Even if we don't feel it: the gravitational field is not equally strong everywhere on the earth's surface.
The thickness and composition of the earth's crust, the tides, varying ice masses and the groundwater level influence the earth's gravitational field - globally and locally.
With sensitive instruments, so-called gravimeters, geologists can measure such tiny fluctuations in the earth's gravitational field and deduce their causes.
You can explore the interior of the earth without having to drill any holes.
However, many sensors are only suitable for stationary applications.
Researchers from the University of Birmingham are now presenting a mobile gravimeter that was specially developed for field measurements of gravity in the journal "Nature".
It works with extremely cold atoms in free fall.
Manfred Lindinger
Editor in the department "Nature and Science".
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All gravimeters work on the same principle: they measure the force that the earth's gravitational field exerts on a test mass.
The simplest gravimeter is the spring balance, where gravity pulls on a mass suspended from a spring.
The instrument deflects when the force of gravity changes, for example due to tides or as a result of polar movements or earthquakes.
Superconducting gravimeters are currently among the most sensitive sensors.
Here a magnetic field keeps a small superconducting hollow sphere in limbo.
Forces that act on the sphere and change its position are compensated by a control system and the electrical signal required for this is registered.
Superconducting gravimeters, such as those used by the Federal Agency for Cartography and Geodesy, achieve a high level of measuring accuracy.
They can still detect gravity fluctuations of less than a billionth of the acceleration due to gravity.
However, the devices must be cooled with liquid helium.
They are therefore only suitable for stationary use, for example to measure the influence of the tides or other geophysical processes on the gravitational field.
The absolute strength of gravity is determined from the acceleration of objects in the gravitational field.
To do this, for example, a freely falling mirror is irradiated with laser light.
The reflected beam is superimposed on the oncoming original beam.
The fall distance of the mirror and from this the gravitational acceleration can be determined from the interference pattern.
However, such laser interferometers are less sensitive than superconducting sensors.
Gravity tugs at falling atoms
A slightly better resolution can be achieved if, like the researchers from Birmingham, you let a large number of quantum-physically superimposed atoms fall freely instead of using a mirror.
The particles, which have previously been cooled with laser beams, are catapulted upwards and divided into two packages of roughly the same size.
These are left to fall freely.
After the fall, the two atomic clouds are superimposed.
Because if suitably measured, the cold atoms behave like waves, and so an interference signal is also observed here, from which the value of the gravitational acceleration and thus the absolute value of gravity can be derived.
Because such quantum gravimeters are extremely sensitive to vibrations and stray magnetic fields and require a great deal of technical effort, they have only rarely been used outside of laboratories.
Three years ago, researchers from the University of California presented a mobile atom interferometer that they could use to measure the gravitational field on the hills around Berkeley.
The data allowed the mean density of the rock to be determined and even showed the effects of earthquakes that had occurred in South America during the measurements.
Each measurement lasted 15 minutes.
The aim of the British researchers led by Ben Stray is to create an atom interferometer that works reliably even under the harsher conditions of field use.
Their measuring device consists of a one and a half meter high vacuum tube in which freely falling rubidium atoms are superimposed.
According to the researchers, the 75-kilogram instrument, which is fixed to a mobile frame, is extremely robust and therefore also suitable for measurements in the city.
The instrument was tested on a street between two multi-story buildings.
The task was to explore a two by two meter tunnel at a depth of one meter.
The result: the quantum gravimeter was actually able to detect the cavity based on the gravity reduction it caused.
By placing the device in different places, the tunnel could be mapped.
The resolution was around 0.2 meters.
The sensor needed less than two minutes for one measuring point.
Larger areas should be able to be measured quickly.
Now the researchers have to show whether their gravimeter can look even deeper into the earth.
Then it could help to detect oil fields or mineral resources.