Discovering
Nature’s clocks

On neutron stars, U.S. radarmen, LGM,
and looking at the Universe in a different way.


In July 1967, U.S. Air Force Staff Sergeant Charles Schisler was sitting at his monitoring workstation inside Clear Air Force Station, in the middle of the Alaskan tundra. The airmen at this station operated three large radars, all pointed west toward the Soviet Union. It was the height of the Cold War, and the purpose of these radars was to detect intercontinental nuclear ballistic missiles soon after they left their silos, so that the U.S. military could mount an immediate response and assure, if not victory, at least an even outcome of mutual destruction.

Clear Air Force Station [U.S. Army Corps of Engineer]

Radar antennae work by emitting “pings” of radio waves that are reflected by targets in their field, and then detected as they return to the antenna. The two-way time of flight of each ping indicates the distance of the target. Because of this mode of operation, radar electronics are designed to ignore steady radio signals and to seize on those that are pulsed. (This detail is important to our story, and we will come back to it). On that summer evening, a weak but unusual signal appeared on Schisler’s screen. Thankfully, it did not have the signature of a missile launch; the target did not appear to rise from the ground to the sky, and the radar could not compute any one distance for it. Schisler made a note of the event, and continued his monitoring shift.

Boeing B-47 Stratojet [U.S. Air Force]

The following evening, he detected the signal again; while it came from the same direction, it appeared four minutes earlier. Schisler had been a navigator on B-47 high-altitude bombers, so he knew that such behavior is caused by the motion of the Earth around the Sun, and it meant that the source of the signal was not on Earth, but out among the stars.

Schisler drove to the University of Alaska at Fairbanks, and by consulting a catalog of known astronomical radio sources he established that his source corresponded to the Crab Nebula, the spectacular remnant of the 1054 supernova explosion recorded by Chinese astronomers. He returned to Clear Station with a list of other radio sources that he thought he may be able to observe; and several he did observe over the rest of his tour of duty.

The Crab nebula [Hubble Space Telescope/NASA]

Schisler tried to interest his superiors in performing more careful follow-up observations of “his” sources, but his request was denied as extraneous to the mission of the Station. (One may wonder whether this incuriosity and singlemindedness was or not a desirable trait in those servicemen, whose scrupulousness ultimately made the difference between history as it unfolded and, well, kaboom.) All activities at Clear Station were classified, so Schisler could not publicize his results. Furthermore, he thought that what he had seen were systems already known to radioastronomers. He did not realize that he was observing them using rather unusual instrumentation. Crucially, the Clear Station radars were only sensitive to time-varying signals.

The Cambridge Radio Scintillation Array [University of Cambridge]

Only a few months later, University of Cambridge graduate student Jocelyn Bell was busy analyzing the recordings of a new large radiotelescope, which she had helped build by stringing 120 miles of wire across 1,000 posts over a large field. This radiotelescope was different than most, in that it could resolve the time variability of signals. At the beginning of radioastronomy, a basic assumption was that all interesting astronomical signals would have a continuous, steady nature, whereas intermittent signals should be discarded as the product of interference from humanmade source. Thus, most radiotelescope receivers were built to record the total radio energy received over relatively long times, but not its fluctuations.

Jocelyn Bell Burnell in 1967 [R. Scagell/SPL]

However, not this radiotelescope. Bell and her advisor Tony Hewish were interested in determining the size of quasars, the most distant objects in the Universe, by observing the flickering of their radio waves as they move across the turbulent solar wind in the Solar System. Each day, their instrument would produce 30 meters of tape, recording the time variations of radio sources that came into view as the telescope scanned the sky. Bell would lay the tape on the floor of her office, and painstakingly analyze it by hand. She identified several quasars, and she dismissed many episodes of interference caused by more mundane sources, such as pirate radio stations and the local police force.

She also noticed a recurrent signal that looked neither like flickering quasars, nor like interference. Furthermore, the signal appeared with the tell-tale four-minute daily advances, which showed that it came from a fixed position in the sky. It was an astronomical source! Bell and Hewish decided to inspect it more closely, by performing “fast” recordings that would show the minute details of its time variation. But for an entire month, the source did not show. Still, she persisted.

High-speed chart observation of the first pulsar [University of Cambridge]

When it finally came back, Bell was astonished at what she saw in the recordings. This signal consisted of a series of equally spaced pulses, appearing exactly every 1.337 seconds. Ping, ping, ping. Stars (let alone galaxies) could not pulse like this! Even if it could be imagined that stars could somehow sustain a regular coherent motion to generate the pulses, repetitions every few seconds were just too fast for an object as large as a star. The source was confirmed by another radiotelescope, and it was established, by observing the slowing effects of interstellar gas on the pulses, that the source was well outside the Solar System, but inside the Galaxy.

The New York Times, Oct 31, 1938

Another student in Bell’s office nicknamed the extraordinary signal “LGM,” for “Little Green Men.” Could these regular pulsations have been created by another civilization? Bell discounted such a hypothesis as a distraction that she must investigate and exclude before she could finish her thesis. Hewish took the idea more seriously, and entertained long discussions with other senior researchers about how one would responsibly announce the news that life had been discovered elsewhere in the Universe. Indeed, as Cocconi and Morrison had reasoned already in 1959, if other scientifically advanced civilizations existed in the Galaxy, they would have likely chosen radio waves as the most efficient channel to establish communication with other intelligent beings. Hewish and his colleagues were worried that the news that “LGM” exist would surely prompt attempts to contact them — which could result in disaster if, say, the aliens were looking for a handsome green planet to invade.

