Why Does E=mc2_ - Brian Cox [92]
Today Einstein’s theory has been tested to a high accuracy using some of the most remarkable objects in the universe: spinning neutron stars called pulsars. We met neutron stars and pulsars at the end of Chapter 6, and they are abundant in the universe. Of all the objects we can study accurately from the earth using telescopes, spinning neutron stars are special in that they provide us with large distortions of spacetime and a precise time stamp that rivals the stability of the world’s best atomic clocks. If you wanted to dream up an object that would provide the perfect environment in which to test general relativity, you might well come up with a pulsar. Pulsars deliver their time stamp by beaming out radio waves as they spin. You might like to imagine a lighthouse, shining out a narrow beam that scans around once every second or so. These wonderfully useful objects were discovered quite by accident in 1967 by Jocelyn Bell Burnell and Tony Hewish. If you’re wondering how it is possible to stumble across a spinning neutron star by accident, Bell Burnell was looking for fluctuations in the intensity of radio waves emitted by distant objects known as quasars. The fluctuations were known to be caused by the solar winds in interstellar space. Being a good scientist, however, she was always on the lookout for interesting things in her data and, one November night, she detected a regular signal that she and her supervisor, Hewish, naturally thought was of man-made origin. Subsequent observations convinced them that this could not be the case and that the signal must come from a source beyond our planet. “I went home that evening very cross,” Bell Burnell later said of her observations. “Here was I trying to get a PhD out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us.”
Although pulsars are fairly commonplace in the universe, there is only one known instance where two pulsars are circling each other. The existence of this double pulsar was established by radio astronomers in 2004, and subsequent observations have led to the most precise test to date of Einstein’s general theory.
The double pulsar is a remarkable thing. We now know that it consists of two neutron stars separated by a distance of around 1 million kilometers. Imagine the violence of this system. Two stars, each with the mass of the sun compressed into the size of a city, spinning hundreds of times a second and careering around each other at a distance only three times greater than that from the earth to the moon. The advantage of having two pulsars for Einstein-testers is that the radio waves from one of them sometimes pass very close to the other pulsars. This means that the ultraregular radio beam passes through a region of heavily curved spacetime, which delays its transit. Careful observations can measure the delay and in that way confirm the correctness of Einstein’s theory.
Another virtue of the double pulsar system is that as the stars orbit around each other, they induce ripples in spacetime that propagate outward. The ripples take energy away from the rotational motion of the pair and cause them to slowly spiral inward. The ripples have a name. They are called gravitational waves and their existence is also a prediction of Einstein’s theory (they do not exist in Newtonian gravity). In one of the greatest achievements in experimental science, astronomers using the 64-meter Parkes telescope in Australia, the 76-meter Lovell telescope at Jodrell Bank in the UK, and the 100-meter Green Bank telescope in West Virginia have measured the rate at which the pulsars spiral inward to be just 7 millimeters each day, which is in accord with the prediction of general relativity. The achievement is breathtaking. These are spinning neutron stars orbiting around each other at a distance of a million kilometers and located 2,000 light-years from earth. Their behavior was predicted to millimeter precision using a theory developed in 1915 by a man who wanted