Introduction to LIGO & Gravitational Waves

The Potential of Gravitational Waves

Gravitational waves will usher in a new era in astronomy.  Most of the astronomy done in the past has relied on different forms of electromagnetic radiation (visible light, radio waves, X-rays, etc.), but electromagnetic waves are easily reflected and absorbed by any matter that may be between their source and us.  Even when light from the universe is observed, it is often transformed during its journey through the universe.  For example, when light passes through gas clouds or the Earth's atmosphere, certain components of the light will be absorbed and cannot then be observed.

Gravitational waves will change astronomy because the universe is nearly transparent to them: intervening matter and gravitational fields neither absorb nor reflect the gravitational waves to any significant degree.  Humans will be able to observe astrophysical objects that would have otherwise been obscured, as well as the inner mechanisms of phenomena that do not produce light.  For example, if stochastic gravitational waves are truly from the first moments after the Big Bang, then not only will we observe farther back into the history of the universe than we ever have before, but we will also be seeing these signals as they were when they were originally produced.

The physics that went into the creation of a gravitational wave is encoded in the wave itself.  To extract this information, gravitational wave detectors will act very much like radios—just as radios extract the music that is encoded on the radio waves they receive, LIGO will receive gravitational waves that will then be decoded to extract information on their physical origin.  It is in this sense that LIGO truly is an observatory, even though it houses no traditional telescopes.  However, the data analysis that is required to search for gravitational waves is much greater than that associated with traditional optical telescopes, so real-time detection of gravitational waves will usually not be possible. Therefore, LIGO creates a recorded history of the detector data.  This provides an advantage when cooperating with traditional observatories, because LIGO has a ‘rewind’ feature that telescopes do not.  Consider a supernova that is only observed after the initial onset of the explosion.  LIGO researchers can go back through the data to search for gravitational waves around the start time of the supernova.

Gravitational wave astronomy will help explore some of the great questions in physics: How do black holes form?  Is General Relativity the correct description of gravity? How does matter act under the extremes of temperature and pressure in neutron stars and supernovae?