringent constraints on strong-field GR, using the so-called Extreme Mass Ratio Inspirals (EMRIs). These are objects where a supermassive black hole captures, and eventually merges with a small black hole. The chaotic orbit of the small black hole as it inspirals into the larger body, allows us to map the gravitational field of the supermassive body, and hence test GR in this (as of yet, un-tested) regime.

You should read the paper, but you won't find the answer there ;)

The truth is we are still investigating the source of that line. It is likely an electrical interference in the phase measurement system or on the modulated light we use to make the phase measurement. It's pretty likely to be a high frequency line aliased (or beat down) into our band, and therefore is difficult for us to diagnose with our routine low-frequency telemetry. Specific measurements are planned to investigate this using specialised telemetry. All that said, the line is very narrow in frequency, and does not disturb our main measurement.

Thanks for the good question!

M. H.

Although, in principle, all accelerating mass (even you waving your hands about) produces gravitational waves, it takes the extreme acceleration of really enormous masses to produce signals that are detectable by the kinds of detectors we can build today. The kind of signals we expect to see with a future Gravitational Wave observatory like LISA, are produced by pairs of blackholes orbiting each other, each with masses millions of times that of our Sun.

It's highly unlikely that in any kind of foreseeable future, detectors sensitive enough to see gravitational waves from asteroids can be built. However, with LISA, we would see the influence of passing asteroids on the gravitational field in the neighbourhood of the observatory.

M. H.