Richland, WA -- More than one hundred years ago, Albert Einstein changed the face of physics forever. In 1905, his "annus mirabilis," Einstein published four landmark papers that established the quantum theory of light, ended a debate on the existence of atoms, established the equivalence of mass and energy, and introduced a theory of space and time called the Special Theory of Relativity.

In the desert of southeastern Washington state, physicists tested Albert Einstein's crowning theoretical achievement--the General Theory of Relativity. 50 years after his death and more than 100 years after his annus mirabilis, this icon of physics is still challenging our conception of how the universe operates. To commemorate Einstein's work, 2005 was declared the World Year of Physics (WYP 2005).

So, how did you celebrate WYP 2005? Ah, you visited the local science museum for some light edutainment? No? I see. You seriously considered cracking open that dusty physics test to review Newton's Laws--really, seriously thought about it but you couldn't remember where you put the book? Hmm. So, you slumped on your couch and lacklusterly toasted Einstein with a six-pack? Aw, c'mon.      

Here's a better idea. Visit a physics lab. Chances are, there's one close by. It's with this motivation that I drove 225 miles from Portland, Oregon to Richland, Washington to tour LIGO: a state of the art laboratory nestled in the sagebrush of the Columbia basin to learn about gravity waves

Theories of Gravitation

We are born knowing gravity. Of the four forces in the universe (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force), gravity is the only force that appears tangible to us. A dropped object falls. What goes up must come down. It all seems to make sense. Or does it?

In the late 17th century, Isaac Newton published the Universal Law of Gravitation. This law states that all objects attract all other objects with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. It is an immensely successful law if you're considering the orbits of planets or satellites, or an apple falling to the earth. However, the law has a fatal flaw. It does not provide a way for objects to communicate with each other in a finite amount of time. Instead, it assumes that objects communicate instantaneously. Instantaneous communication violates the universal speed limit given by the speed of light (186,000 miles/second). Despite its successes, Newton's theory is wrong.

Enter Albert Einstein. In 1915, Einstein developed the General Theory of Relativity (GR). As the name suggests, this theory is a generalized version of the Special Theory of Relativity that incorporates gravity and the acceleration of objects.

Einstein's view of gravity is that objects distort the space-time in which they exist much like a bowling ball placed on a trampoline distorts its surface. In GR, objects are attracted to each other not by a force, but by "falling" into the distortion via a straight line in space-time.

Moreover, GR includes a mechanism by which objects communicate with each other: gravity waves. Think of gravity waves as ripples in space-time that propagate at the speed of light. GR predicts that gravity waves are emitted by all objects undergoing acceleration. The bigger the mass and acceleration, the bigger the wave. To detect a wave using the state of the art technology at LIGO, the masses and accelerations must be huge. Objects that meet this criterion include black holes, neutron stars, and other stellar objects that are undergoing collisions and explosions.

Physicists have demonstrated the validity of GR many times. In 1919, Arthur Eddington verified the curvature of space-time when he observed gravity bend the path of starlight as it passed the sun. Demonstrating gravity waves took a little longer. In 1974, Russell Hulse and Joseph Taylor discovered an exotic stellar object: a binary pulsar named PSR 1913+16. PSR 1913+16 consists of two neutron stars (one a pulsar) rapidly orbiting each other. In other words, two huge objects undergoing huge accelerations--a perfect system for testing the existence gravity waves.

If gravity waves exist, the orbits of the two neutron stars should decrease and the orbital period should increase as energy is carried away by the waves. This behavior has been observed over many years and the data are in excellent agreement with the predictions of GR. In 1993 Hulce and Taylor won the Nobel Prize for their work.

To date, PSR 1913+16 offers the strongest evidence for the existence of gravity waves. However, the evidence is indirect because the waves themselves have not been measured.

Measuring gravity waves is LIGO's raison d'être.

About LIGO

Located on the Department of Energy's Hanford Site a few miles north of Richland, Washington, LIGO is the most advanced facility in the world for the detection of gravity waves.

Physicists have a penchant for acronyms. LIGO (pronounced with a hard I) stands for Laser Interferometer Gravitational-Wave Observatory. Gravitational-Wave was hyphenated and the W left out of the acronym because, after all, LIGWO sounds silly.

