Tuesday, January 29, 2008

News From LIGO

LIGO announced earlier this month that they did not detect gravitational wave signals from what was the most expected source of a recent intense gamma ray burst. Scientists theorized that the burst came from the collision of two neutron stars or two black holes, but now that LIGO has ruled that possibility out, they believe it must have come either from a magnetar in the Andromeda galaxy, or something behind the spiral arm of that galaxy. Even though LIGO didn’t detect gravitational waves, that information still gives astrophysicists a lot to work with. This is also an important step for the large detector because this data could not have been found with any other available methods.

The Laser Interferometer Gravitational-wave Observatory (LIGO) is searching for gravitational waves, which are most likely created by violent and/or massive events in the universe. Einstein predicted the existence of gravitational waves when he determined that time and space make up a single fabric. His idea of gravity can be analogous to bowling balls on a trampoline. When objects (mainly planets and stars) rest on the fabric of space time, their mass causes the fabric to dip and bend. The more massive the object (heavier the bowling ball), the deeper it sinks on the fabric. This was Einstein’s idea of gravity. So continuing with the idea of a trampoline, massive gravitational events, like the collision of two black holes, are believed to cause ripples in the space-time fabric. These ripples are gravitational waves. If two colliding objects were responsible for an intense gamma ray burst, they would also emit gravitational waves. [As a side note, this analogy is inherently flawed because a bowling ball on a trampoline is under the influence of gravity, so the analogy contains itself.]

Gamma rays are perhaps the most energetic kind of light in the universe. In just seconds, hard gamma rays can emit more energy than the sun will emit in 10 billion years. It can thus safely be guessed that an event causing such a huge energy release will also create gravitational waves. You may have seen a chart of the electromagnetic spectrum, which displays all the spectrum of light that we know of. When light behaves as a wave, it always travels at the same speed, but has different frequencies. This translates into larger amounts of energy. Radio waves have the least energy and the lowest frequency, and gamma waves have the highest energy and highest frequency. Visible waves are somewhere in between. There are “hard” gamma rays of particularly high energy, and “soft” gamma rays, which are less intense.

Gamma ray bursts are also mysterious. While astronomers have multiple theories on where gamma ray bursts can come from, the results from LIGO show that our methods of discerning them are somewhat limited. This particular burst was observed on an eye-line with a spiral arm of the Andromeda galaxy, but it could have come from within the galaxy, or beyond it. By merely observing the light waves from that direction, (whether you use visible, infrared, x-ray or gamma), scientists are limited by the fact that it is difficult to determine how far away a light source is. LIGO offers a different method for observing astronomical events, thus opening a whole new window of observation. Gamma ray bursts will most likely pair with events that LIGO will focus on, so the two should assist each other for years to come.

Magnetars, which I mentioned earlier, are a key source of the most intense gamma rays. Read this article from Scientific American for an absolutely breathtaking story about magnetars and the most intense gamma ray bursts in the universe.

An update on LIGO events are usually released about once a month, either through CalTech or MIT. A recent article by Kathy Svitil explains the science of LIGO very well. At the moment, LIGO is anticipating the start of work on Advanced LIGO, an addition to the current project that will make it 10 times more sensitive. LIGO representatives expect that gravitational waves will most likely not be detected before completion of Advanced LIGO, sometime around 2010.

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