Using cold atoms in space to weigh the Himalayas

Kangchenjunga from Pelling, Sikkim (Image used for representation only)
| Photo Credit: glowform

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The amount of gravitational force you experience on the earth’s surface is uneven. This is because it depends on the amount of mass nearby: if there is more mass in one area, for example in the form of a mountain range, the force you’ll experience near that area will be higher than, say, in the middle of a city.

This said, the difference between one place and the next is too small for anything other than the most sensitive instruments to notice. One such instrument is the gravity gradiometer. Say you drop a ball from the top of a building. Newton’s second law states that the force acting on a body is equal to its mass multiplied by its acceleration (F = m · a). As the ball drops towards the ground, its acceleration can be calculated by dividing the force acting on the ball — which depends on the mass nearby — by its mass.

Similarly, a gravity gradiometer measures how much faster or slower a ball falls in one place compared to another. For example, an oil company can find out where a hydrocarbon deposit is located and how it is distributed underground by using a gravity gradiometer to understand the density of the ground at various depths and combining this information with what its scientists know about the properties of different types of rocks. Since hydrocarbons are less dense than rocks, a gravity gradiometer will reveal that the ball accelerates less when dropped above the location of the deposit.

In a March 14 paper in EPJ Quantum Technology, a team of NASA scientists proposed a novel idea: that an advanced quantum gravity gradiometer (QGG) could be placed onboard a satellite and launched into low-earth orbit. From that lofty perch, the instrument could study small changes in the earth’s distribution of water, ice, and rocks to inform studies of climate change as well as help countries improve national security.

In a QGG, atoms of a particular element (typically rubidium) are cooled to near absolute zero in a vacuum so that they start behaving like waves rather than particles, and are manipulated by lasers. In this state, they experience a phase shift that’s directly proportional to the strength of the gravitational force acting on them. The shift is extremely sensitive. So by using a pair of such setups say 1 m apart, a QGG can deduce a difference in acceleration lower than 10^-15 m/s2 across a distance of 1 m on the earth’s surface.

In other words, a clump of ultra-cold atoms and lasers in space can measure how much the Himalayas weigh.

According to the NASA team, such a QGG onboard a satellite could weigh around 125 kg, have a volume of 250 litres (like an oil drum), and consume 350 W (like an old Intel CPU).

In the paper, the team has proposed a technology demonstration to verify a QGG can be sent to space, followed by an operational mission. “We need to fly it so that we can figure out how well it will operate, and that will allow us to not only advance the quantum gravity gradiometer, but also quantum technology in general,” Ben Stray, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory and one of the team members, said in a statement.