Two Trinities

By Daniel Holz | April 8, 2007 10:20 pm

Yesterday I took a pilgrimage to two holy sites:

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.

CATEGORIZED UNDER: Science, Science and Society
  • Ryan

    Lucky! When I went to White Sands it wasn’t the one or two days a year when the Trinity site was open. The other people on the tour, who planned months ahead to see a bomb site must have been very “interesting.”

  • Kieran

    I drove past the VLA a few years ago on the way back to Arizona. It was shortly after my daughter was born and I remember thinking, hey, in fourteen or fifteen years I can book some time at this place so I can try to communicate with her.

  • Geoff

    What preciseley does a physicist or astronomer mean when they use the term holy to describe a site? Possibly a failed pun in reference to the first one?

  • Trinifar

    In any case New Mexico is a very beautiful piece of the earth. Other wonderful places within its borders are the cliff dwellings. The ones I saw are at Bandelier National Monument.

  • michael pierce

    Yeah, I’m not quite sure I’d use the adjective holy to describe the trinity site. I’ve been to both the VLA and the nondescript area SE of Socorro. Seeing the VLA the first time (I was 20 or so) was truly what I’d call inspiring and spiritual (as spiritual as I geT). I’d include that in the count of things that inspired me to grad school. Trinity on the other hand just gave me the creeps and left me silent.
    Same thing for Los Alamos (there’s a “trinity” street there). I’ve been to that lab a few times, even interviewed for a job there. But I just cannot get past what is done there, what has been done there, and the responsibility that comes with it.

    Using “holy” to describe the trinity test site runs very close to pushing my buttons about science, education, and responsibility. Grad school is great at teaching us to do science, but very rarely do we ask whether or not we should be doing a particular thing. There are some scientists that, while I may not agree with them or find myself capable of doing their work, have at least thought a good deal about the implications of their research, are passionate/patriotic about it, and I’m capable of having some respect for them. There are others…. Ok, I’ve digressed far enough from the topic and need to put my soap box away. As you say both are testaments to human ingenuity: one to our beautiful dreams, one to our terrible nightmares.

  • Geoff

    Now that I think about it, doesn’t designating one (or two) sites as holy violate the “sacred” asumptions of homogeneity and isotropy?

  • Lab Lemming

    Thank you for describing your trinity experience, explaining interferometry and posting nice pictures all in one great post! This is the best science blog entry I’ve read this year.

    A question: What is the advantage of placing the dishes any closer than their maximum spread? I don’t see the advantage of lower resolution.

    Can I assume you didn’t drive from upstate NY? There are some roadcuts through sub-economic thorium deposits in NY that will set off a geiger counter in cars driving down the highway. But unlike trinity, those sites have a halflife just a shade over the age of the universe.

    Also, anyone assuming isotropy on a crystalline planet needs to take a mineralogy course.

  • Joseph Smidt

    Great Post. That Geiger counter information was interesting.

  • IseFire

    I actually spotted a “The VLA – 20 Years” fired but unglazed enamel label pin (shows three of the dishes) at Hell’s Kitchen Flea Market the other day. I actually knew what the heck the VLA was, so I bought it for a whopping $.50 and pinned it to my backpack. I notice quite a few people looking at it curiously–in the grocery store, in the gym locker room, on the subway (yes, my backpack goes with me pretty much everywhere: such is the practical urban lifestyle). So, I’m doing my small part to raise awareness about the VLA. :)

    (I think you’re maybe crediting the VLA with a bit too much “star” status. *grin* My guess would be that it is by no means as well known as Arecibo, which is less remote, is close to a tourist destination city, has a visitor center, and has probably been in more TV shows, movies, and commercials than the VLA. But I could be wrong.)

  • IseFire

    er–that should have been “lapel pin.”

