LISA in a nutshell
The following pages provide information about the LISA mission - from the scientific background to LISA technology.
Spacetime - from Einstein to cutting-edge physics
At the beginning of the 20th century, Albert Einstein revolutionized our ideas of space, time, and the universe. His theory of general relativity has fascinating consequences: It predicts the existence of black holes - regions of space from which nothing, not even light, can escape -, and of gravitational waves - disturbances of the fabric of space travelling through the cosmos like ripples on a pond.
In Einstein's universe, time and space do not exist as separate entities. Instead, they are woven together into a four-dimensional “spacetime”. Massive bodies distort spacetime, akin to a bowling ball that is placed on a rubber sheet. The motion of any nearby bodies will be influenced by these distortions - similar to the way a marble rolling on the rubber sheet will change direction as it encounters the depression caused by the bowling ball. This interplay of distortion and motion is how gravity manifests itself in Einstein's cosmos.
Ripples of spacetime
In Einstein's theory of general relativity, gravity is an aspect of the curvature of space and time. Moving masses - such as two stars in orbit around each other - can cause changes in that curvature which propagate outwards as gravitational waves, stretching and compressing space as they pass by. The above image is a visualization of gravitational waves generated by the collision of two black holes.
Just as electromagnetic waves (such as light or radio waves) carry information about distant cosmic objects, gravitational waves could give us unique insight into regions of the cosmos that are inaccessible to today's astronomers - from the interior of a supernova explosion to the dance of two merging black holes. In addition, there are situations in which gravitational waves can help us create maps of the space-time geometry around black holes, for instance when a small black hole falls into the central, supermassive black hole of a far-away galaxy.
Although there is a strong indirect evidence for the existence of gravitational waves, they have – due to the weakness of their interactions - not yet been detected directly.
Detecting gravitational waves
In order to detect gravitational waves, scientists search for tell-tale signs of the stretching and squeezing of space which heralds the passing of such waves. To this end, LISA will be using laser light to monitor the distances between its three satellites, which orbit the sun in a triangular formation. The image above shows one of the LISA satellites bathed in laser light from one of its companion crafts. The amount of stretching produced by gravitational waves reaching us from the depths of space is very small. An example for a gravitational wave signal which LISA is meant to detect is a wave produced by two white dwarf stars orbiting each other somewhere in our galaxy which causes distances to change by only one part in a thousand billion billions (1021) as it reaches the earth.
The fact that the changes wrought by a passing gravitational wave are so exceedingly small shows that such waves interact only very weakly with objects that might lie in their path. This makes the waves very hard to detect, but it also constitutes a great advantage for gravitational wave astronomy: Just as the gravitational waves have only a minute effect on our detectors, they have hardly any effect at all on any matter they encounter en route from their source to earth. Thus when scientists measure the exact shape of a gravitational wave, they obtain a perfectly clear "image" of the gravitational wave source. In contrast, electromagnetic radiation such as light or radio waves reaching us from distant objects will usually have interacted with matter such as gas or dust on its way to earth, resulting in less than perfect images.
Continue with the next part of "LISA in a nutshell": The science of LISA