Dr. Lee Samuel Finn
Professor of Physics, Astronomy and Astrophysics
Research Interests
Research in my group, which is part of the Penn State LIGO Science Collaboration Research Group, focuses on the challenges and rewards of gravitational wave detection. The challenges are experimental, phenomenological and theoretical; the rewards are the prospect of new tests of fundamental physics and a fundamentally new way of looking at the Universe.
Gravity is the least understood or tested of the four fundamental forces of Nature. Gravitational waves have never been directly detected. Einstein's Theory of General Relativity describes gravity as a manifestation of space-time curvature. In General Relativity, gravitational waves are dynamical, propagating waves of spacetime curvature. A changing spacetime curvature changes the distance between neighboring objects, without moving them. Gravitational wave detectors work by measuring that small change in distance.
How small is that change? It varies as the separation between the neighboring objects. For the LIGO detectors, the separation is 4 Km: over this distance the size of the change is, for the most intense waves that are expected to bathe Earth with any significant frequency, less than one one-thousandth the diameter of a proton. Gravitational wave detectors are thus, at heart, precision mechanical experiments, requiring some of the most precise measurements ever made.
This is part of the aesthetic appeal of gravitational wave detection: it is a 19th Century experiment performed with 21st Century precision and technology.
The challenge of Gravitational wave detection is not limited to experiment, however. Even with the most sensitive instrumentation that we can build today evidence for gravitational waves will be weak. There is no hope of building a terrestrial gravitational wave generator that can excite the detectors we can build: the gravitational waves we will observe arise from astronomical sources at cosmological distances. What we know of the cosmos, however, is limited to how we observe it: with light. Many of the most intense sources of gravitational radiation, however, may emit no light - black holes, for example. For these sources we know very little, either of the rates of occurrence or the details of the signal that we should expect. To detect this these weak signals, of unknown structure and occurring at random times with an unknown rate, will require sophisticated data analysis techniques and strategies.
Beyond the challenges of detecting gravitational radiation are its rewards. The direct detection of gravitational radiation is, by itself, a test of fundamental physics: dynamical gravitation. Einstein's theory of gravity predicts exotic phenomena, such as black holes, and makes unique and unambiguous predictions for the nature of the gravitational radiation they emit. Observing that radiation will provide as clean and pure a proof of the existence of black holes and test of strong field, dynamical gravity as one might hope for.
Finally, the gravitational waves observed we observe will be of astronomical origin. Astronomers rely on a multiplicity of observational perspectives in order to infer the nature of the Universe. Progress in Astronomy has naturally and historically been associated with new observational perspectives or improved observational techniques: Brahe's careful observations provided Kepler the raw information needed to reformulate our model of planetary motion, and Galileo's use of the telescope revolutionized our understanding of the Sun and the planets. Reber's study of the sky in the radio, Penzias and Wilson's discovery of the Cosmic Microwave Background, the Vela satellite observations of gamma-ray bursts, and the Hubble Space Telescope's observations of the early stages of galaxy formation are all more recent examples of the same.
Thus, new and improved observational perspectives are accompanied by the prospect of new insights and understandings, not just of exotic astrophysical processes, but of "bread-and-butter"astrophysics (e.g., stars, galaxies and their formation and evolution) as well our very conception of the heavens. Gravitational wave detectors like LIGO will provide us with the kind of new perspective on the sky not seen since Galileo first turned a telescope to the heavens and saw more than his imagination prepared him for.