measure for measure The slightness of Michel Van Camp and Olivier de Viron are attracted to the fluctuations in the Earth’s gravitational pull.

hen asked to give his age, the 1 by measuring how much force is required French humourist Alphonse Allais 0 to do so. Spring relative gravimeters are an ) Wfamously replied “I cannot, it −2 −1 example; they are portable devices that report changes all the time”. This is all the more true gravity changes with respect to a reference.

(nm s −2 for g, the gravitational felt near the g In superconducting gravimeters, a hundred Earth’s surface, which varies in both time and −3 times more precise and much more stable –2 −4 space (between 9.78 and 9.83 m s ). These 0 10 20 30 40 50 than spring instruments, a superconducting 6 variations are a result of the gravitational Time (min) sphere is made to levitate in a magnetic field , attraction by the mass inside and around resulting in a highly sensitive instrument the Earth, centrifugal effects due to the ) enables a determination of with a drift of a few 10–9g per year — spring Earth’s rotation, the distance to the centre g by measuring the pendulum’s period of instruments typically have a drift of 10–9–10–7g of the Earth, geographical latitude, the mass oscillation1. Realizing a free-fall experiment per day. Superconducting gravimeters are distribution in the Earth’s interior, and the for determining g with sufficient precision — also capable of measuring variations in g with relative positions of the Earth, the Moon, the as physics students may experience — was a precision that is a hundred times better Sun and the planets. Precise determination practically impossible because it requires very than that of absolute gravimeters for periods of g is essential in several scientific domains. accurate measurements of both distance and shorter than a day. In contrast to spring As geophysical processes are associated with time. In 1971, ballistic absolute gravimeters gravimeters, the most common mode of deformations of the solid Earth and mass based on optical interferometry became the operation of superconducting gravimeters is distribution changes in its climate system, new standard1; today, these enable an accuracy continuous at a fixed location. The levels of monitoring local variations in gravity provides for g of 10–8 m s–2 or, as gravimetrists would precision achievable today — a few 10–11g — information on tectonic deformations, past say, 1 µGal. (The , named after Galilei, is are such that the effect of rain showers or the and present ice-mass changes, , the the cgs unit of acceleration.) The working motion of a person operating the instrument dynamics of the oceans and the hydrosphere principle lies in repeatedly measuring the are detectable (as seen in the plot after about and the structure of the Earth1. time and distance along a 20–30 cm path 22 minutes when the operator sits above the Gravity surveys are also useful in travelled by a freely falling mass in a gravimeter for 5 minutes). geology and mineral exploration, and are chamber. In the 1990s cold-atom gravimeters When realized that a falling apple indispensable for the determination of the were developed4; one of their advantages is obeys the same law as the Moon, could he — the reference equipotential surface that they allow for continuous measurements have imagined that atoms play the same game4, of the Earth’s gravity field — and hence of because they have no moving parts and, now verified with an accuracy of 10–9g? ❐ altitude. In volcanology, gravity measurements consequently, do not experience wear. Atom complement deformation monitoring to gravimeters are now being miniaturized to MICHEL VAN CAMP is at the - discriminate between intrusions of lava, facilitate the portability of precise absolute Gravimetry Service, Royal Observatory water or gas. In metrology, g is key in the gravimeters, useful for field studies. of Belgium, Avenue Circulaire 3, new realization of the with the Watt Since the 1970s, the motion of satellites has 1180 Uccle, Belgium. balance experiment weighing electrical and been used to compute global models of the e-mail: [email protected] mechanical powers2. Consequently, g plays a mean Earth’s gravitational field5. In 2002, the major role in state-of-the-art measurements Gravity Recovery and Climate Experiment OLIVIER DE VIRON is at Littoral, of Planck’s constant. Finally, gravimeters (GRACE) twin satellites started providing Environnement et Sociétés, UMR7266, have also been useful in investigations of free information on the temporal variation of Université de La Rochelle and CNRS, 2 rue oscillations of the Earth (global vibration gravity, with a precision equivalent to a few Olympe de Gouges, 17000 La Rochelle, France. modes excited by major earthquakes) through per square on spatial scales e-mail: [email protected] the monitoring of inertial acceleration3. of ~400 km at a sampling rate of once per Measuring local gravity is conceptually month. The temporally constant part of the References simple: one drops an object and measures the gravity field is now known at a level of 1 part 1. Niebauer, T. in 2nd edn (ed. Herring, T. A.) Ch. 4, 43–64 free-fall time over a given distance. However, in 106 with a spatial resolution that can reach (Treatise on 3, Elsevier, 2015). 2. Stock, M. Metrologia 50, R1 (2013). before the 1960s, the best way of measuring 200 m in the best-covered areas by combining 3. Agnew, D. et al. EOS 67, 203–211 (1986). g (with an accuracy of one part in 105) was data from satellite and ground measurements. 4. Peters A. et al. Nature 400, 849–852 (1999). by means of Kater’s reversible pendulum. In relative gravimeters, which only 5. Beutler, G., Drinkwater, M. R., Rummel, R. & von Steiger, R. (eds) Earth Gravity Field from Space—from Sensors to Earth Sciences First introduced in 1817, this instrument measure changes in g, a mass is prevented (Springer, 2003). (essentially a compound two-pivot-point from falling by holding it and g is determined 6. Goodkind, J. M. Rev. Sci. Instrum. 70, 4131–4152 (1999).

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