However, Bell, Hewish, and their colleagues were able to dismiss the LGM hypothesis within few weeks. Here’s how. Hewish thought that communications from living beings must come from a planet orbiting a star, but he failed to detect any imprint of the orbit on the signal. Most convincingly, Bell found three more pulsating sources at different positions in the sky, with different pulse spacings. What was the chance that multiple sets of LGM would choose the same frequency, at the same time, to signal the Earth?

Little Ghost nebula, with a white dwarf at the center [Hubble Heritage Team]

If not LGM, then what was the source of these signals, which had by then come to be known as pulsars? Bell and Hewish reasoned that such regular pulses must be generated by the oscillations of an entire star; and that the rapidity of the pulses suggested that they were emitted by a very small system. Perhaps it could be a white dwarf — an old star that has exhausted its fuel, and collapsed to a much denser state, where atomic nuclei are kept apart only by electrons pushing against each other to, as it were, maintain their individual identity. A white dwarf packs the mass of the Sun in a region the size of the Earth, so that a pea-sized amount of white-dwarf matter would weigh 200 kg.

Or perhaps it could be a neutron star — the hypothesized, but so far never observed stellar remnant that results after a massive star exhausts its fuel, explodes, and collapses under its own gravitational pull. When neutron stars form, atoms are crammed so densely that they dissolve into a sea of their constituent neutrons. Neutron stars can pack the mass of the Sun in a sphere twenty kilometers across, so that a pea-sized amount of neutron-star matter could weigh as much as the great Pyramid of Giza, perhaps 6 million tonnes. Even more striking, imagine such a sphere spinning so fast that it goes through hundreds of rotations in a second. That is a neutron star!

Artist’s conception of a neutron star [ESO/L. Calçada]

Nevertheless, the conventional wisdom was that a star so small, no matter how hot, could never be seen. Astronomers Franco Pacini and Thomas Gold, who occupied nearby offices at Cornell University, thought otherwise. (Amazingly, they came to the conclusion independently, and without interaction.) Pacini showed that the rapid rotation and very strong magnetic field of a collapsed neutron star could power a very energetic electric generator, providing the energy for the radiation from a surrounding nebula, such as the Crab. Gold suggested that the signals observed by Bell and Hewish were emitted as a strong beam of radiation from a small region on a rapidly spinning neutron star. The beam would sweep over the Earth once every rotation of the star, producing a pulse, much like the rotating beacon of a lighthouse; the star’s steady rotation could account for the regularity of the signal.

Artist conception of matter swirling around a black hole [NASA/Dana Berry/SkyWorks Digital]

Pacini and Gold’s imaginative explanation was soon confirmed by the discovery of a very rapidly rotating pulsar (30 times per second) in the middle of the Crab nebula. The discovery of the first pulsar opened a new era for astronomy, revealing realms dominated by extreme densities of mass and energy, by the strongest gravitational fields, by unimaginable velocities approaching the speed of light itself, by violent encounters and blinding emissions. This dynamic side of the Universe all but required description in terms of general relativity, Einstein’s theory of gravitation; and the knowledge that the incredibly dense neutron stars existed in Nature made it plausible that even black holes, the outrageous knots of pure gravity predicted by general relativity, could be lurking somewhere in the dark sky.
In short, it was a triumph, and a great new beginning. In the decades since, the analysis of pulsar signals provided the most accurate confirmation that gravity indeed behaves as predicted by Einstein’s theory. Pulsars offered proof of the existence of gravitational waves — the fluctuations of the very fabric of spacetime, again predicted by general relativity. Today, the extreme regularity of the fastest-spinning pulsars makes it possible to use them as clocks (indeed, Nature’s clocks) in building gravitational-wave detectors that literally span a fraction of our Galaxy.

Jocelyn Bell Burnell today [Sigma Pi Sigma]

In 1974, the Nobel Prize for physics was awarded for the discovery of pulsars to Hewish and to his colleague Martin Ryle (who had developed the technique behind the Cambridge radiotelescope), but not to Bell, controversially. While she did not conceive or design the experiment, her curiosity, brightness, and persistence in carrying it out were absolutely crucial to the discovery: it would have been fitting to award her a prize that is meant to encourage those very qualities. Nevertheless, Bell went on to a brilliant career in astronomy. In 2007, Charles Schisler attended a conference celebrating the 40th anniversary of pulsars, and told the tale of his quasi-discovery to an audience of amazed and amused radioastronomers.

If there is a moral to draw from the events of 1967, it is perhaps that when we find the courage and imagination to look at the world in new ways to and to question the assumptions behind the usual way of looking, we will sometimes glimpse what has so far remained unseen. It may take strenuous work, more talent than most of us can muster, and an environment that will encourage such inquiry (like Cambridge, rather than the Air Force), but the rewards can be extreme. So keep looking.

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