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When you study physics, they teach you a bunch of rules. Things like how you can't create energy, you can't travel faster than light — it's like, stop telling me what to do, man!

These rules are helpful, though, because they give you a framework for how the universe works. Like, no matter how complicated things get, at least it's not a lawless wasteland out there. Except, there is one law of physics that's fundamentally breakable, even though scientists don't usually treat it that way: the second law of thermodynamics.

To put it simply, this law says that a system will never become more orderly on its own. The amount of disorder will always increase or stay the same. It's a cornerstone of physics, and it tells us how everything from car engines to galaxies should behave.

But if you look closely at it, you can see that it's a law that was made to be broken. And for the world of nanotechnology, that could lead to unexpected consequences. Although you might have a pretty intuitive idea of what disorder is — like, if you've looked at your sibling's bedroom recently — scientists have a more specific way of thinking about it.

They measure the amount of disorder with a concept called entropy. Entropy can be considered a measure of randomness — or how orderly a bunch of particles are — but it can also be considered the amount of energy that's unavailable to do work. Like, imagine running a car engine.

No matter what you do, the engine will always turn some amount of fuel into waste, like heat. This is unavailable energy, because it can't be used to make your car go forward. According to the second law of thermodynamics, the longer you run your car, the more unusable energy you'll get.

In other words, the more entropy will increase. This observation is actually what led to the discovery of the second law in the first place — although it was discovered in the 1800s, so it was about steam engines, not cars. And today, we know that concept applies everywhere.

It's why engineers still can't make a perfectly efficient machine, and — on a much larger scale — it's why the universe will eventually run out of use-able energy. It's even why the layers of milk and espresso in your cappuccino will blend together if you leave the cup out long enough. The two liquids might start out all neat and all artistically separated, but they'll eventually become more random and will mix.

Essentially, the second law says that entropy will never decrease on its own. Even if you do create order in one system, you'll always create disorder elsewhere, like by generating heat. But for as confident as this law sounds, it isn't bulletproof.

If you look closely at why the second law exists, you start to realize that it has a pretty big loophole. At the most basic level, the second law is true because molecules move around randomly, due to their thermal energy. And they're equally likely to move in any direction.

Ultimately, this means there are way more opportunities for particles to end up disordered than ordered, so disorder is more likely to increase. Think about that cappuccino again. There are probably around a few septillion molecules — that's a one followed by 24 zeroes — in the drink.

And there are only a few ways those molecules can be arranged so that all of the milk is on one side, and all of the espresso is on the other. So assuming one arrangement is just as likely as any other, the odds of your cappuccino getting more orderly are really, really tiny. But the thing is… “really, really tiny” is not quite impossible.

It could happen. Of course, the odds of this happening for a cup of coffee are so slim that they're kind of ridiculous, and they don't mean much to physicists. That's why the second law can so confidently say that the entropy of a system never decreases on its own.

But in reality, that law is just a statement about the statistics of particle motion. In other words, it's so likely for entropy to increase — or at least stay the same — that we can functionally say it does it all the time. Except, for small systems, we can't assume that.

And by “small”, I don't only mean systems of, like, two atoms. In recent decades, scientists have found that in systems with up to 100 particles, it's not too unlikely for random chance to decrease entropy, just for a little bit. One of the biggest experiments came in 2002, when Australian researchers published evidence for entropy decreasing in a collection of micrometer-sized beads suspended in water.

The random thermal jiggling of water molecules was constantly pushing the beads around, and for the most part, as expected, the beads were moved randomly. But then the researchers looked harder. And they found that sometimes, by pure chance, the beads were pushed in a specific direction, causing them to noticeably speed up.

In other words, the beads were turning the disordered, random heat energy of the water into ordered kinetic energy — energy that can do work. And it was all for free. This effect only persisted for a few tenths of a second, but it proves that on a small scale, you can cheat the system, at least briefly.

This wasn't the only paper to investigate this idea, either. There was another in 2005 that suggested these effects might be present in photosynthesis, and a few more from 2013 and 2016 that explored how this might apply on the quantum level. So it's something researchers are actively exploring.

While this might sound like a random technical detail, these studies are actually really significant. Because as nanotechnology continues to develop, we're going to be building things like engines and transistors on these smaller and smaller scales. Realistically, this loophole in the second law doesn't mean we'll be able to make a bunch of tiny, 100% efficient engines or anything.

But it does mean they might sometimes pick up energy from their environment, or their molecules might move differently than expected. And that's definitely important for us to know. The rules developed back in the 1800s will have to be re-thought to control things at these tiny scales.

Otherwise, we might be in for some surprises. Thanks for watching this episode of SciShow! If you want to learn more about the laws of physics and how they apply to the world around you, you can check out Crash Course Physics, an amazing series put together by one of our sister channels.

You can find it at youtube.com/crashcourse. [♪ OUTRO].