THE LIGO EQUIPMENT

LIGO operates from two sites in the US: one is near Hanford in eastern Washington, and another near Livingston, Louisiana. The sites are selected so that they are as far from any seismic noise and traffic as possible. The photograph on the front cover of this issue is of the Livingston detector site. The two 4-km arms of the interferometer, set at right angles, are seen.

have grasped their significance. What is sure, though, is that these are now numbers connecting theory, after decades of speculation about gravitational waves, to solid physical observations many thought could never be made.

The paper says that the above finding assume that the massless gravitons are distributed in the vacuum of space as though they were massive particles. And that in itself would seem to be a matter of much significance to the nature of physics. However, in their analysis, the authors - there are several hundred of them - conclude GW 170104 is consistent with General Relativity.

The false alarm rate for the signals detected at aLIGO is one in 70,000 years, and the signal to noise ratio is 13.

LIGO works by detecting the infinitesimally small force exerted by a gravitational wave on a detector comprising two 4-kilometres long vacuum tubes set at right angles to one another (see cover of this issue). There are masses at the ends of the tubes, and at the right-angle junction. Laser light passes back and forth between mirrors in the tubes of equivalent length. Impact of gravitational waves alters the length of the tube, and thus the distance travelled by the laser light. The result is to create an interference pattern, from which alteration in the length of either tube, caused by the gravitational wave, can be calculated. It is that strain measurement which allow the gravitational wavelength to be analysed. Strain is a measure of the ratio of the original length to a length as compressed or extended - in this case by gravitational waves. Optics within each arm mean that the laser light repeatedly reflects to and fro within the vacuum tube, and thus extends the length of the instrument way beyond its actual 4-kilometre length. That optically created additional length makes the instrument much, much more sensitive than it would otherwise be, and so LIGO can detect gravitational waves. The two distantly placed LIGO detectors in Washington State and Louisiana enable cross checks, improved calibration, and greater accuracy.

This paper in Physical Review Letters, and its concepts, and its place at an intersection of general relativity, the standard model, special relativity and quantum mechanics is hard to grasp in the depth and breadth it actually has. I would guestimate I have truly followed only between 40 and 50 percent of its implications and what it is communicating. In that I may be being overly generous to myself. Even specialist will need to read this paper several times. But I am sure I can see that physics and science are now on the brink of something truly extraordinary

5 SCIENCE, PEOPLE & POLITICS [ISSN 1751-598X]