Artificial gravity: the search for gravity created by humans

In early 2021, NASA presented a feasibility study for a kilometer-scale space structure. The US space agency is not wrong: to plan long-haul flights, humanity will have to carry its own gravity with it. Without this “pulling down” force, health problems are likely to build up: muscle atrophy, bone loss, impaired vision, immunosuppression, and so on. The co-directed work by Professors Jeffrey Lipton (University of Washington) and Zachary Manchester (Carnegie-Mellon University) accepts the challenge of rotation to achieve a simulation of gravity equivalent to that felt on Earth. But other artificial paths are conceivable … and are foreseen, especially on the side of science fiction works.

© Zachary Manchester, drawing by Tzipora Thompson

The structure imagined by NASA would provide a more comfortable living environment for the crew.

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From gravity to weightlessness

Gravity can be defined as a force exerted between two bodies of mass. In the framework of Newton’s theory formulated in 17i century, counteracting the perceived attraction of an object is tantamount to accelerating its motion enough to orbit it. “If it were possible to throw an apple hard enough, it would begin to revolve around the Earth in an ellipse, as does the Moon,” explains Alexandre Le Tiec, astrophysicist in charge of CNRS research at LUTH (Laboratory of the Universe and its theories). Then the gravity, og, of an object is proportional to the mass of the body that attracts it and inversely proportional to the square of the distance between the two. At the Earth’s surface, this value is about 9.8 m / s2. But as soon as we move away from it, the force of gravity decreases rapidly, until it is almost imperceptible at 408 km, the area where the International Space Station orbits. Astronauts face the effect of weightlessness, or microgravity.

Beyond the long-term critical health issues, the absence of seriousness is problematic on a practical level. In space, the legs are useless and the arms are used for grabbing and propelling. “Two films wanted to take advantage of zero-G parabolic flights, which mimic weightlessness for a few seconds, to add value to the end result,” says Richard Taillet, professor of astrophysics at the University of Savoy-Mont Blanc . The first is Apollo 13 (1995) directed by Ron Howard, and the second is … a pornographic film. When the idea is not to play with the opportunities for the disappearance of gravitational feeling, it is a matter of finding solutions to do without them. As early as 1929, in Fritz Lang’s silent film The woman on the moon, an idea is proposed: use copper hoops fixed to the ground to slide your feet. A fairly common choice in science fiction are also magnetic boots. “More original is the use of this technique with nanomagnetic combinations in the 2009 Canadian series Defying gravity “, Points out the professor-researcher.

The Moon orbits the Earth in an ellipse (image courtesy of NASA).

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The Moon orbits the Earth in an ellipse (image courtesy of NASA).

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The spectacular centrifugal force

Operating without gravity remains a major difficulty if humanity dreams of swarming across the galaxy. The most promising track still includes the idea of ​​artificial gravitation, but this time resulting from rotational motion. Whether it’s the boat DiscoveryOne of 2001, A Space Odyssey (1968), elAvalon of passengers (2016) or even O’Neill’s extraterrestrial cylinder from the literary series rama of Arthur C. Clarke, each of these ships has at least one of its parts placed in rotation. “The rotational movement of the cylinder relative to the so-called” inertial frame of reference “produces an apparent acceleration, also called centrifugal force,” explains Alexandre Le Tiec. This force, for example, will tend to “stretch” the bodies inside the cylinder outward. Just like a children’s tourniquet tends to kick them out. In the case of the cylinder, the goal is to approach an acceleration of 1 g to walk on its inner surface. “Unlike the Earth, where it is necessary to move away from its center so that the attraction decreases, this time it is moving towards the central axis that a possible passenger will feel the attenuated force,” specifies the astrophysicist.

How to transcribe these effects on the screen for viewers? The film crews of all time have simply competed in ingenuity! Grab the feather that seems to float in Stanley Kubrick’s Odyssey. In fact, it is only attached to a window with a decoration placed in the background. “This trick first appeared in the weightless striptease scene Barbarella, in 1968, ”says Richard Taillet. Often the ships themselves were nothing more than cable-suspended models, withdrawn in post-production. Of course, there were some hiccups: glue dots visible on the screen, windows next to the flying saucers (where the crew would normally walk …). But this is not the most important thing according to the professor-researcher: “these films were not intended to educate the public about human physiology in space but to highlight the spectacular side of these techniques. A successful bet! “.

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Giant magnets for shrinking

In 1930, in his novel The last and the first, British author Olaf Stapledon introduced “an ‘artificial gravity’ system based on the properties of the electromagnetic field”. A suggestion that speaks again of magnetogravure field. “These are gravitational fields generated or associated with electromagnetism. Their principle is based on Einstein’s equivalence, according to which all forms of energy produce gravitation in the same way,” describes André Füzfa, a professor at the University. of Namur, Belgium. In his 2016 article ” How space-time curves and solenoids curve lists the hardware needed to set up a technical demonstration experiment and successfully detect the severity thus produced. Because this is where the main difficulty of this work lies: in the extreme weakness of gravity coupling. “Storage solenoids with current technology will provide such weak fields that even their analysis would be problematic,” the professor argues. Therefore, it would be necessary to have access to a very high intensity magnetic source associated with ultrasensitive detection means, of the type used in the monitoring of gravitational waves.

“The magnets to be considered should be able to create a very powerful magnetic field, concentrating a colossal amount of energy in as small a size as possible,” insists André Füzfa. Currently, the closest thing to it is called SMES, for superconducting magnetic energy storage. Magnetic field-shaped energy storage system activated by direct current circulation in a superconducting ring. The whole set would be reminiscent of the giant magnets of CERN’s particle physics laboratories. Precisely, the professor reminds us: “gigantism cannot be avoided, and therefore have artificial gravity systems such as spacecraft of star trek Because gravity is a macroscopic force, it is also the fundamental interaction with the weakest intensity. Thus, the few grams of magnetic material of a fridge magnet are enough to counteract the gravity of the entire planet! and so the result of a magnetic field fi c could be observed by sensors designed for gravitational waves … A mandatory first step if we want to believe in a future where the human species will fly between the stars.

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