Think about this: You drop a pen from your hand, and it falls to the ground. Simple, right? But when students — especially those studying at a reputed bsc college in Jaipur — are asked “Why did it fall?”, the answers often sound confusing:
- Because the Earth pulls it down.
- Because objects are heavy.
So, why do things fall? And how can we explain gravity in a way that makes sense both in the classroom and in real life?
When you drop a ball, it falls to the ground. When you jump, you come back down. Planets move around the Sun, and the Moon circles Earth. These cosmic motions are all governed by one force — gravity. It is invisible and universal.
To truly understand the answer, we must explore two scientific revolutions — Newton’s Law of Universal Gravitation and Einstein’s Theory of General Relativity. Together, they transformed our understanding of the universe.
Newton’s Law of Universal Gravitation
According to Newton, the idea began with a falling apple. Around 1665, as he watched an apple drop from a tree, he wondered: why does it fall straight down and not sideways? More importantly, could the same force that pulls the apple to Earth also keep the Moon in its orbit?
From this simple question, Newton built one of the cornerstones of modern physics — the Law of Universal Gravitation. He proposed that every object in the universe attracts every other object with a force that depends on their masses and the distance between them.
In mathematical form:
F = G (m₁m₂) / r²
Where:
- F is the gravitational force,
- m₁ and m₂ are the masses of two objects,
- r is the distance between their centers, and
- G is the gravitational constant.
The Limitations of Newton’s Gravity
Newton’s theory worked beautifully for centuries. It could predict the motion of planets, comets, tides, and even the trajectories of cannonballs. However, as science advanced, small cracks appeared in this elegant picture.
- The orbit of Mercury didn’t match Newton’s predictions exactly — it wobbled slightly over time.
- Newton couldn’t explain how gravity acted across empty space. He described what it did, not why.
These puzzles hinted that gravity might be deeper and stranger than a simple pull between masses. It took nearly 250 years — and the genius of Albert Einstein — to redefine our understanding.
Einstein and the Curvature of Spacetime
In 1915, Einstein introduced his General Theory of Relativity, which changed everything we thought we knew about gravity. Instead of being a force between masses, Einstein proposed that gravity is the curvature of spacetime caused by mass and energy.
To visualize this, imagine spacetime as a stretched rubber sheet. Place a heavy ball (like the Sun) on it — the sheet bends around it. Now roll a smaller ball (like Earth) nearby — it moves in a curved path around the heavier one, not because of an invisible pull, but because the surface itself is warped.
Einstein’s famous equation:
E = mc²
taught us that mass and energy are interchangeable — and both can bend spacetime. So, when you drop a ball on Earth, it’s not being “pulled” by a force; rather, it’s following the curved path of spacetime toward the Earth’s center. Falling is simply moving along the geometry of the universe.
Under Einstein’s view, the reason things fall becomes beautifully simple:
They’re following the natural curves of spacetime created by massive objects.
- The Earth bends spacetime around it. You perceive that motion as “falling.”
Einstein realized that when you are in free fall — say, in an elevator or orbiting spacecraft — you actually feel weightless. That’s because gravity is not a force pulling you down; it’s the shape of the space you’re moving through.
This insight led to one of Einstein’s greatest ideas: the equivalence principle, which states that being in free fall and being weightless in space are fundamentally the same experience.
Testing Einstein’s Theory
Einstein’s ideas were radical, but experiments soon confirmed them:
- In 1919, astronomers observed the bending of starlight around the Sun during a solar eclipse — exactly as Einstein predicted.
- Later, the Global Positioning System (GPS) had to account for general relativity; without it, your phone’s location would drift by several kilometers each day.
- Even today, massive discoveries like gravitational waves continue to prove Einstein right.
Newton’s gravity remains an excellent approximation for most everyday uses — from engineering to planetary motion — but in extreme conditions (like near black holes or at high speeds), Einstein’s relativity is essential.
From Falling Apples to Falling Stars
What began with a falling apple led humanity to the edges of black holes and the birth of the universe. Newton explained that things fall because of a force between masses; Einstein revealed that they fall because mass tells space how to curve, and space tells mass how to move.
Even today, gravity remains a source of wonder and mystery. Physicists are still searching for a deeper theory that unites Einstein’s gravity with quantum mechanics — the physics of the very small. Perhaps one day we’ll discover the “quantum” side of gravity, completing the story that began under an apple tree.
Conclusion
So, why do things fall? Because the universe is curved. Because mass shapes space and time. Because the same invisible dance that brings a dropped pen to your desk keeps the stars in their cosmic orbits.
Every time something falls, it’s a quiet reminder that we’re part of a vast and interconnected fabric — one that stretches from the apple in Newton’s orchard to the galaxies swirling billions of light-years away.
Gravity, in the end, isn’t just a force or a curve — it’s the story of how everything holds together.
Blog by:
Diksha Gupta
Assistant Professor of Physics
Department of Science, Biyani Girls College