Gravity: The weight of expectation
Nature 421 , 899 - 900 (2003)
C. D. HOYLE
C. D. Hoyle is in the Department of Physics, University of Trento, Via Sommarive 14, 38050 Povo (TN), Italy. e-mail: hoyle@science.unitn.it
Newton devised his universal law of gravitation for planets, but does it work at small scales? A search for a deviation from the expected behaviour could provide the first evidence in support of string theory.
Gravity, one may think, is a rather well understood subject. It has been more than 300 years since Isaac Newton determined that the magnitude of the gravitational force, F, between two bodies of masses Mand m(separated by astronomical distances) depends on the inverse-square of the distance, r, between them: F=GMm /r2. And we have recently passed the two-century mark since Henry Cavendish made his first measurement 1of the gravitational constant, G, which sets the fundamental strength of the interaction.
But developments in theoretical physics predicting possible deviations from the inverse-square law at small distances 2have brought about a renewed enthusiasm for precise laboratory tests of this most familiar fundamental force. The results reported by Joshua Long and colleagues 3on page 922 of this issue describe an investigation of gravity at a previously unexplored scale. Furthermore, Long et al . have devised a new apparatus, in place of the low-frequency torsion balance that has been the workhorse of laboratory-scale gravitational experiments since the time of Cavendish 4-6 -- they use a kilohertz, resonant-oscillator technique to test the gravitational force experienced by test masses that are separated by just 108 µm ( Fig. 1 ).
Gravity may have been the first of the fundamental forces to be described mathematically, but it is still the most poorly characterized. The inverse-square-distance dependence established by Newton had been assumed to hold true at short distances, but only recently has gravity actually been shown to exist between objects separated by less than one millimetre 5.
Why is it so difficult to measure the properties of gravity, when it obviously exerts a strong influence on us every day? The reason is that gravity is incredibly weak compared with the other known fundamental forces of nature (electromagnetism, and the strong and weak forces). We feel gravity only because, unlike electromagnetism, it cannot be shielded with 'negative' masses -- there is no gravitational repulsion to balance the attraction. To put the weakness of gravity in context, consider the classic example of the hydrogen atom: for the electromagnetic force between the proton and electron to be as small as the gravitational force they experience in the atom, they would have to be separated by about 2.5 million kilometres -- roughly six times the distance between the Earth and the Moon.
Expressed another way, in any gravitational experiment the 'cancellation' of the electromagnetic interaction between test masses must be at the level of roughly one part in 10 40 to leave any sensitivity to gravity -- not an easy task on short distance scales, as local charge inhomogeneities and magnetic impurities in the materials of the experiment quickly become important. A careful analysis of subtle systematic effects is crucial for any such measurement.
Elegant experiments such as that of Long et al .3have a broad appeal. Cosmologists and high-energy-physics theorists, for example, are interested in the short-range properties of gravity, and its theoretical aspects present at least as many difficulties as those encountered in the laboratory. Despite years of effort, it has not been possible to devise a quantum theory of gravity, even though most theoretical physicists believe that such a theory must exist to describe it in harmony with the other three forces. Because gravity is so weak, its quantum behaviour is usually predicted to become important only at inaccessible distances, at the scale of the 'Planck length' (10 -33 cm). It is discomforting to many theorists that there is such a large discrepancy between the characteristic energy scale associated with this distance (10 19 GeV) and that of the other three forces (10 3GeV; 1 GeV = 10 9electron volts). This is known as the 'hierarchy' or 'naturalness' problem.
Recently, string theories (so called because the fundamental particles are described as vibrations of one-dimensional strings) 2have become the leading candidates for the successful integration of quantum mechanics and gravity. Such models require the existence of extra spatial dimensions, in addition to the three dimensions in which we live. In particular, one class of string-inspired theories 7attempts to solve the hierarchy problem by suggesting that gravity is just as strong as the other interactions, but that its strength is diluted because it propagates through many spatial dimensions; the other fundamental forces are confined to our three-dimensional world, or 'brane'.
These extra dimensions would be curled up, or 'compactified', but could still be as large as a millimetre. If one were to look closely enough to begin to 'see' these large extra dimensions, gravity would behave as if it were propagating in a higher-dimensional world, and the inverse-square attraction would no longer be observed. In addition, different string-theoretical models predict as yet unseen fundamental particles that could mediate new interactions with strength comparable to that of gravity at sub-millimetre distances (the range of a possible new force would depend inversely on the particle's mass) 8. Evidence of these new forces could also appear in short-range tests of the inverse-square law. So searching for exotic gravitational behaviour at the laboratory scale, instead of at the Planck length, could yield the first direct evidence for string theory.
The experiment by Long et al .3is the latest search for deviations from the inverse-square law. Their results set the best constraints so far on any departure from that law between 10 and 100 µm, and have ruled out much --although not all -- of the remaining parameter space for new forces. So far, Newton is holding his ground. Although this first-generation apparatus could not have detected any new effects that have the same strength as gravity, the relatively high-frequency technique has advantages that may enable it eventually to probe shorter distances with higher sensitivity. For example, many sources of environmental noise, such as temperature and seismic fluctuations, have an intrinsically 'red' (or low-frequency) tendency; the high frequency of this experiment serves to suppress the effects of these disturbances.
The development of a promising technique to measure the unexplored properties of one of the fundamental forces in nature should always be welcomed, especially when the force in question is gravity. Having a 'gene pool' of experimental techniques involving scientists from many backgrounds contributes to the growth and refinement of the field as a whole. With advances in high- and low-frequency methods, we will be better able to understand and verify any surprises that gravity may hold.
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