How can we explain gravity?

How Can We Explain Gravity? Einstein's Curved Spacetime

How can we explain gravity? This fundamental force governs everything from falling apples to orbiting planets, yet its true nature remains partially mysterious. Understanding gravity is crucial for technologies like GPS, which rely on relativity to function accurately. Moreover, most of the universes mass—dark matter and dark energy—interacts only through gravity, making it a key to cosmic mysteries.

Defining the Invisible: Why Gravity is a Mystery

Gravity is the fundamental force of attraction between objects with mass or energy, acting as the cosmic glue that keeps planets in orbit and our feet on the ground. It can be explained through two main frameworks: Isaac Newtons concept of an invisible pulling force and Albert Einsteins theory that mass warps the fabric of space and time. While it feels powerful when we drop a glass or climb a hill, gravity is actually the weakest of the four fundamental forces in nature.

I remember the first time I saw a demonstration of gravity using a large spandex sheet and a heavy bowling ball. The way the sheet dipped looked so simple, yet the math behind it is notoriously complex. Lets be honest: gravity is the most famous thing in the universe that we still do not fully understand. We can measure it with extreme precision, but we are still debating what it actually is at its smallest level.

But there is one catastrophic mistake we would make if we used the wrong theory of gravity - a mistake that would make your phones GPS fail by several kilometers every single day. I will reveal exactly why that happens in the section on practical applications below.

The Newtonian Legacy: A Force of Attraction

In 1687, the publication of the Law of Universal Gravitation changed how we see the world by describing gravity as a predictable force. This theory suggests that every particle of matter in the universe attracts every other particle with a force that depends on two things: how much mass the objects have and how far apart they are. If you double the mass of one object, the pull doubles; if you double the distance between them, the pull drops to one-fourth of its original strength.

This model is incredibly accurate for most human activities. We use it to launch satellites, calculate bridge loads, and predict the moons phases. However, it treats gravity as an action at a distance, meaning it happens instantly.

When I was in school, I found this idea a bit strange - how could the Earth know the Sun was there without anything touching? Newton himself admitted he did not know the mechanism; he just knew the math worked. For over two centuries, no one could find a flaw in this logic until astronomers noticed a tiny, nagging error in the orbit of Mercury that Newtons equations simply could not explain.

The Limit of the Pulling Force

Newtonian physics predicts the paths of planets with nearly perfect accuracy, but it fails in environments with extreme gravity. In the case of Mercury, the point of its orbit closest to the sun - known as the perihelion - shifts slightly every century. Astronomers found a discrepancy of 43 arcseconds per century that Newtonian math could not account for ^[1]. It was a tiny number, but in science, a small error often hides a massive truth. This gap eventually paved the way for a complete rethink of what gravity actually is.

Einstein’s Spacetime: The Fabric of Reality

Albert Einsteins General Theory of Relativity, introduced in 1915, explains gravity not as a force, but as a geometric property of space and time itself. According to this view, space and time are joined in a four-dimensional fabric called spacetime. Massive objects like the Sun do not pull on planets; instead, they curve the spacetime around them. Planets then follow the straightest possible path through that curved space, much like a marble rolling around a bowl.

This shifted our understanding from a tug-of-war to a landscape. Space is not empty. It is a flexible medium. Gravity is just the shape of that medium. When light from a distant star passes near our Sun, it actually bends because the space it travels through is curved. This effect was famously proven during a solar eclipse in 1919. It was a total game-changer. My mind still struggles to visualize four-dimensional curves, but the evidence is undeniable. Einsteins math solved the Mercury problem perfectly, accounting for that missing 43 arcseconds of orbit shift without any extra adjustments.

Why Your Phone Needs General Relativity

Here is the critical mistake I mentioned earlier: if we ignored Einsteins theories, global positioning systems (GPS) would be useless. GPS satellites orbit about 20,000 kilometers above the Earth where gravity is much weaker than it is on the ground. Because gravity warps time as well as space, clocks closer to a heavy mass tick more slowly than clocks further away. This is known as gravitational time dilation.

