Perhaps the most powerful aspect of physics — and indeed perhaps the most amazing thing about the cosmos as a whole — is the universality of physical laws and theories.

A few scant equations — small enough to fit on your favorite T-shirt — can explain a variety of phenomena from one edge of __the universe__ to the other, and from the earliest moments of the Big Bang to the unfathomable future. Let's get a taste for just how powerful modern physics can be.

**Related: ****The universe: Big Bang to now in 10 easy steps**

## Games of gravity

Albert Einstein's __general theory of relativity__ is our modern theory of how gravity works: matter and energy bend space-time, and in turn the bending of space-time tells matter how to move. The math is a bit complex: it takes a suite of 10 interrelated equations to describe all this bending and warping and moving. But those equations contain enormous power.

For example, in the limit of weak gravity, Einstein's equations reduce to the more familiar expressions of Newtonian __gravity__, which is used to explain everything from the trajectories of thrown baseballs to hydroelectric dams. Beyond the surface of the Earth, Einstein takes more control, where the equations of relativity are used to provide accurate positioning with the GPS system and precisely predict the orbits of all the planets.

Those very same equations, without a single modification, go on to greater feats, revealing the existence of __black holes__ and their workings, the growth of the biggest structures in the universe, the presence of dark matter inside galaxies and the __Big Bang__ itself.

All of that from a set of 10 equations, spanning both cosmic space and cosmic time — indeed, showing that the universe has a finite age in the first place.

**Related: ****How old is the universe?**

## Nuclear energies

When physicists began to crack the nuclear code in the 1940s, they had no idea that their machinations would end of unlocking one of the most perplexing mysteries of astronomy: __how stars work__. Prior to this time, scientists had tried all sorts of attempts to reconcile the age of the Earth as revealed by geology and paleontology (billions of years) with all the known physical ways to keep the sun burning so brightly. These attempts, as a rule, failed quite miserably, with even the best explanations only reaching a few million years.

But nuclear physics was a whole new ball game, and once physicists figured out the conditions necessary to ignite nuclear fusion (namely, insanely high pressures, temperatures, and densities), they realized that such conditions aren't always human-made (inside __nuclear bombs__ and reactors) but can be found in nature itself: in the hearts of stars.

Nuclear fusion of hydrogen is how stars power themselves for billions of years, and the equations that physicists use to understand that process are the exact same ones they use to turn nuclear reactions into usable energy. From the tiniest of atoms to the largest of stars, nuclear physics — a relative newcomer in the world of physics — unites the cosmos in a surprising way.

## Laws of motion

But you don't have to use esoteric equations of relativity or complicated calculations of nuclear reactions to discover the universality of physics. It can be as simple and straightforward as, say, a car crash.

When two vehicles collide, the laws of conservation of energy and momentum apply: the total amount of energy and momentum before the collision must equal the total amount of energy and momentum after the collision. Using these simple statements, investigators can reconstruct the scene of the accident, figuring out which driver was at fault and what led to the collision.

And cars aren't the only thing in the universe that smash together.

Colliding stars. Merging galaxies. Mixing gas clouds. It's rare to find a paper in astronomy or physics that doesn't mention, in some way, the conservation of energy and momentum. Scientists use these principles to understand just about everything in the cosmos.

Why is that gas cloud radiating energy? Conservation of energy and momentum. Why is that __neutron star__ changing its rotation speed? Conservation of energy and momentum.

What will happen when those __galaxies collide__? Conservation of energy and momentum.

The next time you get in a car accident, take a moment to think about momentum, and how it applies throughout the universe, wherever you are.

*Paul M. Sutter** is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of **Ask a Spaceman** and **Space Radio**, and author of "**How to Die in Space**." He contributed this article to **Space.com's Expert Voices: Op-Ed & Insights**. *

*Learn more by listening to the episode 'What's the most amazing thing about the universe?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to @iva_987* *for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.*