The LIGO mission is to observe gravitational waves of cosmic origin out to (initially) about 70 million light years. The LIGO facility in Hanford operates in conjunction with a LIGO facility in Livingston, LA. And, in 2007, LIGO formed a groundbreaking collaborative research partnership with a similar facility--the Virgo Interferometric Gravitational wave detector of the European Gravitational Observatory near Pisa, Italy--the first of several international facilities with which it is likely to partner.

International partnerships are crucial for LIGO. Sharing information between facilities ensures the accuracy of the data and will enable researchers to accurately pinpoint the source of gravity waves. In the words of researcher Benoit Mours, ""Combining the data from the collaborations is a classic example of 'the whole being more than the sum of the parts.'"

The Hanford Site facility consists of two 4 kilometer (2.5 mile) arms housing vacuum systems and configured at right angles to each other much like a carpenter's square. This configuration was chosen because gravitational waves cause space to contract in one direction (objects get closer to each other) and expand in a perpendicular direction (objects get farther from each other).

At the end of each arm a mirror hangs inside the vacuum system. Laser light is introduced into each arm where it stays for about 1/100 second. If there is no gravitational wave present, the two light beams travel exactly the same distance and are detected at the same time. If a gravity wave is present, the two light beams travel different distances and are detected at different times.

The system is sensitive to a change in length of about 1/1000 of the diameter of a proton, which corresponds to less than one trillionth of the diameter of a human hair!

The Tour

I arrived for the free tour 15 minutes before the scheduled 1:30 start and lingered in the small lobby with about 12 others. The lobby contains an informative display that describes the theory of gravity waves, the science and engineering principles of interferometers, and some of the experimental data acquired at the lab so far. There is also a model of a gravity well and some exhibits of lab equipment.

At 1:30, we were met by LIGO staff physicist Dr. Vern Sandberg who led us into the spacious auditorium. Dr. Sandberg delivered a 35 minute presentation that provided a solid overview of the history and physics of gravity waves, the construction of LIGO, and the data collected as of August 2005. Additionally, there was a table-top interferometer that modelled the basic operation of LIGO. Our host was very enthusiastic and answered many questions from the curious audience. After the talk, we went outside to view one arm of the interferometer. We then walked to the counting room where LIGO staff monitor and control the facility, and analyze and display data.

As you might expect with such a sensitive device, accurately identifying sources of noise plays a crucial role in obtaining quality data. Traffic, wind, micro-seismic activity, and even waves crashing on the Pacific coast are some common culprits that wiggle the lab and introduce noise into the system. In fact, some of the most interesting data I saw was the anthropogenic noise sources originating from human activity in and around Richland. By looking at this data over the course of the day, you can determine the change in traffic volume and even when farm vehicles are in operation. After spending just a few minutes at LIGO, you realize that much of the effort and expense is associated with the ultra high vacuum, mirror supports, elaborate servo controls, and other devices that help isolate the lab from wiggles.

What about gravity waves? Of course, nobody can predict when a gravity wave will be detected, and when I was at LIGO  Sandberg noted, with a smile, that the probability of detection is unity within the S5 timeframe. Although no waves have been detected directly as of May 2009, the influence of gravitational waves on a binary pulsar system (two neutron stars orbiting each other) has been measured accurately and strongly agrees wth the predictions. And, in 2008 the National Science Foundation allocated $205.12 million to upgrade LIGO. This investment will significantly enhance the sensitivity of LIGO's instruments by a factor of 10 and will mutliply the number of astrophysical candidates for gravitational wave signals by a thousand.

It is a hallmark of many modern physics experiments that you can't directly experience what's happening. That's the case with LIGO. When a gravity wave is detected, there is no perceptible motion-nothing at all for your senses to hang onto. A gravity wave might be passing through you right now and you would never know. The proof of success comes only after careful computer analysis and expert evaluation. The truth appears only as bumps on a graph. Still, think how amazing it will be when a gravity wave is detected, the champagne is uncorked, and the ultimate experimental proof of Einstein's General Theory of Relativity is recorded for posterity. The news will make all the papers.

I came to LIGO expecting to learn something about gravity waves. What I didn't expect to learn was how relevant Einstein's work is today, more than 50 years after his death and more than 100 years after his miracle year.

Free LIGO tours are offered on the 2nd Saturday of each month starting at 1:30. Special group tours can be arranged at other days and times.

Resources

LIGO Hanford Web site
http://www.ligo-wa.caltech.edu/

Participate in gravity wave detection via the Einstein@Home project.
http://einstein.phys.uwm.edu

Explore the work of Hulse and Taylor
http://nobelprize.org/physics/laureates/1993/index.html