Gravitational Interferometers
Gravitational Interferometers are the instruments for Gravitational Astronomy. They consist of three or more satellites in precise positions that accurately measure the distance between them via interference in laser beams. Alternate expansion and contraction of perpendicular distances record the passing of gravitational waves.

The General Theory of Relativity predicted the existence of gravitational waves, in analog with electromagnetic theory predicting waves emitted by charged particles. The basic value, however, is mass rather than charge. Due to the scale of G, the gravitational constant, only massive and/or relativistic bodies emit measurable gravitational waves, which only interact weakly with matter. As a result, gravity waves traverse the cosmos in almost pristine condition, providing information about the sources in much the same way that visible light provides information about the stars. Unlike forms of EM Astronomy, gravitational detectors are emitted from space-time bulk rather than points, and so provide some information about the mass currents of the source. The signal quality of a GI is governed by its frequency and strength. Because gravitational waves are spatially larger than the objects they measures, GI is more akin to sonar than telescopes.

As a gravitational wave travels through the Universe, it alternately compresses and expands objects in perpendicular directions. As a bulk phenomenon, gravitational waves are only observable in the context of two or more masses -- a single point can never measure a gravitational wave, since locally (at a point) space-time is flat. Only with respect to another body can the induced gravitational strain, h, be measured.

In general non-relativistic gravitational wave production is negligible. Practical measures of gravitational waves measure strain, or change in length divided by length, for which the generalized gravitational quadrupole formula yields:

h ~ (2G M/c^2)*(v/c)^2*(1/r)

Where G is Newton's constant, M is the mass of the wave-generating object, c is the speed of light, v is the velocity of the wave-generating object, and r is the detection range. Some convenient numbers are:

G = 6.673*10E-11 meters^3/(kg seconds^2)
c = 2.997*10E8 meters/second
v/c should be a ratio
for r in meters, 1 light year = 9.46*10E15 meters

For intelligences possessing nanotech, the smallest possible scale at which differences can be measured is the nuclear scale, at 1E-15 meters. Hence the "arms" of the interferometer are designed to be as long as possible, to maximize the detectable strain threshold. Since for large cosmic events, h is of magnitude 1E-17, gravitational interferometry is accessible even to baseline intelligences. (Circa AD2000 LISA measures strains to 1E-22).

For higher toposophic grades, gravitational astronomy becomes a penetrating tool towards unravelling the mysteries of the cosmos.

Sophisticated GI use micro-wormholes to compare laser links traveling in space with reference beams through the wormholes. These sensors can be segmented, so that strong signals are processed quickly with fainter signals detected over the whole array. Such sensors can stretch to light-years in length, and are located well above the ecliptic plane of a solar system, to minimize local gravitational disturbances. Over the span of decades to centuries, these observatories can measure cosmic phenomena such as universal expansion and cosmic string/domain walls/monopole production, and map out the global structure of the universe. They can detect the mass current density of local galactic formations, observe in detail the ongoing collision of the Milky Way and Sagittarius, and predict stellar phenomena such as novae, pulsars, and black hole collapse/formation.

The increasing size of the observatory coupled with photon redshift, quantum noise, and the challenges of keeping the satellites stationary and synchronized, and local gravitational signals presents a formidable challenge. However, those curious and/or extrospective sophonts are rewarded by an unparallelled instrument for the discovery and testing of cosmological theory.

Gravitational Interferometry has made another, political contribution to the galaxy, rivalling the establishment of the wormhole networks. A sophisticated GI sensor is capable of detecting an attacking relativistic war fleet at distances of light-hours to light years, depending upon the mass of the vessels, giving early warning against relativistic attack which would otherwise surprise standard sensors due to time of flight restrictions imposed by c. They are also capable of detecting the formation of certain types of wormholes, and can be used to track their construction and movement. Because of this extreme sensitivity, and the nature of gravitational waves themselves, it is nearly impossible to militarily surprise an entity possessing sufficiently sensitive GI coupled to efficient wormhole-grade communications.

In the early stages of expansion, colonies are susceptible to interstellar war. But as important planetary systems acquire significant defenses in the form of long-range relativistic missiles and mass-drivers along with advanced GI sensors, they become difficult to assault without enormous cost. Even an exceedingly determined attacker, bringing along a wormhole for re-supply, cannot hide this from sensitive GI observatories.

The demarcation lines become well entrenched, with the battles moving fluidly across the colonies. Warfare becomes a matter of stellar geographical position and economics, and of the acquisition of resources faster than the opponent balanced against the need to defend those resources.

The extraordinary long lives of the archai coupled with this penetrating vision of the universe has significantly reduced the threat of interstellar war in the core regions of the galaxy. Such warfare flares up only in the less-developed fringes, or in rare cases, as part of titanic millennia-spanning conflicts.


Mathematical Details (an appendix for the truly curious)

Formally, power carried by a gravitational wave is proportional to the square of the third time derivative of the gravitational quadrupole tensor. Gravitational dipoles do not exist, unlike their electric analogs, due to conservation of momentum. A gravitational monopole is equivalent to the standard Newtonian or Special Relativistic (Covariant) treatment. Gravitational waves are strictly a prediction of General Relativity (Covariance).

Links:

Laser Interferometry Space Antenna, http://lisa.jpl.nasa.gov/WHATIS/intro.html "Gravitational Astronomy: The high frequency window", Nils Anderson and Kostas Kokkotas, PDF

"General Relativity ", Robert M. Wald
 
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Development Notes
Text by Adam Getchell

Initially published on 07 October 2004.

page uploaded 7 October 2004, last modified 2 July 2007