What’s Up in the Sky – March, 2016
In the Middle Ages (before science) it was generally believed that heavier objects would fall to Earth faster than lighter objects. Some of my students still believed that twenty years ago. Galileo Galilei thought otherwise and just to demonstrate his theory (according to legend) he went to the top of the leaning tower of Pisa and dropped a 10-pound and a 5-pound cannon ball simultaneously. They both hit the ground at the same time.
His little demonstration probably did more to get him in trouble than to convince anyone their common sense was wrong. And, thanks to Edmund Halley’s prediction that a comet would return on a certain date (it did) it would take another hundred years or so before science was generally accepted.
Fast forward to now. One of modern science’s best known prediction makers was Albert Einstein. His list of hits is impressive: the bending of light by gravity, time dilation, anomalies in the orbit of Mercury, black holes. All of them have been verified many times. One prediction, however, was so extremely difficult to test that it took over a hundred years to verify. But it has been.
On February 11, scientists working with the Laser Interferometer Gravitational-wave Observatory (LIGO) announced that gravitational waves had been detected by the instrument on September 14, 2015. The time lag is how long it took them to verify the observation.
This is a monumental confirmation of general relativity, right up there with the 1919 test of the deflection of starlight by the eclipsed Sun, which made Einstein famous (he was right that time, also). Not only has this major prediction been verified, but also a new and unprecedented window into the cosmos has been opened.
Gravitational waves can be described as “ripples in the fabric of spacetime” and arrive at Earth after traveling for billions of years from the distant universe. Their existence was first demonstrated in the 1970s and 80s when scientists observing a pulsar and neutron star orbiting each other noticed that the orbit of the pulsar was slowly shrinking. They also showed that this was due to the release of energy in the form of gravitational waves and that measurements of gravitational waves would now be possible.
And that’s exactly what happened. The LIGO detectors are rather amazing pieces of technology. Each consists of two 4 km long, 4-ft diameter tubes kept at an almost perfect vacuum at right angles to each other. Two beams of laser light travel the lengths of the tubes to measure the distance between two precisely placed mirrors at the ends of the arms. According to Einstein, a gravitational wave passing the detector will cause the distance between the mirrors to change infinitesimally. The instrument is so sensitive that it can measure changes as small as one ten-thousandth the diameter of a proton!
Two such instruments are used, one in Washington and one in Louisiana, to determine the direction from which the waves originated and to rule out other possible sources. Interestingly, each had just undergone a major upgrade that increased its sensitivity and they were on their first observational run. Not bad for a first try.
Scientists are anxious to have more such devices at locations around the globe to give them an even better understanding of what’s up in the sky.