On the first day of skating, my daughter told me that the most important thing is to learn how to fall. With my professional deformity, I thought: See, skating is similar to physics, where the most important and complicated thing is learning how things fall. And the key to our existence here, to the existence of our planet, our sun, our galaxy, is that part of the matter that makes up the universe has “fallen” to a certain place. I put this verb in quotes because the concept of falling is not too simplistic on a cosmic scale. I explain it even though this is one of the great questions that humanity has been asking itself for millennia and that astrophysicists are asking themselves today, and we still (I’m optimistic) don’t understand it well. The subject is so complicated that it will give me several weeks, and not even then. I start today by talking about why things fall?
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Let’s start with the most mundane, which may seem simplest to us: why do things fall to earth? Aristotle said: “Everything tends towards the natural and a stone falls to the ground because they are of a similar nature.” They will say millennia later: light, because of its weight, and that’s what the RAE says, falling is “moving from top to bottom by the action of one’s own weight.” But if they know a little more about physics, they’ll say: It’s falling because of the effects of gravity or the gravitational force associated with the mass of the Earth. All three answers are correct to some degree, although they do not fully describe reality, and indeed the most advanced physical explanation today goes beyond these concepts of nature, weight and force.
But I don’t want to stop here, never really, because I want to see falling things stop. What I want now is to go forward in time and ask myself what happens after something starts to fall. Every body increases its speed under the effect of gravitational acceleration. Aside from the effects of air resistance, which must slow objects down (in different ways depending on their shape), a body in what is known as free fall always moves faster. Let’s drop that for a moment and move on to a different concept.
The history of the universe is like a novel by George RR Martin, an eternal song of ice and fire (to this day). Only when there are icy gas clouds (meaning very cold, with temperatures of -200 degrees Celsius or less) with hydrogen can stars form, which are super-hot structures compared to this primordial gas. The stars die and their material, which contains elements such as oxygen, phosphorus or iron synthesized within them, cools down again and is in fact surrounded by a cosmos that has an average temperature of about -270 degrees Celsius absolute zero (less than 3 degrees above). These elements re-form cold clouds, which warm up again, creating planets with temperatures we consider benign for our lives. Hot and cold, concepts we refer to when we talk about temperature.
Temperature is actually a measure of the speed of the air molecules in our atmosphere, or the atoms in the gas clouds that make up stars. And here we present what we needed to continue our story that things fall. Things fall, they go faster and faster, so their temperature rises. And when the temperature of a gas increases, how can cold clouds form? Without cold clouds, there are no areas where matter becomes dense, which is necessary to form stars or planets.
Returning to Earth, the faller gains speed – they say kinetic energy – and the end result, whether it knows how the skater is falling or not, is to stop its motion with an impact on the planet’s surface. If its energy is not high enough, the result of the collision will be that all the energy it carried will be converted into deformation, vibration (and therefore sound), heat… That is, into energy transfer to the atoms of the ground and the falling object that they move faster, so much so that macroscopically they may no longer count as solids, but instead become a liquid (where the atoms or molecules move more) or even a gas.
Once we understand what it means to fall to earth, let’s apply it to the universe. In order for stars to form, and with them planets and life, there must be cold clouds where the density is as high as we see on the sun or our planet. For these clouds to form, the material has to fall to a specific location because the universe as a whole is much less dense than the clouds that form stars. Imagine the atoms falling due to the effects of gravity, although it is important to note that there is no up or down to help understand the word “fall”. But what is gravity connected to? And every time the atoms fall, they have a higher speed and a higher temperature. So how do they cool to form the clouds that form stars? Because there is no planetary surface for them to stand on. There are more questions: Does all matter fall equally into these clouds to form stars, or are there things that fall faster? Does matter fall at stellar scales in the same way as at larger scales, similar in size to galaxies? As you can see, many more questions than answers, I have already warned that the subject is the most complicated in the world, in fact some of us in the universe devote our lives trying to understand it.
Cosmic Void is a section in which our knowledge of the universe is presented qualitatively and quantitatively. It aims to explain how important it is to understand the cosmos not only from a scientific point of view, but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is mostly empty, with less than one atom per cubic metre, although paradoxically there are trillions of atoms per cubic meter in our environment, inviting us to wonder about our existence and to contemplate the presence of life in the universe. The section consists of Pablo G. Perez GonzalezResearchers at the Center for Astrobiology and Eva VillaverResearchers at the Center for Astrobiology.
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