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 interferometry at optical wavelengths,
using computers and state-of-the-art light sensors, have only matured within
the past 15 years.
The sections below go into more detail about the principles of optical
interferometry. For an interactive tutorial, click on the link at right.
And if you want to see how these principles are applied in the lab, try out
the virtual interferometer.
Light and Waves
Visible light and radio waves represent different segments of the same
electromagnetic spectrum. Both travel at the speed of almost 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.
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.