Comments on: Two Trinities

By: The AstroDyke

The AstroDyke — Thu, 19 Apr 2007 23:44:51 +0000

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.

By: Dragging on | Cosmic Variance

Dragging on | Cosmic Variance — Sun, 15 Apr 2007 21:33:59 +0000

[…] GP-B is probably the oldest space experiment alive. The mission was first proposed in 1959, and funding began in 1964 (Francis Everitt, the Principal Investigator, has been involved from the very beginning). The science goal is eminently worthwhile: to measure the Lense-Thirring precession (also known as frame dragging) due to the Earth’s rotation. In general relativity a rotating mass will drag space along with it, leading to effects which would be completely absent in Newtonian gravity. For example, a gyroscope in polar orbit about the Earth will show an extra precession due to the Earth’s one-revolution-per-day spin. One of the problems with general relativity is that gravity is much too weak. Every time we come up with some cool effect (gravitational waves, frame dragging, time dilation), it turns out that it’s almost impossible to see the effect. Frame dragging is no exception. If we were near a rapidly rotating black hole, frame dragging would jump out at us: a gyroscope would wobble all over the place. But the Earth’s frame dragging, for an object in orbit 650 km up, adds up to a miniscule 39 milli-arcseconds per year (mas/yr). For some sense of how small this is, consider your average visible, bright star. For generations we’ve considered the stars to be fixed on the sky. As we now know, this isn’t entirely accurate, and the stars do indeed move. The record-holder is Barnard’s star, which moves by 10,000 mas/yr. Typical stars have proper motions closer to 100 mas/yr. In comparison to the effects of frame-dragging, the “fixed” stars are moving all over the place, which emphasizes the difficulty of measurement. GP-B monitors the orientation of the spin axis relative to a particular star (IM Pegasi). This star was specifically chosen because it is bright in both optical and radio, allowing its motion (against a background, fixed frame of distant quasars) to be exquisitely well-measured using radio telescopes (through Very Long Baseline Interferometry, incorporating data from the VLA). (If you’re wondering about GP-A, it was launched in 1976. It carried an atomic clock, and directly measured the time dilation due to the gravitational redshift, confirming relativity at the 0.01% level.) […]

By: Fermi-Walker Public Transport

Fermi-Walker Public Transport — Thu, 12 Apr 2007 16:06:35 +0000

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.

By: Clark

Clark — Wed, 11 Apr 2007 00:22:53 +0000

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.

By: Clark

Clark — Wed, 11 Apr 2007 00:21:04 +0000

“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.

By: Ben

Ben — Tue, 10 Apr 2007 21:35:38 +0000

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.

By: adam

adam — Tue, 10 Apr 2007 20:05:20 +0000

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.

By: daniel

daniel — Tue, 10 Apr 2007 16:57:48 +0000

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

By: Lab Lemming

Lab Lemming — Tue, 10 Apr 2007 11:25:34 +0000

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

By: Julianne

Julianne — Tue, 10 Apr 2007 07:08:11 +0000

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.