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Sunday, January 2, 2011




One of the ways in which exposure to unusual circumstances serves as a catalyst in the process of conceptual transformation is that it can help turn theoretical science into experimental science. To illustrate how space science plays that role, let us consider Einstein's General Theory of Relativity. Perhaps the most important insight of this theory is that the geometry of space and time (spacetime) is affected by the amount and distribution of matter (or rather, mass-energy) throughout it. This point is difficult to grasp but it can be illustrated by means of the following analogy. On the surface of a regular coffee cup draw two points and trace the shortest line between them. The length of this line is going to be independent of the amount of coffee in the cup. Similarly, we would expect the properties of space and time to be independent of the "stuff" that makes up the universe. But consider now that the cup is made of some highly elastic material (say, the rubber used in toy balloons). In such a case not only the length of the line but the very shape of the path will be determined by the amount of coffee and by the way we squeeze the liquid about in the cup. Just as the geometry of the cup is affected by the liquid in it, Einstein might say, the geometry of spacetime is affected in a similar way by the matter in it. Thus the paths that light follows (geodesics) will vary from region to region; the universe as a whole will be curved; and time will slow down in the presence of strong gravitational fields (for it will be as if light had fallen into a deep hole-- and so it will take longer to come out of it--which means that even the most efficient clock, using light signals, would measure time more slowly).

To put it in a nutshell, the theory claims that the geometry of spacetime tells matter how to behave and that matter tells spacetime how to curve. Massive objects thus distort the geometry of spacetime (this is the effect of gravitational fields). And such geometry (or "shape", to state it colloquially) determines, for instance, how light can move through different regions of the universe. Near the Earth spacetime is almost flat, and thus it is very difficult to perform experiments to test Einstein's insights. As a result, for a long time there were only three tests of the theory. The most famous checked the prediction that stars directly behind the sun should appear shifted to the side of the sun. This should be expected, if the theory is correct, because the mass of the sun carves a sort of a basin out of the fabric of spacetime. Light coming very near the sun would follow the contours of the basin, and thus be deflected; but it would also be "delayed", for it now has longer to travel. Unfortunately, the sunlight does not permit a check of this prediction except during eclipses. Even then tests were difficult. Nonetheless, they favored Einstein's theory, since during eclipses the stars in very close proximity to the sun were seen shifted away from it within the range of Einstein's prediction.

With the advent of space exploration, however, relevant and far more precise tests can become almost routine. Einstein's prediction is not really limited to light, but extends rather to all electromagnetic radiation, of which visible light is only one form. Thus we can test Einstein’s theory by tracking the radio signals of satellites that circle the sun, or the radio signals from the Viking landers on the surface of Mars when Mars goes behind the sun. That is, we can check not only the prediction about the deflection of light but also about its delay.

We can also check some other startling predictions of the theory, such as the claim that gravity slows time (for clocks should then move faster the further away from massive bodies). Although it is possible to make this last kind of test using only terrestrial science, space offers much greater sophistication and permits far more ambitious developments along these lines. Already it permits us to use very precise clocks in spaceships whose distance from several points is determined to high accuracy by tracking stations using lasers. The advances in chronometry are respectable enough that the entire field goes beyond applied science into engineering.[i]

Of extraordinary importance would be the discovery of gravity waves, for they would provide an entire new window to the universe since matter is transparent to them. Joseph Weber and others have tried to detect gravitational waves by means of extremely sensitive aluminum cylinders and other ingenious devices. Unfortunately the problem of discriminating between true gravitational waves and "noise" caused by other factors has rendered inconclusive the results of most experiments along these lines. In space, however, the availability of very large baselines, e.g. between a station near the earth and a spaceship, permit us to search for gravity waves of different frequencies. Just as Weber expected his cylinders to vibrate whenever a gravity wave went by, we could expect the station-spaceship baseline to oscillate whenever we observe gravitational events of cataclysmic proportions. The opportunity to observe such events will be offered by space- based telescopes trained on pulsars, the vicinity of possible black holes, and other cases of very dense matter, all of which would allow us to see just how well Einstein's ideas fit the world, and to correct or replace them as circumstances and our imagination dictate.

In addition to all this, space exploration makes possible what some like to call "crucial experiments" between General Relativity and some of its sophisticated competitors. Dicke's theory of gravitation, for example, predicted many of the results of Einstein's that we have already discussed. But it differed in some respects. In particular, it predicted that the Moon's orbit is deflected toward the sun. The predicted change in that orbit was so small, however, that a test was out of the question until the landing of Apollo 11 on the Moon. The first humans on the Moon installed a reflector (the first of a series now in place) for a laser beam from Earth. This special equipment has permitted extremely precise measurements of the distance between the Moon and the Earth, thus setting up an experiment that pits Dicke's theory against Einstein's. Einstein's won.

In this new era of telescopes we have also been able to perform some striking confirmations of Einstein’s theory. For example, the theory predicts that large masses may serve as “lenses.” Essentially, a large mass conveniently situated in the line sight between a source of light and the observer will split the light around its body. We have now been able to observe this gravitational lensing on many occasions.

As in these cases, such experimental work is not only entirely new but often requires the contribution of space science. And the most fascinating experiments are yet to come. Of particular significance is Gravity Probe B, the orbiting gyro experiment directed by Francis Everitt at Stanford University. This experiment is now testing the general theory of relativity in two most significant respects (it was originally scheduled to be launched by the shuttle in the late 1980s, but it has finally be placed in orbit only recently). According to Einstein's view, the distribution of mass determines the geometry of space-time. But when a big mass rotates, it places space-time in motion as well, that is, it drags space-time along. To make a very precise calculation of the value of the drag we have to know very precisely the values of the mass and the rotation involved. These values have been determined for the Earth, which also has a mass big enough to permit, according to the theory, a small but detectable amount of space-time drag. Four small, nearly perfect spheres – gyroscopes – placed in polar orbit would have their rotational motions affected by such a drag. The object of the experiment is to measure that effect on the rotation--more specifically, the precession of the spheres with respect to the fixed stars--and see how it agrees with the values predicted by the theory.

