A dream. Always the same. In the imagination, with Icarus or the superheroes of the fantastic series. In real life, for pole vaulters, airplane pilots or spacemen. This dream, to fly, to get away from the force of gravity that relentlessly binds us to the ground, keeps us confined to the surface of the Earth. To be free from gravity, what freedom it would be!
For astronauts, this dream is almost a reality, thanks to the International Space Station.
Does Earth’s gravity just immobilize us?
To answer this question, it must be taken into account that gravitational attraction acts on the mass of objects, whatever they may be. If we see a marble fall in the air, we have to imagine that each fraction of the marble is attracted towards the center of the Earth. The gravitational force is applied to the entire marble, at its volume. It acts in the same way on the gases that make up the surrounding air, creating the protective atmosphere. Without gravity, there is no atmosphere, and there is probably no life.
Let’s look at it from the perspective of physics. The motion of any object (which we call the body) depends on the forces acting on it. Since it is imposed on any body with mass, the force of gravity is found in many, if not all, of our everyday phenomena. To eliminate this force would be to inhibit the phenomenon that originates it. We have already mentioned the existence of our atmosphere. The same will happen with the thrust of Archimedes’ theorem. Does it exist in space?
Due to gravity, the pressure in a fluid (air, water) increases with depth. Therefore, if an object is submerged in water, the pressure below it will be greater than the pressure above it. This difference causes the object to be pushed upwards. If its density is less than that of water, this buoyancy will cause it to rise to the surface. Fleet. In the absence of gravity, the buoyancy is over … and the objects do not sink either! The hot air masses that rise to the colder air are over, and with them the hot air balloons are over, the heating with radiators is over, the combustion (candles, fire, etc.) maintained by the renewal of the air is over. constantly heated surrounding area, the boiling of the water stopped letting gas bubbles escape to the surface, the ocean currents stopped, all that was finished.
All of these assumptions, and many others, are the subject of scientific experimentation. The objective is to determine the role that gravity plays in this or that phenomenon. In these experiments, scientists see gravity as one force among others, which we can modify: a bit like pushing an object more or less hard.
The problem is that it is impossible to get rid of gravity . For this reason, various means have been created to simulate their absence: sounding rockets, free-fall towers, parabolic flights, the International Space Station (ISS). In all these experimental platforms, the goal is to “drop” the experiment, including the laboratory, to cancel the weight of the assembly. The duration of this situation of apparent weightlessness depends directly 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 performed in weightlessness?
Weightlessness makes it possible to study objects by making them float in the air without touching them. This is especially suitable for cases where the object in question cannot be touched, because it is charged with electricity, for example.
Like gravity, the electrical force affects the volume of bodies. For electrons, which are very light, gravity dominates. However, for larger objects, such as water droplets, this is no longer the case. Today, we can find electrically charged droplets both in industry (metal and paint sprays) and in fundamental research (electrically charged droplet gases).
In everyday life, clouds house droplets of water charged with electricity. This electricity is the source of the rays. However, the mechanism by which the drops charge and the interactions they undergo (collisions, mergers, breaks…) are relatively little known. By conducting experiments in weightlessness, it is possible to make the 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 the electric charge on the size of the raindrops.
In certain situations, it is useful to perform experiments in weightlessness to highlight a force less important than gravity.
Weightlessness to reveal capillarity
With its action on the entire volume of bodies, gravity acts at great distances: the Earth is attracted by the Sun, which is very far away. On the contrary, the field of action of the force responsible for the spherical shape of raindrops is limited to the surface of liquids. This force is called surface tension. It only occurs at the boundary between two fluids: air and water, for example. We can verify its existence in some specific situations. For example, we need to blow to produce a soap bubble. The little energy that this effort costs us serves to counteract the surface tension.
For most objects, gravity dominates surface tension. To reverse this trend, we must consider small liquid objects: water droplets, for example. In this case, the surface tension is capable of imposing the spherical shape of the drop, even if it is placed on a table. Unfortunately, if the volume of the drop increases a little (about 10 mm³ is sufficient), gravity takes over again and the drop flattens, eventually turning into a puddle.
To take advantage of the effect of surface tension, several studies on weightlessness focus on soap suds. With their hundreds of bubbles, foams have a large liquid surface area and maximize the effect of surface tension. Due to the effect 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 a better understanding of the physicochemistry of these particular materials. The results of this research provide useful information in many industrial fields (for example, for the development of lightweight and resistant materials) and in fundamental science (confined fluid flows).
Space exploration less than 400 kilometers from Earth
By trying to hide the action of gravity, research in microgravity and weightlessness makes the conquest of space a means, not an end. It complements programs aimed at understanding the vastness of the universe, and offers the opportunity to approach manned flights with a better understanding of the environment in which the astronauts will immerse themselves. All these results are obtained, therefore, staying very close to the surface of the Earth: a parabolic flight takes place at a height of about 10 kilometers and the ISS is only 400 kilometers from the Earth.
Hervé Caps, Professor of Physics, Director of Science Museum, Université de Liège
This article was originally published on The Conversation. Read the original.
