Earth's gravity is essential to life since it “holds back” the gases that make up the atmosphere, according to our partner The Conversation.
However, human beings have always dreamed of freeing themselves from this force of gravity.
The analysis of this phenomenon was carried out by Hervé Caps, professor of physics and director of the Science Museum of the University of Liège (Belgium).
A dream.
Always the same.
In the imagination, with Icarus or the superheroes of fantastic series.
In real life, for pole vaulters, airplane pilots or astronauts.
This dream, to fly, to extract oneself from this force of gravity which, relentlessly, sticks us to the ground, keeps us confined to the surface of the Earth.
Free oneself from gravity, what freedom that would be!
Thomas Pesquet “floating” in the ISS © Wikimedia / NASA CC BY-SA
For astronauts, this dream is almost reality, thanks to the International Space Station that Thomas Pesquet, Megan McArthur, Shane Kimbrough and Akihiko Hoshide joined on April 24, 2021. In this spacecraft, everything seems to float, as finally freed of its weight .
But, by the way, does Earth's gravity have the only effect of pushing us down to the ground?
To answer this question, it should be noted that gravity attraction acts on the mass of objects, whatever they are. If we watch a marble drop in the air, we have to imagine that every little piece of the marble is attracted towards the center of the Earth. The gravitational force is applied on the whole of the ball, on its volume. It acts in the same way on the gases composing our surrounding air, thus creating our protective atmosphere. Without gravity, no atmosphere, and probably no life.
Let's look at it from a physics perspective.
The movement of each object (we speak of a body) depends on the forces acting on it.
As it imposes itself on any body having a mass, the force of gravity is found in very many phenomena of our daily life, if not in all.
To withdraw this force would amount to inhibiting the phenomena of which it is at the origin.
We have already mentioned the existence of our atmosphere.
It will be just as much for the push of Archimedes.
Does it exist in space?
Experiment on the interaction between electrically charged drops, during a parabolic flight © ESA, CC BY-SA / A. Le Floc'h (via The Conversation)
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. This difference causes the object to be pushed upwards. If its density is lower than that of water, this Archimedean thrust will cause it to rise to the surface of the water. He floats. In the absence of gravity, no more flotation… and no more objects that sink either! No more hot air masses rising in the colder air and with them, no more hot air balloons, no more heating with radiators, no more combustion (candle, 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, none 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 a particular 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 overcome 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 the weight of the whole.
The length of time this apparent weightlessness situation lasts depends directly on the time during which this "fall" can be maintained: from 10 seconds in a free-falling tower, to several months in the ISS.
What experiences to carry out in zero gravity?
Weightlessness makes it possible 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 precisely because it is charged with electricity, for example.
Like gravity, 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 in industry (metal and paint sprays) as well as in fundamental research (electrically charged drop gas).
Foams behave much differently when there is no longer gravity to deform them. Astronaut Frank De Winne poses in the ISS with the “FOAM Stability” experiment © ESA, CC BY-SA (via The Conversation)
In everyday life, it is in the clouds that we find drops of water charged with electricity.
This electricity is the source of lightning and lightning.
However, the mechanism by which the drops charge as well as the interactions they undergo (collisions, mergers, breaks, etc.) is relatively poorly understood.
By performing experiments in zero gravity, it becomes possible to make drops interact and observe their dynamics for several seconds, without touching them and without 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 demonstrate 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, however 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 manifests itself at the border 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 costs us has served to counteract the surface tension.
Under the action of gravity, the foams dry up. The top bubbles become fine and angular; only the bubbles close to the water remain spherical © D. Terwagne / University of Liège (via The Conversation)
For most objects, gravity dominates the surface tension.
To reverse the trend, 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 deposited on a table.
Unfortunately, if the volume of the drop increases a little (about 10 mm3 is sufficient), gravity takes over and the drop flattens out, eventually becoming a puddle.
In order to take maximum advantage of the effect of surface tension, several weightless studies have been carried out on soap foams.
With their hundreds of bubbles, foams have a large liquid surface and maximize the effect of surface tension.
Under the effect of gravity, the liquid in the foam tends to descend and the foam dries up, eventually dying.
Our "SPACE" file
In weightlessness, this phenomenon disappears and it is possible to study wet foams.
The characteristics (stability, mechanical resistance, etc.) of these wet foams make it possible 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 hide the action of gravity, research in microgravity and weightlessness makes the conquest of space a means, and not a goal.
They complement the programs aimed at understanding the vastness of the universe, and offer the opportunity to approach on-board flights with a better knowledge of the environment in which the astronauts would be immersed.
All these results are however obtained while remaining, finally, very close to the surface of the Earth: a parabolic flight takes place at approximately 10 kilometers of altitude and the ISS is only at approximately "only" 400 kilometers from the sea. Earth.
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This analysis was written by Hervé Caps, professor of physics and director of the Science Museum of the University of Liège (Belgium).
The original article was published on The Conversation website.
Declaration of interests
Hervé Caps works as a physics professor at the University of Liège (Belgium). It receives funding from ESA, ULiège and Belspo. He is responsible for "La Maison de la Science", the science museum at ULiège.
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