Rocks, Rules, and the Fabric of Reality

So, I stumbled across this video the other day that asked a question that seems simple but kinda breaks your brain if you think about it too much: If you throw a rock in the deep emptiness of space, why doesn't it just... stop?

My first thought after watching the video was like, the big bang? universe expanding? Symmetry? – What does these even has to do with a rock flying in space. We all now what happen in space if you are let's say "given some space" - you float around and you float around until either you hit something or something hits you, Classic newtonian physics right? But what does 'frame of reference matching the universe's stretch' even has mean Well, i did some digging and after 23 firefox tabs , It turns out the answer digs into some really fundamental ideas about how reality even works.

The Obvious-But-Not-So-Obvious Answer: Inertia!

The most direct reason the rock keeps trucking along is something we probably learned in school but maybe didn't fully absorb the weirdness of: Newton's First Law, the law of inertia. Basically, stuff keeps doing what it's already doing unless something messes with it. An object at rest stays put, and an object in motion keeps moving at the same speed and in the same direction, unless some force pushes or pulls on it.

In space, far from planets or stars or even much gas, there's practically nothing to mess with the rock. No air resistance, negligible gravity. So, once it's thrown, it just... keeps going. Its own inertia carries it forward. It doesn't need fuel or a constant push; it just is in motion.

Okay, But Why Does Inertia Work? Meet Momentum.

This idea of inertia is tied up with momentum. Momentum is basically how much "oomph" a moving object has (technically, mass times velocity). In an isolated system (like our lonely rock in space), the total momentum doesn't change – Anindya would say it's conserved. Since no forces are changing the rock's momentum, its velocity stays constant.

The Really Deep Reason: It's All About Symmetry (Whoa)

Now, here’s where my mind started to bend a little. Why is momentum conserved? Is it just some arbitrary rule? Nope. It comes from something incredibly deep and beautiful: symmetry.

Specifically, it's linked to spatial translational symmetry. That's a mouthful, but it just means the laws of physics are the same everywhere. If you do an experiment here on Earth (ignoring gravity and air for a sec) and an identical one in a galaxy billions of light-years away, the fundamental rules governing how things move and interact are identical. Space itself doesn't change the rules.

Emmy Noether: German Physics to the rescue

The person who connected these dots was the brilliant mathematician Emmy Noether. Around 1915, she proved something amazing, now called Noether's Theorem. It basically says that for every continuous symmetry in nature, there's a corresponding quantity that's conserved.

Spatial Translational Symmetry (physics is the same everywhere) → Conservation of Linear Momentum. (This is our rock!)
Time Translational Symmetry (physics laws don't change over time) → Conservation of Energy.
Rotational Symmetry (physics laws don't depend on direction) → Conservation of Angular Momentum.

So, the rock keeps its momentum because space itself has this fundamental symmetry. How cool is that? Symmetry isn't just about pretty patterns; it's woven into the very fabric of physical law.

Quick Detour: It's All Relative (Frames of Reference)

Before we tackle the expanding universe bit, we need to touch on frames of reference. How we describe motion always depends on our viewpoint. If you're on a smoothly moving train, you feel stationary relative to your seat, but someone on the ground sees you speeding past. There's no absolute "rest" frame in the universe.

Physics laws look simplest in inertial frames – these are viewpoints that aren't accelerating. Newton's First Law (our inertia law) holds perfectly in these frames. Our rock's steady motion is described relative to one of these local, non-accelerating inertial frames.

The Big Bang & The Expanding Universe: A Whole Different Ball Game

Okay, so what about the Big Bang and the expanding universe? That's definitely happening! The universe started incredibly hot and dense about 13.8 billion years ago and has been expanding ever since.

And it's not like galaxies are just flying through space away from a central explosion point. Space itself is stretching, carrying galaxies along for the ride. Think of dots on an inflating balloon – the dots get farther apart because the rubber between them stretches. Hubble's Law tells us that the farther away a galaxy is, the faster it seems to recede from us, because there's more expanding space between us.

Local vs. Cosmic frame of reference: Why Expansion Doesn't "Push" the Rock

This is the crucial distinction. This is what i was initially mistaken when i was actually reading about this, My assumption was that as we are basically using a static frame of reference for the rock relative to the expanding universe, the rock will slowly match the speed of the expanding universe and will appease still.

The cosmic expansion (the Hubble flow) dominates on huge scales, between clusters of galaxies. But locally – within a solar system, or even within a galaxy or galaxy cluster – gravity holds things together. These bound systems don't expand along with the universe. The motion of objects within these local systems (like our rock, or planets orbiting a star, or even the Andromeda galaxy moving towards us) is determined by local gravity and inertia. This local motion, separate from the overall cosmic expansion, is called peculiar velocity.

For our thrown rock, the effect of the space between it and us stretching is incredibly tiny, completely swamped by its own inertial motion. Its constant velocity isn't because its frame is "locked" to the expansion; it's because it's following the local law of inertia in its local inertial frame. The rock is like the ant walking steadily on the balloon's surface – its own walking is governed by the rubber right under it, even as the whole balloon inflates and carries it away from distant points.

So, That Video...?

The Veritasium actually did a decent job hitting the core point: the rock keeps moving because of momentum conservation, which is linked to spatial symmetry via Noether's theorem. Where it could be a bit clearer is separating the direct cause (inertia/momentum conservation) from the fundamental reason that law holds (symmetry). And it didn't really get into the whole local motion vs. cosmic expansion thing, which was part of my initial confusion.

Wrapping Up

So, there you have it. That rock thrown in space keeps going because:

Inertia: Things in motion stay in motion unless stopped.
Momentum Conservation: Its "oomph" stays constant because no forces are acting on it.
Spatial Symmetry: Momentum is conserved because the laws of physics are the same everywhere (thanks, Noether!).

The Big Bang and the expansion of the universe are real and super important, but they describe the grand scale. They don't directly dictate the rock's local, inertial coasting through the void.

It's kind of amazing how a simple question about a rock can lead you down a rabbit hole involving the fundamental symmetries of spacetime and the history of the entire cosmos. Makes you think, doesn't it?