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Exploring the Universe's Stage

In November, the Physics Olympiad camp received a visit from the National Center of Competence in Research PlanetS and the Center for Space and Habitability at the University of Bern. Now you can read what the participants experienced and learn some fascinating facts about planets yourself. In part two, Jehan Alsawaf teaches us a trick to easily unlock the secrets of gravity in your classroom or home.

The participants of the Physics Olympiad camp gathering around the gravity table. (Image: Sebastian Käser, Physics Olympiad)

Little balls being spread across the gravity table. (Image: Sebastian Käser, Physics Olympiad)

Apart from the inflatable planetarium, the participants of the physics camp also got to learn about how the University of Bern developed and built an instrument for the ESA space probe Juice. (Image: Sebastian Käser, Physics Olympiad)

Metal balls, a round table, and a sizable fabric sheet. With these seemingly ordinary ingredients, anyone can unlock the understanding of basic concepts of the universe. Flip the table upside-down, drape it with a tightly stretched fabric sheet, and voilà— an interactive gravity table materializes. In physics, spacetime is a fundamental concept that merges three dimensions of space with one dimension of time into a unified four-dimensional continuum. In the Swiss Physics Olympiad, spacetime is a hands-on gravity table that represents spacetime interactively.


Are you a teacher looking to introduce your students to the fascinating science behind planets? Apart from making your own gravity table, you can find a number of  teaching materials on the website of NCCR PlanetS.


Of course, you might think: how can a three-dimensional representation truly capture the intricacies of a four-dimensional model? The answer is, well, it can't. The gravity table is an analogy, not a description. Similar to studying circles to learn about spheres, or studying squares to learn about cubes, we visualize spacetime in three-dimensional models to learn about gravity. It is an effective educational method that offers a broad introduction to complicated concepts and serves as a starting point to deeper exploration as it becomes a playground for those who haven't ventured beyond the notion of gravity as just pulling things toward Earth.


Spacetime Curvature. (Copyright: ESA via https://www.esa.int/ESA_Multimedia/Images/2015/09/Spacetime_curvature)


Thanks to the revolutionary redefinition of gravity by general relativity, astronomers' perspectives underwent an epochal transformation. Gravity, once perceived as a pulling force, is now understood as the elegant curvature of space-time. Your journey through space mirrors its curvature. In the absence of warping, particles move in straight lines, along the "flat" spacetime. Warped spacetime, however, curves particle paths. The mastermind behind this cosmic choreography? Mass, the silent architect dictating spacetime's graceful dance.


Here we imagine space as a giant, flexible fabric. Now, we throw in a mass – any mass, really. This mass creates a dent in the fabric, and that is the curvature of space-time. The more massive an object, the deeper the dent in the fabric, the stronger its gravitational pull. The center of gravity is the balancing point of an object. When we discuss radial velocities, we're talking about the speed and direction at which an object moves away from or towards a central point, like a star or a planet. 


Apart from the gravity table, the participants of the physics camp also got to see an inflatable planetarium and learn about how the University of Bern developed and built an instrument for the ESA space probe Juice. This mission has been on its way to Jupiter since April 2023 to explore the largest planet in our solar system and three of its more than 80 moons: the large, icy moons Ganymede, Callisto and Europa.


Why does this matter? Well, it turns out that many distant worlds we're aware of today owe their discovery to radial velocity. Stars, instead of remaining completely static, exhibit subtle movements in a circular or elliptical pattern, responding to the gravitational pull of their smaller celestial companions. Now, here's where it gets interesting. When we look at this star from afar, these subtle moves mess with its usual light spectrum or color signature. If the star is edging closer to us, its spectrum takes a dip toward the bluer, shorter wavelengths. But, if it's moving away, the spectrum shifts toward the redder, longer wavelengths. The way the star is giving us a colorful hint about its planetary dance partner is what we call the radial velocity method. This method, one of the pioneers in detecting exoplanets, remains an indispensable tool for astronomers to further characterize the enigmatic worlds scattered across the cosmos around us. As of 2023, NASA reports 5550 confirmed exoplanets and 4130 planetary systems, all discovered with the help of radial velocity and other methods. But how did it come into existence?


Detecting exoplanets with radial velocity. (Copyright: ESA via www.esa.int/ESA_Multimedia/Images/2019/02/Detecting_exoplanets_with_radial_velocity)


Approximately 4.5 billion years ago, our solar system took shape from a cloud of interstellar gas and dust drawn together by their gravitational pull. This choreography also imparted a slight rotation, creating a disk around the central collapse. Over time, this central core birthed the Sun, while small clumps of gas and dust in the disk migrated across it and merged together in a process called planetesimal accretion, giving rise to planets and their moons. 


By scattering many tiny balls across the fabric and running a heavy metal ball in it, we picture dust and gas swirling and coalescing to form planets and other celestial bodies, giving birth to entire solar systems and other planetary systems. However, our understanding of this process is limited and there remains much to uncover, prompting the dedicated study of exoplanets. The exploration of a wide array of exoplanets and planetary systems, spanning from the small to the colossal, from Earth-like to the extraordinarily peculiar, serves as a crucial endeavor. This exploration not only deepens our insights into the formation and evolution of diverse systems but also offers vital clues in the quest to discern the potential existence and locations of life beyond our celestial neighborhood. These extraordinary systems, beyond our earthly realm, contribute indispensable insights into unraveling the mysteries of our own Solar System's formation and the genesis of our home planet, Earth.


Current artist’s conception of the planet-forming disk around Beta Pictoris, with rings of material, planets, and comets. (Copyright: Courtesy NASA/FUSE/Lynette Cook via imagine.gsfc.nasa.gov/features/bios/roberge/roberge_image.html)


We spent an entire evening in shared awe of these mysteries and many more. To understand gravity is to understand the universe. In fact, a participant approached me and exclaimed, 'Who would have imagined that the key to understanding the universe lies right beneath my plate?' So, as you gather around the table with friends and family this upcoming Christmas, seize the opportunity to engage in intriguing discussions about gravity—because you never know where the secrets of the cosmos might emerge!


This article was written by Jehan Alsawaf, Physics Olympiad volunteer and CSH research assistant. The series was supported by Sophie Krummenacher from NCCR PlanetS/CSH. CSH is an international research team at the University of Bern which fosters interdisciplinary interactions on the formation, detection and characterization of other worlds within and beyond the Solar System. NCCR PlanetS is a project funded by the Swiss National Science Foundation involving the Universities of Bern, Geneva and Zurich as well as ETH Zurich.


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