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Gravity, also known as gravitation, is the best known of all basic physical forces.
After all, she ensures, for example, that precious red wine glasses and sticky jam sandwiches land unhappily on the floor.
But we also owe the force of gravity that the earth can move its orbit around the sun and that the moon remains bound to our planet.
The existence of gravity is so obvious that even politicians sometimes use this physical phenomenon when they want to illustrate the irrefutable nature of a scientific truth.
The British physicist Isaac Newton formulated the law of gravitation as early as the 17th century, according to which all material objects - depending on their distance and their masses - always attract.
According to an anecdote, he got the idea when an apple fell from a tree on his head in the garden.
Yes, it is the gravitational force of the planet earth that tugged at this apple and finally tore it down.
Although the effects of gravitational force are present everywhere in our everyday life, it is a comparatively weak force.
Two metal balls lying on a smooth table surface do not roll towards each other.
The gravitational force between them is simply too small for that.
They just stay where they are.
Such balls could, however, be set in motion with magnetic or electrical forces.
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In space, when it comes to the interaction of massive objects, to the gravitational force of stars, planets or even black holes, gravity is, as it were, the king of physical forces.
The measurement of gravitational forces between earthly objects, however, is very difficult because they are so tiny.
Nevertheless, some will remember that in physics class the force of attraction between two massive metal balls weighing a few kilograms was determined.
In this way, the so-called gravitational constant in Isaac Newton's law of gravity can be determined.
With a similar experimental set-up, physicists at the University of Vienna have now set a new world record.
In the journal "Nature" they report that they have succeeded in measuring the smallest gravitational force ever registered - between two gold spheres with a radius of only around one millimeter each.
The masses of the two balls were only about 90 milligrams, i.e. 90 thousandths of a gram.
One of the two gold balls on the glass pendulum (left) is brought closer to another gold ball (right).
Source: Tobias Westphal / dpa
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The two gold balls were connected to each other by a four centimeter long and half a millimeter thick glass rod.
Similar to a mobile, this dumbbell was hung in the middle on a wafer-thin fiberglass.
The diameter of this fiber was only a few micrometers, i.e. thousandths of a millimeter.
The researchers positioned another very close to one of these two spheres.
It exerted a gravitational force and thus caused a tiny twist of the dumbbell, which led to a so-called torsion in the suspension.
By periodically moving the third gold ball, the researchers were able to stimulate the torsion pendulum to vibrate.
Vibrations can be measured more easily than a linear deflection.
With the help of a laser beam, the team of physics around Tobias Westphal
Measure the oscillation of the gold ball mobile and use this to calculate the force of attraction between the two balls.
Within the error tolerance of this experiment, the gravitational constant determined by them agreed with the previous literature value.
Two physicists from the University of Vienna are working on the experimental setup.
The pendulum with the gold balls is in a high vacuum during the measurement.
Source: Barbara Mair / dpa
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What then is the gain in knowledge?
Is it just a sport to measure the smallest possible gravitational forces?
Of course, scientists are not free from the ambition to set records.
But there is actually more to it.
On the one hand, it is a legitimate and important question whether the law of gravity is exactly as valid for very small masses as it is for cosmic mass constellations.
In fact, there has been speculation for years whether Newton's law of gravity might have to be modified a bit to explain the mysterious dark matter in the universe.
Perhaps there is no special extra matter that has not yet been captured.
Perhaps the effects ascribed to dark matter can also be explained by an adapted law of gravity.
The physicists, however, have another, and probably even greater, goal in mind.
So far, scientists have not been able to combine gravitational force with quantum theory.
That means: In the world of particles there is still no theory that can also be used to describe gravitational forces.
So there is no quantum theory of gravity.
If it were possible to measure the gravitational forces of ever smaller objects, then gravity could possibly be understood in the context of quantum systems.
That would be a great scientific breakthrough.
The Viennese researchers want to make the spheres smaller and smaller until quantum effects also play a role. But until then it is still a long way. Theoretical considerations show that such effects can only be expected when the globules in the measuring apparatus are only a hundredth of a milligram in weight - that is, they have almost a thousand times less mass than the gold globules now used.