What Keeps Our Feet on the Ground and the Universe in Motion?

6 min readAug 15, 2023
Gravity | Quanta Magazine

Gravity: It’s a force that shapes our world and influences the way celestial bodies move through space. But have you ever wondered where this force comes from? At the heart of the captivating realm of astronomy lies gravity, and the journey to understand it is nothing short of fascinating.

Isaac Newton: Unraveling the Threads of Gravity

In the bustling world of the 17th century, a brilliant mind emerged — Isaac Newton. A luminary in both physics and astronomy, Newton brought us the initial theory of gravity that continues to stand the test of time. Picture a young Newton, armed with a telescope and an abundance of mathematical prowess. It was with these tools that he crafted his theory of gravity, a theory that remained solid for centuries until it received its final refinements in the form of Albert Einstein’s general relativity.

Newton’s theory of gravity was groundbreaking, yet it wasn’t without its problems. Even as he crafted his remarkable theory, Newton was faced with aspects of gravity that eluded his complete understanding. Imagine the scene: Newton contemplating a force that seemingly operated instantaneously across the vacuum of space. When questioned about the intricacies of this natural force, he humbly replied, “I frame no hypothesis.” This serves as a reminder that even the brightest minds can find certain mysteries of the universe beyond their grasp.

Gravity’s Universal Reach

Newton’s theory, while not all-encompassing, proved to be an invaluable tool for comprehending the cosmos. It thrives in scenarios involving relatively weak gravity — think of the objects within our solar system, the stars twinkling within our galaxy, and the dance of galaxies across the vast expanse of the universe. So enduring was Newton’s theory that it even earned a spot on Apollo 8’s monumental journey to the moon. When asked who was at the spacecraft’s helm, one of the astronauts answered, “Isaac Newton’s driving, son.”

The Elegance of Newton’s Law

Central to Newton’s theory is his universal law of gravity, a simple yet profound equation that shapes the interaction between celestial bodies. Imagine this equation as a cosmic handshake between two objects: the force between them equals the gravitational constant multiplied by the masses of the objects, all divided by the square of the distance that separates them. This equation is a testament to the unity of our world, bridging the gap between events on Earth and those in the celestial heavens.

From Apples to Orbits: Unifying Forces

You might have heard the story of the apple that inspired Newton’s revelations about gravity. While the apple tree incident is more fiction than fact, Newton’s understanding of gravity indeed unified earthly experiences with cosmic phenomena. The motion of a falling apple shares a mathematical connection with the moon’s orbit around Earth. Remarkably, if you crunch the numbers, you’ll find that an apple falling covers the same distance in one second that the moon deviates from a straight path during its orbit.

Diving into the Depths of Gravity

However, Newton’s law isn’t the ultimate solution when multiple objects come into play. The law holds for isolated pairs of objects but falters in the face of more complex situations. For this reason, the law requires numerical approximations, as the intricacies multiply with the addition of more objects. This is a crucial point to understand the limitations of Newtonian gravity and the importance of advancements in understanding, like Einstein’s general relativity.

The Weighty Matter of Mass and Gravity

Newton’s genius also reshaped our perception of mass and weight. Weight, as we commonly understand it, isn’t static but varies with local gravity. Imagine an elevator ride that makes you feel heavier or lighter than usual. This change in weight doesn’t alter your mass, which is the sum of all the atoms within you. Mass remains a constant, whereas weight dances to the tune of gravity’s local rhythm.

Breaking Free: Orbital and Escape Velocities

Imagine firing a cannon off a towering mountain, and you’ll find yourself in a world of orbital velocities. Isaac Newton, the visionary mind behind gravity’s first principles, pondered the speed at which a cannonball would match the curvature of Earth’s surface as it fell. This speed, the orbital velocity, unlocks the path to a parabolic trajectory that mirrors the planet’s curve, a journey of perpetual freefall.

But Newton’s curiosity didn’t stop there. He delved into the energy required to free an object entirely from Earth’s gravitational grip. This energy called the escape velocity, is the ticket to launch objects beyond Earth’s embrace, sending them on a journey to infinity. For any object, the escape velocity is about 40 percent higher than the circular velocity needed for orbital motion. These fundamental concepts underpin the realm of space travel and telecommunications, propelling humanity into the cosmos.

The Dance of Lagrange Points

As we venture further into space, we encounter the intriguing Lagrange points. These celestial sweet spots, theorized by a French mathematical physicist two centuries ago, represent the balance of gravitational forces. Imagine these points as cosmic equilibrium zones, where minimal energy is needed to keep spacecraft or probes in place. Some Lagrange points are stable, while others require delicate maneuvers to maintain position. NASA and ESA missions often set their sights on these points, including L2, a favored location for satellite missions like WMAP, Herschel, and Planck.

Harnessing Gravitational Assists

Space travel holds its share of ingenious tricks, and one of the most fascinating is gravitational assist. By strategically positioning a fast-moving object behind a larger, more massive body, gravitational interactions can transfer kinetic energy, boosting the smaller object’s speed. This gravitational slingshot effect allows space probes to gain momentum without expending excessive propellant. This technique is a staple for outer solar system missions, where probes utilize close passes by inner solar system objects to catapult themselves toward distant targets.

The Mighty Tidal Force

Gravity’s reach extends far beyond mere attraction; it also births the tidal force. In the embrace of an extended celestial body, like a planet or moon, gravity on the near side dwarfs that on the far side due to the inverse square law. This asymmetry births a stretching force known as the tidal force, shaping the very fabric of celestial bodies. Tides, both in the ocean and on land, ebb, and flow in response to the pull of the moon and sun. Even seismic activity feels the tug of tides, as earthquakes exhibit slight increases during new and full moons.

Gravity: The Cosmic Conductor

In the grand symphony of the universe, gravity stands as the maestro. Despite its title as the weakest of the four fundamental forces, gravity’s far-reaching influence is undeniable. It governs the very architecture of our cosmos, orchestrating the formation of galaxies, stars, and planets. Isaac Newton laid the foundation for our understanding of gravity, and though Einstein expanded the theory to shine in strong gravitational scenarios, Newtonian gravity remains steadfast and precise for most cosmic scenarios.

The Calculations of Cosmic Harmony

Newton’s universal law of gravitation is the cornerstone of this cosmic ballet. It elegantly states that the force between two isolated objects is dictated by their masses and the inverse square of the distance between them. Yet, when it comes to extended objects or interactions between more than two bodies, computers step in to tackle the complex calculations required for an accurate result.

In Conclusion

The history of our understanding of gravity is a tale of brilliance, curiosity, and the relentless pursuit of knowledge. From Newton’s insightful equations to Einstein’s revolutionary insights, the evolution of gravity theory has illuminated the cosmos like never before. As we continue to explore the vastness of space, we stand on the shoulders of these intellectual giants, pushing the boundaries of human understanding to new horizons. So, the next time you gaze at the stars, remember the profound impact of gravity — the force that binds us all in an intricate cosmic ballet.

Gravity isn’t just a force that binds us to the Earth — it’s the symphony that shapes the cosmos. From orbital velocities to Lagrange points, gravitational assists to tidal forces, this force of nature guides our exploration of space and our understanding of the universe’s intricate dance. As we gaze upon the stars and ponder the mysteries of the cosmos, let us never forget the enduring influence of gravity, the cosmic conductor that orchestrates the beauty of the cosmos.