universe or freedom
Image of the Sun taken by the 'Solar Orbiter' spacecraft at perihelion.
Image of the Sun taken by the ‘Solar Orbiter’ spacecraft at perihelion.Sebastian Carrasco (Europa Press)

The type and degree of freedom, which says a lot about a system, seems to be measured in Madrid in terms of not meeting your ex. That idea about what “freedom” means can be transferred to the universe, to get some use out of it.

Not meeting your ex in physics is expressed by what is known as the mean free run or, rather, mean free time. The mean free time of a particle, statistically speaking, is the time interval between two interactions with some other particle (same as it or different). According to the president of Madrid, the average free time of 2 ex-boyfriends in Madrid is infinite, or at least greater than the life expectancy of the aforementioned. In short, they never meet or, rather, the probability that they will is very low, with what happens it can happen, but it is not normal.

What does its mean free path depend on? Well, going back to our analogy where the birds visit the psychiatrist and the stars forget to go out, as the poet said, meeting an ex depends on how the 2 subjects move, on the density of “exes” you have around and what you consider “meeting” with that person. Trying to be clearer, it is not the same that your speed when crossing Madrid is 30 km/h, and you always go from east to west, or that your exes spend the day at home, that you spend the whole day wandering around the street. Also the density of ex (now well written in plural) matters and being a typical person is not the same as being Julio Iglesias. Nor is it the same to be myopic and run into an ex a few meters away, that you don’t even know about, than to go looking for a topic in every corner, or to crash than to cross paths on the opposite sidewalk. In short, you have to define the distance, or the area around you, in which if your ex gets involved, you will have to consider the president’s words false (and I will leave my comment there). In physics, the former is described by the speed (with its direction and direction) of the particles, which depends, for example, on temperature and pressure. The second translates into the density of particles with which you can interact. And the third thing is called the effective section, which can be considered as the area around a particle that you have to pass through to consider that you have interacted (and it is noticeable in something), and this can depend on the types of particles that we consider , of its energy, etc…

The path or mean free time is a physical property of a system that is applied in countless occasions in astrophysics. For example, inside the Sun, photons are constantly being produced as hydrogen fuses at the rate of 4 atoms to form one helium. It is estimated that every second 620 million tons of hydrogen are converted into 616 million tons of helium. To get an idea, that is approximately 10 times the amount of crude that Spain imports but not in a second but in a year! Some of that 4 million-ton mass difference is released as photons, which travel at the speed of light in random directions. But the density in the interior of the Sun, which is not rocket science either but not bad, about 150 grams per cubic centimeter, about 7 times denser than the densest we have on Earth but tens of billions of times less dense than a neutron star, it is tall enough for a photon to travel no more than a centimeter before interacting with an atomic nucleus (hydrogen or helium, above all) or an electron. The interacting thing means (interpreted in a classical way) that it passes through an area of ​​about 10-²¹ m², compared to the 3.14 m² (the area around us defined by a meter of distance) that our ex has to pass through for almost collide with us. The centimeter of mean free path translates to the photon taking no more than 30 picoseconds (0.00000000003 seconds) to meet its “ex” (or someone who looks like their ex, a proton or an electron, come on). That is the mean free time of a photon inside the Sun, which could also be interpreted (with quite a few weak points) as meaning that a photon takes on the order of thousands if not hundreds of thousands of years to travel from the interior of the Sun to its surface, to then travel freely (with a tremendously higher mean free path) towards us.

In contrast, the neutrinos that are also created in the nuclear reactions of the Sun’s core are more like ex-boyfriends in Madrid, they must travel on the order of… a trillion centimeters to interact with someone in an environment as dense as the interior of the Sun! ! That is a one with 18 zeros or about a quarter of the distance to the closest star to the Sun. And it is that neutrinos interact with particles like the neutron with an effective section billions of times smaller than photons with electrons. As the interstellar and intergalactic space is quadrillion times less dense than that solar nucleus, we are not going to get lost among so many zeros and we will say that the result is that it is very difficult for a neutrino to meet an ex, or anyone, and “stop” to interact with any “person” anywhere in the universe (there are exceptions, but there are several other stories). They are not very Madrilenian, they don’t drink beers with anyone, but they are very free in a certain sense.

We conclude: in order not to meet your ex in Madrid you have to have a very large average free path. That is achieved if you have very few exes to meet, as happens to photons in the intergalactic and interstellar medium. Or if you decide with your freedom or lack of it not to move much and live without “pressure” (physically speaking, the higher the pressure, the lower the mean free path), as would be a universe expanding forever. Or if your definition of meeting is very restrictive, only colliding with it counts, as happens to neutrinos with everything, both the neutrinos that are created in the Sun and the (many more) that were created a few seconds after the Big Bang, those before did pay taxes and interact with everything, but that’s another story.

cosmic void it is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding 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, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up Pablo G. Perez Gonzalezresearcher at the Center for Astrobiology; Patricia Sanchez Blazquez, full professor at the Complutense University of Madrid (UCM); Y Eve Villaverresearcher at the Center for Astrobiology.

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