If our eyes could see gravitational waves
Copyright: NASA/C. Henze
Copyright: NASA/C. Henze
Picture the scene: two gigantic black holes, each one a good fraction
of the size of our Solar System spiralling around each other. Closer
and closer they draw until they touch and merge into a single, even more
gigantic gravitational prison.
But what would you actually see?
For such a cataclysmic event, it might all take place with remarkable
stealth because black holes by their very nature emit no light at all.
Rather than light, it would be a different story if our eyes could see
gravitational waves.
This is what the merger of two black holes
would look like. It is a computer simulation of the gravitational waves
that would ripple away from the titanic collision, a bit like the
ripples on a pond when a pebble drops into the water.
In the case
of gravitational waves, the disturbances are not in water but in the
spacetime continuum. This is the mathematical ‘fabric' of space and time
that Albert Einstein used to explain gravity.
Gravitational
radiation has been indirectly observed but never seen directly. Its
detection would open a whole new way of studying the Universe. As a
result, astronomers are working on both ground-based and space-based
detectors. And it is a real challenge.
Gravitational radiation is
incredibly difficult to measure. The ripples cause atoms to ‘bob’ about
to just 1 part in 1000 000 000 000 000 000 000. Building a detector to
notice this is like measuring the distance from Earth to the Sun to the
accuracy of the size of a hydrogen atom.
Following decades of
technology development and experiments, detectors on the ground are
nearing the required sensitivity. The first detections are expected in
the next few years. But these detectors can see only half of the
picture. The mass of the colliding black holes determines the frequency
of the gravitational radiation.
The merger of small black holes,
each about a few times the mass of the Sun, will create high-frequency
gravitational waves that could be seen from the ground. But the giant
black holes that sit at the heart of galaxies with masses of a million
times that of the Sun will generate gravitational waves of much lower
frequency. These cannot be detected with ground-based systems because
seismic interference and other noise will overwhelm the signals. Hence,
spaceborne observatories are needed.
ESA has selected the
gravitational Universe as the focus for the third large mission in the
Cosmic Vision plan, with a launch date of around 2034.
Unlocking the gravitational Universe will require a highly ambitious mission. In preparation, ESA will launch LISA-Pathfinder
this November to test some of the essential technologies needed to
build confidence in future spaceborne gravitational wave observatories.
This image is from a simulation of two black holes merging and the resulting emission of gravitational radiation, published by NASA in 2012.
Source: ESA
