Wednesday, October 22, 2014

Interferometry must be a pretty good idea. Everyone does it.

Many years ago, I read a cool article about very long baseline interferometry (VLBI). This is a technique where very sensitive radio telescopes are positioned on opposite sides of the world and are trained on a powerful and very distant radio source, such as a quasar. Quasars are thought to be some of the oldest and brightest galaxies in the universe. They're in the same position in the sky throughout the years because they're so far away.

VLBI is used to image very distant sources. As the entry for the subject in Wikipedia says, "This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes." The idea is to simulate the biggest telescope in the world by using the size of the world to do it.

The article that I read years ago taught me that VLBI can be used to measure the rotational speed of the earth, that the speed of the earth's rotation changes with respect to the speed of the global jetstream, and that the shape of the earth can be measured very accurately with VLBI. The example in that article used two telescopes.

What I find interesting is that, as far as I know, all vertebrate animals have two eyes. Two eyes use a form of interferometry. The brain takes two simultaneous images or streams of images that can be used to calculate it's relative position in space and time. Somewhere, long ago, I learned that the brain uses two eyes to calculate distance within about 10 feet. For everything beyond that the brain uses one eye.

By the same token, many years ago, Scientific American ran an article about how owls hear. Owls are nocturnal creatures and have each ear pointed in a different direction. They can use their ears to listen for their prey and pinpoint it's location in the dark. This again, is a form of interferometry. The brain uses two different signals to determine the location of an object, like a mouse.

We have been graced with two eyes and two ears, probably for redundancy just so that we can see and hear if we lose one. But as we have evolved, it seems Nature has found unique benefits to having two ears and two eyes. One of them being a way to fix the location of things we see and hear. Nice work, Mother Nature.
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