From apples to atoms: measuring gravity with ultra cold atomic test masses

Abstract

Over 250 years ago Sir Isaac Newton, inspired by an apple falling from a tree in his orchard (Stuckeley 1752), made the mental leap to conjecture that the same force that caused this apple to fall also held the Moon to the Earth. This stimulated him to develop his Law of Gravitation, and led to the principle that all objects fall with the same acceleration irrespective of their mass, as observed by Galileo Galilei. Over 250 years ago, these scientists understood gravity as well as many people do today. In reality, we still measure gravity by dropping a proverbial apple – a falling test mass whose trajectory we measure through space–time. However, developments over the past two centuries have led to a vast improvement in our measurement precision. With the advent of the optical laser and atom interferometers over the past 50 years, we have far superior rulers, and far superior clocks with which to make such a measurement. Mankind's most precise instruments are those that measure space and time. At the heart of these measurement devices is the phenomenon of wave interference. For example, the most precise rulers to date are optical interferometers, built for the detection of gravitational waves using very long baseline interferometers such as the Laser Interferometer Gravitational Wave Observatory (LIGO). This device measures distance with a sensitivity up to 1 part in 10 24 (The LIGO Scientific Collaboration and The Virgo Collaboration 2012). On the other hand, the most precise keeper of time is an atomic clock. With its ceaseless ringing, a caesium atom is an oscillator that defines the International System of Units (SI) second at the level of 1 part in 10 16 (Heavner et al. 2005). Precise measurement of the absorption of radiation at 9 192 631 770 Hz by caesium again relies on interference, in this case the interference of matter-waves in an atom interferometer.