Question 4. Did Einstein have the Last Word on Gravity?
Black holes are ubiquitous in the universe, and their intense gravity can be explored. The effects of strong gravity on the early universe have observable consequences. Einstein’s theory should work as well in these situations as it does in the solar system. A complete theory of gravity should incorporate quantum effects – Einstein’s theory of gravity does not – or explain why they are not relevant.
Ripples in space-time
Albert Einstein predicted the existence of gravitational waves in 1916 as part of the theory of general relativity. He described space and time as different aspects of reality in which matter and energy are ultimately the same. Space-time can be thought of as a "fabric" defined by the measuring of distances by rulers and the measuring of time by clocks. The presence of large amounts of mass or energy distorts space-time -- in essence causing the fabric to "warp" -- and we observe this as gravity. Freely falling objects -- whether a soccer ball, a satellite, or a beam of starlight -- simply follow the most direct path in this curved space-time.
When large masses move suddenly, some of this space-time curvature ripples outward, spreading in much the way ripples do the surface of an agitated pond. Imagine two neutron stars orbiting each other. A neutron star is the burned-out core often left behind after a star explodes. It is an incredibly dense object that can carry as much mass as a star like our sun, in a sphere only a few miles wide. When two such dense objects orbit each other, space-time is stirred by their motion, and gravitational energy ripples throughout the universe.
In 1974 Joseph Taylor and Russell Hulse found such a pair of neutron stars in our own galaxy. One of the stars is a pulsar, meaning it beams regular pulses of radio waves toward Earth. Taylor and his colleagues were able to use these radio pulses, like the ticks of a very precise clock, to study the orbiting of neutron stars. Over two decades, these scientists watched for and found the tell-tale shift in timing of these pulses, which indicated a loss of energy from the orbiting stars -- energy that had been carried away as gravitational waves. The result was just as Einstein's theory predicted.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) / see www.ligo.caltech.edu
LIGO’s mission is to observe gravitational waves of cosmic origin. LIGO searches for gravitational waves created in the supernova collapse of stellar cores to form neutron stars or black holes, the collisions and coalescences of neutron stars or black holes, the wobbly rotation of neutron stars with deformed crusts and the remnants of gravitational radiation created by the birth of the universe. LIGO is operated by the California Institute of Technology and the Massachusetts Institute of Technology for the National Science Foundation.
LIGO Observatory facilities in Hanford, WA and Livingston, LA house laser interferometers, consisting of mirrors suspended at each of the corners of a gigantic L-shaped vacuum system, measuring 4 kilometers (2-1/2 miles) on a side. Precision laser beams in the interferometers will sense small motions of the mirrors that are caused by a gravitational wave. Gravitational waves that originated hundreds of millions of lights years from earth are expected to distort the 4-kilometers mirror spacing by about a thousandth of a fermi, less than one tenth of a trillionth of the diameter of a human hair.
Observing runs began in 2002. In 2008, the NSF approved funding Advanced LIGO. The Advanced LIGO program will place upgraded detector subsystems into the existing infrastructures at the sites, generating a ten-fold sensitivity improvement and yielding a thousand-fold increase in the volume of space that LIGO will survey.
A Deep Underground Gravity Laboratory (DUGL)
Scientists who are studying gravitational waves are looking beyond Advanced LIGO to the next generation experiments. There are many advantages to building an even longer interferometer deep underground: reduced seismic noise, a more stable gravitational field, and a more stable environment in general (temperature, pressure, etc.). The DUGL collaboration is measuring these parameters at several levels of the Homestake Mine. The photo is a gravity gradiometer that measures changes in the gravitational field.
More about the search for gravity waves :
Documentary video about the Laser Interferometry Gravity Observatory (LIGO)
http://www.nsf.gov/news/mmg/mmg_disp.cfm?med_id=58443&from;=vid.htm
Keep up to date with happenings at Sanford Underground Laboratory at Homestake by visiting http://www.sanfordundergroundlaboratoryathomestake.org/