It is perhaps ironic that this experiment so much resembles one of the most celebrated experiments of the 19th century, the Michelson-Morley experiment, which by failing to detect any ether drag later made more acceptable to many physicists Einstein's special theory of relativity. By giving real properties to space-time in his general theory, it seems that Einstein introduced into physics something that performed functions in the new theory not unlike those of the ether in the old. Now his own general relativity will be put to a similar test. Unfortunately for Everitt and his team, space shuttle delayed his experiments so much that different experiments, using telescopes, have apparently demonstrated this drag already.

In any event, Everitt’s experiment has a double purpose. It also aims to measure a most startling prediction of the general theory of relativity. When an electrically charged body rotates, it produces a magnetic field. Because of the structure of general relativity, we can ascribe a similar effect to rotating gravitational masses. That is, a field should be produced that would create torques on the orbiting gyroscope (this would lead to what is called the "gravito-magnetic precession of the gyroscope"). A positive result of this experiment would confirm the existence of magnetic-like properties of gravity, a discovery of the most extraordinary importance.

A great deal of advanced technology has been necessary in this experiment, involving sometimes a million-fold improvement in precision over previous technology. The four spheres so perfect that, if you blew one up to the size of the Earth, it would not show a deviation from the average radius larger than two feet. Such sphere must be suspended without friction, and its minute precessions must be measured (one of 6.9 arcseconds per year, the other 0.05 arcseconds per year). These specifications can be met in space after considerable ingenuity and effort. The quartz spheres are coated with metal and suspended by the action of small electric fields. Their rotation is monitored thanks to the fact that since it will be electrically charged and rotating it will produce a small magnetic field.

On the Earth thousands of volts would be required to keep the spheres suspended, with many unwelcome side-effects added to a variety of disturbances typical of the environment--atmospheric, seismic, human, and so on. But even if all those things are accounted for, gravity alone would demand a degree of perfection in the spheres simply unattainable. The reason is that insofar as the spheres are less than absolutely perfect their individual center of mass will vary slightly from the center of a perfect sphere of the same radius. The stronger the gravitational field, the larger the rotational distortion produced by such a variation. Given the desired experimental performance – 3 x 10-11 degrees per hour – under the gravitational acceleration of the Earth, the deviation in radius at any one point cannot be larger than 10-16 of the radius, which in this case it means that the deviation must be less than 1/1000 of a nuclear diameter. That is plain out of the question. By contrast, in orbit the deviation must be less than 10-6, which is difficult but possible to achieve.

The Chinese physicist Ning Li has suggested a radically different way of investigating this prediction drawn from the mathematics of Einstein’s theory. If her analysis is correct, the effect could be shown by a far smaller mass (at the atomic level) made to rotate extremely fast by a superconductor. Her experiment can be carried out on Earth, but even if she is successful, we should acknowledge that space science has blazed the trail that she later followed to recreate the experiment in a different setting.

The full-fledged development of space astronomy and space physics will enhance in a myriad of ways the dialectical relationships between different levels of studying the universe. As we have already mentioned, for example, in the solar system we find clues about the rest of the universe, and our general cosmology affects our ideas of the solar system. In a similar fashion, terrestrial physics and astrophysics have been intertwined since the beginnings of the Copernican revolution. Space physics promises to make that relationship even more intimate. And this is all we need to answer the queries about the long-term consequences of the changes in points of view that space may bring about.

The future contribution of space to this new approach to experimental gravitation will not be independent of the explosive growth expected of space astronomy. An accurate determination of the relevant masses is essential to the study of these peculiar gravitational phenomena. But to make such determination we must also determine, for example, stellar distances and magnitudes very accurately. This will be the minimum benefit from the more powerful and more varied astronomy that will supplement from space the work of astronomers on the ground. And of course we wish to examine the instances of massive gravitational collapse. Even though a black hole does not permit radiation to escape – the curvature of its disturbance of space-time is so pronounced that we might say a black hole is a bottomless pit – it nevertheless makes itself known by the collapse of the matter around it. The gravitational force is so strong that great energy is released as matter sinks into the black hole. That energy can be detected with X-ray telescopes, and so the search for strong sources of X- rays becomes greatly significant.

Space science is thus becoming a significant factor in the ingenious attempt to turn a very theoretical subject into an experimental science, and in some cases into a branch of engineering. We can see the emergence of a familiar pattern: theory leads to new practical concerns, and those practical concerns in turn put us in a position to test theory – sometimes the same theory, sometimes theories in other fields. It is this rather complicated connection between theory and practical concern that eventually forces theory to work under circumstances different from those of its origin. Our views of the universe have permitted us to build the spaceships and the instruments to challenge many other views that we also hold. Ultimately the ensuing transformation will lead to a new round of challenge, and the dynamic character of science will be preserved. This task will be aided in no small measure by the other great advantage of space science: by escaping from the confines of our own planet it removes barriers and gives us a larger perspective.

[i] For an account see Carroll O. Alley, “Proper Time Experiments in Gravitational Fields with Atomic Clocks, Aircraft, and Laser Light Pulses,” a lecture published in Quantum Optics, Experimental Gravitation, and Measurement Theory, P. Meystre and M. O. Scully, eds., Plenum Publishing Co., New York: 1982.

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