200 most important Astronomy topics - Sykalo Eugen 2023

The Geodesics

Try it. Go ahead. Take a pen, puff up a balloon, and trace what you think is a straight path from one point to another. What you'll find is not a line but an arc—a curve that defies Euclid’s sensibilities. And yet, this curve is the shortest path between those two points. In physics, we call it a geodesic. In spacetime, we call it destiny.

Let me walk you into this idea. Not with equations—but with awe.

The Straightest Curve You’ll Ever Meet

In flat space, a geodesic is just a straight line. Obvious. But space isn’t flat. Not here. Not near a planet, a star, or a black hole. Our Universe is like a cosmic trampoline, where anything massive—like the Sun—sinks the surface. Try rolling a marble on that dip. Its path will bend. That bend isn’t a force pulling it. That’s just the marble following the shortest path it can—its geodesic.

Einstein shattered the Newtonian apple cart with this concept. He said: gravity is not a force. It’s the geometry of spacetime itself. Mass tells space how to curve; space tells mass how to move. And the movement it dictates? That’s the geodesic.

Imagine driving through the mountains. You twist and turn, hugging cliffs and valleys. Now remove the road signs and say, “Let’s follow the straightest route possible without turning the wheel.” You’ll naturally trace the mountain’s shape. That’s what light does around a star. That’s what Earth does around the Sun.

Even when you’re sitting still, you’re traveling—through time. Because time is not separate from space; they’re fused into spacetime. And your trajectory through it, your personal geodesic, is determined by every whisper of gravity around you.

The Dance of Light and Stars

Remember the first time light was caught bending around the Sun? It was during a solar eclipse in 1919. Eddington took a picture of starlight skirting the edge of the Sun and found what Einstein had predicted: light doesn’t travel straight in the presence of mass. Not because it’s pulled, but because the fabric it moves through is curved.

That moment turned geodesics from an abstract mathematical idea into a photograph. It made spacetime real.

Light always follows a geodesic—a so-called “null” one, in fact, which means it has no length in the way we measure distances. This idea is wild: photons don’t age. For them, the entire journey from the Big Bang to your retina happens in an instant. That’s the kind of madness geodesics bring to the table.

So next time you see the Sun, remember: every beam of light reaching your eye has traced the warped curves of spacetime, dancing a perfect arc along the invisible architecture of the Universe.

Black Holes: Where Geodesics Go to Die (or Loop Forever)

You know what's terrifying? A black hole doesn’t “pull” things in. There’s no cosmic vacuum cleaner. Instead, spacetime itself is so warped that every geodesic leads inward.

You could be a photon—flying free, carefree—and then bam, you cross the event horizon. Beyond that? All roads, all geodesics, point to the singularity. There’s no route out.

Even weirder: some geodesics near a black hole just loop. Light trapped in orbit. Imagine shining a flashlight sideways and watching the beam orbit you like a satellite, forever.

Astronomers recently captured a shadow of a black hole in M87—an image made not of the black hole itself (you can't see that), but of light geodesics skimming its edge, twisted into rings of flame by gravity so intense, spacetime ties itself into knots.

You Are Riding a Geodesic Right Now

“But I’m not moving,” you say.

Yes. You. Are.

Earth is careening around the Sun at 30 kilometers per second. The Sun is doing cartwheels around the Milky Way at 828,000 km/h. And the Milky Way is barreling toward Andromeda at 110 km/s. And all of it—every swirl and pirouette—is dictated by geodesics.

We often talk about gravity like it’s a thing. A pull. A force. But it’s just your attempt to walk a straight line in a world that isn’t flat. What you feel when you fall isn’t a force dragging you—it’s you returning to your natural path, your geodesic.

And standing on Earth? That’s you not following your geodesic. The ground resists it. That’s why you feel your weight.

The Geometry of Destiny

There’s something poetic here, isn't there?

That in a Universe so vast, so chaotic, there are these quiet rules beneath it all. Paths etched not in stone, but in the very weave of space and time. Every planet, every particle, every photon—it all dances to the same silent geometry.

And yet, there’s freedom too. Because spacetime’s shape is not fixed. It evolves. It listens to mass and motion. Which means: the Universe doesn’t just guide us—it responds to us.

You bend spacetime, too. Your body, your thoughts, your atoms—they all nudge the Universe, ever so slightly. Maybe that’s not a lot. But it’s not nothing. And that gives me chills.

A Thought to Carry

I don’t know if the Universe has a purpose. Maybe it doesn’t. But I do know this: it has structure. Not cold, mechanical structure—but curved, dynamic, evolving beauty.

And in that structure, we trace our paths. Not imposed from outside, but emergent from the way the cosmos is shaped.

So next time you trip, or float, or dream—think of geodesics. Think of the straightest path in a curved world.

Think of how even in the deepest darkness, light follows a path. And so can we.