Two Trinities
Yesterday I took a pilgrimage to two holy sites:
Trinity
This was where the first atomic bomb was detonated, on July 16, 1945. The site is located on the White Sands Missile Range, and is open to the public twice a year. Needless to say, it’s in the middle of nowhere. You drive for miles across desert scrubland, to arrive at a fenced in area the size of a soccer field.
One morning over 60 years ago, the desert floor glowed brighter and hotter than the surface of the Sun. The bomb was detonated at the top of a 100ft steel tower. A small piece of twisted steel, one of the footings of the tower, is all that remains. As you walk the site, you notice little pieces of mottled greenish glass (think tiny shards of a beer bottle). This is trinitite: sand from the desert floor melted into glass by the explosion. After the explosion the entire crater floor was covered with trinitite, forming a green glassy bowl. Since then the trinitite has been bulldozed, though scattered pieces remain. We brought a Geiger counter, which provided the main indication that this patch of Earth is unlike your average backyard. At the epicenter the radiation level is roughly an order of magnitude higher than background levels. It is unnerving to be exploring a nondescript patch of desert while your Geiger counter clicks up a storm.
One becomes contemplative at the site. Holding a piece of trinite, you realize that it was forged at the instant of the birth of the atomic age. That this tiny piece of glass is a physical remnant of humanity’s loss of innocence.
Very Large Array
A couple of hours away from Trinity sits the Very Large Array (VLA), part of the National Radio Astronomy Observatory. The VLA is perhaps the single most publicly recognizable scientific installation. It is extraordinarily photogenic; the film Contact moved the observatory into the “A” list of movie stars. It is hard not to be impressed by its sheer scale: 27 radio dishes, each of them 25 meters (82 feet) in diameter and weighing 230 tons. The dishes move along 21 kilometer (13 mile) long train tracks, allowing for various configurations trading off resolution and field-of-view. These tracks are arranged into three arms radiating from a central point, forming a scientific trinity. This trinity has led to great enlightenment.
The receivers are in the 70 Mhz–50 Ghz frequency range, corresponding to wavelengths of 400–0.7 cm. Because these radio wavelengths are long, a much larger dish is needed to produce a resolution on the sky equivalent to optical telescopes. The angular resolution of a telescope can be approximated by: θ = λ/d, where θ is the angular resolution (in radians), λ is the wavelength of the observed radiation, and d is the diameter of the telescope. For reference, the full moon is ~0.5 degrees = 30 arcminutes = 1800 arcsec across, and 1 arcsec ~ 5e-6 radians. The center of the visible (optical) band of light, corresponding to the color green, has λ ~ 500 nm = 5e-5 cm. To image something green on the sky to 1 arcsecond (which optical telescopes routinely do) thus requires a telescope of size at least 10 cm. To make an equivalent image in radio frequencies (which have wavelengths roughly 100,000 times longer) requires a dish 100,000 times bigger: instead of 10cm, we need a dish 10 km across. There are two ways to address this: (1) Make a humongous dish. The Arecibo dish in Puerto Rico is 300 meters across. (2) Make use of interferometry. The VLA combines the data streams from 27 dishes to produce a single image, corresponding to a much larger single observatory. Each individual pair of dishes can be thought of as sampling an interference pattern of a point source, or measuring a Fourier component of the full brightness distribution of an extended source. With sufficient numbers of pairs, a detailed image can be reconstructed. The VLA has 27 dishes, and thus 26+25+24+..+1 = 351 separate pairs. In the A configuration, the dishes are placed at their furthest positions, leading to a maximum pair separation of 36km. This corresponds to the resolution of a single dish 36km across, with a collecting area (and thus sensitivity) of a single dish 130 meters in diameter. In its highest frequency band, and in its widest observing mode, the VLA has an effective resolution of 0.05 arcsec. At present the VLA is in D configuration, which is its most tightly-packed: all the dishes are within 1km of each other. In addition to having great resolving power, the VLA is extraordinarily sensitive. If you were sitting on the Moon trying to make a cellphone call, and the VLA pointed at you, you would completely overwhelm its detectors. Needless to say, all cellphones must be turned off on the VLA grounds. In addition, computers need to be shielded in metallic rooms (Faraday cages). Most importantly, the observatory has to be far from all possible interference. It is in a remote part of New Mexico, surrounded by mountains which act as natural shields. The VLA has been responsible for many spectacular discoveries, on everything from magnetars in our Milky Way to quasars at the far reaches of the observable Universe.
Both the Trinity Test site and the VLA are located in the New Mexican desert. Both are deliberately remote. And both are testaments to human ingenuity. They remind us of the tremendous and the terrible power of science.
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