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How Does SIM Lite Work?
What is an Interferometer? A New Window on the Universe
 Astronomers used interferometry to measure the shape of the star Altair, viewed here from the Mount Wilson Observatory. |
It is very difficult to take a picture of a planet as small as Earth in orbit around a distant star. The tiny planet is very faint because it only reflects the light from the star and does not shine on its own. In addition, this faint planet appears to be so close to the much brighter star that they are almost impossible to tell apart. If we still want to find out what these alien Earths might look like we need to develop other methods than traditional telescopes. One of these methods, called optical interferometry, has been identified as a key technology in NASA's search for new worlds.
Essentially, optical interferometry combines the light of multiple telescopes to perform the work of a single, much larger telescope. This is possible due to the interaction of light waves, also called interference. Their interaction can be used to cancel out the blinding glare of bright stars or to measure distances and angles precisely. The word interferometry itself illustrates this idea: interfere + measure = interfer-o-metry.
Interferometry at radio wavelengths has been employed to observe the structure of distant galaxies by their radio emissions for almost half a century. But techniques to perform astronomical interferometry at optical wavelengths, using computers and state-of-the-art light sensors, have only matured within the past 15 years.
The section below goes into more detail about the principles of optical interferometry.
Light and Waves
Visible light and radio waves represent different segments of the same electromagnetic spectrum. Both travel at the speed of more than 186,000 miles per second and share properties that are familiar from waves on the ocean. It is this wave nature of light that makes interferometery possible.
The two keywords to understanding interferometry are wavelength and amplitude. The wavelength of a wave is the distance between two neighboring crests of a wave. Imagine you are a surfer on the top of a crest and a friend of yours is right behind you on the next crest; then the wavelength of the wave you are riding is the distance between you and your friend. For light, different wavelength means different colors. Red light has a longer wavelength than blue light.
The second keyword is the strength, or amplitude of a wave. The amplitude is half of the height the bottom to the crest of a wave. Bright lights have larger amplitudes than dim lights.
If two rays of light match each other perfectly in color, they can interact in a surprising way. Because all the crests of one wave have the same wavelength as the second ray the crests of the two waves can be lined up with each other. As each wave crest of one ray coincides with the crest of the other ray, the two amplitudes of the waves add up to twice the amplitude and the result is a single, much brighter light ray. This is called constructive interference. (Probably the only time when it is considered constructive to interfere!)
However, if we shift one light ray by just half a wavelength the result will be very different. Now all the crests of one wave coincide with the troughs of the other wave and the two rays cancel each other out. Suddenly two rays of light add up to darkness.
Through very subtle adjustments to the two light rays we can change light into dark and light again. Astronomers can, for example, tune an interferometer to block out the light from a bright star and still collect all the light from a faint planet in orbit around this star.
Studying Planets Around Other Stars
So far, this discussion and illustrations have assumed light rays of the same wavelength (color) are being added together. The result of adding rays of white light, which contain all the colors from violet to red is quite different. Because all the peaks of all the colors only line up in one place, there will be one high peak and many lesser peaks as troughs line up more and more, reducing the combined heights. With all wavelengths (white light), there are both kinds of interference occurring in all the colors. This produces an intensity (fringe) pattern that, projected on a screen and measured with a light meter, would create a graph like this.
 Intensity fringe graph, as measured by a light meter. Astrometric measurements made with an interferometer start with the position of mirrors in the optical system based on the appearance of the fringe pattern on the detector in the interferometer. The mirror position is different for different stars, and even for the same star if it has moved.
It is very difficult to take a picture of a planet as small as Earth in orbit around a distant star. The tiny planet is very faint because it only reflects the light from the star and does not shine on its own. In addition, this faint planet appears to be so close to the much brighter star that they are almost impossible to tell apart. If we still want to find out what these alien Earths might look like we need to develop other methods than traditional telescopes. Optical interferometry has been identified as a key technology in NASA's search for new worlds. To find out more about how optical interferometry can be used to cancel out starlight so that nearby planets or other features can be observed, see Technology > Planet Imaging.
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