The Search for Gravity Waves

This is a paper I wrote for my first Calc Physics Class.



Throughout history, great advances in science have been sparked by discoveries in astronomy. Optical telescopes gave Isaac Newton a view of the heavenly bodies that would inspire his work in physics. Radio telescopes led to the discovery of pulsars and quasars and microwave background radiation, which world-renowned physicist Stephen Hawking has called the greatest discovery of the century. Now, scientists are searching for a new way to look into the universe that could yield equally spectacular results.

Einstein's general theory of relativity explains gravity as the effect caused by objects moving in a straight path through a 3D space that is curved. This curvature results from the fact that all objects with mass distort the 3D space around them, just like bowling balls placed on a tightly stretched bed sheet. The theory also states that gravity has an effect on the way the object experiences time and that, because of this, space and time are closely related and often described as spacetime. Einstein's equations also leave room for an interesting consequence of this definition of gravity: a "gravity wave."

A gravity wave is a wave that propagates through spacetime just as a ripple travels through the water in a pond. It is described as a "wave" because of the rhythmic stretching and compressing that a spectator hit with the wave observes. Just like any wave, gravity waves possess amplitude, frequency, wavelength and speed. The amplitude of a gravity wave describes the amount of stretching or compressing that an object experiences. Gravity waves travel at roughly the speed of light, though this varies depending on their amplitude.

Gravity waves are caused by massive objects undergoing certain movements. One of the best examples is a binary neutron star system. In fact, it was a binary neutron star system (known as PSR B1913+16 or the Hulse-Taylor binary pulsar) that provided some of the first evidence for the existence of gravity waves. The system was discovered in 1974 by Joseph Hooton Taylor, Jr., and Russell Alan Hulse of Princeton University. Radio wave data that they collected about the stars' orbits around one another showed that the orbits were decaying. The amount of decay agreed with the amount of energy loss that Einstein's equations had predicted would occur because of gravity wave emissions. For this discovery, Hulse and Taylor were awarded the Nobel Prize in Physics in 1993.

Just like radio waves, gravity waves are expected to contain information about the event that caused them, information that can be analyzed once the wave is detected. But detecting a gravity wave could prove to be problematic. Scientists have theorized that gravity waves could be detected from sources as far away as 70 million light years. This would be an unprecedentedly deep look into the universe but complications arise from the fact that by the time they reached the earth, the rhythmic distortion of objects by which they are expected to be identified would be very small. Extremely precise measurement techniques would be required.

One of the first attempts to make these measurements came in 1968, when Joseph Weber of University of Maryland began his construction of an instrument he called a "Weber bar." The device consisted of large aluminum cylinders and took advantage of piezoelectricity, which is the tendency of certain materials to experience a minor change in electric potential when they undergo mechanical stress from being stretched or compressed. As a gravity wave passed through the earth, it was expected to rhythmically stretch and compress the aluminum cylinders by a very small amount. The change in electric potential would be measured by piezoelectric sensors attached to the cylinders. Over the course of the experiment, Weber claimed to have identified numerous instances of passing gravity waves. These claims were the source of much controversy and his experiment was repeated by several other groups of physicists with Weber bars of their own construction. None of the other groups claimed to have been able to detect a gravity wave with the device and they criticized Weber's methods of data analysis.

The search for gravity waves has continued but the methods employed have changed significantly. Scientists now favor an approach using a device called a Michelson interferometer. An interferometer is a tool for studying how two waves interact. A Michelson interferometer works by splitting a beam of light into two beams and having them reflected back at each other. If the synchronization of the waves has not been altered, they will cancel one another out. However, if the synchronization of the waves has been affected by, say, a change in the distance one of the beams traveled compared to the other beam, the waves will not completely cancel and the byproduct will be observed by a detector.

One of the largest of these projects is known as LIGO (Laser Interferometer Gravitational-Wave Observatory.) The LIGO projected, sponsored by the National Science Foundation and run by scientists at Caltech and MIT, consists of two large Michelson interferometers (one in Washington State and the other in Louisiana) connected to 4 km long vacuum tubes, down which the laser beams it uses are reflected. When a gravity wave hits the earth, it is expected to distort the distance that the beams travel which will cause the beams to become slightly out of sequence. LIGO will record information about the way the beams paths were altered. The data that LIGO collects will be analyzed using computers and will, hopefully, positively identify a passing gravity wave and its source. LIGO has yet to successfully detect a gravity wave, but its scientists, aware of the potential of this unprecedented discovery, remain hopeful.

The road to identifying a gravity wave is a long one and there are many difficulties. A gravity wave is only expected to alter the 4 kilometer spacing of the mirrors by about 10−18 of a meter. Observing such a miniscule change is forcing LIGO scientists to push the boundaries of measurement techniques. Another major difficulty in analyzing data collected by LIGO is isolating the system from and detecting outside interference or "noise." Earthquakes, vehicles passing by, wind, or even the movement of waves against beaches hundreds of miles away all interfere with LIGO's measurements. Because of this sensitivity to interference and because of the precision of measurement the experiment requires, LIGO utilizes extremely sophisticated equipment capable of measuring distances one thousandth the diameter of a proton and with the ability to detect and identify the vibration due to the natural motion of atoms inside its mirrors.

Such impressive equipment seems likely to carry a large price tag -- and it does. LIGO's total cost is estimated at $300 million and its yearly cost is approximately $30 million. This cost must be placed in perspective, however. NASA's space shuttle cost nearly $1.7 billion to construct, with operational costs of $430 million per launch, with multiple launches per year and over 120 launches total since its initial construction.

But in the end, LIGO's value (and other similar detectors, like VIRGO, GEO 600, and TAMA 300) should not be measured in dollars, but in the possible contributions to science and our understanding of the universe. Einstein's general theory of relativity revolutionized scientific thought. Gravity waves could provide us with the best opportunity yet to put Einstein's theory to a rigorous test. Being able to detect and analyze them could enable us to look farther out into space than ever before. If any scientific endeavor is worth being funded, the search for gravity waves certainly is.

Works Cited
LIGO Livingston. Science of LIGO. http://www.ligo-la.caltech.edu/contents/overviewsci.htm

National Science Foundation. LIGO: The Search for Gravitational Waves. http://www.nsf.gov/news/news_summ.jsp?cntn_id=103042

National Aeronautics and Space Administration. Space Shuttle and International Space Station FAQ. http://www.nasa.gov/centers/kennedy/about/information/shuttle_faq.html#1

Physical Review Focus. A Fleeting Detection of Gravity Waves. http://focus.aps.org/story/v16/st19