A Triumph in Physics
Guy Consolmagno*

Einstein's gravitational waves confirmed

On Thursday morning, 11 February [2016], scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first observation of gravitational waves. This discovery was a significant confirmation of a prediction first made by Einstein’s Theory of General Relativity, which itself was published just over 100 years ago, in November 1915.

Newton’s physics had invoked the concept of gravity as a force to explain the motions of the planets around the sun, or of an apple falling on Earth. However, Newton only described how gravity behaved; he never attempted to understand what gravity actually was. When challenged to explain this mysterious force, he famously replied, “I feign no hypotheses”.

More than 200 years later, however, Einstein did propose an explanation for gravity in his Theory of General Relativity. Space and time, he suggested, were merely different dimensions of a reality to be called space-time; and gravity, he proposed, was the warping of space-time.

It’s hard for us to picture how ordinary three-dimensional space can be “warped”. But if you imagine that space were just a flat two-dimensional plane, the presence of a massive object would bend that space just like a heavy weight placed on a sheet of rubber would pull the sheet into a distorted shape. An ant walking across the distorted sheet would wind up walking around and around the weight, as the bend in the rubber would turn its path. In the same way, Einstein proposed, planets orbit a star because the star’s mass has bent the space nearby, turning straight-ahead motion into a path around the star.

But if space-time can be warped, is it possible for that warp to act like a ripple traveling away from the source of the distortion? Einstein himself was not sure at first; after proposing just such an effect when he first described General Relativity, he changed his mind several times, before finally concluding mathematically that such waves were inevitable.

Such waves could occur if something occurred to change the position of a massive object; as it moved, it would cause the space-time around it to also move. In order for such waves to be noticed across the enormous distances of space, however, the moving mass would have to be very massive indeed.

In 1974, a pulsar (a massive star which emits pulses of radio waves at precise intervals) was found in orbit around a massive neutron star; over time, its orbit was seen to decay at a rate that matched calculations for a system emitting energy in the form of gravitational waves. However, the waves themselves were not directly detected.

To see such waves themselves, which would be very small, a pair of identical detectors were constructed at opposite ends of the United States, in Louisiana and. Washington. (A similar detector called VIRGO is being developed near Pisa.) They both have lasers shining down 4-kilometer tunnels and reflecting off high-precision mirrors. The lasers and mirrors are so delicately balanced and insulated from other forms of vibration that any fluctuation in local space-time could be detected as changes in the distance down the tube, a distance that the lasers could measure with extreme precision — as little as one thousandth the diameter of a proton. Each detector has two such tunnels set at right angles to detect such waves coming from any direction.

The first version of the LIGO experiment was set up 10 years ago, but it was only with a recent upgrade that scientists felt they had any hope of actually detecting a wave. In fact, the detection reported on Thursday was an event that occurred while the new system was still being tested last September. That same event was seen, identically, in both Washington and Louisiana; and the nature of the fluctuations matched exactly what had been predicted for the collision of two black holes, each 30 times more massive than our sun, which converted roughly half their combined mass into a massive burst of energy.

The detection is a triumph of both theoretical and experimental physics. The theorists were able to calculate just what sorts of signals this detector could find, and what would be needed to detect them; the experimentalists were able to devise just the sort of high precision instrument needed to find them.

More than just confirming Einstein’s theory, the experiment is also already living up to its designation as an “observatory”. By this detection, the LIGO team have not only proved that gravitational waves exist; they have also learned something new about black holes, objects that could never be seen directly because their mass and density prevent light or radio waves from escaping their gravity.

*Director of the Vatican Observatory

L'Osservatore Romano
Weekly Edition in English
26 February 2016, page 6

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