[♩INTRO].

It's usually not a great idea to break laws. But breaking the laws of science is an exception in fact, it's often how we make progress.

See, scientific laws are just formulas that do a good job of describing how the stuff in the world interacts. Like, there are laws to describe how objects with mass attract, or how objects with the same charge repel. And we call them laws because everything we observe seems to obey them.

But even after laws are in place, scientists keep looking for conditions that break them. Because when laws break down, they almost always tell us something completely new about reality. In fact, there's one law scientists have been testing for over 200 years, even though it has never failed a test:.

Coulomb's law for the force between two charged particles. Because if it ever breaks, it could have an enormous impact on what we believe to be true about our universe including concepts as fundamental as the speed of light. Coulomb's law was invented by the French physicist.

Charles-Augustin de Coulomb in 1785. And it describes the way force is related to the distance between two electrically-charged particles. It says that, as you increase the distance between two charged particles, the force between them drops off in a way that's proportional to that distance, or radius, squared.

Which basically means that as the particles move farther apart, the force between them gets smaller fast. Generations of physicists have used and tested this formula, and it's never failed them. But history gives us a pretty good reason to keep testing the laws of physics, even the ones that have held up as long as this one.

See, Coulomb's law looks a lot like another familiar law one you've probably seen if you've taken high school physics:. Newton's law of gravity. Like a lot of formulas in physics, they both have that radius-squared in the bottom of their fractions, and they both describe how forces drop off with distance.

And they do a really good job. Most of the time. But back in the mid-1800s, astronomers discovered that the planet Mercury's orbit didn't quite follow Newton's law.

Over time, the point where the planet passed closest to the Sun was happening at slightly different points in the orbit. For a while, no one knew what to make of that. Some astronomers even suggested that there was a hidden planet tugging Mercury around.

Another astronomer, named Simon Newcomb, attempted to fix Newton's law by tweaking that r-squared exponent. Instead of two, he suggested that it could be 2.0000001612. But even he knew that was just a Band-Aid.

Because, sure, it helped solve the problem with Mercury, but it didn't address why Newton's law broke down. Then, in 1915, Albert Einstein came up with his theory of general relativity, which fundamentally changed how we look at our universe. With general relativity, Einstein introduced the idea that mass warps spacetime.

And Newton's law doesn't quite cut it when you're near a very massive object that's warping the spacetime around it. For example: the Sun. Newton's law broke down because he didn't have the full picture of how mass affects spacetime.

And that's a good reminder that things we call laws work only under the right conditions. So when we find the conditions that break the law, we can learn some really radical things. So, scientists have a decent reason to question Coulomb's law, which is almost a mirror image of Newton's.

And they've put a ton of effort, across centuries, into testing it. One way physicists do that is by measuring the precision of that two in the exponent. Like what happened with Mercury, one sign of a problem in Coulomb's law would be if certain conditions required an exponent other than exactly two.

That could be a sign that the whole formula needs an overhaul. For now, it seems to be exactly two, as close as we can tell, but scientists have been testing it ever since Coulomb published his law in 1785. In fact, Coulomb himself was already thinking about it.

He knew his measurements couldn't prove that the exponent was exactly two... there was some uncertainty. The exponent could be two plus or minus a smidge, and he put that smidge at 0.04. Over time, scientists designed better experiments with better equipment, and our confidence in the value of that two improved.

In the 20th century, researchers shrank the smidge down to about one billionth. Today, it's less than a quadrillionth. That means we know that the exponent in Coulomb's law is two to nearly 20 digits.

So Coulomb's law has stood the test of time, so far. But if it ever does break down, it would mean our universe is wilder than even we realize. For example, physicists have shown that if Coulomb's law breaks, it would tell us that photons, or particles of light, have mass.

Photons are what carry the electromagnetic force, which Coulomb's law describes. And theorists have shown that the exponent can only be two when the mass of a photon is exactly zero. The inverse is also true: In a universe where that exponent is off by just a little, photons have mass.

A really tiny mass, but mass nonetheless. If that were the case, it would mean photons can't travel at the speed of light. Because nothing with mass can reach the speed of light.

Meaning… light would not travel at the speed of light. That would change what we understand about how electricity, magnetism, and quantum mechanics are related and it would likely take radically new ideas about reality to reconcile them. Seriously, the universe becomes a bizarre place if Coulomb's law breaks down if that little number 2 has anything other than a bunch of zeros after it.

And scientists have found a lot of zeros, but their job is to be skeptical, and to test even the things they think are true. Because when laws break down, they show us the seams in our understanding of reality and force us to rethink the entire nature of our world. Thanks for watching this episode of SciShow!

If you enjoyed this video, you might also like our episode about another law of physics one that was made to be broken. You can check it out right after this. [♩OUTRO].