A PASSION FOR SPACE
The Sky at Night presenter hails a new age of astronomy in the wake of the latest gravitational wave detection
Following the first announcement that gravitational waves had been detected in February 2016, I speculated about the timescales needed for this new area of astronomy to take off. It took the two LIGO instruments 14 years to detect their first wave and having done the trick once, I was interested in how long it would take to detect the next one.
My wait was a short one: a faint signature of these ripples in space-time was detected just four months later and three more confirmed detections were announced within 18 months of the first – one by the European counterpart of LIGO, an instrument called Virgo. The latest gravitational wave detection, announced on 16 October, is the most impressive of all: not only was the gravitational wave detected, but within seconds of it an electromagnetic signature was also recorded from an overlapping part of the sky.
Rather excitingly, this electromagnetic signature was a gamma-ray burst, the brightest source of electromagnetic waves in the Universe. Its detection marks a milestone in astronomy since it means that the event was not just observed by the LIGO and Virgo gravitational wave detectors, but also independently witnessed by NASA’s Fermi and ESA’s Integral space telescopes.
The joint detection sheds light on the mystery that is gamma-ray bursts. These highly energetic emissions are transitory, lasting from a few milliseconds to a few hours, and there has been much speculation about their nature. But from this joint detection the source of the eruption appears to be the collision of two neutron stars.
The age-old question
Gravitational waves were a prediction of Einstein’s 1915 general theory of relativity, so how come it took over 100 years for the first observation to be made? The answer lies in the size of the signal that the gravitational wave detectors are trying to pick up. The passing of a gravitational wave causes a very distinct signature in space-time: first it is elongated in one dimension while being compressed in another, then the reverse happens. While this may be unique, the size of the movement is smaller than a proton. Detecting a signal this size above any noise present is challenging, but it is achieved using a device called an interferometer. In this, a beam of light travels down two perpendicular, L-shaped arms in the detector. The length of this path (4km for LIGO, 3km for Virgo) means the instrument can detect minute perturbations: it’s the equivalent of measuring the distance between us and Alpha Centauri (some 41 trillion km away) and being able to detect a change in that distance equivalent to the width of a human being!
It is challenging stuff, but now we have detected gravitational waves multiple times and have independent verification, it is time to hail a whole new way of doing astronomy. I am sure that Einstein would have been very pleased.
Discover more about the historic October 2017 detection of gravitational waves on page 35 and page 106