THE world is in free-fall. This seems obvious enough in a world in which billionaires have collectively got richer by $3.9 trillion during a pandemic, exceeding by $0.2 trn the global combined wage income lost by workers in the same time.

However, beyond the inability of governments to regulate capital, the Earth itself is falling towards the Sun, always “overshooting” but being pulled constantly down in a way that creates its orbit.

This is the effect of gravity, the force that keeps us stuck on Earth, the Earth spinning round the Sun, and the Sun moving through space as part of an enormous galaxy. 

Gravity was the first to be identified of the physical forces that we still recognise today (unlike earlier forces which were theorised but abandoned, like forces named by Empedocles as Love and Strife).

The tendency of objects to fall towards each other has a long and famous history in scientific discovery.

Galileo is credited in European history with the discovery that objects of different weights fall with the same acceleration towards Earth, provided they experience the same air resistance due to their shape.

Isaac Newton formulated an equation describing gravitational attraction in the 17th century. This equation describes the force of attraction between any two objects.

Applied to falling objects, it says that all objects fall towards the Earth at the same rate and that this rate depends on the mass of Earth.

The gravitational force between two objects is proportional to their masses. That is why astronauts are so much lighter on the Moon, although they are still gently pulled towards its centre by gravity.

This gravitational force acts on both the falling object and the attracting object, so as a feather falls to the ground by gravity, the Earth feels a force as the weight of the feather attracts the Earth towards the feather. This force is of course so tiny that we don’t see the Earth move towards the feather.

Gravity is extremely weak. In modern physics there are three other fundamental forces in the whole universe, which are responsible for the interactions binding atoms together and the behaviour of light and electric fields.

Gravity is the odd one out: it is  22 billion, billion, billion times weaker than the weakest of these three other forces.

The main reason that we notice it at all is the enormous masses that make up the stars and planets of the universe.

Despite the extreme weakness of gravity, these huge masses mean that the force is the main event on the planetary scale. Its radically different strength from the other forces also means that it can be easily distinguished from them.

Although we notice gravity at work on the biggest objects in space, gravity works on everything. It is the gravitational attraction of small particles in clouds of dust and gas that draws this material together to form stars.

Gravity, when left to act on dust floating in space, can bind atoms of gas together so tightly that it causes the nuclear fusion that powers our Sun.

Our ancient fascination with the stars and with falling objects led from Newton to the “modern” conception of gravity created by Einstein, which is now over a century old.

The extremely successful theory of “general relativity” has been shown to account for things that Newton’s simple law of attraction can’t.

Along with it came the new concept of space-time. In this new theory, masses bend space-time like a child standing on a trampoline.

Where the mass is centred, there is an increased curvature and as on a trampoline, any light objects will slide inexorably towards the stretched out lowest point. This image gives a good idea of how physicists now think about gravity.

Just as in the earlier theory, any mass therefore bends space-time, even if only to a tiny extent.

As you move your hands through space, they bend space-time around them.

If your hands were dense enough, nearby objects would visibly move towards them in curved space-time.

Unlike static electricity, this would not be selective: anything with a mass would accelerate towards your hands.

The theory works amazingly well to understand large gravity-dominated objects. Unfortunately, a problem arises at smaller scales. There is no good theory to understand how atoms interact with gravity and experiments are far beyond the reach of experimental physics.

However, this month, physicists in Austria reported that they have measured the gravitational pull of a tiny metal sphere. Not an atom, but far smaller than the gravitational field of anything that has been measured before.

The tiny ball-bearing made of gold is only 2mm wide and has  about the same mass as a ladybird (90mg).

The team used a scaled-down version of an experiment that was used originally to measure the force of gravity from the Earth.

The apparatus is a pendulum with two little weights on it that is allowed to hang downwards towards the Earth. A third mass is brought towards the hanging pendulum.

The amount of deflection from the pendulum’s normal position is measured to understand how much force is produced from the proximity of the new mass.

The biggest problem in this experiment is isolating the effect of gravity from the effects created by all the other forces.

Even forces we don’t feel will ruin the experiment: imperceptible vibrations from roads and weather, movement of the air, or electric fields caused by charging of the metal spheres.

The team took precautions against all of these sources. They even did the experiments at night over the Christmas holidays so as to minimise the amount of traffic noise.

They used a Faraday cage — a metal screen that shields the experiment from electromagnetic forces (you can’t use a mobile phone in a Faraday cage).

The experiment is so sensitive that gravitational noise causes a problem too — for example, someone moving in the lab or a bus going past outside.

This noise has to be filtered out and the measurement repeated many times.

The deflection in movement was extremely small and measured by bouncing a laser off the pendulum to measure its position. 

The weight of a small ball-bearing is far from that of an atom and in fact it seems unlikely that the experiment can be scaled down to the most important and tiniest scale — although the researchers have set their sights on a smaller experiment.

However, evidence of a gravitational field around this tiny ball is exciting because it’s a demonstration of the real existence of forces created by all material, no matter how tiny.