  • Fermi-Walker Public Transport

    Lab Lemming,

    There are two reasons why the VLA has a compact configuration. The most important reason is that too much resolution makes it impossible to see the extended structure in your target. Each antenna pair contributes a Fourier component to the brightness distribution and is inversely related to the angular scale That is small baselines are needed to image extended structure. If you don’t have them, the image of your source will be incomplete. There are many papers in the literature that are simply wrong because of this mistake. Worse still, often the lack of small baselines will create false artifacts in the image.

    On the other hand, if one wants to see if that point source truly is a point source, that is when one cranks up the resolution using the “A” configuration if one is observing with the VLA. Observing say the extended hydrogen gas in an nearby galaxy means using the most compact “D” or next to most compact “C” configurations.

    Imaging both the fine and extended scale structure usually means combining data from different configurations and that is usually not straightforward.

    Finally, often one wants to compare two images made at different frequencie and it is very helpful then that both images be made at comparable resolution. Since resolution varies as lamda/d, changing lamda means observing with a different configuration.

  • Fermi-Walker Public Transport

    OOps, forgot to add that a small baseline means two antennas being close.

  • Neil B.

    About Trinity, and nukes:
    I used to work as a civilian tech in USN nuclear vessels. I remember how sobering it was, to sit at the fire-control chair of a “boomer” submarine, fingering “the button”, and think of what that meant: armageddon at the touch of the fingertips.

  • daniel

    Ryan, the crowd at Trinity site was indeed quite “interesting”. Over 2,500 people visited covering quite a broad spectrum, ranging from red-blooded military types to liberal peaceniks. All ages. The parking lot was sprinkled with both “God bless the USA” and “Visualize world peace” bumper stickers. There were no events or lectures. I’m guessing protests (and, presumably, celebrations) would be discouraged by the many armed guards roaming the area. The overall mood was somber and subdued. It was unusually cold and overcast, which seemed entirely appropriate.

    Lab Lemming, Fermi-Walker hit the nail on the head. It’s a trade-off between resolution and field-of-view. You get a high-resolution image of a tiny part of the source (“A” configuration), or a lower-resolution image of the entire thing (“D” configuration), or something in between.

    Geoff, michael et al., I was deliberately provocative with “holy”. But I feel it’s not entirely inappropriate. Certainly this patch of earth is somehow distinct and special. The ground itself, radioactive and laden with trinitite, bears witness to an event that has had a profound impact on subsequent human development. I’m having a hard time coming up with a comparable site. Many “holy” sites are of dubious provenance, and have led to dubious contributions to humankind. At Trinity, the ground itself is scarred and sacred. At other “holy” sites the ground is physically no different from that down the street. But I think my main reason for using the term is to encourage people to take a pilgrimage there. It is useful for all of us (perhaps particularly scientists) to reflect on the terrible power we’ve unleashed. And the barren, unforgiving landscape, and the unremitting click of the Geiger counter, are certainly as effective a reminder of man’s place in the Universe as any ancient church or smoldering heap of rocks.

  • beezle

    I hope you stopped into White Sands too. Unfortunately for my friend who was driving (a little fast), the state trooper didn’t buy into the ‘we need to get to the VLA before sunset’. We had to settle for the self-guided ‘tour’ and then beers at a little dive bar in Quemado. We thought we might be able to crash there but apparently the ‘lightning farm’ is nearby and all local rooms were booked. We ended up crashing on lawn chairs checking out the night sky half way between Quemado and Grants. I deny any and all rumors there was vodka involved :)

  • Carl Brannen

    Here’s a photo of the VLA I took last summer. I think it’s most beautiful on cloudy days.

  • Ian B Gibson

    …this tiny piece of glass is a physical remnant of humanity’s loss of innocence.

    Because we’d all been so nice to each other until then?

  • assman

    I was thinking that you could also do this for optical telescopes. I imagine a whole massive array of optical telescopes in outer space, with lasers fired from some reference to each telescope. The reference lasers would be necessary to establish extremely accurate time-space distance between the telescopes in order to do interferometry. Maybe the laser could interfer in some complicate way with image data at each telescope. This information could then be stored optically and later combined to do get a high resolution image.

