Physicist Trevor Weekes chose a specialty that fit his personality.
He wasn’t fond of working with large groups of scientists, so he steered away from a big particle-accelerator project and toward the smaller field of ground-based gamma-ray astronomy.
He chose correctly. For decades, Weekes was among a handful of people trying to establish the field. He’s now regarded as its father. He is also the architect of the VERITAS telescope array on the western slopes of Mount Hopkins, south of Tucson. VERITAS, at the Smithsonian Institution’s Fred Lawrence Whipple Observatory, stands for Very Energetic Radiation Imaging Telescope Array System.
It is a science that explores the most extreme phenomena in the cosmos — low-wavelength, highly energetic particles emanating from cataclysmic events.
“The types of things we look for are pathological — stars that have exploded, the centers of galaxies where massive black holes are accreting matter,” Weekes said. “We are looking at energies and conditions that cannot be duplicated on Earth.”
Weekes compares the science to peering down the beam of a massive particle accelerator, except that nobody could manufacture a physics experiment that could produce that much energy.
“The amount of energy is really quite phenomenal,” he said.
In 1966, Weekes, then a postdoctoral researcher newly arrived from Ireland, was part of a team from the Smithsonian Astrophysical Observatory that experimented with gamma-ray detection in the Santa Ritas, originally using parts scavenged from searchlight mirrors.
The team eventually built a 10-meter collector atop Mount Hopkins.
Two of Weekes’ partners in that effort — Giovanni Fazio and George Rieke — moved into the more promising field of infrared astronomy after a few years of trying without success to document a gamma-ray source.
“For three years we tried to get a signal from the Crab Nebula and didn’t get one,” said Fazio, who had started the gamma-ray program and hired Weekes as a researcher. “I had enough of that.”
Rieke, who left the team to join the pioneering effort in infrared astronomy led by Gerard Kuiper at the University of Arizona, said he too had despaired of making gamma-ray detection work.
Other astronomical fields exploring wavelengths outside visible light — X-ray, radio and infrared — were making discovery upon discovery. “There was this feeling that you go into a new spectral region and lightning is going to strike.”
It just wasn’t happening with gamma rays. “The most interesting question scientifically is why Trevor hung in there,” Rieke joked.
But he did. And now he has seminars held to honor him. At the recent “Trevorfest” in Tucson, Fazio — now a senior physicist at the Harvard Smithsonian Center for Astrophysics — called his former colleague the “creator of a new branch of astronomy.”
DEVELOPING THE DISCIPLINE
The technology to record gamma-ray signals, and to separate them from the background noise of cosmic rays, did not exist when Weekes and his colleagues began their search.
Weekes had to develop the tools, push to have them built, and wait for computing technology to improve.
As the evolving fields of X-ray, radio and infrared astronomy made discovery after discovery, gamma ray waited for its first verified results.
Weekes proposed larger telescopes with more light-gathering photomultiplier tubes spread over a wider area on the ground.
The problem with detecting gamma rays from Earth is that they don’t make it through the atmosphere. They disperse in a flash of Cherenkov radiation — a blue light that lasts a few billionths of a second — too ephemeral to be recorded by our eyes or by anything less than a very sensitive instrument.
Meanwhile, less energetic cosmic rays are bombarding the Earth at more than 1,000 times the frequency of the gamma rays, producing “noise” in the detectors.
“The problem is they cannot get here,” Weekes said. “They collide at the top of the atmosphere and give off a flash of light that happens only at pretty high energy TeV (teraelectron volts) — a million, million times the energy of a photon of visible light.
“Light is spread over an area the size of a football field. What makes it very difficult is that there are 1,000 cosmic rays for every gamma ray. The trick is trying to differentiate the gamma rays from the background. For many years, we were not successful.”
For some of those years, they weren’t even trying — a period from 1978 to 1982 when gamma-ray astronomy was suspended at the Smithsonian, deemed “the Dark Ages” in the Trevorfest presentation. Weekes moved into optical astronomy and resisted attempts to recruit him into the Smithsonian’s X-ray-astronomy program. That would have meant moving to Cambridge, Mass., when he and his family were comfortably settled in Sahuarita.
Weekes kept pushing for gamma-ray research. The program was restored, and in 1989, the hard work paid off. He and colleagues pinpointed a source in the Crab Nebula.
A bigger array was greenlighted, but there were more hurdles to come. The original site chosen for the VERITAS array was opposed by a Native-American group that conducted cultural ceremonies at a sweat lodge on Forest Service land nearby.
Kitt Peak offered a site, but it was opposed by the Tohono O’odham Nation.
Eventually the array of four 12-meter mirrors was built near the Whipple visitors’ center, halfway up the road to Mount Hopkins.
“This was a compromise ... well, a desperate measure,” Weekes said. “We needed to get it up.” NASA was launching a telescope designed to work in tandem with the new array.
Weekes said the decision to build the array near the visitors’ center turned out to be “a wise move.”
The mirrors, each one an assemblage of 350 offset hexagons, create interesting optical illusions during daylight hours when they must be pointed away from the sun. They are a hit with Whipple visitors, Weekes said.
VERITAS began observations in 2007. The technology had finally caught up with Weekes’ vision.
More than 200 sources have been pinpointed since that first discovery in 1989. Arrays similar to VERITAS have been built across the globe, and European physicists are now planning a massive gamma-ray observatory at two sites in the Northern and Southern hemispheres. Each site would have hundreds of collectors and be spread over 15 square kilometers.
Scientists expect to answer fundamental questions of particle physics and learn more about the characteristics of dark matter by observing the “pathological” regions of the universe.
Two sites in northern Arizona have been proposed and are among the finalists for the Northern Hemisphere site.
Pascal Fortin, operations manager at VERITAS, said a prototype telescope for the proposed Cherenkov Telescope Array, or CTA, will be built at Whipple in the coming year — part of a bid by a coalition of U.S. scientists to bring the observatory to Arizona.
Now retired, Weekes visits his office at Whipple Observatory three or four times a week from his home in Sahuarita. He works with students and runs the observatory’s public lecture program.
He’s keeping his hand in recent developments, but he’s happy he does not have to involve himself in the new CTA, which has already signed on 1,500 scientists.
That’s much too large a group for a guy who used to have the field pretty much to himself.