What Would Happen If Gravity Vanished?

[Un article de The Conversation écrit par Hervé Caps – Professeur de physique, directeur du musée des sciences, Université de Liège]

For astronauts, this dream is almost reality, thanks to the International Space Station, to which Thomas Pesquet, Megan McArthur, Shane Kimbrough and Akihiko Hoshide are taking off this Thursday, April 22, 2021. In this spaceship, everything seems to float, as if finally freed from its weight.

But, in fact, does the earth's gravity have the sole effect of pinning us to the ground?

To answer this question, it should be noted that gravitational attraction acts on the mass of objects, whatever they may be. If we watch a marble fall through the air, we must imagine that each little piece of the marble is being drawn toward the center of the Earth. The gravitational force applies to the entire ball, to its volume. It acts in the same way on the gases making up our surrounding air, thus creating our protective atmosphere. Without gravity, no atmosphere, and probably no life.

Let's approach things from a physics perspective. The movement of each object (we speak of a body) depends on the forces which act on it. As it is imposed on any body having mass, the force of gravity is found in many, if not all, phenomena in our daily lives. Removing this force would amount to inhibiting the phenomena it causes. We have already mentioned the existence of our atmosphere. The same will be true of Archimedes' thrust. Does it exist in space?

Due to gravity, the pressure in a fluid (air, water) increases with depth. Therefore, if we immerse an object in water, the pressure it will experience below it will be greater than that above it. This difference causes the object to find itself pushed upwards. If its density is lower than that of water, this Archimedes' push will have the effect of making it rise to the surface of the water. It floats. In the absence of gravity, no more buoyancy… and no more sinking objects either! No more hot air masses rising into the colder air and with them, no more hot air balloons, no more heating with radiators, no more combustion (candles, fire, etc.) maintained by the renewal of the surrounding air constantly heated, no more water boiling letting gas bubbles escape to the surface, no more ocean currents, no more of that.

All of these assumptions, and many more, are the subject of scientific experiments. The goal is to determine the role played by gravity in this or that phenomenon. In these experiments, scientists see gravity as one force among others, which can be varied: a bit like pushing more or less hard on an object.

The problem is that it is impossible to free ourselves from gravity. Several means have therefore been created to simulate its absence: sounding rockets, free-fall towers, parabolic flights, the International Space Station (ISS). In all these experimental platforms, the objective is to “drop” the experiment, including the laboratory, in order to cancel out the weight of the whole. The duration during which this situation of apparent weightlessness persists directly depends on the time during which this “fall” can be maintained: from 10 seconds in a free fall tower, to several months in the ISS.

What experiments can be carried out in weightlessness?

Weightlessness allows you to study objects by making them float in the air without touching them. This is particularly suitable for cases where the object in question cannot be touched, because it is charged with electricity for example.

Just like gravity, the electric force acts on the volume of bodies. For electrons, which are very light, it dominates gravity. On the other hand, for larger objects like water drops, this is no longer the case. However, electrically charged drops are found both in industry (metal and paint sprays) and in fundamental research (gas from electrically charged drops).

On a daily basis, it is in the clouds that we find drops of water charged with electricity. This electricity is the cause of lightning and lightning. However, the mechanism by which the drops charge as well as the interactions they undergo (collisions, mergers, breakages, etc.) are relatively poorly understood. By carrying out experiments in weightlessness, it becomes possible to make drops interact and observe their dynamics for several seconds, without touching them and without them being disturbed. It is also possible to study the influence of electric charge on the size of raindrops.

In certain situations, it is useful to carry out experiments in weightlessness in order to highlight a force of less importance than gravity.

Weightlessness to reveal capillarity

With its action on the entire volume of bodies, gravity acts over long distances: the Earth is attracted by the Sun, although it is very distant. On the contrary, the field of action of the force which is responsible for the spherical shape of raindrops is limited to the surface of liquids. This force is called surface tension. It only appears at the boundary between two fluids: air and water, for example. We can notice its existence in certain specific situations. For example, you have to blow to produce a soap bubble. The little energy that this effort cost us was used to counteract the surface tension.

For most objects, gravity dominates surface tension. To reverse the trend, we must consider small liquid objects: drops of water, for example. In this case, the surface tension is able to impose the spherical shape on the drop, even if it is placed on a table. Unfortunately, if the volume of the drop increases a little (around 10 mm³ are enough), gravity takes over and the drop flattens, finally becoming a puddle.

In order to take maximum advantage of the effect of surface tension, several studies in weightlessness focus on soap foams. With their hundreds of bubbles, the foams have a large liquid surface and maximize the effect of surface tension. Under the influence of gravity, the liquid in the foam tends to sink and the foam dries up, eventually dying. In weightlessness, this phenomenon disappears and it is possible to study wet foams. The characteristics (stability, mechanical resistance, etc.) of these wet foams allow us to better understand the physico-chemistry of these particular materials. The results of this research provide useful information in many industrial fields (for example for the development of light and resistant materials) and in fundamental science (confined fluid flows).

Space exploration, less than 400 kilometers from the ground

By trying to obscure the action of gravity, research into microgravity and weightlessness makes space conquest a means, not a goal. They complement programs aimed at understanding the immensity of the universe, and offer the opportunity to approach on-board flights with better knowledge of the environment in which astronauts would be immersed. All these results are however obtained while remaining, ultimately, very close to the surface of the Earth: a parabolic flight takes place at around 10 kilometers of altitude and the ISS is only around “only” 400 kilometers from the Earth. Earth.