  • Julianne

    I was thinking that you could also do this for optical telescopes. I imagine a whole massive array of optical telescopes in outer space, with lasers fired from some reference to each telescope. The reference lasers would be necessary to establish extremely accurate time-space distance between the telescopes in order to do interferometry. Maybe the laser could interfer in some complicate way with image data at each telescope. This information could then be stored optically and later combined to do get a high resolution image.

    This is actually being planned by SIM (“Space Interefometry Mission”; see link here for a summary). However, it’s only 2 telescopes, and recent NASA funding shenanigins have done some serious damage to the project.

  • Lab Lemming

    Does anyone know which isotopes are responcible for the remaining radioactivity?

  • daniel

    A graph of some of the radioactive elements in trinitite is available here.

  • adam

    No argument from me on the significance of the bombsite. Amazing achievement of a group of scientists working towards one end for a purpose. Admittedly a purpose other than the one in which the bomb saw its first use, but there’s a lesson in that, too.

  • Ben

    The previous comments mentioned two reasons why one might observe with a compact configuration of a radio array: detectability to structure on large scales (sometimes called the problem of missing short spacings), and matching resolution at different wavelengths. There is a third, which is overall sensitivity. Larger spacings mean a smaller beam, in arcseconds on the sky, which means smaller flux per beam from an extended source. For the VLA, where the ratio between A/B/C/D arrays is about a factor of 3, this means you take a factor of 9 hit in signal per beam going from C to B array. This is huge for weak transitions like the 21cm line of atomic hydrogen. (This is why people who use the VLA to observe e.g. galaxy rotation curves at 21cm usually use C array; it would be nice to use B array and clean up the short spacings with a bit of C array data, but the B array observations would take a ton of telescope time.)

    Combining the data from two different array configurations of the VLA isn’t that difficult. Combining data from two different radio interferometers would be harder.

    It’s a lot easier to do interferometry in the radio than in the optical because (1) the wavelength is physically longer, and (2) radio receivers preserve phase information and optical detectors don’t, so you need to do optical beam combining before hitting the detector, and that requires extreme stability and precise metrology.

  • Clark

    “It is unnerving to be exploring a nondescript patch of desert while your Geiger counter clicks up a storm.”

    I can point you to plenty of places in the Utah desert where it’s like that with no nuclear weapons effects. All those signs warning about not going into caves because of radiation aren’t lying.

  • Clark

    BTW – the big 3: Trinity, the VLA, and White Sands really ought be on every physicist’s vacation time. A whole bunch of us students working at LANL did it and had a blast of a roadtrip.

  • Fermi-Walker Public Transport

    Ben is correct, combining VLA configurations is not that hard. What I meant to say, is that making images from the data, wheather from single or muliple configurations, usually takes some thought in that one has the flexibility to emphasize certain antenna pairs over others. That is, does one emphasize the close baselines and go for the extended stucture, or the more further apart baselines and get better resolution at the expense of some sensitivity. The former is called “natural weighting” and the latter is called “uniform weighting”. It all depends on the science one wants.

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  • The AstroDyke

    Great post, Dan. Being at the VLA really does feel vaguely, strangely spiritual. Especially at night. Nobody sleeps over anymore, since the observing’s automated. But I asked to spend the night (in Jodie Foster’s room, be still my heart) and wandered the grounds for most of the night, inspecting the Milky Way bulge with big fat binos. The seeing’s crap, but what an astonishingly dark site. And then out of the darkness, you hear the antennas smoothly grind into motion, turning together to look somewhere else.

    It’s a shame the Trinity site is only open 1 day per year. I’d like to see it. I doubt we lost any innocence at Trinity but certainly, we gained an enormous, awful power.


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