Deciphering Ripples In The Fabric Of Space-Time

By David Giraud

Imagine the energy of the sun, the energy which saw Pangea begin to split into continents millions of years ago, the energy which has seen the rise of the Rocky Mountains, extinction of dinosaurs and the energy which saw the development of rudimentary life. Imagine the sun's energy, which has shone down upon earth over its entire lifetime of 4.5 billion years.

Go further: imagine sun's energy produced over its entire predicted lifetime of 9 billion years. Now, imagine that energy compressed into a burst which, instead of billions of years, lasts seconds. Such an unbelievably intense burst of energy exists and can be found throughout our universe at vast distances.

"They are one of the greatest mysteries in our universe,” says University of Oregon physics professor Raymond Frey. These remarkable yet perplexing events are known as gamma ray bursts (GRBs). They typically occur when two very massive systems collide, such as neutron stars or black holes. Such a burst (GRB070201) was detected on February 1, 2007 in the direction of the Andromeda galaxy 2.5 million light years away.

While GRBs are not rare, this one is particularly special. When neutron stars or black holes interact, they are detectable by several means. Gravitational waves provide such a window into cosmic events and scientists in the Northwest led by Frey have played an integral role understanding how they do so.

Gravitational waves were first proposed by Albert Einstein as a consequence of his theory of general relativity published in 1916. Among many other significant developments related to understanding the propagation of light and the passage of time, Einstein redefined the idea of space. Known as "space-time,” this new geometry accounts for time as an additional dimension to the three dimensions to which we are all accustomed. While visualizing four dimensions may induce headache among some, physicists have enjoyed utilizing the concept to unveil understanding of physical behavior from the level of the smallest known particle to the universe as a whole. An elegant and powerful application of the construct of space-time can be seen in terms of something with which we are all innately familiar: gravity. Einstein described gravity as being caused by a curvature of space-time.

While the "curvature” of a medium may sound exotic and foreign, we are familiar with many examples. One example is sound. When listening to music emanating from a loudspeaker, we hear by sensing rapid changes in the density of air caused by the movement of the speaker cone. Just as vibration of air can be described in terms of wave behavior, it is thought that some events will create detectable waves in space-time. The detection of these gravitational waves, or gravitational radiation, is precisely the goal of the Laser Interferometer Gravitational-wave Observatory (LIGO).

LIGO is a project led by the California Institute of Technology and the Massachusetts Institute of Technology. It consists of two sites, one on the Hanford Nuclear Reservation near Richland, Wash. and another in Livingston, La. Funded by the National Science Foundation, LIGO was designed not only to detect gravitational radiation but as an astronomical tool for understanding our universe.

When GRB070201 was detected by satellites orbiting the earth, LIGO was collecting data on the ground. Interestingly, gravitational radiation was not detected at the time satellites recorded GRB070201. This non-detection is significant and as a result, Frey and other scientists from the University of Oregon Center for High Energy Physics have proposed that the lack of gravitational radiation rules out neutron stars and black holes as a cause.

At the 12th Gravitational Wave Data Analysis Workshop in Cambridge, Mass., University of Oregon physicists suggested two possible alternative causes. One possibility is that an event beyond the Andromeda galaxy may have been mistaken for one within it. Another possibility is that the GRB may have resulted from neutron stars with very large magnetic fields that emit gamma rays at irregular intervals. These objects are known as soft gamma ray repeaters. However, the sensing capabilities of LIGO continue to be refined.

While our bodies possess finely tuned "instruments” perfectly capable of detecting sound and light to our satisfaction, constructing an instrument to detect gravitational waves is quite difficult. LIGO relies on a technique called interferometry where small changes in distance can be detected from the interference of light. While LIGO has yet to detect gravitational waves directly, evidence if their existence has been observed from the interaction of a binary pulsar (two neutron stars orbiting each other). Inspired by this, scientists have diligently honed the device to an incredible level of sensitivity; it can detect a change less than a thousandth of the diameter of an atomic nucleus.

However, such sensitivity comes at a price. While scientists work their hardest to minimize them, events unrelated to gravitational radiation can also be detected by the instrument. Thus, in addition to being sensitive to the detection of valuable data, it is also sensitive to the detection of unwanted data or "noise.” Seismic vibration is one such example and is unavoidable even though the Hanford location is relatively remote from common sources.

In fact, this is exactly why two installations are necessary. Noise due to regional events may be mistaken for gravitational wave at each site separately. However, given their distance of separation, it is unlikely that the same noise would occur simultaneously at both sites.

Robert Scholfield, a colleague of Frey at the University of Oregon, is an expert in determining the source of noise at the Hanford site. It can come from the ocean, water as it passes through dams on the Columbia river, or even the nearby artillery range. Noise can also exist in electric form resulting from lighting. "Noise reduction is the name of the game,” notes Frey. Characterizing noise and determining the source enables scientists to concentrate on important data.

In addition to sifting through large amounts of data, University of Oregon physicists are also responsible for maintaining the instrument itself. In fact, they are already working on improving the detectors at each site. A new version of LIGO, known as Advanced LIGO, aims to increase sensitivity by a factor of 10 and should commence observations in 2013.

Along with this new increase in sensitivity comes an increase in the volume of space in which gravitational waves can be detected. This space is so large that in 3 hours, Advanced LIGO will have collected the same amount of data the original LIGO collected in a year.

Perhaps with this new capability and sharp minds sift even larger amounts of data, Northwest scientists will play an even larger role in understanding what gravitational radiation can tell us about our universe. Furthermore, interpreting the fluctuations in the fabric of space-time may unlock mysteries beyond GRBs. Scientists are tantalizingly close.

David Giraud is an undergraduate studying physics, applied and computational mathematic sciences and music at the University of Washington.

Images:

Top: An artist conception of a gamma ray burst. Photo: NASA/SkyWorks Digital

Bottom: An aerial view of the LIGO instrument in Hanford, Wash. Photo: LIGO Laboratory