How black holes explain traffic pileups – 03/20/2024 – Fundamental Science

In England in 1783, naturalist philosopher John Michell would become the first person to propose the existence of celestial bodies known today as “black holes”, in an article published in the oldest scientific journal in the world, Philosophical Transactions of the Royal Society of Londonfounded in 1665. At the time, already considered a great scientist, he imagined that, just as there were visible stars in the sky, there could be stars so compact and dense that their gravity would prevent light from escaping.

Michell was one of the main people responsible for trying to understand the behavior of these “dark stars”, and even though at that time the necessary technology for their direct observation did not exist, he postulated that the universe was full of them, a hypothesis that would later be proven by astronomers.

Over the almost 250 years that have passed since its publication, many mysteries about the functioning of this celestial body have already been clarified. However, photographic visualization, for example, was only made possible in April 2019, when the EHT project (Event Horizon Telescope) managed to capture the first image of a black hole.

Thiago Gonçalves, astronomer and director of the Valongo Observatory/UFRJ, says that the particularities of black holes are not yet fully understood. “The image that was produced in 2019 and other data that were observed help us understand how it works, but there are some details that physics does not understand. For example, it is imagined that there is a point of infinite density in them, a characteristic that the dynamics can’t explain.”

Because of this difficulty, the adoption of analogies is a strategy to build associations between astronomical phenomena and common everyday situations. In 2022, physicist George Matsas, together with his then master’s student at the Institute of Theoretical Physics (IFT) at Unesp, Luanna K. de Souza, proposed something along these lines, a hypothesis that associates traffic pileups with black holes. In the article “Black-hole analog in vehicular traffic“published in American Journal of Physicsthe researchers compared the origin of the pileups to a particular characteristic of black holes, using traditional general relativity methods.

Imagine the following situation: different cars travel in a straight line along a road, one behind the other. The vehicles then enter a region of fog. Due to low visibility, the first driver in line is forced to slow down. By doing this, it transmits the message to reduce speed to the car behind, as the brake light is activated. Based on this information, the car behind also reduces speed and immediately applies the brake, when the third car also receives the message. And so this deceleration starts to occur successively along the line of cars.

The scenario changes when one of the vehicles activates the brake light after having already entered the fog region, preventing the spread of information to greater distances, which ends up causing the collision with the car behind. The time for the brake light to activate between one car and another will be shorter and shorter, resulting in the beginning of a pileup.

Just like in car pileups, in black holes there is a physical delimitation that prevents the message from evolving, an aspect observed through the study of the so-called “information non-return zone” — the event horizon, a region in which intense gravity generates an infinite curvature of space-time, preventing the application of conventional laws of physics. In traffic, the delay in light propagation is conditioned by a region of fog, while in the black hole what exists is the so-called Schwarzschild radius — known as the black hole radius.

This radius functions as a kind of “border”, or geographic limit, for the propagation of any and all matter. This happens because, starting from the Schwarschild radius, the gravitational force is too great for the escape velocity — the lowest speed required that an object needs to reach to be able to “escape” the gravitational attraction exerted by celestial bodies — to be greater than the speed of light. “For a stationary black hole, the Schwarzschild radius is the distance to the event horizon,” explains Gonçalves.

Throughout the history of scientific and philosophical communication, scientists and science communicators have sought ways to translate abstract concepts. “Einstein, for example, used mental experiments, the ‘Gedankenexperiments’, precisely to try to understand a ‘non-experiable’ problem, adds the astronomer. “The objective is always to try to develop a mathematical equation that, based on a model that works , explain what is being observed and also predict future behavior.”

Gonçalves comments on the idea proposed by Matsas and Souza. “The dynamics of a car pileup serve perfectly to illustrate transmission within a black hole,” he says. “The goal is to make an analogy of how the signal propagates. If the cars are too close together, the braking signal doesn’t reach the vehicle behind quickly enough, so one car will crash into the back of the other. Just like if you If it were inside the event horizon and sent, for example, a radio signal saying ‘help, I’m trapped’, that message would not be able to leave the black hole. The analogy is there.”

Parallels like this are a manifestation of the universality of physics, says Gonçalves. “It is the same throughout the Universe. You can apply knowledge from different areas in apparently different situations, but which ultimately represent how physics and the Universe work.”

Although it is not yet possible to immediately predict how this study might be applied to everyday life, scientific analogies stimulate curiosity about how nature works. Basic research, that which is not focused on immediate applications, is essential to pave the way for future innovations. GPS, for example, would not exist without deep fundamental knowledge about how relativity works. Feeding scientific curiosity can, therefore, play an essential role in solving everyday problems.

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Maíra Vallejo and Mariana Coutinho are students at the UFRJ School of Communication. They participated in a scientific journalism workshop promoted by the Serrapilheira Institute and the blog Ciência Fundamental for journalism students at the university, in October 2023. The duo’s text was selected in the final stage of the workshop, which included a practical exercise to suggest an agenda and the production of an article for publication on the blog. The text was edited by Clarice Cudischevitch and Maria Emilia Bender, and checked by Nathália Afonso.

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