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Dr. Colwell's Remarks


"LIGO - A New Window on the Universe"

Dr. Rita R. Colwell
National Science Foundation
Session on "Probing the Universe for Gravity Waves: A First Look with LIGO"
(Laser Interferometer Gravity Wave Observatory)
Annual Meeting of the American Association for the Advancement of Science
Denver, Colorado

February 17, 2003

See also slide presentation.

If you're interested in reproducing any of the slides, please contact
The Office of Legislative and Public Affairs: (703) 292-8070.

[Title slide]
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Good afternoon and welcome to what may be the weightiest event of the entire annual meeting. Our subject today is gravity - the force that is always with you. I felt compelled to add a little levity to all this gravity.

Gravity has been studied with intense interest ever since the first proto-human or Neanderthal fell on his duff. Of the four fundamental forces of nature, it is the one most palpably felt in our daily lives. And it was the first to be described mathematically, in Isaac Newton's conceptual triumph.

[Newton portrait, equation]
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It is safe to say that very few products of human intellect have served us so well or so dependably as the notion that gravitational force between any two objects varies directly in proportion to the product of the masses, and inversely to the square of the distance between them.

And yet, here at the dawn of the 21st century, nearly 350 years after Newton's epochal insight, gravity remains one of the most obstinate mysteries confronting modern science. That may seem odd, since the Newtonian method works so well that it led to the discovery of Neptune and Pluto, and has sent our spacecraft to the remotest corners of the solar system with exquisite accuracy.

[Einstein slide]
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But Sir Isaac's grand view was displaced in 1916, with the advent of one of the most revolutionary ideas since the Copernican cosmos. Gravity, according to Einstein's general theory of relativity, is not a force instantaneously communicated between masses. Instead, it is a geometric pas de deux jointly performed by mass and the compound entity we call spacetime.

[gravity curvature slide]
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The presence of mass imposes a curvature on spacetime, while simultaneously spacetime imposes constraints on how mass can behave. Or, in John Archibald Wheeler's memorable formulation, matter tells spacetime how to curve, and spacetime tells matter how to move. Those of you who attended the lecture by Dr. Barry Barish of Caltech earlier have already obtained a splendid overview of that concept.

Interestingly, the general theory of relativity makes another crucial prediction - the one that brings us here today. Accelerating mass should radiate gravitational waves - that is, ripples in spacetime - in somewhat the same way that accelerating charges give off electromagnetic radiation.

[ripple slide]
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A stationary mass will not produce these waves; nor will a mass, no matter how huge, that is traveling at a constant velocity. But in theory, acceleration of a sufficiently large celestial mass will generate ripples that should be detectable on Earth - although drastically attenuated by distance. And, indeed, although no one has yet observed a gravitational wave directly, we have excellent reason to believe that they exist and function as Einstein described.

Gravitational waves, he predicted, should carry off some energy from a dynamic massive system, such as two stars that orbit one another in a "binary" arrangement. In 1974, Russell Hulse and Joseph Taylor, using the Arecibo radio telescope in Puerto Rico, discovered the first binary pulsar system.

They also discovered that the orbital period was decreasing: That is, the two stars are rotating faster and faster around one another (well, faster by 75 one-millionths of a second per year), presumably because gravitational waves emitted by the system are draining off a bit of its energy.

[slide of pulsar; slide of Hulse and Taylor]
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Hulse and Taylor's findings agreed with the theoretically calculated value to an accuracy of 99.5 percent. The pair won the Nobel Prize in Physics in 1993 for their work.

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But long before that discovery, physicists were pondering ways to detect gravity waves on Earth. In the 1950s, the late Joseph Weber of the University of Maryland, and a former colleague of mine, pioneered the idea that the peculiar nature of the waves could be made measurable by their effect on large test masses.

Gravity waves affect the shape of spacetime on axes that are transverse to the direction of the waves' propagation.

[slide and/or animation of effect]
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That is, if a wave is moving on the z axis of a coordinate system - in this illustration, coming out of the screen - then the effects will be felt as an alternating extension and compression of space on the x and y coordinates as the wave passes through.

Weber used one-ton suspended aluminum bars and piezoelectric transducers to measure strain in an attempt to detect the passage of waves through the bars.

The results, though intriguing, were, unfortunately, inconclusive. Among other things, they indicated an effect a couple orders of magnitude larger than theory predicts - which is, to say the least, not much. Well, he was a man before his time.

Unlike electromagnetic waves, gravity waves are neither scattered nor absorbed by interstellar dust or other celestial debris. But their strength does fall off inversely with distance.

For me, a molecular biologist, to understand how minuscule the effect is on astronomical scales, I consider the following: The gravitational signature from the inspiral of two neutron stars at the center of our galaxy - each in the range of 1.4 solar masses - would amount to a dimensional change in a test mass on Earth of about one part in 10 to the minus 21. [10-21] That is, one part in a billion trillion.

So, even in a test frame several kilometers long, such as LIGO's, the change in size would amount to about 10 to the minus 18 meter. [10-18 m] That's about 10 million times less than the diameter of a respectable atom. In fact, it's a thousand times smaller than a proton. And we molecular biologists think a nanometer is small!

Still, as you will hear in the succeeding presentations, distortions on that scale can be detected through sophisticated optical interferometry. Weber and his coworker Robert Forward floated the idea about 30 years ago.

By the mid-1970s, trailblazing researchers like Ray Weiss of MIT, who is here today, and others had brought their ideas for interference-based detectors to NSF, which has traditionally been the primary sponsor of ground-based gravitational research - and, indeed, for numerous high-risk projects at the very frontier of understanding.

The Foundation became an essential catalyst for encouraging the community to recognize the extraordinary potential of interferometric detection to produce the sensitive measurements.

The result became a plan for the Laser Interferometry Gravitational-Wave Observatory, or LIGO, which Dr. Weiss will explain in his presentation.

In 1984, the Foundation concluded the proposal was ready for planning and feasibility studies.

[slide of schematic diagram]
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At least two detectors, separated by a considerable distance and located within quite different geophysical environments, would be necessary to discern concurrently the faint signal from the cacophony of background noise. The final design called for an L-shaped configuration with an arm length of 4 kilometers.

After examination of 19 potential sites in 17 states, construction began in Hanford, Washington

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and Livingston, Louisiana during the 1990s.

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I will NOT go into the decision-making for site location! I'll leave that to the other speakers (I was merely a National Science Board member at the time, not Director!).

In a tour-de-force of science and engineering, the NSF-Caltech-MIT consortium brought this ambitious and unprecedented project in on budget and on schedule.

By last year, the LIGO interferometers, although still in a commissioning phase, became the most sensitive gravitational wave detectors on Earth.

This prompted the LIGO researchers to undertake their first scientific data run, and today Albert Lazzarini of Caltech will present the very first scientific results from LIGO, marking the beginning of LIGO's search for gravitational waves.

Finally, David Shoemaker of MIT will give you the prospect for the future development of this facility and its plans for ever-evolving technological improvements and increased scientific reach.

LIGO, after all, is not simply a masterpiece of instrumentation and design. It is also a new kind of observatory that may reveal unprecedented views of astrophysical phenomena inaccessible by other means.

[End slide]
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And, of course, it is ultimately intended to reach the absolute limits of Earth-based detection. And so even after the first detection of gravity waves, we expect the LIGO team to keep making waves for many years to come. We at NSF are proud of LIGO, the LIGO team, and this investment in science at the furthest cutting edge.



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