The legendary radio telescope at Arecibo Observatory in Puerto Rico collapsed on Dec. 1, sending shock waves throughout the astronomy and astrophysics communities. The telescope, the world's most powerful radar that was used by scientists for almost six decades to send beams to and receive signals from outer space to elucidate the ways of the universe, also is cemented in the history of the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL).
It was at Arecibo in 1974 that Russell Hulse, a University of Massachusetts graduate student, along with his advisor, James Taylor, discovered the first binary pulsar - a pulsar comprised of two stars in very close proximity that rotate around each other. Hulse was a physicist at PPPL from 1977 to 2007 and Taylor became a Princeton physicist. Their discovery, for which the pair received the Nobel Prize in 1993 while at Princeton, confirmed Albert Einstein's general theory of relativity, that predicted a pulsar would emit energy in the form of gravitational waves. Gravitational waves were confirmed by LIGO in 2015.
Something indeed amiss
Hulse described his discovery in the Nobel Prize Lecture he delivered exactly 27 years ago, Dec. 8, 1993: "Something was indeed amiss for this data - and quite seriously so! Instead of the two Doppler-corrected periods being the same to within some small expected experimental error, they differed by 27 microseconds, an enormous amount. My reaction, of course, was not 'Eureka - it's a discovery,' but instead a rather annoyed 'Nuts - what's wrong now?' After a second attempt to carry out the same observation two days later resulted in even worse disagreement, I determined that I was going to get to the bottom of this problem, whatever it was, and finally get a good period for this one recalcitrant pulsar." But after further examination, "within a short period of time I was sure that the period variations that I had been seeing were in fact due to Doppler shifts of the pulsar period produced by its orbital velocity around a companion star."
He continued, "… at Arecibo I did have two critical advantages ... In the first place, the telescope's enormous sensitivity allowed me to first discover and then reobserve the pulsar in relatively short integrations, short enough so the pulsar could still be detected despite its varying period. A more subtle advantage was that at Arecibo I had the rare opportunity to use a 'big science" instrument in a hands-on 'small science' fashion. I had extensive access to telescope time, and I was able to quickly set up, repeat, and change my observations as I saw fit in pursuing the binary pulsar mystery."
In his official autobiography for the Nobel Prize Organization, Hulse, now at the University of Texas - Dallas, described his work at PPPL:
"Starting at the lab in 1977, my first task was developing new computer codes modeling the behavior of impurity ions in the high temperature plasmas of the controlled thermonuclear fusion devices at PPPL. I had never really done computer modeling before and the art and science of computer modeling is one of the most valuable things which I have learned in the 16 years which I have now been at the lab.
Code still used today
"The multi-species impurity transport code which ultimately grew out of this initial work at PPPL is still in use to this day. It models the behavior of the different charge states of an impurity element under the combined influences of atomic and transport processes in the plasma. I oriented my development of this code very much towards its practical use by spectroscopists and other experimentalists in interpreting their data and one of my greatest satisfactions has been that this code has become widely used over the years both at PPPL as well as at other fusion laboratories.
"My own research with this code included determining transport coefficients for impurity ions by modeling spectroscopic observations of their behavior following their injection into the plasma. In connection with modeling impurity behavior, I also worked on investigating the atomic processes themselves, for example, by helping to elucidate the importance of charge exchange reactions between neutral hydrogen and highly charged ions as an important recombination process for impurities in fusion plasmas. In a rather different sort of contribution, I more recently developed a computer data format which has been adopted by the International Atomic Energy Agency as a standard for the compilation and interchange of atomic data for fusion applications."
He continued, "In another recent new direction, I have been working to establish a new effort at PPPL in advanced computer modeling environments. The objective of this research is the development of novel approaches to creating modular computer codes which will make it much easier to develop and apply computer models to an extended range of applications in research, industry and education."
Like a gigantic ear
The Arecibo telescope was built in a natural bowl in central Puerto Rico in the early 1960s. Like a gigantic ear tuned to the heavens, the 305-meter telescope (about 1,000 feet) had scoured the skies to find pulsars, galaxies and other objects in the solar system and examine Earth's ionosphere. The antenna dish is so large - about the size of three football fields - that the height of the Empire State Building fits in its diameter; the Washington Monument would sit snug at the dish's focal point. With such a large collecting area, Arecibo could detect extremely weak radio signals, such as those from millisecond pulsars. And indeed, it did.
The telescope, now operated by the University of Central Florida for the National Science Foundation, listened day and night to the natural clatter throughout the universe, and was used extensively by SETI, the search for extraterrestrial intelligence, by examining signals from afar (although there were several tantalizing candidates, none have been found). Arecibo found its place in popular culture as well, as the telescope had a central role in the movies "Contact," based on the Carl Sagan novel, and the James Bond thriller "Goldeneye."
Among its other achievements, the telescope was used by NASA in 1974 to famously send a radio message, devised by Cornell astronomers Frank Drake and Carl Sagan, about humanity and its home planet to potential extraterrestrials in globular star cluster M13. M13 is a cluster of several hundreds of thousands of stars in the constellation Hercules. Largely symbolic, the purpose was to call attention to the tremendous power of the newly installed radar transmitter at the observatory, and human technological achievement.
You can read Hulse's Nobel Prize Lecture, describing his discovery using the Arecibo telescope, here: https://www.nobelprize.org/uploads/2018/06/hulse-lecture.pdf