On a typical day, the clocks on GPS satellites gain about 38 microseconds compared to clocks on the ground.^[2]

While a microsecond feels like nothing, signals for GPS travel at the speed of light. Without correcting for this time difference, the location data on your phone would drift by approximately 10 to 12 kilometers every single day.^[3] Within a week, your phone would think you are in a different city entirely. Your Uber would never find you. Your food delivery would end up in the next state. We literally have to program relativity into our technology just to navigate to the grocery store.

The Quantum Gap: Why Physics is Broken

Despite its success on a cosmic scale, general relativity does not play well with quantum mechanics - the science of the very small. When we try to apply Einsteins equations to atoms and subatomic particles, the math breaks down and produces nonsensical answers like infinity. This suggests that our current explanation of gravity is incomplete. We have two separate rulebooks for the universe that do not agree with each other.

Gravity is also shockingly weak compared to other forces. To put it in perspective, gravity is roughly 10 to the power of 36 times weaker than the electromagnetic force.^[4] You prove this every time you use a tiny refrigerator magnet to lift a paperclip. That small magnets electromagnetic pull is stronger than the gravitational pull of the entire Earth. It is a David and Goliath story where David wins every time. Physicists are currently searching for a theory of Quantum Gravity - perhaps involving hypothetical particles called gravitons - to bridge this gap, but so far, the answer remains elusive.

The Dark Side of Gravity

As we look at the larger universe, gravity presents even deeper mysteries. When we observe how galaxies rotate, they seem to have much more gravity than their visible stars can account for. This led to the discovery of dark matter, an invisible substance that provides extra gravitational pull. Current data suggests that ordinary matter - the stuff made of atoms that makes up you, me, and every star - only accounts for 5 percent of the universes mass and energy ^[5].

The rest is a mix of dark matter, which makes up about 27 percent, and dark energy, which accounts for roughly 68 percent.^[6] Dark energy actually acts like a sort of anti-gravity, pushing the universe to expand faster and faster. I find it humbling that 95 percent of everything out there is still a total mystery to us. We have spent centuries explaining how things fall, only to realize we have barely scratched the surface of why the universe stays together.

Newtonian vs. Einsteinian Gravity

Newtonian Physics

• An invisible pulling force between two masses
• Engineering, sports, and simple orbital mechanics
• Instantaneous (happens at infinite speed)
• Fails near massive objects or at high speeds

General Relativity (Recommended for precision)

• The curvature of spacetime caused by mass and energy
• GPS technology, black holes, and cosmology
• Travels at the speed of light
• The most precise description of gravity to date

The GPS Timing Crisis

In the early days of satellite navigation, engineers in California struggled with a persistent timing drift. The onboard atomic clocks were perfectly calibrated on the ground, but once they reached orbit, they began running fast by several billionths of a second every hour.

The team initially thought it was a hardware defect or radiation interference. They wasted weeks shielding components and running diagnostics, but the error remained. The satellites simply wouldn't stay synchronized with the ground stations.

The breakthrough came when they applied General Relativity. They realized the clocks weren't broken; time itself was literally moving faster in the weaker gravity of space. They had to intentionally de-tune the clocks before launch to make them run "wrong" on Earth so they would be "right" in space.

Once they implemented this offset of 38 microseconds per day, the system stabilized. Today, every GPS receiver on Earth relies on this adjusted math to provide location accuracy within a few meters.

LIGO and the Whisper of Black Holes

A team of researchers spent decades building the LIGO detector to catch gravitational waves - tiny ripples in spacetime. For years, they heard nothing but noise from passing trucks and distant earthquakes, leading many to doubt the billion-dollar project.

The friction was immense: they needed to measure a change in distance smaller than 1/10,000th the width of a proton. The first few trial runs in 2015 were so quiet that the team felt the weight of potential failure.

Then, a clear "chirp" signal appeared. They realized it was the collision of two black holes 1.3 billion light-years away. It wasn't a glitch; it was the first direct proof that space can ripple like water.

This discovery opened a new era of astronomy, allowing us to "hear" the universe for the first time. It confirmed Einstein's 100-year-old prediction with a precision that was previously thought impossible.